Contents 21.1 Protection of a domestic or an industrial single-phase system 21/775 21.1.1 Effects of current passing through a human body and the body’s tolerable limits 21/775 21.1.2 Use of ground leakage circuit breakers (GLCBs) 21/775 21.1.3 Principle of operation of a ground leakage circuit breaker (application of a core-balanced CT) 21/778 21.2 Ground fault on an LV system 21/778 21.2.1 System protected through over-current releases and HRC fuses 21/778 21.2.2 System protected through relays 21/779 21.3 Ground fault protection in hazardous areas 21/780 21.4 Ground fault protection of MV and HV systems 21/781 21.4.1 Ground fault protection of EHV systems 21/783 21.4.2 Ground fault protection of generators 21/783 21.4.3 Ground fault protection of electronic circuits 21/783 21.5 Core-balanced current transformers (CBCTs) 21/783 21.5.1 Design parameters for a CBCT 21/783 21.5.2 Insulation level 21/784 21.5.3 Mounting of CBCTs 21/784 21.6 Ground fault (G/F) protection schemes 21/786 21.6.1 Protection through a single CT 21/786 21.6.2 Restricted ground fault protection 21/787 21.6.3 Unrestricted ground fault protection 21/788 21.6.4 Directional ground fault protection 21/789 21.6.5 Current setting of a ground fault relay 21/790 21.7 Grounding systems 21/791 21.7.1 Choice of grounding system 21/793 21.7.2 Protection from lightning strikes, transferred surges and over-voltages 21/793 Relevant Standards 21/794 List of formulae used 21/794 Further Reading 21/795 21 Grounding theory and ground fault protection schemes 21/773
Transcript
Contents
21.1 Protection of a domestic or an industrial single-phasesystem 21/77521.1.1 Effects of current passing through a human body
and the body’s tolerable limits 21/77521.1.2 Use of ground leakage circuit breakers (GLCBs)
21/77521.1.3 Principle of operation of a ground leakage circuit
breaker (application of a core-balanced CT)21/778
21.2 Ground fault on an LV system 21/77821.2.1 System protected through over-current releases
and HRC fuses 21/77821.2.2 System protected through relays 21/779
21.3 Ground fault protection in hazardous areas 21/780
21.4 Ground fault protection of MV and HV systems 21/781
21.4.1 Ground fault protection of EHV systems 21/78321.4.2 Ground fault protection of generators 21/78321.4.3 Ground fault protection of electronic circuits 21/783
21.5 Core-balanced current transformers (CBCTs) 21/78321.5.1 Design parameters for a CBCT 21/78321.5.2 Insulation level 21/78421.5.3 Mounting of CBCTs 21/784
21.6 Ground fault (G/F) protection schemes 21/78621.6.1 Protection through a single CT 21/78621.6.2 Restricted ground fault protection 21/78721.6.3 Unrestricted ground fault protection 21/78821.6.4 Directional ground fault protection 21/78921.6.5 Current setting of a ground fault relay 21/790
21.7 Grounding systems 21/791
21.7.1 Choice of grounding system 21/79321.7.2 Protection from lightning strikes, transferred
Grounding theory and ground fault protection schemes 21/775
21.1 Protection of a domestic or anindustrial single-phase system
21.1.1 Effects of current passing through ahuman body and the body’s tolerablelimits
An electric current, rather than voltage, through a humanbody may cause shock and can damage vital organs ofthe body as follows:
1 It may cause muscular contraction, unconsciousness,fibrillation of the heart, respiratory nerve blockageand burning. These are all functions of body weight.The muscular contraction makes it difficult to releasean energized object if held by the hand and can alsomake the breathing difficult. The heart, being themost vulnerable organ of a human body, is damagedmost, mainly by ventricular fibrillation, which mayresult in an immediate arrest of the blood circulation(electrocution). In Table 21.1 we provide likelyintensities of body currents when lasting for morethan a heartbeat (nearly 60–300 ms or 3–15 cyclesfor a 50 Hz system).
2 It is generally seen that a human body can sustain amuch higher current at a lightning or switchingfrequency (5 kHz or above) due to the extremelyshort duration of such surges (30 ms or less).
The current can pass through the heart, when the currentpasses through hand to hand, or through one hand and afoot. Current flowing between one foot to the other maynot be considered dangerous, but may cause muscularcontraction and pain. The subsequent body fall, if it occurs,may, however, be fatal as now the current can also flowthrough the hand involving the heart. Ground faultprotection emphasizes keeping the fault current belowthe fibrillation threshold and for a period of less than aheartbeat, in the range of 60–300 ms. It has beenestablished that the electric shock energy which a humanbody can endure, without damage has a relationship with
the leakage current through the body and its duration,i.e.
S I tb b2
b = ◊
or IStb
b
b = (21.1)
whereSb = shock energy in watt-seconds (Ws)Ib = r.m.s value of the leakage current through the
body in Amperes (A)tb = duration of leakage current in seconds (s)
The energy, Sb, for a 50 kg body is regarded safe at0.0135 Ws and for a 70 kg body at 0.0246 Ws
\ It t
bb b
(50 kg) = 0.0135 = 0.116 (21.2)
and It t
bb b
(70 kg) = 0.0246 = 0.157 (21.3)
Figure 21.1 as in IEC 60479-1 illustrates the variouszones of ground leakage current versus duration of faultand its effect on a human body. For safe leakage currentin terms of touch voltages as a function of time seeFigure 22.12.
To enable the body sustain a high fault current it isessential that the fault interrupting device (relay or release)is quick responding. For domestic applications, it isrecommended to be less than a heartbeat.
NotePrevention from ground leakage as such is important, as it causescorrosion through electrolysis and may damage the insulation ofwires due to ageing.
21.1.2 Use of ground leakage circuit breakers(GLCBs)*
It is likely that smaller installations, such as a domesticlight or power distribution network, and single-phaseindustrial or light loads may not always meet therequirements, as discussed in Section 21.2, Table 21.2,due to the circuit’s own high impedance (other than theimpedance of the ground conductors). It is thereforepossible that the fuses, if provided for the short-circuitprotection of the system, may be too large to detect aground leakage. The ground leakage circuit breakers(GLCBs) usually called as residual current devices (RCDs)provide an effective solution to this problem and are easilyavailable and extensively used. They are extremely sensitiveto very feeble ground leakages, of the order of 10 mA ormore, and trip the faulty circuit within a safe period on thesmallest leakage current and provide effective protectionto the human body. Figure 21.2(a) illustrates the trippingscheme for such a breaker and the principle of operation is
Table 21.1 Likely intensities of body currents when lasting formore than a heartbeat (� 60–300 ms)
Sr. Body At frequency Intensityno. current
(mA) (Hz)
1 100 50–60 Lethal2 100 3–10 kHz Tolerable (because of
extremely short duration)3 60–100 50–60 Fatal4 16–60 50–60 Breathing may become
difficult due to muscularcontraction
5 10–16 50–60 It is the let-go current andmakes it hard to release anenergized object if held byhand
6 5–9 50–60 No dangerous effects7 1–4 50–60 Threshold of sensation
*More commonly known as earth leakage circuit breakers (ELCBs),residual current circuit breakers (RCCBs) or residual current devices(RCDs). They operate on the principle of residual current.
21/776 Electrical Power Engineering Reference & Applications Handbook
discussed below. Normally four types of RCDs are in use,10 mA, 30 mA, 100 mA and 300 mA (IEC 61008, 9).Figure 21.2(b) shows some views of RCDs.
The selection of RCDs depends on the type ofapplication i.e. to protect a circuit, equipment or device.The 10 mA and 30 mA RCDs are to protect the humanbody from shocks and electrocution, while the others areto protect the circuit from fire risk. Normally the 300mA RCD is used as the incomer and 30 mA as the outgoingfor the individual feeding circuits.
The RCDs can be with or without over-current (o/c)protection. Most household appliances and officeequipment are fixed type loads and may not call for o/cprotection. But loads using electric motors like, mixers,grinders and washers may sometimes be overloaded dueto excessive load or under-voltage and may call for o/cprotection too. To meet this requirement RCDs with built-in o/c protection are also available and called RCBOs(residual current breakers with o/c protection).
Discrimination
To ensure that residual ground fault current in a sub-circuitcauses the RCD of this circuit only to trip and not theupstream RCD, the tripping time of the upstream RCDshould be higher than the downstream RCD. As a rule ofthumb tripping time of the upstream RCD should beminimum 3 times the tripping time of the downstream RCD.
