1
Abstract
Based on the analysis of the startup of pumpedstorage generator under motor circumstance, it is found out thatthe conventional 20Hz injection component is influenced by thelow frequency component when ground faults take place in thestator windings for static frequency converter (SFC) startupgenerator. The conventional relay is just simply blocked in casemaloperation during 20Hz working condition. To overcome thedisadvantage, a novel scheme is proposed to improve groundingresistance(R) calculation accuracy and enhance ability to limitimpact of 20Hz component from generator. The simulationresults proved its effectiveness and practicability of the scheme.
Index Terms
Grounding, injection voltage source, protection,pumped storage, windings.
I. I
NTRODUCTION
HE construction of pumped storage power station hasentered a new period of fast development in recent years.It plays an important role in power network because of theeffect of peak clipping and valley filling, frequencymodulation, phase modulation, load adjusting and spinningreserve, which improve the flexibility and reliability of power network operation. The great social and economic benefits produced by pumped storage plants have been recognized bymore and more countries. It puts forward higher requirementsfor the protective relaying because of flexible operation mode.The Subsynchronous overcurrent protection, underpower protection are the special protection of pumped storage units[1]. The starting process of a pumped storage generatormotor unit which is started by a static frequency converter issimulated in the paper [2], the results show that motor havelots of low frequency components during startup. In somecountries, the ground fault protection is blocked in case maloperation, therefore, the stator windings are not protected. Thestartup is frequent during one day and duration is rather long,sometimes between 510 minutes [3]. The generator will bedamaged if ground fault occurs near the neutral point of thestator windings. So it needs to improve the injection protectionscheme to ensure operation safety of generator.In this paper, the 20Hz power injection scheme for groundfault protection of pumped storage generator are analyzed, and
Zheng Zhu, Yuping Lu are with the school of electrical engineering,Southeast University, Nanjing, 210096 P.R. China.(email: zhuzheng317@gmail.com).
the methods employed in starting pumped storage units in the pumping mode are discussed. From the analysis, it isconcluded that the protection is influenced by the lowfrequency component when ground fault occurs in stator windings for SFC startup generator. A new algorithm is proposed from the equivalent circuit. The simulation resultsshow that grounding resistance accuracy is improved by thenovel algorithm when ground fault occurs, and it is notaffected by the 20Hz component from generator, the trippingis reliable and sensitive.II. S
TART

UP
M
ETHOD
A
ND
A
NALYSIS
O
F
P
UMPED
S
TORAGE
G
ENERATOR
A. Startup Method
The pumped storage generator is under the synchronousmotor circumstance in the pumping mode. The commonmethods of startup are as follows: synchronous start (back to back start), semisynchronous start, asynchronous start, woundrotor induction motor start and SFC start.In asynchronous start, the motor is connected to griddirectly when three phase short circuit occurred in excitationwindings, then asynchronous torque which generated bydamping windings accelerates the rotor, when reaches therated speed, the motor is synchronized by exciting. Thismethod adapt to the medium and small capacity units.Similarly, in semisynchronous start, the pumping unit isstarted as an induction motor. When the motor reachesapproximate generator speed(about 50% of rated speed), themotor field is applied to bring it into synchronism with thegenerator. So there has not low frequency zone becauseexcitation is not existed at the beginning of startup.In SFC start, synchronous start and wound rotor inductionmotor start, excitation is required to provide full field at zerospeed, so it exists low frequency component of voltage andcurrent in stator windings.
B. SFC Startup and Brake of Pumped Storage Generator
With the development of Silicon Controlled Rectifier (SCR)and electronic devices, the use of SFC has increasedsignificantly and is the preferential method in pumped storage plants with more than two units [4]. The advantage of SFC issteady, high reliability, economical, small in space, andefficient. Fig. 1 shows the connection of SFC startup.
