Research and Thesis on Earthing System in LV Networks

CHAPTER 1

NEUTRAL EARTHING, EARTHING SYSTEM STANDARDS IN LV NETWORKS & NEUTRAL FLOATING

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1.1 Introduction In power system, Grounding and Earthing means connecting frame or enclosure of electrical equipment ( non-current carrying part in normal conditions) or some electrical part of the power system (e.g. neutral point in a star connected system or one conductor of the secondary of a transformer etc.) to earth i.e. soil to provide a low impedance path for the fault current. This connection may be through a conductor or some other circuit element (i.e. resistor, a circuit breaker etc.) depending upon situation. The term earthing and grounding have the same meaning and it is a means of making a connection between the system or equipment and the general mass of the earth. Regardless of the method of connection to earth, grounding or earthing provides two major advantages. Firstly earthing provides protection to the power system. For example, if the neutral point of a star-connected system is grounded through a circuit breaker and phase to earth fault occurs on any one line, a large fault current will flow through circuit breaker. The circuit breaker will open to isolate the faulty line. This protects the system from harmful effects of the fault. This type of earthing is called system or neutral earthing. Secondly earthing of electrical equipment (e.g. domestic appliances, hand-held tools, industrial motors etc.) ensures the safety of the persons handling equipment. If insulation fails, there will be a direct contact of the live conductor with the metallic part (i.e. frame) of the equipment. Any person in contact with the metallic part will be subjected to a dangerous electric shock that can be fatal. This type of earthing is called equipment earthing.

1.2 Neutral conductor of LV networks The intermediate conductor in a polyphase (three-phase) electrical system usually grounded or made intentionally at zero potential. A conductor (when one exists) of a polyphase circuit, or a single phase 3-wire circuit, which is intended to have a voltage such that the voltage differences between it and two or more ungrounded conductors are approximately equal in magnitude and equally spaced in phase. The neutral conductor is normally connected to the neutral point of a system that is intended to carry current under normal conditions. There is differentiation between the ―neutral conductor‖ and the ―equipment grounding conductor‖ which are in fact both ultimately connected to the neutral point of a system. The differentiation is that under some normal conditions, the ―neutral conductor‖ is expected to be current carrying while under normal conditions the equipmentgrounding conductor is never a current carrying conductor.

1.2.1 Importance of Neutral conductor in LV Networks. A neutral conductor is very important in LV voltage networks as it provide two different supply voltage levels.

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In three phase applications: Most big appliances such as air conditioners, dryers, some kitchen ranges, etc., which need to draw high power, are nowadays supplied not from a 120 volt "leg" but at a full 240 volts by connecting them to both of the legs, "+120" and "-120". In Single-Phase Applications: However many of these appliances also need a 120 volt feed for such things as lamps, time-clocks, program control circuits, etc. To get those 120 volts they must use the neutral conductor which comes from the center tap of the pole transformer. For safety purpose: An electrical circuit requires at least two wires, whether it is ac or dc. In mains ac, one of the wires is connected to ground for safety. This is called the neutral. So the circuit will still work without the neutral connected to ground but not be so safe. The reason the neutral makes it safer is that a current will flow in the event of a fault on the hot wire and cause the protection (fuse or breaker) to operate. Act as a Return Path: Commercial city electricity comes from the power plant in the form of three-phase current. Each phase requires one wire, and the neutral is the common return path for all three. Unbalance loads: if there is any unbalance loads on three phase conductors then neutral will act as return path for the current towards the source.

1.3 Neutral Earthing System in LV Networks. A system in which the neutral is connected to earth, either solidly, or through a resistance or reactance of low enough value to reduce materially transient oscillations and to give a current sufficient for selective earth fault protection. The neutral earthing, that is to say the connection between the transformer neutral points and earth, is of high importance to the behavior of a power system during an unsymmetrical fault. Two important functions of neutral earthing are to detect earth faults and to control the fault current, since large fault currents can cause the potential rise of exposed parts of the power system to reach dangerous levels. In early power systems were mainly neutral ungrounded due the fact that the ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries. These power systems required ground detection systems, but locating the fault often proved difficult in unearthed systems. Although achieving the initial goal of continuity of supply, the ungrounded system provided no control of transient over-voltages. A capacitive coupling exists between the system conductors and ground in a typical distribution system.

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Figure 1.1: Ungrounded system with a single line-to-ground fault. As a result, their series resonant L-C circuit can create over-voltages well in excess of line-toline voltage when subjected to repetitive re-strikes of one phase to ground fault shown in figure 1.1.This in turn reduces insulation life in possible equipment failure and also provide hazardous environment for workers. Neutral earthing systems are similar to fuses in that the do nothing until something in the system goes wrong. The, like fuses, they protect personal and equipment from damage. Neutral earthing provides protection to the power system. For example, if the neutral point of a star-connected system is earthed through a circuit breaker and phase to earth fault occurs on any one line as shown in figure 1.2. A large fault current will flow through circuit breaker. The circuit breaker will open to isolate the faulty line. This protects the system from harmful effects of the fault.

Figure 1.2: Neutral Earthed system with single-line-to-ground fault. System grounding refers to the intentional connection of a phase or neutral conductor to earth, the purpose of which is to control the current to earth or to keep this within predictable limits. It also provides a path for current to flow, which allows the detection of unwanted connection between the system and ground. Sufficiently large current are able to activate or operate protective device when the first phase-to-ground fault occurs as shown in figure 1.2.

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There are many neutral grounding options available for both Low and medium voltage power systems.

1.4 Earthing Standards in LV Networks Earthing systems are governed by standard IEC 60364-3. There are three types of systems: IT, TT and TN. 1.4.1 TT Earthing System In this system, the supply source has a direct connection to earth. All exposed conductive parts of an installation also are connected to an earth electrode that is electrically independent of the source earth. 1.4.3 IT system (isolated neutral) This earthing system has the following features:  

No intentional connection is made between the neutral point of the supply source and earth. All Exposed and extraneous-conductive-parts of the installation are connected to an earth electrode.

1.4.2 TN Earthing System In a TN earthing system, the supply source (transformer neutral) is directly connected to earth and all exposed conductive parts of an installation are connected to the neutral conductor. The several versions of TN systems are shown below. 1.4.2.1 TN-C Earthing System TN-C system has the following features:  

Neutral and protective functions are combined in a single conductor throughout the system. (PEN—Protective Earthed Neutral). The supply source is directly connected to earth and all exposed conductive parts of an installation are connected to the PEN conductor.

1.4.2.2 TN-S Earthing System TN-S system has the following features: 

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A TN-S system has separate neutral and protective conductors throughout the system. The supply source is directly connected to earth. All exposed conductive parts of an

installation are connected to a protective conductor (PE) via the main earthing terminal of the installation. 1.4.2.3 TN-C-S Earthing System TN-C-S earthing system has the following features:   

Neutral and protective functions are combined in a single conductor in a part of the TNC-S system. The supply is TN-C and the arrangement in the installation is TN-S. Use of a TN-S downstream from a TN-C. All exposed conductive parts of an installation are connected to the PEN conductor via the main earthing terminal and the neutral terminal, these terminals being linked together.

1.5 Neutral Floating in LV Networks If The Neutral Conductor opens, Break or loose at either its source side (Distribution Transformer, Generator) or at Load side (Distribution Panel of Consumer), the distribution system‘s neutral conductor will ―float‖ or lose its reference ground Point. The floating neutral condition can cause voltages to float to a maximum of its Phase voltages RMS relative to ground, subjecting to its unbalancing load Condition. If the Star Point of Unbalanced Load is not joined to the Star Point of its Power Source (Distribution Transformer or Generator) then Phase voltage do not remain same across each phase but its vary according to the Unbalanced of the load. As the Potential of such an isolated Star Point or Neutral Point is always changing and not fixed so it‘s called Floating Neutral. Floating Neutral conditions in the power network have different impact depending on the type of Supply, Type of installation and Load balancing in the Distribution. Broken Neutral or Loose Neutral would damage to the connected Load or Create hazardous Touch Voltage at equipment body. Important: Broken neutrals can be difficult to detect and in some instances may not be easily identified. Sometimes broken neutrals can be indicated by flickering lights or tingling taps. If you have flickering lights or tingly taps in your home, you may be at risk of serious injury or even death.

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CHAPTER 2

EARTHING IN ELECTRICAL NETWORK, PURPOSE AND LV EARTHING SYSTEM STANDARDS

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2.1 Introduction The whole of the world may be considered as a vast conductor which is at reference (zero) potential. In the UK we refer to this as 'earth' whilst in the USA it is called 'ground'. People are usually more or less in contact with earth, so if other parts which are open to touch become charged at a different voltage from earth a shock hazard exists. The process of earthing is to connect all these parts which could become charged to the general mass of earth, to provide a path for fault currents and to hold the parts as close as possible to earth potential. In simple theory this will prevent a potential difference between earth and earthed parts, as well as permitting the flow of fault current which will cause the operation of the protective systems. (IEE)   



In LV power distribution, safety of people from electric shocks is crucial. Therefore, some protection methods must be applied in LV distribution systems. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply. The conductor that connects the exposed metallic parts of the consumer's electrical installation is called protective earth (PE). A protective earth avoids electric shocks by keeping the exposed conductive surfaces at earth potential if insulation fails. (In USA, it is called grounding) In normal condition, no current allowed to flow in PE conductor. During a fault, high short circuit current will trip the circuit breaker or blow the fuse. In case of high impedance line-to-ground fault, a residual-current device (RCD) may operate.

2.2 Purpose, Advantages and Disadvantages of Earthing An effective earthing system is a fundamental requirement of any modern structure or system for operational and/or safety reasons. Without such a system, the safety of a structure or system, the equipment contained within it and its occupants is compromised. 1. Safety for Human life/ Building /Equipment. 

   

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To save human life from danger of electric shock or death by blowing a fuse i.e. to provide an alternative path for the fault current to flow so that it will not endanger the user. To protect buildings, machinery and appliance under fault conditions. To ensure that all exposed conductive parts do not reach a dangerous potential. To provide safe path to dissipate lightning and short circuits currents. To provide stable platform for operation of sensitive electronic equipments i.e. to maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.

2. Over-Voltage Protection. Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the system. 3. Voltage stabilization. There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent (everywhere present at the same time) conductive surface, and so it was adopted in the very beginnings of electrical distribution systems as a nearly universal standard for all electric systems.

Disadvantages of Earthing System The two important disadvantages are: 1. - Cost: the provision of a complete system of protective conductors, earth electrodes, etc. is very expensive. 2. - Possible safety hazard: It has been argued that complete isolation from earth will prevent shock due to indirect contact because there is no path for the shock current to return to the circuit if the supply earth connection is not made (see fig 2a). This approach, however, ignores the presence of earth leakage resistance (due to imperfect insulation) and phase-to-earth capacitance (the insulation behaves as a dielectric). In many situations the combined impedance due to insulation resistance and earth capacitive reactance is low enough to allow a significant shock current (see fig 2b).

Fig 2: Danger in an unearthed system a) b)

apparent safety: no obvious path for shock current actual danger: shock current via stray resistance and capacitance)

Source: From 16th Edition Electricians Guide by IEE Regulations.