Electromagnetic Compatibility (EMC)
Since these devices are highly sensitive, they are alsosusceptible to EM interferences in the vicinity. It isimperative that they are made immune to such effectsand are EM compatible to avoid nuisance tripping (IEC61543). Many manufacturers provide a high frequencyL-C filter circuit as an EM suppressor to make it immunefrom tripping due to spurious voltages and currents causedby lightning, transferred and switching surges and presence
Figure 21.1 Effects of current leakage through a human body
Current passing through the human body in mA (rms)
Effects of a.c. currents (50 Hz) on a personat least 50 kg in weight.
Zone-1 – Usually no reactionZone-2 – Usually no dangerous physiological
effectZone-3 – Usually no danger of heart failure
(ventricular fibrillation)Zone-4 – Probability of heart failure up to
50%Zone-5 – Probability of heart failure more
than 50%
Figure 21.2(a) Schematic drawing of an RCD
i� i�
i�
R
N
B
Y
N
InTest*
i�
i� i�
i�
i�
i�
GL
Appliance
CBCT
Detectorcoil
ELCBtrip coil
P2P1
IA
i�
Zg – Ground fault loop impedanceIA – Leakage current through the appliance. But for simplicity the
whole current is considered through the body onlyi� – Leakage current through the human body
* – Test button to check periodically the operation of the ELCB
G
R
i�
Zg
S
ELCBtrip
contacts
Grounding theory and ground fault protection schemes 21/777
of harmonics. Surges can cause electrostatic dischargesgiving rise to high frequency leakage currents. Also dueto switching of inductive and capacitive loads causingheavy in-rush currents and phase-to-phase faults etc. SeeSections 6.13 and 17.7 for sources and causes of over-voltages and surges.
Usually a RCD may feed a number of load points inthe same circuit such as clubbing of a few light andpower points. It is possible that the RCD may trip evenunder healthy condition due to spurious currents. Forinstance devices like computers, printers and officeworkstations having built-in EM suppressors through L-C filter circuits may have constant ground leakage currentsvia the filter circuits. It may cause nuisance trippingeven on small line disturbances in the same circuit(switching of devices causing surges and harmonics).Similarly fluorescent tubes having electronic ballasts(reactors) generate high frequency currents during aswitching operation and that may find their path to theground through their built-in capacitors to cause nuisancetripping. In such cases loads may be divided into morecircuits. Experience of the project engineer is the bestguide. As a rule of thumb when leakage ground currentof a device (computer or fluorescent tube) is known fromits manufacturer, the total leakage current of all suchdevices connected on one circuit may be kept aroundone third of RCD threshold value. Like for a 30 mARCD it may be kept at about 10 mA and the appropriatenumber of devices in a circuit be determined accordingly.
To account for line disturbances and high frequencycurrents some manufacturers adjust their EM L-C filters
to suit a particular application such as x-ray devices,circuits associated with frequency converters or variablespeed drives (PWM inverters) etc. User may contact themanufacturer for the right choice of RCDs. For moredetails see reference 7 and 8 in the Further Reading.
Figure 21.2(b) Two-pole and four-pole ground leakage circuit breakers (RCDs) and layout of a DB
100 200 0.8125 250 0.64150 300 0.53175 350 0.46200 400 0.4300 600 0.27400 etc. 800 etc. 0.2
21/778 Electrical Power Engineering Reference & Applications Handbook
Influence of SPDs
The ground fault severity may mitigate when an SPD(Section 17.13) is provided upstream of an installationfor surge protection. But, when the SPD is provideddownstream to an RCD it is possible that while SPD isdischarging the arriving surge to the ground, the RCDmay detect the ground discharge current and trip when itis not wanted. In such cases special types of RCDs maybe employed that play immune up to certain level ofground discharge currents. To meet this, RCDs aredesigned with the following immunity levels,
– General type RCDs with a minimum surge immunityof 200 A with 0.5 ms/100 kHz wave.
– S type RCDs with minimum surge immunity of 3kAwith an 8/20 ms wave.
It is advisable to use SPDs upstream of RCDs or selectRCDs as noted.
Other applications
Ground leakage device can also be built into or associatedwith a circuit breaker (MCCB or ACB) to detect feebleground leakages due to bearing currents, electrostaticdischarges or insulation faults of a motor or generator,or insulation fault in cables.
Using the principle of RCD some manufacturers havedeveloped the device into a ground leakage relay thatcan be incorporated in a protective scheme through abreaker in all current ranges. Using a filter circuit theyare made immune to spurious voltages and currents toavoid nuisance trippings, as noted already.
21.1.3 Principle of operation of a groundleakage circuit breaker (application of acore-balanced CT)
Refer to Figure 21.2(a) showing a single-phase circuitbreaker having a thermal over-current and a magneticshort-circuit element, OCR. A core-balanced transformeris arranged in the circuit as shown. When the system ishealthy, the current through the phase, IR, and the neutral,In, will be equal and the magnetic effect of the loadcurrent on the primary windings, P1 and P2, will be almostzero. In the event of a ground leakage, I�, this balance isdisturbed and some of the leakage current will also flowthrough one of the primary windings of this core-balancedCT, as shown in the circuit. This will cause a magneticflux in the core, inducing a voltage in the secondarywinding S. In the circuit shown, it feeds a rectifier circuitprovided on the secondary side of the circuit. This unitfacilitates a direct current flow through the leakage tripcoil, which trips the breaker. These breakers are extremelyfast operating on fault (operating time may be less than5 ms), and have a low let-through energy. Refer toI2 – t characteristics (Figure 21.1). The ground fault currentwill also be comprised of a current through the appliance,IA (Figure 21.2(a)). But since the basic purpose of thisdevice is to protect the human body from an electricshock or electrocution, we have considered the leakagecurrent, I�, through the body alone to be safer. To ensure
total safety, the device should be able to detect this leakage,assuming there was no ground current through theappliance. When the device will be able to do so, thecircuit in any case will be protected.
21.2 Ground fault on an LV system
21.2.1 System protected through over-currentreleases and HRC fuses
The value of ground loop impedance is alwayspredetermined, depending upon the system requirementand the type of protection available to the system, itsaccuracy to detect the fault and time to operate. Forsystems protected through over-current releases or HRCfuses only, the ground loop must have a comparativelylow impedance. It will allow a high ground fault currentthrough the faulty circuit, sufficient to trip the over-current-cum-short-circuit releases of the breaker if the circuit isprotected through such releases of the breaker or blowout the HRC fuses, if the circuit is protected throughHRC fuses. Such a requirement is more desirable forhigher rating systems, where discrimination between ahealthy and a faulty condition by such devices may bedifficult. Medium-rating systems may cause a relativelymuch higher fault current and be automatically protected,as the normal ground fault current would be sufficient totrip the short-circuit releases or blow out the HRC fuses.
The rule of thumb to determine the ground loopimpedance is to consider the ground fault current as oneand a half times that of the over-current setting of thecircuit breaker for breaker-controlled systems (a faultcondition for a breaker) or three times the rating of thefuses, for fuse-protected systems (an over-currentcondition for the fuses). Based on this rule, Table 21.2suggests the optimum values of ground loop impedancesfor circuits of different current ratings for an LV system.At these values of currents, the over-current releaseswill trip in about 130–370 seconds. Refer to ‘I2 – t’characteristic curves of such releases as shown in Figure21.3. The HRC fuses will blow out in about 40/60 seconds.Refer to ‘I 2 – t’ characteristic curves of fuses, as shownin Figure 21.4.
Notes1. Ground fault protection through over-current releases or HRC
fuses is not a reliable practice. It requires a very low groundcircuit resistance to maintain a high ground fault current, whichmay not be possible in the long run. As a consequence, in certaincases the system may remain intact on a ground fault and damagethe healthy circuits. It is therefore good practice to provide aground leakage MCB, ground leakage MCCB or a conventionalMCCB with a CBCT and a ground fault leakage relay for low-rating incoming feeders.
For domestic light and power distribution this practice iscommon. Modern industrial installations can now adopt a fuse-free system and use MCCBs and provide a more sensitive groundleakage protection scheme for more safety and reliability.
2. For clarity one may note that single-phase loads or unevenlydistributed loads on a balanced three-phase system do not leadto a faulty condition as a result of unbalanced currents. Thesingle-phase load currents will always have the return path throughthe neutral and not the ground. The current through the groundcircuit will flow only when there is a ground fault and the circuitcompletes through the ground loop. See Figures 21.4(a) and (b).
Grounding theory and ground fault protection schemes 21/779
21.2.2 System protected through relays
A system protected through a separate ground fault relaywill require a different concept from that discussed above.A relay can be
• Electro-magnetic (being quickly outdated)• Static, based on discrete ICs or• Solid-state microprocessor based.
