A novel series-in series hybrid active power filter

Liqing Tong1, Zhaoming Qian1, Lingxiao Xue 1, Fang Z. Peng1,2 1Zhejiang University, Hangzhou, China, 310027 2Michigan State University Department of Electrical and Computer Engineering

Abstract-In order to decrease the passive filter rating, suppress the large resonant peak when the passive part is used alone for the series-in series hybrid active power filter (SHAPF), a novel series-in SHAPF topology is proposed in this paper. It is a combined system of a traditional SHAPF and a capacitor which is connected in series with inverter and resonant with the inductor of the output filter at the fundamental frequency. It can decrease the harmonic voltage on the inverter, increase harmonic impedance of the main circuit and avoid the affection of the line and passive filter variations. Simulation and experimental results from a 10kVA prototype verify that the proposed SHAPF can promote the efficiency of the shunt passive filters, decrease the rating of the inverter and improve the system filtering performance and reliability.

Index terms-resonant peak; series-in series hybrid active power filter.

Ⅰ.INTRODUCTION

The traditional series hybrid active power filter (SHAPF) first proposed by Fang Zheng Peng in 1989 [4] is a more advantage active power filter (APF) [1-5]. The load harmonic current is mainly filtered by the shunt passive filters, while the series active filter is not directly to compensate for the harmonics of the load, but to improve the filtering performance of the shunt passive filters and to solve the problems of the shunt passive filters used alone for the SHAPF. To acquire the desired filter performance of the SHAPF, the active filter must to be controlled as current controlled voltage source converter (CSVS). It presents the impedance source characteristic, where the series active filter is controlled to present no impedance at the fundamental frequency and a K resistance to the source and the load at the harmonics frequency. But there exist two main problems for the SHAPF [6-7]: 1) The value of the K is confine to the stability. 2) The SHAPF is not applied to high power and high voltage power system, for its large rating of the active filter which endures all the source current and harmonic voltage.

Therefore, to solve the above problems of the SHAPF, a series-in SHAPF is proposed which is a combined system of a traditional SHAPF [4] and an fundamental frequency series-resonance circuit that is connected in series between the couple transformer and the inverter [8]. Additional, it can decrease the harmonic voltage on the inverter, increase harmonic impedance of the active part and avoid the resonance between the line and passive filters. But there also have some disadvantages for the series-in SHAPF, such as: the fundamental frequency series-resonance circuit increasing the rating of the passive filters, and the large resonant peak value when the inverter is out of work.

To decrease the passive filters rating, suppress the large resonant peak when only passive part is on work and retain all

the advantages of the series-in SHAPF, a new series-in SHAPF topology is proposed in this paper, which unites the fundamental frequency series-resonance circuit and the output filter. Simulation and experimental results from a 10kVA laboratory prototype verify that the proposed series-in SHAPF can promote the efficiency of the shunt passive filters, decrease the rating of the inverter and improve filter performance.

Ⅱ. GENERAL PRINCIPLE OF THE SHAPF

A. The traditional SHAPF The traditional SHAPF topology and its equivalent one-

phase circuit is shown in Fig.1, where sZ is the source impedance, FZ is the equivalent impedance of the shunt passive filters. As is shown in Fig.1(b), the active filter is controlled as the controlled c shV KI= . Let’s assume that a source sV is sinusoidal, the filter characteristic of the traditional SHAPF can be expressed as:

Fsh lh

s F

ZI I

Z Z K=

+ + (1)

where, lhI is the load harmonic current and shI is the source harmonic current. If the K impedance is much larger than the source impedance sZ and the equivalent impedance FZ , it will no any harmonic source current and not be influenced by the variations in the source impedance and excellent harmonic isolation effect, i.e., no harmonic current flowing from the load side into the source side or from the source side into the load or into the shunt passive filters. The series active filter also acts as a damping impedance which can eliminate the parallel resonance and harmonic sink problems inherent to the shunt passive filters.

