Earthing of Electrical Systems

SAIEE TUTORIAL PROTECTION APPLICATION CONSIDERATIONS FOR EARTHING SYSTEMS AND

SECTION ON EARTHING SYSTEMS BY D G DUNCAN

Ron Slatem and Associates (Acknowledgement to Ron Slatem and previous Eskom personnel)

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SYSTEM GROUNDING AND ASSOCIATED PROTECTION ASPECTS 1.0 SYSTEM NEUTRAL GROUNDING (OR EARTHING) 1.1 Introduction The principal objective for any power system, whether industrial or utility, is to continue to function in its entirety for as great a percentage of its operating life as possible. If it could be arranged that the system continued to function satisfactorily even when a fault has occurred, without disconnecting the faulty element, this would be ideal. In the hope of achieving this objective the earliest power systems were operated without the intentional connection to ground of any part of the system. The theory was that an accidental connection to ground would have no adverse effect on plant as no ground fault currents could flow. With no ground fault currents there could be no damage. Therefore the plant could be kept in service. Unfortunately, it is physically impossible to have a system without any connection whatsoever to ground. Even if there is no intentional connection, there is always the capacitance of the system to ground which provides a path for current flow if a further connection to ground occurs. See fig 1a. Such currents would, of course, be of relatively low magnitude. If the further, unintentional, connection to ground is a solid, "bolted" connection, the situation is still tolerable as the effect of grounding one phase of an otherwise "ungrounded" system is merely to lift the other two phases to line voltage above ground. (Fig.lc). If the insulation to ground is designed to withstand such voltages continuously, the system can be operated in spite of the ground "fault" condition. The first problem that is likely to arise is if a second phase should also develop a ground fault. Now there is a solid path for the flow of large fault currents and the condition is equivalent to a phase-tophase fault. The second fault is highly unlikely to occur on the same circuit as the initial fault. This type of double ground fault on different circuits is called a "cross-country" fault and is often very difficult to relay satisfactorily. It also requires the disconnection of two items of plant. The second, and more serious problem arises if the ground fault is not a solid bolted fault, but an arc. The arc is of a low current level and tends to be highly unstable so it tends to quench and restrike as the system voltage alternates. Such restriking in a capacitively grounded circuit is called an "arcingground" and can give rise to extremely high transient overvoltages. These overvoltages can severely stress the insulation of the whole of the associated system and cause multiple insulation failures on other items of plant, leading to far greater disruption of the system than the removal of the initial faulty item would have caused. The disruption is also likely to be more prolonged and more costly. Because of bitter adverse experience of multiple insulation failures in many isolated neutral systems, it has become increasingly common practice to provide systems with an intentional grounding connection. The primary purpose of system grounding is to minimise the overvoltage and/or thermal damage to the system in the event of an earth-fault. System grounding reduces the severity of dynamic and transient overvoltages which develop if the system is subjected to a ground fault, particularily and can reduce thermal damage at the point of fault if the ground fault current is restricted.

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The grounding connection is usually made via available neutral points on the apparatus, such as the transformer neutral points on star connected windings, or via artificially derived neutral points. See fig 1b. In exceptional cases, at lower voltages up to 33kV, one corner of the system delta voltage triangle, instead of a neutral point, can be grounded. See fig 1c. This is normally only considered for systems of very limited extent e.g., the delta connected, stabilising winding of an auto-transformer which is either not used to supply load or which only supplies the local station auxiliary load. The various available ways of grounding the system can each result in different levels of current and voltage stress. It is the purpose of this article to assist the planning engineer in deciding how to select the grounding method most suited to his needs. The basic reasons for grounding or not grounding are presented, and the general practices and methods of grounding are reviewed. The SAIEE Workshop on neutral earthing at CSIR in June 1980 resulted in the recommendation of 300A earthing at medium voltages.in South Africa under the SABS Code of Practice 0200-1985 for Neutral Earthing of MV Industrial Power Systems.

1.2 Definitions The following varieties of system grounding are possible:-

1.2.1 Solid grounding No intentional impedance in the grounding path (see note 2 on page 4)

1.2.2 Effective grounding Based on the criterion that reduced insulation levels and reduced rated (80%) surge arresters can be used. (See note 1 on page 4) .

RO

≥

X1 and X0 ≤

RO

=

Zero sequence resistance

X0

=

Zero sequence reactance

X1

=

Positive sequence reactance

3X1

1.2.3 Reactance grounding Intentional insertion of reactance in the grounding path.

X0

≤

10 X1

4

1.2.4 Low Resistance Grounding Intentional insertion of resistance into the system grounding path.

R0

≥

2 X0

1.2.5 High Resistance Grounding The insertion of nearly the highest permissible resistance in the grounding connection.

R0

≤

XC0/3

1.2.6 Grounding for serving line-ground loads Z0

≤

Z1

1.2.7 Ungrounded No intentional system grounding connection.

1.2.8 Effects of method of Grounding The typical levels of available ground fault current that can be expected from these types of system grounding are:1)

Solidly grounded, effectively grounded and grounded for serving line-ground loads can result in the same order of ground fault current as is available for three-phase short circuits usually of the order of several thousands of amperes.

2)

Reactance grounded. Ground fault currents range from 25% to 60% of the three-phase level. These correspond to X0/X1 ratios of 10 and 3

3)

Low resistance grounded:

4)

High resistance grounded: ground fault currents up to 50 amperes. Usually less than 10 amperes.

from as low as 50 amperes up to several thousand amperes.

It must immediately be obvious that, the higher the level of ground fault current that can be expected from the grounding method, the faster the protection must operate if excessive thermal damage is to be avoided. If the grounding method limits the ground fault currents to very low levels, the protection can be slower in operation but the need for greater sensitivity increases. However, the protection must not be too slow as to allow the ground fault to develop into a phase to phase fault.

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There is always a trade-off between current stress and voltage stress. If you limit the current stress, you run the risk of introducing voltage stress and also increase the relaying difficulty. The prime objective of system grounding is to limit current stress without introducing unacceptable voltage stress or relaying difficulty. 1.2.8.1..Effectively Grounded. Grounded through a grounding connection of sufficiently low impedance (inherent or intentionally added or both) that ground faults which may occurcannot build up voltages in excess of limits established for apparatus, circuits, or systems so grounded. NOTE 1 . An A.C. system, or portion thereof, may be said to be effectively grounded when, for all points on the system or a specified portion thereof, the ratio of zero-sequence reactance (X0) to positive-sequence reactance (X1) is not greater than 3 and the ratio of zero-sequence resistance (R0) to positive-sequence reactance (X1) is not greater than 1.0 for any condition of operation and for any amount of connected generator capacity. This definition is basically used in the application of line-to-neutral surge arresters. Surge arresters with less than line-to-line voltage ratings (80% arrestors) are applicable on effectively grounded systems. Insulation cost can be reduced by the use of effective earthing, but at the expense of relatively high fault current levels. 1.2.8.2..Grounded System. A system of conductors in which at least one conductor or point (usually the middle wire or neutral point of a transformer or generator winding) is intentionally grounded, whether solidly or through a current-limiting device. NOTE 2 Various degrees of grounding have been used, from solid or effective to the high-impedance grounding obtained from a small grounding transformer erroneously used only to secure enough ground current for relaying, or to the high-resistance grounding which secures control of transient overvoltages but may not furnish sufficient current for ground-fault relaying. Figs,lb and lc show two points at which a system may be grounded and the corresponding voltage relationships.

1.2.8.3 Solidly grounded. Grounded through an adequate ground connection in which no impedance has been inserted intentionally. NOTE 3. This term, though commonly used, is somewhat confusing since a transformer may have its neutral solidly connected to ground, and yet the connection may be so small in capacity as to furnish only a very-high-impedance ground to the system to which it is connected. In order to define grounding positively and logically as to degree, the term effective grounding has come into use. The term solidly grounded will, therefore, be used only in referring to a solid metallic connection from system neutral to ground; that is, with no impedance intentionally added in the grounding circuit. It does not imply that the system is necessarily effectively grounded.

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1.2.8.4 Ungrounded. A system, circuit, or apparatus without an intentional connection to ground except through potentialindicating or measuring devices or other very- high-impedance devices and the system capacitance. NOTE 4. Though called ungrounded, this type of system is in reality coupled to ground through the distributed capacitance of its phase windings and conductors. In the absence of a ground fault, the neutral of an ungrounded system, under reasonably balanced load conditions, will usually be close to ground potential, being held there by the balanced electrostatic capacitance between each phase conductor and ground. Fig. la shows an ungrounded system with voltage relations for balanced phase-to-ground capacitances.

1.3 Factors Influencing Choice of Grounded or Ungrounded System 1.3.1 Service Continuity As already indicated, a great number of industrial plant distribution systems have been operated ungrounded at one or more voltage levels for may years. In most cases this was done with the thought of gaining an additional degree of service continuity. The fact that any contact occurring between one phase of the system and ground is unlikely to cause an immediate outage to any load may represent an advantage in many plants, varying in its importance according to the type of plant. For the first grade shut-down supplies at Koeberg nuclear power station it is vital that the supplies are available even if a ground-fault should occur. The licensing authorities insist that the auxiliary systems providing these supplies must not be grounded. They claim that the zero-sequence capacitance of the system is so low that arcing fault currents will be self extinguishing. There should, therefore, be no danger of arcing grounds causing excessive overvoltages, and multiple ground-faults. Grounded systems, in most cases, are designed so that circuit protective devices will remove a faulty circuit from the system regardless of the type of fault. A phase-to-ground fault generally results in the immediate isolation of the faulted circuit with the attendant outage of the loads on that circuit. However, experience has shown (4), in the greatest number of systems, that greater service continuity may be obtained with grounded-neutral than with ungrounded-neutral systems. 1.3.2 Multiple Faults to Ground We have seen that while a ground-fault on one phase of an ungrounded system generally does not cause a service interruption, the occurrence of a second ground fault on a different phase, before the first fault is cleared does result in an outage. If both faults are on the same feeder, that feeder will be opened. If the second fault is on a different feeder, both feeders may have to be de-energized. The longer a ground is allowed to remain on an ungrounded system, the greater is the likelihood of a second ground occurring on another phase, resulting in an outage. The advantage of an ungrounded system, in not immediately dropping load upon the occurrence of a ground fault, may largely be destroyed by the practice of ignoring a ground until a second one occurs and repairs are required to restore service. With an ungrounded system it is extremely important that an