The relay is very sensitive and quick responding and canreliably actuate at low leakage currents (0.1 A or less). Itis therefore operated through the secondary of a CT (1 or5 A), provided in the ground circuit. Figures 21.5(a) and(b) show different methods of CT connections for a groundfault protection. The fault current in this case need notbe very high. It should rather be limited to a low value,to limit damage to the equipment. As a rule of thumb itmay be up to the rated current of the feeding transformer,or the rating of the incoming feeder, or twice the ratingof the largest outgoing feeder, depending upon the circuitto be protected.
The most common practice, however, is to limit theground fault current to only half the rated current of thesystem, or the circuit that is being protected. This is alsoin line with the universal practice of having the neutral ofhalf the size that of the phases. The neutral is normallygrounded to form a complete circuit through theground conductor in the event of a ground fault. Refer to
Figure 21.4 Figure 12.19 reproduced for illustration (Courtesy: Siemens)
Figure 21.3 I 2–t characteristics of an OCR (Figure 12.13(a),reproduced for illustration) (Courtesy: L & T)
Maximum
Minimum
104
864
2
103
864
2
�370
�130102
864
2
10864
2
0.7
0.8
0.9 1 1.2 2 3 4 5 6 7 8 9 10
Tim
e in
sec
onds
On 3-f healthy operation
150% Ir ¥ Ir
21/780 Electrical Power Engineering Reference & Applications Handbook
more single-phase light or power loads, a higher settingwould be necessary, depending upon the likely single-phase loads. This is to avoid false tripping on a healthysystem, as all the out-of-balance current will flow throughthe neutral only.
The theory of operation of such a protection scheme isbased on the principle that in a balanced circuit the phasorsum of currents in the three healthy phases is zero, asillustrated in Figure 21.7, and the current through thegrounded neutral is zero. In the event of a ground fault,i.e. when one of the phases becomes grounded, this balanceis upset and the out-of-balance current flows through thegrounded neutral. A healthy three-phase circuit, however,does not necessarily mean that each current phasor R, Yor B individually is equal in magnitude and phase. Evenif it has unbalanced currents in the three phases (whichis a likely situation in a three-phase system, see alsoSection 12.2(v)), it will not cause a current to flow throughthe ground circuit, as illustrated in Figure 21.4(a). Thecurrent through the ground circuit will flow only whenany one or more of the phases has a ground fault andforms a complete circuit through the ground loopconductor, as illustrated in Figure 21.4(b), and disturbsthe balance of the system. Similarly, on a single phasing,although the balance is upset, there is no current throughthe ground circuit and this scheme would not detect asingle phasing.
21.3 Ground fault protection inhazardous areas
These are highly sensitive areas and a little higher levelof a ground fault current can be catastrophic. It is thereforemandatory at such locations to keep their ground leakagecurrent (rather than the ground fault currents) low bymaintaining a certain level of ground loop impedanceand then be able to detect and isolate these currentspromptly.
Figure 21.6, showing a typical distribution networkillustrating the grounding circuits.
The preferred normal settings in a ground fault relayare 10–40% or 20–80% of the rated system current,depending upon the application. For a balanced load, asetting of, say, 10–20% is sufficient. For a system having
R
B
Y
N
G L1 L3L2Single phase
loads
(a) Healthy system
Inl1 + Inl2 + Inl3 Inl2 + Inl3 Inl3
R
B
Y
N
L1 L3L2
Inl1 + Inl3 Inl3
Inl3
Inl1
Ig
Inl3
Ig
IgIg
Ig
G G
Single phaseloads
(b) Faulty system
Figure 21.4 In a healthy system the unbalanced current (otherthan a ground fault or phase to phase and ground fault) will flowthrough the neutral and not the ground
Figure 21.5(a) Unrestricted G/F protection schemes for three-phase three-wire systems
G/F O/C O/C O/C G/F O/C O/C G/F
System or equipmentunder protection
N
G G G
Relayunits
R
Y
B
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
(a) (b) (c)
(a) Only for a ground fault (G/F) protection(b) For 3–O/C and 1–G/F protections(c) For 2–O/C and 1–G/F protections
G
Grounding theory and ground fault protection schemes 21/781
15% of the rated current and the recommended maximumground loop impedances.
The use of core-balanced CTs is quite common for suchapplications. They are specially designed to detect the groundleakage current of a circuit. This ground leakage is thenused to trip the faulty circuit through a ground faultrelay, wired at the secondary of such CTs, as shown inFigure 21.8.
The theory of operation of a ground leakage circuitbreaker (GLCB) is also the same as the combination ofa core-balanced CT and a ground leakage relay. Forindustrial application, use of a core-balanced CT with aground leakage relay and for domestic application use ofa GLCB is more common.
21.4 Ground fault protection of MVand HV systems
Now the situation is different from that in LV due tohigh phase to ground voltage Vg. It is now capable ofcausing high ground fault currents. Too large a groundfault current is not desirable for reasons of safety topersonnel from high ground touch voltages. To contain
GG
N
G G
G G
HV
LV
Figure 21.6 An HV to LV distribution system showing groundingcircuits
I R
IR
IYI B
IB
2I Y
2
IR = IY = IB
\ I I IR Y B + + = 0
Figure 21.7 Phasor sum of a balanced three-phase system iszero
Figure 21.5(b) Unrestricted G/F protection schemes for three-phase four-wire systems
G/F O/C O/C O/C G/F O/C O/C G/F
G G G
R
Y
B
S1 S2
S1 S2
S1 S2
(a) (b) (c)
(a) Only for a ground fault (G/F ) protection(b) For 3–O/C and 1–G/F protections(c) For 2–O/C and 1–G/F protections
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
S1 S2
N
Relayunits
G
System or equipmentunder protection
N
The leakage current at hazardous locations such asrefineries, petrochemical plants and mines should notexceed 15% of the rated current of the circuit or 5 A,whichever is greater. Table 21.3 indicates the maximumpermissible ground leakage currents for such areas at
21/782 Electrical Power Engineering Reference & Applications Handbook
these currents in such a system the normal practice is toincrease the ground impedance by inserting someimpedance into the ground circuit through the groundedneutral as discussed in Section 20.4.2. These systemsare protected through relays for different protectiveschemes and not through fuses or over-current releasesas for LV. Since a relay is much more sensitive and isquicker to respond, even a small ground leakage currentwill be enough to actuate it and isolate the faulty circuit.The ground loop impedance in these systems are thereforekept much higher than on an LV system. Moreover, sincemodern relays are able to sense a current as low as 0.1 Aor less, the universal practice is to have as large a ground
loop impedance as possible to limit the ground leakagecurrent, such that under no conditions will the touch andstep voltages exceed the permissible tolerable levels asdefined in Section 22.9.
In Chapter 20 we have analyzed the behaviour andcharacteristics of a system when grounded or left isolated.The ground fault factor (GFF) plays a very significantrole in the selection of insulation level (BIL) and its co-ordination with the different equipment connected onthe system. The application of a particular method ofgrounding would thus depend upon
• The ground protection scheme envisaged to decideon the magnitude of the ground fault current
• Criticality of the supply system, e.g. whether animmediate trip on fault is permissible
• Insulation level of the main equipment connected inthe system
• Where a neutral is not available (case of a three phasethree wire system) an inter-connected transformer isused to provide an artificial neutral as discussed inSection 20.9.
The grounding of a generator, for instance, whichmay be designed for 6.6, 11, 15 or 21 kV, and all otherequipment connected on this system may be solidlygrounded to have the least GFF and hence, add no extracost to the machine for a higher level of insulation or alarger size of machine. But at such voltages the groundfault current will be excessive as noted in Section 16.13.2to cause an extra burden to the windings of the machineor in the selection of protective devices. It is, however,noted that a few application engineers may prefer anisolated neutral system at certain installations wherecontinuity of supply is mandatory, even on a groundfault, until an alternative arrangement is made. Examplesare auxiliary drives in a power generating unit or essentialdrives in a process plant, essential public services railways,airports, community centres etc. At such installations, itis imperative to ensure that the generator and all theequipment connected in the system are designed for thehigher GFF and a larger size or greater cost of the machinesare immaterial. But the occurrence of another groundfault before clearing the first will lead to fatality andcause total damage of the faulty equipment. The groundcurrent can now find its way through the earlier groundfault and cannot be prevented, as there is no protectionavailable. Isolated systems are therefore generally notrecommended. Instead, for such requirements, a resonancegrounding system may be adopted, limiting the groundfault current to a desired low value to protect the machinefrom heavy fault currents and prevent the system fromtripping on a ground fault. Such a system is more prevalentin overhead transmission or long distribution networksto save the whole system from an outage on a groundfault. The terminal equipment and the windings of allthe machines may now be designed for a voltagecorresponding to the relevant GFF.
