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A New Selective Transient Protection for the Ground

Fault of Large Unit-connected Generators

Bi D a q ia ng , W a ng We i j i a n , Gui Lin a nd Wang

Xiangheng

Abstract-Based on the mu lti-loop circuit method of A C

machine a mathematic model is established for the large

unit-connected generators, namely the outlet terminal o f

generators connected to a common busbar, and the transient

zero-sequence current and voltage of these generators are

simulated and analyzed at different grounding location. Further,

making use of the wavelet transform sensitively detecting the

singularity o f signal, the properties o f their value of wavelet

transform are analyzed, and a selective transient protection is

proposed according to the sign and value of the modu lus maxima

o f wavelet transform. The results o f simulation and experiment

verify the new p rotection can correctly detect the gene rator with

ground fault, and discriminate between the internal and external

Faults.

Index Terms--generator, 'ground fault, zero-sequence

componen t, transient protection, wavelet transform

1. INTRODUCTION

enerator stator is most frequently subject to the damage

G aused by the ground faults, and they often precede other

worse winding faults such as turn-to-turn and phase-to-phase

faults, therefore ground-fault protection is the key element of

the protection system for generators [4]. At present, the

protection sch emes against the ground fault mainly include the

zero-sequence fundamental voltage scheme, the third

harmonic voltage schem e and subharm onic voltage injection

scheme, their combination

can

provide 100 coverage of

stator windings.

But these schem es aren't able to discriminate which

generator suffers the ground fault if several medium or

small-sized generators are connected to the common busbar.,

And they also can't distinguish between an internal and

external .ground fault for

a

unit-connected generator. To avoid

the unnecessary stopping of no-fault generator in the large

unit-connected generators, it is desired for the protection

scheme w ith selectivity.

In fact, when a fault occurs, some relevant signals appear

singular. These transient signals contain extensive fault

information such as type, direction, location and sustained

time. The information covers the entire frequency domain

including DC, power frequency and high frequency [l].

Extracting and utilizing these transient information can

provide a new way to resolve the problem about the selectivity

of protection scheme.

This paper establishes a mathematic model for several

circuit method of AC machine, transient zero-sequence current

and voltage of these generator are simulated and analyzed

under this model o f connect at different grounding location, a

selective protection

is

proposed by using the wavelet

transform to extract these transient information.

11. ESTABLISHINGHE S l M UL Al l ON MODEL

A. Assuming

Condi t ions

The diagram of large unit-connected system

is

showed in

Fig. 1. In the procession of establishing the simulation model

of whole system, self-conduction and mutual-conduction of

generator windings are calculated by using the multi-loop

circuit method of AC m achine [5]. Th e distributed capacitance

of stator winding to ground is substituted by several lumped

capacitances with uniform distribution along the stator

windings, and the additional capacitance

at

terminal, low side

winding capacitance of step-up transformer to ground and the

grounding impedance on neutral are considered. The step-up

transformer is represented by its equivalent circuit.

At present the neutral of generator mainly grounded

through Petersen coil or resistor, the variation o f stator current

caused by ground fault is so small that the damping winding

can be ignored during simulation, which means a generator

with damping windings can be regarded as a non-damping

windings generator during the ground fault.

L

L

Fig. I. he diagram oflarge unitsannected

system

B.

Establishing

the

state

equations

Choosing the current of inductance and the voltage of

capacitance as the state variables, the last form of state

eouations

is

derived in

n

trix

as

follows.

l ~ ~ ~ ~ ~ ~

generators connected to a comm on busbar using the multi-loop

The

authors are with the department of Electrical E ngineering and Applied

Electronic Technology, Tsinghua University, Beijmg 100084 China (e-mail:

bidaqiang990mails.tsinghuaeducn .

where,

p M + r

A

I

=[-

B

F][U]+[ ]

0-7803-7459~2/0~ 17.002002

EEE

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p

is the differential operator;

M C and r is the inductance, capacitance and resistance

matrix of .generator winding s, terminal and neutral

respectively;

A

and B are the associated matrix between node and

branch;

U s the vector of voltage source;

and

U

re the vector of state variables;

F is the associated matrix of between the fault node and

branch, under normal condition

F

=

O]

when ground fault

occurring, a zero element on the leading diagonal ought

to

be

substituted by

- l / R g

according to the fault location, here

Rg

is the transient resistance.

111. SIMULATION

ND

ANALYSIS

Using the above mathematic model, this part simulates a

system, which includes two same generators (No.1 and No.2)

connected to the common busbar. Their main data are as

follows.

Rated power lZkW, rated voltage

400V,

pole pair number 2,

parallel branches per phase 2, coil number per branch 7,

excitation current under no-load and rated voltage 8.24 A, rated

excitation current 22.6A. For genera tor 1 and generator 2, The

capacitance of stator windings to ground is 0.12 P F and

0.08

P F and the additional capacitance at terminal is 0.03 P F and

0.02

P

F respectively. The neutral is grounded through the

Petersen coil and resistor with small value in series. The

sampling frequ ency is

IO .

