m6-6307

TATA CONSULTING ENGINEERS SECTION: TITLETCE.M6-EL-GL-816-6307 SYSTEM NEUTRAL EARTHING SHEET (i) OF (iii)

REV.NO R0 R1 R2 ISSUE

INITIALS SIGN INITIALS SIGN INITIALS SIGN INITIALS SIGN R2

PPD.BY MNB -Sd- RRD -Sd- RRD -Sd-

CKD.BY DVM -Sd- BSK -Sd- BSK -Sd-

APP.BY VNM -Sd- UAK -Sd- UAK -Sd-

DATE 76-04-02 98-09-08 99-05-14

FORM NO. 020R2

DESIGN GUIDE

FOR

SYSTEM NEUTRAL EARTHING

TATA CONSULTING ENGINEERS73/1, ST. MARKS ROADBANGALORE 560 001

FLOPPY NO : TCE.M6-EL-FP-DOC-005FILE NAME : M6-6307.DOC

TATA CONSULTING ENGINEERS SECTION:REV. STATUSTCE.M6-EL-GL-816-6307 SYSTEM NEUTRAL EARTHING SHEET (ii) OF (iii)

ISSUER2

FORM NO. 120 R1

REVISION STATUS SHEET

REV. NO. DATE DESCRIPTION

R1 1998-09-08 a) Annexure-1 & 3 included

b) rtificial grounding methodsdesignated as Annexure-2

c) Generally revised

R2 1999-05-14 Table-I Sl.No. 1.d and 1.e revisedand a note is added in annexure-1

TATA CONSULTING ENGINEERS SECTION: CONTENTSTCE.M6-EL-GL-816-6307 SYSTEM NEUTRAL EARTHING

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CONTENTS

__________________________________________________________________________________

SL.NO. TITLE SH. NO.

1.0 SCOPE 1

2.0 DEFINITION OF SYSTEM EARTHING 1

3.0 RECOMMENDED PRACTICE 2

4.0 DISCUSSION 2

5.0 EQUIPMENT RATING 7

6.0 REFERENCES 9

TABLE : 1RECOMMENDED TYPES OF

SYSTEM EARTHING 10

TABLE : 2RECOMMENDED GENERATOR EARTHINGAPPLICATIONS 12

ANNEXURE-1 SAMPLE CALCULATION OF GROUNDING TRF. 13

ANNEXURE-2 ARTIFICIAL GROUNDING METHODS 15

ANNEXURE-3 16

____________________________________________________________

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1. SCOPE

This design guide covers methods of neutral earthing to be adoptedfor various systems and incorporates calculations for earthingequipment ratings.

2. DEFINITIONS OF SYSTEM EARTHING

2.1 Effectively Earthed

A system earthed through an earthing connection of sufficiently lowimpedance (inherent or intentionally added or both) such that groundfaults that may occur cannot build up voltages in excess of limitsestablished for apparatus circuits, or systems so earthed. The criteriafor meeting these requirements are

i) R0 < X1ii) X0 < 3X1 The coefficient of earthing (cl.2.8) of such systems

< 0.8Where, R0= Zero sequence resistance of the system

X0= Zero sequence impedance of the systemX1= Positive sequence impedance of the system

2.2 Solidly Earthed

A system earthed through an adequate earth connection, in which noimpedance has been inserted intentionally, (i.e. a solid metallicconnection from system neutral to earth).

2.3 Unearthed

A system, circuit or apparatus without an intentional connection toearth except through potential indicating or measuring devices orother very high impedance devices.

2.4 Medium Resistance Earthed

A system where a resistance is intentionally added into the systemearthing connection such that R0 2X0.


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2.5 High Resistance Earthed

A system where nearly the highest permissible resistance is insertedin the earthing connection such that R0 < XCO / 3,Where XCO is theZero sequence capacitive reactance of the system.

2.6 Reactance Earthed

A system where a reactor is intentionally added into the systemearthing connection such that XO < 10X1.

2.7 Earthed for Serving Line to Neutral loads

System so earthed that Z0 < Z1.

2.8 Co-efficient of Earthing

Coefficient of earthing is the ratio ELG/ELL expressed as percentage ofthe highest r.m.s. line to earth power frequency voltage ELG on asound phase at a selected location, during a fault to earth affectingone or more phases, to the line to line power frequency voltage ELLwhich would be obtained at the selected location, with the faultremoved.

