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6-9 April 2008 Nanjing China
Influence of HVDC Ground Electrode Current
on AC Transmission System and Development
of Restraining Device
Sheng Wang, Chengxiong Mao, Member, IEEE, Jiming Lu, Guihua Mei and Yancun Liu
Abstract-- HVDC power systems operating in ground-return
mode produce DC voltages around the electrodes. During
Monopolar operations of the HVDC electrode, DC biases of the
transformers caused by the DC current through neutral points
may damage the transformers and the whole network. The noises
and the neutral point DC currents of the main transformers with
earthed neutral points in Yihe substation and LN power plant
were measured and recorded during the commissioning of the
HVDC system from Three Gorges power station to Guangdong
power network. Compared to injecting reverse direct current
and inserting in series with the neutral point a resistor, the
strategy of installing capacitors to the main transformer neutral
point is more effective. According to the DC current values
measured at the transformer neutrals of a power station, this
method was studied based on its corresponding module of power
system with ground grid proposed. Simulations and laboratory
tests have been carried out and some main technology
parameters were put forward according to the research results.
Index Terms--DC bias; HVDC; transformer neutral point
I. INTRODUCTION
W
ITH the high speed development of electric power
industry in China, according to the feature of China
Southern Power Grid, which is with long-distance, heavy
capacity of transmitting power, and equipped with the
structure of six-AC-main lines and three-DC-main lines,
which is the combined operation of AC/DC and concentrated
landing in Guangdong Province for the HVDC lines, a lot of
problems [1]-[3] occurred seriously such as DC bias which
caused by large DC currents flowing through the neutral
earthed transformers. Therefore, it is worthwhile to study the
impact brought by the DC transmission on the AC
transmission network.
During Monopolar operations of the HVDC electrode,
significant DC currents can be observed in the transformer
neutrals of substations in the vicinity of the electrode of the
HVDC converter. There are no doubts on the origin, cause and
mechanisms of the DC currents that appear in the neutral of
This work was supported in part by Program for New Century Excellent
Talents in University of P.R.C. (NCET-04-0710).
Sheng Wang, Chengxiong Mao and Jiming Lu are with the College of
Electric and Electronics Engineering, Huazhong University of Science &
Technology, Wuhan 430074, Hubei province, China.
Guihua Mei and Yancun Liu are with the Electrical Power System
Technique Department of the Guangdong Electric Power Research Institute
Guangzhou 510600, Guangdong Province, China
978-7-900714-13-8/08/ ©2008 DRPT
the high voltage transformers. A portion of the DC current that
is flowing between the two HVDC electrodes is captured by
the grounding systems of the transmission and distribution
lines and substations located in zones where the earth
potentials are high and is discharged back to soil by the
grounding systems of the electric network at the locations
where the earth potentials are lower. In other words, Part of
the DC current injected into the electrode finds an easier
return path through the AC system via the grounded neutrals
of the transformers. This DC current through transformer
neutrals may make the transformers DC biases [4]-[7] and
therefore, increases the noise of transformer. Furthermore, it
can cause many problems due to excessive core vibration,
overheating regarding the integrity and longevity of the
transformers and other related instrumentation problems. So
the abnormal operation of transformer will cause the unsafe
operation of substation and power plant. The voltage total
harmonic distortion of 200kV and 500kV AC transmission
network will increase sharply and these will impact other
equipments operation. Sequentially, it may damage the
transformers and the whole network. In a word, the HVDC
ground electrode current makes the transformer with earthed
neutral point as a harmonic source.
In 2009, the HVDC transmission system of YunnanGuangdong will begin to be commissioned, the grounding
current must influence the normal running of AC system
seriously, unless some measures are put into effect.
Consequently, designing appropriate measure to restrain the
DC neutral current has become a very active and important
research topic. Section II deals with the testing of neutral point
direct currents and noises of the main transformers earthed
neutral point in Yihe substation and LN power plant.
II. DIRECT CURRENT AND NOISE TESTING
The test on HVDC transmission system of the Three
Gorges-Guangdong began in December of 2003 and lasted for
half of a year. In Mar 30th, 2004, the monopolar ground
circuit operation mode of the Three Gorge-Guangdong HVDC
transmission system with the transmitting power 15000 MW
was tested [8]. The noises and the neutral point direct currents
of the main transformers with earthed neutral point in Yihe
substation and LN power plant were measured and recorded
presented in Table I.