sV

sZ

LI

FI

FZ

sIcV lV

li

cv

lv

fLfC

nTsisv

sL sR

dL

dR

dU

c shV KI=

(a) Topology (b) The equivalent one-phase circuit

Fig.1 The traditional SHAPF

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But with the increment value of the K the traditional SHAPF system will oscillate and even lead to the system instability[6-10], so the filter performance is confine to its close-loop control capability. Additional, the series active filter endures the whole source current and harmonic voltage, which will cause the inverter rating larger. So there are many APF topologies [7] are developed to improve or solve the problems, the series-in SHAPF is one of the proposed topology as shown in Fig.2. B. The series-in SHAPF

The series-in SHAPF topology and its equivalent one-phase circuit is shown in Fig.2, and its filter characteristic is given by

1

Fsh lh

s F

ZI I

Z Z K Z=

+ + + (2)

where, 21 1 1

1

jZ j L R nCωω

⎛ ⎞= − +⎜ ⎟⎝ ⎠

and n is the turn ratio of

the couple transformer. The 1L 1C fundamental frequency series resonance circuit presents the zero impedance at the fundamental frequency and the linear increment impedance characteristic at the harmonic frequency, especially very high impedance at high frequency. So it can increase the loop harmonic impedance gain for the harmonic load current and the source harmonic voltage suppression. For it will endure the most harmonic voltage of the series part, the harmonic voltage of the inverter can be decreased. Additional, it also can increase harmonic impedance gain of the main circuit and avoid the parameters variation of the line and passive filters. But the larger rating of the passive part and the higher peak value at the lower resonant frequency when only the passive part is on work are inevitably disadvantages for the series-in SHAPF.

li

cv

lv

fLfC

1L

1C

nT

sisv

sL sR

dU

dL

dR

sV

sZ

LI

FI

FZ

1Z sIcV lV

c shV KI=

(a) Topology (b) The equivalent one-phase circuit Fig.2 The series-in SHAPF

C. The proposed SHAPF

To decrease the rating of the passive part, suppress the resonant peak value and retain all the advantages of the series-in SHAPF, a new series-in SHAPF is proposed in this paper, as shown in Fig.3. Its main idea is

Unite the two main part of the series-in SHAPF, the 1L

1C fundamental frequency series-resonance circuit and the output filter, which is applied to suppress the switching current and voltage ripple.

Make fL and 1C be the fundamental frequency resonant circuit, remove 1L and combine 1C and fL be the output filter, as shown in Fig.3.

li

cv

lv

fLfC

1C

nT

sisv

sL sR

dU

dL

dR

Fig.3 The proposed series-in proposed SHAPF

From the previous analysis, the parameter of the new series-in SHAPF can be designed as fL 107.9mH (with Q=25), 1C 94 Fμ and fC 0.1 Fμ in this paper. It can be seen obviously that its total rating is smaller than the series-in SHAPF.

For 31 33.88( ) 3.18 10 ( )

f fc L cX X X= = Ω = × Ω at the

fundamental frequency, 10f fL Cω ω≥ and 2f sL n L at the

harmonic frequency, the proposed SHAPF can keep all the advantages of the series-in SHAPF and decrease the rating of the passive part. Its equivalent one-phase circuit is illustrated in Fig.4.

sV

sZ

LI

FI

FZ

'1Z sI

cV lV

c shV KI=

sZ

LfIFfI

FZ

sfI

sfV

sZ '1Z shI

shVLhI

FhI

FZ

cV

c shV K I= ⋅

Fig.4 One-phase Equivalent circuit for the proposed SHAPF

Its filter characteristics can be formulated from the Fig.4 and the source harmonic current can be expressed as

'1

Fsh lh

s F

ZI IZ Z K Z

=+ + +

(3)

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where, ' 21

1f f

jZ j L R nCωω

⎛ ⎞= − +⎜ ⎟⎝ ⎠

. While the passive part

is on action alone, the source current can be expressed as

211

Fsh lh

s F

ZI I

Z Z n Cω=

+ + (4)

D. The filter performance comparison From the preview analysis and equation (1)-(4), Fig.5 and

Fig.6 show the filter characteristics of the three SHAPF topologies in case of 0.23sL mH= , the vertical axis of which indicates the ratio of the source harmonic current to the load current. In the case of the passive part use alone (K=0), the

source impedance will parallel resonance with the passive part, as shown in Fig.5. It can be seen obviously that the resonant peak value is highest and the resonant frequency is lowest for the series-in SHAPF. While the resonant peak value of the proposed SHAPF is smaller than the series-in SHAPF even the traditional SHAPF. In case of the active filter is on work, the filter characteristics are all improved, and no parallel resonance occurs. And the proposed SHAPF have the same excellent filter performance with the series-in SHAPF, as shown in Fig.6.