3.0 RECOMMENDED PRACTICE

3.1 It is necessary to provide system neutral earthing at each voltagelevel except where a certain voltage level is intentionally keptunearthed.

3.2 Recommended type of earthing for various systems is given in Table-I.

3.3 Recommended generator earthing applications are given in Table-II.

4.0 DISCUSSION

4.1 Location of Earthing Connection

Earthing should normally be provided at the source end. This isnecessary because loads may consist of transformers with deltaprimaries or delta connected motors and neutral point may not bereadily available. Also earthing at load end may result in wide


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variation in fault current depending upon system operating conditionand selective relaying may be difficult.

4.2 Generator Earthing

The maximum stress, which a generator is normally designed towithstand, is that associated with the currents of a three phase fault atthe machine terminals. Because of relatively low zero sequenceimpedance inherent in most synchronous machines, a solid line toearth fault at machine terminals results in a machine winding currenthigher than three phase fault current. Therefore, some impedance inthe generator neutral earthing is necessary.This impedance will beused to limit the generator ground fault currents within the safe limitsas specified /recommended by the generator manufacturer.

4.3 Multiple Power Source

4.3.1 When there are two or more major bus sections, each bus sectionshould have at least one earthed neutral point.

4.3.2 When there are two or more generators,without the generatortransformers at one station, which operate in parallel, only one neutralearthing resistor is some times used. Each power source is thenconnected to the resistor through a neutral bus and neutral switchingequipment (preferably load break switch). It is desirable in such casesto operate with only one generator neutral switch closed at a time toeliminate any circulating harmonic or zero sequence currents (Referfig 1 in Annexure-3). In the case of multiple transformers all neutralisolating devices may be normally closed because presence of deltaconnected windings (which are nearly always present on at least oneside of each transformer) minimises circulation of harmonic currentbetween transformers.However,in the case of an emergency DGset,normally they do not operate in parallel,in which case seperateneutral grounding resistor is necessary for each generator.

4.4 Artificial Grounding

Where one machine only is tied to a bus with feeders requiring apermanent system earthing, generator neutral earthing of any type isusually inadequate,as removal of the generator from service for anyreason also removes the only earth point. This also may be the casewith several machines on the bus where, for most economicscheduling of generation or other reasons, all the generators on thatbus may at some time be shut down. In such cases, when generator


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neutral is inaccessible and for delta connected systems, an earthingtransformer should be provided on the generator bus(one for eachbus section/island). Either a zig-zag or wye-delta transformer may beused. These grounding transformers provide a low impedance pathfor the flow of zero sequence currents associated with ground faultconditions.The impedance of grounding transformer to a normal 3phase current is high so that when no fault exists on the system, onlysmall magnetising current flows in the transformer winding.

4.4.1 Zig Zag Transformers

Zig Zag transformers provide a stable neutral point which makes themsuitable for grounding an isolated system neutral. A commonly usedzig-zag transformer is a 3 phase transformer without a secondarywinding. The neutral of this transformer is then connected solidly orthrough a resistance or reactance to ground. For a given short timerating, a zig-zag earthing transformer is somewhat lower in cost andmore frequently used.Zig Zag transformers with a star connectedsecondary winding can be used as a grounding cum substationtransformer.

4.4.2 Wye-delta earthing transformer

Wye-delta Earthing Transformer is also used for obtaining a neutralpoint for delta connected systems and it is usually recommended forexisting systems. A more readily available star/delta transformer ofadequate rating (probably in stock) may be used.

In case of single phase to ground fault on non effectively earthedsystems, earthing transformer used on delta side of power systemshall be either 3 nos. of single phase transformers used in star/opendelta formation with secondary loading resistor or a 3 phase earthingtransformer with 5 limbed construction. As far as possible singlephase earthing transformer should be recommended for thisapplication. In case adequate space is not available then 3 phase, 5-limb transformer should be specified. For secondary resistor loadedon open delta side of start/open delta earthing transformer,Power lossshall be made equal to 1.5 times the system. capacitive kVAR with a10% positive tolerance on the capacitance of the system. This willform the basis for arriving at the rating of grounding transformer andassociated loading resistor(Refer Annexure-1 for sample sizingcalculations). Each phase of the high voltage winding (i.e. Primary)shall be designed to continuously withstand the system phase-phasevoltage. The calculated max. Zero sequence current in each phase


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(both in primary as well as in secondary) shall be clearly specifiedinstead of specifying kVA rating of the transformer. It is recommendedthat broken delta winding shall have only two terminals.

