d-statcom modeling and simulation applying ...this paper focuses to design and analyze the simulink...

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Int. J. Elec&Electr.Eng&Telecoms. 2015 Kajal Rathore et al., 2015

D-STATCOM MODELING AND SIMULATIONAPPLYING CONTROL SCHEMES FOR POWER

QUALITY IMPROVEMENT

Kajal Rathore1*, Dhananjay Kumar1 and Sunil Yadav1

*Corresponding Author: Kajal Rathore, [email protected]

This paper focuses to design and analyze the Simulink model for the control of Distributed staticsynchronous compensator (D-STATCOM). It is important for a distribution system to havecontrollability, voltage stability and good power transfer capability. The voltage source converterbased D-STATCOM achieves voltage regulation in the system by absorbing or supplying therequired reactive power. The system is modeled using two control strategies, i.e., direct andindirect control which includes d-q transform. The comparative analysis of the control strategieshas been done. The results of simulation are demonstrated and analyzed using MATLAB.

Keywords: Reactive power compensation, D-STATCOM, dq-model, Phase shift control, Voltagecontrol, VSC

INTRODUCTIONThe rapidly developing power electronicstechnology provides an opportunity fordeveloping new power equipment forimproving the performance of the powersystem. Flexible AC Transmission Systemtechnology (FACTS) uses the latest powerelectronic devices and methods to controlelectronically the high-voltage side of thenetwork. FACTS devices can be used forpower flow control, voltage regulation, transientstability improvement, and damping of poweroscillations. FACTS devices can be of shuntor series or combination of shunt and series

ISSN 2319 – 2518 www.ijeetc.comVol. 4, No. 2, April 2015

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Int. J. Elec&Electr.Eng&Telecoms. 2015

1 Jawaharlal Institute of Technology, Borawan, Madhya Pradesh 451228, India.

types. The shunt devices can be used forvoltage regulations, while series devices canbe used for regulation of line impedance andseries-parallel combination can be used forreal and reactive power compensation inaddition to regulation of voltage and regulationof line impedance (Hingorani, 1991).

The family of emerging power electronicsdevices being offered to achieve these custompower objectives is:

• Distribution Static Compensator (D-STATCOM) that protects the distributionsystem from the effects of fluctuating

Research Paper

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voltage, voltage sags and swells and non-linear loads,

• DVR that protects a critical load fromdisturbances like sags, swells, transientsand harmonics originating on theinterconnected distribution system,

• Unified Power Quality Conditioner (UPQC),a combination of series and shunt controller,which compensates supply voltage andload current imperfections in the distributionsystem.

The DSTATCOM is a versatile device forproviding reactive power compensation in acnetworks. The control of reactive power isachieved via the regulation of a controlledvoltage source behind the leakage impedanceof a transformer (Lehn and Iravani, 1998). It issimilar to a conventional synchronouscompensator, which is essentially asynchronous generator where the field currentis used to adjust the regulated voltage. TheDSTATCOM uses Voltage Source Converter(VSC) to achieve the regulation task. Whenused in low or medium voltage distributionsystems the STATCOM is normally identifiedas Distribution STATCOM (D-STATCOM). Itoperates in a similar manner as the STATCOM(FACTS controller), with the active power flowcontrolled by the angle between the AC systemand VSC voltages and the reactive power flowcontrolled by the difference between themagnitudes of these voltages. As with theSTATCOM, the capacitor acts as the energystorage device and its size is chosen basedon power ratings, control and harmonicsconsiderations. The D-STATCOM controllercontinuously monitors the load voltages andcurrents and determines the amount of

compensation required by the AC system fora variety of disturbances.

The D-STATCOM is a shunt device. It shouldtherefore be able to regulate the voltage of abus to which it is connected. The operatingprinciple of a D-STATCOM in this mode hasbeen termed as the D-STATCOM in voltagecontrol mode. This report shows that eventhough the structure of D-STATCOM used inboth current control and voltage control modesis the same, its operating principle is different.In the current control mode it is required tofollow a set of reference currents while in thevoltage control mode it is required to follow aset of reference voltages. This paperdiscusses the reference voltage generationscheme and the control of D-STATCOM in thevoltage control mode.

BASIC MODEL OF D-STATCOMThe D-STATCOM system comprises of a VSC,a set of coupling reactors (leakage reactanceof the transformer) and a controller. The D-STATCOM generates a controllable acvoltage from the Voltage Source Inverter (VSI)connected to a dc capacitor (energy storagedevice). The ac voltage appears behind thetransformer leakage reactance.

