Lightning Arrester

Lightning Arrester Summary -

-

-

What is Lightning Arrester Types of Lightning Arrester o Expulsion Type o Valve Type o Gapless Metal-oxide Type Classification of Lightning Arrester o Station Class o Intermediate Class o Distribution Class o Secondary Class Identification Discharge Current Service Condition o Normal Service Condition o Abnormal Service Condition

What is Lightning Arrester -

Lightning Arrester: A device designed to protect electrical equipment from high transient voltage and to limit the duration and frequency, the amplitude of flow of current. Surge arrester is usually connected to the electrical conductors of a network and earth.

Types of Lightning Arrester Originally there are three types of surge arresters. They are: -

Expulsion Type Valve Type Gapless Metal – Oxide Type

E pulsio T pe: This t pe of a este is also alled p ote to tu e a d is o o l used o s ste operating at voltages up to 33kv. It is essentially consists of a rod gap in series with the protector tube. The upper electrode of protector tube is connected to rod gap and to the lower electrode to earth. Valve Type: Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. It consists of two assemblies i) Series spark gaps and ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps and earth. Gapless Metal-Oxide Type: These arresters are most widely used today. The metal oxide lightning arrester is the most advanced over-voltage protector. It is widely used as protective devices against switching and lightning over voltage.

Classification of Lightning Arrester There are four classifications of surge arresters: -

Station Class Intermediate Class Distribution Class (Heavy, normal and light duty) Secondary Class

The station class surge arrester is the best because of its cost and overall protective quality and durability.

Identification Surge arrester shall be identified by the following minimum information which shall appear on the rating plate: -

Rated Voltage Rated Frequency Nominal Discharge Current (Specifying for the 5,000A arrester whether series A or B and for the 10,000A arrester, whether light or heavy duty) Long duration discharge class (for 10,000 A heavy duty arrester) Manufacturers name or trademark, type.

Discharge Current Normal System Voltage (kV) 66 132 220 400 750

Highest System Voltage (kV) 72.5 145 245 420 765

Surge Current (A) 5,000 5,000 10,000 15,000 20,000

Service Condition -

Normal Service Conditions: Surge arresters which conform to this standard shall be suitable for normal operation under the following normal service condition :o Ambient air temp. within the ranges of -40 C to +40 C o Solar Radiation o Altitude not exceeding 1000m o Frequency of the AC power supply not less than 48Hz and not exceeding 62 Hz. o Wind speed < 34 m/s

-

Abnormal Service Conditions: Surge arrester subjected to other than normal application or service conditions may require special consideration in design, manufacture or application. The use of this standard in case of abnormal service conditions is subjected to agreement between the manufacturer and the purchaser. A list of possible abnormal service condition is given below. o Temp. in excess of +40C or below -40C o Altitude higher than 1000 m. o Nominal system frequency below 48Hz or above 62Hz. o Unusual transportation or storage.

Thyrite Arrestors Thyrite is a material obtained by a special type of clay mixed with carborundum (Silicon Carbide). Thyrite is used widely in lightning arresters. Thyrite is a non-linear resistor. i.e. it has high resistance at low voltages and low resistance at high voltages. A two times increase in voltage causes the current to increase by nearly 12 times. Hence, heavy currents can be discharged during voltage strikes and other surges. This heavy discharge of current enables quick reduction of the surge voltage preventing flashovers.

The Thyrite arresters are usually arranged in parallel with the primary winding of the Transformer. The Thyrite inside the arrester is arranged in the form of discs. The sides of the discs are metal plated to decrease resistance between the discs. Once the surge has been discharged, the Thyrite quickly returns to its high resistance state.

Zinc Oxide Lightning Arrester In modern era, gap less ZnO or zinc oxide surge arresters are mainly used for surge protection. Let us discuss zinc oxide type gap less arresters.

