M. Koteswara Rao, T.Ganeshkumar and … 7/Issue1/8...M. Koteswara Rao, T.Ganeshkumar and PappuPawan International Journal of Emerging Trends in Electrical and Electronics (IJETEE –

M. Koteswara Rao, T.Ganeshkumar and PappuPawan

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: Vol. 7, Issue. 1,

Mitigation of Voltage Sag and Voltage Swell byUsing D-STATCOM and PWM Switched

AutotransformerM. Koteswara Rao, T.Ganeshkumar and PappuPawan Puthra

Abstract— This paper proposes a novel distribution-levelvoltage control scheme that can compensate voltage Sag andSwellconditionsin three-phase power systems. Faults occurringin power distribution systems or facilities in plants generallycause the voltage sag or swell. Sensitivity to voltage sags andswells varies within different applications. For sensitive loads,even the slightest voltage sag for short duration can causeserious problems. Normally, a voltage interruption triggersaprotection device, which causes shut down the entire load.. Inorder to mitigate power interruptions, this paper proposes avoltage sag support based on a pulse width modulatedautotransformer and D-STATCOM. The proposed devicesquickly recognize the voltage sag and voltage swell conditionsand correct the voltage by either boosting the input voltageduring voltage sag events or reducing the voltage during swellevents. Simulation analysis of these devices is performed inPSCAD/EMTDC and performance analysis of the system ispresented for various levels of sag and swell. Simulation resultsare presented for various conditions of sag and swelldisturbances in the supply voltage to show the compensationeffectiveness.

Index terms—D-STATCOM, Pulse Width Modulation (PWM)

I. INTRODUTION

With an increase in the use of sensitive loads, the powerquality issues have become an increasing concern. Poordistribution power quality results in power disruption for theuser and huge economic losses due to the interruption ofproduction processes. According to an Electric PowerResearch Institute (EPRI) report, the economic losses due topoor power quality are $400 billion dollars a year in theU.S. alone [1]. Many power quality surveys have been done,which show that voltage sags have been identified as themost serious power quality problem facing industrialcustomers today.Voltage sag is a momentary decrease of the voltage RMSvalue with the duration of half a cycle up to many cycles.Voltage sags are given a great deal of attention because ofthe wide usage of voltage-sensitive loads such as adjustablespeed drives (ASD), process control equipment, andcomputers.Sag can cause serious problem to sensitive loads that usevoltage-sensitive components such as adjustable speed

drives, process control equipment, and computers [2], [3].Power systems supply power for a wide variety ofdifferent user applications, and sensitivity to voltage sagsand swells varies widely for different applications. Someapplications such as automated manufacturingprocesses are more sensitive to voltage sags and swellsthan other applications. For sensitive loads, even voltagesag of short duration can cause serious problems in themanufacturing process. Normally, a voltageinterruption triggers a protection device, which causesthe entire branch of the system to shut down.

Various voltage sag mitigation schemes are based on invertersystems consisting of energy storage and switches. The D-STATCOM has emerged as a promising device to providenot only for voltage sag mitigation but a host of otherpower quality solutions such as voltage stabilization, flickersuppression, power factor correction and harmonic control[4].The D-STATCOM has additional capability to sustainreactive current at low voltage and can be developed as avoltage and frequency support by replacing capacitors withbatteries as energy storage. The D-STATCOM, whichconsists of a thyristor-based voltage source inverter [5], canprovide fast capacitive and inductive compensation and isable to control its output current independently of the ACsystem voltage. This feature of the compensator makes ithighly effective in improving the transient stability.In an effort to achieve the advantages of a fast responsetime, but at a significantly lower cost, the PWM switchedautotransformer is proposed here [6]. The proposed systemhas only one PWM switch per phase with no energy storage,which is a very low cost solution for voltage sag mitigation.Any power electronic switch for a high voltage applicationis expensive, and the peripheral circuits such as gate driversand power supplies are even more expensive than the deviceitself. The overall cost of power electronics-based equipmentis nearly linearly dependent on the overall number ofswitches in the circuit topology. Hence, this paper suggests ascheme that uses only one PWM switch with no energystorage. Here the control circuit based on RMS voltage isused to identify the sag and swell disturbances. Simulationof the compensator is performed using PSCAD/EMTDC andperformance results are presented.