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organised maintenance programme be provided so that grounds are located and removed as soon as possible after their detection. An adequate detection system, possibly in conjunction with an audible alarm, is considered essential for operation of an ungrounded system. In addition, it is advisable in ungrounded systems to employ ground-fault tracing equipment which permits maintenance personnel to locate a ground with the system energised and without the necessity of interrupting service on any circuit during the process of fault locating. Experience has shown that multiple “cross country” ground faults are rarely, if ever, experienced on grounded-neutral systems. A possible exception to this rule is the case of double circuit overhead lines where high power footing resistances can result in a ground fault on both circuits due to a lightning stroke to the tower. This is a double “back-flash”. 1.3.3 Arcing Fault Burndowns In recent years, especially in low-voltage power systems, there have been many reported cases of arcing fault burndowns in which severe damage to or complete destruction of electrical equipment has been caused by the energy of arcing fault currents (5). In typical cases an arcing fault becomes established between two or more phase conductors in an ungrounded system, or between phase conductor(s) and ground in a solidly grounded-neutral system. The fault arc causes the release of enormous amounts of energy at the fault site, resulting in the violent generation of hot gases and arc plasma. The accompanying heat is so intense that it vaporises copper or aluminium conductors and surrounding steel enclosures and distills toxic and flammable gases from organic insulation systems. Frequently, the devastation is so complete that the equipment involved must be totally replaced. It is characteristic of arcing fault burndowns that the normal phase-overcurrent devices do not operate to remove the initial fault quickly. Arcing fault current levels may be so low that such devices are either not actuated at all (fault currents are below pickup settings) or are actuated only after a long period of time, too late to prevent burndown. It is generally recognised that prevention of arcing fault burndowns, at the present state of protective system design, must rely upon fast and sensitive detection of the arcing fault current, accompanied by interruption of the faulty circuit within approximately 5 to 20 cycles, (100-400ms). In the solidly grounded-neutral system, this fast, sensitive detection is possible since an arcing fault will produce a current in the ground path, either because the fault begins as a line-to-ground fault or because it will almost instantly involve ground even though initiated as a line-to-line arcing fault. Under normal (nonground fault) conditions there is no significant current in the ground return path; therefore, monitoring the solidly grounded-neutral system for currents in the ground circuit provides an easy means for detecting and removing destructive arcing faults to ground. This type of protection is universally applicable throughout the power system, and the sensitivity and speed of such relaying is independent of load current values and phase-overcurrent device settings. Relays are being introduced which recognise the high frequency signature of an arcing ground fault Thus, the solid and low-resistance grounded-neutral systems provide a basis for easily securing protection against ruinous (phase-to-ground) arcing fault burndowns. The ungrounded and highimpedance grounded systems, on the other hand, while generally being less likely to experience arcing faults, provide less protection against arc blast and flash hazard. No system has completely reliable and universally applicable means of protection against low-level line-to-line arcing fault burndowns. 1.3.4 Location of Faults

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On an ungrounded system, a ground fault does not open the circuit. Some means of detecting the presence of a ground on the system should be installed. Voltage transformers connected to indicate the potential from each phase to ground will show the presence of a ground fault and which phase is involved, but will not show on which feeder the fault has occurred. Locating a ground fault on one of the several feeders may require the removal from service of one feeder at a time until the ground detector indicates that the faulted feeder has been removed from the system. Should it happen that the same phase of two different feeders becomes faulted to ground at the same time, the faulted feeders cannot be located by removing them from the system one at a time. It may be necessary to remove all feeders and restore them to service one at a time, checking the ground detector as each feeder is restored. Such detection methods inevitably result in considerable disruption of the supply continuity. The location of a grounded feeder on an ungrounded system may be facilitated by use of various types of locating apparatus. For example, an interrupted direct voltage or superimposed audio signal may be applied to the feeder bus and the tracing current detected in the grounded feeders. Some operators have reported success using 1ocation apparatus not requiring the deenergizing of system feeders. This, of course, has the advantage of permitting location of grounds without waiting for light load periods on the system. An accidental ground on a grounded system is both indicated and at least partially located by an automatic interruption of the accidentally grounded circuit or piece of equipment.

1.3.5 Safety Many of the hazards to personnel and property existing in some industrial electrical systems are the result of poor or nonexistent grounding of electrical equipment and metallic structures. It is important to note that regardless of whether or not the system neutral is grounded, safety considerations require thorough grounding of equipment and structures. Proper grounding of a low voltage (600 V or less) distribution system may result in less likelihood of accidents to personnel than leaving the system supposedly ungrounded. The knowledge that a circuit is grounded generally will result in greater care on the part of the workman. It is erroneous to believe that on an ungrounded system a person may contact an energized phase conductor without personal hazard. As Fig. la shows, an ungrounded system with balanced phase-toground capacitance has normal line-to-neutral voltage existing between any phase conductor and ground. To contact such a conductor accidentally or intentionally, may present a serious, perhaps lethal, shock hazard in most instances. The capacitve current which can flow may be in the order of amps at medium voltages. During the period a ground fault remains on one phase of an ungrounded system, personnel contacting one of the other phases and ground are subjected to voltage 1,73 times that which would be experienced on a solidly neutral-grounded system. The voltage pattern would be the same as shown in Fig. lc. Other hazards of shock and fire may result from inadequate grounding of equipment in either grounded or ungrounded systems. Accidental grounds are inevitable. The current path to ground for a windingto-frame insulation breakdown in a motor may include greasy shavings or other materials that would be ignited by sparks or localised heating. Such a high-impedance ground circuit may not permit enough current flow to operate protective devices, with the result that a potential fire and safety hazard may exist for some time. There is the hazard of shock to personnel from such a condition should they bridge

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all or part of the high-impedance ground path, for example, contacting the frame of the faulty machine. This hazard is particularly bad because there are more possible victims than in the case of persons familiar with electric systems working on a circuit. 1.3.6 Abnormal Voltage Hazards The posssible overvoltages on the ungrounded system may cause more frequent failure of equipment than if the system were grounded. In some cases these overvoltages have caused failures on more than one unit of equipment at the same time. These multiple failures are not necessarily confined to one feeder but may involve equipment on several different feeders. A fault on one phase of an ungrounded system places a sustained increased voltage on the insulation of ungrounded phases in a three-phase system. This overvoltage is 1,73 times the voltage normally on the insulation. This or other sustained overvoltages or the transient overvoltages on the ungrounded system may not immediately cause failure of insulation, but may tend to reduce the life of the insulation. The reduced overvoltages experienced on grounded systems are less likely to damage equipment or insulation.

1.4 Power System 0vervoltages (See Refs. 9, ch 5). Some of the more common sources of overvoltage on a power system are the following: (1) Lightning (2) Switching surges (3) Static (4) Contact with a higher voltage system (5) Line-to-ground faults (6) Resonant conditions (7) Restriking ground faults 1.4.1 Lightning (See Ref,10) Most industrial systems are effectively shielded against direct lightning strokes. Many circuits are either underground in ducts or within grounded metal conduits or race-ways. Even open-wire circuits are often shielded by adjacent metallic structures and buildings. Surge arresters applied at the incoming service, limit the surge voltages within the plant resulting from strokes to the exposed service lines, other arrester applications may be necessary within the plant to protect low impulse strength devices such as rotating machines. Where a plant is supplied from a substation stepping down from some higher voltage, surge arresters may be required on the low side of the transformer to protect plant equipment from surges passed by the high side arresters and reflected through the transformer. On the power system there are many thousands of kilometres of overhead lines and transformers etc., directly exposed to lightning. The protection of this plant against lightning overvoltages is a separate study. The system grounding does, however, have some influence on lightning arrester application since it affects the system insulation level required. All forms of grounding, other than effective grounding, require the use of 100% arresters. The arrester ratings are based on the "sound phase" voltages which can arise during a ground-fault on one phase. These voltages exceed 80% of the normal system line-line voltage for all forms of grounding other than effective grounding.

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For the latter case, arresters having 80% of the reseal value of those for non-effectively grounded systems, can be used. This permits the use of lower system insulation levels.

1.4.2 Switching Surges Normal switching operations in the system can cause overvoltages. These are generally not more than three times normal voltage and are of short duration. Overcurrent devices such as circuit breakers or fuses, in general, interrupt a circuit at a normal current zero at which time the stored energy in the inductance of the circuit is zero. The overvoltages thus developed result from transient oscillation in the circuit capacitance and inductance, there being stored energy in the circuit capacitance at the time of current interruption. More serious overvoltages can be produced by devices which interrupt by forcing current zero. Such devices as current-limiting fuses must be carefully applied because of this overvoltage problem. In low-voltage systems, however, the standards for current-limiting fuses require that the maximum peak voltage occurring during circuit interruption shall not exceed 3000 V; this is less than the crest of the normal high-potential voltage applied to 600-V class equipment. Neutral grounding is not likely to reduce the total magnitude of overvoltage produced by lightning or switching surges. It can, however, distribute the voltage between phases and reduce the possibility of excessive voltage stress on the phase-to-ground insulation of a particular phase.

1.4.3 Static Buildup of overvoltage on power system conductors due to static charge is not usually a problem in modern plants with metal-enclosed circuits and equipment. Static charge on moving belts can build up voltages which can be transmitted to the power system unless motor frames are properly grounded. 0verhead open-wire lines may be subject to static overvoltages resulting from certain atmospheric conditions. A system ground connection, even of relatively high resistance, can effectively prevent static voltage buildup. 1.4.4 Contact with Higher Voltage System Contact with a higher voltage system may be caused by a broken high-voltage conductor falling on a lower voltage conductor where both lines cross or are carried on the same poles, or by breakdown between the high- and low-voltage windings of a transformer. If the low-voltage system is ungrounded, the high voltage will remain on the low-voltage system causing breakdown of insulation, possibly at several points. An effectively grounded low-voltage system, though resulting in high values of fault current for this condition, will hold the system neutral close to ground potential and greatly reduce the overvoltages to ground on the low-voltage system.