A Novel 20Hz Power Injection ProtectionScheme for Stator Ground Fault of PumpedStorage Generator
Zheng Zhu, Yuping Lu,
Senior Member, IEEE
T
DRPT2008 69 April 2008 Nanjing China
9787900714138/08/ ©2008 DRPT
2
Fig. 1 Schematic diagram of SFC startup
The main part of frequency converter is two rectifier bridgeworked as rectifier/converter. First close CB2, then close CB1to take power from an auxiliary source, the excitation isrequired to provide full field at zero speed. And acdcacconverter provides a variable frequency current to the motor by transformer or ac reactor. The rotating magnetic fieldinteracts with rotor to accelerate the motor, so the inverter frequency follows motor speed and the motor acceleratestoward synchronism, close CB3 and open CB2 while reachesthe rating speed.The process of electrical braking is disconnect the motor from power grid, deexcite at first, Let threephase short circuitoccurs after the speed lower than 50% rated speed, then themotor are excited in order to shutdown in a short time.After analyzing the spectrum of voltage and current byusing FFT algorithm, it found that the current and voltagecontain not only fundamental but also
61
k
±
th harmonicwave during startup. The zero sequence circuit does not havethese components under normal condition, so the protectionwill avoid maloperation.III. A
NALYSIS
A
ND
I
MPLEMENTATION
O
F
I
NJECTION
P
ROTECTION
S
CHEME
F
OR
P
UMPED
S
TORAGE
G
ENERATOR
A. Equivalent Circuit and Principle of Injection Scheme
At present, stator ground fault protection by injecting 20Hzvoltage are adopted in some pumped storage generator, thesubharmonic injection scheme can provide 100% coverage of stator windings and sensitivity is irrelative to the position of ground fault. It can also detect the insulation deterioration of the stator windings.
Fig. 2 Schematic diagram of 20Hz stator singlephase ground protection
Fig. 2 is the schematic diagram of microprocessorbased20Hz injection scheme [7]. The principle of this scheme isconnecting 20Hz injection voltage source to the secondaryside of a distribution transformer, the stator ground resistanceis calculated from the
U
SEF
and
I
SEF.
If motorgenerator started by asynchronous or semisynchronous method, it has a little residual voltage becausethere is no excitation on the rotor until it reaches the rated
speed, the injection scheme is not influenced, so it is notnecessary to block the protection. However, the motor hasexcitation at the beginning by static converter start, when inthe low speed, the zero sequence components of motor willenter the circuit which interfere the 20Hz injected signals.Especially if the frequency of motor is 20Hz, the current in thecircuit and the voltage across the loading resistance of thedistribution transformer have 20Hz component generated bythe injection source and generator, they can influence theadmittance criterions seriously.
Fig. 3 Equivalent circuit of injection scheme
In order to analyze the influence of zero sequencecomponents which generated by generator,
suppose groundfault occurs at stator windings of phase A through resistance
R
f.
Fig. 3 is the equivalent circuit of the Fig. 2.
R
f ’
is faultresistance,
C
g ’
is each phase capacitance to ground ingenerator stator windings.
R
n
is the resistance loadedsecondary of the distribution transformer, r is the internalresistance of the injection voltage source,
U
is the injectedvoltage.
U
SEF
is measured through voltage divider.
I
SEF
ismeasured from the secondary of TA. All above parametershave been resolved to the secondary of the groundingtransformer.
α
is the fault position in stator windings.
''0
A
UE
α
=−
(1)
Where
'0
U
is zero sequence voltage of fault point.
Suppose ground fault occurs while the frequency of generator is
f
1
（
0<
f
1<50Hz
）
during startup. from Fig. 3, itis obtained that
U
SEF
is not the voltage of parallel branch of
R
f
’
and 3
C
g
’
, but the voltage drop on
R
n
from
U
0
'
and
U
,
I
SEF
is nolonger the current inflow the parallel branch. Therefore, theresistance calculated by algorithm deviates from the truevalue, which causes protection maloperate.
B. Proposed Algorithm
According to Superposition Theorem, The circuit shown inFig. 3 can be divided into two parts, as it is shown in Fig. 4.
