A New Converter Transformer and a Corresponding Inductive Filtering Method for HVDC Transmission System

A New Converter Transformer and a Corresponding
Inductive Filtering Method for HVDC
Transmission System


Abstract

—A new converter transformer and an inductive filtering
method are presented to solve the existing problems of
the traditional converter transformer and the passive filtering
method of the high-voltage direct current (HVDC) system. It
adopts the ampere-turn balance of the transformer as the filtering
mechanism. A tap at the linking point of the prolonged winding
and the common winding of the secondary windings is connected
with the LC resonance circuit. It can realize the goal that once the
harmonic current flows into the prolonged winding, the common
winding will induct the opposite harmonic current to balance it by
the zero impedance design of the common winding and the proper
configuration of LC parameters, so there will be no inductive
harmonic current in the primary winding. Moreover, the reactive
power that the converter needs can be partly compensated in the
secondary winding. Simulation results have verified the correctness
of the theoretical analysis. The new converter transformer
can greatly reduce the harmonic content in the primary winding,
loss, and noise generated by harmonics in the transformer, and
the difficulty of the transformer’s insulation design.
Index Terms—Filtering mechanism, harmonic, high-voltage
direct current (HVDC), inductive filtering, new converter
transformer.

I. INTRODUCTION

THE high-voltage direct current (HVDC) transmission
system has been widely used in remote and large power
transmission, submarine cable transmission, and domain electric
network interconnection [1]–[3]. HVDC transmission
system is always made up of a rectifier station, a dc line, and
an inverter station. During the commutating process, a large
number of harmonics will be generated by the nonlinear load.
Therefore, it is necessary to carry out harmonic suppression.
The traditional HVDC ac passive power filters (PPF) are always
placed at the converter transformer’s primary side (grid side),
and the transformer will be adversely affected by harmonics,
which causes a series of problems, such as additional harmonic
loss, heat, vibration, and noise [1], [4]–[7]. In addition, in order
to avoid series/parallel resonance between parallel PPF and
(b) Voltage phasor diagram.

system impedance, the traditional PPF cannot reach its tuned
point, which greatly affects the filtering effect [8]–[10]. The active
power filter (APF) has better filtering effect than the passive
power filter (PPF), but APF needs a complex regulation and
control system, especially a large power harmonic-generating
source, which is inapplicable in current HVDC transmission’s
ac system [11]–[13]. A patent named coupling-compensation
and harmonic-shielding converter transformer, that is, the new
converter transformer, proposes an ideal solution to harmonic
suppression.

II. PROBLEMS OF TRADITIONAL CONVERTER TRANSFORMER’S
WIRING MODE AND AC FILTERING SCHEME
the traditional converter transformer and ac
passive filtering method are commonly used in 12-pulse HVDC
system. It is clear that the transformer adopts wye/wye/delta
wiring, and ac filters are placed at the transformer’s primary
side. Although this kind of converter transformer and passive
filters are widely applied in HVDC systems, these structures and
designs still have some disadvantages.
1) In HVDC transmission systems, the converter is the
main harmonic-generating source. A three-phase bridge
converter usually generates characteristic
harmonic currents at the ac side because of
the turning of the thyristors [14]. The noncharacteristic
harmonic currents can also be generated due to some
factors, such as various unbalances in ac voltages, system
impedance, and transformer parameters [15]. All the
harmonic currents will flow in the primary and secondary
windings of the traditional converter transformer, which
increases the transformer’s additional heat, vibration, and
NEW CONVERTER TRANSFORMER AND A CORRESPONDING INDUCTIVE FILTERING METHOD 1427
New converter transformer and corresponding inductive filtering
system. (a) Wiring mode. (b) Voltage phasor diagram. (c) Arrangement of
filters.


noise. As a result, it increases the added loss, the difficulty
of insulating design, the capacity of the transformer, and
the margin of the design capability, which increases the
cost of the traditional converter transformer.
2) In the ac system of HVDC, traditional passive filtering
is the main method of harmonic suppression. However, it
still has several disadvantages. The most serious one is
that the series/parallel resonance may occur between the
system impedance and the passive power filters. This series/
parallel resonance will result in the amplification of
harmonic current and harmonic voltage, and it may damage
the passive power filters and neighboring power equipment
[16], [17]. To avoid the resonance of the passive
power filters, the tuned frequency of passive power filters
is designed slightly away from the dominant harmonic frequency.
However, it will degrade the performance of the
passive power filter, and the filtering effect of the traditional
passive filter cannot be optimal.