II.DISTRIBUTIONSTATIC COMPENSATOR(DSTATCOM)

In its most basic function, the DSTATCOM configurationconsist of a two level voltage source converter (VSC), a dcenergy storage device, a coupling transformer connected inshunt with the ac system, and associated control circuit [7,8] as shown in Fig 1. More sophisticated configurations usemultipulse and/or multilevel configurations as discussed in[9]. The VSC converts the dc voltage across the storagedevice into a set of three phase ac output voltages. Thesevoltages are in phase and coupled with the ac system throughthe reactance of the coupling transformer. Suitableadjustment of the phase and magnitude of the DSTATCOMoutput voltages allows effective control of active and reactivepower exchanges between the DSTATCOM and the acsystem.

Fig. 1.Schematic diagram of the DSTATCOM as a custom powercontroller

The VSC connected in shunt with the ac system provides amultifunctional topology which can be used for up to threequite distinct purposes [10]:

i. Voltage regulation and compensation ofreactive power;ii. Correction of power factor;iii. Elimination of current harmonics.

The design approach of the control system determines thepriorities and functions developed in each case. In this case,DSTATCOM is used to regulate voltage at the point ofconnection. The control is based on sinusoidal PWM andonly requires the measurement of the rms voltage at the loadpoint.

A. Basic configuration and function of D-statcom

The DSTATCOM is a three phase and shunt connectedpower electronics based device. It is connected near the loadat the distribution systems. The major components of theDSTATCOM are shown in Fig 2. It consists of a dccapacitor, three phase inverter module such as IGBT orthyristor, ac filter, coupling transformer and a controlstrategy. The basic electronic block of the DSTATCOM isthe voltage sourced converter that converts an input dcvoltage into three phase output voltage at fundamentalfrequency.Referring to Fig 2, the controller of the DSTATCOM isused to operate the inverter in such a way that the phase

angle between the inverter voltage and the line voltage isdynamically adjusted so that the DSTATCOM generates orabsorbs the desired VAR at the point of connection.

Fig. 2.Building blocks of DSTATCOM

The phase of the output voltage of the thyristor basedconverter, Vi, is controlled in the same way as thedistribution system voltage, Vs. Figure 3 shows the threebasic operation modes of the DSTATCOM output current, I,which varies depending upon Vi. For instance, if Vi is equalto Vs, the reactive power is zero and the DSTATCOM doesnot generate or absorb reactive power. When Vi is greaterthan Vs, the DSTATCOM ‘sees’ an inductive reactanceconnected at its terminal. Hence, the system sees theDSTATCOM as a capacitive reactance. The current, I, flowsthrough the transformer reactance from the DSTATCOM tothe ac system, and the device generates capacitive reactivepower. Furthermore, if Vs is greater than Vi, the system‘sees’ and inductive reactance connected at its terminal andthe DSTATCOM ‘sees’ the system as a capacitive reactance,then the current flows from the ac system to theDSTATCOM, resulting in the device absorbing inductivereactive power.

Fig. 3.Operation modes of a DSTATCOM



III. APWM SWITCHED AUTOTRANSFORMER

The proposed device for mitigating voltage sag and swell inthe system consists of a PWM switched power electronicdevice connected to an autotransformer in series with theload. Fig. 4 shows the single phase circuit configuration ofthe mitigating device and the control circuit logic used in thesystem. It consists of a single PWM insulated gate bipolartransistor (IGBT) switch in a bridge configuration, athyristorbypass switch, an autotransformer, and voltagecontroller.

Fig. 4. Block diagram of the voltage sag/swell mitigation scheme.

A. Principle of operation

An IGBT is used as power electronic device to injectthe error voltage into the line so as to maintain the loadvoltage constant. Four power diodes (D1 to D4) connected toIGBT switch (SW) controls the direction of power flow andconnected in ac voltage controller configuration. Thiscombination with a suitable control circuit maintains constantrms load voltage. In this scheme sinusoidal PWM

This control voltage is then compared with the triangularvoltage Vtri to generate the PWM pulses VG which areapplied to the IGBT to regulate the output voltage. Hencethe IGBT switch operates only during voltage sag or swellscondition and regulates the output voltage according to thePWM duty-cycle. To suppress the over voltage when theswitches are turned off, RC snubber circuits are connectedacross the IGBT and thyristor.

