New Hybrid Static Var CompensatorWith Series Active Filter

Ayumu Tokiwa, Hiroaki Yamada,and Toshihiko Tanaka

Department of Electrical and Electronic EngineeringYamaguchi University

Ube, Japane-mail: totanaka@yamaguchi-u.ac.jp

Makoto Watanabe, Masanao Shirai,and Yuji Teranishi

The Chugoku Electric ManufacturingCompany, Incorporated

Hiroshima, Japane-mail: a001033@pnet.gr.energia.co.jp

Abstract—This paper proposes a new hybrid static var com-pensator (SVC) with a series active filter (AF). The proposedhybrid SVC consists of a series AF and SVC. The series AF,which is connected in series to phase-leading capacitors in theSVC, performs a resistor for source-side harmonic currents. Asinusoidal source current with a unity power factor is obtainedwith the series AF, although the thyristor-controlled reactorgenerates harmonic currents. A digital computer simulation isimplemented to confirm the validity and high-practicability of theproposed hybrid SVC using PSIM software. Simulation resultsdemonstrate that sinusoidal source currents with a unity powerfactor are achieved with the proposed hybrid SVC.

Index Terms—thyristor-controlled reactor, phase-leading ca-pacitor, static var compensator, series active filter, harmonicscompensation

I. Introduction

Large-capacity electric arc furnaces and rolling mills causerapid reactive-power fluctuations in distribution feeders. Theserapid reactive-power fluctuations lead to reactive powerinterferences such as flickers and voltage fluctuations indistribution feeders. Static var compensators (SVCs) withthyristor-controlled reactors (TCRs) and phase-leading capac-itors (PLCs) are widely used to solve the reactive powerinterferences in distribution feeders because of low costs [1].However, TCRs generate harmonic currents on the sourceside [2]. Many topologies were proposed to improve thecompensation characteristics of the source-side harmonic cur-rents [3]–[6]. Fig. 1 shows a power circuit diagram of the pro-posed hybrid SVC topology. The passive filters, which consistof the 5th- and 7th-tuned filters and a high-pass filter (HPF),are combined with the TCR. A three-phase voltage-sourcepulse-width-modulated (PWM) inverter is connected in seriesto the passive LC filters through matching transformers (MTs).The series-connected three-phase PWM inverter improves thecompensation characteristics of the passive filters.

Hybrid active filters (AFs) were also proposed by many re-searchers [7]–[11]. The proposed hybrid active power topolo-gies were basically a series connection of the LC tuned filterand a three-phase shunt AF. The rating of the shunt AF can bereduced because the fundamental reactive and designate-orderharmonic currents are compensated by the series-connectedLC tuned filter. A hybrid SVC topology with the hybrid AF

AF

Load

TCR

5th

7th

HPF

Fig. 1. Power circuit diagram of the previously proposed hybrid static varcompensator (SVC) with a series active filter (AF) [3]–[6].

TCR

Shunt

passive filter Shunt AF

Load

Fig. 2. Power circuit diagram of the previously proposed hybrid SVC withthyristor-controlled reactor (TCR) and phase-leading capacitor (PLC) withseries AF [12, 13].

was proposed [12, 13]. Fig. 2 shows a power circuit diagramof the proposed hybrid SVC. The PLCs with TCRs controlthe fundamental reactive power on the source side. As theshunt AF compensates only harmonic currents, the requiredrating of the shunt AF is small. In [14], a static synchronouscompensator (STATCOM) is combined with TCRs. The STAT-COM performs PLCs compensating harmonic currents on thesource side. Thus, the required rating of the parallel-connectedSTATCOM is large. A combined system of shunt-passive andseries AFs was also proposed [15]–[17]. While the proposedtopologies are practical and cost-effective, the fundamental

978-1-5090-2364-6/17/$31.00 ©2017 IEEE

IEEE PEDS 2017, Honolulu, USA12 – 15 December 2017

901

Load

TCR

STATCOM

Fig. 3. Power circuit diagram of the previously proposed hybrid SVC witha static synchronous compensator (STATCOM) [14].

TABLE IPreviously Proposed Hybrid SVC Topologies and Functions of Added AF.

