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Zero-sequence components in unit-connected generator

with ungrounded neutral during ground-faults

M.Fulczyk

ABB Corporate

Research

Krakbw.

Poland

Abstract: An analysis of voltage zero-sequence components in

generator during normal operation (pre-fault conditions) and during

ground-faults in stator winding of generato r with ungrounded

neutral is presented. The changes in voltage zero-sequence

components of fundamental frequency and

3d

harmonic during

generator pre-fault conditions, ground-faults in generator stator

winding and ground-faults in power system are shown. The analysis

was done for generator with ungrounded neutral for different values

of the generator and main transformer parameters and additional

capacitance to ground

of

the generator breakers. Different

resistances of a breakdown channel and different locations

of

the

ground-faults in the generator stator winding were considered in

analysis. It has

been

found that

in

the generator with ungrounded

neutral, the parameters of generator and transformer, additional

capacitance to ground of the generator breakers, phase

of

interrupted

arcing ground-fault (level of the fault resistance) and the ground-

fault location have a substantial influence on the zero-sequence

voltages feeding different generator protection systems. The voltage

zero-sequence components

in

generator neutral and the distribution

of 3d harmonic voltages in generator stator winding are determined

mainly by a l l these parameters.

Keywords: generators, grounding, protection, windings.

I. INTRODUCTION

The ground-faults in the g enerator stator windings a re very

dangerous for the unit-connected generator. These faults are

the most frequent causes of dam age

to

the stator winding of

the unit-connected generator and also the direct cause of

phase-to-phase faults. Faults that are not d etected can cause,

that fault transforms into phase-to-phase fault what may

immediately damage generator. The additional capacitance to

ground of the generator breakers connected into the zero-

sequence circuit of the unit-connected generator increases the

value of ground-fault current.

So

high current values can

cause very extensive damage to the generator magnetic

circuit. Therefore it is necessary to reduce or even totally

eliminate such dangers. To ensure maximum safety for the

generator stator magnetic circuit, a system grounding the

generator neutral should operate with a ground-fault

protection covering 100 of the ge nera tor stator windings.

Considering the results of investigations of the ground-

fault processes and the results of analysis of failures of

currently used unit-connected gen erators it is thought that the

ground-fault protections of the stator windings should detect

ground-faults at any point of the winding, including the

generator neutral [1,2,3]. Moreover, in order to m inimise the

possibility of improper operation

of

the generator ground-

fault protection system, the particular types of protections

forming this system should use different excitation

parameters [4,5,6]. Additionally it would ensure maximum

redundancy in protection for th e generator stator.

By influencing the parameters of the sy stem grounding the

generator neutral it is possible to create conditions under

which erosion of the magnetic circuit caused by a ground-

fault arc is insignificant or is even totally eliminated and

ground-fault overvoltages are not dangerous to the stator

main insulation. Then the occurrence of phase-to-phase faults

in the generator circuits, if the ground-fault protection

operates property, is practically impossible.

The grounding methods of the generator neutral and the

capacitance to grou nd of the generator breakers can imp rove

the operating conditions of the particular ground-fault

protection schemes. Howev er these parameters influence the

level of voltages and currents in generato r neutral and in the

breakdown channel at ground-fault location. To ensure the

proper operation of ground-fault protection schemes it is

necessary to know the levels of these voltages and currents

and to determine the relation between elements grounding

neutral and voltages and c urrents feeding these systems.

In this paper, the influen ce parameters of the generator and

transformer, the additional capacitance to ground of the

generator breakers on voltage and currents zero-sequence

components in generator with ungrounded neutral was

determined. The analysis of zero-sequence components was

carried out for unit-connected gene rators of power up to 1110

MVA.

The voltage zero-sequence components feeding the

measuring element of the protections were determined at

ground faults along the whole length of the generator stator

winding.

Results of experimental studies of ground-fault

phenomena, carried out in real conditions on the unit-

connected generators, were taken into accou nt in the analysis.

This refers mainly to the fault resistance of the breakdown

channel in the main insulation of the generator stator

windings during interrupted arcing ground-faults. The

breakdown channel resistances in range from

1052

(the

resistance in the quasi-galvanic phase of the ground-fault

after carbonisation of the organic parts in the main insulation

of the stator winding) to

2k 2

(the resistance of the ground-

fault during first

arc

ignitions) were assumed in analysis

W I

II.