III. TECHNICAL CHARACTERISTICS OF NEW CONVERTER
TRANSFORMER AND CORRESPONDING
INDUCTIVE FILTERING METHOD

the new converter transformer and the corresponding
inductive filtering system, in which, (a) shows the
wiring mode of the transformer, and its secondary winding
adopts prolonged-delta wiring. To facilitate our discussion,
the winding of , , is called
prolonged winding, and the winding of , , ,
, , is called common winding. (b) shows
the transformer’s voltage phasor diagram, which is used to
discuss the phase-shifting of the new transformer. (c) shows
the arrangement of the inductive filters. As can be seen from
(c), a tap at the linking point of each single-phase prolonged
winding and common winding is connected with double-tuned
(DT) filters. The inductive filtering method will be discussed
later on in this paper.
A. Phase-Shifting Principle
In order to satisfy the demand of 12-pulse HVDC, the converter
transformer has to supply 12-phase commutating line
voltage. The secondary winding of the traditional transformer
adopts wye/delta wiring, and the phase angle difference between
the wye winding’s line-voltage and the delta winding’s
line-voltage has to be 30 , which is shown in Fig. 1(b). As for
the new converter transformer, according to Fig. 2(b), we can
set the phase angle difference between the line-voltage
and the to , and set the phase angle difference
between the line-voltage and the to . In
this way, the phase angle difference between the line-voltage
and the is 30 . So we can deduce that the
phase-shifting angle should be 15 . Set that the
voltage value of the primary winding of the new converter
transformer is , the voltage value of the secondary prolonged
winding is , and the voltage value of the secondary common
winding is ; then, according to Fig. 2(b) and sine rule, the
following can be obtained:
(1)
According to the above equation, the turn-ratio can be obtained
as follows:
(2)
in which , , respectively, represent the turn-ratio of the secondary
prolonged winding and the common winding to primary
winding. , , and are the turn number of the primary
winding, the secondary common winding, and the prolonged
winding, respectively.
In the actual HVDC systems, the new converter transformer
can adopt the single-phase three-winding method. As long as the
relation of the turn-ratio satisfies (2), the new converter transformer
can supply 12-phase commutating line voltage and satisfy
the commutating demand of the 12-pulse converter.
B. Self-Coupling Action
The secondary prolonged winding and the common winding
of the new converter transformer adopt self-coupling connection,
which is similar to the series winding and the common
winding of autotransformer [17], [18]. According to Fig. 2(c),
set that the output line-voltage is , the voltage of the common
winding is and the voltage of the prolonged winding is ;
then, the following voltage phasor diagram in Fig. 3 can be
obtained.
According to cosine rule, the output line-voltage can be expressed
as follows:
(3)
. Voltage phasor diagram for secondary winding’s analysis.
New converter transformer’s single-phase harmonic model.
Then, the voltage of the secondary prolonged winding is deduced
as follows:
(4)
The secondary prolonged and common winding of the new
converter transformer is electromagnetic coupling, which is
similar to the series andcommonwinding of the autotransformer.
When the prolonged winding and the common winding maintain
magnetic force balance, we can obtain the following relation:
(5)
in which and are the root-mean-square (RMS) current
of the secondary prolonged winding and the common winding,
respectively.
(c) shows that the current of the secondary prolonged
winding is equal to the output current , and its electromagnetic
capacity can be expressed as follows:
(6)
Meanwhile, the output capacity can be expressed as follows:
(7)
Then, the ratio coefficient can be obtained as follows, which
is used to analyze the material utilizing ratio of the transformer:
(8)
Assuming that the output line-voltage value of the new
converter transformer is 110 kV, and voltage value of the secondary
common winding is 35 kV, then, according to (4)–(8), we
can obtain the ratio coefficient , which indicates
that new converter transformer is material saving.
C. Inductive Filtering Mechanism
shows the single-phase model of the new converter
transformer, which is used to analyze the inductive filtering
mechanism. In this figure, indicates the harmonic current
source, which is also the harmonic current of the secondary
Single-phase model of the new converter transformer. (a) Winding arrangement.
(b) Equivalent circuit.
prolonged winding. and indicate the harmonic current
of the primary winding and the secondary common winding, respectively.
Because of the harmonic current of the secondary
prolonged winding, the primary winding and the secondary
common winding will induce harmonic current and to
balance . According to magnetic force balance, the following
results:
(9)
in which , , and are the turn number of the primary
winding, the secondary prolonged winding, and the common
winding, respectively.
If the harmonic ampere-turns of the secondary prolonged
winding and those of the common winding can keep balance,
then , that is, there will be no induction harmonic
current in the primary winding. That is to say, the harmonic
currents only flow in the secondary winding of the new transformer.
To realize the inductive filtering method, it not only
needs the full tuning design of the tapping filter, but also needs
the zero inductance design of the secondary common winding
of the new converter transformer, which will be analyzed in
the following equivalent circuit of the single-phase transformer
shown in Fig. 5.
Fig. 5(a) shows the winding arrangement of the single-phase
model of the new converter transformer. According to short-circuit
test, we can measure the short-circuit impedance , ,
and . Then, the equivalent impedance can
be expressed as follows:
(10)
By regulating the winding arrangement shown in Fig. 5(a),
it can realize the goal that the impedance of the secondary