The analysis of these data shows that monopolar metallic
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6-9 April 2008 Nanjing China
circuit operation mode or the bipolar symmetric mode of
HVDC transmission system has few impact on the noise and
neutral point direct current of transformers with earth neutral
point; on the contrary, it shows that monopolar ground circuit
operation mode or the bipolar asymmetric mode of HVDC
transmission system has great impact on the noise and neutral
point direct current of these transformers. In the test, both of
the two main transformer of Yihe substation operate with
earthed neutral point , at the worst condition, the neutral point
direct currents of these two main transformers still reach 21.7
A. If only one of the main transformers operates with earthed
neutral point, the neutral point direct current could reach 35 A,
which is harmful for the transformer’s safe operation.
TABLE I
THE MEASUREMENT OF THE MAIN TRANSFORMERS NEUTRAL POINT DIRECT
CURRENTS AND NOISES
Transmitting
Power (MW)
(HVDC of the
Three GorgesGuangdong)
Transmitting Power
PolarⅠ (polarⅡ
)(MW) (HVDC of
TianshengqiaoGuangzhou)
Neutral point Direct
Current of
Transformers (A)
Noise of
Transformers (dB)
Yihe
Substation
LN
Power
Station
Yihe
Substation
LN
Power
Station
600 Monopolar
ground circuit
450（450）
14.3
12.8
90.6
90.2
600 Monopolar
ground circuit
600（300）
16.0
7.2
91.8
87.6
750 Monopolar
metallic circuit
450（450）
0.2
-0.2
80.0
83.6
750 Monopolar
ground circuit
600（300）
19.5
7.8
93.7
87.4
900 Monopolar
ground circuit
750（150）
13.5
3.5
91.2
83.6
1250 Monopolar
ground circuit
750（150）
18.05
10.9
93.5
86.5
1500 Monopolar
ground circuit
750（150）
21.6
16.0
94.8
87.2
1500 Monopolar
metallic circuit
750（150）
2.8
12.0
88.7
85.9
Three main restraining measures of neutral point DC
current are presented in Sections III A-C.
III. RESTRAINING MEASURES OF HVDC GROUND
ELECTRODE CURRENT ON TRANSFORMER
In an attempt to prevent or minimize these harmful effects,
many restraining devices have been studied in the past [9]-[22]
and some even installed in the neutrals of transformers. In
principle, there are three main restrain measures: injecting
reverse direct current, inserting in series with resistor and
inserting in series with capacitor.
A. Injecting reverse direct current
The DC current was injected from the outer compensationearthing pole to the neural point of the transformer. It partly
flows back to the compensation-earthing pole via the
transformer winding and the power network.
It must be admitted that the measure does not change any
transformer’s wire connection. Thus it is safe and reliable to
the transformers and around substations. But the measure isn’t
flawless and some shortcomings are as follows:
ƒ
It can only compensate for part of DC current and
the DC bias of transformer can’t eliminate
thoroughly.
ƒ
According to correlation studies and experiences, it
has such a low efficiency that the DC current from
current supply is larger than the compensation
current (typically several to tens of times larger).
Thus the loss of energy is so large that it increases
the running cost by far.
ƒ
It needs to install an additional mesh-form earthing
device, so the extra charge of requisitioning of land
must be added to the running cost.
B. Inserting in series with the neutral point an appropriate
resistor
This method is to insert in series with the neutral point a
resistor. So the neutral point DC current of transformer will
decrease as the impedance of the resistor connected to neutral
point increasing.
It must be admitted that the application of the neutral series
resistor provides an inexpensive method to restrain the HVDC
ground current flowing into transformer. But its drawbacks are
obvious:
ƒ
This method can only compensate for part of DC
current and the DC bias of transformer can not
eliminate thoroughly just like injecting reverse direct
current.
ƒ
To calculate the resistances of different transformers
in the power network involves complex geological
structures and soil characteristics, the extent of the
power network, pipeline network. Uncertainties
make modeling the whole system accurately very
difficult. It becomes very difficult task if all
grounding factors including all above.
ƒ
Resistance will vary with different transformer which
is located different site. So if the value of resistor is
not so big, it will not affect the system protections
and relays setting seriously due to changing its zerosequence resistance network, but some protection
parameters should be reconfigured. While the value
is big, it will not only influence protections and relay
setting but also couldn’t make the neutral point of the
transformer connect to the ground effectively. Thus it
may cause a relatively over voltage for the
transformer neutral point referred to the earth when
system fault.