1.4.5 Line-to-ground Faults A common source of sustained overvoltage on an ungrounded system is caused by one phase of a three-phase system becoming grounded. In this case the insulation of the other phases is subjected to 1.73 above normal voltage. An effectively grounded-neutral system would not result in this overvoltage. While this voltage seldom approaches the insulation levels of equipment and circuits, the cumulative effect of higher than normal voltage stresses may somewhat reduce insulation life.

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1.4.6 Resonant Conditions An ungrounded system may be subjected to resonant overvoltages. With the high phase-to-ground capacitance of larger systems, there may be a condition of approximate circuit resonance during a lineto-ground fault through an inductance such as a faulty coil in a motor starter. The voltage to ground of the unfaulted phases will then be considerably in excess of line-to-line voltage. An overvoltage due to resonant or near-resonant conditions can be encountered on a small system where tuned inductivecapacitive circuits are used for such purposes as operation of welders. For example, if the welder is equipped with a series capacitor for power factor improvement, the voltage across the capacitor and across the transformer winding are each many times the supply line-to-line voltage. A fault between the capacitor and the welder transformer imposes this high voltage on the insulation of the ungrounded system. A grounded-neutral system would prevent this overvoltage by holding the phases to their approximate normal voltage to ground. Ferro-resonant conditions can also cause excessive overvoltages if the system is grounded through a high inductance which can saturate. Such an inductance might arise if the system is grounded through a relatively small transformer such as a voltage transformer or neutral earthing compensator (NEC, now called a neutral electromagnetic coupler). Transient saturation of the iron core of the grounding device results in a value of inductance which resonates with the system zero-sequence shunt capacitance. The overvoltages are produced by a phenomenon known as "neutral inversion". Due to the resonant condition, the system neutral moves outside of the system voltage triangle and the phase-ground voltages can be as high as 5-8 times the normal values. The phenomenon is illustrated in Figs 2 a & b Reference 14,15. 1.4.7 Restriking Ground Faults Field experience and theoretical studies have shown that arcing, restriking, or vibrating ground faults on ungrounded systems, can under certain conditions, produce surge voltages as high as six times normal. The conditions necessary for producing these overvoltages require that the dielectric strength of the arc path builds up at a higher rate after each extinction of the arc than it did after the preceding extinction. This phenomenen is unlikely to take place in open air between stationary contacts because such an arc path is not likely to develop sufficient dielectric recovery strength. It may occur in confined areas where the pressure may increase after each conduction period. Neutral grounding is effective in reducing transient voltage buildup from such intermittent ground faults by reducing neutral displacement from ground potential and reducing the destructive effectiveness of any high-frequency voltage oscillations following each arc initiation or restrike. Resistance grounding is particularly effective since it provides the necessary damping to prevent voltage build up.

1.5 Cost Grounded-neutral systems may cost from nothing to several percent more than ungrounded-neutral systems, depending upon system voltage and whether the system to be grounded is an existing or a new system. Equipment for 380/220-V star-connected systems almost universally provides for system neutral grounding and therefore does not involve any extra cost. Star-connected unit substations for

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grounding 500-V systems are standard but may cost up to 6 percent more than delta-connected substations. In addition, there is the cost of the resistor or other grounding impedance, if used, and the possible additional cost of relaying. For system voltages of 3,3/6,6/11/22/44 kV delta-connected transformers are common. Therefore, there is generally a transformer price increase for grounding these systems. The grounding resistor and ground relaying represent an extra cost. Systems from 66kV upwards are usually star-connected and effectively grounded in this country. Considerable savings in cost of the transformer insulation can be effected at these voltages by using graded insulation. There is no additional cost associated with grounding such systems. To change existing systems to grounded operation may be very expensive because it may be necessary to add grounding transformers and their protective equipment, grounding impedance if required, and possibly a third current transformer in all circuits to obtain ground-fault relaying on all three phases.

1.6 Trends in Application of System Grounding The basic reasons for system grounding are the following: (1)

To limit the difference of electric potential between all uninsulated conducting objects in a local area.

(2)

To provide for isolation of faulty equipment and circuits when a fault occurs.

(3)

To limit overvoltages appearing on the system under various conditions.

As previously stated many industrial power system operators believe that an ungrounded system offers greater service continuity than a grounded system, because a line-to-ground fault does not cause immediate tripping of the faulted circuit. On the other hand, a second ground fault on another phase of another circuit from the original fault causes a phase-to-phase fault, large short-circuit current flow (with attendant hazards), and tripping of both faulted circuits. Consequently, a major factor to consider in selecting a grounded or ungrounded system is the quality of electrical maintenance available. Well-maintained ungrounded systems, in which the first ground fault is promptly located and corrected, probably have greater service continuity than solidly grounded systems. Great care must be taken to ensure that the zero sequence capacitance of such ungrounded systems is low or serious overvoltages could arise (Figs.3a & 3b & 4a) (Refs. 14 chapter 1 and Refs. 16 chapter 6). Ungrounded systems are definitely not recommended unless a full transient overvoltage study indicates that they will not produce excessive overvoltages. However, many users whose maintenance practices are not quite so extensive feel that a groundedneutral system gives them more continuous service than an ungrounded system. There has been an increasing trend toward grounding industrial systems in order to overcome some of the disadvantages attributed to ungrounded operation. In recent years a substantial percentage of new

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industrial substation transformers have been purchased with star-connected low-voltage windings with insulated neutral brought to external termination suitable for neutral grounding. In new installations, these transformers offer the advantage that they can be operated ungrounded, while having the neutral available for grounding, if desired, at some future time. After the SAIEE Workshop of 1980, the recommended practice is resistive grounding of MV systems, limited to 300A per transformer or grounding point

1.6.1 High-Voltage systems (above 66 kV) Systems above 66kV are nearly always solidly grounded, because these are usually transmission circuits with open lines in which, in most cases, grounded-neutral-type (80%) lightning arresters are desirable for better overvoltage protection and lower cost. In addition, rotating equipment is seldom connected directly to these systems; hence, limited ground-fault current, to prevent burning of laminations, is a less important factor than in the medium-voltage systems. In addition, voltages above 66kV are not usually carried inside buildings; hence shock hazards due to high fault currents are not a factor. Finally, the cost of grounding resistors at these voltages is high. 1.6.2 Industrial vs. Utility Practice The characteristics and operation of industrial power systems differ in some respects from those encountered in utility systems; therefore it may be expected that grounding practices would also be different. Such is the case, although the basic principles of neutral grounding are followed in both types of systems. A comparison of the pertinent characteristics of the two types of systems is given and summarized in Table I. Utility practice in recent years has followed the recommendation of 300A resistance grounding. Many such networks feed industrial or municipal systems. This method requires the use of ungroundedneutral-type lightning arresters and reduced ground fault currents. Solid grounding offers savings in the use of graded insulation in transformers at 66kV and above. A large percentage of ground faults on utility systems occurs as insulator flashovers on overhead lines and the high ground-fault current due to solid grounding does not cause expensive damage to equipment at the point of fault. In the case of industrial power systems, resistance grounding is preferred for voltage levels from 2,2 to 33kV. The principal reason for this practice is to ensure reduced magnitudes of ground-fault current and consequent reduction in possible damage at the point of fault. This is a particularly important factor in the case of ground faults in the windings of motors and generators. Although a ground fault of limited magnitude and duration may cause sufficient damage to require the replacement of several coils, the desired result is obtained when the damage is confined to the coils and the machine laminations are left intact.

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TABLE 1

Industrial vs. Utility Grounding Practice

Desirability of high continuity of power Predominant method of conducting power Percent of system subject to lighning hazard Investment in lightning arresters Predominance of voltage levels above 11kV Rotating equipment at distribution and transmission levels

Industrial Yes

Utility Yes

Cable

Overhead lines

Small

Large

Small Few systems

Large Many systems

Yes in most cases

Not in the usual case

1.7 Methods of System Grounding of the System Neutral Most grounded systems employ some method of grounding the system neutral at one or more points. These methods (see Fig.4) are referred to as the following: (1) Solid grounding see fig 4b (2) Resistance grounding see fig 4e (3) Reactance grounding see fig 4 d (4) Ground-fault-neutralizer grounding

see fig 4c

Table II Sytem Characteristics with Various Grounding Methods Ungrounded

Essentially solid grounding Solid

Current for phaseto-ground fault in percent of threephase fault current Transient overvoltages Automatic segregation of faulty zone Lightning arrestors

Less than 1%

Remarks

Not recommended due to overvoltages and nonsegregation of fault

Very high No

Ungroundedneutral type

Varies, may be 100% or greater Not excessive Yes

Groundedneutral type

Low-value reactor Usually designed to producte 25 to 100% Not excessive Yes

Groundedneutral type if current is 60 % or greater Generally used on systems (1) 600 volts and below and (2) over 15 kV

Reactance grounding High-value reactor 25 to 60 % for X0/X1 of 10 to 3

Ground-fault neutralizer

Resistance grounding Resistance

Nearly zero fault current

5 to 20 %

Very high

Not execessive

Yes

No

Not execessive Yes

Ungroundedneutral type

Ungroundedneutral type

Ungroundedneutral type

Not recommende d due to excessive overvoltages

Best suited for high-voltage overhead lines where faults may be selfhealing

Generally used on industrial systems of 2,2 to 33kV

Table II gives the characteristics of the different methods of grounding.

15

Each method, as named, refers to the nature of the external circuit from system neutral to ground rather than to the degree of grounding. In each case the impedance of the generator or transformer, whose neutral is grounded, is in series with the external circuit. Thus, a solidly grounded generator or transformer may or may not furnish effective grounding to the system, depending on its impedance.

