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

As Fig. 1 shows, 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

Fig. 2. 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

Fig. 2 shows 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)

Fig. 3. Voltage phasor diagram for secondary winding’s analysis.

Fig. 4. 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.

Fig. 2(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

Fig. 4 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

Fig. 5. 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 shown in Fig. 6(b) 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

Fig. 5(b), 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.

Fig. 7. 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 Fig. 6, 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

Fig. 7 shows the phase current FFT of the secondary terminal

of the traditional converter transformer and that of the

Fig. 8. Phase current FFT of the primary terminal of the traditional and the

new converter transformer. (a) FFT of primary phase current I corresponding

to Fig. 1. (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

Fig. 8(b) 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.

Fig. 9 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

Fig. 9(b), 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.