Construction of Zinc Oxide Lightning Arrester This type of arrester comprises of numbers of solid zinc oxide disc. These discs are arranged one by one to form a cylindrical stack. The number of zinc oxide discs used per lightning arrester depends upon the voltage rating of the system. This stack is kept inside a cylindrical housing of polymer or porcelain. Then the stack is placed inside the housing and highly pressed by heavy spring load attached to end cap at top. The equipment connection terminal for line is projected from top cap and connection terminal for earth is projected from the bottom cap.

Working Principle of Zinc Oxide Lightning Arrester The normal operation is defined as condition when no surge is presented and the surge arrester is subjected to normal system voltage only. The zinc oxide has highly non-uniform current voltage (I – V) characteristics. This typical I-V characteristic makes zinc oxide very suitable for designing gap less zinc oxide lightning arrester for surge protection. The non linear resistance of the block is an inherent bulk property and consists of mainly zinc oxide (90 to 95%) with relatively small amounts of several additives of other metal oxide (5 to 10%) like alumina, antimony tri-oxide, bismuth oxide, cobalt oxide, zirconium etc. On a macroscopic scale the additives are almost homogeneously distributed throughout the arrester blocks. But the micro structures of the metal oxide block represents a network of series and parallel arrangements of highly doped zinc oxide (ZnO) grains separated by inter granular junctions. The non linear behavior is the super imposition of non linear characteristics of individual junctions. The current carrying capacity of the surge arrester block is proportional to the total cross-section of the block. The non linear resistance characteristics of ZnO block can be expressed as,

where, Ir and Vr are the reference current and voltage respectively of the lightning arrester or surge arrester block. The value of x is 30 to 40 in case of metal oxide block. For normal system, the voltage and current increase. For normal system, the voltage and current increases linearly, i.e. for increasing system voltage at this range, current is increased in linear proportionate. The current at this region of characteristics is in range of micro ampere. But beyond a certain voltage level, leakage current voltage level, leakage current starts increasing very rapidly it is of KA range. The voltage beyond which the current through the LA becomes such high, is referred as reference voltage and the current at reference voltage is known as reference current. Sudden draining of huge current through lightning arrester just beyond reference voltage level, prevents the system from transient over voltage stress. The voltage-current relation in a metal oxide block highly depends upon temperature. Metal oxide block has negative temperature co-efficient. That means with increase in temperature, resistance of the surge arrester decreases hence for some system voltage, the leakage current through the instrument increases with increase in temperature.

As we know that, there would be a continuous leakage current through the LA. This leakage current generates heat. This generated heat should be dissipated properly otherwise the temperature of the LA may rise which further increases the leakage current. Because of this the proper thermal design of surge arrester housing plays an important role. There is a critical temperature depending upon the voltage rating of the metal oxide block beyond which joule heat generated in the block which joule heat generated in the block cannot be dissipated at required rate and which finally leads to thermal runaway of lightning arrester. Now we can understand that, the working principle of LA or surge arrester used for surge protection fully depends upon non linear V-I characteristics of metal oxide (ZnO) blocks inside the insulator housing of the arrester.

Definition of Electrical Noise An AC power line disturbance caused by sudden changes in the load. Electrical noise is problematic to solid state devices because they cannot differentiate between an intended electrical pulse and an unintended electrical spike.

Definition of Earth Loop as a cause of Noise In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops are a major cause of noise, hum, and interference. They can also create an electric shock hazard, since ostensibly "grounded" parts of the equipment, which are often accessible to users, are not at ground potential. Types of Coupling Electrical conduction: 

Hard-wire



Resistive

Electromagnetic induction: 

Electrodynamic -- commonly called inductive coupling, also magnetic coupling



Electrostatic -- commonly called capacitive coupling

Electrostatic and Electromagnetic Coupling Coupling is the transfer of electrical energy from one circuit segment to another. For example, energy is transferred from a power source to an electrical load by means of conductive coupling, which may be either resistive or hard-wire. An AC potential may be transferred from one circuit segment to another having a DC potential by use of a capacitor. Electrical energy may be transferred from one circuit segment to another segment with different impedance by use of a transformer. This is known as impedance matching. These are examples of electrostatic and electrodynamic inductive coupling.