Two typical faults, occurring inside and outside generator,

are simulated and analyzed. Defining

io

and io2 is the

zero-sequence current of generator

1

and ,2, and uo is the

zero-sequence voltage of the busbar.

Fig. 2(a) gives the simu lation results of

io o

and

u

when

a ground fault takes place at 42.9 of the first branch of A

phase of generator 2 close

to

the neutral via

5000

transient

resistance. It is showed the tran sient zero-sequence current and

voltage is discontinuous, and the direction of change between

h a n d

io2

is contrary after the ground fault occurring. Because

the zero-sequence current through the outlet terminal of the

fault generator is equ al to the all grou nd capacitance current of

external voltage network expect that of itself, the transient

zero-sequence current of the fault generator is greater than that

of the normal generators, which can be seen from Fig. 2(a).

Fig. 3(a) shows the s imulation results of an external ground

fault at

K,

n Fig.

1

through

500 R

transient resistance.

io

and

io change in the same direction, and uo produces a sharp

change during the transient. Because the ground fault

is

outside generators,

u

has a larger variation than normal

condition.

In

this case, the zero-sequence currents through the

outlet terminal of generators are the ground capacitance

current

of

themselves individually.

IV.

SELECTIVE ROTFCTIONSCHEME For GROUNDAULT

A . Analysis of the wavelet transform

Conventional Fourier transform can only give the

information in frequency domain and has no ability of time

resolution so that it is not fit to analyze the transient signal,

and cant detect the singularity of signal in time field. Rather

than as a developing time-frequency method, the wave

transform is a powerful

tool

in proces sing the transient signal

because of its ability to extract information from transient

signal simultaneou sly in both time and frequency domain.

According to the s ingularity detection theory of the wavelet

transform [2] and [6], if a sudden change or discontinuation of

signal appears, its wavelet transform values have the local

modulus maxima, which

carry

some important information.

The fault type and location can be discriminated by

comprehensively using the sign and value of these local

modulus maxima. This paper chooses Daubechies 5

orthogonal wavelet as the mother wavelet and adopts the.

atrous algorithm [3] to analyze the zero-sequence components

by the detection of singularity with m ultiscale transforms, and

the values of wavelet transform at scale 2 are used in the

protection scheme at the sampling frequency IOkHz. Here

WTI, and WTUo are defined

as

the values of wavelet

transform of zero-sequence current of generator

i

and

zero-sequence voltage of busbar respectively.

During the normal condition, the variation of WTI WTI

and W Uo is very flat in Fig. 2(b). But when the ground

occurring inside generator 2, their values sharply change and

there are the modulus maxima. Compared with the results of

transform, the modulus maxima of WTI a n d . WT12 occur

simultaneously and have the same sign. The value of the

modulus maxima of

WTI

is more than that of W I Z .

When the ground fault happening outside generators, there

appear the modulus maxima of the zero-seq uence components.

Comparing the results of transform, the modulus maxima

occur simu ltaneously but their signs are contrary in Fig. 3(b).

B.

Elective p rotection scheme

Through the above analysis, the elective protective scheme

is advancedas follows,

1) Choose the values of the wavelet transform of the

zero-sequence current and voltage of generators at scale 2 as

the characteristics of ground fault at the lokHz sampling

frequency;

2 ) Calculate the modulus maxima WTI,,k of zero-sequence

currents (including the positive and negative,

i = l - n

is the

number of generator, k

is the

position of maximum . If the sign

of mI h of generator i is contrary to that of all other

generators, then the ground fault occurs inside a generator;

(3)

Under the condition 2, if the number of generators

connected together is more than 2, then generator

i

is the fault

generator. When there are only

two

generators, if

IWTI I

is

greater than that of the other generator additionally, then the

generator i is the fault generator;

(4) If all of sign

is

identical amon g WTIj,i

i= l -n ) ,

then the

ground fault is external;

5 ) To improve the reliability of detecting external faults,

supplement

I W T I I , k I > W T I , , .

For the external ground fault,

the range of the zero-sequence voltage variation

is

very large,

so that the value of W U,*,is easily

set;

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'I

021

(a) Transient zero-sequence comp onents

4

os

I

019

0 2

021 022 on ou

2s

t ( 3 )

(b) The results of wavelet transform

at

scale

2

Fig.

2 The simulationresultsof

an

internal pound fault

(6) To enhance the reliability of whole protection scheme,

only after more than three points of the modulus maxima

continuously meet the conditions, the protection scheme

decides whether and where the ground fault happens.

V. RESULTS O F WERIMENT

To verify the above elective protection scheme, some

experiments were carried on a large unit-connected system

with

two

generators, an d its diagram likes Fig.

1.

The main

data of

two

generators are

as

follows.