GENERATOR-TRANSFORMER

UNIT

SCHEME

The analysis of the zero-sequence voltages feeding the

generator protection system s were carried out fo r a generator-

transformer unit equipped with the additional capacitance to

ground of ge nerator breakers and without this capacitance.

0-7803-6338-8/00/$10.00(~)2000EEE

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The voltage zero-sequence compon ent feedin g the m easuring

element of the generator protection system at a ground fault

in the generato r stator and in power system were determined

on the base of the system equivalent scheme shown in F ig.1.

In

this scheme the generator neutral is not connected with

ground. The simplifications introduced into

t h i s

scheme do

not have any significant effect on the voltages feeding the

measuring element of the prote ction system.

CT1-2

Rg

.

Sche me of u nitconn ected generatorwth ungrounded neutral;

G-equivalent phase capacitance of statorwndi ngto grou nd , C g c - p h a s e

capacitanceto ground seen

h m

enerator erminal, C@-phase apacitance

to

groundof generator breaker,N-neutral, Rrfault resistan ce,

CT,.Z-capacitance

between

low and high Windings

of transformer,

III.

ZERO-SEQUENCE VOLTAGES

IN

GENERATOR

A.

Voltage zero-sequence com pone nt in generator neutral

during faults in gen erator stator winding

The voltage zero-sequence in generator neutral in

primary

winding of voltage transformer can be determined using the

simplified equivalent scheme of the unit-connected gen erator

shown in

figure

2.

Fig.2 . Equivalent scheme of generator during ground-fault;; ,-generator

phase voltage, x-lw ation of ground-faultin generator stator winding (01).

U~ zemsequenceoltage in generator neutral,

Zero-sequence component IJ ) in secondary winding of

grounding transformer con nected in generator neutral (Fig.3)

can

be

recalculated using formula (1).

where:

Un

.B -

ratio of grounding transformer

(

-

.

100

&-

Fig.3. Scheme of generator wth grounded neutral

The effective value of the voltage zero-sequence component

in generator neutral, when there is no element connected

between generator neutral and groun d, during steady state of

the ground-fault in stator winding after carbonisation of

insulation organic parts is determined by follo wing relation:

1

R f +

where:

w

-

pulsation for fundamental frequency.

It is seen from

(2)

that the voltage zero-sequence

component in generator neutral depends only on the

parametas of generator and parameter of elements seen from

generator terminal (transformer, generator breaker and

elements connected directly to the buses con necting generator

and transformer). But it also significantly depends on the

resistanc e of breakdown c hann el and ground-fault location in

the stator winding. These zero-sequence components in

secondary winding of grounding transformer are shown in

Fig.4. For low fault resistance this voltage depends linearly

on number of shorted coils during fault (fault location). In

case of fault w ith h igher fault resistance

this

is also linear

relation but maximum voltage reach only part of total zero-

sequence component in generator.

gmundfautt location [XI

gmun&fault

resistance

[Ohm]

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ground-fault ocation [ I

ooo-0

ground-fault esistance [Ohm]

c)

generator capacitance

IF]

gmund-fault resistance [ Ohm]

Hg.4. Voltage zero-sequence component n generator neutral during ground-

fault in generator statorwinding; a) C .2

p

C e O

@;

b)C 8 . 6

@,

C .4 @; c) cgt=o.2 F,x= l

During faults at particular locations in the stator winding the

voltages take higher values in the systems with lower total

capacitance to ground (Fig.4a,4b). For lower capacitance to

ground the voltages in generator neutral de crease linearly as

fault resistance increases, whereas for higher cap acitance this

decreasing is not linear (Fi g.4 ~).

B. Voltage zero-sequence component in generator neutral

during faults in power system

During faults in power system zero-sequence component

may transform

from

power system to unit-connected

generator through capac itance between winding of main

transformer. The level of transformed voltage is determined

by parameters of particular component forming this system.

The voltage zero-sequence component in generator neutral

during ground-fault in power system can be determined using

the simplified equivalent scheme of the unit-connected

generator shown in figu re 5

Rg.5.