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2.3 Definition of standardized earthing systems in LV networks. Electrical power was actually used in 1900. Today electrical installation standards are highly developed and cover all major aspects for a safe installation. In LV, the reference standard is IEC 60364; other standards are also of great importance such as IEE (institution of electrical engineers) regulations and BS 7430 which are following nearly IEC 60364. Standard makers have paid particular attention to the measures to be implemented to guarantee protection of personnel and property. The choice of these methods governs the measures necessary for protection against Indirect-contact hazards. The earthing system qualifies three originally independent choices made by the designer of an electrical distribution system or installation:  The type of connection of the electrical system (that is generally of the neutral conductor) and of the exposed parts to earth electrode(s)  A separate protective earth conductor or protective conductor and neutral conductor being a single conductor  The use of earth fault protection of overcurrent protective switchgear which clear only relatively high fault currents or the use of additional relays able to detect and clear small insulation fault currents to earth. In practice, these choices have been grouped and standardized as explained below. Each of these choices provides standardized earthing systems with three advantages and drawbacks:   

Connection of the exposed conductive parts of the equipment and of the neutral conductor to the PE conductor results in equipotentiality and lower overvoltage‘s but increases earth fault currents. A separate protective conductor is costly even if it has a small cross-sectional area but it is much more unlikely to be polluted by voltage drops and harmonics, etc. than a neutral conductor is. Leakage currents are also avoided in extraneous conductive parts. Installation of residual current protective relays or insulation monitoring devices are much more sensitive and permit in many circumstances to clear faults before heavy damage occurs (motors, fires, electrocution). The protection offered is in addition independent with respect to changes in an existing installation.

2.4 LV Earthing System The IEC 60364 standard, in 3-31 and 4-41, BS standard 7430 and the Institution of Electrical Engineers (lEE) regulations has defined and developed 3 main types of earthing systems (ES). The philosophy of the IEC standard is to take into account the touch voltage (Uc) value resulting from an insulation fault in each of the Systems.

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  

TN system TT system IT system

As far as earthing types are concerned, letter classifications are used. The first letter indicates that how earthing is done on source side (Generator or Transformer).  T: Direct connection of one point to earth  I: isolated from earth or high impedance-earthed neutral (e.g. 2,000 Ω)  The second letter indicate the relationship of exposed conductive parts to earth 

T: Direct electrical connection of exposed conductive parts of the installation to earth, independently of the earthing of any point of the power system  N: Direct electrical connection of the exposed conductive parts to the neutral point or conductor of the power system. Subsequent letter if any shows the arrangement of neutral and protective conductor:  

S: separated protective and neutral conductor (5wire); C: Neutral and protective function combined in a single conductor (4wire).

2.4.1 TT System. One point at the supply source is connected directly to earth. All exposed- and extraneousconductive-parts are connected to a separate earth electrode at the installation. This electrode may or may not be electrically independent of the source electrode. Figure 2.1, show the TT system.

Figure 2.1: Directly Earthed Neutral (TT system) in LV System. Source: BSI Standards Publication BS 7430:2011 Code of practice for protective earthing of electrical installations.

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Key: 1. Source of energy 3. Source earth 5. Exposed conductive parts

2. Consumers’ installations 4. Equipment or Loads in installations 6. Installation earth electrode

2.4.1.1 Specific Characteristics of TT system     

The installation of RCDs is compulsory. All exposed conductive parts protected by the same protective device should be connected to the same earth. The neutral earth and the exposed conductive part earth may or may not be interconnected or combined. The neutral may or may not be distributed. Earth electrode must have a low resistance to be able trip the circuit breaker. But sometime it is difficult to achieve low resistance. Therefore, RCD device must be used to protect for leakage current in the circuit.

2.4.1.2 Fault Behavior in the TT Earthing System Figure 2.2 explain fault occurs in TT earthing system. When an insulation fault occurs, the fault current Id is mainly limited by the earth resistances (Ra and Rb). At least one residual current device (RCD) must be fitted at the supply end of the installation. In order to increase availability of electrical power, use of several RCDs ensures time and current discrimination on tripping [16].

Figure 2.2: The fault behavior in the TT earthing system Source: ―Comparison the Performances of Three Earthing Systems for Micro-Grid Protection during the Grid Connected Mode‖ Scientific research paper by Rashad Mohammedeen Kamel, Aymen Chaouachi, Ken Nagasaka.

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2.4.1.3 Advantages:    



  

Simplest solution to design and install. Used commonly in installation supplied directly by the public LV distribution network. Does not require continuous monitoring during operation (a periodic check on RCD may be necessary). Protection is ensured by special devices, the residual current devices (RCD). Which also prevent the risk of fire when they are set to ≤ 300 mA. Each insulation fault results in an interruption in the supply of power, however the outage is limited to the faulty circuit by installing the RCDs in series (selective RCDs) or in parallel (circuit selection). Loads or parts of the installation which, during normal operation, cause high leakage currents, require special measures to avoid nuisance tripping i.e. supply the loads with a separation transformer or use specific RCDs. Protection of persons is ensured by RCD device, RCD causes the de-energizing of switchgear as soon as the current has a touch voltage greater than safety voltage Uf. Easy location of faults. Upon occurrence of an insulation fault, the short-circuit current is small.

2.4.1.4 Disadvantages:    

Switching upon occurrence of the first insulation fault Each customer needs to install and maintain its own ground electrode. Safety and protection depends on the customer, thus complete reliability is not assured. High over voltages may occur between all live parts and between live parts and PE conductor. Possible overvoltage stress on equipment insulation of the installation.

2.4.2 TN Systems (exposed conducting parts connected to neutral) In a TN earthing system, the supply source (transformer neutral) is directly connected to earth and all exposed conductive parts of an installation are connected to the neutral conductor. Safety of personnel is guaranteed, but that of property (fire, damage to electrical equipment) is less so. The three sub-systems in TN earthing system are described below.

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2.4.2.1 TN-C System It has combined PE and N conductor all the way from source to the device. In a TN-C system (Figure 2.3) the neutral and protective functions should be combined in a single conductor (PEN) throughout the entire system. Multiple connections to earth are recommended along the PEN conductor and the source is solidly earthed.

Figure 2.3: TN-C system Source: BSI Standards Publication BS 7430:2011 Code of practice for protective earthing of electrical installation Key: 1. Source of energy 3. Additional source earth

2. Consumers’ installations 4. Source earth

5. Equipment in installations 6. Exposed conductive parts PEN combined protective and neutral conductor

The need for multiple earth connections is because if the neutral becomes open-circuit for any reason, the exposed-conductive-parts will rise to line to earth voltage in the case of single-phase connections and a value up to line to earth voltage in the case of three-phase connections, depending on the degree to which the load is unbalanced. Therefore PEN conductor must therefore be connected to a number of earth electrodes in the installation. Earthing connections must be evenly placed along the length of the PEN conductor to avoid potential rises in the exposed conductive parts if a fault occurs. This system must not be used for copper cross-sections of less than 10 mm² and aluminium cross-sections of less than 16 mm², as well as downstream of a TNS system (see IEC 60364-5, section 546-2).

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2.4.2.2 TN-S system In a TN-S (Figure 2.4) the Neutral and Protective conductors should be kept separate throughout the system and the source is solidly earthed. PE and N conductors are never connected together in TN-S system. Earthing connections must be evenly placed along the length of the protection conductor PE to avoid potential rises in the exposed conductive parts if a fault occurs. This system must not be used upstream of a TNC system.

Figure 2.4: TN-S system Source: BSI Standards Publication BS 7430:2011 Code of practice for protective earthing of electrical installations. Key 1. Source of energy 3. Additional source earth 5. Equipment in installation

2. Consumers’ installations 4. Source earth 6. PE protective Earth

A TN-S system has a particular disadvantage that in the event that the protective conductor becomes open circuit, there is no indication that a fault has occurred and installations can unknowingly be left without an earth. In the event of an earth fault all of the exposedconductive-parts within a consumer installation may be raised to a hazardous potential. Earth fault protection devices will not operate as there will be no flow of current to earth.

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2.4.2.3 TN-CS system In a TN-C-S system, (Figure 2.5) the neutral and protective functions should be combined in a single conductor (PEN) from the source (solidly earthed) up to the consumers intake. Multiple connections to earth are recommended along the PEN conductor. Within the consumer‘s installation the neutral and protective conductors should be kept separate. Means the supply is TN-C and the arrangement in the installation is TN-S. This system uses TN-S downstream from a TN-C. All exposed conductive parts of an installation are connected to the PEN conductor via the main earthing terminal and the neutral terminal, these terminals being linked together.

Fig 2.5: TN-CS system Source: BSI Standards Publication BS 7430:2011 Code of practice for protective earthing of electrical installations. Key: 1. Source of energy 3. Additional source earth 5. Equipment in installation PEN

2. Consumers’ installations 4. Source earth 6. Exposed-conductive-parts

Combined protective and neutral conductor

This system is also known as PME (protective multiple earthing). In the UK, this system is also known as a protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia and New Zealand being designated as multiple earthed neutral (MEN).

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2.4.2.4 Specific characteristics of TN system Generally speaking, the TN system:  Requires the installation of earth electrodes at regular intervals throughout the Installation.  Requires that any modification or extension be designed and carried out by a qualified electrician.  May result, in the case of insulation faults, in greater damage to the windings of rotating machines.  May, on premises with a risk of fire, represent a greater danger due to the higher fault currents. 2.4.2.5 Fault Behavior in the TN Earthing System Figure 2.6 shows the fault behavior in the TN earthing system and the path of the fault current. When an insulation fault is present, the fault current Id is only limited by the impedance of the fault loop cables. Short circuit protection devices (circuit breaker or fuses) generally pro-vide protection against insulation faults, with automatic tripping according to a specified maximum breaking time (depending on phase-to-neutral voltage Uo).

Figure 4: A fault behavior in the TN-S earthing system. Source: ―Comparison the Performances of Three Earthing Systems for Micro-Grid Protection during the Grid Connected Mode‖ Scientific research paper by Rashad Mohammedeen Kamel, Aymen Chaouachi, Ken Nagasaka.

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2.4.2.6 Advantages of the TN Earthing System 

 

    

The TN earthing system always provides a return path for faults in the LV grid. The grounding conductors at the transformer and at all customers are interconnected. This ensures a distributed grounding and reduces the risk of a customer not having a safe grounding. Lower earthing resistance of the PEN conductor. TN system has the advantage that in case of an insulation fault, the fault voltages (touch voltages) are generally smaller than in TT earthing systems. This is due to the voltage drop in the phase conductor and the lower impedance of the PEN conductor compared with the consumer earthing in TT systems. No overvoltage stress on equipment insulation. TN-S system has the best Electromagnetic Compatibility (EMC) properties for 50 Hz and high frequency currents, certainly when LV cable with a grounded sheath is applied. In TNS, due to the separation of the neutral and the protection conductor provides a clean PE (computer systems and premises with special risks). TN earthing system could work with simple over current protection. High reliability of disconnection of a fault by overcurrent devices (i.e. fault current is large enough to activate the over current protection devices).

2.4.2.7 Disadvantages of the TN Earthing System         

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Faults in the electrical network at a higher voltage level may migrate into the LV grid grounding causing touch voltages at LV customers. A fault in the LV network may cause touch voltages at other LV customers. Potential rise of exposed conductive parts with the neutral conductor in the event of a break of the neutral network conductor as well as for LV network phase to neutral and phase to ground faults and MV to LV faults. The utility is not only responsible for a proper grounding but also for the safety of customers during disturbances in the power grid Protection to be fitted in case of network modification (increase of fault loop impedance). Third and multiples of third harmonics circulate in the protective conductor (TNC system). The fire risk is higher and, moreover, it cannot be used in places presenting a fire risk (TNC system). TN-C system is less effective for Electromagnetic Compatibility (EMC) problems. Upon occurrence of an insulation fault, the short-circuit current is high and may cause damage to equipment or electromagnetic disturbance.

