simple peak detection control algorithm of distribution static compensator for power quality...

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Published in IET Power ElectronicsReceived on 31st December 2012Revised on 10th December 2013Accepted on 25th December 2013doi: 10.1049/iet-pel.2013.0494

736The Institution of Engineering and Technology 2014

ISSN 1755-4535

Simple peak detection control algorithm ofdistribution static compensator for power qualityimprovementBhim Singh, Sabha Raj Arya, Chinmay Jain

Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi, Delhi 1100116, India

E-mail: [email protected]

Abstract: This study deals with the control of a three-phase, four-wire distribution static compensator (DSTATCOM) based onthe peak detection of load currents for power quality improvement under linear/non-linear loads. This control algorithm is simpleand easy in real time implementation. In this control algorithm, an extraction of active and reactive power components of loadcurrents is based on a low-pass filter and the voltage unit vector in time domain. It is based on the mathematical formulationsfor fast and accurate estimation of reference supply currents. A four-wire DSTATCOM is modelled and implemented for thecompensation of linear and non-linear loads using a digital signal processor. Performance of DSTATCOM is found quitesatisfactory under balanced and unbalanced loads in three-phase, four-wire distribution system.

1 Introduction

As the continuous depletion of conventional energy resourcessuch as oil, coal and gas, which are used for generation ofelectricity, an attention has been focused towards powerquality problems caused by non-linear nature of linear/non-linear type consumer loads [1]. Generally non-linearloads, which are based on the static power converters, arewidely used in the refrigeration, paper, transportation,cement and textile industries. Shunt active line conditionersare applicable for compensation of current-based distortionssuch as current harmonics, reactive power and neutralcurrent [2]. Advancement in power electronics convertersand development of various fast and accurate controlalgorithms in digital signal processing has made theimplementation of them in compensating devices [3–5].Performance of line conditioners at PCC (point of commoncoupling) are observed under the guidelines of variousstandards [6, 7]. El-Habrouk et al. [8] have presented areview on active filters which includes classification basedon power level, structure and number of phases. Khan et al.[9] have described dynamically decoupled topology whichis combination of parallel passive and an active filter toreduce the power rating of an active filter. Routimo et al.[10] have presented the benefits and drawbacks of voltagesource converter (VSC) and current source converter-basedactive filters. Active filters are also useful in powergenerating systems especially in renewable powergeneration [11–13]. Nishida et al. [11] have describedhybrid application of the active filter for voltage regulationand harmonics elimination in an induction generator.Sharma and Singh [12] have described an application of

VSC as voltage and frequency controller in permanentmagnet synchronous generator-based standalone windenergy conversion system. Patel and Agarwal [13] havedescribed solar energy conversion system with an interfaceof an active filter and its configuration. As the advancementin digital signal processing, various digital platforms areavailable to process the designed control algorithm.Performance of distribution static compensator(DSTATCOM) which is the new version of an active filterdepends upon its control algorithm used for extraction ofreference currents. Garcia-Cerrada et al. [14] have describedrepetitive control for current harmonics compensation whichis justified the improvement of the tracking of the referencesupply currents. Ramos and Costa-Castello [15] havedescribed the limitation of repetitive control and haveproposed new second-order odd-harmonics repetitivecontrol when the system frequency is not fixed or varyingwith time. Other control algorithms are also reported asadopting Lyapunov control approach used in integralcontroller for better harmonic current suppression [16],proportional-resonant controllers for better trackingperformance [17], improved hysteresis current control forthree-level DSTATCOM with limited band of switchingfrequency and reduced losses [18] and multilayer neuralnetwork for harmonic compensation [19]. Mikkili andPanda [20] have reported id−iq control algorithm forharmonic compensation and dc bus voltage regulation usingdifferent fuzzy membership function such as Gaussian,triangular and trapezoidal. In addition, a hybrid algorithmbased on proportional–integral (PI) control and repetitivecontrol in the synchronous reference frame (SRF) isreported for harmonics elimination [21]. A predictive

IET Power Electron., 2014, Vol. 7, Iss. 7, pp. 1736–1746doi: 10.1049/iet-pel.2013.0494

Fig. 1 Schematic diagram of VSC-based three-phase DSTATCOMin four distribution system