Topology Functions of Added APFFig. 1 Thyristor controlled reactor Improvement of harmonic

(TCR) and passive LC filter voltage compensationwith series active filter characteristics of passive(AF) LC filter

Fig. 2 TCR and PLC Harmonic currentswith series AF compensation

Fig. 3 TCR and parallel-connected Fundamental reactive powerSTATCOM control and harmonic currents

compensation

reactive power on the source side cannot be controlled. Table Ishows summaries of the previously proposed hybrid SVC.Thus, a hybrid SVC topology consisting of TCRs and purePLCs with a small-rated voltage-source PWM inverter has notbeen reported as long as the authors know.

This paper proposes a new hybrid SVC topology comprisinga series AF and SVC, which consists of TCRs and purePLCs. The series AF is connected in series to the purePLCs. Thus, the authors offer a simple, practical, and cost-effective hybrid SVC. In [17], Prof. H. Fujita, et al. previouslyproposed a combined system consisting of a shunt passivefilter and series AF for a current-source harmonic-producingload. Considering both the three-phase load and the TCRas a current-source harmonic-producing load, the previouslyproposed control strategy for the series AF is applicable tothe newly proposed hybrid SVC. The basic principle of theproposed hybrid SVC is discussed in detail. The compensationcharacteristics of the harmonic currents are shown in detail,and they are then confirmed by digital computer simulationusing PSIM software. Simulation results demonstrate thatthe sinusoidal source currents with a unity power factor areobtained. From the simulation results, the required-capacity ofthe series AF is 2.7 % as compared to that of the rating of thethree-phase load. This demonstrates that the proposed hybridSVC is useful for practical distribution feeders.

II. Newly proposed hybrid static var compensator

Fig. 4 shows a circuit diagram of the proposed hybrid SVC.The proposed hybrid SVC comprises a series AF and SVC,which consists of the ∆-connected TCR and ∆-connected purePLCs. The series AF consists of a three-phase voltage-source

PWM inverter with insulated-gate bipolar transistors (IGBTs).The series AF is connected in series to the three-phase PLCsthrough MTs, where the turns ratio is 1:2. The small-rated LCfilter Lf and Cf suppresses switching ripples that are generatedby the PWM inverter, which performs the series AF. Thepurpose of this paper is to demonstrate the compensationperformance of the reactive and harmonic currents for theproposed hybrid SVC. Thus, ideal models for IGBTs, MTs, in-ductors, and capacitors are used. A three-phase load generatesthe fundamental reactive currents, 5th- and 7th-order harmoniccurrents. A three-phase current source is used to demonstratethe three-phase load. The rating of the three-phase load is176 MVA. The ratings of the three-phase source voltages are33 kVrms, 60 Hz, while the rating of the inductor of TCRs is80 MVA and that of the PLCs is 140 MVA, respectively. Thepure PLCs with TCRs compensate the fundamental reactivepower on the source side. The series AF performs a resistorof KC Ω. This series AF compensates the source-side harmoniccurrents, and the sinusoidal source currents with a unity powerfactor are achieved on the source side. As quantitatively shownin the simulation results, the required capacity of the series AFis 2.7 % as compared to that of the rating of the three-phaseload. Thus, the proposed hybrid SVC is practical and cost-effective. Prof. H. Fujita, et al. proposed a combined system ofa shunt-passive and series-AF for the current-source harmonic-producing load with a novel control method of the seriesAF [17]. Note that there are two current-source harmonic-producing loads, namely the TCR and three-phase load, inFig. 4. Therefore, the control method proposed in [17] isapplicable to the proposed hybrid SVC in Fig. 4. This is asimple and practical idea for the control strategy of the seriesAF in Fig. 4.

Here, the basic principle of the harmonics-compensationstrategy of the series AF is briefly introduced. A three-phasephase-locked loop (PLL) is used to detect the electric angleθC of a-phase PLCs current [18]. Note that any sensors for theexternal voltages and currents are not needed in the proposedhybrid SVC shown in Fig. 4. The authors have offered apractical hybrid SVC.

The source currents iSa, iSb, and iSc are expressed as

iSa =√

2ISF cos(ωSt − φF) +√

2∞∑

h=2

ISh cos(hωSt − φh)

= iSaF + iSah,

iSb =√

2ISF cos(ωSt −23

hπ − φF)

+√

2∞∑

h=2

ISh cos(hωSt −23

hπ − φh)

= iSbF + iSbh,

iSc =√

2ISF cos(ωSt +23

hπ − φF)

+√

2∞∑

h=2

ISh cos(hωSt +23

hπ − φh)

= iScF + iSch. (1)