2.4.3 IT system (isolated or impedance-earthed neutral) In IT system (Figure 2.9) has the source either connected to earth through earthing impedance or is isolated (Insulated) from the earth. All of the exposed-conductive parts of an installation are connected to an earth electrode in a similar manner to a TT arrangement. The IT system can have an unearthed supply, or one which is not solidly earthed but is connected to earth through current limiting impedance.

Figure 2.9: IT system Source: BSI Standards Publication BS 7430:2011 Code of practice for protective earthing of electrical installations Key: 1 Source of energy

5 Equipment in installation

2 Earthing impedance

6 Exposed-conductive-parts

3 Consumers‘ installations

7 Installation earth electrode

4 Source earth 2.4.3.1 Specific characteristics:  

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Switching upon occurrence of a double fault is usually generated by phase-to-phase fault protective devices (circuit-breakers, fuses, etc.). If the short-circuit current is not large enough to activate protection against phase-tophase faults, notably if the loads are far away, protection should be ensured by residual current devices (RCDs).



  

It is compulsory to install an overvoltage limiter between the MV/LV transformer neutral point and earth. If the neutral is not accessible, the overvoltage limiter is installed between a phase and earth. It runs off external over voltages, transmitted by the transformer, to the earth and protects the low voltage network from a voltage increase due to flashover between the transformer‘s medium voltage and low voltage windings. A group of individually earthed loads must be protected by an RCD. Generally an IT system would be chosen in locations such as medical centre‘s and mines where the supply has to be maintained even in the event of a fault, and where the connection with earth is difficult (for example a mobile generator). The total lack of earth in some cases, or the introduction of current limiting into the earth path, means that the usual methods of protection will not be effective. For this reason, IT systems are not allowed in the public supply system in the UK.

2.4.3.2 Fault Behavior in the IT Earthing System 2.4.3.2.1 First Fault in the IT earthing system Figure 2.10 shows the occurrence of the first fault in the IT earthing system. The fault voltage is low and not dangerous. Therefore it is not necessary to disconnect an installation in the event of a single fault. However it is essential to know that there is a fault and need to track and eliminate it promptly, before a second fault occurs. To meet this need the fault information is provided by an Insulation Monitoring Device (IMD) monitoring all live conductors, including the neutral [16].

Figure 2.10: First insulation fault current in the IT earthing system Source: ―Comparison the Performances of Three Earthing Systems for Micro-Grid Protection during the Grid Connected Mode‖ Scientific research paper by Rashad Mohammedeen Kamel, Aymen Chaouachi, Ken Nagasaka.

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2.4.3.2.2 Second fault in the IT earthing system Figure 2.11 shows the occurrence of the second fault in the IT earthing system. Maximum disconnection times for the IT earthing system are given in Table 2 (as in IEC 60364 tables 41B and 48A) [16]. The IT earthing system used when safety of persons and property, and continuity of service are essentials

Figure 2.12: Second insulation fault current in IT system (distributed neutral). Source: ―Comparison the Performances of Three Earthing Systems for Micro-Grid Protection during the Grid Connected Mode‖ Scientific research paper by Rashad Mohammedeen Kamel, Aymen Chaouachi, Ken Nagasaka. 2.4.3.3 Advantages  

System providing the best service continuity during use. When an insulation fault occurs, the short-circuit current is very low.

2.4.3.4 Disadvantages      

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Requires maintenance personnel to monitor the system during use. Overvoltage limiters must be installed. Requires a high level of insulation in the network. Protection of the neutral conductor must be ensured Locating faults is difficult in widespread networks. When an insulation fault in relation to the earth occurs, the voltage of the two unaffected phases in relation to the earth takes on the value of the phase-to-phase voltage. Equipment must therefore be selected with this in mind.

2.5 Earthing Regulations Various earthing regulations standards using on the earth planet are described below: 

  





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In the United States National Electrical Code and Canadian Electrical Code the feed from the distribution transformer uses a combined neutral and grounding conductor, but within the structure separate neutral and protective earth conductors are used (TN-C-S). The neutral must be connected to earth only on the supply side of the customer's disconnecting switch. In Argentina, France (TT) and Australia (TN-C-S), the customers must provide their own ground connections. Japan is governed by PSE law, and uses TT earthing in most installations. In Australia, the Multiple Earthed Neutral (MEN) earthing system is used and is described in Section 5 of AS 3000. For an LV customer, it is a TN-C system from the transformer in the street to the premises, (the neutral is earthed multiple times along this segment), and a TN-S system inside the installation, from the Main Switchboard downwards. Looked at as a whole, it is a TN-C-S system. In Denmark the high voltage regulation and Malaysia the Electricity Ordinance 1994 states that all consumers must use TT earthing, though in rare cases TN-C-S may be allowed (used in the same manner as in the United States). Pakistan and many Asian counties are follow up British Standards.

Chapter 3 Installations, Measurements of earth electrodes and Earth electrode Resistance and Soil Resistivity testing methods

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3.1 Introduction to IS Specifications Regarding Earthing of Electrical Installations The various important specifications regarding earthing as recommended by Indian Standards are given below: 1. Distance of earth From Building. An earthing electrode shall not be situated within a distance of 1.5m from the building whose installation system is being earthed. 2. Size of Earth Continuity Conductor. The conductor, by means of which the metal body of an electrical equipment or appliance is connected to the earth, is known as earth continuity conductor (ECC). The earth continuity can be ensured either through metal conduit, metal sheathing of metal sheathed cables or by a special earth continuity conductor. The cross-section of earth continuity conductor should not be less than 2.9mm2 or half of the installation conductor size. 3. Resistance of Earth. The main principle regarding earth resistance is that the earth resistance should be low as possible to cause flow of current sufficient to operate the protective relays or blow fuses, in the event of fault. The value earth resistance is not constant but changes with the weather, as it depends upon the moisture content of the soil, and is maximum during dry season. As a general rule the lower the value of earth resistance better it is but even the following values of earth resistance (maximum permissible values) will give satisfactory results. Large power station – 0.5 Ω Major power station – 1.0 Ω Small substation – 2.0 Ω In all other cases – 5 Ω Maximum Earth continuity inside an installation i.e. from the earth plate to any point in the installation is normally 1.0 Ω. 4. The earth wire and earth electrode will be same material. 5. The earth wire shall be taken through GI pipe of 13 mm diameter for at least 30 cm length above and below ground surface to the earth electrode to protect it against mechanical damage. 6. It is not necessary that earth wire connected to an earth electrode is run along the whole wiring system. All the earth wires run along the various sub-circuits shall be terminated and looped firmly at the main board and from main earth wire shall be taken to the earth electrode. The loop earth wires shall not be either less than 2.9 mm2 or half of the size of the sub-circuit conductor. 7. The earthing electrode shall always be placed in vertical position inside the earth or pit so that it may be in contact with all the different earth layers.

3.2 Why must we have earth electrodes and earthing lead? Any wire, rod, pipe or metal plate embedded in earth for the purpose of making an effective connection with the general mass of the earth is known as earth electrode. The wire which connects overhead earth wire or any other apparatus to be earthed to the earth electrode is known as earthing lead.

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The principle of earthing is to consider the general mass of earth as a reference (zero) potential. Thus, everything connected directly to it will be at this zero potential or above it by the amount of the volt drop in the connection system (for example, the volt drop in a protective conductor carrying fault current). The purpose of the earth electrode is to connect to the general mass of earth.(IEE) 3.2.1 Earth Electrode Types Acceptable electrodes are rods, pipes, tapes, wires, plates and structural steelwork buried or driven into the ground. The pipes of other services such as gas and water must not be used as earth electrodes although they must be bonded to earth. The sheath and armour of a buried cable may be used with the approval of its owner and provided that arrangements can be made for the person responsible for the installation to be told if the cable is changed, for example, for a type without a metal sheath. The effectiveness of an earth electrode in making good contact with the general mass of earth depends on factors such as soil type, moisture content, and so on. A permanently-wet situation may provide good contact with earth, but may also limit the life of the electrode since corrosion is likely to be greater. If the ground in which the electrode is placed freezes, there is likely to be an increase in earth resistance. In most parts of the UK an earth electrode resistance in the range 1 Ohm to 5 Ohms is considered to be acceptable.(tlc) The resistance of a ground electrode to current has 3 basic components: 1) The resistance of the ground electrode itself and the connections to the electrode. Rods, pipes, masses of metal, structures, and other devices are commonly used for earth connections. These are usually of sufficient size or cross-section that their resistance is a negligible part of the total resistance. (Megger) 2) The contact resistance of the surrounding earth to the electrode. If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible. Rust on an iron electrode has little or no effect; the iron oxide is readily soaked with water and has less resistance than most soils. But if an iron pipe has rusted through, the part below the break is not effective as a part of the earth electrode. 3) The resistance of the surrounding body of earth around the ground Electrode. An electrode driven into earth of uniform resistivity radiates current in all directions. Think of the electrode as being surrounded by shells of earth, all of equal thickness (see Fig. 3.1). The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance. The next earth shell is somewhat larger in area and offers less resistance. Finally, a distance from the electrode will be reached where inclusion of additional earth shells does not add significantly to the resistance of the earth surrounding the electrode. It is this critical volume

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of soil that determines the effectiveness of the ground electrode and which therefore must be effectively measured in order to make this determination. Ground testing is distinct when compared to more familiar forms of electrical measurement, in that it is a volumetric measurement and cannot be treated as a ―point‖ property. The resistance of the surrounding earth will be the largest of the three components making up the resistance of a ground connection.

Figure: 3.1. Shows Components of earth resistances in an earth electrode

Source:

A practical guide to earth resistance testing ―Getting Down to Earth‖ by Megger

The resistance to earth should be no greater than 220 Ohms. The earthing conductor and its connection to the earth electrode must be protected from mechanical damage and from corrosion. Accidental disconnection must be avoided by fixing a permanent label as shown in {Fig 3.2} which reads:

Figure: 3.2. Connection of earthing conductor to earth electrode Source: Electricians Guide 16th Edition of IEE Regulations.

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The quality of an earth electrode (resistance as low as possible) depends essentially on two factors:

 

Installation method of earthing Type of soil

3.3 Installation Methods for Conventional Earthing: Four common types of conventional earthing methods of installation will be discussed (cost and estimation book)

3.3.1 Plate Earthing.    

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In this type of earthing plate either of copper or of G.I. is buried into the ground at a depth of not less than 3 meter from the ground level. An earthing plate either of copper of dimensions 60 cm × 60 cm ×3 mm or of galvanized iron of dimensions 60 cm × 60 cm ×6 mm is buried into the ground with its face vertical. The earth plate is embedded in alternative layer of coke and salts for a minimum thickness of about 15cm. The earth wire copper wire for copper plate earthing and G.I. wire for G.I. plate earthing) is securely bolted to an earth plate with the help of bolt nut and washer made of copper, in case of copper plate earthing and of G.I. in case of G.I. plate earthing.

Figure: 3.4. Pipe/Rod Earthing Source: Electrical Installation Estimating & Costing by J.B. GUPTA

The plates may be:  Copper of 3 mm thickness  Galvanized (1) steel of 3 mm thickness  Galvanized iron of 6 mm thickness The resistance R in ohms is given (approximately), by:

L = the perimeter of the plate in meters ρ = resistivity of the soil in ohm-meters (see ―Influence of the type of soil‖ below)

3.3.2 Pipe Earthing 

    

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Pipe earthing is best form of earthing and it is cheap also in this system of earthing a GI pipe of 40 mm diameter and 2.5 meters length is embedded vertically in ground to work as earth electrode but the depth depends upon the soil conditions. The earth wire is fastened to the top section of the pipe with nut and bolts. The pit area around the GI pipe for a distance of 15 cm is filled with salt and coal mixture for improving the soil conditions and efficiency of the earthing system. It can take heavy leakage current for the same electrode size in comparison to plate earthing. In summer season to have an effective earthing three or four bucket of water is put through the funnel. Figure 3.4 below shows pipe earthing detail.