common winding is approximately equal to 0 (the resistance
can be ignored for high-capacity converter transformers). In
the solid arrow and the virtual arrow, respectively, indicate
basic frequency current and harmonic frequency current.
Under the specific harmonic frequency of the harmonic current
that needs suppressing, both of the double-tuned filter and the
harmonic impedance of the secondary common winding are
approximately 0, so the harmonic current mainly flows into
the branch of the secondary common winding, and there is
approximately no harmonic current in the primary winding.
In addition, under the fundamental frequency, the impedance
of the filter is capacitive, thus providing reactive power
compensation.
LUO et al.: NEW CONVERTER TRANSFORMER AND A CORRESPONDING INDUCTIVE FILTERING METHOD 1429
Fig. 6. New HVDC transmission analogy system with new converter transformer
in rectifier station and traditional converter transformer in inverter
station.
Phase current fast Fourier transform (FFT) of the secondary terminal of
the traditional and the new converter transformer. (a) FFT of secondary phase
current I corresponding to Fig. 1. (b) FFT of secondary phase current I
corresponding to Fig. 2.
IV. SYSTEM SIMULATION STUDY
A. Simulation Model
In order to prove the correctness of the above analyses, according
to the new HVDC transmission testing system shown
in we have established a system simulation model by
using MATLAB/SIMULINK. Fig. 6 shows the rectifier station
with the new converter transformer and in the corresponding inductive
filtering method and the inverter station with the traditional
converter transformer and in the passive filtering method.
It is necessary to clarify that the double-tuned filter (DT5/7)
is not needed when we consider suppressing fifth and seventh
harmonic currents in the wiring method of the new converter
transformer in the rectifier station. Here, due to the high content
of fifth and seventh harmonics, in order to remove their negative
effect of fifth and seventh harmonics on the converter transformer,
we have designed the DT5/7. In Fig. 7, HP2 indicates
the second-order high-pass filter; Zr and Zi, respectively, indicate
the system impedance of the rectifier and the inverter side,
and and , respectively, indicate the inductance of the rectifier
and the inverter side.
B. Contrast Analysis of Simulation Results
shows the phase current FFT of the secondary terminal
of the traditional converter transformer and that of the
Phase current FFT of the primary terminal of the traditional and the
new converter transformer. (a) FFT of primary phase current I corresponding
to (b) FFT of primary phase current I corresponding to Fig. 2.
TABLE I
COMPARISON OF THE HARMONIC CONTENT OF THE SECONDARY SIDES
OF THE NEW AND THE TRADITIONAL CONVERTER TRANSFORMERS
New converter transformer. It can be seen that the harmonic content
of each order of the traditional and the new converter transformers
is similar, which is determined by the nonlinear load,
that is, the converter. However, as for the primary phase current
of the transformer shown in Fig. 8, it can be seen that the primary
phase-current waveform of the new converter transformer
is better than that of the traditional one, which is determined
by the wiring method of the transformer and by the filtering
method. We can see that adopting the new winding wiring and
the inductive filtering method can effectively suppress the 5th,
7th, 11th, and 13th harmonic currents that only flow in the secondary
winding of the new transformer, so the THD shown in
is lower than that in Fig. 8(a). Table I shows the FFT
value of the exact harmonic contents of Figs. 7 and 8, which further
proves the correctness of the above analysis.
shows the phase current FFT at the grid side of the
rectifier and the inverter station, respectively.We can see that the
waveform of the phase current in Fig. 9(a) is better than that in
which is caused by the new inductive filtering method.
Considering the effect of the system impedance, the resonance
point of the passive filters cannot be reached. So the filtering
effect is not ideal, as shown in Fig. 9(b). While adopting the
inductive filtering method, the harmonic currents are confined
by the coupling-windings of the new converter transformer, so
the resonance point of the tap filters can be reached. Therefore,
we can obtain the ideal phase current waveform at the grid side
shown in Fig. 9(a).Acomparison of the exact harmonic contents
is shown in Table II.
1430 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 3, JULY 2008
Fig. 9. Phase current FFT at the grid side of the rectifier station and the inverter
station, respectively. (a) FFT of phase current I at the grid side of the rectifier
station corresponding to Fig. 6. (b) FFT of phase current I at the grid side of
the inverter station corresponding to Fig. 6.
TABLE II
COMPARISON OF THE HARMONIC CONTENT OF THE GRID SIDES
OF THE RECTIFIER STATION AND THE INVERTER STATION
V. CONCLUSION
In 12-pulse HVDC transmission systems, the secondary
windings of the new converter transformer adopt prolonged
delta wiring, which brings about good symmetrical characteristics
to its structure. Each phase short-circuit impedance can be
equal. It can facilitate the reliable commutation and the sound
operation of the converter. The equivalent impedance of the
secondary common winding is approximately 0, which provides
good conditions for inductive filtering. The resonance point of
the tap filters of the new transformer can be reached without the
consideration of the effect of the system impedance. Simulation
results verify the correctness of our theoretical analysis, and
show that the filtering effect of the inductive filtering method
is better than that of the traditional passive filtering method.
Adopting the new converter transformer and the corresponding
inductive filtering method can optimize the structure of HVDC
transmission systems, greatly reducing the negative effect of
harmonic on the operation of the transformer and improving
the filtering effect at the ac side of HVDC systems.