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control for high-performance active shunt filters [22], p−qtheory [23], single phase p−q theory using direct andindirect approach [24] are also used for harmonics andreactive power compensation under non-ideal AC mains etc.Zhou and Liu [25] have described the modification ofconventional PI control technique to overcome samplingand computation time delays under high di/dt operation. Itis designed based on deadbeat control. Rahmani et al. [26]have described direct and indirect control algorithms withmodified pulse width modulation (PWM) switching scheme.Cardenas et al. [27] have reported comparative analysis ofcontrol algorithms such as instantaneous reactive powertheory, SRF theory and peak detection method (PDM). ThePDM control algorithm has a low frequency oscillationproblem in DC bus voltage of the active power filter.Herrera and Salmeron [28] have reported a discussion oninstantaneous reactive power theory. It is based on itsapplication in AC mains, basic origin in term of power andreformulation under polluted AC mains. In four wiredistribution system, the neutral current is the majorcomponent which is generated because of non-linear loadsand unbalanced loading. It directly affects the rating of thesystem because of excess flow of neutral current [29].Transformer-based topologies in different connections havealso been applied for the neutral current compensation [30,31]. Generally these transformers are shunt connected withloads. Some of these transformers connections used forneutral current compensation are star–delta, star–hexagon,star/polygon, Scott and T connection, zig-zag etc. Therating of zig-zag, Scott and T-connected transformers arelow compared with star–delta, star–hexagon, star/polygonconnected transformers for neutral current compensation.Again among zig-zag, Scott and T-connected transformers;a zig-zag connected transformer has less rating for samevalue of neutral current. However, Scott and T-connectedtransformers require only two single-phase transformerswith different voltage rating. The zig-zag connectedtransformer is designed using three single-phasetransformers with a neutral conductor which provides a pathfor zero sequence load currents and splits this among thethree phases. This zig-zag connected transformer has alsoan enhanced capability of VSC for load balancing becausethe neutral current is distributed uniformly among all threephases. Reduced complexity and ruggedness are theadvantages of this type of neutral current compensationcompared with active compensation techniques. Some othertechniques used for neutral current compensation areVSC-based active compensation and a synchronousmachine [32–34]. The VSC-based DSTATCOM topologiesfor neutral current compensation are four leg VSC, three-legsplit-capacitor topology and three H-bridge VSC topologyetc. All these topologies require a control algorithm forcompensation of neutral current, balancing the capacitorvoltages and extra hardware components. Performance ofthese active compensation techniques depends upon thetuning of PI controller and effectiveness of the controlalgorithm. In three H-bridge VSC topology, number ofVSC legs are just double compared with three legs VSC. Asynchronous machine used for neutral compensation is notuseful because it requires maintenance for rotating part andcreates the noise problem.In this paper, a modified simple peak detection control
algorithm is implemented for load balancing, reactive powercompensation and harmonics elimination withself-supporting DC bus of VSC in a three-phase four-wiredistribution system. It is also extended for zero voltage


regulation at PCC. A zig-zag connected transformer is usedfor compensation of neutral current because it hasadvantage of low rating as well as enhanced the capabilityof VSC for load balancing [31]. It has equal distribution ofneutral currents among all three phases. Simple structureand fast extraction of reference supply currents are theadvantages of this control algorithm. Direct estimation ofreference supply currents from load currents without use ofany reference frame is the main characteristics of thisalgorithm. It is based on mathematical formulation usingbasic theory. As the simple structure of the controlalgorithm, uncertainty in response is drastically reduced.This algorithm requires only few components in real timeimplementation so its practical implementation is easy.

2 Configuration of DSTATCOM

Fig. 1 shows the schematic diagram of a DSTATCOMconnected to AC mains feeding the four-wire linear/non-linear loads. Three-phase loads may be an unbalancedload, a lagging power factor load, non-linear load orcombination of them. Non-linear loads are represented by athree-phase rectifier with a resistive load and an inductivefilter. The VSC performs an inverter operation andgenerates three-phase AC output PWM voltages forrequired compensation. For reducing ripple in compensatingcurrents, interfacing inductors (Lf) are used at AC side ofthe VSC. A small series connected capacitor (Cf) and aresistor (Rf) represent a ripple filter and it is connected atPCC to filter the high frequency switching noise of thePCC voltage. The rating of the IGBT (insulated gate bipolartransistor) switches of VSC is based on the voltage andcurrent rating of VSC for necessary compensation. Azig-zag transformer is used to compensate the load neutralcurrent (iLn). It is designed using three single-phasetransformers with a neutral conductor which provides a pathfor zero sequence load currents. For real timeimplementation, designed and selected values of the DCbus voltage, DC bus capacitor, AC inductors, the ripplefilter and three single-phase zig-zag transformers are givenin Appendix 1.