Figure: 3.4. Pipe/Rod Earthing Source: Electrical Installation Estimating & Costing by J.B. GUPTA

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3.3.3 Rod Earthing     

In this system of earthing 12.5mm diameter solid rods of copper 16mm diameter solid rod of GI or steel or hollow section of 25mm GI pipe of length not less than 3 meters are driven vertically into the earth. In order to increase the embedded length of electrode under the ground, which is some time necessary to reduce the earth resistance to desired value more than one rod section are hammered one above the other. This system of earthing is suitable for areas which are sandy in character. This system of earthing is very cheap. Figures 3.4 and 3.5 shows typical connection of earth rod.

Figure: 3.5. Shows Earthing Rod connection and Rods connected in parallel Paths. Source: low Voltage Expert Guide by Schneider Electric Industries. The approximate resistance R obtained is:

If the distance separating the rods > 4L Where L = the length of the rod in meters ρ = resistivity of the soil in ohm-meters (see ―Influence of the type of soil‖ below) n = the number of rods

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3.3.4 Strip or Wire Earthing.     

In this system of earthing strip electrodes of cross-section not less than 25 mm × 1.6 mm of copper or 25 mm × 4 mm of GI or steel are buried in horizontal trenches of minimum depth of 0.5 meter. If round conductor are used their cross sectional area shall not be smaller than 3.0 mm2 if of copper and 6 mm2 if GI or steel is used. The length of buried conductor shall be sufficient to give the required earth resistance and however should not be less than 15 meters. The electrode shall be as widely distributed as possible in a single straight or circular trenches radiating from a point. This type of earthing is used in rocky soil earth bed because at such places excavation work for plate earthing is difficult.

Figure: 3.6. Shows GI Strip Types of Electrodes Source: Metal Gems Company

3.4 Soil Resistivity Effects on Earthing and its Variability with Types of Soil Soil resistivity directly affects the design of a grounding (earthing) electrode system and is the prime factor that determines the resistance to earth of a grounding electrode or grounding electrode system. Therefore, prior to the design and installation of a new grounding electrode system, the proposed location shall be tested to determine the soil's resistivity (BS 7430:1998, IEEE STD 81). Soil resistivity varies widely by region due to differences in soil type and changes seasonally due to variations in the soil's electrolyte content and temperature. Therefore, it is recommended that these variations be considered when assessing soil resistivity. To ensure expected grounding (earthing) electrode system resistance values are achieved throughout the year, worst-case soil resistivity values should be considered when designing a grounding electrode system.

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Table 3 SOIL RESISTIVITY FOR VARIOUS SOIL* TYPES

Types of Soil

Moist humus soil, moor soil, swamp Farming soil loamy and clay soils Sandy clay soil Moist sandy soil Dry sand soil Concrete 1: 5 Moist gravel Dry gravel Stoney soil Rock

Soil Resistivity RE ( Ωm) 30 100

150 300 1000 400 500 1000 30,000 107

Source: (F. Wenner, A Method of Measuring Earth Resistivity; Bull, National Bureau of Standards, Bull 12(4) 258, p. 478-496; 1915/16.

3.4.1 Factors Affecting Soil Resistivity Following are the main factors which affect soil resistivity and they must be considered during installations of earth electrodes. 3.4.1.1 Resistivity decrease with increase in moistures and electrolytes (salts) In soil, conduction of current is largely electrolytic. Therefore, the amount of moisture and salt content of soil radically affects its resistivity. The amount of water in the soil varies, of course, with the weather, time of year, nature of sub-soil, and depth of the permanent water table. Table 4 shows typical effects of water in soil; note that when dry, the two types of soil are good insulators (resistivity greater than 1000 x 106 ohm-cm). With a moisture content of 15 percent, however, note the dramatic decrease in resistivity (by a factor of 100,000). Actually, pure water has an infinitely high resistivity. Naturally occurring salts in the earth, dissolved in water, lower the resistivity. Only a small amount of salt can reduce earth resistivity quite a bit. (See Table 5), this effect can be useful to provide a good low-resistance electrode, in place of an expensive, elaborate electrode system.

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Table 4: Effect of Moisture Content on Earth Resistivity* Moisture contents,

Table 5: Effects of salt contents on Resistivity.

Resistivity (Ω-cm)

Percent by Weight 0.0 2.5

Top Soil 6 1,000 x 10 250,000

5.0 10.0 15.0 20.0 30.0

165,000 53,000 21,000 12,000 10,000

Sandy Loam 6 1,000 x 10 150,000 43,000 22,000 13,000 10,000 8,000

Added Salt Percent by Weight of Moisture 0.0 0.1 1.0 5.0 10.0 20.0

Resistivity (Ω-cm) 10,700 1,800 460 190 130 100

Source: *From ―An Investigation of Earthing Resistance‖ by P.J. Higgs, I.E.E. Journal, vol. 68, p. 736, February 1930 3.4.1.2 Effect of Temperature on Earth Resistivity Two facts lead to the logical conclusion that an increase in temperature will decrease resistivity: (1) water present in soil mostly determines the resistivity, and (2) an increase in temperature markedly decreases the resistivity of water. The results shown in Table 6 confirm this. Note that when water in the soil freezes, the resistivity jumps appreciably; ice has a high resistivity. The resistivity continues to increase temperatures go below freezing. Table 6: Effect of Temperature on Soil Resistivity. Temperature

Resistivity

C

F

(Ohm-cm)

20

68

7,200

10

50

9,900

0

32 (water)

13,800

0

32 (ice)

30,000

-5

23

79,000

-15

14

330,000

Source: Soares Book on Grounding and Bonding, 9th addition (ISBN 1890659-36-3

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3.4.1.3 Seasonal Variations in Earth Resistivity Because the resistivity of soil is directly affected by its moisture content and temperature, it is reasonable to conclude that the resistance of any grounding electrode system will vary throughout the different seasons of the year. Figure shows the seasonal variations of the resistance to earth of a grounding electrode. This is particularly true in locations where there are more extremes of temperature, rainfall, dry spells, and other seasonal variations. Temperature and moisture content both become more stable as distance below the surface of the earth increases. Therefore, in order to be effective throughout the year, grounding electrode system should be installed as deep as practical. Best results are achieved when ground rods, or other grounding electrodes, reach permanent moisture as seen from the figure 3.8.

Seasonal Variations

Figure: 3.8. Shows Resistance of Test Electrode verses Seasonal Variations Source: Soares Book on Grounding and Bonding, 9th addition (ISBN 1890659-36-3).

3.4.2 Methods of Improving Earth Resistance (Reducing Earth Resistance) When we find that our earth electrode resistance is not low enough, there are several ways you can improve it:  Lengthen the earth electrode in the earth.  Use multiple rods.  Treat the soil. 3.4.2.1 Effect of Rod Size:

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As we might suspect, driving a longer rod deeper into the earth, materially decreases its resistance. In general, doubling the rod length reduces resistance by about 40 percent. The curve of figure 3.9 shows this effect. For example, note that a rod driven 2 ft down has a resistance of

88 Ω; the same rod driven 4 ft down has a resistance of about 50 Ω. By using the 40 percent reduction rule, 88 x 0.4 = 35 Ω reduction occurs. By this calculation, a 4-ft deep rod would have a resistance of 88 - 35 or 53 Ω — comparing closely with the curve values. We might also think that increasing the electrode diameter would lower the resistance. It does, but only a little. For the same depth, doubling the rod‘s diameter reduces the resistance only about 10 percent.

Figure: 3.9. Earth Resistance Decreases with Depth of Electrode in Earth Source: ―Ground Connections for Electrical Systems,‖ O.S. Peters, U.S. National Bureau of Standards, Technological Paper 108, June 20, 1918 (224 pages - now out of print). 3.4.2.2 Use of Multiple Rods: Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly; that is, the resultant resistance is not one-half the individual rod resistances (assuming they are of the same

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size and depth). Actually, the reduction for two equal resistance rods is about 40 percent. If three rods are used, the reduction is 60 percent; if four, 66 percent.

Figure 3.10: Average Results Obtained From multiple-rod Earth Electrodes.

Figure 3.11: Comparative Resistance of multiple-rod Earth Electrodes.

Source: ―Practical Grounding Principles and Practices for Securing Safe Dependable Grounds,‖ Publication of Copperweld Steel Co., Glassport, Pa. 3.4.2.3 Treatment of the Soil Chemical treatment of soil is a good way to improve earth electrode resistance when you cannot drive deeper ground rods because of hard underlying rock, for example. It is beyond the scope of this manual to recommend the best treatment chemicals for all situations. Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable noncorrosive materials. Magnesium sulfate is

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the least corrosive, but rock salt is cheaper and does the job if applied in a trench dug around the electrode (see Fig. 3.12). It should be noted that soluble sulphates attack concrete, and should be kept away from building foundations. Another popular approach is to backfill around the electrode with a specialized conductive concrete. A number of these products, like bentonite, are available on the market. Chemical treatment is not a permanent way to improve your earth electrode resistance. The chemicals are gradually washed away by rainfall and natural drainage through the soil. Depending upon the porosity of the soil and the amount of rainfall, the period for replacement varies. It may be several years before another treatment is required.

Figure: 3.12. Trench Method of Soil Treatment Source: ―Practical Grounding Principles and Practices for Securing Safe Dependable Grounds,‖ Publication of Copperweld Steel Co., Glassport, Pa.

3.5 Earth Electrode and Earth Resistance Testing Methods

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The resistance to earth of any system of electrodes theoretically can be calculated from formulas based upon the general resistance formula: R=L/A Where  is the resistivity of the earth in ohm-cm L is the length of the conducting path A is the cross-sectional area of the path The theory behind resistance measurement gets quite involved but is simplified by assuming that the earth‘s resistivity is uniform throughout the entire soil volume under consideration. The earth tester generates an ac signal, which is fed into the system under test. The instrument then checks the status of the circuits for good connection and noise. If either of these variables is out of specification then the operator is informed. Having checked that the conditions for test are met, the instrument automatically steps through its measurement ranges to find the optimum signal to apply. Measuring the current flowing and the voltage generated the instrument calculates and displays the system or electrode resistance. Two most common methods are discussed here for obtaining accurate earth electrode resistance.

 

Clamp-On Method (Stakeless Method) Fall-Of-Potential Method

3.5.1 Fall-Of-Potential Method The Fall-of-Potential method (sometimes called the Three-Terminal method) is the most common way to measure earth electrode system resistance, but it requires special procedures when used to measure large electrode systems (see Measuring Large Electrodes below). For small electrodes, such as one or a few ground rods or a small loop, the Fall-of-Potential method requires only simple procedures (see Measuring Small Electrodes below). This three-terminal test is the method that can be carried out with three or four terminal earth testers. Although four terminals are necessary for resistivity measurements, the use of either three of four terminals is largely optional for testing the resistance of an installed electrode. 3.5.1.1 Basic Procedure The basic procedure for the Fall-of-Potential method is to first connect the test set terminals C1 and P1 to the earth electrode under test, connect the test set C2 terminal to a current probe located some distance from the earth electrode and finally connect the test set P2 terminal to a potential probe located a variable distance between. The two probes normally are located in a straight line. At each potential probe location, the resistance is recorded (a form is provided in Appendix I for this purpose). The results of these measurements are then plotted to graphically determine the electrode resistance. 3.5.1.2 Measuring Small Electrodes 1. Connect C1 and P1 terminals on the test set to the earth electrode (Fig. 3.14).