1.7.1 Solid Grounding Solid grounding (fig 4b) refers to the connection of the neutral of a generator, power transformer, or grounding transformer, directly to the station ground or to the earth. Because of the reactance of the grounded generator or transformer in series with the neutral circuit, a solid ground connection does not provide a zero-impedance neutral circuit. It is most important to realise that if the reactance of the generator or transformer is too great with respect to the total system reactance, the objectives sought in grounding, principally freedom from transient overvoltages, may not be achieved. Thus, it is necessary to determine the degree of grounding provided in the system. A good guide in answering this question is the magnitude of ground-fault current in relation to the three-phase fault current, the higher the ground fault amount in relation to the three-phase current, the greater the degree of grounding in the system. In terms of resistance and reactance, effective grounding of a system has been defined in Note 1. In most generators, solid grounding, that is without external impedance, may permit the maximum groundfault current from the generator to exceed the maximum three-phase fault current which the generator can deliver and for which its windings are braced. Consequently, neutral-grounded generators should be grounded through a reactor which will limit the ground-fault current to a value no greater than the generator three-phase fault current. A generator has a lower zero sequence impedance than positive sequence impedance. Most modern large generators are unit-connected with their step-up transformers. The generator transformer LV winding is delta-connected and, together with the generator, forms an "island" whose earthing can be decided on the basis of minimising the stator iron damage caused by earth faults. High resistance or resonant grounding is most common. Refs. (1) ....... Surge arresters for grounded-neutral service (rated near but not less than 80% of line-to-line voltage) require that the system be effectively grounded. This will carry with it a line-to-ground circuit current of at least 60% of the three-phase short-circuit value.

1.7.2 Resistance Grounding In resistance grounding (fig 4e), the neutral is connected to ground through one or more resistors. In this method, with the resistors normally used and excepting for transient overvoltages, the line-toground voltages which exist during a line-to-ground fault are nearly the same as for an ungrounded system. A system properly grounded by resistance is not subject to destructive transient overvoltages. For resistance-grounded systems at 15 kV and below, such overvoltages will not ordinarily be of a serious nature, unless the resistance exceeds the following boundary limits:

16

R0

≤

XC0 /3,

R0

≥

2 X0 .

Resistance grounding may be either of two classes, high resistance or low resistance, distinguished by the magnitude of ground-fault current permitted to flow. Both types are designed to limit transient overvoltages to a safe level (within 250% of normal); however, the high-resistance method usually does not require immediate clearing of a ground fault since the fault current is limited to a very low level. This low level, typically in the order of 5 A, must be at least equal to the system total capacitance-to-ground charging current, 3 x IC0/phase. The protective scheme associated with highresistance grounding is often detection and alarm rather than immediate tripout. Generally, the use of high-resistance grounding with alarm only should be avoided on systems where the line-to-ground fault current exceeds 10 A. The low-resistance method has the advantage of immediate and selective clearing of the grounded circuit but requires that the minimum ground-fault current be large enough, usually 300A or more, to actuate the applied ground-fault relay positively. High-resistance grounding is a method that can be applied to existing medium-voltage ungrounded systems to obtain the transient overvoltage protection without the modification expense of adding ground relays to each circuit. Systems grounded through resistors generally require surge arresters suitable for use on ungroundedneutral circuits, that is, with a voltage rating at least equal to line-to-line voltage. The reasons for limiting the current by resistance grounding may be one or more of the following: (1)

To reduce burning and melting effects in faulted electric equipment such as switchgear, transformers, cables and rotating machines.

(2)

To reduce mechanical stresses in circuits and apparatus carrying fault currents.

(3)

To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path.

(4)

To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault.

(5)

To reduce the momentary line voltage dip occasioned by occurrence and fault.

(6)

To secure control of transient overvoltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high-resistance grounding).

(7)

Slower and therefore cheaper ground-fault protection can be used.

(8)

Practically negligible fault-breaking duty on circuit breakers clearing ground-faults - therefore less maintenance.

(9)

Reduced telephone interference.

clearing of a ground

17

1.7.3 Reactance Grounding. The term reactance grounding (fig 4e) describes the case in which a reactor if connected between the system neutral and ground or where a neutral electromagnetic coupler (NEC) or grounding transformer, having its neutral directly grounded, is provided. The value of the zero-sequence reactance of these grounding devices is usually chosen to satisfy the requirement that X0/X1

≤

10.

Since the ground-fault current which may flow in a reactInce-grounded system is a function of the neutral reactance, the magnitude of ground-fault current is often used as a criterion for describing the degree of grounding. In a reactance-grounded system, the available ground-fault current should be at least 25 percent (X0/X1 ≤ 10) and preferably 60 percent (X0/X1 ≤ 3) of the three-phase fault current to prevent serious transient overvoltages (both satisfy X0 ≤ l0X1 ). This is considerably higher than the minimum fault current desirable in a resistance-grounded system and, therefore, reactance grounding is usually not considered an alternative to resistance grounding.

1.7.4 Ground-Fault Neutralizer (Petersen coil) (See Ref.11). A ground-fault neutralizer (fig 4c) is a special case of a reactor connected between the neutral of a system and ground and having a specially selected, relatively high value of reactance. It should be noted that failures in solid insulation such as paper, varnished cambric, and rubber are not self-healing, as insulator flashovers can be, and are not extinguished by the use of the ground-fault neutralizer. See Fig. 5 A line-to-ground fault causes line-to-neutral voltage to be impressed across the neutralizer, which passes an inductive current. This current is 180ø out of phase and is approximately equal in magnitude (when the neutralizer is tuned to the system) to the resultant of the system-charging current of the capacitance of the two unfaulted phases. The inductive and capacitive components of current neutralize each other, and the only remaining current in the fault is due to resistance, insulator leakage, and corona. This current is relatively small, and, as it is in phase with the line-to-neutral voltage, the current and voltage pass through a zero value at the same instant. In addition, the rate of rise of recovery voltage on the faulted phase is very low. Therefore, the arc is extinguished without restriking, and flashovers are quenched without removing the faulted line section from service. On systems for which ground-faults in air are relatively frequent, ground- fault neutralizers may be very useful by reducing the number of circuit breaker operations required to remove faults, thus improving service continuity. They have been used primarily on systems above 15 kV consisting largely of overhead transmission lines. They have also been used on unit connected generators and generation transformers. In some cases, where it has not been deemed desirable by the plant operators to trip a circuit on the occurrence of a ground fault, special arrangements have been used to limit the damage due to the flow of charging current and yet be able to locate the faulty feeder easily. One method is to use a groundfault neutralizer in the neutral to limit the ground-fault current and to reduce switching surges to safe values. In some cases it may be desirable to pass enough ground-fault current to operate relays that

18

give a signal or trip the breaker of the faulty feeder. This is done by a current-sensing relay in combination with a resistor in parallel with the neutralizer. Because of the current to be switched, a power circuit breaker should be used for switching the resistor. The resistor and relay are selected as if only the resistor were used, such a scheme is expensive and is employed only in very special cases. One of the characteristics of resonance-grounded systems is that care must be taken to keep the ground-fault neutralizer tuned to the system capacitance to minimize the development of transient overvoltages. Thus, when sections of the system are switched on or off, it may be necessary to adjust the neutral reactance by changing the neutralizer tap. This operation may be accomplished by providing in the powerhouse an ammeter and control switch for remote control of a motordriven tap changer on the neutralizer, so that when parts of the system are switched, the neutralizer may be readjusted at that time. Self tuning reactors are now available in the market, but the cost needs to be weighed against the advantages of the earthing with a neutralizer.

1.8 Grounding at Points other than System Neutral In some cases, systems, particularly low-voltage systems (600 V and below), are grounded at some point other than the system neutral. This has been done to obtain a grounded system at a minimum expense where existing delta transformer connections do not provide access to the system neutral.

1.8.1 Corner-of-the-Delta Grounding In some systems of limited physical extent, grounding of one phase or"corner-of-the delta" has sometimes been done as a means of obtaining a grounded system. (see Fig 1c) A particular case where this has been fairly extensively used in the Eskom network is for grounding the delta stabilising winding of HV auto-transformers. By this means the LV system is grounded and ground faults can be detected and relayed without the additional expense of NECS or other grounding devices. Usually the corner is connected to ground via a current transformer which is often built into the power transformer. It has the disadvantage that the normal line-ground voltages on two of the phases are higher than they would be for a neutral grounded system. Also, a fault to ground on one of the other phases results in fault currents of phase-fault magnitude.

1.9 Selection and Design of System Grounding Arrangements The best way to obtain the system neutral for grounding purposes in three-phase systems is to use source transformers or generators with wye-connected windings. The neutral is then readily available. The alternative is to apply grounding transformers.

19

1.9.1 Grounding Transformers System neutrals will usually not be available in systems of 40kV and less. When existing deltaconnected systems are to be grounded, grounding transformers may be used to obtain the neutral. Grounding transformers may be either of the zig-zag or star-delta connected type. The type of grounding transformer most commonly used is a three-phase zig-zag transformer with no secondary winding. The internal connection of this transformer is illustrated in Fig.6a The impedance of the transformer to three-phase currents is high so that when there is no ground fault on the system, only a small magnetizing current flows in the transformer winding. The transformer impedance to ground current, however, is low so that it allows high ground current to flow. The transformer divides the ground current into three equal components; these currents are in phase with each other and flow in the three windings of the grounding transformer. The method of winding is seen from Fig 6b to be such that when these three equal currents flow, the current in one section of the winding of each leg of the core is in a direction opposite to that in the other section of the winding on that leg. This tends to force the ground-fault current to have equal division in the three lines and accounts for the low impedance of the transformer-to-ground currents. A star-delta-connected three-phase transformer or transformer bank can also be utilized for system grounding. As in the case of the zig-zag grounding transformer, the usual application is to accomplish reactance or resistance-type grounding of an existing ungrounded system. The delta connection must be closed to provide a path for the zero sequence current and the delta voltage rating is selected for any standard value. A resistor inserted between the primary neutral and ground, as shown in Fig.7 provides a means for limiting ground-fault current to a level satisfying the criteria for resistance-grounded systems. For this arrangement, the voltage rating of the star winding need not be greater than the normal line-to-neutral system voltage. For high-resistance grounding it is sometimes more practical or economical to apply the limiting resistor in the secondary delta connection. Three single-phase distribution class transformers are used, with the primary star neutral connected directly to ground. The secondary delta is closed through a resistor which effectively limits the primary ground-fault current to the desired low level. For this alternative application, the voltage rating of each of the transformer windings forming the star primary should not be less than the system line-to-line voltage. The rating of a grounding transformer, measured as the available system energy in kilovoltamperes, is equal to rated line-to-neutral voltage in kilovolts times rated neutral current. Most grounding transformers are designed to carry their rated current for a limited time only, such as 10.0 s or 1.0 min. Consequently, they are much smaller in size than an ordinary three-phase continuously rated transformer with the same rating for the available system energy measured in kilovoltamperes. A grounding transformer should be connected to the system in such a manner that the system will always be grounded. Fig 8a shows a grounding transformer with an individual line breaker for connection directly to the main bus of the system. Fig 8b shows a means of connecting a grounding transformer to a system without an individual line breaker. In this case, the grounding transformer is connected between the main transformer bank and its breaker. If the grounding ransformer is connected as shown in Fig 8b, there should be one grounding transformer for each delta-connected bank supplying power to the system, or enough grounding transformers to assure at least one grounding transformer on the system at all times. When the grounding ransformer is so connected, it is included in the protective system of the main transformer. If all grounded sources should trip, and the system grounding connection lost, any ungrounded sources should be disconnected automatically. Thus ungrounded sources would be tripped if all grounding transformer breakers in arrangement of Fig 8a should trip. If there are three or more transformers

20

feeding a bus and only two of these transformers have grounding sources, the ungrounded transformers would be disconnected automatically if those with grounding transformers as per Fig 8b should have their associated breakers opened.