Shielded Isolation Transformer A shielded transformer is a two-winding transformer, usually delta–star connected and serves the following purposes: 1. – Voltage transformation from the dist i utio

oltage to the e uip e t s utilization voltage.

2. – Converting a 3-wire input power to a 4-wire output thereby deriving a separate stable neutral for the power supply wiring going to sensitive equipment. 3. – Keeping third and its multiple harmonics away from sensitive equipment by allowing their free circulation in the delta winding. 4. – Softening of high-frequency noise from the input side by the natural inductance of the transformer, particularly true for higher frequency of noise for which the reactance becomes more as the frequency increases. 5. – Providing an electrostatic shield between the primary and the secondary windings thus avoiding transfer of surge/impulse voltages passing through inter-winding capacitance.

Figure 1 - Principle of a shielded two winding transformer

Figure 1 shows the principle involved in a shielded transformer. The construction of the transformer is such that the magnetic core forms the innermost layer, followed by the secondary winding, the electrostatic shield made of a conducting material (usually copper) and finally the primary winding.

Regulating Transformer Transformer having one or more windings excited from the system circuit or a separate source and one or more windings connected in series with the system circuit for adjusting the voltage or the phase relation or both in steps, usually without interrupting the load.

Earth Leakage Circuit Breaker (ELCB) An Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power. 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). Voltage Base ELCB Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end. 



If the voltage of the Equipment body is rise (by touching phase to metal part or failure ofinsulation of equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. When voltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock. The ELCB detects fault currents from live to the earth (ground) wire within the installation it p ote ts. If suffi ie t oltage appea s a oss the ELCB s se se oil, it ill s it h off the po e , and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.

 



These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts. These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of meters of metal service pipes buried in the ground. The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30 A ‘CD s fo a applia es a d e te sio leads that ay be used outside as a minimum.

Advantages   



ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer nuisance trips. While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus there are situations in which an ELCB can nuisance trip. When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough voltage to cause it to trip. If the i stallatio s ea th od is pla ed lose to the ea th od of a eigh o i g uildi g, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the ELCB.



 

 

If there is an accumulated or burden of currents caused by items with lowered insulation resistance due to older equipment, or with heating elements, or rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a some mA which is equal to ELCB rating than ELCB may give nuisance Tripping. If either of the earth wires become disconnected from the ELCB, it will no longer trip or the installation will often no longer be properly earthed. Some ELCBs do not respond to rectified fault current. This issue is common for ELCBs and RCDs, but ELCBs are on average much older than RCB so an old ELCB is more likely to have some uncommon fault current waveform that it will not respond to. Voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connection to earth on the protected system can disable the detector. Nuisance tripping especially during thunderstorms.

Disadvantages     

They do not dete t faults that do t pass u e t th ough the CPC to the ea th od. They do not allow a single building system to be easily split into multiple sections with independent fault protection, because earthing systems are usually use common earth Rod. They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth. As electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB to trip. ELCBs introduce additional resistance and an additional point of failure into the earthing system.

Current-operated ELCB (RCB) 

Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earth leakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents means the magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact.



When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity current operated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate at much higher currents.

Typical RCB circuit:



The supply coil, the neutral coil and the search coil all wound on a common transformer core.



On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.



In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero. This net remaining flux is what we call a residual flux.



The periodically changing residual flux inside the transformer core crosses path with the winding of the search coil. This action produces an electromotive force (e.m.f.) across the search coil. An electromotive force is actually an alternating voltage. The induced voltage across the search coil produces a current inside the wiring of the trip circuit. It is this current that operates the trip coil of the circuit breaker. Since the trip current is driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current devise.



With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker (RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated.



RCD sensitivity is expressed as the rated residual operating current, noted IΔ . P efe ed alues have been defined by the IEC, thus making it possible to divide RCDs into three groups according to thei IΔ alue. High sensitivity (HS): 6- 10- 30 mA (for direct-contact / life injury protection) Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current. Type AC: RCD for which tripping is ensured for residual sinusoidal alternating currents

  