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3 Control algorithm
Various functions of DSTATCOM depend upon the controlalgorithm for accurate and fast detection of non-linearquality/disturbances. Generally DSTATCOM is based onVSC and it is controlled as a current source by use ofPWM switching. Fig. 2 shows the block diagram of a peakdetection control algorithm for extraction of referencesupply currents which is based on the mathematicalformulation. Basic equations of this control algorithm forthe estimation of various control signals are given as follows.

3.1 Conversion of line voltages to phase voltages

Sensed PCC line voltages vab and vbc are converted tothree-phase PCC voltages vsa, vsb and vsc as

vsavsbvsc

⎡⎣

⎤⎦ = 1

3

2 1−1−1

1−2

( )vabvbc

( )(1)

3.2 Estimation of amplitude of PCC voltages andvoltage unit vectors

The amplitude of PCC voltages (Vt) and in-phase unit vectorsare computed as

V t = 2 v2sa + v2sb + v2sc( )

/3{ }1/2

(2)

where vsa, vsb and vsc are three phase voltages at PCC.

Fig. 2 Peak detection-based control algorithm for extraction of referen


In-phase unit vectors of three-phase PCC voltages (upa, upband upc) are estimated after dividing respective phase voltagewith amplitude of three-phase PCC voltages (Vt) as [12]

upa =vsaV t

, upb =vsbV t

, upc =vscV t

(3)

The quadrature unit vectors (uaq, ubq and uqc) are estimated as

uqa =−ubp + ucp

( )��3

√ , uqb =3upa + upb − upc

( )2

��3

√ ,

uqc =−3upa + upb − upc

( )2

��3

√ (4)

3.3 Estimation of fundamental active and reactivepower components of load currents

Generally, the load current is lagging and distorted because ofapplication of reactive and non-linear loads. As the thesecomponents, loads current iLa(t) can be expressed as

iLa(t) = Io + ILpa(t)+ ILqa(t)+ Ih(t) (5)

where Io is the DC component which amplitude is very lowbecause of the half wave symmetry. The term ILpa is thefundamental active power component of load current forphase ‘a’. The term ILqa is the fundamental reactive powercomponent of load current for phase ‘a’ and Ih correspondsto harmonics present in iLa.

ce supply currents


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Again (5) can be rewritten as
iLa(t) = Io + ILpa sinvt + ILqa cosvt

+∑1m=1

I2m sin(2vmt + f2m)

+∑1n=1

I2n+1 sin (2n+ 1)vt + f2n+1

{ }(6)

In order to extract peak amplitude of fundamental activepower component of the load current for phase ‘a’ (ILpa),the in-phase vector upa (sinωt) is multiplied with the iLagiven in (6) for phase ‘a’ and it results in followingexpressions as [27] (see (7))

After product of phase ‘a’ load current (iLa) and in phase unitvector (upa), DC and oscillatory components are presented in(7). It is proportional to ILpa with the factor of 1/2. Theamplitude of fundamental active power component of theload current (ILpa′) is extracted using a low-pass filter witha cut off frequency (15 Hz) less than supply frequency. Anamplification factor of 2 as a gain is used to determineILpa′. The amplitude of fundamental active powercomponent ILpb′, ILpc′ of the load currents for phases ‘b’and ‘c’ are extracted using same procedure. The averageamplitude of fundamental active power component of loadcurrents (ILpA) is calculated as

ILpA = (I ′Lpa + I ′Lpb + I ′Lpc)/3 (8)

The amplitude of fundamental reactive power component ofthe load current (ILqa) for phase ‘a’ is extracted bymultiplying (6) with quadrature unit vector uqa (cos ωt) asfollows (see (9))