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2. Drive a probe into the earth 100 to 200 feet from the center of the electrode and connect to terminal C2. This probe should be driven to a depth of 6 – 12 inches. 3. Drive another probe into the earth midway between the electrode under test and probe C2 and connect to terminal P2. This probe should be driven to a depth 6 – 12 inches. 4. Record the resistance measurement. 5. Move the potential probe 10 feet farther away from the electrode and make a second measurement. 6. Move the potential probe 10 feet closer to the electrode and make a third measurement. 7. If the three measurements agree with each other within a few percent of their average, then the average of the three measurements may be used as the electrode resistance. 8. If the three measurements disagree by more than a few percent from their average, then additional measurement procedures are required (see Measuring Large Electrodes).

Figure: 3.14. Fall of Potential Method for Measuring Small Electrodes

Source: from ―Principles and Practice of Earth Electrode Measurements‖ by Whitham D. Reeve

3.5.1.3 Measuring Large Electrodes

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A quick set of measurements can be made as follows (Fig. 3.15). This method eliminates many tedious measurements but may not yield good accuracy unless the current and potential probes are outside the electrical influence of the electrode system: 1. Place the current probe 400 – 600 feet from the electrode 2. Place the potential probe 61.8% of the distance from the electrode to the current probe 3. Measure the resistance 4. Move the current probe farther away from its present position by, say, 50 – 100 feet 5. Repeat steps 2 and 3 6. Move the current probe closer by, say, 50 – 100 feet 7. Repeat steps 2 and 3 8. Average the three readings

Figure: 3.15. Fall of Potential Method for Measuring Large Electrodes Source: From ―Principles and Practice of Earth Electrode Measurements‖ by Whitham D. Reeve 3.5.2 Clamp-On Method (Stakeless Method) Unfortunately, the Fall of Potential method also comes with several drawbacks:   

It is extremely time consuming and labor intensive. Individual ground electrodes must be disconnected from the system to be measured. There are situations where disconnection is not possible.

During the last decade, a new technology appeared in the market, in the form of the clamp-on ground tester. This testing device was developed specifically for improving the speed and convenience of the ground test. The clamp-on ground tester performs a „stakeless‟ test which is a ground resistance test performed without disconnecting the ground. Based on Ohm‘s Law (R=V/I), the stakeless test induces a known voltage in a loop circuit that includes ground,

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measures resultant current flow and calculates the loop resistance of the circuit. Four pole earth testers also perform a stakeless test; however they use two clamps, a voltage clamp (V clamp) and a current clamp (I clamp), and keep the clamps separate to prevent interaction between the two. The operator must be certain that earth is included in the return loop and be aware that the tester measures the complete resistance of the path (loop resistance).

Figure: 3.16. Shows the Fluke 1630 Earth Ground Clamp for Stakeless Method Source: www.Fluke.com 3.5.2.1 What is stake-less testing? Stake-less testing is one of many methods of measuring earth electrode resistance. However what sets this method apart from all other earth electrode test methods is that it is the only method that does not require the use of auxiliary test electrodes or test leads. Since many earth electrodes are in locations surrounded by concrete or tarmac this is of real benefit. The lazy-spike method works well, but can easily be influenced by steel reinforcement or buried metal pipes. 3.5.2.2 Test Procedure. This measurement method is innovative and quite unique. Illustration in figure 3.17 shows a simplified ground distribution system. The equivalent circuit is shown in Figure A. If R1, R2, R3, Rn are simplified to Req, then only Rg and Req are left in the circuit (refer to Figure B). If a constant voltage is applied to the circuit, the following equation is traversed,

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= Rg + Req

∑ If Rg and R1, R2, Rn are similar values, and n is a large number (such as 200), then Req will be much less than Rg and approaches zero. Rg >> Req (Reg → 0)

Figure: 3.17. Shows simplified ground distribution system. Source: Earth resistance testing techniques by common sense testing (protecequip.com)

Figure: A. Equivalent circuit of figure 3.17.

Figure: B. Equivalent circuit of Figure A.

Source: Earth resistance testing techniques by common sense testing (protecequip.com).

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3.5.2.3 Practical Example: If Rg and R1, R2, Rn are all 10 ohms, respectively and n = 200. Then Req by calculation equals

= Rg + Req = 10 + 0.05 = 10.05  Rg In this example, we can see that as long as the number of multiple electrodes is large enough, the equivalent resistance is negligible with respect to the ground resistance to be measured.

3.5.3 Soil Resistivity Measurement by Four Pin Wenner Method It is well known that the resistance of an earth electrode is heavily influenced by the resistivity of the soil in which it is driven and as such, soil resistivity measurements are an important parameter when designing earthing installations. In 1915, Wenner demonstrated that field test measurements of soil resistivity are commonly obtained by use of the four-pin method. Soil resistivity is defined as the resistance between the opposite faces of a cube of soil having sides of length one meter and can be expressed in Ohm-meters. Soil resistivity is the key factor that determines what the resistance of the charging electrode will be and to what depth it must be driven to obtain low ground resistance. The resistivity of the soil varies widely throughout the world and changes seasonally. Soil resistivity is determined largely by the content of its electrolyte which consists of moisture, minerals, and the dissolved salt. It figures has a direct impact on the overall sub-station resistance and how much earth electrode is required to achieve the desired values. The lower the resistivity the fewer the electrodes required to achieve the desired earth resistance value. The apparent resistivity for the Wenner array can be obtained from the field measurements using the following formulae  = 2a

1.

 = 2aR

2.

Where  is resistivity of the local soil (Ω-cm) ΔV is the voltage measured (V) a is distance between probes(m)

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R is resistance determined by the testing device or instrument Ω) Earth Resistance (Rg) of a single spike, of diameter (d) and driven length (L) driven vertically into the soil of resistivity (ρ), can be calculated as follows [

]

3.

Where ρ is the soil resistivity in Ω-m L is the buried length of the electrode in m d is the diameter of the electrode in m

3.5.3.1 Testing Procedure Wenner array method and Fall-of-Potential technique were used to determine the soil resistivity and tower footing earth resistance respectively at each selected tower along the transmission line. The following listed equipment was used for the field measurements in this work. Equipments:      

A 4-Pole Digital Meter – (Megger or any other Earth Tester) Four probes or stakes Four insulated wire conductors Measuring tape Hammer (to drive probes) User‘s Manual for Meter

As the ―4-point‖ indicates, the test consists of 4 pins that must be inserted into the earth. The outer two pins are called the Current probes, C1 and C2. These are the probes that inject current into the earth. The inner two probes are the Potential probes, P1 and P2. These are the probes that take the actual soil resistance measurement. 1. Connect up the equipment as shown in figure 3.18. A four terminal earth resistance tester is required as it will indicate directly the value of mutual resistance 'R' in ohms. Use test leads with a cross sectional area of at least 2.5mm2. 2. Ensure the rods are in a straight line with an equal spacing of ‗a‘ meters and inserted to a depth of not more than ‗b‘ = 1/20th their spacing. 3. Keeping the centre position the same, take resistance measurements at various rod spacing‘s. Always ensure that the spacing between individual rods are identical. 4. For each new spacing‘s, calculate the apparent soil resistivity using the equation given above. The depth to which the soil resistivity is measured is approximately the same as the spacing.

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5. This type of measurement provides the average resistivity to a depth equal to the probe separation distance. For example, if the probes are spaced 10 ft (3.05 m), the measurement provides the average resistivity to a depth of about 10 ft.

Figure: 3.18. Soil Resistivity Measurement Setup (Wenner 4 Pin Method) Source: From ―Principles and Practice of Earth Electrode Measurements‖ by Whitham D. Reeve In this method all four electrodes are moved for each of the test with the spacing between each adjacent pair remaining same. Finally put all measured values of resistance and their corresponding resistivity in the table 7 below.

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3.5.3.2 Soil Resistivity Test. Date:

/

/

Time:

:

Location: SPACING “a” METERS

RESISTANCE READING “R” ohms

APPARENT RESISTIVITY

ρ = 2πaR ohm-meters

Table: 7. Soil Resistivity Test Results Source: Field Work

At last we will draw a graph between spacing and resistivity.

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CHAPTER 4 Neutral Earthing In Low Voltage Networks

47

4.1 Introduction The neutral or system earthing, that is to say the connection between the transformer neutral points and earth, is of high importance to the behavior of a power system during an unsymmetrical fault. The earthing design is considered the single most important parameter to determine the earth fault behavior in a power system. A power system can have more than one neutral point. All neutral points of one system do not have to be connected to earth, using the same earthing method. Two important functions of neutral earthing are to detect earth faults and to control the fault current, since large fault currents can cause the potential rise of exposed parts of the power system to reach dangerous levels. In practice, the system earthing consists of the connections between transformer neutral points and earth. The connections, i.e. the neutral point equipment, can differ between different transformers in the same system. The connections influence the zero sequence equivalent impedance of the system and by that, the unsymmetrical fault current. The fault current in its turn determines the voltage at the transformer neutral, i.e. the neutral point displacement voltage. The system earthing is designed to limit the maximum earth fault current, in order to avoid dangerous step and touch voltages. At the same time, it must also see to that fault currents and displacement voltages are high enough to facilitate high-impedance earth fault detection. In order to achieve this, different earthing designs must be used depending on the capacitive strength of the system. Since it is the type and the length of the distribution lines that determine the capacitance of the system, they are of vital importance to the choice of system earthing design.

4.2 Neutral Earthing Methods Neutral grounding methods can be classified into the effective neutral grounding (or solidly neutral grounding) method and the non-effective neutral grounding method. The difference between the two practices is the difference of the zero-sequence circuit from the viewpoint of

48

power network theory. Therefore all power system behavior characterized by the neutral grounding method can be explained as phenomena caused by the characteristics of the zerosequence circuit. Accordingly, neutral grounding methods have a wide effect on the actual practices of various engineering fields, for example in planning or operational engineering of short-circuit capacity, insulation coordination, surge protection, structure of transmission lines and towers, transformer insulation, breaker capability, protective relaying, noise interference, etc. The neutral grounding method of power systems can be classified as follows: a) Effective neutral grounded system: 

Solidly grounded system

b) Non-effective neutral grounded system:    

Resistive neutral grounded system Arc-suppression coil (Peterson coil) neutral grounded system Neutral ungrounded system (may be called neutral minute-grounded system), but only adopted for distribution systems. Earthing Transformer earthing

The features of each method can be explained as features based on the zero-sequence circuit. By using a plain expression for the non-effective grounded system, grounding fault currents can be reduced considerably, but on the contrary higher temporary over voltages would be caused during faults. The effective neutral grounded system (solidly grounded system) has the opposite features. 4.2.1 Standardized Definitions In order to establish a common perspective, some definition and short terms explanations must be presented. The definitions are taken from IEEE ―Green Book‖. Ungrounded System A system, circuit, or apparatus without an intentional connection to ground except, through potential indicating or measuring devices or other very high impedance devices is said to be ungrounded system. Note: Although called ungrounded, this type of system is in reality coupled to ground through the distributed capacitance of its phase windings and conductors Grounded system

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A system of conductors in which at least one conductor or point (usually the middle wire or neutral point of transformer or generator winding) is intentionally grounded, either solidly or through impedance. Grounded Solidly Connected directly through an adequate ground connection in which no impedance has been intentionally inserted. Resistance Grounded. Grounded through impedance, the principle element of which is resistance Inductance Grounded Grounded through impedance, the principle element of which is inductance

Effectively Grounded Grounded through sufficiently low impedance, such that Xo / X1 are positive and less than 3.0 and R0/ X0 is positive and less than 1.0. 4.2.2 Ungrounded Neutral Systems or Isolated Neutral Systems. A system where all transformer neutrals are unearthed is called an isolated neutral system. The only intentional connection between an unearthed neutral and earth is via high impedance equipment for protection or measurement purposes [2] such as surge arresters or voltage transformers. In a power system there are however always capacitive connections between the phases and earth. The strength of the capacitive connection depends on type and length of the power system circuit. Consequently, the ―ungrounded system‖ is, in reality, a ―capacitive grounded system‖ by virtue of the distributed capacitance. Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral point for the system; As a result, the phase conductors are stressed at only lineto-neutral voltage above ground. When an earth fault occurs in the system, the capacitance to earth of the faulty phase is bypassed. But problems can rise in ground fault conditions. A ground fault on one line results in full lineto-line voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all insulation in the system. Figure 4.1 shows an earth fault in a system on one phase in unearthed neutral.