Equivalent scheme of generator during ground-fault

in

power system;

Uos-zero-sequencevoltage

in

power system recalculated

to

gener tor

voltage, Uw-z ereseque nce voltagein generator neutral

The value of the voltage zero-sequence component in

generator neutral during faults in power system through low

resistance can be determined using following relation:

After some sim plification can be transformed to following

form:

Then the zero-sequence component in generator neutral

transformed from power system is given per units in relation

to the zero-sequence voltage at fault location in power system

recalculated to generator level.

It is clearly seen from (4) that in the generator with

ungrounded neutral the voltage zero-sequence component in

generator neutral durin g faults in power system depends only

on the capacitance to ground of generator, transformer,

generator breaker and elements connected directly to the

buses connecting generator and transformer.

These zero-sequence com ponen ts in generator neutral

are

show n in Fig.6. For lower capacitance to ground of generator

the zero-sequence voltages take higher values. Influence

of

capacita nce between windings of main transformer is more

visible in genera tors with low er capacitanc e to ground

equipped with generator breaker without additional

cap acit anc e to groun d Oi;ig.6a, 6b). In gene rato rs with hi ghe r

capacitance to ground

of

stator winding the additional

capacita nce to ground of generator breakers do not influence

analysed phenomena (Fig.6b). Zero-sequence component in

generator with ungrounded neutral transformed from power

system during faults in this system assumes the lowest values

in generators with high ca pacitanc e to ground of stator

winding and additional capac itance to ground of generator

break ers (Fig.6b).

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I

breaker capacitance

F1

transformator capacitance [F]

4 0.5

Y 1u

transformator capacdance [F]

Hg.6. Voltage mo-sequence

in

generator neutral during ground-fault n

power

system;

a)

C d . 2

pF,

b) w . 6 F

x 10.

breaker capscdance

[F]

C .

Third harm onic voltage in generator neutral

In the unit-connected generator the total 3rd harmonic

phase voltage between the generator neutral and generator

terminals is a vector s u m of the voltages induced in the

particular bars of a one pha se of the g enerator stator winding.

Because the g enerator current influences the resolution of the

curve of the magnetic induction in the generator air-gap, the

3rd harmonic voltages also change in adequate proportions

with changes in generator load [6,7,8]. The third harmonic

voltages in gener ator neutral and a t generator terminals were^

determined on the base of detailed analysis of the 3rd

harmonic voltag e distribution in the generator stator windings

[3,4]. The analysis was made using an equivalent scheme of

the unit-connected generator for third harmonic component

during pre-fault conditions and during ground-faults in the

stator winding (fig.7). The voltages in generator neutral and

at its terminal during generator normal operation (pre-fault

conditions) and during gro und -faults in stator winding were

analysed considering all param eters of the unitconnected

generator having a significant effect on the value of the

voltage 3rd harmonics. Figure 7 shows the simplified real

distribution of 3rd harmonic voltage vectors in the generator

stator winding during pre-fault conditions and during ground

fault in stator winding at point x through fault admittance

L

Hg.7 Equivalent scheme of un itcamaected generator for

3rd

harmonic and

distributionof voltage 3rd harmonic vectors in generator stator; a ) during

normal

operation

b) during ground -fault at point x; Emi-total3rd

harmonic

voltage betw een neutral and termnals E3n-voltages 3rd harmonic

between neutral and fault locatio n and between fault location and terminals,

Ym, Yn-admittauce

in

neutral and at

terminals

Yrfault adm ittance at

ground-fault ocation, N,T-generator neutral and

terminal.

The equivalent admittance Y N 3 and

Y n

of fault admittance

connected in generator neutral and at generator terminals can

be evaluated using the fo llowing relations:

1

3

xT3y G f

+j(2w3c,

- 3 c z

6)

where:

G -ph ase conductance of g enerator stator winding,

- pulsation for 3Tdarmonic.