After multiplication of phase ‘a’ load current (iLa) withquadrature phase unit vector (uqa), DC and oscillatorycomponents are presented in (9). It is proportional to ILqawith a factor of 1/2. The amplitude of fundamental reactivepower component of the load current (ILqa′) is extractedusing a low-pass filter with a cut off frequency (15 Hz) lessthan supply frequency. An amplification factor of 2 as a

iLpa = iLa(t) · upa = iLa(t) sinvt

= Io sinvt + [(ILpa/2){1− ( cos 2

+∑1m=1

I2m2

[ cos{(2m− 1)vt + f

+∑1n=1

I2n+1

2[ cos{(2nvt + f2n+

iLqa = iLa(t) · uqa = iLa(t) cosvt

= Io cosvt + [(ILqa/2){1+ ( cos

+∑1m=1

I2m2

[ sin{(2m+ 1)vt + f

+∑1n=1

I2n+1

2[ sin(2n+ 2)vt + f


gain is used to determine ILqa′. This control algorithmbelongs in the category of classical control based on thestrong mathematical expression. The fundamental reactivepower component of the load currents for phases ‘b’ and‘c’, ILqb′, ILqc′ are extracted in the same manner. Theaverage amplitude of load reactive power component ofcurrents (ILqA) is calculated as

ILqA = (I ′Lqa + I ′Lqb + I ′Lqc)/3 (10)

3.4 Extraction of active power component ofreference supply currents

The active power component of supply currents (Itsp) is anaddition of fundamental active power component of loadcurrent (ILpA) and a current required for self-supporting DCbus of DSTATCOM (Icp). The control DC bus voltage ofDSTATCOM is achieved by adjusting a small amount ofactive power flowing into the DC capacitor of VSC. Acurrent required for self-supporting DC bus of VSC isextracted using PI controller over the DC bus voltage whichis used to eliminate the steady-state error in the DC voltageof VSC of DSTATCOM. At nth instant, it is expressed as

Icp(n) = Icp(n− 1)+ kdp ve(n)− ve(n− 1){ }+ kdive(n)

(11)

where ve = v∗dc − vdcis the error in DC bus voltage. v∗dc and vdcare the reference voltage and sensed filtered voltage of DCbus of DSTATCOM, respectively. kdp and kdi are theproportional and integral gain constants of the DC bus PIvoltage controller.Total value of active power component of reference supply

current (Itsp) is as

Itsp = ILpA + Icp (12)

3.5 Extraction of reactive power component ofreference supply currents

The DSTATCOM can be operated in either power factorcorrection (PFC) or ZVR modes. The ZVR mode is used

vt)}]+ {(ILqa/2} sin 2vt}

2m}− cos{(2m+ 1)vt + f2m}]

1)}− cos{(2n+ 2)vt + f2n+1}]

(7)

2vt)}]+ {(ILpa/2} sin 2vt}

2m}+ sin{(2m− 1)vt + f2m}]

2n+1 sin(2nvt + f2n+1)]

(9)


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when PCC voltage is not at rated value. The PCC voltage mayreduce because of loading. In case of ZVR, the PCC voltageis regulated by supplying extra reactive power locally. Forregulating PCC voltages, the VSC has to supply extrareactive power of the load as well as to compensate thedrop because of supply impedance. The reactive powercomponent of load current is estimated from load currents.A voltage PI controller is used to estimate total reactivepower component for ZVR to be supplied by DSTATCOM.The output of PCC voltage PI controller includes the extrareactive power component of supply current required forZVR. The reactive power component of load current issubtracted from output of PCC voltage PI controller asshown in Fig. 2 to estimate net reactive power componentof reference supply current. This reactive power componentof supply current (Icq) is to regulate the amplitude of thePCC voltage at nth sampling instant is expressed as
Icq(n) = Icq(n− 1)+ kqp Vte(n)− Vte(n− 1){ }+ kqiVte

(13)

where Vte = V ∗t − Vt is the error in amplitude of the PCC

voltage. V ∗t and Vt are the amplitude of PCC reference

voltage and amplitude of PCC voltage, respectively. kqp andkqi are the proportional and integral gain constants of thePCC voltage bus PI voltage controller.Total value of reactive power components of reference

supply current (Itsq) is as

Itsq = Icq − ILqA (14)