(a) The traditional SHAPF (b) The series-in SHAPF (c) The proposed series-in SHAPF

Fig.5 Filter characteristics comparison of different SHAPF only passive filters

(a) The traditional SHAPF (b) The series-in SHAPF (c) The proposed series-in SHAPF

Fig.6 Filter characteristics comparison for different SHAPF

Ⅲ SIMULATION RESULTS

As shown in Fig.3, a capacitor 1C is in series between the inverter and the couple transformer. The passive filters consisting of 5th-, 7th- and 11th-tuned LC and a high-pass of rating 5kVA series resonant filters are shown in Table.1. A typical current-source harmonic producing load is a three-phase six-pulse diode rectifier of rating 10kVA, where dL is 7.5mH and dR 25Ω . The turn ratio of the couple transformers n is 6, and the dc capacitor the PWM inverter is 1410 Fμ . So the simulation results of the three different SHAPF topologies by Saber are shown in Fig7.

Table.1 Shunt passive filters

5th( 5Z ) L=8.63mH C=47uF Q=9

7th( 7Z ) L=9.41mH C=22uF Q=7.5

11th( 11Z ) L=8.38mH C=10uF Q=7

HPF( hZ ) L=1.41mH C=32uF R=20

It can be observed from the Fig.7 that the filter performance the traditional SHAPF is the worst in the three SHAPF, and the total harmonic distortion (THD) of the source current si attains 9.875% when the load current THD is 25.7%. While, the series-in and the proposed SHAPF have the same filter performance and the THD of the source current si are 2.98% and 2.96% respectively.

( )si A

( )Li A

(a) The traditional SHAPF

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( )si A

( )Li A

( )si A

( )Li A

(b) The series-in SHAPF (c) The proposed SHAPF

Fig7 Simulation waveforms of three SHAPF

. EXPERIMENT RESULTSⅣ

To verify the validity of the proposed SHAPF topology, some experiments have been done with sL =0.23mH and dcV =200V. Fig.8, Fig.9 and Fig.10 show the experiment results of the steady characteristics for the traditional, the series-in and the proposed SHAPF. It is obvious from Fig.8-Fig.10 that the experiment waveforms is similar to the

simulation waveforms, and the THD of the source current is 10.963%, 3.247% and 3.095% respectively. Therefore, from the previous theoretical analysis, simulation results and the experimental verification that the proposed SHAPF can promote the efficiency of the shunt passive filters, decrease the rating of the inverter and improve the system filtering performance and reliability.

(a) Source current and load current (b) Source current spectrum 10.963%

Fig8 Waveform and spectrum before compensation

(a) Source current and load current (b) Source current spectrum 3.247%

Fig9 Waveform and spectrum before compensation

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(a) Source current and load current (b) Source current spectrum 3.095%

Fig10 Waveform and spectrum before compensation

. COⅤ NCLUSION

The authors have proposed a new series-in SHAPF in this paper. It is a combined system of a traditional SHAPF and a capacitor which is connected in series with inverter and resonant with the inductor of the output filter at the fundamental frequency circuit. The new APF topology proposed in this paper was verified theoretically and experimentally and its main features are summarized as follows, 1. Parallel and series resonance between the source and

passive part can be suppressed much when active filter is out of work.

2. The required rating of the active filter is much smaller than the traditional SHAPF, and the rating of the passive part is also smaller than the series-in SHAPF.

3. Filter performance is better than the traditional SHAPF and have the same excellent performance with the series-in SHAPF.

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