The magnitude of the ground fault current is a matterof system design and will largely depend upon the systemvoltage and the ground loop impedance. The requiredground impedance may be determined on the followinglines, if
R
CBCT
R Y B N
Figure 21.8 Schematic of a core-balanced CT (CBCT)
Table 21.3 Maximum impedances of ground loop for protectionby ground leakage relays in hazardous areas
Current rating Maximum permissible Recommended maximumof circuit ground leakage ground loop impedance
currenta ‘Ig’ on a 240 V phase toAmp Amp ground circuitb
Notesa Highly sensitive ground leakage relays can sense a current as lowas 0.1 A and less and at a much higher ground loop impedance.b Calculated to allow at least 150% of the maximum permissiblecurrent to be on the safe side. For example, for 15A maximumground leakage current, maximum impedance at 240 V
= 2401.5 15
= 10.7 ¥ W
Grounding theory and ground fault protection schemes 21/783
kVA = rated capacity of the supply source, which can bea generator or a transformer
V� = system rated line voltage in voltsIr = system rated full-load line current in amperesZg = impedance of the grounded neutral circuit in ohmsIg = required level of the ground fault current in amperes
Then
Z
V
Ig
g=
3 l
◊W
and I
Vr = 1000 kVA
3 Amp◊
◊ lIg is generally defined in terms of Ir, such as 10–40% or20–80% of Ir, depending upon the protection scheme. IfIg is, say, n in units of Ir then
Z
V V
ng =
3
3 1000 kVAl l◊ ◊
◊ ◊ ◊
or Z
V
ng
2
= 1000 kVA
�
◊ ◊ W (21.4)
From this equation one can determine the requiredvalue of neutral circuit impedance for a particular levelof ground fault current. The external impedance will beZg, less the ground impedance. In HV systems one canalso determine the likely value of a ground inductor coilto achieve a near-resonance condition, to eliminate thearcing grounds, on the one hand, and facilitate a strike-free extinction of an arc by the interrupting device, onthe other.
Example 21.1For a 1600 kVA, 11/0.415 kV transformer, considering the LVside:
Zng
2 2=
(0.415) 1000 1000 1600
¥
¥ ¥W
For an industrial power distribution network, if the setting ofthe protection relay is considered to be 20% of Ir then
Z g
2 2=
(0.415) 10000.2 1000 1600
0.54
¥¥ ¥
W
W�
The natural zero phase sequence inductive reactance of thegrounded neutral may be considered to be too small comparedto this and ignored for ease of calculations. Thus the resistanceR0 of the grounded neutral circuit may be considered as itsimpedance, i.e.
Ig = 0.415 1000
3 0.54
444 Amps
¥¥
�
The impedance calculated thus will form the basis ofdetermining the adequacy of the grounding stations provided.Probably, for such a low value of grounded neutral impedancethe grounding stations may have to be more elaborate andgreater in numbers (arranged in parallel). This is to ensure
that at no stage will the impedance of the system increasebeyond 0.54 W (inclusive of the impedance of the ground).Otherwise it will render the protective scheme ineffective andallow the ground fault to persist.
The above is the case for an LV system. For an HV system,similar calculations may be carried out to determine the groundneutral impedance. Now it will be much higher than the abovedue to a high Vg to limit the fault current and thus also theground potential difference to below a dangerous level at anypoint on the ground circuit. The impedance of the groundcircuit in such cases may be increased through a resistanceor an inductor coil.
21.4.1 Ground fault protection of EHV systems
Protection of EHV and power generating stations callsfor special considerations due to very high ground voltagesand is dealt with separately in Section II, Chapter 22.
21.4.2 Ground fault protection of generators
This subject is dealt with in Section 16.13.2.
21.4.3 Ground fault protection of electronic circuits
Isolated or clean ground system is necessary for electroniccircuits and serial data transmission to comply with EMC/EMI requirements. This subject is dealt with in Section6.13.3.
21.5 Core-balanced currenttransformers (CBCTs)
These are generally employed to detect small amounts ofground leakage currents, such as in mines and other sensitiveinstallations. They are also used to protect sensitive equipmentagainst small ground leakages. Installations having isolatedneutral or using ground resistance or impedance, to limitthe ground leakage currents, may also require this typeof fault detection.
A core-balanced CT is in a toroidal (circular) orrectangular form, like a conventional protection CT, exceptthat it is designed, with a large core opening toaccommodate all the 3-, 3 -1
2 or 4-core feeder cablespassing through it (Figure 21.8). The basic differencebetween this and conventional protection CTs is the lowunbalance or leakage current at which a CBCT operates.A normal protection CT would operate between its ratedand accuracy limit currents, as discussed in Section 15.6.5.The important design parameter in a CBCT is itsmagnetizing current at the relay operating voltage, ratherthan the class of accuracy and accuracy limit factor.
21.5.1 Design parameters for a CBCT
As these CTs have to detect small to extremely feebleground leakage currents their operating region is requiredto be very low, near the ‘ankle point’ (almost 10% of theknee point voltage) on the magnetizing curve. The designcriterion is thus the minimum exciting current requiredat the relay operating voltage (details available from therelay manufacturer) to actuate the relay.
21/784 Electrical Power Engineering Reference & Applications Handbook
Consider the equivalent tripping circuit diagram ofthe CBCT in Figure 21.9. If
RCT = resistance of the CBCTRL = resistance of the connecting leadsRr = resistance of the protective relayIre = relay operating currentVr = relay operating voltage at the relay current
settingIp = primary unbalance or ground fault current
= I I IR Y B + +
n = turns ratio of the CBCT
Inl = magnetizing (leakage) or no-load current ofthe CBCT at the relay operating voltage Vr
Then Vr = Ire(RCT + R� + Rr) andThe primary ground fault current
I n I Ip re n = ( + )�
Since on a fault the power factor is normally low, Ire andIn� may be considered almost in phase, when
I n I Ip re n ( + )� �
Since Vr and In� are interdependent parameters the optimumdesign is achieved when In� at Vr is of the same magnitudeas the relay operating current, i.e. In� � Ire.
For high current systems, using more than one cablein parallel, the number of CBCTs will also be the sameas the number of cables, as illustrated in Figure 21.10, inwhich case
Ip = n[Ire + N · In�] for N number of CTs
All such CBCTs have to be identical in turns ratioand magnetizing characteristics to avoid circulatingcurrents among themselves. To order a CBCT thefollowing information will be essential:
• Minimum primary ground leakage current• Nominal CT ratio. This may be such that on the smallest
ground fault the current on the secondary is sufficientto operate the relay. Normal Ip = 50, 100 and 200 A.It is recommended to be such that Vr � 0.1 ¥ kneepoint voltage of the CBCT
• Relay setting• CT secondary current, 1A or 5A• Minimum excitation current required at the relay
operating voltage• Knee point voltage• Number of cables in parallel• Limiting dimensions and internal diameter (ID) of
the CT. ID will depend upon the size of the cable.
21.5.2 Insulation level
Irrespective of the system voltage, a CBCT may bedesigned for an insulation level of only 660 V. The cableinsulation of the HV conductor is sufficient to provide therequired insulation between the conductor and the CBCT.
21.5.3 Mounting of CBCTs
The following is the correct procedure for the propermounting of these CTs:
1 It is necessary to pass all the 3, 3 12 or 4 cores of the
cable through the core of the CBCT to detect the
Groundleakage
relay
To indication,alarm or trip
circuit
S1
S2
S1
S2
S1
S2
S1
S2
S1
S2
S1
S2
a1
a2
b1
b2
c1
c2
*
A number of cables connectedto a common load
A number of cables connectedto a common load
* The relay contactsare wired in seriesfor indication, alarm
or trip circuit.
(a) Separate CBCT for each cable. (b) Separate protective circuit for each cable
Figure 21.10 Methods to wire a protective circuit through CBCTs
R rR �R CT i rei re + I n�
CBCT Vr
I R I YI B
In�
Relay
Ip = I I IR Y B + + ire = Relay operating currentin� = CT no load or excitation current
i re
Figure 21.9 Equivalent circuit of a CBCT protection circuit
Grounding theory and ground fault protection schemes 21/785
unbalance or the ground leakage in 3-core cables andonly ground leakage in 3 1
2 - and 4-core cables. Toexplain this see Figures 21.11(a) and (b). A 3-corecable will detect an unbalance in the three phases,whether this is the result of unequal loading in thethree phases or a ground fault. However, 3 1
2 - or 4-core cables will detect only a ground leakage as theamount of unbalance, when it occurs, will be offsetby the flow of this unbalanced current through thereturn path of the neutral circuit. When using only 3-core cables, the load must be almost balanced otherwiseit will send wrong signals or a higher setting of therelay will become essential to account for the out-ofbalance currents due to the feeders’ unequal loading.