3.6 Estimation of reference supply currents andgeneration of devices gating pulses

Three-phase reference active power components of supplycurrents are estimated using its amplitude (Itsp) and in phaseunit voltage vectors (upa, upb and upc) as

isap = Itspupa, isbp = Itspupb, iscp = Itspupc (15)

Similarly, reference reactive power components of supplycurrents are estimated using its amplitude (Itsq) andquadrature unit voltage vectors (uqa, uqb and uqc) as

isaq = Itsquqa, isbq = Itsquqb, iscq = Itsquqc (16)

Total reference supply currents are obtained after addition ofreference active and reactive powers components of supplycurrents as

i∗sa = isap + isaq, i∗sb = isbp + isbq, i∗sc = iscp + iscq (17)

These three-phase reference supply currents (i∗sa, i∗sb and i∗sc)are compared with sensed supply currents (isa, isb and isc) toestimated current errors. These current errors (iea, ieb andiec) are amplified using PI controllers and output of the PIcontrollers is used in the PWM controller to generate gatingpulses of IGBTs of VSC used as DSTATCOM.

4 Simulation results

The performance of DSTATCOM using proposed peakdetection control algorithm is simulated using developedMATLAB model for PFC and ZVR modes of operation


under linear and non-linear loads. The performance ofDSTATCOM is presented for linear and non-linear loads asfollows.

4.1 Performance of DSTATCOM under linear loadsin PFC mode

Fig. 3a shows the response of DSTATCOM for reactivepower compensation under lagging power factor linear load.The performance of DSTATCOM is shown as phasevoltages at PCC (vs), balanced supply currents (is), supplyneutral current (isn), load currents (iLa, iLb and iLc), loadneutral current (iLn), compensator currents (iCa, iCb and iCc)and DC bus voltage (vdc) at the time of load injection onphase ‘a’ load at t = 1.45 s. It shows the unity power factorafter compensation of load reactive power demand throughDSTATCOM and balanced supply currents during loadunbalancing.

4.2 Performance of DSTATCOM under non-linearloads in PFC mode

A diode bridge rectifier is modeled as a non-linear load and itis connected at PCC. The dynamic performance ofDSTATCOM with PCC voltages (vs), supply currents(is),source neutral current (isn), load currents (iLa, iLb and iLc),load neutral current (iLn), compensating currents (iCa, iCband iCc) and DC bus voltage are shown in Fig. 3b duringload injection. Function of neutral current compensation canbe observed from values of isn and iLn. After compensation,value of isn is very small compared with iLn. The totalharmonic distortions (THDs) of phase ‘a’ voltage at PCC(vsa), supply current (isa) and load current (iLa) are shownin Table 1. The THD of the phase ‘a’ at PCC voltage,supply current, load current are 2.16, 3.23 and 35.96%,respectively. These results provide satisfactory performanceof DSTATCOM for load balancing and harmonicselimination according to IEEE-519 standard guidelines.

4.3 Performance of DSTATCOM under linear loadsin ZVR mode

Fig. 3c shows the dynamic performance of the DSTATCOMused for voltage control and load balancing when a phase loadat t = 1.45 s is switched on. Performance indices such as PCCphase voltages (vs), balanced supply currents (is), supplyneutral current (isn), load currents (iLa, iLb and iLc), loadneutral current (iLn), compensator currents (iCa, iCb and iCc),amplitude of voltages at PCC (Vt) and DC bus voltage (vdc)are shown under linear loads where supply currents areleading compared with PCC voltage. The DSTATCOMsupplies some extra leading reactive power components inaddition to load reactive power demand in ZVR mode. Thisextra leading reactive power is required to regulate PCCvoltage up to desired level. Basically, it is generated byDSTATCOM by the action of PCC voltage PI controller.

