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7/24/2019 falla a tierra devanado de campo
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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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