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Figure 4.1, Earth fault in a network with an unearthed neutral Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure 4.2 shows the Thevenin equivalent of the network with an unearthed neutral.

Figure 4.2, Thevenin equivalent of a network with an unearthed neutral. Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

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Figure 4.3 Sequence network equivalent of earth fault in isolated neutral system. The zero sequence capacitance, Co is equal to the capacitance between phase and earth, Ce. Source: Earth Faults in Extensive Cable Networks Electric Distribution Systems Anna Guldbrand.

In the case of a solid earth fault, the resistive connections between phase and earth are small enough to be neglected. The earth fault current, as well as the neutral point displacement voltage, depends only on the phase to earth voltage and capacitances. Equation 1 gives the, therefore solely capacitive, earth fault current.

I f  I c  j 3wc0 E

…….. 1

The maximum earth fault current of an isolated system is small providing the system‘s capacitive connection to earth is weak. The presence of a fault resistance means a resistive part is added to the systems equivalent impedance. The reduced fault current will therefore consist of a resistive and a capacitive part. Equation 2 gives the earth fault current in case of a non-solid earth fault.

I ef  I r  jI c

R f  3wC0  .E 2

Ir 

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1   R f 3wC0 

2

Ic 

 3wC0 

2

.E

1   R f 3wC0 

2

…… 2

The fault current gives rise to a zero sequence voltage across the capacitances. This voltage is called the neutral point displacement voltage. In case of a solid earth fault this voltage equals the pre-fault phase to earth voltage of the faulty phase. If the earth fault is non-solid, part of the phase to earth voltage will be a across the fault resistance. Equation 3 gives the neutral point displacement voltage.

Un 

If 3wC0

……. 3

Figure 4 shows the pre-fault phase voltages, the neutral point displacement voltage and the voltage of the healthy phases during a phase-to-earth fault in an isolated system. The voltage between the neutral point and the healthy phases will remain unchanged during the fault. A neutral point displacement voltage therefore remands a change in the healthy phase to earth voltage level. The maximum voltage of the healthy phases is 105 % of the pre-fault phase-tophase voltage.

Figure 4.3, Pre-fault voltages UA, UB, UC, neutral point displacement voltage U0 and voltage of healthy phases U‟B, U‟C during a phase-to-earth fault in an isolated system.

Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

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In isolated neutral systems some phase-to-earth faults are cleared without involving any relay operation. This is normally a good thing but can, in case of intermittent faults and neutral point displacement voltage, lead to over voltages and additional faults in the power system. 4.2.2.1 Fault detection If the unsymmetrical current and the neutral point displacement voltage measured during earth faults differ sufficiently from normal operation values, they can be used to detect earth faults in the system. Typically, over voltage relays are used to detect the neutral point displacement voltage and directional residual over current relays are used for selective fault detection. The relay settings, i.e. the relay operation thresholds, decide the sensitivity of the earth fault detection. Since high-impedance faults give relatively low fault currents and neutral point displacement voltages, high-impedance fault detection requires low relay operation thresholds. However, there are always natural unbalances in the systems. Natural unbalances give rise to a neutral point displacement voltage and unsymmetrical currents equivalent to those of very highimpedance faults. The voltage and currents can cause unwanted relay operation during normal operation if the thresholds are set too low. Earth fault current calculations carried out in Matlab confirm that the difference in earth fault current between solid earth faults and very high impedance earth faults is small for power systems with very weak capacitive connection to earth, that is small IC, see Figure 4.4 The flatter the curve between desirable fault resistance detection level and normal operation asymmetry, the harder to detect the fault.

Figure: 4.4. Fault current as function of fault resistance, for different capacitive connections to earth

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Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University. 4.2.2.2 Advantages of Isolated Systems. 

 

After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation, permitting continued production until a convenient shut down for maintenance can be scheduled. Small earth fault currents, providing limited capacitive connection to earth. Large share of the faults are self-clearing.

4.2.2.3 Disadvantages of Isolated Systems. 

   

In ungrounded system, the available ground fault is very low, but the voltage on normal line-to-ground insulation is increased from line-to-ground value to a full line-to-line magnitude. Strong capacitive connection to earth generates extensive earth fault currents, and is therefore not a suitable earthing method in systems with extensive use of cable. Too weak capacitive connection to earth will result in difficulties detecting the earth faults, it is neither suitable for system earthing in small systems consisting of overhead lines. Risk of over voltages. Because of the risk of over voltages the use of isolated neutral is restricted to low and medium voltage. The cost of equipment damage.

4.2.3 Resistance earthed systems A system where at least one of the neutral points is connected to earth via a resistor is called a resistance earthed system. To improve the earth fault detection in a power system a resistance can be connected between a transformer neutral point and the station earthing system. In order to facilitate high-impedance earth fault detection in systems with weak capacitive connection to earth, the difference between high-impedance earth fault currents and voltages, and those during normal operation must be increased. One way to increase the margin between high-impedance earth fault currents and currents due to normal operation unbalances is to connect a neutral point resistance to the neutral points of some of the transformers in the system. Figure 4.5 shows an earth fault in a system with a resistance earthed neutral.

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Figure: 4.5. Earth fault in a network with a resistance earthed neutral Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University. Figure 7 shows the corresponding Thevenin equivalent.

Figure: 4.6. Thevenin equivalent of a network with a resistance earthed neutral Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure 4.7 shows the corresponding sequence networks equivalent

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Figure 4.7 Sequence network equivalent of an earth fault in a resistance earthed system. Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University. In a system with very weak capacitive connection to earth the reactance of the earth capacitance will be large compared to the neutral point resistance. The neutral point resistance, instead as for the isolated systems the capacitive connection to earth, will therefore determine the maximum earth fault current. Equation 4 gives the earth fault current in case of a solid earth fault.

I ef  I R  I C 

E  j 3wCo E Re

In overhead line systems with weak capacitive connection to earth, the capacitive shunt reactance is very large compare to the parallel neutral point resistance and the maximum earth fault current is therefore determined almost exclusively by the neutral point resistance:

I ef   1 R

C

E Re

…… 4

Fault detection The presence of a fault resistance reduces the earth fault current. Equation 5 gives the earth fault current in case of a non-solid earth fault, the phase to earth capacitance neglected.

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I ef 

E Re  R f

…… 5

As mention in previous section, systems with a very weak capacitive connection to earth are normally resistance earthed. To make the difference between fault currents of isolated and resistance earthed systems visible, earth fault current calculations have been carried out using Matlab. The resulting current as functions of fault resistance is shown in Figure 4.8. It is an obvious difference in earth fault current for fault resistances around 5 k ohm, while the difference in fault current for very high impedance faults, and hence unsymmetrical condition during normal operation, is small. If the systems capacitive earth fault current instead is 2 A there will hardly be any difference in earth fault current of isolated and resistance earthed systems for fault resistances above a couple of thousands ohm.

Figure: 4.8. Fault current as function of fault resistance, for an isolated system Ic and resistance earthed system Ir. Source: Lehtonen, M. & Hakola, T.‖Neutral earthing and power system protection‖, ISBN 95290-7913-3, ABB Transmit Oy, Vaasa 1996

As in the case of a fault in an isolated system, the fault current gives rise to a neutral displacement voltage across the system’s impedance to earth. In the case of a resistance earthed system the impedance to earth is the neutral point resistance in parallel to the phase

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to earth capacitances. Equation 6 gives the neutral displacement voltage which in case of a solid earth fault equals the pre-fault phase to earth voltage of the faulted phase.

Un 

I ef 2

 1  2  R    3wC0   e

……. 6

4.2.3.1 Importance of Resistance Grounding Systems Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of the problems associated with solidly grounded and ungrounded systems. Resistance Grounding Systems limits the phase-to-ground fault currents. Grounding Resistors are generally connected between ground and neutral of transformers, generators and grounding transformers to limit maximum fault current as per Ohms Law to a value which will not damage the equipment in the power system and allow sufficient flow of fault current to detect and operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced. As a result of this fact, protection devices may not sense the fault. Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors to approximately rated current of transformer and / or generator. In addition, limiting fault currents to predetermined maximum values permits the designer to selectively coordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault. The main reasons for limiting the phase to ground fault current by resistance grounding are:       

To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating machines. To reduce mechanical stresses in circuits/Equipments carrying fault currents. To reduce electrical-shock hazards to personnel caused by stray ground fault. To reduce the arc blast or flash hazard. To reduce the momentary line-voltage dip. To secure control of the transient over-voltages while at the same time. To improve the detection of the earth fault in a power system.

4.2.3.2 Types of Resistive Grounding Systems There are two categories of resistance grounding:

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1. Low resistance Grounding 2. High resistance Grounding

Ground fault current flowing through either type of resistor when a single phase fault to ground occur will increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and surge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two and three pole breakers installed on resistance grounded low voltage systems.

Figure: 4.9. Resistance neutral earthing Source: Electrical Engineering Portal (―IEEE Standard 141-1993, ―Recommended Practice for Electrical Power Distribution for Industrial Plants‖). Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arcflash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and reduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current. The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A. Low resistance grounding indicates that NGR current would be above 100 A.

1. Low Resistance Grounded

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Low Resistance Grounding is used for large electrical systems where there is a high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact

and it is not commonly used in low voltage systems because the limited ground fault current is too low to reliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, low resistance grounded systems are not suitable for 4-wire loads and hence have not been used in commercial market applications. A resistor is connected from the system neutral point to ground and generally sized to permit only 200A to 1200 amps of ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit and trip it off-line but not so much current as to create major damage at the fault point.

Figure: 4.9. Low Resistor neutral Grounding Source: Electrical Engineering Portal (―IEEE Standard 141-1993, ―Recommended Practice for Electrical Power Distribution for Industrial Plants‖). Since the grounding impedance is in the form of resistance, any transient over voltages are quickly damped out and the whole transient overvoltage phenomena is no longer applicable. Although theoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system voltage dropped across the grounding resistor, there is not enough voltage across the arc forcing current to flow, for the fault to be reliably detected. For this reason low resistance grounding is not used for low voltage systems (under 1000 volts line to-line).

Advantages 

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Limits phase-to-ground currents to 200-400A.

 

Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current conditions only. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings.

Disadvantages:     

Does not prevent operation of over current devices. Does not require a ground fault detection system. May be utilized on medium or high voltage systems. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer. Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

2. High Resistive Method. High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes two things. The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need not be tripped off-line when the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. In this respect, it performs just like an ungrounded system. The second point is it can control the transient overvoltage phenomenon present on ungrounded systems if engineered properly. High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts or arcs to ground, but at lower levels than low resistance systems. In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.