4.4 Performance of DSTATCOM under non-linearloads in ZVR mode

Performance of the control algorithm of DSTATCOM is alsostudied under current fed type non-linear load. The dynamicperformance of DSTATCOM in terms of waveforms suchas PCC phase voltages (vs), balanced supply currents (is),supply neutral current (isn), load currents (iLa, iLb and iLc),load neutral current (iLn), compensator currents (iCa, iCb and


Fig. 3 Dynamic performance of DSTATCOM under varying

a Linear loads in PFC modeb Non-linear loads in PFC modec Linear loads in ZVR moded Non-linear loads in ZVR mode

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iCc), DC bus voltage (vdc) and amplitude of voltages at PCC(vt) are shown in Fig. 3d. It shows the smooth variation ofsupply currents (is), DC bus voltage (vdc), terminal voltage(vt) during load perturbation. The THDs of phase ‘a’voltage at PCC (vsa), supply current (isa) and load current(iLa) are shown in Table 1. The THDs of the phase ‘a’ atPCC voltage, supply current, load current are 2.28, 3.34and 35.9%, respectively, where THD of supply current isunder the limit of IEEE-519 standard. Three-phase PCCvoltages are regulated to the desired value which hasreduced because of loading in PFC mode. These results,demonstrate the satisfactory performance of DSTATCOMfor harmonics elimination, load balancing, reactive powercompensation in three-phase distribution system.


5 Experimental results

A peak detection control algorithm used in DSTATCOM isapplied for real time implementation. Its steady anddynamic performances are studied using a DSP(d-SPACE-1103). A full scale laboratory model ofDSTATCOM using a VSC is developed to validate theproposed control algorithm. It is connected in shunt at PCC.The ripple filter is connected parallel to PCC which is theseries connected resistances and capacitances forsuppression switching noise. Reference supply currents areestimated from sensed PCC voltages (vab and vbc), loadcurrents (iLa, iLb and iLc) and the DC bus voltage ofDSTATCOM (vdc). Hardware implementation data are


Table 1 Simulation performance of DSTATCOM

Operatingmode

Performanceparameters (RMS

value)

Linearload

Non-linear load(three-phase dioderectifier with filter

inductance)

PFC mode PCC voltage, V, %THD

238.012(1.97%)

238.012 (2.16%)

supply current, A,%THD

23.12(2.93%)

23.13 (3.23%)

compensatorcurrent, A

16.36 6.73

load current, A, %THD

27.61(0.13%)

21.35 (35.96%)

ZVR mode PCC voltage, V, %THD

239.71(2.15%)

239.42 (2.28%)

supply current, A,% THD

23.36(2.62%)

23.31 (3.34%)

compensatorcurrent, A

19.32 8.93

load current, A, %THD

27.80(0.18%)

21.48 (35.90%)

DC bus voltage, V 700 700

Fig. 4 Extraction of phase ‘a’ reference supply current duringload injection using peak detection-based control algorithm

a vab, iLa, iLpa, ILpab ILpA, Itsp, isap, isa

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given in Appendix 2. The performance of the peak detectioncontrol algorithm in the three-phase four-wire DSTATCOMis analysed under linear/non-linear and unbalanced loadsand test results are discussed as follows.

5.1 Performance of control algorithm

Figs. 4a and b show the various intermediate signals of theproposed control algorithm which include PCC voltage(vab) load current (iLa), amplitude of active powercomponent of load current (iLpa), filtered amplitude ofphase ‘a’ load current (ILpa

′), average load current (ILpA),total reference active power component of supply current(Itsp), reference active power component of supply current(i∗sap) and phase a supply current (isa), respectively. In PFC,reference supply active power components (isap) isconsidered as reference supply current (i∗sa). It shows anaccurate and fast extraction of control signals and trackingof reference active power component of supply current (i∗sa)and supply current (isa) under non-linear loads during loadinjection in PFC mode even at small variation in loads.

5.2 Steady-state and dynamic performances ofDSTATCOM under linear loads

The power factor at AC mains is improved and it ismaintained near unity after load reactive powercompensation using DSTATCOM. It is illustrated in

Fig. 5 Steady-state load balancing performance of DSTATCOM under


Figs. 5a and b where supply kVAR and load kVAR are0.13 and 3.39, respectively. The reactive power ofDSTATCOM is shown in Fig 5c which is leading in nature.In steady-state operation, three-phase supply currents (isa,isb and isc), compensator currents (iCa, iCb and iCc) and loadcurrents (iLa, iLb and iLc) are shown in Table 2 underbalanced and unbalanced operation. The magnitude ofthree-phase supply currents and load current are 5.63, 5.31,5.49 A and 0.077, 9.38, 8.52 A, respectively under

linear load (a–c) Ps, PL and Pc


Table 2 Experimental performance of DSTATCOM under linear load

Performance parameters (RMS value) PFC mode

Linear load (three-phase resistive and inductive load)