2 In armoured cables, armouring must be removed
before passing the cable through the CBCT to avoidan induced e.m.f. through the armour and thecorresponding magnetizing current which may affectthe performance of the CT.
3 As such CTs are required to detect small out-of-balancecurrents, the connecting leads should be properlyterminated and must be short to contain the leadresistance as far as possible.
4 For high-rating feeders using more than one cable,there must be one CBCT for each cable. Not morethan one multi-core cable must pass through suchCTs. The secondary of all such parallel CTs may,however, be connected in series, across the commonrelay (Figure 21.10(a)).All CBCTs being used in parallel and intended for the
Figure 21.11 Detecting the unbalance and the ground leakage currents through a CBCT
A A
A A
R
Y
B
N
Iub
Iua
Iua Iub
Iub
Iua
G
3–f load
Load-A
3–f four-wire load
Load-B
(a) On a healthy system
R
Y
B
NIub
Iua
Iga Igb
Iub
Iub
G
3–f load
Load-A
3–f four-wire load
Load-B
(b) On a ground fault
Iua
Igb
Igb(Iua + Iga)
Iga
G G
IgbIga Igb
Iga
Igb
Iga
Igb
This CBCT will detect the unbalancedcurrent
This CBCT will detect nothing asthe unbalance of the 3-phases isoffset by the return current throughthe neutral circuit
Iua – Unbalanced current due to load–AIub – Unbalanced current due to load–B
This CBCT will detect the unbalance aswell as the ground leakage currents
This CBCT will detect only theground leakage current
Iua – Unbalanced current due to load–AIub – Unbalanced current due to load–BIga – Ground fault current of load–AIgb – Ground fault current of load–B
Iga
N
N
21/786 Electrical Power Engineering Reference & Applications Handbook
same feeder must have identical magnetizingcharacteristics and calibration in order to relay identicalsignals. Even then small variations in the output mayoccur which could affect the sensitivity of a circuitusing more than one CBCT. If such a reduction insensitivity is considered detrimental to the protectionof the system or the equipment, it is advisable to usea separate relay with each CBCT (Figure 21.10(b)).For a common indication, alarm or trip, the relays’trip contacts may be wired in series.
5 When using only single-core cables, the same methodshould be adopted, as discussed above. Groups maybe made of 3, 3 1
2 or 4 cores (R, Y, B or R, Y, B andN) of single-core cables, and one CBCT used incircular or rectangular form, whichever is moreconvenient, for each group.
NoteIn an HV system it is recommended that each circuit be protectedseparately for a ground leakage as far as possible. This is tolocalize the effect of the fault and to trip only the feeder of thefaulty circuit. A common protection in the incoming is notadvisable, except for cost considerations, which also will benominal for such a protection scheme compared to the cost ofthe main equipment. Otherwise a ground leakage in thedownstream may trip the whole system, which may not bedesirable. It is also possible that at the downstream, the groundleakage is very feeble due to high ground loop impedance andmay not be sufficient to be detected by the common groundleakage protection provided at the incoming. It is also possiblethat the setting of the relay is not so low as to detect this. Thus toprovide a reliable ground leakage protection for each individualcircuit will be worthwhile without any serious cost implications.Such a philosophy, however, will not hold good for a systemwhich is protected only through its incoming feeder and all theoutgoing feeders are merely isolators. In this case the groundleakage protection will have to be centralized for the entiresystem and provided in the incoming feeder only.
21.6 Ground fault (G/F) protectionschemes
A scheme for a ground fault protection will depend uponthe type of system and its grounding conditions, i.e. whetherthe system is three-phase three-wire or three-phase four-wire. A three-wire system will require an artificial groundingwhile for a four-wire system the type of grounding mustbe known, i.e. whether it is effectively (solidly) groundedor non-effectively (impedance) grounded.
Grounding protection will depend upon themeasurement of the residual quantities (V0 or I0) thatwill appear across the ground circuit in the event of aground fault. As discussed above, in a balanced three-phase system the voltage and current phasors are 120∞apart and add up to zero in the neutral circuit. In theevent of an unequally distributed system or a groundfault, this balance is disturbed and the out-of-balancequantities appear across the neutral or the ground circuitrespectively. The current through the ground circuit willflow only when there is a ground fault, the fault currentcompleting its circuit through the ground path. The normalunbalanced current, due to unevenly distributed single-
phase loads or unequal loading on the three phases, willflow only through the neutral circuit.
In a phase-to-phase fault, however, the system willbe composed of two balanced systems, one with positivesequence and the other with negative sequencecomponents. The phasors of these two systemsindividually will add up to zero, and once again, as inthe above case, there will be no residual quantities throughthe neutral or the ground circuit, except for the transientand spillover quantities.
The ground fault current may be detected throughthree or four CTs, one in each phase and the fourth in theneutral circuit (Figures 21.5(a) and (b)). Through theneutral to discriminate the fault, as discussed later.
NoteIn a G/F the three CTs will also measure the unbalanced loadcurrent, if any, in addition to G/F current. For an appropriate settingof the relay, therefore, it will be essential that the likely systemunbalanced current be measured and the relay set in excess of thisto detect a G/F. For systems feeding single-phase or unbalancedloads, prone to carrying high and widely fluctuating unbalancedneutral currents, it may be difficult to determine the likely amountof unbalance and provide a suitable setting for the G/F relay. Insuch cases use of four CTs or core balanced CTs would be moreappropriate.
Below we discuss the more widely adopted practicesto detect a ground fault, i.e.:
• Protection through a single CT• Restricted G/F protection• Unrestricted G/F protection• Directional G/F protection and• Differential G/F protection (high impedance differential
protection is discussed in Section 15.6.6(1)).
21.6.1 Protection through a single CT
This is the simplest method to protect an equipment againsta G/F (Figure 21.12). It can, however, be applied only atthe source, which is a generator or a transformer, providedthat the source has no other parallel grounding paths inthe vicinity. This is to avoid sharing of the fault currentand false or inadequate detection of the fault current bythe relay. This scheme is therefore more functional at themain generating source, such as at the generator or thegenerator transformer, having a low impedance solidlygrounded neutral.
Downstream
GFR
Neutralsolidly
grounded
Figure 21.12 Ground fault protection through a single CT
Grounding theory and ground fault protection schemes 21/787
For any other equipment or system, such as shown inFigure 21.13, the fault current may be shared by the variousgrounding stations in the vicinity and the relay may notsense the real extent of the fault, even when the system iseffectively grounded. A part of the fault current, may nowflow through the other nearby grounding stations. Moreover,for a fault on another feeder spill currents may also passthrough such relays and trip them (unwanted) when therelays are highly sensitive or have a low setting. Such ascheme will also not discriminate when required, and hencewill have limitations in its application. Nevertheless, it iscommon practice to apply single CT protection throughneutral circuits of the grounded transformers anywhere inthe system, generation, transmission or distribution. Multiplegroundings may cause problems, but this is taken into accountat the design/planning stage.
21.6.2 Restricted ground fault protection
When it becomes essential to discriminate between afault within the circuit to be protected from one outsidethe circuit, this scheme may be adopted. While doing so,it must be ensured that adequate ground fault protectionis available to the remaining feeders, if connected on thesame system.
For a three-phase three-wire system(generally HV systems)
The scheme for a three-phase three-wire artificiallygrounded system will require four CTs, identical in designparameters, turn ratio, error and magnetizingcharacteristics. Otherwise spill currents may occursometimes sufficient to operate inadvertently a low-settingor highly sensitive ground fault relay. The fourth CTthrough the ground circuit is used with the same polarityas the three CTs of the phases. Then only would theresidual current of the phase CTs fall 180∞ apart fromthat of the ground circuit CT. Such an arrangement willprovide the desired discrimination, to detect the faultoccurring within the protected zone. Figures 21.14(a)and (b) illustrate this discrimination through the use ofthe fourth CT. The residual current of the three-line CT,in a healthy condition, in the event of an unbalance inthe system, is taken care of by raising the setting of therelay to account for the unbalance, as in a core-balancedCT. The fault current through the relay will flow only on
a G/F occurring within the restricted zone as illustratedin Figure 21.14(a). For faults occurring outside therestricted zone, as shown in Figure 21.14(b), the faultcurrent through the ground circuit CT will be offset bythe residual current of the three-phase CTs and thus therelay will remain immune to such a fault.