4.5 Parallel Operation of Transformers

When an earthed star/star transformer with delta tertiary is operated inparallel with similar transformer with neutral unearthed, the rating ofthe tertiary of the former should be checked for being adequate tocarry zero sequence current under fault condition. The usual 33%rating of the tertiary winding may not be adequate.

4.6 Expected Earth Fault Currents

4.6.1 The typical levels of available earth fault current that can be expectedfrom various types of system earthing are as follows :

(a) Solidly earthed, Effectively: Same order of fault current as is earthed, Earthed for servingavailable for the three phase short line to neutral loads circuit current (more than 60% of three phase fault current)

(b) Reactance earthed : Nearly as high as the three phaseshortcircuit current (25% to 60% of3 phase fault current)

(c) Medium resistance earthed: 50A to several thousand amperes depending on the value of

resistance

(d) High resistance earthed : Upto 15A level (current throughresistor more than system chargingcurrent)

4.6.2 The medium resistance method has the advantage of immediate andselective clearing of the earthed circuit but requires that the minimumearth fault current be large enough, usually 400A or more, topositively actuate earth fault relay. High resistance earthing is amethod that can be applied to limit transient over voltages withoutrequiring to add earth fault relays to each circuit.


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4.7 Criteria for Limiting Transient Over-voltages

4.7.1 In resistance earthed systems, the resistor earth fault current shouldbe at least equal to, but preferably greater than the capacitivecharging current of the system.

4.7.2 In reactance earthed system the ratio X0/X1 < 10 where X0 is thezero sequence inductive reactance of the system including that of theneutral reactor.

4.7.3 Where a combination of earthing transformer and neutral earthingresistor is used, the earthing transformer impedance should be lowrelative to the neutral resistance.

R0/X0 2, where R0 and X0 are inclusive of neutral resistor andearthing transformer.

4.8 Arrester Application

4.8.1 Arrester application discussed below is only to bring out arresterratings required with adoption of different methods of system neutralearthing. For actual selection of arrester reference should be made toTCE.M6-SY-G-LA-6702.

4.8.2 The minimum required arrester rating is the maximum operatingvoltage times coefficient of earthing, which is 80% for effectivelyearthed systems, and more than 80% for non-effectively earthed orunearthed systems. The earth fault current in this case is more than60% of three phase fault current.

4.8.3 Systems, which employ some form of resistance system earthing, arenon-effectively earthed system having coefficient of earthing of 100%for arrester application purposes.

4.8.4 Many high voltage systems may exhibit coefficients of earthing as lowas 70% and certain multi-earthed distribution systems may be evenslightly less.

4.8.5 It may not be practicable to provide an earthing transformer of the sizeand impedance necessary to achieve an X0/X1 ratio of 3 or less.However, if the generator neutral is also earthed by means of asuitable reactor, the earthed neutral type (80%) of arrester may beapplied at the machine terminals.


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4.9 System earthing in case of generator circuit breakers

In case of generator system having a generator circuit breaker and ifthe generator circuit breaker is open for some reason, the zonebetween generator circuit breaker and generator transformer primarywill be left ungrounded. In such cases a star-star voltage transformerwith an open delta tertiary winding loaded with resistor should beused.(Refer fig-2 in Annexure-3)

5.0 EQUIPMENT RATING

5.1 Earthing Transformeri) Voltage - Line to Line voltage of the systemii) Reactance XET, Such that

(X0/X1) (System Line to Line voltage in kV)2

XET = ------------------------------------------------------------------ (System Symmetrical 3 phase short circuit level in MVA)

iii) Current - Maximum neutral current

3E E = Line to Neutral voltageIe = ---------------------------- Xep = Reactance of earth return

X1 + X2 + X0 + 3 Xep path

Note : Where resistor/reactor is inserted between the neutral and the earthconnection the transformer should be rated to carry the requiredcurrent for rated time.

iv) Time Continuous rated if it is wired for Alarm

--10 or 60 Seconds if it is wired for Trip depending upon fault clearing time.

