Download A Novel 20Hz Power Injection Protection Scheme for Stator Ground

DRPT2008
6-9 April 2008 Nanjing China
1
A Novel 20Hz Power Injection Protection
Scheme for Stator Ground Fault of Pumped
Storage Generator
Zheng Zhu, Yuping Lu, Senior Member, IEEE
Abstract--Based on the analysis of the start-up of pumped
storage generator under motor circumstance, it is found out that
the conventional 20Hz injection component is influenced by the
low frequency component when ground faults take place in the
stator windings for static frequency converter (SFC) start-up
generator. The conventional relay is just simply blocked in case
mal-operation during 20Hz working condition. To overcome the
disadvantage, a novel scheme is proposed to improve groundingresistance(R) calculation accuracy and enhance ability to limit
impact of 20Hz component from generator. The simulation
results proved its effectiveness and practicability of the scheme.
Index Terms--Grounding, injection voltage source, protection,
pumped storage, windings.
T
I. INTRODUCTION
HE construction of pumped storage power station has
entered a new period of fast development in recent years.
It plays an important role in power network because of the
effect of peak clipping and valley filling, frequency
modulation, phase modulation, load adjusting and spinning
reserve, which improve the flexibility and reliability of power
network operation. The great social and economic benefits
produced by pumped storage plants have been recognized by
more and more countries. It puts forward higher requirements
for the protective relaying because of flexible operation mode.
The Sub-synchronous overcurrent protection, underpower
protection are the special protection of pumped storage units
[1]. The starting process of a pumped storage generator-motor
unit which is started by a static frequency converter is
simulated in the paper [2], the results show that motor have
lots of low frequency components during start-up. In some
countries, the ground fault protection is blocked in case maloperation, therefore, the stator windings are not protected. The
start-up is frequent during one day and duration is rather long,
sometimes between 5-10 minutes [3]. The generator will be
damaged if ground fault occurs near the neutral point of the
stator windings. So it needs to improve the injection protection
scheme to ensure operation safety of generator.
In this paper, the 20Hz power injection scheme for ground
fault 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.
(e-mail: [email protected]).
978-7-900714-13-8/08/ ©2008 DRPT
the methods employed in starting pumped storage units in the
pumping mode are discussed. From the analysis, it is
concluded that the protection is influenced by the low
frequency component when ground fault occurs in stator
windings for SFC start-up generator. A new algorithm is
proposed from the equivalent circuit. The simulation results
show that grounding resistance accuracy is improved by the
novel algorithm when ground fault occurs, and it is not
affected by the 20Hz component from generator, the tripping
is reliable and sensitive.
II. START-UP METHOD AND ANALYSIS OF PUMPED STORAGE
GENERATOR
A. Start-up Method
The pumped storage generator is under the synchronous
motor circumstance in the pumping mode. The common
methods of start-up are as follows: synchronous start (back to
back start), semi-synchronous start, asynchronous start, wound
rotor induction motor start and SFC start.
In asynchronous start, the motor is connected to grid
directly when three phase short circuit occurred in excitation
windings, then asynchronous torque which generated by
damping windings accelerates the rotor, when reaches the
rated speed, the motor is synchronized by exciting. This
method adapt to the medium and small capacity units.
Similarly, in semi-synchronous start, the pumping unit is
started as an induction motor. When the motor reaches
approximate generator speed(about 50% of rated speed), the
motor field is applied to bring it into synchronism with the
generator. So there has not low frequency zone because
excitation is not existed at the beginning of start-up.
In SFC start, synchronous start and wound rotor induction
motor start, excitation is required to provide full field at zero
speed, so it exists low frequency component of voltage and
current in stator windings.
B. SFC Start-up and Brake of Pumped Storage Generator
With the development of Silicon Controlled Rectifier (SCR)
and electronic devices, the use of SFC has increased
significantly and is the preferential method in pumped storage
plants with more than two units [4]. The advantage of SFC is
steady, high reliability, economical, small in space, and
efficient. Fig. 1 shows the connection of SFC start-up.