When determining the 3rd harmonic voltages between

generator neutral and ground-fault locationx (or ground

z

it

is necessary to take into co nsideration the real resolution of

t h i s

voltage along the stator winding and non-linear

dependence on the number of shorted coils [3,4,7]. Therefo re

admittances of breakdown channel which are splinted into

two separated equivalent admittances

(YN3f

and

Ymf)

can be

calculatedfrom he follow ing equations:

Y N 3 f =[1-k(x)I.Y,

9

x3Tf = d X ) - x f 9 7)

where:

k-coefficient reflecting non-linear distribution of 3d harmonic

in generator stator winding:

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0 x 5

0.5

0.5

.

8)

- x) =

p 2 5 (1

-

ix'2n)

0.25. (3 eJ'Xzn)

1) generator normal operation (p re-fau lt conditions)

The relative value of the third harmonic voltages during

generator normal operation were determined in relation

to

the

total third harmonic voltage induced in generator stator

winding which is represented by

U

voltage between

generator neutral and terminals. The relative value of the

voltages U,, in generator neutral dur ing pre-fault conditions

can be determined using formula:

In Fig.8 the voltage

3d

harmonic in generator neutral during

normal operation (pre-fault conditions) for different

capacitance to ground of generator stator winding insulation

and capacitance to ground of generator breaker is shown. It is

clearly seen that in generator without fault the

3d

harmonic

voltage in generator neutral is higher then 50% of total

3d harmonic voltage induced in generator stator windings.

Additionally it can be noticed that capacitance to ground of

the generator breakers increases this voltage for any

generator, but this influence is m ore visible for the generators

with lower capacitan ce to ground of the stator winding

insulation (Fig.8).

1

7 0 8

-

.-

f O

g 0.4

m

0.2

0

breaker

capacltance [Fl

10

generator capacltance

[FI

Fig.8. Voltage

3d

harmonicin

generator neutral during normal operation

2) ground-fault in stator winding (fault conditions)

During faults in th e generator stator the distribution of 3d

harmonic voltages is mainly influenced by th e fault resistance

(admittance) at fault location. Then the 3d harmonic in stator

winding between the generator neutral and fault location

x

(or

ground Z was determined applying relation:

In F ig 9 the voltage 3d harmon ic in the neutral of generator

during ground faults along the whole length of the stator

winding for different capacitance to ground of generator

stator winding insulation and capacitance to ground of

generator breaker is show n. For low fault resistan ce this

voltage varies from minimum at faults close to the generator

neutra l to maximum du ring ground faults at the terminals. At

these faults 3d harmonic voltages in the generator neutral

reach values of total 3d harmonic voltage induced in the

generator stator winding. Th e fault resistan ce influences more

significantly

3d

harmonic voltages in generators with lower

capacitance to ground of stator winding insulation and

without additional capacitan ce to ground of th e generator

break ers (Fig.9).

1

-

2 0 8

-

.-

E 06

0 4

B

0 2

0

2m

m

m

m

>

'0 -

fault location I

ault

resistance

[Ohm]

b)

1

3

0.8

a

y

f

0.6

E 0.4

-

L

m

m

g 0.2

0

p m

im

0 -0 -

fault

location I

ault resistance Ohm]

Fig.9. Voltage

3d

harmonic

in

generatorneutral during faults;

a) CF0.2P C g 4

PEb) W 6PE d . 4 PF

The 3d harmonic in generator neutral changes significantly

during fault in stator winding. Fig.10 shows the absolute

differe nce in voltages 3rd harmonic in generator neutral

during normal operation and during fault in stator winding for

differe nt capac itance to ground of generator stator and

capac itance to ground of g enera tor breaker at different fault

locatio ns and fault resistances. The maximum d ifferen ces in

these voltages occur for low fault resistance in neutral of

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generator with higher stator capac itance to ground and

equipped with the generator breakers with capacitance to

ground. In such systems during faults till 20 of generator

stator length these differences reach

50

of total

3d harmonic voltage induced in ge nerato r stator (Fig.lOb).

a)

1

-

2o.e

-

0.6

0 4

c

m

-o

2

0

0

0

fault location [ ]

fault resistance [Ohm]

b)

1

-

2 0 8

50 2

-

6 6

0.4

m

0

0

0

w

faun location

I

fault resistance [Ohm]

Rg.10. Absolute difference in voltages 3 harmonic in generator neutral

during

norm l

operation and during ground-fault in generator;

a) C ~ 0 . 2

p

&

I@

b) C .6

p

6 . 4

V.