(a) At balanced condition (b) At unbalanced condition

Phase a Phase b Phase c Phase a Phase b Phase c

PCC phase voltage, V 233.94 234.00 233.94 233.24 233.30 233.19supply current, A 7.63 7.70 7.67 5.63 5.31 5.49compensator current, A 5.68 5.31 5.49 3.309 3.15 6.24load current, A 8.47 8.51 8.72 0.077 9.38 8.52DC bus voltage, V 700 700

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unbalanced linear loads. From these test results, it is observedthat the magnitude of supply current is less compared withload current after reactive power compensation underbalanced linear load operation.Figs. 6a and b show the waveform of supply currents (isa,

isb and isc) and load currents (iLa, iLb and iLc) with PCC line

Fig. 6 Dynamic performance of DSTATCOM during injection of phase

a vab, isa, isb and iscb vab, iLa, iLb and iLcc vdc isa, iLa and iCad vab, isa, iLn and iTn


voltage (vab) as a reference under unbalanced linear loads.Unbalanced load is realised by injection of load in phase‘a’. The supply current (isa), load current (iLa) andDSTATCOM current (iCa) are shown with DC bus voltage(vdc) in Fig. 6c under varying load conditions. It shows thebalanced supply currents when load currents are

‘a’ in linear loads


Table 4 Experimental performance of DSTATCOM undernon-linear

Performance parameters(RMS value)

PFC mode

Non-linear load underunbalanced operation

(three-phase diode rectifierwith filter inductance)

Phase a Phase b Phase c

PCC phase voltage, V 241.15 241.27 240.58supply current, A, %THD 5.09

(4.3%)5.19(4.5%)

5.05(4.6%)

compensator current, A 3.255 2.157 3.94

Table 3 Experimental performance of DSTATCOM undernon-linear loads

Performance parameters(RMS value)

PFC mode

Non-linear load under balancedoperation (three-phase dioderectifier with filter inductance)

Phase a Phase b Phase c

PCC phase voltage, V 236.42 236.82 236.94supply current, A, %THD 7.39

(3.0%)7.39(2.9%)

7.33(2.8%)

compensator current, A 2.383 2.332 2.346load current, A, %THD 7.55

(34.2%)7.54

(34.0%)7.56

(33.3%)load per phase power, kVAR 0.70 0.68 0.68supply power, kVAR 0.00DC bus voltage, V 700supply neutral current, A 0.043load neutral current, A 5.56zig-zag transformer current asa compensator current, A

5.54

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unbalanced. The function of neutral current compensation canbe observed from Fig. 6d where load neutral current (iLn) andzig-zag transformer neutral current (iTn) are equal andopposite in phase at the time of load removal. Both currentsare cancelling each other and compensate load neutralcurrent even at small variation in the load current.
5.3 Steady-state performance of DSTATCOMunder non-linear loads

Phase ‘a’ supply current (isa) with its harmonic spectrum andload current (iLa) with its harmonic spectrum are shown inFigs. 7a–d under steady-state operation. After PFC, THDsof supply currents and load currents are 3.0 and 34.2%,respectively. In steady-state operation, three-phase supplycurrents (isa, isb and isc) with harmonic distortion,compensator currents (iCa, iCb and iCc) and load currents(iLa, iLb and iLc) with harmonic distortion are shown inTable 3 under balanced operation. The function of neutralcurrent compensation can be observed from this table wheresupply neutral current (isn) is almost negligible. From thesetest results, it is concluded that proposed control algorithmis effective for harmonics suppression with improvedsupply power factor near to unity without affecting DC busvoltage.