DRPT2008
6-9 April 2008 Nanjing China
Fig. 1 Schematic diagram of SFC start-up
The main part of frequency converter is two rectifier bridge
worked as rectifier/converter. First close CB2, then close CB1
to take power from an auxiliary source, the excitation is
required to provide full field at zero speed. And ac-dc-ac
converter provides a variable frequency current to the motor
by transformer or ac reactor. The rotating magnetic field
interacts with rotor to accelerate the motor, so the inverter
frequency follows motor speed and the motor accelerates
toward synchronism, close CB3 and open CB2 while reaches
the rating speed.
The process of electrical braking is disconnect the motor
from power grid, deexcite at first, Let three-phase short circuit
occurs after the speed lower than 50% rated speed, then the
motor are excited in order to shutdown in a short time.
After analyzing the spectrum of voltage and current by
using FFT algorithm, it found that the current and voltage
contain not only fundamental but also 6k ± 1 th harmonic
wave during start-up. The zero sequence circuit does not have
these components under normal condition, so the protection
will avoid mal-operation.
III. ANALYSIS AND IMPLEMENTATION OF INJECTION
PROTECTION SCHEME FOR PUMPED STORAGE GENERATOR
A. Equivalent Circuit and Principle of Injection Scheme
At present, stator ground fault protection by injecting 20Hz
voltage are adopted in some pumped storage generator, the
subharmonic 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.
2
is calculated from the USEF and ISEF.
If motor-generator started by asynchronous or semisynchronous method, it has a little residual voltage because
there is no excitation on the rotor until it reaches the rated
speed, the injection scheme is not influenced, so it is not
necessary to block the protection. However, the motor has
excitation at the beginning by static converter start, when in
the low speed, the zero sequence components of motor will
enter the circuit which interfere the 20Hz injected signals.
Especially if the frequency of motor is 20Hz, the current in the
circuit and the voltage across the loading resistance of the
distribution transformer have 20Hz component generated by
the injection source and generator, they can influence the
admittance criterions seriously.
Fig. 3 Equivalent circuit of injection scheme
In order to analyze the influence of zero sequence
components which generated by generator, suppose ground
fault occurs at stator windings of phase A through resistance
Rf. Fig. 3 is the equivalent circuit of the Fig. 2. Rf’ is fault
resistance, Cg’ is each phase capacitance to ground in
generator stator windings. Rn is the resistance loaded
secondary of the distribution transformer, r is the internal
resistance of the injection voltage source, U is the injected
voltage. USEF is measured through voltage divider. ISEF is
measured from the secondary of TA. All above parameters
have been resolved to the secondary of the grounding
transformer. α is the fault position in stator windings.
U '0 = −α E 'A
(1)
Where U '0 is zero sequence voltage of fault point.
Suppose ground fault occurs while the frequency of
generator is f1（0<f1<50Hz）during start-up. from Fig. 3, it
is obtained that USEF is not the voltage of parallel branch of Rf’
and 3Cg’, but the voltage drop on Rn from U0' and U, ISEF is no
longer the current inflow the parallel branch. Therefore, the
resistance calculated by algorithm deviates from the true
value, which causes protection mal-operate.
B. Proposed Algorithm
According to Superposition Theorem, The circuit shown in
Fig. 3 can be divided into two parts, as it is shown in Fig. 4.
Fig. 2 Schematic diagram of 20Hz stator single-phase ground protection
Fig. 2 is the schematic diagram of microprocessor-based
20Hz injection scheme [7]. The principle of this scheme is
connecting 20Hz injection voltage source to the secondary
side of a distribution transformer, the stator ground resistance
(a)
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6-9 April 2008 Nanjing China
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Where I S 0 U S 0 are measured from TA and VD before the
ground fault occurs, so the phase angle of U Ad is obtained.
