a hysteresis current controller based dstatcom for power ...tating d-q reference frame. the time...

A Hysteresis Current Controller basedDSTATCOM for Power-Quality

Improvement

M.Kala RathiAssistant Professor

Electrical and Electronics Engineering DepartmentMepco Schlenk Engineering College,

Sivakasi, [email protected]

November 2, 2018

Abstract

There is a huge growth in Power Electronics applica-tions. The power electronics systems offer highly non -linearcharacteristics. An increase in such non linearity causes var-ious undesirable features such as increased power qualityproblems such as voltage sag and voltage swell. In orderto overcome these problems custom power devices such asDistribution Static Compensator (DSTATCOM) have beenanalyzed and developed. The main challenge is to develop acontrol strategy to generate a reference voltage for DSTAT-COM to mitigate the power quality problems and to supplythe reactive power need produced by nonlinear loads. Theimplementation of the control strategy is done in a threestep process. The first is the sensing of signals. The sec-ond step is the derivation of compensating signals based oncontrol methods and filter configurations. Among the var-ious current strategies available for deriving compensatingsignals, Synchronous Reference Frame (SRF) theory is usedhere. The final stage is the generating gating signals for the

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International Journal of Pure and Applied MathematicsVolume 120 No. 6 2018, 1257-1271ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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bridges of the DSTATCOM using hysteresis current con-troller. The effectiveness of the proposed system is verifiedby conducting simulations in MATLAB.Key Words::DSTATCOM, Power Quality, voltage regula-tion, PI controller, Hysteresis current controller.

1 Introduction

Power electronics is a branch of electrical engineering devoted to thecontrol and conversion of electrical power using electronic convert-ers based on power semiconductor devices. The advent of Thyris-tors led to the development of area of application of power electron-ics. Power electronics applications span the whole field of electricalpower systems with power ranging from a few VA/Watts to sev-eral MVA/MW. The main task of power electronics is to controland convert electrical power from one form to another. Because ofthe proliferation of sensitive power electronics devices, the powersystem meets a lot of power quality problems. Utility distributionnetworks, sensitive industrial loads and critical commercial opera-tions suffer from various types of outages and service interruptionswhich can cost significant financial losses. With the restructur-ing of power systems and with shifting trend towards distributedand dispersed generation, the issue of power quality is going totake newer dimensions. Voltage sag and voltage swell are some ofsuch problems. Voltage sag is caused by a fault in the utility sys-tem, a fault within the customers facility or a large increase of theload current, like starting a motor or transformer energizing. Typ-ical faults are single-phase or multiple-phase short circuits, whichleads to high currents. The high current results in a voltage dropover the network impedance. At the fault location the voltage inthe faulted phases drops close to zero, whereas in the non-faultedphases it remains more or less unchanged. To mitigate the powerquality problems, many research works are developed using cus-tom power devices. Custom power devices embrace a family ofpower electronics devices or a toolbox which is applicable to thedistribution systems to provide power quality problem solutions.This technology has been made achievable due to other extensiveavailability of cost effective high power semiconductor devices suchas Gate Turn Off (GTO) Thyristors and Insulated Gate Bipolar