The active and reactive power transferbetween the power system and the D-STATCOM is caused by the voltage differenceacross this reactance. The D-STATCOM isconnected to the power network at the Pointof Common Coupling (PCC), where thevoltage-quality problem is a concern. Allrequired voltages and currents are measuredand are fed into the controller to be comparedwith the reference. The controller then performsfeedback control and outputs a set of switching

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signals to drive the main semiconductorswitches (IGBTs) of the power converteraccordingly (Introducing Custom Power, 1995).The basic diagram of the D-STATCOM isillustrated in Figure 1.

WORKING PRINCIPLEOF D-STATCOMD-STATCOM is inserted in transmission lineto suppress voltage variation due to deviatingload conditions and control reactive power.Basically it is connected in shunt totransmission line with Point of CommonCoupling (PCC) in phase with system voltage.It can compensate for inductive and capacitivecurrents linearly and continuously. Figure 2shows the vector diagram at the fundamentalfrequency for capacitive and inductive modesand for the transition states from capacitive toinductive and vice versa. The terminal voltage(Vbus) is equal to the sum of the inverter voltage(Vvsc) and the voltage across the couplingtransformer reactive VL in both capacitive andinductive modes. I mean that if output voltage ofDSTATCOM (Vvsc) is in phase with bus terminalvoltage (Vbus) and (Vvsc) is greater than Vbus D-STATCOM provides reactive power to system.

And if Vvsc is smaller than, D-STATCOMabsorbs reactive power from power systemVbusand Vvschave the same phase, but actuallythey have a little phase difference to componentthe loss of transformer winding and inverterswitching, so absorbs some real power fromsystem.

Figure 2 is DSTATCOM vector diagrams,which show inverter output voltageVI, systemvoltage VT, reactive voltage VL and line currentI in correlation with magnitude and phase .Figures 2a and 2b explains how VI and VTproduces capacitive or inductive power bycontrolling the magnitude for inverter outputvoltage VI in phase with each other.

MATHAMATICAL MODELINGOF D-STATCOMModeling Equations are given as:

scsaa

faf VVdtdiLIR ...(1)

Figure 1: Basic Block Diagramof D-STATCOM

Figure 2: Vector Diagram of D-STATCOM(a) Capacitive Mode (b) Inductive Mode

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cbsbb

fbf VVdtdiLIR ...(2)

ccscc

fcf VVdtdiLIR ...(3)

Here,

Vsa, Vsb, Vsc : Voltage at the PCC

Vsa, Vcb, Vcc : Inverter output voltage

Lf : Filter inductance

Rf : Equivalent filter resistance

C : DC Link Capacitor

Designing Equations

a) CSTAT VIQ 3

b) MaVVDC 3/22

c)pps

DCAC Iaf

MaVL

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d) Capacitor designing

2 12/3 DCDCLSDC VVTIVC Here,

IC = D-STATCOM Line Current

V = Line Voltage

Ma = Modulation Index

VDC = DC Link Voltage

QSTAT= Power Rating

LAC = AC Inductor

a = Over Current Factor (1.2)

Park’s Transformation: This block performsthe abc to dq transformation on a set of three-phase signals. It computes the direct axis Vd,quadrature axis Vq, and zero sequence V0quantities in a two axis rotating reference frameaccording to the following transformation:

3/2sin3/2sinsin*3/2 tVtVtVV cbad...(4)

3/2cos3/2coscos*3/2 tVtVtVV cbaq...(5)

cba VVVV *3/10 ...(6)

where = rotation speed (radian/second) ofthe rotating frame. This transforms threequantities (direct axis, quadrature axis andzero-sequence components) expressed in atwo axis reference frame back to phasequantities. The following transformation isused:

0cossin VtVtVV qda ...(7)

03/2cos3/2sin VtVtVV qda ...(8)

03/2cos3/2sin VtVtVV qdc ...(9)

where = rotation speed (radian/second) ofthe rotating reference frame.

VARIOUS CONTROL SCHEMESSatisfactory performance, fast response,flexible and easy implementation are the mainobjectives of any compensation scheme. Thecontrol strategy of D-STATCOM is mainlyimplemented in the following steps (Masandet al., 2006; and Singh, 2006):

Figure 3: Simplified Single Line Diagramof D-STATCOM

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• Measurements of system variables andsignal conditioning

• Extraction of reference compensatingsignals

• Generation of firing angles for switchingdevices

Basically all the above steps will be appliedin every control scheme but methods ofmeasurement of signal variables, extraction ofreference, and generation of firing pulses willbe different for different control schemes. Twocontrol schemes are implemented andcompared here are Phase Shift Control andDecoupled Current Control.