'
arg(U Ad ) = arg( I SEF − U SE
F ⋅
IS 0
)
U S0
(8)
'
and E 'A ; β is the
Suppose γ is the angle between U SEF
angle between U Ad and E 'A . Based on the Sine Theorem, it is
obtained that
(b)
Fig. 4 Separation of the equivalent circuit
U Ad
From Fig. 4(a), (b):
U '0
U1 =
R +r
+ j 3ω Cg' ) + 1
R 'f ⋅ ( n
Rn r
U ⋅r
Rn ⋅ R 'f
U2 = U −
Rn + R 'f + j 3ω C g' R 'f Rn
(2)
(3)
'
Where U SEF
is the voltage across the loading resistance of
the distribution transformer.
i
EA
•
U Ad
i
α E A'
γ i
U ' SEF
i
i
EC
EB
Fig. 5 Vectorial relations of voltages for units
Fig.5 is phasor diagram of voltage in pumped storage units.
If the ground fault occurs, U Ad is the voltage of fault point.
α E 'A can be obtained from (4)
U⋅ Rn ⋅ R −U ⋅ r ⋅ R +(1+ j3ω C R ) ⋅ Rn  −Rn ⋅ R ⋅U
Rn ⋅ r
It is obvious obtained from the Fig.5 that
αE'A =
'
f
'
SEF
'
f
'
U Ad = α E 'A + USEF
=
' '
g f
'
f
'
U ⋅ Rn − USEF
⋅ ( r + Rn + r ⋅ j3ωCg' Rn )
Rn ⋅ r
'
SEF
⋅ R'f
(5)
(6)
From the relation of currents in Fig. 3, (6) becomes
IS 0
) ⋅ R 'f
US0
sin β
Equation (9) can be written as
'
U SEF
∴ U Ad =
⋅ sin γ
sin β
(9)
(10)
U Ad
(11)
IS 0
I SEF − U
⋅
U S0
Where the parameters in (11) is 20Hz sine wave after
digital filtering.
The resistance in the stator side is
(12)
R f = N 2 R 'f
R 'f =
'
SEF
Where N is the ratio of grounding transformer.
C. Proposed Scheme
In this scheme, the current and voltage data are sampled
and filtered; the frequency of generator is measured by
frequency 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. The
tripping signal is achieved while the value meets the operation
criterion.
The resistance calculation scheme can be derived as below:
Case 1: tripping an alarm signal (Rf<Rset.h)
Case 2: tripping a shutdown signal (Rf<Rset.l)
Where Rset.h=5kΩ, Rset.l=1kΩ.
According to the above analysis, the proposed algorithm
could reflect the insulation deterioration and provide sensitive
protection for 100% of stator windings.
IV. SIMULATION OF THE PROPOSED ALGORITHM
'
U − USEF
U'
'
=(
− SEF − USEF
⋅ j3ωCg' ) ⋅ R'f
r
Rn
'
U Ad = ( I SEF − U SEF
⋅
'
U SEF
Hence U Ad is obtained from (8) and (10)
+r
Where U1 and U 2 are the voltage across the loading
resistance.
Hence from (1) (2) (3),
U ⋅ Rn ⋅ R 'f − α E 'A ⋅ r ⋅ Rn
'
(4)
U SEF
= U1 + U 2 =
r ⋅  R 'f + (1 + j 3ωC g' R 'f ) ⋅ Rn  + Rn ⋅ R 'f
β
sin γ
=
(7)
The simulation of the proposed protection scheme is
implemented 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. Rn=2.54Ω; the
ground fault occurs at 0.4s. The results are as follows:
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6-9 April 2008 Nanjing China
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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 the
conclusion that the conventional algorithm is not suitable for
start-up of pumped storage units in the pumping mode; the
resistance is calculated accurately by proposed scheme after
one cycle.
