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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:[email protected])
.
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