Phase Shift ControlIn this control algorithm the voltage regulationis achieved in a D-STATCOM by themeasurement of the RMS voltage at the loadpoint and no reactive power measurementsare required. Figure 4 shows the blockdiagram of the implemented scheme (Masandet al., xxxx).

necessary phase shift between the outputvoltage of the VSC and the AC terminalvoltage. This angle is summed with the phaseangle of the balanced supply voltages,assumed to be equally spaced at 120 degrees,to produce the desired synchronizing signalrequired to operate the PWM generator. In thisalgorithm the DC voltage is maintainedconstant using a separate dc source.

Decoupled Current Control p-qTheoryThis algorithm requires the measurement ofinstantaneous values of three phase voltageand current. Figure 5 shows the block diagramrepresentation of the control scheme. Thecompensation is achieved by the control ofdirect axis and quadrature axis currents and .Using the definition of the instantaneousreactive power theory for a balanced threephase three wire system, the quadraturecomponent of the voltage is always zero, thereal (P) and the reactive power (Q) injectedinto the system by the D-STATCOM can beexpressed under the dq reference frame as:

qqdd IVIVP ...(10)

qddq IVIVQ ...(11)

Since Vq = 0, id and iq completely describesthe instantaneous value of real and reactivepowers produced by the D-STATCOM whenthe system voltage remains constant.Therefore the instantaneous three phasecurrent measured is transformed by abc to dq0transformation block. The decoupled d-axiscomponent id and q-axis component iq areregulated by two separate PI regulators. Theinstantaneous id reference and theinstantaneous iq reference are obtained by thecontrol of the dc voltage and the terminal

Figure 4: Block Diagram of Phase ShiftControl

Sinusoidal PWM technique is used whichis simple and gives a good response. Theerror signal obtained by comparing themeasured system RMS voltage and thereference voltage, is fed to a PI controller whichgenerates the angle which decides the

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voltage measured. Thus, instantaneouscurrent tracking control is achieved using fourPI regulators. A Phase Locked Loop (PLL) isused to synchronize the control loop to the acsupply so as to operate in the abc to dq0reference frame.

MODELLING ANDSIMULATION RESULTSIn this work, the performance of VSC basedpower devices acting as a voltage controller

is investigated. The D-STATCOM model canbe seen in Figure 6 which comprised ofdistribution bus, a VSC and the controller. Thecontroller scheme is applied to generate theswitching pulse for VSC.

Phase Shift ControlIt does not have a self supporting dc bus andrequires a separate dc source to pre-chargethe dc side capacitor and maintain its voltageduring the operation of D-STATCOM. Itassumes that the supply side voltage isbalanced and without harmonics, sincefundamental wave form is used to obtain thephase angles of the supply wave form. Thereis no provision for harmonic suppression in

Figure 5: Block Diagram of DecoupledTheory Based Control of D-STATCOM

Figure 6: Simulink Model for Phase ShiftControl

Figure 7: Simulink Model of Controllerfor Phase Shift Controller

Figure 8: 3-Phase Source Voltage at PCC

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Figure 9: Voltage of SeparatePre-Charged Capacitor

Figure 10: Inverter Output Voltage

Figure 11: Compensated Output Voltageand Current

Figure 12: Total Harmonic Distortion(Linear Load) (a) Before Compensation

(b) After Compensation

Figure 13: Total Harmonic Distortion(Nonlinear Load) (a) Before

Compensation (b) After Compensation

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case the load connected to PCC is nonlinear.This method results in generation of activepower by the VSC along with the var.

Decoupled Current ControlThe advantageous features of this scheme are:

It incorporates a self supporting dc bus. Theactive and reactive power control achievedthrough and control are decoupled from eachother. The dc bus control or regulation aredecoupled from the ac bus control like voltageregulation or power factor correction and loadbalancing.

Switching of devices of VSC is done at fixedfrequency. Thus switching losses can be

limited within the rating of the devices. Thistype of control is inherently linear and robustand uses PI or PID controls.

The results can be seen for variousparameters like voltage, current before andafter applying the controller for compensation.