(a) Rf=1kΩ
V. CONCLUSION
(b) Rf=5kΩ
Fig. 6 calculation results of conventional scheme
Fig. 6(a),(b) show that when ground fault occurs at terminal
through 1kΩ and 5kΩ, the calculation of resistance deviate
from the true value. From (a), Rf=1kΩ, the calculation value is
1267Ω, the protection will refuse to trip; from (b), Rf=5kΩ,
the calculation value is 1157Ω, the protection will maloperate.
The paper proposed a 20Hz power injection protection new
scheme used in pumped storage generator. The resistance
calculation accuracy can be greatly improved under 20Hz low
frequency zero sequence component of motor. A narrow band
pass digital filter is designed to mitigate influence from the
low frequency component of motor. The simulation shows its
reliability and sensitivity improvements.
VI. REFERENCES
[1]
[2]
[3]
[4]
[5]
(a) Rf=1kΩ
[6]
[7]
[8]
[9]
(b) Rf=5kΩ
Fig. 7 calculation results of proposed scheme
Fig. 7(a), (b) show the results of resistance calculation by
using proposed algorithm. Before the ground fault occurs, the
value is much higher than setting value. After the ground fault
occurs, an accurate ground resistance can be obtained without
regard to transient in data window.
TABLE I
COMPARISON OF CONVENTIONAL AND PROPOSED SCHEME WHEN GROUND
FAULT OCCURS IN D IFFERENT POSITION
α
20%
1000 Ω
Weijian. W. "Principle of Large Generator-units Protection", China
Electrical Power Publish House, 2002.
Lijun Tian, Yuping Lu, Heng Chen. "Digital simulation for starting
process of a pumped storage generator/motor," Automation of Electric
Power Systems. 1997, 21 (7):38-41.
Zuyan Mei, "Pumped storage station technology" china machine press.
2000.
Konidaris, D.N. "Investigation of back-to-back starting of pumped
storage hydraulic generating units," IEEE Trans on Energy Conversion.
2002，17 (2):273-278.
Ji'an Wu; Hui Wan; Yuping Lu. "Study of a fresh subharmonic injection
scheme based on equilibrium principle for hydro-generator stator ground
protection," IEEE PES Winter Meeting Proceedings. 2002, Vol2: 924929.
Zielichowski, M.; Fulczyk, M, "Analysis of operating conditions of
ground-fault protection schemes for generator stator winding," IEEE
Trans on Energy Conversion. 2003. Vol.18. No.1:57-62.
SIPROTEC Multifunctional Machine Protection 7UM62 Manual
C53000-G1176-C149-5, V4.6, SIEMENS AG, 2005.
NengLing Tai, XiangGen Yin, Zhe Zhang, DeShu Chen, "Research of
subharmonic injection schemes for hydro-generator stator ground
protection," Power Engineering Society Winter Meeting, 2000. IEEE,
Volume: 3, 2000 Page(s):1928-1932.
Qixue Zhang, Kangqing Xi, Jiasheng Chen, Xiang Wang, Quanrong
Shen, "Field application and analysis of the stator earth fault protection
with voltage injection for large-sized generator" Automation of Electric
Power Systems. 2007, 31 (11):103-107.
VII. BIOGRAPHIES
Zheng Zhu was born in Nantong, China, in March,
1983. He has received the Bachelor’s degree in
electrical engineering from Nanjing University of
Aeronautics and Astronautics in June 2005. Now he
is pursuing his M.E. in Southeast University. His
current interesting area is power system protection.
5000 Ω
Conventional
New
Conventional
New
scheme
scheme
scheme
scheme
818 Ω
1002 Ω
2522 Ω
5011 Ω
Yuping Lu was born in Danyang, China, in Oct,
1962. He has received the Ph.D. degree in electrical
engineering from the City University, UK in 2003.
DRPT2008
6-9 April 2008 Nanjing China
He is working as a professor in Southeast University of China. His research
interests are power system protection, especially in digital relaying of
generator-transformer unit and control technique in distribution system with
DGs.
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