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Figure: 4.9. High Resistance neutral Grounding Source: Electrical Engineering Portal (―IEEE Standard 141-1993, ―Recommended Practice for Electrical Power Distribution for Industrial Plants‖). Advantages        



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Enables high impedance fault detection in systems with weak capacitive connection to earth Some phase-to-earth faults are self-cleared. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage. Limits phase-to-ground currents to 5-10A. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing current conditions only. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinery windings. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground). May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that ―high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding relaying is employed‖. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.

Disadvantages  

Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Cost involved. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred.

4.2.4 Resonant Earthed Systems. A system in which at least one of the neutrals is connected to earth via an inductive reactance or a Petersen coil/ arc suppression coil/ Earth fault neutralizer, and the current generated by the reactance during an earth fault approximately compensates the capacitive component of the single phase earth fault current, is called a resonant earthed system. Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the maximum 3 phase short circuit capacity (thousands of amperes) to a relatively low value (200 to 800 amperes). The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive earth fault current of the system. A system in which the inductive current is slightly larger than the capacitive earth fault current is over compensated. A system in which the induced earth fault current is slightly smaller than the capacitive earth fault current is under compensated. Figure 9 shows the earth fault current phasors of a slightly over compensated system.

Figure: 4.10. Earth fault current phasors of a slightly over compensated power system Source: Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University. The neutral point reactor is often combined with a neutral point resistor. In a resonant earthed system the resulting reactive part of the earth fault current is too small for the relay protection to measure. By using a neutral point resistance a measurable resistive earth fault current is created as explained in the section about resistance earthed systems. In addition to this, there will always be active losses in the neutral point generator, which contributes to the active part of the earth fault current.

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Typical examples of power systems with strong capacitive connection to earth, suitable for resonant earthing, are systems consisting of an extensive amount of cables. If the high capacitive earth fault current of such systems is not compensated, the risk of dangerously high potential rise of exposed parts of the power system is evident. Figure 4.11 shows an earth fault in a system with a resonance earthed neutral. Figure 4.12 shows the corresponding Thevenin equivalent.

Figure: 4.11. Earth fault in a network with a resonant earthed neutral Source: Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure: 4.12 Thevenin equivalent of a network with a resonant earthed neutral Source: Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

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Figure 4.13 shows the corresponding sequence network equivalent in a system with negligible series impedance.

Figure 4.12 Sequence network equivalents of earth fault in resonance earthed system. Source: Earth Faults in Extensive Cable Networks Electric Distribution Systems by Anna Guldbrand. The earth fault current is made up of the capacitive current due to the phase to earth capacitances of the system, the inductive current generated in the neutral point reactor, the resistive current due to losses in the reactor parallel to the neutral point resistor. Equation 7 gives the single-phase earth fault current in case of a solid earth fault.

I ef  I RL  I R 0  I L  I C

I ef 

 RL  R0  E  j.  3wC RL .R0

 

0



1  E wC 

…… 7

In case of complete compensation the solid earth fault current, given by Equation 8, is solely resistive.

I ef 

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 RL  R0  E RL .R0

……. 8

In case of complete compensation the earth fault current is solely resistant as given by Equation 9. I ef 

 RL  R0  E

……. 9

RL .R0  RL .R f  R f .R0

Equation 10 the neutral point displacement voltage,

Eno 

I ef 2

 1  1   R    3wCo  wL    e 

…… 10 2

4.2.4.1 Petersen Coils A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitive current in the earth fault is compensated by an inductive current passed by the Petersen Coil. A small residual current will remain, but this is so small that any arc between the faulted phase and earth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin insulator, could be held on the system without the supply being interrupted. Transient faults would not result in supply interruptions. Although the standard ‗Peterson coil‘ does not compensate the entire earth fault current in a network due to the presence of resistive losses in the lines and coil, it is now possible to apply ‗residual current compensation‘ by injecting an additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced to practically zero. Such systems are known as ‗Resonant earthing with residual compensation‘, and can be considered as a special case of reactive earthing. Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively act as a high impedance NER, which will substantially reduce any earth fault currents, and hence also any corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, including any EPR hazards impressed onto nearby telecommunication networks). Advantages of resonant earthed systems:  

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Small reactive earth fault current independent of the phase to earth capacitance of the system. Enables high impedance fault detection.

Disadvantages   

Risk of extensive active earth fault losses Complicated relay protection High costs associated.

4.2.5 Solidly Neutral Earthed Systems (Effectively Earthed Systems) A system in which at least one of the transformer neutrals is directly connected to earth is called a solidly earthed system (IEC 2008). During an earth fault in a solidly earthed system the capacitive connection between phase and earth is bypassed as shown in Figure 4.13. Figure 4.14 shows the corresponding equivalent circuit.

Figure: 4.18. Earth fault in solidly earthed system and corresponding equivalent circuit Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure 4.19 Equivalent circuit of earth fault in solidly earthed system Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

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4.2.5.1 Importance and Application Solidly grounded systems are usually used in low voltage applications at 600 volts or less. Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current. However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved. While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. According to NEC (article 250-5) to maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very low impedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces the shock hazard to personnel! If the system is not solidly grounded, the neutral point of the system would ―float‖ with respect to ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability. The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitude of the current depends on the fault location and the fault resistance. Advantages 

The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high voltage levels (HV).

Disadvantages   

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This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances. There is no service continuity on the faulty feeder. The danger for personnel is high during the fault since the touch voltages created are high.

Applications   

Distributed neutral conductor 3-phase + neutral distribution Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole



Used when the short-circuit power of the source is low

4.2.6 Grounding Transformer For cases where there is no neutral point available for Neutral Earthing (e.g. either for ungrounded Wye or for a delta winding), an earthing transformer may be used to provide a return path for single phase fault currents. So far this section has discussed system grounding where the neutral point of the source has been readily available. But what does one do when the neutral point is not available? The best way to ground an ungrounded delta system (existing or new) is to derive a neutral point through grounding transformers. This may be accomplished in one of two ways as shown in Fig. 4.19. In Fig. 4.19a, high resistance grounding is accomplished through three auxiliary transformers connected Wye-broken delta.

Figure: 4.19a. 3 Auxiliary transformers/ Wye Broken Delta Figure: 4.19b Zig-Zag Figure: 4.19. Transformer Earthing Source: Baldwin Bridger, ―High-Resistance Grounding‖, IEEE Transactions on Industry Applications, Vol. IA-19, NO.1, January/February 1983.

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The resistor inserted in the "broken delta" leg is reflected to the primary underground fault conditions and limits the current to a nominal value as dictated by its design. Also, sensing the voltage drop across the resistor (device 59G) can be used to signal an alarm advising that a ground fault has occurred. The three lights across each individual transformer will constitute a version of the normal ground detection scheme currently employed on ungrounded systems. High resistance grounding can also be achieved alternately by a zigzag grounding transformer as shown in Fig. 4.19b. The scheme in Fig. 9a uses the flux in the transformer's iron core to produce secondary voltages with their respective phase relationships. With the zigzag transformer, the windings are connected in a zigzag fashion such that the flux in the iron is vectorially summed opposed to vectorially summing the secondary voltages. Consequently it behaves on the system just as the three auxiliary transformers do. It appears "transparent" to the system except underground fault conditions. The resistor makes it resistance grounded. Advantages of deriving a neutral:    

Previously ungrounded delta systems can be retrofitted with grounding transformers. Transient over voltages due to arcing ground faults are controlled. Both Low Resistance and High Resistance Grounding can be achieved. Limit damage to system

Disadvantages of deriving a neutral:   

Can be more costly than an original equivalent wye connected systems. Is difficult to find ground faults on high resistance grounded systems unless a ground fault detection scheme is employed. High resistance grounded systems are not practicable above 5 kV until tripping is provided.

4.3 Neutral system – Single earthed or Multi earthed In distribution system three phase load is unbalance and non linear so the neutral plays a very important role in distribution system. Generally, distribution networks are operated in an unbalanced configuration and also service to consumers. This causes current flowing through neutral conductor and voltage dropping on neutral wire. The unbalance load and excessive current in neutral wire is one of the issues in three phase four-wire distribution systems that causes voltage drop through neutral wire and makes tribulations for costumers.

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The existence of neutral earth voltage makes unbalance in three phase voltages for three phase customers and reduction of phase to neutral voltage for single phase customers. Multi-grounded three-phase four-wire service is widely adopted in modern power distribution systems due to having lower installation costs and higher sensitivity of fault protection than three-phase three-wire service.

4.3.1 Multi Grounded Neutral System (MEN) The multiple earthed neutral (MEN) system of earthing is one in which the low voltage neutral conductor is used as the low resistance return path for fault currents and where its potential rise is kept low by having it connected to earth at a number of locations along its length. The neutral conductor is connected to earth at the distribution transformer, at each consumer‘s installation and at specified poles or underground pillars. The resistance between the neutral conductor of the distribution system and the earth must not exceed 10 ohms at any location.

Figure: 4.20. Three-phase four wire multi grounded neutral Source: Electrical Engineering portal (Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book NFPA 70)

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Advantages of Multiple grounded neutral Systems The reasons for the development of the three phase, four-wire, multi-grounded systems involve a combination of safety and economic considerations.    

Optimize the Size of Surge Arrestor The zero sequence impedance is lower for a multi grounded system than the single point grounded neutral system. Cost of Equipment for the multi-grounded system is lower. Safety Concerns on Cable Shields

Disadvantages of Multiple grounded neutral systems  

Less Electrical Safety in Public and Private Property. Earth Fault Protection Relay setting is complicated.

4.3.2 Single Grounded Neutral Figure 4.21 shows single grounded neutral which is different from multi grounded system. Figure shows the neutral is also connected to earth at source side only, but the neutral conductor is extended along with the phase conductors. The configuration shown in figure allows electrical loads, transformers to be placed between any of the three phase conductors, phase-to-phase and/or phase-to-neutral.

Figure: 4.21. Three-phase four wire single neutral grounded Source: Electrical Engineering portal (Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book NFPA 70)

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This connection, phase to neutral will force electric current to flow over the neutral back to the transformer. So far, this electrical connection is acceptable, as long as the neutral is insulated or treated as being potentially energized, but modifications will be made in the future that will negate safety for the public and animals. Advantages of Single Grounded Neutral System   

More Reliable and Safe System. Protection Relay Setting is more easy in Single Grounded Neutral: Sensing of Ground Fault current:

4.4 Conclusion Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient over voltages. High resistance grounding systems may also be employed to maintain service continuity and assist with locating the source of a fault.

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Floating Neutral in LV Networks, its Causes and Elimination Methods

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5.1 Introduction If the Neutral Conductor is opened, broke or lost at either of its source side (Distribution Transformer, Generator) or at Load side (Distribution Panel of Consumer), the distribution system‘s neutral conductor will ―float‖ or lose its reference ground Point. The floating neutral condition can cause voltages to float to a maximum of its Phase volts RMS relative to ground, subjecting to its unbalancing load Condition. Floating Neutral conditions in the power network have different impact depending on the type of Supply, type of installation and Load balancing in the Distribution. Broken Neutral or Loose Neutral would damage to the connected load or create hazardous Touch Voltage at exposed conductive parts. Note: Here we are trying to understand the Floating Neutral Condition in T-T distribution System.

5.2 What is Floating Neutral? If the Star Point of Unbalanced Load is not joined to the Star Point of its Power Source (Distribution Transformer or Generator) then Phase voltage do not remain same across each phase but its vary according to the Unbalanced of the load. As the Potential of such an isolated Star Point or Neutral Point is always changing and not fixed so it‘s called Floating Neutral.