Figure 21.13 Limitation in using a single CT for a G/F protectionwhen the equipment has more than one parallel ground path
G
GFR
G G
F
Ir
Iy
IgIg
S1
S2
ig
Ig
GGFR
To tripcircuit
Directionof residual
inducedcurrent
Impedancegroundedsystem
S1
S1
S1
S2
S2
S2
1
2
3
ir
iy
G
Note All the four CTs must be wired with the same polarity
(a) Relay operates when the fault occurs within the protected zone
R
Y
B
4
Note All the four CTs must be wired with the same polarity
* Induced residual fault current through the phase CTs,
( = + + + )f r y b gi i i i i– – – – –
is equal and falls opposite to theresidual current if through the ground circuit CT.
( + + )r y bi i i– – –
is considered zero under healthy condition.For small unbalances the relay is set a little higher.
(b) The relay stays immune to a fault occurring outside theprotected zone.
Figure 21.14 Scheme for a three-phase three-wire restrictedground fault protection for an HV system
Equipment tobe protected
Direction ofphase inducedcurrents in theCTs
Direction ofresidualinducedcurrent
Impedancegroundedsystem
Ir
Iy
Ib + Ig
S14
S2
ifif ig *
G GFR
To tripcircuit
Ig
G
S1
S1
S1
1
2
3
ir
iy
R
Y
B
S2
S2
S2
(ib + ig)Ig
21/788 Electrical Power Engineering Reference & Applications Handbook
R
Y
B
N
Ir
Iy
Ib
Iu
S1
S1
S1
S1
S2
S2
S2
S2
ir
iy
ib
iu
iu
iu
iu
iu
S1
S2
GGFR
Under healthy condition the unbalance residual current of the 3 phaseCTs is nullified by the equal and opposite current in the neutral CT.There is thus no current through the relay.
I I I I–
r–
y–
b–
u + + =
Figure 21.15 Scheme for restricted G/F protection for a three-phase four-wire system. Healthy condition
Figure 21.16 Scheme for restricted G/F protection for a three-phase four-wire system. Fault occurring within the protected zone
R
Y
B
N
Ir
Iy
Ig
S1
S1
S1
S1
S2
S2
S2
S2
ir
iy
ig
if
S1
S2
Ig
GFRG Ig
G
The residual currents are additive and the relay can be set low.
If + igIg
Ig
Figure 21.17 Scheme for restricted G/F protection for a three-phase four-wire system. Fault occurring outside the protectedzone
R
Y
B
N
Ir
Iy
Ib + Ig
S1
S1
S1
S1
S2
S2
S2
S2
ir
iy
ib + ig
ig
IgG
G
IgGFR
S1
S2
* The residual currents are equal and opposite, hence nullify andthe relay stays inoperative.
ig*
ig*
Ig
For a three-phase four-wire system(generally LV systems)
A three-phase three-wire system is generally a balancedsystem and has negligible unbalanced residual currentthrough the three-phase CTs. The relay for a groundfault protection can thus be set low. The scheme discussedabove is thus satisfactory for a G/F. But in three-phasefour-wire LV systems, which are generally unbalanced,the above scheme poses a limitation as there may nowbe a substantial residual unbalanced current through therelay. For G/F protection, therefore, such a scheme willrequire a higher setting of the relay to avoid a trip in ahealthy condition. It is possible that this setting maybecome sufficiently high, for highly unbalanced systemsto detect an actual G/F and defeat the purpose of G/Fprotection. Such a situation can be averted by providinga fifth CT in the neutral circuit as shown in Figure 21.15,which obviously will have its excitation current directionopposite to the residual current of the three-phase CTs.Hence, it will offset the same, and the relay can be setlow. It is therefore mandatory to use five CTs in LVsystems for adequate restricted G/F protection.
Application1 A restricted ground fault is recommended for
equipment that is grounded, irrespective of its methodof grounding. Unless the protection is restricted, theequipment may remain unprotected. Generally, it isan equipment protection scheme and is ideal for theprotection of a generator, transformer and all similarequipment or circuits, requiring individual protection.Figure 21.16 illustrates the operation of the relay whenthe fault occurs within the protected zone. The schemewill prevent isolation of the equipment for faultsoccurring outside the restricted zone (Figure 21.17).
2 A delta-connected or an ungrounded star-connectedwinding should also be protected through a restrictedground fault scheme, otherwise it will remainunprotected. There is no zero sequence or residualcurrent in such a winding to detect a groundfault. The arrangement will be similar to that fordirectional protection of a delta side of a transformer,as discussed later, with the use of a groundingtransformer (Figures 21.18 and 21.19).
21.6.3 Unrestricted ground fault protection
The CT provided in the ground circuit is now removedand the same scheme becomes suitable for an unrestricted
Grounding theory and ground fault protection schemes 21/789
Supply-A Supply-B
G a G b
Iga
G
Iga A¢G
Iga
Igb
Iga
Igb
B ¢
G
Igb
Iga
IgaIga
G a¢
Iga
Iga
Notea, b – are non-directional GFRsa¢, b¢ – are directional GFRs
* Reverse direction current, Iga through relay b ¢ would trip breakerB ¢ and reduce Iga to zero.
*
G b¢
Igb
Figure 21.18 System using directional ground fault relays
R
Y
B
Primarywindings
G
G
Tertiaryopen deltawindings
Secondarywindings
Residual VT withstar connectedsecondary
DirectionalGFR
Residualvoltage
polarizing coil
Residualcurrent
coil
Ve
G G
S1 S2
S1 S2
S1 S2
N
Figure 21.19 A typical scheme for a directional G /F protectionrelay
G/F or a combined G/F and phase fault protections. It istrue for a three-phase three-wire or a three-phase four-wire system. This scheme may also be arranged for acombined O/C and G/F protections as illustrated in Figures21.5(a) and 21.5(b).
ApplicationThis is the most common scheme in normal use for anypower system with more than one feeder, connected to acommon bus, such as for distribution and sub-distributionpower networks, having a number of load points,controlled through a main incoming feeder. In a switchgearassembly, for instance, common protection may beprovided at the incoming for a ground fault or combinedO/C and G/F protections as discussed above. In suchcases, a restricted G/F protection may not be appropriateor required, as the protection now needed is system groundprotection, rather than individual equipment groundprotection. The incomer must operate whenever a groundfault occurs at any point on the system. Moreover, for anLV system, where it may not be desirable or possible toprovide individual ground protection to each feeder, sucha scheme is adopted extensively.
21.6.4 Directional ground fault protection
In the previous section we discussed non-directionalprotection of an equipment or a system. But for systemswith more than one source of supply in parallel, such asa power grid, receiving power from more than one source(Figure 13.21) or an industrial load, having two or moresources of supply, one of them being a captive powersource, it is possible that a fault on one may be fed bythe other sources and may isolate even a healthy system,rendering the whole system unstable. Such a situationrequires discrimination of a ground fault and can beprevented by the application of directional G/F protection.The primary function of a directional G/F protection isthus discrimination.
In the above situation, even an overspeeding motoron a fault elsewhere would feed back the supply sourceand require such protection. The protective scheme isolatesthe faulty source from being fed by the healthy sources.Figure 21.18 illustrates a simple power circuit providedwith a directional G/F relay. In the event of a fault insystem B, source B alone would isolate. Source A wouldnot feed the fault as relay b¢ would trip the breaker B¢and eliminate Iga. The relays are necessarily set at lowersettings and at lower tripping times than the non-directionalGFR to isolate the faulty feeder quickly than to wait forthe trip by the non-directional relay. This is to avoid atrip of the non-directional relay ‘a’ on system A.
A directional G/F relay basically is a power-measuringdevice, and is operated by the residual voltage of the systemin conjunction with the residual current detected by thethree CTs used for non-directional protection, as shown inFigure 21.19. To provide directional protection, therefore,a residual VT is also essential, in addition to the threeresidual CTs. The voltage phasor is used as a reference toestablish the relative displacement of the fault current. Inhealthy conditions, i.e. when the current flows in the right
21/790 Electrical Power Engineering Reference & Applications Handbook
direction, Ve = 0 (refer to Section 15.4.3 for details), andthe relay remains inoperative. The relay operates onlywhen the current flows in the reverse direction.
NoteThis relay may be used only under unrestricted fault conditions,with three CTs as shown. If the scheme is used under a restrictedfault condition, with the fourth CT in the neutral, the directionalrelay will remain immune to any fault occurring outside the zoneof the three CTs, as the fault current through the fourth CT willoffset the residual current, detected by the three CTs (Section 21.6.3),rendering the whole scheme non-functional.
Current polarization
Voltage polarization depends upon the location of therelay and the location of the fault. It is possible that theresidual voltage, at a particular location in the system, isnot sufficient to actuate the voltage coil of the directionalG/F relay. In such an event, current polarization is usedto supplement voltage polarization. Current polarizationis possible, provided that a star point is created on thesystem, even through a power transformer, if sucha transformer is available in the same circuit, Figure21.20. Else a grounding transformer may be provided asillustrated in Figure 21.21, and grounded neutral utilised,to provide the required residual current polarization, toactuate the ground fault relay.