5.2 Earthing Resistance

5.2.1 High ResistanceThe resistor is designed to dissipate kW equal to 1.5 times chargingkVA of distributed capacitance continuously. In calculating thecapacitance to earth, the important items are the machine windingcapacitance, the surge protective capacitors and the load carryingpower cable and surge suppressor capacitance for HT Motor feeders,if used. The capacitance to earth of bus duct, lightning arrester and


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transformer low voltage windings is relatively small (1 or 2 percent ofthe total) and need not be considered.

Resistance Value

a) For direct connection to Neutral

RN = 706/C ohms for 50 Hz

b) For connection to secondary of distribution transformer

Rs = 706/C.N2 ohms for 50 Hz

c) Resistor kW rating

P = 0.472V2C for 50 Hz

Where V = Line to line voltage C = Distributed capacitance to earth in mf

N = Turns ratio of distribution transformer

iii) Temperature rise of grounding resistors shall be specified asper IEEE-2

iii) Distribution transformer Voltage rating :

HV : Standard rating near generator Line to Line voltageLV : 240 Volts

iv) Distribution transformer kVA rating :

3 X Resistor kW rating.

v) Time : Continuous rated ,if it is wired for alarm 10 or 60 seconds, if it is wired for trip depending upon fault clearing time

5.2.2 Medium Resistance : (Earth fault current equal to full load current-Iof largest feeder or machine).

EL EL.Ii) Resistor R = --------- ohms, P = -------- Watts

3.I 3


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Where, EL= Line to Line voltage

ii) Time 10 or 60 seconds depending upon fault clearing time

5.3 Reactor

X1 X0 X1 = Generator positive sequence sub5.3.1 Reactance Xn = ---------- transient reactance

3 X0= Generator zero sequence reactance

5.3.2 Thermal Current Rating

3EIE = ------------------------------------

X1 + X2 + X0 + 3 (Xn + Xep)

Note: For X1 of generators and synchronous motors usesubtransient reactance. For X2 of generators, synchronousand induction motors, use transient reactance (X1, X2, X0 etc.refer to source to point of fault reactances).

5.3.3 Time 10 or 60 seconds depending upon fault clearing time.

5.4 Earth Fault Neutralizer

5.4.1 Voltage : Line to Neutral voltage

5.4.2 Current I = 3E/XCO E = Line to neutral voltage XCO = Zero sequence capacitive reactance per phase

5.4.3 Time 10 or 60 seconds depending upon fault clearing time.

6.0 REFERENCES

6.1 IEEE Standard 142-1982 (Revision)

IEEE Recommended Practice for Grounding of Industrial &Commercial Power System.

6.2 CET 1941 A (Xerox 261b) Generator neutral grounding

6.3 IEEE Standard-2.

TATA CONSULTING ENGINEERS SECTION: TABLETCE.M6-EL-GL-816-6307 SYSTEM NEUTRAL EARTHING

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TABLE-I

Sl.No.

System RecommendedMethod ofEarthing

Ref.Fig.

Remarks

1. Industrial Plants

a) Upto 415V

b) Above 415V upto 11 kV

c) Above 11 kV

d) For dieselgenerator setsof voltagerating above415V andemergency DGsets of allvoltages

e) For dieselgenerator setsof voltagerating of 415V

Solidly earthed

Mediumresistanceearthed

Solidly earthed

High resistanceearthed

Solidly earthed

Earth connector should be ofadequate capacity

Limit earth fault current tolargest feeder current rating &permit tripping of circuit


High resistance grounding isrecommended for emergencyDG sets to enable continuedoperation.


2. Power Plants

a) 415V Auxiliarysystems (New)

b) 415V Auxiliarysystems(Extension)

c) Above 415V &upto11 kV

Solidly earthed

High resistanceearthed

Adequate earth fault currenthelps to locate faults andisolating the faulty section bymeans of residually connectedearth fault relays or fuses.

Retain the existing system ofearthing

Limit earth fault current to 1.5times capacitive charging

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Current & permit tripping of thecircuit

3. SynchronousGenerators

a) Unit system &TG rating upto 120 MW

b) Generalmachines and /or feeders onthe bus & TGrating above200 MW

i) Highresistance Earthed

Or

ii) Reactorearthed

Current limited to 1.5 timescapacitive charging current(usually distribution transformerand secondary resistor)

Earth fault current to be limitedto 25%-60% of three phasefault current to prevent serioustransient over voltages (X0

m6-6307

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