ƒ
When the network configurations change, for
example, a transmission line is added or removed; a
transformer is installed or removed, etc., the
designed resistor must be modified and replaced
accordingly for meeting the new system structures.
C. Inserting in series with the neutral point a capacitor
This method is to insert in series with the neutral point a
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capacitor. For the capacitor’s character, the DC current is
blocked while the AC current is not. Compared to injecting
reverse direct current and inserting in series with the neutral
point a resistor, the strategy of installing capacitors to the main
transformer neutral point is more effective.
Section IV mainly deals with the development of the
capacitor blocking device.
IV. CAPACITOR BLOCKING DEVICE DESCRIPTION
The device is designed to be installed in series with the
transformer neutral connection to electrical system ground. Its
purpose is to provide a low impedance path for steady state
AC current while blocking the flow of DC current under
normal system operating conditions. The impedance of the
capacitor has no influence on the limited DC current. Thus the
main transformer neutral point capacitor should be as low as
possible in order to avoid the ferromagnetic resonance or other
overvoltage.
returns to original status and the capacitor is put in
automatically.
Thus the key work for the capacitor and protection
equipment is presented as the following:
ƒ
The action of bypass IGCTs must be rapid and
reliable;
ƒ
The action of bypass mechanical switch must be
rapid and reliable;
ƒ
When the IGCTs are triggered into conduction, we
must take the current change rate, current peak value
and flow equalization into account.
220Vdc
Power-supply system
I
220Vdc
±12V
+5V
+24V
Power-supply system
II
Manual
operation
Pulse
amplifier
segregatin
g unit I
Guidance
penal
output
circuit
I
Measurement
component Ι
Measurement
component II
Integrated
circuit
(overvoltage
and
overflowing
protection)
output
circuit
II
Microprocessor
(overvoltage
and
overflowing
protection)
IGCT
Pulse
amplifier
segregatin
g unit II
K3
switching
on logic
circuit
K3
switching
off logic
circuit
By-pass
switch
K3
Fig. 1. Diagram of capacitor blocking device main circuit.
Fig. 2. Basic block diagram of dual redundant circuit.
The capacitor connected to the main transformer should
operate continuously and ensure the neutral pointed to be
connected to the ground effectively. In the event of an
abnormal condition, such as an AC fault, lightning, switching
transient and similar momentary events, which could damage
or fail the capacitor, it is advisable to use some means of
bypassing the capacitor, so a high current by-pass path is
provided by the bypass IGCTs installed on the rectified side of
a full-wave diode bridge rectifier, as illustrated in Fig. 1. This
figure illustrates the principle underlying the DC currentblocking circuit. The IGCTs are triggered into conduction
when the voltage exceeds a predetermined primary voltage
threshold or when the current exceeds a predetermined current
threshold, whichever occurs first. The capacitor is shorted or
bypassed very rapidly by the IGCTs. Once the IGCTs have
been triggered into conduction by either means, the capacitor
remains shorted for about 20 seconds (This time can be
adjusted if necessary). In addition, since the planned IGCT ontime is longer than an AC fault duration, this period gives the
IGCT time to cool down, thereby better enabling them to
withstand the re-applied voltage. At the same time, the parallel
mechanical switch is closed for protecting IGCTs and
capacitor. After the fault is cleared away, the bypass device
A. Rapid and reliable action of IGCTs
To meet the rapidity and reliability of bypass IGCTs, some
measures must be done.
ƒ
Rapid measurement
To reduce the response and delay time of sensor, the device
adopts rapid sensor group to measure current and voltage. The
response time of quick sensor is less than or equal to 3µs.
According to the simulation analysis, the response time will
decrease by about 500µs to 1000µs if using the current
criterion.
ƒ
Rapid judgment
The device’s dual redundant circuit, as illustrated in Fig. 2,
which is total of pure hardware logic circuit and microprocessor intelligent controller, ensure its rapidity and
reliability. The pure hardware circuit is just composed of
operational amplifiers and logic gate circuit, so its response
time is less than 3µs. The micro-processor intelligent unit,
adopting 150MHz DSP processor, needs more time about
10µs to perform interrupt subroutine, A/D transmission and
parallel outputting, which is slower than the pure hardware
circuit but has already met the specification.
ƒ
Rapid execution
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Execution unit is composed of rapid triodes and pulse
transformer. Execution signal is amplified by the rapid triode
and sent to IGCTs’ drive units through insulation pulse
transformer. The total delay time of execution unit could be
limited to 5µs, including the IGCTs’ conducting time 3µs.