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Transistors (IGBT), a low cost microprocessors and techniques de-veloped in the area of power electronics. This problem can bemitigated by installing a custom power device called DistributionStatic Compensator (DSTATCOM)[1-4]. DSTATCOM is a shunt-connected custom power device and is often referred to as shuntor parallel active power filter. It is used for voltage regulation ata distribution bus. This paper proposes a new algorithm to gen-erate a reference voltage for the Distribution Static Compensator(DSTATCOM) operating in Voltage Control Mode. The compen-sator injects lower current and therefore reduces losses in the feederand voltage source inverter. Further, a saving in the rating of theDSTATCOM is achieved which increase its capacity to mitigatevoltage sag and reactive power compensation. Thus the proposedsystem improves the power quality based on generated power anddemand. Designing a suitable controller for the DSTATCOM isvery important. A number of control strategies such as instan-taneous reactive power theory [5-6], synchronous reference frametheory [7-9], modified synchronous reference frame theory [10] hasbeen studied for the development of three phase active filter andDSTATCOM. Suleiman Musa et. al proposed the use of modifiedSRF extraction with fuzzy logic controller for Pulse Width Mod-ulation (PWM) current control of voltage source inverter in shuntactive power filter[11]. Bhuvaneswari et. al developed a new Icosalgorithm for designing and implementing three phase shunt activepower filter[12]. Ravi Kiran Dasari et.al employed DSTATCOM forimproving power quality in Brushless Permanent Magnet and Non-Permanent Magnet Machines [13]. An adaptive control strategy isemployed for STATCOM in ship electric power system [14]. A newtype of STATCOM suitable for high voltage applications is devel-oped using cascading inverters by Liang et. al[15]. The stabilityanalysis of power system containing series and shunt compensatorswas done by Ghosh et.al[16]. This work uses SRF theory for gener-ating reference signals. The DC voltage regulation is an importantpart of STATCOM operation and a simple PI controller is usedfor this. The pulses are generated using simple hysteresis controlwhich can be implemented easily. Simulations are performed to ver-ify the performance of the developed system and its results showthe efficacy of the system.

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2 SYSTEM DESCRIPTION

The Static Synchronous Compensator (STATCOM) is a shunt con-nected reactive compensation equipment which is capable of gener-ating and/or absorbing reactive power whose output can be variedso as to maintain control of specific parameters of the electric powersystem[17-19]. The STATCOM provides operating characteristicssimilar to a rotating synchronous compensator without the me-chanical inertia, because the STATCOM employs solid state powerswitching devices it provides rapid controllability of the three phasevoltages, both in magnitude and phase angle. STATCOM providevoltage support to buses by modulating bus voltages during dy-namic disturbances in order to provide better transient character-istics, improve the transient stability margins and to damp out thesystem oscillations due to these disturbances The DSTATCOM isconnected in distribution system at the Point of Common Couplingknown as PCC.

Figure 1. Voltage Source Inverter based DSTATCOM

A STATCOM is comparable to a Synchronous Condenser (or Com-pensator) which can supply variable reactive power and regulatethe voltage of the bus where it is connected. Figure 1 shows thevoltage source inverter based DSTATCOM.

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3 REFERENE SIGNAL GENERATION

Figure 2. Block diagram representation of SRF theory

Figure 2 shows the block diagram representation of SRF theory.Here for the generation of reference currents synchronous referenceframe theory is employed. Using parks transformation, the threephase load currents and source voltages are converted into the ro-tating d-q reference frame. The time domain based synchronousreference frame theory is utilized in this work to extract the refer-ence current from the distorted line current. The SRF control strat-egy operates in steady-state as well as in dynamic-state perfectly.Another important characteristic of SRF theory is the simplicityof the calculations, which involves only algebraic calculation. Inthis method, angle is calculated directly from main voltages usingPLL circuit and thus enables the method to be frequency inde-pendent. Thus large numbers of synchronization problems withunbalanced and non-sinusoidal voltages are also avoided. The syn-chronous frame method uses Parks transformation to transform thethree phase ac quantities into synchronous rotating direct, quadra-ture and zero sequence components which are dc components andare easy to analyze. The direct and quadrature components repre-sent the active and reactive powers respectively. The higher orderharmonics still remain in the system but can be eliminated by usinglow pass filters. If theta is the transformation angle, the transfor-mation is defined by: The basic structure of SRF method consists ofPhase Locked Loop(PLL) -circuit for vector orientation ( sin θ andcos θ ) and a controller for dc-link capacitor voltage regulation. The

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three-phase load currents iLa , iLb, iLc in stationary coordinatesare converted into the two-phase direct axis (d) and quadratic axis(q) rotating coordinates currents id-iq, using parks transformationas shown in Equation 1.

Equation.2 and 3 gives the average value or dc component and anoscillating value or ac component contained by the each currentcomponent (id , iq).

For the elimination of dc components in the non-linear load cur-rents, the currents (id, iq) obtained from parks transformation arepassed through a Butterworth type low pass filter. The PI con-troller is used to reduce the steady state error of the dc componentand maintains the dc-capacitance voltage of the inverter. The out-put current from this controller is added to the positive sequencefundamental frequency active component of the d-q current so asto abate the inverter losses. The desired reference currents areobtained using inverse parks transformation given in Equation 4,which transforms two phase synchronous frame currents (id , iq)into three-phase stationary (a-b-c) reference frame currents.