CONCLUSIONS

The zero-sequence voltage transformed from power

system

to

generator during faults in power system takes

higher values for generators with lower capacitance to

ground. The minimum values are reached in the

genera tors with high capa citance to ground of stator

winding and additional capacitance to ground of the

genera tor breakers.

The

3d

harmonic voltag e in generator neutral depends on

the capacitan ce to gro und of th e generator breakers and

capacita nce to ground of the stator w inding insulation.

The fault resistance in fluenc es more significantly neutral

3d

harmonic voltages in gene rators with lower stator

capacita nce o ground and without additional capacitance

to ground of the generator breakers. The change in the

neutral 3d harmonic caused by fault takes maximum for

low resistance fault in the neutral of generator with

higher stator capa citance to ground and equipped with

the generato r breakers with capacitance o ground.

VI. REFERENCES

J.W.Pope, A comparison

of

100% stator ground fault protection

schemes for generator stator

windings ZEEE Trmuction on Power

Apparafus andSystems, vol. PAS-103, no.4. April 1984. pp.832-840.

X.G.Ym, O.Malik, G.Hope., D.Chen, Adaptive ground fault

protection schemes for turbogenerators based

on third

harmonic

voltages,

ZEEE Tr-ctzons on Power Del ive ry,

~01.5,

no.2,

1990,

SShiwen, S.Binhua, Analysis of ground protection

of

unit connected

generator using third harmonic,

Fourth Znternational Conference on

Developments

UI

ower Protection,

Edinburgh, UK 1989, pp.254-258.

W.W.Xie Xiaoping, Zxiling, New developments of third harmonic

ground fault protection schemes

for

turbine-generator stator

windings,

Fourth Znternational Conference

on

Developments in

Powe r System Protection,

Edinburgh, UK 1989, pp. 250-253.

MZelichowski., M.Fulczyk, Influence

of

voltage transformers on

operating conditions of ground-fault protection system for unit-

connected generator,

Inremational

ournal of

Electric Power &

Energy Systems, ~01.20, o .5,1998, pp.313-319.

J. Basilesco, J. Taylor.

Report

on methods for earthing of generator

step-up mansformer and generator winding neutrals as practised

throughout the word. CIGRE. N0.121,pp.86-101,1988.

M. Zelichowski. Erosion du circuit magnetique des stators de

turbogeneratem pendant les courts-circuits a l terre. Revue

Electricite.vol. IX o.12

pp.

226-234 980.

G.W.Buckley, R.L.Schalke. Performance of

third

harmonic ground

fault protection schemes for generator stator windings.

IEEE

saction on Power Apparatus and Systems, vol. PAS-100, No.7,

pp. 595-603.

pp. 3195-3202,1981.

W IOGRAPHY

In genera tor with ungrounded neutra l, the parameters of

Marek Fulczyk (1968) received the M.Sc. and

generator and transformer,

additional capacitance

to,

Ph.D. degree in Elect~icalEngineering from the

ground of th e generator breakers, fault resistance and the

Wroclaw University of Technology, Poland in

1993 and 1997, respectively.

In

1997 he joined

groun d-fault location have a su bstantia l influence on the

ABB Group

as

a research scientist. Now he is a

zero-sequence componen ts in generator during pre-fault

leader of Engineering Systems

&

Automation

cond itions and during ground-faults.

Group at ABB Corporate Research in Krak6w,

The voltage zero-sequence component in generator

Poland. His fields of interests include power

system protection, voltage stability,

neutral during faults in stator sig nificantly depe nds on

collaborative technology, 3D mcdelling and

the fault resistance and g round-fault location in the stator

simulations

of phenomena in power system.

winding. For low fault resistance this voltage depends

(ABB Corporate Research, Starowisha 13A,

mainly linearly on fault locatio n, whereas for higher fault

31-038 Krakow, Poland,

resistance it depends

also

on the total capac itance to

Phone 4 8 2 14295027,

ground of th e system.

Fax.

8- 12 I4224906, E-mail:arek.fulczyk@pl.abb.com)

.

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