5.4 Load balancing and dynamic performance ofDSTATCOM

Table 4 shows the root-mean square (RMS) magnitude ofthree-phase supply currents (isa, isb and isc) and their THDs,compensator currents (iCa, iCb and iCc), load currents (iLa,iLb and iLc) and their THDs, and load per phase powerunder unbalanced non-linear loads. Under unbalancedloads, three supply currents and load currents are 5.09,5.19, 5.05 A and 0.033, 7.49, 7.57 A, respectively. The

Fig. 7 Performance of DSTATCOM under non-linear loads

a isa with vabb Harmonic spectrum of isac iLa with vabd Harmonic spectrum of iLa

load current, A, %THD 0.033 7.49(30.0%)

7.57(30.3%)

load per phase power, kVAR 0 0.62 0.70supply power, kVAR 0.00DC bus voltage, V 700supply neutral current, A 0.016load neutral current, A 7.82zig-zag transformer current asa compensator current, A

7.82


unbalanced load currents are compensated by theDSTATCOM and it maintains the supply currents balanced.During this condition, THDs of three-phase supply currents(isa, isb and isc) and load currents in phases ‘b’ and ‘c’ are4.3, 4.5, 4.6% and 30.0, 30.3%, respectively, with improvedpower factor. After compensation supply neutral current is0.016 A which is very small compared with load neutralcurrent. These test results show satisfactory performance ofthe control algorithm used in DSTATCOM for harmonicselimination, neutral current compensation and loadbalancing under unbalanced non-linear loads.Figs. 8a and b show the waveforms of supply currents (isa,

isb and isc) and load currents (iLa, iLb and iLc) in dynamiccondition with respect to line PCC voltage (vab) undernon-linear loads. Fig. 8c shows the waveform of supplycurrent (isa), load current (iLa) and compensating current(iCa) with DC bus voltage (vdc) in unbalanced loadcondition. The function of neutral current compensation isshown in Fig. 8d where phase ‘a’ supply neutral current(isn), load neutral current (iLn), zig-zag connected


Fig. 8 Dynamic performance of DSTATCOM during injection of phase ‘a’ in non-linear loads

a vab, isa, isb and iscb vab, iLa, iLb and iLcc vdc isa, iLa and iCad vab, isn, iLn and iTn

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transformer neutral point current (iTn) are shown with linePCC voltage (vab). These test results demonstrate thesatisfactory performance of proposed control algorithm ofDSTATCOM under dynamic conditions.

6 Conclusion

A peak detection control algorithm of DSTATCOM has beenimplemented for the load compensation in a three-phasefour-wire distribution system. It has been used forextraction of fundamental active power and reactive powercomponents of load currents. These load currentcomponents have been used for estimation of referencesupply currents with DC bus voltage control ofDSTATCOM. The performance of DSTATCOM has beenfound quite satisfactory for load balancing, reactive powercompensation and harmonics elimination under limit ofIEEE-519 and IEC-61000 standards. Proposed controlalgorithm of DSTATCOM has been found satisfactoryunder dynamic and steady-state conditions even smallchange in loads. The DC bus voltage of the DSTATCOMhas also been regulated to desired value under varying loadconditions.


7 References

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8 Appendix

8.1 Appendix 1

AC mains: three-phase, 415 V (L-L), 50 Hz; sourceimpedance: Rs = 0.07 Ω, Ls = 2 mH; load: (i) linear: 20 kVA,0.8 pf lagging (ii) non-linear: three-phase full bridgeuncontrolled rectifier with R = 8 Ω and L = 100 mH; ripplefilter: Rf = 5 Ω, Cf = 10 μF; DC bus capacitance: 7500 μF;reference DC bus voltage: 700 V; frequency of low-passfilter used in DC bus = 12 Hz; frequency of low-pass filterused in AC bus = 12 Hz; interfacing inductor (Lf) = 3.5 mH;7 kVA 50 Hz, three single-phase zig-zag connectedtransformer (maximum worst condition).

8.2 Appendix 2

AC mains: three-phase, 415 V (L-L), 50 Hz; load: (i) linear:6.10 kVA, 0.83 DPF lagging, (ii) non-linear loads: threesingle-phase full bridge uncontrolled rectifier with R = 29 Ωand L = 100 mH; gains of PI controller for DC bus voltage:kdp = 0.15, kdi = 0.01; DC bus capacitance: 2350 μF;reference DC bus voltage: 700 V; interfacing inductor(Lf) = 7 mH; passive ripple filter: Rf = 5 Ω, Cf = 5 μF; 10 kVA200 V, 50 Hz, three single-phase zig-zag transformer; cutoff frequency of LPF used in DC bus = 12 Hz.


simple peak detection control algorithm of distribution static compensator for power quality...

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