902

abc

dqiCd

ω =2πfS

s1

= 0ICq* PI

vSa

vSb

vSc

iSa

iSb

iSc

iLa

iLb

iLc

Cab Cbc Cca

Lab

Lbc

Lca

Ls

iTCbiTCaiTCc

iSa

iSc

3-phase Load

176 MVA

power factor: 0.7

THD: 11.3 %

vAFa

Q1

Q4

Q3

Q2

Q5

Q6

vDC

iCab

iCcaN1:N2

Cf

Lf

CDC

vTa

vTb

vTc

33 kVrms

60 Hz

TCR

80 MVA

PLC

140 MVA

AF

4.7 MVA

vAFb

vAFcθC

iCab iCca

dq

abc

iSa

iScdq

abciSah

iSbh

iSch

KC Q1

Q2

Q3

Q4

Q5

Q6

Triangular wave

KC

KC

PIVDC*

vDC

vAFb*

vAFc*

vAFa*iSd

iSq

~

~LPF

LPF

iCabiCcaiCbc

6C

πθ +

iSq

iSd

iCq

vab

vbc

iSd

iSq

-

-

Fig. 4. Circuit diagram of the proposed hybrid static var compensator (SVC) with a series active filter (AF).

Three-phase source currents, iSa, iSb, and iSc are detected,and then the detected source currents are transformed into d-qcoordinates using the detected electric angle θC. iSd and iSqare given by

iSd =√

3ISF cos(23π + φF)

+2√

3cos(ωSt +

23π) ·

∞∑h=2

ISh cos(hωSt − φh)

+ cosωSt ·∞∑

h=2

ISh cos(hωSt −23

hπ − φh)

+ cos(ωSt +43π) ·

∞∑h=2

ISh cos(hωSt +23

hπ − φh)

= iSd + iSd,

iSq = −√

3ISF sin(23π + φF)

−2√

3sin(ωSt +

23

hπ) ·∞∑

h=2

ISh cos(hωSt − φh)

+ sinωSt ·∞∑

h=2

ISh cos(hωSt −23

hπ − φh)

+ sin(ωSt +43π) ·

∞∑h=2

ISh cos(hωSt +23

hπ − φh)

= iSq + iSq. (2)

The dc components iSd and iSq in d-q coordinates originatefrom the fundamental components iSaF, iSbF, and iScF of thesource currents iSa, iSb, and iSc, respectively, in a-b-c coor-dinates. The ac components iSd and iSq in d-q coordinatesoriginate from the harmonic currents iSah, iSbh, and iSch ofthe source currents. These ac components are extracted byHPFs with a 2nd-order low-pass-filter (LPF). The extractedac components are then retransformed into a-b-c coordinates.Retransforming iSd and iSq into a-b-c coordinates gives thesource-side harmonic currents iSah, iSbh, and iSch. With theextracted source-side harmonic currents iSah, iSbh, and iSch, thereference values v∗AFa, v∗AFb, and v∗AFc for the series AF aregiven by

v∗AFa = KC · iSah,

v∗AFb = KC · iSbh,

v∗AFc = KC · iSch. (3)

Therefore, the series AF performs a resistor of KC Ω forsource-side harmonic currents iSah, iSbh, and iSch. A sine-triangle intercept technique is used to generate the gate signalsfor the three-phase voltage-source PWM inverter. The switch-ing frequency fS of the three-phase voltage-source PWMinverter is 12 kHz.

Fig. 5 shows per-phase equivalent circuits for Fig. 4.Fig. 5(a) shows a per-phase equivalent circuit for Fig. 4,where VS is the source voltage, IS is the source current, ILis the load current, IC is the PLC current, VCh is the outputvalue of the series AF, ZS is the source-side impedance, and

903

SI&

LI&

CZ& CI&

(b)

(c) (d)

hIS&

SV&

SZ&

CZ& CZ&

SZ&

SZ&

hVS&

hIL&

hIS&

hIC&

Three-phase

load with TCR

SZ&

CZ&Three-phase

loadTCR

(a)

SV&CI&

SI&

LI&

h

h

IK

V

SC

C&

&

×=h

h

IK

V

SC

C&

&

×=

h

h

IK

V

SC

C&

&

×=h

h

IK

V

SC

C&

&

×=

Three-phase

load with TCR

Fig. 5. Per-phase equivalent circuits for Fig. 4 with the control gain KC. (a) Per-phase equivalent circuit. (b) Per-phase equivalent circuit for current-sourceharmonics-producing load. (c) Equivalent circuit for ILh. (d) Equivalent circuit for VSh.