Figure 14: Simulink Model for DecoupledCurrent Controller

Figure 15: Source Current Waveform

Figure 16: Before Compensation (a) LoadVoltage and Load Current (Linear)(b) Load Voltage and Load Current

(Nonlinear)

Figure 17: After Compensation (a) LoadVoltage, Load Current (Linear), (b) Load

Voltage, Load Current (Nonlinear)

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These Power devices provide solutions topower quality at the medium and low voltagedistribution network level. This projectpresents the detailed modeling of one of thecustom power products, D-STATCOM ispresented using instantaneous P-Q theory andphase shift theory used for the control of D-STATCOM are discussed. These controlalgorithms are described with the help ofsimulation results under linear loads andnonlinear loads. It was observed thatundersized capacitor degrades all threeaspects. On the other hand, an oversizedcapacitor may also lead to a PWM control witha sluggish response but it will reduceD-STATCOM harmonic generation and

Figure 18: Total Harmonic Distortion(Linear Load) (a) Before Compensation


Figure 19: Total Harmonic Distortion(Nonlinear Load (a) Before Compensation


CONCLUSIONDetailed modeling is presented and resultsare discussed with different control studies.

1. Reactive power Average Partialcompensation

2. Performance Contains Not Satisfactoryunder nonlinear undesiredloads harmonics

3. Applicable for Yes Nosingle phasesystems

4. Harmonic Less Averagecompensation

5. PWM switching Fixed Fixedfrequency

6. Self supporting No YesDC Bus

7. Generation of Sine PWM Sine PWMfiring pulses

8. Power factor 0.8 to 0.94 0.8 to 0.94Linear Load (linear) (linear)

9. Power factor 0.8 to 0.92 0.8 to 0.95Non Linear Load (Nonlinear) (Nonlinear)

10. Response time Less More

11. Control scheme Easy Complex

Table 1

S.No Parameters

PhaseShift

Control

DecoupledCurrentControl

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transient overshooting. It is concluded that aD-STATCOM though is conceptually similar toa STATCOM at the transmission level; itscontrol scheme should be such that in additionto complete reactive power compensation,power factor correction and voltage regulationthe harmonics are also checked, and forachieving improved power quality levels at thedistribution end.

FUTURE WORK• This work presents a detailed modeling and

analysis of one of the custom power deviceD-STATCOM. Instantaneous DecoupledCurrent Control or instantaneous p-q theoryand phase shift control is discussed andverified through detailed simulations bydeveloping the models in MATLAB Simulinkusing the Simpower system control toolboxes. Now we are posing a challenge tocomplete if the remaining control strategieswhich include the synchronous frame theory,regulation of Bus and DC link voltage, andANN based Adaline theory. These controlstrategies are implemented and studied indetail through various simulations then itwould be of immense help for the real timeimplementation of the D-STATCOM acrossall over the globe.

• If thrown light on other custom powerdevices like the Dynamic VoltageRegulator (DVR), and Unified Power QualityConditioner (UPQC), applying differentstrategies then we can bring a revolution inthe control of power in the distributionsystems.

REFERENCES1. Hingorani N (1991), “FACTS—Flexible ac

Transmission Systems”, in Proc. IEE 5th

Int. Conf. AC DC Transmission, Vol. 345,pp. 1-7, Conf. Pub., London, UK.

2. “Introducing Custom Power”, IEEESpectrum, Vol. 32, June, pp. 41-48.

3. Lehn P and Iravani M (1998),“Experimental Evaluation of STATCOMClosed Loop Dynamics”, IEEE Trans.Power Delivery , Vol. 13, October,pp. 1378-1384.

4. Masand D, Jain S and Agnihotri G (2008),“Distribution Static CompensatorPerformance Under Linear and NonlinearCurrent Regulation Methods”, J. ElectricalSystem, Vols. 4-1, pp. 91-105.

5. Masand D, Jain S and Agnihotri G (2006),“Control Algorithms for Distribution StaticCompensator”, Dept. of ElectricalEngineering, Maulana Azaad NationalInstitute of Technology, Bhopal, MP, India.

6. Pierre Giroux, Gilbert Sybille and HoangLe-Huy (2001), “Modeling and Simulationof a Distribution STATCOM UsingSimulink’s Power System Block Set”,IECON’01, The 27th Annual Conferenceof the IEEE Industrial Electronics Society.

7. Singh B (2006), “A Comparative Study ofControl Algorithms for D-STATCOM forLoad Compensation”, Department ofElectrical Engineering, Indian Institute ofTechnology, Hauz Khas, New Delhi.

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