4 CONTROL STRATEGY

A. DC Link Voltage Regulation with PI Controller:The controllers are used in a feedback mechanism that continuouslycalculates the error value e (t) as the difference between the desiredset point and a measured process variable and applies a correction

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based on the proportional, integral, and derivative terms. The con-trollers are broadly classified into three types. They are•Proportional controller•Integral controller•Proportional plus integral controller(PI).

Figure 3.PI controller block diagram

PI controller is a conventional control method. This controller isenough to eliminate steady state error and peak over shoot. Thecapacitor voltage at the dc-link should not fluctuate in order toensure proper working of STATCOM[20-22]. In order to maintaindc link voltage constant and to generate the compensating refer-ence currents, PI controller is used in this work. The initial stepin implementing DC voltage regulation is sensing of dc-side capac-itor voltage and comparing with a set reference voltage. The errorvoltage e= Vdc-ref -Vdc , is then processed through a PI controller,which contributes to zero steady state error in tracking the referencecurrent signal. The output of the PI controller (id1h) represents thetotal active current required to maintain Vdc at a constant valuewhich in turn compensates for the power losses occurring insideAPF represented by Equation 5 [20-22].

where Kp and Ki are the proportional and integral gains of the PIcontroller respectively.

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5 GENERATION OF GATING PULSES

Figure 4 shows the gating signal generation block used in this work.The hysteresis block is used for generating gating pulses. It makesthe actual compensating current of DSTATCOM to follow the ref-erence compensating current within a fixed hysteresis band. It pro-vides excellent dynamic performance and simple to implement.

Figure.4 Hysteresis current controller.

6 SIMULATION RESULTS

Computer simulations are developed to analyze and verify the the-oretical models. Three phase three wire system with nonlinear loadis simulated to have a brief study of power quality problems likevoltage sag and voltage swell. The power system is connected to avariable load, which serves as a source of non-linearity creating volt-age sag and voltage swell. The simulation diagram of the systemis shown in figure 5 and the value of the energy storage capacitorand the reference voltage to the capacitor are given in the table 1.

TABLE I System parameters

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Figure 5. Simulation diagram of a three phase system withoutDSTATCOM.

The load voltage depicting voltage source and voltage sag areshown in figure 6.

Figure 6. Voltage sag and swell.

The DSTATCOM is introduced in the system for supplying thereactive power needed, thereby compensating the missing voltage.Here in the proposed system to compensate the missing voltagePI controller is employed. The synchronous reference frame theoryis used for the computation of reference current. The subsystemshowing the extraction of reference current is shown in figure 7.

Fig.7.Extraction of reference current

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Figure 8.DSTATCOM controllerThe sub-system showing DSTATCOM controller is shown in

Figure 8.

Figure 9.Simulink model of DSTATCOM

Figure 9 shows the DSTATCOM employed to mitigate the voltagesag and voltage swell problems: The equation for real and reactivepower injected is given by equation 6 and 7.

The graph showing the real and reactive power injected using theDSTATCOM is shown in Figure 9.

Figure.10 Real and Reactive power injectionUsing PI controller the DC Voltage has been regulated and graph

showing the regulated voltage is shown in Figure 11.

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Figure.11 Voltage at DC bus

Figure.12 Regulated phase voltage

7 Conclusion

In this paper, DSTATCOM is implemented using PI controller.The proportional and integral gains are chosen heuristically. Thevoltage is injected into system to mitigate the voltage sag. ThusDSTATCOM compensates the reactive power needed by the systemsince voltage injected is directly proportional to the reactive powerinjected in to the system. By implementing PI controller there isreduction in distortion of DC voltage. Hysteresis current controlis used to generate pulses for the switches used in DSTATCOM.The control strategy for reference current extraction based on Syn-chronous Reference Frame theory provides a better extraction ofreference compensation currents from the distorted line currents.Also the simulation results show the system works well.

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a hysteresis current controller based dstatcom for power ...tating d-q reference frame. the time...

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