5.3 Loss of Neutral Impact to Consumers The impacts of floating neutral are dependent on the position in the power system where the neutral is broken.

5.3.1 Loss of Neutral at Transformer 5.3.1.1 Single Phase Transformer For installation supplied from a single phase transformer, the impact of broken neutral is not very severe. Customers connected to that particular transformer will not have supply as the return path will have been broken. Hazardous touch voltages might also be experienced on exposed conductive parts.

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5.3.1.2 Three Phase Transformer A broken neutral on a three phase transformer will cause the voltage to float up to the line voltages depending on the load balancing on the system. This type of fault condition may damage the customers equipment connected to the supply.

Figure: 5.1. Three Phase Transformer scheme under normal condition Source: V Cohen, 2002, Application Guide for the protection of L.V Distribution Systems, CBI Ltd, Johannesburg, South Africa, 20.1-20.7 Normal Condition From figure 5.1, it can be seen that under normal conditions, the current flow from the phases to the load and back to the source via neutral. In a balanced system, one phase matches the other two phases, resulting in no current through neutral. Any imbalance of Load will result in a current flow on neutral, so that the sum of zero is maintained. Floating Neutral Condition From figure 5.2, it can be seen that under floating neutral conditions. When the neutral is broken the current from red phase will go back to the yellow or blue phase resulting in line to line voltage between the loads. Similarly the same will applies for the other phases. Neutral Point is not at ground Level but it Float up to Line Voltage. This situation can be very dangerous and customers may suffer serious electric shocks if they touch something where electricity is present.

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Some customers will experience over voltage and others will experience under voltage.

Figure 5.2: Three Phase Transformer with broken neutral condition Source: V Cohen, 2002, Application Guide for the protection of L.V Distribution Systems, CBI Ltd, Johannesburg, South Africa, 20.1-20.7 Broken neutrals can be difficult to detect and in some instances may not be easily identified. Sometimes broken neutrals can be indicated by flickering lights.

5.3.2 Loss of Neutral at the Conductors 5.3.2.1 LV Conductors- ABC or Bare LV Conductors The impacts of broken neutral conductors at low voltage conductors will be similar to when the conductor is broken at the transformer. For singe phase Aerial bundled conductors (ABC), a broken neutral will just result in loss of power supply to customers. No danger will occur to the connected loads. For dual or three phase low voltage conductor will result in supply voltage floating to line voltages instead of phase voltage. This type of fault condition may damage the customers equipment connected to the supply.

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5.3.2.2 Services Conductors A broken service conductor will only result in loss of supply at the customer point; there will be no damage to customer‘s equipment. 5.3.3 Loss of neutral at Pole Top Box If the neutral conductor is broken at pole top box, there impact on customer is that there will be no supply. The fault condition will not damage the customer‘s equipment.

5.4 Types of Neutral Failure and Causes There are several factors which are identifying as the cause of neutral floating. The impact of Floating Neutral is depending on the position where Neutral is broken. 5.4.1 The Three Phase Distribution Transformer Bushings A lot of neutral failures on the transformers were on transformer bushings. The use of Line Tap on transformer bushing is identified as the main cause of Neutral conductor failure at transformer bushing. The Nut on Line Tap gets loose with time due to vibration and temperature difference resulting in hot connection. The conductor start melting and resulting broke off Neutral as shown in figure 5.3.

Figure 5.3 shows Neutral Failure on transformer bushings Source: CIRED Impact of Floating Neutral in Distribution Systems, 19th International Conference on Electricity Distribution paper 0300 by Mashanghu Hudson Xivambu EskomSouth Africa. Poor workmanship of Installation and technical staff also one of the reasons of Neutral Failure.

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5.4.2 Low Voltage conductors The loss of the neutral on low voltage conductors is very common. Investigations reveal that the causes of neutral failures on LV conductors are the following. 5.4.2.1 Incorrect application of Insulation Piercing Connectors IPC‘s are the connectors that are used to connect insulated conductors without removing the insulation. It is important that the connection must be very tight to ensure that the connector is able to go through the insulation and make contact with conductors. If that is not done, a hot connection will develop resulting in conductor failure. Two IPC‘s per connection are used to ensure that the integrity of the connection is maintained at all times as the consequence of loss of neutral is detrimental to our end users.

Figure: 5.4. IPC connection Source: CIRED Impact of Floating Neutral in Distribution Systems, 19th International Conference on Electricity Distribution paper 0300, Vienna, 21-24 May 2007 by Mashanghu Hudson Xivambu Eskom-South Africa. Figure 5.4 above shows an IPC connected on bare and Insulated neutral conductors. The IPC head is not shared off which means that the connection between the connector and conductor is not very good. One has been used instead of two and that increases the risk of failure.

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Figure: 5.5. IPC removed from the conductor Source: CIRED Impact of Floating Neutral in Distribution Systems, 19th International Conference on Electricity Distribution paper 0300, Vienna, 21-24 May 2007 by Mashanghu Hudson Xivambu Eskom-South Africa. Once the IPC is installed on the conductor, it cannot be removed for any reason. Punctured holes will remain on the conductor if the IPC is removed, resulting in short circuit between the phase and neutral to burn off and break. 5. 4.3 Overloading and Load Unbalancing Distribution Network Overloading combined with poor load distribution is one of the most reason of Neutral failure. Neutral should be properly designed so that minimum current will flow in to neutral conductor. Theoretically the current flow in the Neutral is supposed to be zero because of cancellation due to 120 degree phase displacement of phase current. I N  I R 00  I Y 1200  I B   1200

….…… (a)

In Overloaded Unbalancing Network lot of current will flow in Neutral which break Neutral at its weakest point. 5.4.4 Shared neutrals Some buildings are wired so that two or three phases share a single neutral. The original idea was to duplicate on the branch circuit level the four wire (three phases and a neutral) wiring of panel boards. Theoretically, only the unbalanced current will return on the neutral. This allows one neutral to do the work for three phases. This wiring shortcut quickly became a dead-end with the growth of single-phase non-linear loads. The problem is that zero sequence current

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From nonlinear loads, primarily third harmonic, will add up arithmetically and return on the neutral. In addition to being a potential safety problem because of overheating of an undersized neutral, the extra neutral current creates a higher Neutral to ground voltage. This Neutral to ground voltage subtracts from the Line to Neutral voltage available to the load. If you‘re starting to feel that shared neutrals are one of the worst ideas that ever got translated to copper. 5.4.6 Poor Maintenance and Workmanship Practices Normally LV network are mostly not given attention by the Maintenance Staff. Utilities tends to concentrate more on HV networks, Substations and medium voltage networks and ignores the LV networks where the million of end users are connected to. Also bad workmanship was found to be one of the causes of neutral conductor failures in LV networks. 5.4.7 Conductor Theft There is an increase in number of failures that result to conductor theft for resale as scrap metal. This leaves the customers without power and poses a safety risk to the community.

5.5 Proposed Solution for Eliminating Neutral Floating There are number of studies that are in progress, in collaboration with low voltage protection equipment suppliers and academic institutions, to come up with proposed solutions for eliminating or minimizing the impact of floating neutral. 5.5.1 Improved Installation techniques The root cause of the problem is the loss of neutral in the system. Enforcing quality of workmanship and adherence to standards and installations techniques can minimize the risk of having broken neutral conductors. The problem can be greatly reduced by ensuring discipline during network design and construction. Doubling of IPC‘s per neutral connection will ensure that integrity of the neutral connection as the chances of simultaneous failure of both are IPC‘s is very low. Good maintenance practices on low voltage networks can also help to identify and potential problems before they arise.

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5.5.2 Improved Load balancing techniques It has been proved that load balancing contribute to number of neutral failures, if the problem of load balancing can be addressed the impact will be reduced. Even if we can lose a neutral connection, under balanced load condition the voltages will still be within their normal limits hence the impact to the customers will be minimized. Usually a feeder is three phase four wire system in most of the counties. These feeders consist of mixtures of loads, e.g. residential, commercial. Industrial etc. single phase loads are fed by single phase two wire service, while three phase loads are fed by three-phase four wire service. The behavior of the load pattern (daily) depends upon the function of time and types of customers. The resulting power system voltages at the distribution end and at points of utilizations can be unbalanced due to many reasons. The reasons include the following: Unequal voltages magnitudes at fundamental system frequency (under voltages and over voltages), fundamental phase angle deviations, asymmetrical transformer windings impedances, etc. The major cause of this unbalance is uneven distribution of single phase loads that can be continually changing across a three-phase power system due to use. Normally the consumption of consumers connected to a feeder fluctuates, therefore leading to the fluctuation of the total load connected to each phase. The neutral current is the summation of the three phase currents of the transformer. NC 

3

I

p

(1)

p 1

Where IP represents total phase currents of the main transformer that feeds the consumer feeders and NC is the neutral current of main transformer. In the proposed radial distribution system shown below in figure 5.6, a load switch selector to each phase is connected to ensure that load is always balanced at all times.

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Figure: 5.6. Three phase feeder with switch selector Source: CIRED ―Impact of Floating Neutral in Distribution Systems‖, 19th International Conference on Electricity Distribution paper 0300, Vienna, 21-24 May 2007 by Mashanghu Hudson Xivambu Eskom-South Africa.

The proposed functionality of the unit in figure 5.6 is such that each single phase load is connected to the three phase feeder via a switch selector. Only one phase can supply the load at a given period depending on the load balancing of the three phases. Given the topology of switch selector, the equations for phase current can be written as: 3

I ph1k   swk 1i I ki  I ph1( k 1) ,

………. (2)

i 1 3

I ph 2 k   swk 2i I ki  I ph 2( k 1) , i 1

……….. (3)

3

I ph 3k   swk 3i I ki  I ph 3( k 1) , i 1

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……….. (4)

Where I ph1k , I ph 2 k and I ph 3k represent the currents (phasors) per phase (1,2and 3) after the k th point of connection, swk 11 , ….., swk 33 are different switches ( the value of ‗1‘ means that switch is closed and ‗0‘ means that it is open). The constraints of only allowing one breaker in each equations (2) to (4) to be closed can be written as following: 3

 sw

k 1i

i 1

.......... (5)

3

 sw

k 2i

i 1

3

 sw i 1

1  0

k 3i

1  0 ………… (6)

1  0 .

………… (7)

To minimize the neutral current, the objective of this new algorithm is to minimize the difference of the rms value of the phase currents

( I ph1k ( i 1,2,3) ) :

Minimize  I ph1k  I ph 2 k I ph1k  I ph 3k I ph 2 k  I ph 3k  5.5.3 Protection Simply use ELCB, RCBO or 4 Pole Circuit Breaker in the three phase supply system, the unit monitors the current flow in neutral conductor and as soon as neutral opens or break it will trip the complete supply without damaging to the system. An Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power and mainly used in TT earthing systems. There are two types of ELCBs: 1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB) 2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).

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5.5.4 Using Voltage Stabilizer: Whenever neutral fails in three phase system the connected loads will get connected between phases owing to floating neutral. Hence depending on load resistance across these phases, the voltage keeps varying between 230V to 400V.A suitable servo stabilizer with wide input voltage range with high & low cutoff may help in protecting the equipments.

5.6 Conclusion: A Floating Neutral (Disconnected Neutral) fault condition is VERY UNSAFE because If Appliance is not working and someone who does not know about the Neutral Floating could easily touch the Neutral wire to find out why appliances does not work when they are plugged into a circuit and get a bad shock. Single phase Appliances are design to work its normal Phase Voltage when they get Line Voltage Appliances may Damage .Disconnected Neutral fault is a very unsafe condition and should be corrected at the earliest possible by troubleshooting of the exact wires to check and then connect properly.

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