(a)
DRPT2008 69 April 2008 Nanjing China
3
(b)Fig. 4 Separation of the equivalent circuit
From Fig. 4(a), (b):
'01''
(3)1
n fg n
U U Rr RjC Rr
ω
=+⋅ + +
(2)
2''''
3
nf nfgfn
Ur UU RRr RRjCRR
ω
⋅= −⋅++ +
(3)Where
1
U
and
2
U
are the voltage across the loadingresistance.Hence from (1) (2) (3),
( )
'''12''''
13
nfAnSEF fgfnnf
URRErRUUU rRjCRRRR
α ω
⋅ ⋅ − ⋅ ⋅= + =
⋅ + + ⋅ + ⋅
(4)Where
'
SEF
U
is the voltage across the loading resistance of the distribution transformer.
A
E
i
B
E
i
C
E
i
Ad
U
ã
'
SEF
U
i
'
A
E
α
i
β
γ
Fig. 5 Vectorial relations of voltages for units
Fig.5 is phasor diagram of voltage in pumped storage units.If the ground fault occurs,
Ad
U
is the voltage of fault point.
'
A
E
α
can be obtained from (4)
( )
''''''''
13
nfSEFfgfnnfSEF An
URRUrRjCRRRRU E Rr
ω α
⋅ ⋅ − ⋅ ⋅ + + ⋅ − ⋅ ⋅
=⋅
(5)It is obvious obtained from the Fig.5 that
( )
''''''''''
3(3)
nSEFngn AdASEFf nSEFSEF SEFgf n
URUrRrjCRUEUR Rr UUU UjCRrR
ω α ω
⋅ − ⋅ + + ⋅= + = ⋅⋅−= − − ⋅ ⋅
(6)From the relation of currents in Fig. 3, (6) becomes
''00
()
S AdSEFSEFf S
I UIURU
= − ⋅ ⋅
(7)Where
0
S
I
0
S
U
are measured from TA and VD before theground fault occurs, so the phase angle of
Ad
U
is obtained.
'00
arg()arg()
S AdSEFSEF S
I UIU U
= − ⋅
(8)Suppose
γ
is the angle between
'
SEF
U
and
'
A
E
;
β
is theangle between
Ad
U
and
'
A
E
. Based on the Sine Theorem, it isobtained that
'
sinsin
AdSEF
UU
γ β
=
(9)Equation (9) can be written as
'
sinsin
SEF Ad
U U
γ β
∴ = ⋅
(10)Hence
Ad
U
is obtained from (8) and (10)
''00
Ad f S SEFSEF S
U R I IU U
=− ⋅
(11)Where the parameters in (11) is 20Hz sine wave after digital filtering.The resistance in the stator side is
2'
ff
RNR
=
(12)Where
N
is the ratio of grounding transformer.
C. Proposed Scheme
In this scheme, the current and voltage data are sampledand filtered; the frequency of generator is measured byfrequency tracking, if it is far away from 20Hz, narrow band pass digital filter is feasible, otherwise, if frequency is 20Hz,the ground resistance is calculated by proposed algorithm. Thetripping signal is achieved while the value meets the operationcriterion.The resistance calculation scheme can be derived as below:Case 1: tripping an alarm signal (
R
f
<
R
set.h
)Case 2: tripping a shutdown signal (
R
f
<
R
set.l
)Where
R
set.h
=5k
Ω
,
R
set.l
=1k
Ω
.According to the above analysis, the proposed algorithmcould reflect the insulation deterioration and provide sensitive protection for 100% of stator windings.IV. S
IMULATION
O
F
T
HE
P
ROPOSED
A
LGORITHM
The simulation of the proposed protection scheme isimplemented by the Matlab software. The ratio of transformer is 26.4, the capacitance of stator winding to ground is 0.6µF per phase, the frequency of motor is 20Hz.
R
n
=2.54
Ω
; theground fault occurs at 0.4s. The results are as follows:
DRPT2008 69 April 2008 Nanjing China
4
(a) R
f
=1k
Ω
(b) R
f
=5k
Ω
Fig. 6 calculation results of conventional scheme
Fig. 6(a),(b) show that when ground fault occurs at terminalthrough 1k
Ω
and 5k
Ω
, the calculation of resistance deviatefrom the true value. From (a), R
f
=1k
Ω
, the calculation value is1267
Ω
, the protection will refuse to trip; from (b), R
f
=5k
Ω
,the calculation value is 1157
Ω
, the protection will maloperate.