Generator]: rated capacity

5.0kW,

rated voltage 400V. pole

pair number 2, parallel branches per phase 1, excitation

current under no-load 0.162A, the capacitance of stator

windings to ground 0.048 E Ge nera tor2 rated capacity

3.75kW, rated voltage 400V. pole pair number 2, parallel

branches per phas e 1, excitation current under no-load 0.357A,

the capacitance of stator windings to ground 0.036 E

(a) Transient zero-sequence com ponents

-319

62 O i l

oi2

o

ob

ais

C 4

(b)

The results of wa ve la transform at sede 2

Fig 3

The imulstlon

resulls o fan external

p o u n d

fault

There are 6 taps (25 . 33.3 , 41.7 , 45.5 , 47.7 and

49.2 close to the neutral) on the windings of phase A . The

neutrals of two generators are grounded through the potential

transformer, and their outlet terminals are connected to 800V

high voltage system via a step-u p transformer. The record data

is via two level current and voltage transformer at lo&

sampling frequency.

The zero-sequence current of two generators and

zero-sequen ce voltage of busbar are given in Fig. 4(a) when a

solid ground fault

occurs

at terminal of generator

2

K2

n Fig.

1). Fig. 4(b) shows the results of the wavelet transform, their

properties are consistent with th e analysis of simulation results

in part IV. The values of wavelet transform at the time of

ground fau lt are distinctly higher those at normal time or the

zero-sequence components. The sign of the

modulus

maxima

between io] and io2is contrary, and the am plitude of the latter

is higher than that of the former at scale 2.

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(a) Transient zero-sequence components 8) Transient zero-sequence omponents

2-

In Fig. 5(a) the location of ground fault (K, in Fig. 1)

is

external, correspondingly

it

can be seen that at the scale 2 the

sign and position

of

the modulus maxima of iol accords with

that of iO2from ig. 5(b), and

u0

has modulus maxima too.

lfthe fault location

is

close to the neutral

K ,

n Fig.

I ,

the

scheme still can work, because the deviation voltage exists on

the neutral, and the zero-sequence current and voltage still

produce the transient w hen a ground fault occurring.

VI. CONCLUSIONS

Usual protection schemes against the ground fault cant

realize the selectivity protection for the large unit-connected

generators. Based on the simulation and analysis of this

connect model established by the multi-loop circuit method, a

new and selective protection scheme

is

presented. It uses

Daubechies

5

orthogonal wavelet

to

detect the singularities

of

zero-sequence current and voltage, further utilizes the sign and

amplitude

of

the modulus maxima

to

realize the selective

protection. The experimental results verified the s electivity

VII.

RFERENCES

Periodicals:

[ ]

Bo Zhiqian, Transient based protectio- new generation

of

power

system protection, Power Syilem

Technology

~01.20, a.3, pp. 34-36,

1996.

Stephane

Mallet

and WenLiang

Hwang, Singularity

detection and

Processing with uwelets,

IEEE T r am ln orormorion 7heory. Vo1.38,

Na.2,pp. 617-643,Mar1992.

Mark

J.

Shenra, Wedding the ~ U O Y S nd the Mallet algorithm, IEEE

Trow.

SignalProcessing, Va1.40,No.10, pp. 24642482,,Oet.1992.

121

[3]

Books:

[4]

[SI

Wang Wcijan, Principle

and

mpplication

o

electne power

equipmenr

proleclion

Beijing: China Electric

Power

Press,

1998,

pp. 200-203.

Gao Jingde, Wang Xiangheng,

Li

Fahai,

Anolpir of

altemrive cumm

machine

and

{ s

system Beijing: Tsinghua University

Press, 1994,

Yang

Fusheng,

7he

engineering

mly>is

and application

o

wovelet

lronsform,Beijing: Science

Press,

2000, pp.

145.157,

pp.l-43.

161

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VIII. BICGRAPHIES

Bi Daqiang was born

in

Jilin Province, China,

on December

20,

1973. He received his M.S.

degree in Electrical Engineering from Shenyang

University of Technology

in

1996 Now he is

pursuing his Ph.D. at Tsinghua University His

field of interest mainly is the relay protection of

main equipment in

power

system.

Wm g Weijim was barn in Jiangru Province

of

China in 1930. He graduated

from

Electric

Engineering DeparUnent of Tsinghua University

in

1955. Now he is a professor in the Department

of Electrical Engineering, Tsinghua University

He has

been

researching and teaching on

the

r e l w protection for large electric machine

Cui Lin was

barn in

Anhui Province, China, on

July 16, 1974. He received his

B.S.

and M.S.

degree in Electrical Engineering from Hefei

University of Technology in 19 and 1999.

Now he

is

pursuing his Ph.D. at Tsinghua

C ~ ~ $ s v -

University His research interests are the analysis

I?

. . and relay protection of internal fault of

-.

t large-sized generators

W m g Xiangheng

WBS

born in Anhui province of

china, an October 19, ,1940. He gradua ted from

Electrical

Engineenng Depanment of Tsinghua

University in 1964. He worked

at

Dongfang

Electric Machine Works

in

Sishuan Province

from 196 8 to 197 8 and obtained his P.h D

diploma from Tsinghua University in 1986. He s

currently

a

professor at Tsinghua University His

presently conducts research

on

the analysis and

wnbd

for electric machine and its system, fault

analysis and i ts protection for electric machine.

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