NoteThe two currents, residual and polarizing, are capable of operatingthe relay.
21.6.5 Current setting of a ground fault relay
1 Selection of CTs for ground fault protection particularly
R
Y
B
powertransformer
G
S1 S2
S1 S2
S1 S2
CTs
Residualcurrent
coil
Currentpolarizing
coil
Directionalground fault
relay
G
Groundingtransformer
G
Current polarization through the grounded neutral of a groundingtransformer
Figure 21.21 Typical circuit illustrating current polarizationscheme to operate a directional GFR
needs more careful consideration of the fault conditionsand impedance of the ground circuit, in addition tothe location of the CTs. This is due to a rather lowsetting of the G/F relay, 10–40% or 20–80% of thefull load current or even lower, as in mines and othersensitive locations. Too low a setting may even trip ahealthy system due to ground capacitance leakagecurrents (more so in HV circuits) or unbalancedcurrents through the neutral (more on LV circuits). Incore balanced CTs detecting small leakage currents,it is possible that during a phase-to-phase fault theremay be transient spill currents through its residualcircuit, which may operate the low-set and moresensitive G/F relay, which may not be desirable. Toovercome this, the time setting of the relay may be suitablyadjusted so that the over-load relay will operate fasterthan the G/F, or slightly higher setting for the relaymay be provided or time delay in the trip circuitintroduced.
2 In another situation, when the ground circuit has ahigher impedance than designed it may be due topoor soil conditions, dry soil beds, rocky areas, poorgrounding stations or inadequate maintenance of thegrounding stations. In such conditions, the groundcircuit may provide a lower fault current thanenvisaged. Sometimes an overhead conductor maysnap due to strong winds and fall on dry metallicroads, hedges and shrubs causing extremely lowleakage currents, creating a hazard to life and property.This may cause an arcing ground, leading to firehazards. For all such locations and situations, verylow current settings (of the order of 5% of Ir or evenlower) or leakage current detection through core-balanced CTs may be adopted.
Current polarization through the groundedneutral of a power transformer
Powertransformer
R
Y
B
G
S1 S2
S1 S2
S1 S2
CTs
Residualcurrent
coil
Currentpolarizing
coil
Directionalground fault
relay
G
Figure 21.20 Typical circuit illustrating current polarizationscheme to operate a directional GFR
Grounding theory and ground fault protection schemes 21/791
3 For circuits protected by HRC fuses for short-circuitconditions, the G/F relay must be a back-up to thefuses, and trip first on a ground fault. In other words,I2t (relay) < I2t fuses.
21.7 Grounding systems
Based on the protection criteria and ground fault protectionschemes discussed in the previous sections and Chapter20 we now provide a brief reference to the various typesof grounding methods in vogue worldwide for installationscomprising homes, buildings, HV/LV substations,industries, switchyards, power generating stations andover-head lines. The grounding schemes may vary withsystem voltage, type of installation, equipment to beprotected, criticality of installation and exposure to human-body. It may also vary from country to country dependingon their own grounding codes and statutory regulations.Here we provide only broad guidelines on basic groundingsystems as practised for various types of installations.For details on application one may refer to the Standardsnoted under Relevant Standards.
The basic criteria of grounding are,
– protection to a human body from electric shocks andlimiting the touch voltage (body leakage current) withinsafe limits (Section 21.1)
– protection against fire hazards.– power supply continuity on a ground fault when desired.– minimizing EM effects.– protection from lightning strikes, transferred surges,
switching surges and over-voltages.
The nomenclatures used for grounding systems are definedas,
T- Connection to groundI- Isolation from ground.N-Connecting to the neutral and neutral connected to theground.
The following are the grounding systems in practice,
1 TN – The supply source (usually a transformer)neutral is grounded and all metallic frames of equipmentand devices operating on the system are connected to theneutral. The neutral may be connected in the followingpossible ways,
(i) TN-C (Figure 21.22) – Transformer neutral isgrounded and the metallic frames of all load points areconnected to the neutral. The neutral of the supply systemworks as the protective ground (earth) neutral (PGN orPEN). Under normal conditions the N, load metallic framesand the ground are nearly at equi-potential. During aground fault the following may happen,
– During an MV fault a current will flow through theground electrode of the LV neutral and a powerfrequency voltage will appear across the LV bodiesof all equipment and devices connected on the systemand the remote ground.
– During an LV phase fault (insulation fault) also thevoltage between phase and body of the faultyequipment or device may exceed the phase to neutralvoltage (see Section 20.1). It is therefore not arecommended system at locations contaminated withvolatile gas, liquid or vapour prone to fire hazardsand explosions.
– An insulation (equipment) fault is a short circuit faultand the faulty equipment or device must be isolatedquickly using HRC fuses or interrupters, short circuitreleases or relays when used.
– The system is not allowed to be used for conductorsless than 10 mm2 of copper or 16 mm2 of aluminium.
– The touch voltage between any two ground pointsshould be within safe limits as discussed in Section21.1.
– The use of this system must be reviewed for the thirdharmonic quantities that this grounding system is proneto receive from other systems in the vicinity. Thesystem therefore has limitations for electronic devicesand communication circuits when connected on thesystem.
– Neutral in the vicinity of metallic structures and powercables may also become a potential source of EMdisturbances.
Note: To overcome the above short comings (over-voltagesduring a fault) the neutrals of supply source and the LV installationmay be grounded separately.
2 TN-S (Figure 21.23)
– Now the neutral and the protective conductors areseparate and the equipment and devices are grounded
Figure 21.23 TN–S system of grounding
G Load
R
Y
B
N
PG
Figure 21.22 TN–C system of grounding
G Load
R
Y
B
PGN
21/792 Electrical Power Engineering Reference & Applications Handbook
directly with the protective ground (PG). Use ofseparate protective ground and neutral conductor isnecessary for small feeders with, conductors less than10 mm2 of copper or 16 mm2 of aluminium. It is afive-wire system and preferred where quickmaintenance and repair backup is available.
– Neutral is not protected and may cause a risk to lifeand property.
– But it is a low fault current system, the risk to fire anddanger to faulty device is low.
– An MV/LV disruptive break-down in the transformeror the source of supply may cause surge transference(Section 18.5.2) and over-voltages (Section 20.1) andraise the neutral voltage and pose similar risk.
– Since neutral is grounded this scheme also allows thethird harmonics to flow into the system from othersources.
– The system is suitable for loads with low level ofinsulation like furnaces, electronic devices and computernetworks. It is a preferred system for hospitals andpractised by UK and Anglo-Saxon (British) countries.
3 TN-C-S (Figure 21.24) This system is a combinationof TN-C and TN-S at one installation. The neutral andthe protective ground conductors of TN-S, five wire systemare separate from TN-C four wire system. The TN-Stherefore cannot be placed in the upstream. The point atwhich PG conductor separates from neutral is generallyat the origin of installation.
4 TT (Figure 21.25)
– It is the simplest system and practised by mostcountries. The transformer or the source of supplyneutral and its frame are grounded. The metallic bodyof loads are also connected to the ground conductor.The system is prone to over-voltages and transferenceof surge voltages from the MV side and suitableprotection may be installed across the grounded neutral(like a surge protective device). A simpler way toachieve this is to keep source equipment separate fromLV neutral rather than be inter-connected. The groundfault current, Ig is limited by the ground impedanceand the faulty device is protected through a RCD(Section 21.1.2).
– The risk to fire, explosion and damage to equipmentand devices is low.
– The system causes low EM disturbances.
5. IT (Figure 21.26) It is an ungrounded system or hasan impedance grounded neutral. Transformer or sourceof supply neutral is not grounded but load metallic framesare grounded. The system is preferred where continuityof service is more essential such as at critical installationslike power generating stations, process plants and crowdedpublic places (also see Section 20.2.1). It calls for promptfault tracking and removing and demands for efficientengineering and maintenance back up. A second fault onanother phase before the first gets cleared, however, willcause a severe phase to phase fault. Provisions must beavailable to promptly clear the same through short-circuitprotection of interrupting device of the faulty equipment.With the availability of advanced electronic diagnosticinstruments, tracking the first fault and removing thesame promptly even remotely is simple and in alllikelihood a second fault would be rare.
– As there is no current flow during the first fault, therisk to fire, explosion or damage to equipment is low.
– Also low EM disturbances.– In France, for instance, IT system is compulsory in
hospitals, particularly operation theatres to ensure anuninterrupted power supply. With the back-up of UPS(un-interrupted power supply), this requirement maynot be over emphasized, nevertheless it is a safetyrequirement.