1.9.2 Suggested Grounding Methods, Low-Voltage systems (600 V and Below) Low voltage systems are generally operated solidly grounded, one reason for this is that no expense is necessary for reactors. Another reason is that low-voltage switchgear almost universally uses directacting series trip devices, such devices may require high current magnitudes to detect faults to ground, unless supplementary ground- fault relaying is used. For these reasons, solid grounding is used extensively on systems rated 600 V and below. For such systems, it is especially important to provide low-impedance return paths for ground-fault currents in the equipment grounding network. Provided the ground-fault current is not limited to a value less than the pickup value of the circuit breaker instantaneous trip, damage from high-fault-current magnitude is minimized because low-voltage protective devices are extremely fast in operation. If the fault is an arcing one, and the root-mean-square current is less than that required to operate the circuit breaker instantaneous or short-time trip element, then ground-fault sensing may be required to prevent the burndown of equipment, as indicated earlier in this section.

1.9.3 Medium-Voltage Systems (3,3 to 33 kV) Medium-voltage industrial power systems are usually resistance grounded. In this range of system voltages, limiting ground-fault current is highly desirable in the usual industrial power system. Connection of rotating machines directly at voltages of up to 11 kV is common, hence resistance grounding is used to reduce the damage which may result due to a ground fault in the machine windings. In medium-voltage systems, when ground-fault protection is desired, it is provided by using a standard overcurrent relay connected in the residual circuit of the phase-current transformers or to a window or doughnut-type (zero-sequence) core-balance current transformer which encloses all the phase conductors. When any loads are connected line to neutral, then the window or doughnut-type current transformers must also enclose the neutral conductor. The cable sheath earthing strap must not pass through the CT. Positive tripping can be accomplished with low magnitudes of ground-fault current. For this reason resistance grounding is commonly used except for those cases where the size of the system is so small that the maximum available fault current is not too high to be objectionable. At the present time, there are 3,3-11 kV systems in operation without the system neutral adequately grounded. More and more engineers, however, are applying resistance neutral grounding to these systems.The conventional method is to provide sufficient ground-fault current to enable selective relaying through the application of residual or ground relays. Some design engineers feel that it is desirable to ground the system neutral so that the ground-fault current is of a very small magnitude, just enough to stabilize the neutral to prevent severe transient overvoltages upon the occurrence of restriking grounds. This does not cause a load shutdown due to a ground fault, while at the same time it eliminates the transient-overvoltage hazard. It does not, however, eliminate all types of overvoltage. The value of ground-fault current permitted to flow should be the same as, or somewhat greater than, the total system current (3x Ico) resulting from the capacitive coupling to ground inherent in all elements of the system. Capacitively coupled-to-ground elements include cables, overhead lines, transformer and rotating machine windings, surge capacitors

21

(with their star point earthed), and power factor capacitors, Expressed in another manner, the kilowatt rating of the resistor should be the same as, or somewhat greater than, the total available system charging energy to ground measured in kilovoltamperes. The high-resistance grounding practice could be employed with the circuits as shown in Fig.8. The only difference in the various methods of resistance grounding is the amount of ground-fault current that flows. Where selective ground relaying is required, considerably higher values of ground-fault current are required. 1.9.4 High-voltage Systems (Above 66kv) Systems above 66kv are nearly always effectively grounded, because these are usually transmission circuits with open lines in which, in most cases, grounded-neutral service surge arresters are desirable for better overvoltage protection and lower cost. In addition, rotating equipment is seldom connected directly to these systems; hence limiting ground-fault currents to prevent burning of laminations is a less important factor than in medium-voltage systems. In addition, voltages above 11kv are not usually carried inside buildings, hence personnel hazards due to high fault currents are not a factor. Finally, the cost of resistors for resistance grounding at these voltages is high. The use of partially or fully graded insulation of the HV windings of transformers also requires that the associated system is effectively earthed to make full use of grading.

1.10 Criteria for Limiting-Transient-0vervoltages (See Ref,12) Transient overvoltages can be limited effectively to safe values if the following criteria are observed: (1)

In resistance-grounded systems, the resistor ground-fault current should be at least equal to, but preferably greater than, the charging current of the system.

(2)

In reactance-grounded systems the ratio of Xo/X1 should be 10 or less, where Xo is the zerosequence inductive reactance of the system including that of the neutral reactance and X1 is the positive-sequence inductive reactance of the system.

(3)

Where a combination of grounding transformer and neutral- grounding resistor is used, the grounding transformer impedance should be low relative to the neutral resistance. The ratio of Ro/Xo should be equal to or greater than 2, where Ro is the zero- sequence resistance of the circuit, including the neutral resistor, and Xo is the zero-sequence inductive reactance of the circuit, including that of the transformer and resistor.

1.11 Selection of System Grounding Points 1.11.1 Ground at Each Voltage Level It is necessary to ground at each voltage level to achieve the advantages of neutral grounding in all parts of the system. (See Fig.9) For example, if the 11kV system in this diagram were not grounded, this level would have all the characteristics of an ungrounded system; at the same time, the 33-kV and 380- volt levels would have the characteristics of grounded-neutral systems. Each voltage level may be grounded at the neutral lead of a generator, power transformer bank, or grounding transformer. Any generator or transformer used for grounding should, as far as possible, be one which is always connected to the system. Alternatively, a sufficient number of generators or transformers should be grounded to ensure there is at least one ground connection on the system at all times.

22

1.11.2 Ground at the Power Source and Not at the Load The use of an available neutral at the load point, such as a star- delta step-down transformer or a starconnected motor, is not recommended as a point for system grounding. The principal disadvantage is that a number of these load neutrals must be grounded to ensure that the system remains grounded if one or more of these loads is out of service. Consequently, ground-fault current may be excessively high when all grounded points are in service. Since power sources are fewer in number than loads and are less likely to be disconnected, they are preferred as grounding points. (See Fig 10.) Other disadvantages of grounding at the load are the following: (1)

Standard low-voltage unit substations have delta-connected primaries; therefore, special transformers are required if the primaries are to be used as grounding points.

(2)

Since the ground-fault current is dependent on the number of feeders or grounding points in operation, it may vary widely depending on system operating conditions. This makes selective relaying more difficult and may require additional directional ground relaying to avoid false tripping of healthy feeder circuits.

1.11.3 Ground Each Major Source Bus Section When there are two or more major source bus sections, each section should have at least one groundedneutral point, since the bus-tie circuit may be open. If there are two or more power sources per bus section, there should be provision for grounding at least two sources on each section.

1.11.4 Neutral Circuit Arrangements When the method of grounding and the grounding point have been selected for a particular power system, the next question to consider is how many generator or transformer neutrals will be used for grounding and whether (1) each neutral will be connected independently to ground, or (2) a neutral bus with single-ground connection will be established.

1.11.5 Single Power Source When a power system has only one source of power (generator or transformer), grounding may be accomplished by connecting the source neutral to earth either directly or through a neutral impedance. Provision of a switch or circuit breaker to open the neutral circuit is not necessary because neutral circuits have practically zero potential with respect to ground except during the short interval of a fault, hence breakdowns are not likely. Also, it is not desirable to operate the system ungrounded by having the ground connection open while the generator or transformer is in service. In addition, the neutral switching equipment greatly increases the cost of grounding. In the event that some means of disconnecting the ground connection is required in a particular case, a metal-clad circuit breaker should be used rather than an open disconnect switch for indoor installations. The latter is hazardous to personnel if a ground fault should occur at the time the switch is opened or closed.

23

1.11.6 Multiple Power Sources When there are only a few generators or power transformer banks at a station, individual neutral impedances are frequently used. With this arrangement, the neutral of each generator or main transformer bank is connected directly to its neutral impedance without intervening switching equipment. No special operating instructions are required since each impedance is automatically connected whenever this source is disconnected. Where only two sources are involved, use of individual neutral impedances is preferable to the use of a common ground impedance. When several sources are involved, however, the ground current is increased each time a source is added and may be raised to levels which are undesirably high. In the case of resistance grounding, each resistor must be rated for sufficient current to ensure satisfactory relaying when operating independently. Consequently, the total ground current with several resistors will be several times the minimum required for effective relaying. When individual resistors are used, circulation of third-harmonic currents between paralleled generators is not a problem since the resistance limits the circulating current to negligible values. When there are more than two or three generators or power-supply transformer banks at one station, it is possible to use only one resistor. Each power source is then connected to the resistor through a neutral bus and neutral switching equipment. This arrangement keeps the ground-fault current to a practical minimum, since the ground current from the station is never greater than can be supplied through a single resistor. It also ensures the same value of ground current regardless of the number of generators or transformers in use and simplifies ground relaying. The primary purpose of the neutral breakers is to isolate the generator or transformer neutrals from the neutral bus when the source is taken out of service, because the neutral bus is energized during ground faults. This additional switching complication must be weighed against the increase in ground fault curent with many sources Circuit breakers are preferred to disconnecting switches for indoor installations to assure safety to personnel. If disconnecting switches are used, as in some outdoor installations, they should be elevated or metal enclosed and interlocked in such a manner as to prevent their operation except when the transformer primary and secondary switches or generator main and field breakers are open. It is sometimes desirable to operate with only one generator neutral breaker closed at a time to eliminate any circulating harmonic or zero-sequence currents. When the generator whose neutral is grounded is to be shut down, another generator is grounded by means of its neutral breaker before the main and neutral breaker of the first one is opened. However, with similar generators and reasonably equal load division, circulating currents are negligible, and it is often found practical to operate with neutral breakers of two or more generators closed. This simplifies operating procedure and increases assurance that the system will be grounded at all times. In the case of multiple transformers, all neutral isolating devices may be normally closed because the presence of delta-connected windings (which are nearly always present on at least one side of each transformer) minimizes circulation of harmonic current between transformers. When total ground-fault currents with several individual resistors would exceed about 1000 to 4000 A, it is suggested that neutral switchgear and a single resistor be considered for resistance-grounded systems.