0.1 1 10 100

-60

-40

-20

0

20

40

0.1 1 10 100

-60

-40

-20

0

20

40

KC=0

KC=30

KC=0

KC=30

f / f0 f / f0

h

h

V

I

S

SdB

h

h

I

I

L

SdB

(a) (b)

Fig. 6. Gain plots for Fig. 5. (a) Gain plots for Fig. 5(c). (b) Gain plots for Fig. 5(d).

ZC is the impedance of a per-phase PLC. Considering boththe TCR and three-phase load as a current-source harmonics-producing load, Fig. 5(a) can then be represented as Fig. 5(b).In Fig. 5(b), (3) is rewritten as

VCh = KC · ISh. (4)

When no harmonics are included in the source voltage VS,a per-phase base equivalent circuit for load-side harmoniccurrents is shown in Fig. 5(c). In Fig. 5(c), the source-sideharmonic current ISh is given by

ISh =ZC

KC + ZS + ZC· ILh. (5)

If KC (ZS + ZC) in (5),

ISh = 0. (6)

Thus, the sinusoidal source currents iSa, iSb, and iSc areobtained with the series AF connected to the pure PLCs. Thetransfer function G(s) of the LPF in Fig. 4 is considered in

(5). The transfer function G(s) of the LPF is expressed as

G(s) =ω2

C

s2 + 2ξωCs + ω2C

(ωC = 2π fC), (7)

where the cut-off frequency fC is 179 Hz and the dampingratio ξ is 0.7. With the transfer function G(s), the source-sideharmonic currents ISh are rewritten as

ISh =ZC

KC · (1 −G(s)) + ZS + ZC· ILh. (8)

Fig. 6(a) shows gain plots for Fig. 5(c), where the source-side impedance ZS=0.1 pu. In Fig. 6, f0 is the source-voltage frequency, which is 60 Hz. The line with KC=0 showsthe compensation performance for the source-side harmoniccurrents when the series AF is not connected. From the 2nd-to 5th-order components, the source-side harmonic currentsISh generated by the TCRs and three-phase load are magnified.The rating of the three-phase load is 176 MVA. The rated load-side impedance is 6.2 Ω. Thus, the control gain KC of 30 Ω isused with the condition that KC (ZS+ZC) in (8) and (9). Theline with KC=30 shows the compensation performance for the

904

source-side harmonic currents ISh with the series AF, whereKC=30 Ω. The series-connected AF perfectly suppresses thesource-side harmonic currents ISh.

Fig. 5(d) shows a per-phase base equivalent circuit for thesource-voltage harmonics VSh. In Fig. 5(d), ISh is expressedby

ISh =VSh

KC · (1 −G(s)) + ZS + ZC. (9)

Fig. 6(b) shows gain plots for Fig. 5(d). The line with KC=0shows the compensation performance for source-side harmonicvoltages VSh when the series AF is not connected. Fromthe 3th- to 4th-order components, the source-side harmoniccurrent ISh flows into the PLC. The line with KC=30 showsthe compensation performance for the source-side harmonicvoltages VSh with the series AF. The series-connected AFperfectly suppresses the source-side harmonic currents ISh.Thus, the source-side harmonic currents ISh are isolated by theseries AF in the proposed hybrid SVC. Therefore, sinusoidalsource currents with a unity power factor are obtained withthe proposed hybrid SVC. Note that no fundamental sourcevoltage appears across the series AF, and this results in asignificant reduction in the required rating of the series AF.

III. Simulation results

The validity and high-practicability of the proposed hybridSVC are confirmed by digital computer simulation using PSIMsoftware. The RMS value of the fundamental components ofthe load currents is 3.08 kArms. The 5th-order componentsof 334 Arms and 7th-order components of 100 Arms arealso included in the load currents iLa, iLb, and iLc. Thus,total harmonic distortion (THD) values of iLa, iLb, and iLc are11.3 %, respectively. The power factor (PF) is 0.7. Table IIshows circuit constants for Fig. 4, which are used in thefollowing simulation results. The small-rated LC filter Lfand Cf suppresses switching ripples generated by the PWMinverter, which performs the series AF. The constant dc-capacitor voltage control is added in Fig. 4. The dc-capacitorvoltage vDC is controlled using the d-axis component in d-qcoordinates of the PLCs currents iCab, iCbc, and iCca.