Figure 21.25 TT system of grounding
G
R
Y
B
N
Sur
ge p
rote
ctiv
e ar
rest
er(S
PD
) op
tiona
l
G PG
Figure 21.26 IT system of grounding
R
Y
B
N
G
Figure 21.24 TN–C–S system of grounding
G
TN–C
R
Y
B
N
PG
TN–S
16mm2 6mm2
SeeNote
Note: TN-C system is not permitted downstream of TN–S
Grounding theory and ground fault protection schemes 21/793
21.7.1 Choice of grounding system
All grounding systems are similar and aim at achievingthe same objectives as noted before. As there is no onechoice with one grounding system, a power installationmay therefore have a combination of these systemsdepending on location, type of equipment, userrequirements and also local electricity authority statutoryregulations. It is possible that one installation may usemore than one grounding systems to satisfy differentrequirements for different types of loads. For example,
– Special grounding system for sensitive installationshaving electronic circuits, video, computers andcommunication network. Also see Section 6.13.3 onclean grounding.
– IT system may be preferred for critical installationsnot wanting an immediate shut-down. But this toohas its own merits and demerits as noted above.
– TT being the most practised system worldwide forLV grounding.
– Some countries like Norway use IT System.– Anglo-Saxon (British) countries generally use TN-C
system.
All these grounding systems are practised in varyingdegrees by different countries. Table 21.4 gives a generalover-view of the grounding practices of LV installationsas adopted by some countries just for a generalreference.
Most countries have drawn-up their own electricityregulations like National Electric Code (NEC) and onemay refer to the same before deciding on the groundingsystem. For example according to NEC one may useRCD when it meets the following,
– Neutral is directly grounded.
– 150 V< phase to neutral voltage < 600 V.
Similarly various parts of IEC 60364 provide solutionsto grounding of various types of installations and locationssuch as bath tubs, shower basins, swimming pools, saunaheaters, construction and demolition sites, installationsat agriculture and horticulture premises, data processingequipment, caravan parks and caravans etc. Similarlyfor hospitals, schools, navy, mines and work sites.
For brevity we limit our discussions to the above. Forimplementation of these systems one may refer to thevarious parts of IEC 60364, NEC and references 3–6provided in the Further Reading.
21.7.2 Protection from lightning strikes,transferred surges and over-voltages
Limiting potential rise in the LV ground circuit due tolightning surges, surge transferences and over-voltagescaused by faults on MV side is an important requirementfor all LV ground systems. Some remedial measures arenoted below,
– To protect from direct lightning strikes a lightningarrester can be installed at the top most point of thebuilding and grounded separately.
– To protect from transferred surges (Section 18.5.2) asurge protective device (SPD) (Section 17.13) can beinstalled across the neutral and ground on the LV sideof the transformer as marked in Figure 21.25. It shouldbe at the origin of the supply source and not near thereceiving point as far as possible.
NoteSPDs wherever installed must be grounded separately to preventground potential rise while clearing the surge.
– To protect from over-voltages due to faults on theMV side (Section 20.1) many countries practise tokeep the substation LV ground circuit and neutral /ground of LV system separate to prevent MV sideover-voltage raise the voltage of the LV network duringa fault on the MV side, unless LV is also impedancegrounded and its ground impedance is enough to absorbthe over-voltage. The usual practice to accomplishthis is to have an IT system with impedance groundingon the MV side to limit the zero sequence fault currentsin the MV system. IEC 60364-4-442 prescribes thelimiting voltage on the LV system as,
Vr /÷3 + 250 V for more than 5 s.Vr /÷3 + 1200 V for less than 3 s.
In IT system the phase-N voltage, Vr/÷3 rises to Vron a ground fault. LV equipment and devices must besuitable to withstand the same.
To study more on the effects of transferred surges andover-voltages on various LV ground systems refer toIEC 60364 and its various parts and the literatures notedat references 3–6 under Further Reading. Above moreemphasis is provided on LV installations but similarparameters will apply for MV and HV installations also.
Table 21.4 Grounding practices adopted by some countries.
Country LV grounding system
Germany TT and TN-C230/400 VBelgium TT230/400 VSpain TT230/400 VFrance TT230/415 VGreat Britain TT and TN-C240/400 VItaly TT230/400 VJapan TT100/200 VNorway IT230/400 VPortugal TTUSA TN-C and TN-S, IT and impedance120/240 V grounded IT in process industries.
Source: Cahier Technique Merlin Gerin n∞173.
21/794 Electrical Power Engineering Reference & Applications Handbook
Relevant Standards
IEC
60034-1/2004
60050-195/2001
60255-6/1988
60364-1 to 7
60479-1/1994
60479-2/1987
60755/1992
60898/1995
61008/2002
61008-1/2002
61009-1/2003
61024-1/1990
61543/2005
–
–
–
Title
Rotating electrical machines.Rating and performance.
Code of practice for earthing and protection against electricshocks.
Electrical relays – Measuring and protection equipment.
Electrical installation of buildings – code of practice.
Guide on effects of current passing through the human body –General aspects.
Guide on effects of current passing through the human body –Special aspects.
General requirements for residual current operated circuitbreakers.
Electrical accessories – Circuit breakers for over currentprotection for household and similar installations.
Residual current operated circuit breakers.
Residual current operated circuit breakers, without integralover current protection – General rules.
Residual current operated circuit breakers, with integral overcurrent protection – General rules.
Protection of structures against lightning – General principles.
Residual current operated protective devices (RCDs) forhousehold and similar use-Electromagnetic compatibility.
Specification for grounding transformers.
Code of practice for undesirable static electricity.
General requirements for residual current operated protectivedevices. Circuit breakers with integral over current protection.
IS
4722/2001,325/2002
3043/2001
3842-1 to 12/2001
732/2000
8437-1/2002
8437-2/2002
12640-1-2/2001
8828/2001
12640-1/2000
–
–
2309/2000
–
3151/2001
7689/2000
12640-2/2001
BS
BS EN 60034-1/1998
BS 7430/1998,BS IEC 60050-195/1998
BS EN 60255-6/1995
BS 7671/2001
PD 6519-1/1995
PD 6519-2/1988
BS EN 61008-1/1995
BS EN 60898/2003
–
–
–
BS 6651/1999, DD ENV61024-1/1995
BSEN 61543/2006
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Related US Standards ANSI/NEMA and IEEE
ANSI-C37.16/2000
NEMA-280/1990
Low voltage power circuit breakers – preferred ratings and applicationrecommendations.
Application guide for ground fault circuit interrupters.
–
BS 5958-1/1991,BS 5958-2/1991
Notes1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become
available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is acontinuous process by different Standards organizations. It is therefore advisable that for more authentic references, one may consult therelevant organizations for the latest version of a Standard.
2 Some of the BS or IS Standards mentioned against IEC may not be identical.3 The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.
List of formulae used
Current passing through a human body
IStb
b
b = (21.1)
Sb = shock energy in watt-seconds (Ws)Ib = r.m.s value of the leakage current through the body
in Amperes (A)
tb = duration of leakage current in seconds (s)
Safe currents through a human body
(i) for a 50 kg body
It
bb
(50 kg) = 0.116 (21.2)
(ii) for a 70 kg body
Grounding theory and ground fault protection schemes 21/795
It
bb
(70 kg) = 0.157 (21.3)
Selecting a ground fault protection scheme
ZV
ng12
= 1000 kVA
◊ ◊ W (21.4)
kVA = rated capacity of the supply sourceV1 = system rated line voltage in voltsIr = system rated full load line current in Amperesn = unit of Ir
Zg = impedance of the grounded neutral circuit inOhms
Ig = required level of the ground fault current in Amperes
Further Reading
1. General Electric Co. Ltd, Protective Relays and ApplicationGuide, GEC Measurements, St. Leonards Works, Stafford, UK
2. Taylor, H. and Lackey, C.H., ‘Earth fault protection in mines’,The Mining, Electrical and Mechanical Engineer, June (1961)
and Evolutions, Cahier Technique Merlin Gerin, Technical Papern∞ 173, September (1995).
5. Jullien Francois, Heritier Isabelle, The IT Earthing System(unearthed neutral) in LV, Cahier Technique Merlin Gerin,Technical Paper n∞ 178, June (1999).
6. Lacroix Bernard, Calvas Roland, Earthing systems in LV, CahierTechnique Merlin Gerin, Technical Paper n∞ 172, January (2000).
7. Earth leakage protection – The low voltage technical newsletter,Schneider Electric.
8. ‘si’ type earth leakage protection – A new range for ‘sensitiveinstallations’, Schneider Electric.