24

1.12 Calculation of Ground-Fault Currents The magnitude of current which will flow in the event of a line- to-ground fault on a grounded system is usually determined by the reactance of the grounded apparatus, the reactance of the lines or cables leading to the fault, and the resistance and reactance of the ground return path including any intentional grounding resistance or reactance. For interconnected systems, calculation of the current may be rather complicated. For simpler cases, an approximation of the available fault current may be obtained.

1.12.1 Resistance Grounding When a single line-to-ground fault occurs on a resistance- grounded system, a voltage appears across the resistor (or resistors), nearly equal to the normal line-to-neutral voltage of the system. In low resistance grounded systems, the resistor current is approximately equal to the current in the fault. Thus, the current is practically equal to line-to-neutral voltage divided by the number of ohms of resistance used. Resistors have a voltage rating equal to line-to-neutral voltage and an ampere rating equal to the current which flows when this voltage is applied to the resistor. Thus, for example, a maximum ground- fault current of approximately 1000 A will be obtained on a system when using a 1000-A resistor. This very simple method of calculating the ground-fault current is only suitable when the ground-fault current is small compared to the three-phase fault current. The method just outlined applies to faults on lines or buses or at the terminals of machines or transformers. If the fault is internal to a rotating machine or transformer, the ground-fault current will be less. The reduction in current is primarily due to the intemal voltage of the apparatus. In the case of star- connected equipment, this internal voltage is at full value at the terminals and is zero at the neutral. If the fault occurs at the neutral of any apparatus, no voltage will appear across the system grounding resistor, so the fault current will be zero. At intermediate points in the winding between the neutral and a terminal, the fault current will be intermediate between zero and the current due to a terminal fault. For example, at a point 10 percent of the winding length from neutral, the ground-fault current will be approximately 10 percent of the value for a terminal fault. For a fault anywhere between this point and a terminal, the current will be more than 10 percent of the amount for a terminal fault. In the case of delta-connected machines, the internal voltage to neutral may be considered to be 100 percent at the terminals and 50 percent at the midpoint of the windings. The midpoints have the lowest potential with respect to the electric neutral of any part of the winding. Therefore, a ground fault at any point in the winding will produce a ground-fault current of 50 percent or more of the line terminal fault value.

1.12.2 Reactance Grounding In a reactance-grounded system with a single line-to-ground fault, the ground-fault current may be computed from the following expression, where resistance may usually be neglected: . Ig = 3E / [X1 + X2 + X0 + 3 (XN + XGP )] (Eq 1 ) where X1

=

system positive-sequence reactance in ohms per phase

25

X2

=

system negative-sequence reactance in ohms per phase

X0

=

system zero-sequence reactance in ohms per phase

XN

=

reactance of neutral grounding reactor in ohms

XGP

=

reactance of the ground return circuits in ohms

E

=

line-to-neutral voltage in volts

Ig

=

ground-fault current in amperes

1.12.3 Solid Grounding In a system with a solid neutral connectionto ground, the ground fault current for a single line to ground fault may be calculated from the followingequation:Ig

=

3E / [X1 + X2 + X0 + 3 XGP]

(Eq 2 )

where the symbols have the same meaning given above

1.13 Selection of Grounding Equipment Ratings (See IEEE Std 32-1972, Neutral Grounding Devices) Grounding resistors, reactors, and grounding transformers are normally rated to carry current for a limited time only. The standard time interval rating usually applicable with relays arranged to protect the grounding equipment is 10 s. The voltage rating of a grounding resistor should be the line- to-neutral voltage rating of the system. The insulation class of a reactor is determined by the system line-to-neutral voltage. The voltage rating may be less than line-to-neutral voltage and is calculated by multiplying the rated current by the impedance of the reactor. The voltage rating of a grounding transformer should be system line-to-line voltage. Grounding resistors are rated in terms of the current that will flow through the resistor with rated resistor voltage applied. The rated current of a grounding reactor is the thermal current rating. It is the root-mean-square neutral current in amperes that the reactor will carry for its rated time without exceeding standard temperature limitations. If a grounding transformer neutral is solidly connected to ground, the current that will flow during a ground fault is primarily determined by the reactances of the grounding transformer and the system to which it is connected. When a resistor is used between neutral and ground, the current rating of the grounding transformer is based on the resistor rated current. In either case, the transformer is rated to carry the required current for rated time without exceeding its rated temperature limits. Many system grounding devices are short-time rated. Care must be exercised in their application to ensure that current will be automatically interrupted before the thermal limits of components are exceeded.

26

Due consideration must be taken in systems using Auto Reclose on lines, as the total time for all faults before a lock out must be used in calculating thermal requirements.

1.13.1 Resistor Ratings For low-resistance grounded-neutral systems, the determination of the resistor value in ohms, thus the magnitude of groundfault current, is based on the following: (1)

Providing sufficient current for satisfactory performance of the system relaying scheme.

(2)

Limiting ground-fault current to a value which will minimize damage at the point of fault without resulting in system overvoltages.

In most cases, the ground-fault current is limited by the neutral resistor to a value considerably less than that which would flow for a three-phase fault. To determine the minimum ground-fault current required, a diagram of the system must be available, giving ratings of current transformers and types of relays for each circuit. This diagram should include consideration of future changes. The magnitude of ground-fault current must be sufficient for operation of all relays which furnish ground-fault protection. In general, if the current is high enough to operate the relays on the larger circuits, it will be adequate for the smaller circuits. The ground currents required for satisfactory operation of various types of relays, expressed in terms of current transformer rating, are given in Table III. Note that the ground-fault current under all system operating conditions must equal or exceed the minimum required for relaying each circuit connected to the system. This value is established by selecting the highest of those currents which meet the requirements of the several conditions set forth in Table III. Table III

Selection of a Grounding Resistor

Equipment Protected Star connected generators and motors Delta connected generators, motors and transformers L:ines and cables

Ground Overcurrent % 100 40

20 # **

Type of Relaying Differential % 100 *

Other %

40

Pilot wire - 100 Current balance - 100

Buses 50 & Note:- Values given are minimum recommended ground fault currents in percentage of rated primary current of the associated CTs *

If ground fault differential is added to the generator, the ground fault current may be limited to lower values (If other sytem requirements permit)

**

When phase relays must be set to wait for other breakers to clear any through phase-tophase fault, required ground current may have to be as high as 50% of the CT rating to prevent maltripping.

#

Where ground overcurrent relays for lines and cables also serve for back up protection for machines or transformers, this value should be 100%.

27

&

Based on current differential. If voltage differential is provided, ground fault current may be limited to lower values.

1.13.2 Reactor Ratings The reactance of a neutral-grounding reactor should be chosen to limit the ground current and the current in the faulted phase to the desired value. In order to minimize transient overvoltages, the ground-fault current must not be less than 25 percent of three-phase fault current. This corresponds to a ratio of X0 / X1

= 10.

For reactance grounding of generators, the current in any winding must not exceed the three-phase fault current. This corresponds to a ratio of X0 / X1 = 10. This establishes the criteria for maximum and minimum value of neutral reactance. If the neutral reactance is selected in accordance with the following relationship, the current in the winding of the faulted phase will not exceed the three-phase current of the machine regardless of system reactance (ref 9, chs 5-7): XN

=

(X1 - X0 ) / 3

(Eq 3)

where :X1

=

system positive-sequence reactance in ohms per phase

X0

=

system zero-sequence reactance in ohms per phase

XN

=

reactance of neutral grounding reactor in ohms

However, the current that flows through the generator neutral reactor itself is not independent of system constants and may often exceed the three-phase fault current of the machine. The current rating of a neutral reactor is determined by the number and characteristics of system sources and whether they are grounded or ungrounded. This rating (the thermal-current rating) can be calculated by the following equation: Ig

=

3E / [X1 + X2+ X0+ 3 (XN + XGP)

(Eq 4)

X1

=

system positive-sequence reactance in ohms per phase

X2

=

system negative-sequence reactance in ohms per phase

X0

=

system zero-sequence reactance in ohms per phase

XN

=

reactance of neutral grounding reactor in ohms

XGP

=

reactance of the ground return circuits in ohms

E

=

line-to-neutral voltage in volts

where:-

28

Ig

=

ground-fault current in amperes

NOTE:For X1 of generators and synchronous motors, use transient reactance. For X2 of generators, synchronous and induction motors, use subtransient reactance. The neutral-grounding reactor should be selected to carry the available current under all practical operating conditions. With any given condition of connected grounded-neutral sources, the addition of ungrounded-neutral sources and loads will increase the current flow through the grounded-neutral connections.

1.13.3 Grounding-Transformer Ratings The electrical specifications of a grounding transformer are the following: Voltage

the line-to-line voltage of the system

Current

the maximum neutral current

Time

the transformer is usually designed to carry rated current for a short time such as 10 or 60 s; for high-resistance grounding, the rating may need to be continuous.

Reactance

this quantity is a function of the initial symmetrical available system three-phase shortcircuit energy measured in kilovoltamperes

Determination of grounding-transformer reactance, when used to effect reactance-type grounding, is based on the following criterion: The X0 / X1 ratio should not exceed 10, and preferably not exceed 3, in order to eliminate the possibility of transient overvoltages from a forced current zero interruption. This maximum limitation of 3 for the X0 / X1 ratio, will also satisfy the criteria for an effectively grounded system and permit the application of line-to-ground voltage rated surge arresters for greater economy and protection. The ratios specified must be met at any location in the system where the reduced-rating arresters are to be applied. In a system having a grounding transformer, its reactance is the principal part of Xo in the preceding criterion. Also, the positive-sequence reactance X1 is equal to the reactance of the system to initial symmetrical root-mean-square three-phase short-circuit current. The value of X1 used in these limiting ratios should be based on subtransient reactance of rotating machines and the system configuration or connections which will result in the maximum available three-phase short-circuit energy measured in kilovoltamperes. Thus, the grounding-transformer reactance is a function of the initial symmetrical system three-phase short-circuit energy measured in kilovoltamperes. In a system otherwise ungrounded, the grounding-transformer reactance Xgt in ohms per phase required to provide any specified X0 / X1 ratio is given by the following expression: X0 / X1 x EL2 x 1000 __________________________________________________________ System Symnetrical Three-Phase Short-circuit Energy in Kilovoltamperes where EL is the line-to-line voltage.