Fig. 3. Flow process chart of triggering conduction IGCT.
B. Rapid and high-reliability mechanical switch
The delay time of fast mechanical switch could reach about
80ms, including the mechanical vibrating time. The high
reliability depends on not only the device’s dual redundant
circuit, but the switch’s power supply, spark-proof method.
C. Protection of IGCTs
The IGCTs will be damaged by oversize current change
rate and current peak value, unless it has effective flow
restraining and equalization measures. For this, according to
simulations, the 100uH air core reactors to avoid saturation are
in series with the IGCTs.
Trigger control flow process charts of IGCT are shown in
Fig. 3 and Fig. 4.
Fig. 4. Flow process chart of turning off IGCT.
V. SIMULATIONS AND LABORATORY TESTS RESULT
The hardware setup was built in the dynamic analog lab,
the fault current the parameters of 200 A, 0.5 s; 2000 A
asymmetrical, 0.5 s; 200 A, 1 s is obtained from the
transformer neutral point after the equipment is installed. The
following waveforms were obtained as shown in Fig. 5. We
can see that 200 A current on the beginning of curve a. At just
over 1.0 s, this current increase to around 2000 A peak and the
IGCT immediately falls into conduction when it reaches 500
v, as shown on the curve f. Then, the voltage across the IGCT
drops to zero. The fault current last 0.5 s and restored to the
steady-state value of 200 A.
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[6]
Fig. 5. Curves of the results at a fault current.
VI. CONCLUSIONS
When the HVDC power transmission systems operate in
ground return mode, some disadvantage effect to the AC
transmission system will follow, especially to the transformer
with neutral point connected directly with the ground. Part of
the DC current injected into the electrode finds an easier
return path through the AC system via the grounded neutrals
of the transformers. This DC current through transformer
neutrals may make the transformers DC biases and therefore,
increases the transformer noise above normal levels.
Compared to injecting reverse direct current and inserting in
series with the neutral point an appropriate resistance, the
strategy of installing capacitors to the main transformer
neutral point is more effective.
A capacitor DC current-blocking device for transformer
neutrals has been developed. This device consists of a
capacitor inserted in series in the transformer neutral point
with a couple of IGCTs and a mechanical switch for bypass
and service restoration. Simulations and laboratory tests have
been carried out, and the results show that this device could be
installed in the neutral point of transformer for efficiency in
the case of HVDC power transmission systems operating in
ground return mode.
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VIII. BIOGRAPHIES
Sheng Wang was born in Henan, China, in 1979. He received his B.S.
degree in College of Electrical and Electronic Engineering from Huazhong
University of Science and Technology (HUST), Hubei, China, in 2002. Now
he is pursuing the Ph.D. degree in HUST.
His interest is the operation and control of power system. E-mail:
hust_ws@mail.hust.edu.cn
Chengxiong Mao (M’ 1993) was born in Hubei, China, in 1964. He received
his B.S., M.S. and Ph.D. degrees in electrical engineering, from HUST, in
1984, 1987 and 1991 respectively. Presently, he is a professor of HUST.
DRPT2008
6-9 April 2008 Nanjing China
His fields of interest are power system operation and control, the
excitation control of synchronous generator and applications of high power
electronic technology to power system. E-mail: cxmao@mail.hust.edu.cn
Jiming Lu was born in Jiangsu, China, in 1956. He received his B.S. degree
from Shanghai Jiaotong University, Shanghai, China, and received his M.S.
degree from HUST.
His research is focused on the excitation control based on microcomputer.
E-mail: lujiming@mail.hust.edu.cn.
Guihua Mei was born in Hunan, China, in 1964. He received his B.S. and
M.S. degrees in electrical engineering from HUST, in 1984 and 1987
respectively.
He joined the Electrical Power System Technique Department of the
Guangdong Electric Power Research Institute, China in 1987. He has been
involved in the Power System Dynamic Stability Analysis, field
commissioning of electric control system and relay, and mainly involved in
the
Power
Quality analysis
in the last
decade.
E-mail:
mei_gh@sy.gpgc.com.cn
Yancun Liu was born in Henan, China, in 1974. She received her B.S., M.S.
and Ph.D. degrees in Wuhan University, in 1995, 1999 and 2004 respectively.
She joined the Electrical Power System Technique Department of the
Guangdong Electric Power Research Institute, China in 2004. She contributed
to the development of solution of mitigating the impact, by HVDC GR Mode,
on AC system. E-mail: yancun_liu@163.com