(a) R
f
=1k
Ω
(b) R
f
=5k
Ω
Fig. 7 calculation results of proposed scheme
Fig. 7(a), (b) show the results of resistance calculation byusing proposed algorithm. Before the ground fault occurs, thevalue is much higher than setting value. After the ground faultoccurs, an accurate ground resistance can be obtained withoutregard to transient in data window.
TABLE
IC
OMPARISON OF
C
ONVENTIONAL AND
P
ROPOSED
S
CHEME WHEN
G
ROUND
F
AULT
O
CCURS IN
D
IFFERENT
P
OSITION
1000
Ω
5000
Ω
α
Conventionalscheme
Newscheme
Conventionalscheme
Newscheme
20% 818
Ω
1002
Ω
2522
Ω
5011
Ω
50% 1205
Ω
1004
Ω
1120
Ω
5018
Ω
80% 1251
Ω
1054
Ω
1133
Ω
5015
Ω
100% 1267
Ω
1062
Ω
1157
Ω
5058
Ω
Tab. 1 is the comparison of conventional scheme and proposed scheme when ground fault occurs at different position of stator windings. From Tab. 1, we can draw theconclusion that the conventional algorithm is not suitable for startup of pumped storage units in the pumping mode; theresistance is calculated accurately by proposed scheme after one cycle.V. C
ONCLUSION
The paper proposed a 20Hz power injection protection newscheme used in pumped storage generator. The resistancecalculation accuracy can be greatly improved under 20Hz lowfrequency zero sequence component of motor. A narrow band pass digital filter is designed to mitigate influence from thelow frequency component of motor. The simulation shows itsreliability and sensitivity improvements.VI. R
EFERENCES
[1]
Weijian. W. Principle of Large Generatorunits Protection , ChinaElectrical Power Publish House, 2002.[2]
Lijun Tian, Yuping Lu, Heng Chen. Digital simulation for starting process of a pumped storage generator/motor, Automation of ElectricPower Systems. 1997, 21 (7):3841.[3]
Zuyan Mei, Pumped storage station technology china machine press.2000.[4]
Konidaris, D.N. Investigation of backtoback starting of pumpedstorage hydraulic generating units,
IEEE Trans on Energy Conversion
.2002
，
17 (2):273278.[5]
Ji'an Wu; Hui Wan; Yuping Lu. Study of a fresh subharmonic injectionscheme based on equilibrium principle for hydrogenerator stator ground protection,
IEEE PES Winter Meeting Proceedings
. 2002, Vol2: 924929.[6]
Zielichowski, M.; Fulczyk, M, Analysis of operating conditions of groundfault protection schemes for generator stator winding,
IEEE Trans on Energy Conversion.
2003. Vol.18. No.1:5762.[7]
SIPROTEC Multifunctional Machine Protection 7UM62 ManualC53000G1176C1495, V4.6, SIEMENS AG, 2005.[8]
NengLing Tai, XiangGen Yin, Zhe Zhang, DeShu Chen, Research of subharmonic injection schemes for hydrogenerator stator ground protection,
Power Engineering Society Winter Meeting, 2000. IEEE,
Volume: 3, 2000 Page(s):19281932.[9]
Qixue Zhang, Kangqing Xi, Jiasheng Chen, Xiang Wang, QuanrongShen, Field application and analysis of the stator earth fault protectionwith voltage injection for largesized generator Automation of ElectricPower Systems. 2007, 31 (11):103107.
VII. B
IOGRAPHIES
Zheng Zhu
was born in Nantong, China, in March,1983. He has received the Bachelor’s degree inelectrical engineering from Nanjing University of Aeronautics and Astronautics in June 2005. Now heis pursuing his M.E. in Southeast University. Hiscurrent interesting area is power system protection.
Yuping Lu
was born in Danyang, China, in Oct,1962. He has received the Ph.D. degree in electricalengineering from the City University, UK in 2003.
DRPT2008 69 April 2008 Nanjing China