(Eq 5)

29

When a grounding transformer is solidly grounded, care should be taken that its reactance is selected at a value low enough to provide sufficient fault current for tripping relays and circuit breakers.

1.14 AutoTransformers Power Autotransformers are quite frequently used in electric utility power transmission and distribution systems. Their use in industrial power systems as a part of the power distribution system is relatively infrequent. Autotransformers are quite common, however, in control and utilization equipment. Systems using autotransformers may be subject to dangerous fundamental- frequency overvoltage during system faults or to high-frequency or steep wave front transient overvoltages on the lines originating from lightning or switching surges. In general, solidly grounding the neutral of the autotransformer is a satisfactory means of eliminating overvoltages. The disadvantages of solid-neutral grounding is that third-harmonic currents and telephone interference may become excessive in certain cases. These harmonic problems can usually be eliminated by the use of a tertiary delta on the autotransformer. Where auto transformers are used as step-up or step-down transformers in 11 and 22kV systems, problems arise when the auto transformer neutral is grounded in otherwise resistively earthed systems. This additional ground poit must comply with the designed fault current limitation and the associated protection must allow for the bi-directional flow of fault current. In many cases the only solution is to leave the neutral ungrounded as the complication of the protection cannot be easily and cost effectively solved. The zero sequence impedance of the un grounded auto transformer is difficult to determine and may have to be measured on site in order to setb the protection correctly.

1.15 Systems with Utility Supply Some industrial systems are directly connected at their operating voltage to utility systems. When such is the case, the scheme of grounding the industrial system should be properly coordinated with that for the utility system. If two systems are interconnected by means of a transformer bank, at least one winding of the bank will normally be connected in delta, and this delta-connected winding will make each system independent from the standpoint of grounding. Generators with unit-connected transformers usually are equipped with high-resistance neutral grounding for the following purposes: (1)

Minimize transient overvoltages

(2)

Provide a positive indication of a ground fault

(3)

Limit ground-fault current to a value low enough to permit an orderly shut-down of the generator without risking severe arc damage.

As in any other high-resistance grounded system, the grounding resistor is chosen to provide groundfault current at least as great as the system capacitive charging current. Depending on the system

30

voltage and ground current, it may be more economical to ground the generator neutral through the primary of a single phase distribution-type transformer with a low-voltage high-current resistor on the secondary than to ground the neutral directly through a high-voltage low-current resistor. When a distribution transformer is used, its primary voltage should be no less than the line-to-neutral voltage of the generator. In fact, to decrease the transformer-magnetizing inrush current on occurrence of a ground fault, a transformer rated for full line- to-line voltage is often used. In all cases, the zerosequence circuit design should respect the boundary limit of R0 = 2X0 . Ground faults are detected in this scheme either by a current relay in series with the grounding resistor, or by a voltage relay in parallel with the resistor. If the voltage-relay sensor is chosen, possible false operation on third-harmonic voltage, sometimes present, can be avoided by the use of a thirdharmonic- voltage attenuation filter. In either case, no coordination with relays elsewhere in the power system is needed. The physical size of the grounding transformer is influenced by the expected duration of ground current. If the generator will be operated with a ground fault for several hours, the transformer should be rated to carry ground current continuously. However, the short-time overload capacity of transformers permits a considerably smaller and less expensive transformer to be used if the generator is tripped off the line immediately or promptly after a fault is detected.

1.16 Three-Phase Four-Wire Systems In these systems, single-phase loads are connected between phase conductors and the neutral conductor. The neutral conductor is insulated over its entire length, except where it is grounded at its source of supply, in NE systems. There are also systems where there are multiple earths at various points on an otherwise insulated neutral conductor, ie. PME systems. PME systems can increase the difficulty of detecting ground faults. The neutrals of such systems should be grounded such that during a ground fault the voltage between any phase conductor and ground does not appreciably exceed normal line-to-ground voltage. Four-wire systems should thus be effectively grounded in such a manner that the ground-fault currents are approximately equal to three-phase fault currents. This is usually accomplished by direct connection of transformer bank neutrals to ground without any intentional neutral impedance. Where the risk to personnel touching the system is small, as with low voltage systems which are effectively three wire, the use of high resistance or Neutralizer grounding is used to reduce the chance of outages and to minimise damage at the point of fault. This type of application has to be viewed in the light of legislation which may prohobit such instalations at low voltage if the risk to personnel is increased.

31

1.17 Standards References The following standards publications were used as references in preparing this lecture and are useful in the interpretation of its meaning: ANSI Cl-1971 National Electrical Code

(NFPA 70-1971) 2,5

IEEE Std. 32-1972, Neutral Grounding Devices 3 NEMA Std SG 4-1968, AC High-Voltage Circuit Breakers 4 IEEE Std 142-1972 Recozmended Practice for grounding of Industrial & Cozmercial Power Systems "The IEEE Green Book" References (1) (2)

(3) (4) (5)

(6) 7) (8)

(9) (10) (11) (12) (13) (14) (15) (16)

AIEE Committee Report. Application Guide for the Grounding of Synchronous Generator Systems. AIEE Transactions (Power Apparatus and Systems), vol 72, June 1953, pp 517-526. AIEE Committee Report. Application Guide on Methods of Neutral Grounding of Transmission Systems. AIEE Transactions (Power Apparatus and Systems), vol 72, Aug 1953, pp 663-668. AIEE Committee Report. Present-Day Grounding Practices on Power Systems; Third AIEE Report on System Grounding. AIEE Transactions, vol 66, pp 1525-1551. ARBERRY, J.P.E. The Use of 600-Volt and 460-Volt Power Systems with Grounded Neutrals. AIEE National Power Conference Proceedings, Pittsburgh, Pa., Apr 1950. KAUFMANN, R.H., and PAGE, J.C. Arcing-Fault Protection for Low-Voltsge Power Distribution Systems - Nature of the Problem. AIEE Transactions (Power Apparatus and Systems), vol 79, June 1960, pp 160-167. SHIELDS, F.J. The Problem of Arcing Faults in Low-Voltage Power Distribution Systems. IEEE Transactions on Industry and General Applications, vol IDA-3, Jan/Feb 1967, pp 15-25. FORBES, B.G. Locating Grounds on 480-Volt, 3-phase Delta Systems, Power Generation, Sept 1949, pp 60-61. FOX, F.K., GROTTA, H.J, and TIPTON, C.H. High-Resistance Grounding of 1400-Volt Delta Systems with Ground-Fault Alarm and Traceable Signal to Fault. IEEE Transactions on Industry and General Applications, vol IGA-1, Sept/Oct 1965 pp 366-372. BEEMAN, D.L, Ed. Industrial Power Systems Handbook. New York, McGraw-Hill, 1955, chs.5-7. VAUGHAN, H.R. Protection of Industrial Plants Against Insulation Breakdown and consequential Damage. AIEE Transactions, vol 65, Aug/Sept 1946, pp 592-596. AIEE committee Report. Application of Ground-Fault Neutralizers. Electrical Engineering, vol 72, July 1953, pp 606. Electrical Transmission and Distribution Reference Book, fourth edition, Westinghouse Electric Corporation, Sept 1950, chs 11 and 19. Neutral Grounding in HV Transmission, Ilillheim and Waters - Elsevier. Transients in Power Systems. Harold A. Petersen, ch,10 Dover. Transient Performance of Electric Power Systems, Rudenberg chapters 48 and 49. Circuit Analysis of Ac Power Systems. Vol I. - Edith clarke. Whiley.

Additional References BARNETT, H.G. Why Ground Low-Voltage Distribution Systems? Mill and Factory, May 1951.

32

BEEMAN, D.L. System Neutral Grounding in Industrial Plants. AIEE National Power Conference Proceedings. Pittsburgh, Pa., Apr 1950. BLOOMQUIST, W.C. Grounding of Industrial Systems, General Electric Review. Aug 1951. BRERETON, D.S, and HICKOK, H.N. System Neutral Grounding for Chemical Plant Power Systems. AIEE Transactions (Applications and Industry) vol 74, Nov 1955, pp 315-320. JOHNSON, A.A. Grounding Principles and Practice III: Generator- Neutral Grounding Devices. Electrical Engineering, vol 64, Mar 1945, pp 92-99. Interim Report of American Research Committee on Grounding. News Bulletin of the Internaitonal Association of Electrical Inspectors. Mar 1944. STRONG, W.F. Neutral Versus Corner-of-the-Delta Grounding. Electrical World., Sept 25, 1950. THACKER, H.B. Grounded Versus Ungrounded Low-Voltage AC Systems. Association of Iron and Steel Engineers Paper and Discussion. Iron and Steel Engineer, Apr 1954, pp 65-72.