Fig. 7 shows simulation results for Fig. 4 before/afterthe series AF, which is a three-phase voltage-source PWMinverter, was started. vTa, vTb, and vTc are the receiving-endvoltage waveforms; iSa, iSb, and iSc are the source-currentwaveforms; iLa, iLb, and iLc are the load current waveforms;iTCa, iTCb, and iTCc are the TCR current waveforms; iCab, iCbc,and iCca are the PLC current waveforms; vAFa is an a-phaseoutput-voltage waveform of the series AF. In addition, vDC isthe dc-capacitor voltage waveforms. Before starting the seriesAF, the source currents iSa, iSb, and iSc are heavily distortedbecause the TCR and the three-phase load generate harmoniccurrents. The THD values of iSa, iSb, and iSc are 14.7 %,respectively. After the series AF is started, the source currentsiSa, iSb, and iSc are sinusoidal with a unity power factor. TheTHD values of iSa, iSb, and iSc are 2.2 %, 2.1 % and 2.4 %,respectively. The dc-capacitor voltage vDC is well controlled

TABLE IICircuit Constants for Fig. 4.

Item Symbol ValueSource inductor LS 1.64 mHFilter inductor Lf 0.066 mHFilter capacitor Cf 96 µF

Inductors of SVC Lab, Lbc, Lca 108 mHPLCs Cab, Cbc, Cca 114 µF

DC capacitor CDC 4700 µFDC-capacitor voltage V∗DC 10 kVdc

10 ms

cosφ = 1.0vTa

AF was started.

0

50 kV

0

5 kA

0

6 kA

0

0.8 kA

0

4 kA

0

4 kV

9.5 kV

10 kV

10.5 kV

vTcvTb

iSa iSc

iLaiLb iLc

iTCa iTCb iTCc

iCab iCbc iCca

iSb

vAFa

vdc

Fig. 7. Simulation waveforms for Fig. 4 before/after the series AF wasstarted.

to its reference value V∗DC = 10 kVdc. The ripple of the dc-capacitor voltage is 4.0 % in the transient state and ± 1.0 %in the steady state.

Fig. 8 shows simulation results for Fig. 4 with light to heavythree-phase load variation. The three-phase load currents iLa,iLb, and iLc vary from 0.6 pu to 1.0 pu. Before and afterthe load variations, the source currents iSa, iSb, and iSc aresinusoidal with a unity power factor. The dc-capacitor voltagevDC is well controlled to its reference value V∗DC = 10 kVdc.The ripple of the dc-capacitor voltage is 8.9 % in the transientstate and ± 1.0 % in the steady state.

The required rating of the three-phase voltage-source PWMinverter, which performs the series AF, is now calculated. Therequired rating S AF is expressed as

S AF = VAFa · ICab + VAFb · ICbc + VAFc · ICca, (10)

where VAFa, VAFb, VAFc, ICab, ICbc and ICca are all RMS values.From the simulation results shown in Fig. 7, S AF=4.7 MVA.This is 2.7 % as compared to that of the rating of the three-phase load. The addition of the small-rated three-phase andlow-cost voltage-source PWM inverter significantly improvesthe power quality on the source side. It is therefore confirmed

905

vTcvTb

iSa iSciSb

iLb iLciLa

iTCa iTCb iTCc

iCab iCbc iCca

vAFa

vdc

0

50 kV

0

5 kA

6 kA

0

2 kA

0

4 kA

5 kV

9 kV

10 kV

11 kV

20 ms

vTacosφ = 1.0

Three-phase load0.6 pu1.0 pu

Fig. 8. Simulation waveforms for Fig. 4 with light to heavy three-phase loadvariation.

that the proposed hybrid SVC is useful for practical applica-tions.

IV. ConclusionThis paper has proposed a new hybrid SVC topology

comprising a series AF and SVC, which consists of TCRsand pure PLCs. The series AF is connected in series to thepure PLCs. The series AF performs a resistor for source-sideharmonic currents. A sinusoidal source current with a unitypower factor is obtained. The basic principle of the proposedhybrid SVC has been discussed in detail. The compensationcharacteristics of the harmonic currents have been shown,and then confirmed by digital computer simulation usingPSIM software. Simulation results have demonstrated thatthe sinusoidal source currents with a unity power factor areobtained. From the simulation results, the required-capacity ofthe series AF is 2.7 % as compared to that of the rating ofthe three-phase load. It is, thus, concluded that the proposedhybrid SVC is useful for practical distribution feeders.

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