1.18 Symbols Used R0 X0 Z0 X1 R1 Z1 Xc0 RN XN

= = = = = = = = =

Zero sequence resistance of grounding path Zero sequence reactance of grounding path Zero sequence impedance of grounding path Positive sequence reactance to grounding point Positive sequence resistance to grounding point Positive sequence impedance to grounding point Zero sequence capacitive reactance of system Neutral grounding resistor resistance Neutral grounding resistor

33

2.0 PROTECTION ASPECTS OF LIMITED EARTH FAULT 2.1 RESTRICTED EARTH FAULT (REF) The SABS recommendations are for the use of resistance earthing, with a limit of 300A per transformer and a total of 900A with multiple transformers in parallel. With larger transformers, typically 20 MVA at 6.6 kV, the CT ratio would be possibly 2000/1. A problem which needs to be covered with correct design is the requirement to cover 90% of the winding by use of REF (see fig 11). With the typical large CT ratio given above, use of a 5% current relay (with stabilising resistor) would only allow pick up at 100 amps, ignoring CT magnetising current. This is inadequate for covering 90% of the winding as with one transformer and 300A E/F available, only 66% of the winding is protected. The typical REF relay using 20mA pick must be used, as this has a primary pick up of typically 50A with CT magnetising current. This is still only 85% coverage, but is acceptable. However, REF for a 40 MVA transformer is more difficult. 2.2 Pilot wire protection (PW) As given above, PW protection also can have a sensitivity problem (see fig 12). As the typical relay has a worst case sensitivity of 25% on Blue phase, use with a 600/1 CT would result in a pick up of only 150A which is not desireable. This problem is made more unacceptable where the relay does not combine the differential current from both sides to create the operating quantity. Thus for a fault at the remote end of two or more parallel feeders, the pick current could be greater than 300A and no operation would occur for earth faults. 2.3

Multiple Earths When using Auto Transformers

The accepted practice of using low cost auto transformers to step up or down between 11 and 22 kV has resulted in the transformers being positioned at some remote position from the substation with the protection. Where sensitive earth fault protection is used for overhead lines, maloperation can occur when these auto transformers have their neutrals earthed (see fig 13). The fault on a line causes earth fault infeeds from all electrically connected earths. Even though the auto transformer may not have a delta winding for a low impedance source, it can supply sufficient current, typically more than 4 amps, which will cause a healthy line with it’s auto transformer to trip incorrectly. Either no auto transformer neutrals should be earthed when on lines or double winding transformers with suitably low zero sequence impedance for earthing the secondary system must be used.

2.4 Sensitive Earth Fault Relays used on Rings or Hospital Bypas Busbars Where Sensitive Earth Fault (SEF) relays are used, maloperation can occur when a ring or Bypass busbar is closed (see fig 14). This is caused by differences in impedance of especially over head lines and busbars. This difference between phase impedances cuases an apparent earth fault current in both

34

circuits, but of opposite direction. This primary residual current will cause sensitve earth fault relays to mal-operate. The sensitve earth fault needs to be switched off when such switching is undertaken. With limited earth fault currents and the associated increase in sensitity of relay settings, the above malfunction can also occur for normal earth fault relays, when additional impedance such as a poor link contact occurs. This is usually associated with closure of the “ring” as the current is already flowing in the previous system configuration. The line volt drops are small and are unable to break down a surface fim, causing a phase to be “open circuit” and thus creating primary residual currents. For the case where sensitive earth fault protection must be applied to parallel OHL feeders, an additional monitoring relay may be necessary to overcome mal-tripping. 2.5 Sensitive Settings with Earth Fault Relays of Low Burden The modern static relays have very low burdens. This has created a problem with transient stability for motor starting and for transformer magnetising in rush (see fig 15) With low earth fault currents, protection has to be set with adequate sensitivity. Although the current is limited and fault damage is thus reduced, faults should still be clreared as soon as possible so that an earth fault does not develop into a phase-phase fault. This combination of sensitivity, fast operation and low burden causes the relay to malfunction because of CT saturation. STABILISING RESISTORS must be used in many of these applications. 2.6 Contactor Fault Breaking Capability For many motor circuits fused contactors are used when the switching duty is high. As the contactor has a limited fault breaking capability, the High Set instantaneous element of protection relays is disabled. With limited earth fault, the contactor is fully capable of breaking earth fault current. However, if the fault is a phase-phase-earth fault, a fast earth fault relay could drop the contactor out such that it attempts to break the phase phase fault current, which could damage the contactor. A possible solution is to delay the earth fault relay, but this has a cascade effect on all upstream protection with regarg to increased time. The preferred method is to use relays with an internal block funtion if any phase exceeds typically 4x full load current, where overcurrent protection will correct protect the circuit. 2.7 Maltripping Caused By Capacitance Again with the more sensitive settings used with limited earth fault ciurrent, care must be taken with the current derived from cable capacitance or wave shaping capacitors. Wave shaping capacitors of various forms are often used for the protection of motors against transients, with typical values of 0.3 uF. At 11 kV, the typical earth fault infeed from capacitance could be:Wave Shaping Capacitor (0.3uF)

1.8 amps

35

4 kM 120mm² PEX cable (1.33 uF)

7.96 amps

For the above, the earth fault protection must not be too sensitive, as maloperation will occur, especially where ring core Cts are used to improve sensitivity.

2.9 High Voltage Lines With Multiple Earthed Transformer Tees Where 132 kV rings are used in Municipalities, the system is often used without full switching stations (see fig 16) This results in tees off the lines to feed step down transformers, which have the 132 kV neutral earthed as the transformer insulation is usually fully graded. Each of these earthed neutrals feeds earth fault current into the system during an earth fault on the HV system. The source substation for the line thus sees only part of the total earth fault current and this can result in the earth fault relay being unable to correctly clear the fault. Various methods are used to allow correct operation to occur, but many of these compromises may require sequential tripping of tee breakers until the source breaker correctly trips. Fault throwing at a point near the fault is also a possibility, but is not a recommended practice on high fault level lines. The number of teed (earthed) feeders off a ring should be limited with the size of the transformers earth fault contribution also being taken into account.

36

A

A

B

B GROUND

C C

PHASE TO GROUND CAPACITANCE (a) UNGROUNDED SYSTEM

A

A

B

B GROUND

C C

(b) GROUNDED STAR SYSTEM

A

A

B

B

C

GROUND

(c) UNGROUNDED DELTA SYSTEM

VOLTAGES TO GROUND UNDER STEADY STATE CONDITIONS

FIGURE 1

C

37

Xco

STRAY ZERO SEQUENCE CAPACITANCE

EARTHING DEVICE (USUALLY A VOLTAGE TRANSFORMER)

(a) SYSTEM SUBJECT TO FERRO RESONANCE

A V A-G NORMAL SYSTEM NEUTRAL

INVERTED NEUTRAL

V B-G GROUND V C-G

C

B

(c) UNGROUNDED DELTA SYSTEM

VOLTAGES TO GROUND UNDER STEADY STATE CONDITIONS

FIGURE 2

38

39

40

N

Xo

(a) UNGROUNDED

N

Xo

(b) SOLIDLY GROUNDED

Xo X N

N 0

= 3x

Xo X

X

N

(c) REACTANCE GROUNDED NEUTRAL REACTOR or PETERSEN COIL

GROUNDING METHODS

FIGURE 4

X

N 0

N

41

Xo N X

NEC

N 0

Xo X

N 0

(d) REACTANCE GROUNDED GROUNDING TRANSFORMER

Xo R

N 0

= 3x

R

N Xo R

N

(e) RESISTANCE GROUNDED

GROUNDING METHODS

FIGURE 4

R

N 0

N

42

I

TRANSFORMER

I

C

b

c

B

I c

N

I I

I

n A

I

I b

c b

G

n

GROUND-FAULT I

NEUTRALIZER n

I n

I b

I c

V NA V

V BA

I

NA

V CA

c I

I

b

+I

+I

c

2In

c

I

FIG. 5

b

b

GROUND-FAULT-CURRENT PATTERN IN SYSTEM GROUNDED BY MEANS OF A GROUND-FAULT NEUTRALIZER

DGD0001

43

LINE LEADS

NEUTRAL LEAD

SCHEMATIC DIAGRAM OF CONNECTIONS

FIG. 6a

CONNECTIONS OF THREE-PHASE ZIG-ZAG GROUNDING TRANSFORMER

NEUTRAL LEAD LINE LEADS

WINDINGS SHOWN ON CORE

FIG. 6b CONNECTIONS OF THREE-PHASE ZIG-ZAG GROUNDING TRANSFORMER

DGD0002

44

FIG. 7

STAR DELTA TRANSFORMER USED AS GROUNDING TRANSFORMER

GROUNDING TRANSFORMER GROUNDING RESISTOR

GROUNDING TRANSFORMER

GROUNDING RESISTOR

FIG. 8

GROUNDING TRANSFORMER ON A DELTA-CONNECTED OR GROUNDED POWER SYSTEM

DGD0003

45 132kV SYSTEM

GROUND REQUIRED HERE TO GROUND NEUTRAL OF 33kV SYSTEM

33kV SYSTEM

GROUND REQUIRED HERE TO GROUND NEUTRAL OF 11kV SYSTEM

11kV SYSTEM

GROUND REQUIRED HERE TO GROUND NEUTRAL OF 380V SYSTEM

380V SYSTEM

FIG. 9 EACH VOLTAGE LEVEL IS GROUNDED INDEPENDENTLY

GROUND HERE

GROUND HERE

GROUNDING DO

TRANSFORMER

NOT GROUND HERE

MOTOR

IF POWER SOURCES ARE DELTA-CONNECTED, ADD GROUNDING TRANSFORMER RATHER THAN GROUND AT LOAD

FIG. 10 GROUND AT THE SOURCE AND NOT AT THE LOAD

DGD0004

46

66%VPN

ONLY 100A

1/3 VPN

=

RN

33%VPN

RN (300A LIMIT)

FIG. 11

REDUCING EARTHFAULT WRT POSITION OF FAULT ON WINDING

PW

PW

150A

600/1

300A

150A

FIG. 12

PW FAILURE TO OPERATE WHEN TOTAL CURRENT IS NOT USED FOR RELAY OPERATION DGD0005

47

SEF

300A

AUTO-TRFR 75A

SEF

FIG. 13

EARTHED AUTO-TRANSFORMER CAN CAUSE HEALTHY LINE TRIP E/F

IC =0 POOR LINK CONTACT

APPARENT E/F

E/F =

IC

NO E/F CURRENT IN THIS RELAY

FIG. 14 MALOPERATION OF BOTH EARTHFAULT RELAYS WITH A POOR LINK CONTACT IN A CLOSED RING DGD0006

48

XIC

Z = CT RESISTANCE WHEN SATURATED R (1-X)I C

SPLIT OF X AND 1-X DEPENDS ONLY ON Z OF BLUE PHASE TO Z OF EARTHFAULT CIRCUIT

FIG. 15

USE OF STABILISING RESISTOR TO PREVENT EARTHFAULT MALOPERATION WITH LOW BURDEN RELAYS

I

E/F

SMALL

INFEED

INFEED TOTAL

E/F

E/F HIGH

FIG. 16 REDUCTION OF EARTHFAULT CURRENT FROM SOURCE WITH MULTIPLE LINE TEES DGD0007