B. Voltage sag compensation

The ac converter topology is employed for realizing thevoltage sag compensator. This paper considers the voltagemitigation scheme that use only one shunt type PWM switch[11] for output voltage control as shown in Fig. 5. Theautotransformer shown in Fig.5 is used in the proposedsystem to boost the input voltage instead of a two windingtransformer. Switch IGBT is on the primary side of theautotransformer.

Fig. 5. Voltage sag/swell mitigating device.

The voltage and current distribution in the autotransformeris shown in Fig. 6. It does not provide electrical isolationbetween primary side and secondary side but has advantagesof high efficiency with small volume. The compensatorconsidered is a shunt type as the control voltage developedis injected in shunt. The relationships of the autotransformervoltage and current are expressed in (2),

V I N 1Pulse technique is used. RMS value of the load voltage VLIs calculated and compared with the reference rms voltage

L a H , a V H I L

(2)N 1 N 2

Vref. Under normal condition when there is no voltagedisturbance the power flow is through the anti-parallelthyristors used as the ac bypass switch. Output filterscontaining a main capacitor filter and a notch filter are usedat the output side to filter out the switching noise and reduceharmonics. During this normal condition, VL = Vref and theerror voltage Verr is zero. The gate pulses are blocked toIGBT. A sag or swell occurs in the system due to suddenincrease or decrease in the load, or due to faults. The supplyvoltage VS and hence VL decreases. When the sensingcircuit detects an error voltage Verr greater than ±10% of thenormal voltage the voltage controller acts immediately toswitch off the thyristors. Voltage Verr applied to the picontroller gives the phase angle δ. The control voltagegiven in (1) is constructed at power frequency f= 50 Hz..Vcontrol= ma * sin (wt+δ) (1)where ma is the modulation index.The phase angle δ is dependent on the percentage ofdisturbance and hence controls the magnitude of Vcontrol.

where a is the turns ratio,VL= Primary voltageVH = Secondary voltage = Load voltageIL,IH= Primary and secondary currents, respectivelyIS= Source current

A transformer with N1:N2 = 1:1 ratio is used as anautotransformer to boost the voltage on the load side whensag is detected. With this the device can mitigate up to 50%voltage sag. As the turns ratio equals 1:2 in autotransformermode, the magnitude of the load current IL (high voltageside) is the same as that of the primary current IL (lowvoltage side). From (2), it is clear that VL = 2VP and IS = 2IL.The switch is located in the autotransformer’s primary sideand the magnitude of the switch current equals the loadcurrent. The voltage across the switch in the off-state isequal to the magnitude of the input voltage. When sag isdetected by the voltage controller, IGBT switched ON andis regulated by the PWM pulses. The primary voltage VPis such that the



3-Phase100 MVA, 230KV, 50 Hz ,

Transformer 230KV / 11KV / 11KV,100MVA,50Hz

Capacitor

R = 12.1 Ω, L =R= 0.05Ω,

PI controller gain

Switching frequency

Duty cycle

load voltage on the secondary of autotransformer is thedesired rms voltage.

IV. SIMULATIONANALYSIS AND RESULTS

Simulation analysis is performed on 230/11kv three phasesystems in PSCAD version 4.2 to study the performance ofD-STATCOM and PWM Switched autotransformer. Thesystem data as follows

TABLE ISystem Parameters Used For Simulation

Fig. 6. Voltage and current relations in an autotransformer.

C. Ripple filter design

The output voltage VP is given by the IGBT is the pulsecontaining fundamental component of 50 Hz and harmonicsat switching frequency. Hence there is a necessity to designa suitable ripple filter at the output of the IGBT to obtain theload voltage THD within the limits. A combination of notchfilters to remove the harmonics and a low pass filter for thefundamental component as shown in Fig. 1 is used.Capacitor Cr1 in combination with source inductance andleakage inductance form the low pass filter. The notch filteris designed with a center frequency of PWM switchingfrequency by using a series LC filter. A resistor may beadded to limit the current. The impedance of the filter isgiven by (3)

= + −

(A).Voltage sag/swell mitigation by using D- STATCOM:

Fig. 7 shows the test system implemented in PSCAD tocarry out simulations for the D-STATCOM. The test systemcomprises a 230 kV transmission system, represented by aThévenin equivalent, feeding into the primary side of a 3-winding transformer. A varying load is connected to the 11kV, secondary side of the transformer. A two-level D-STATCOM is connected to the 11 kV tertiary winding toprovide instantaneous voltage support at the load point. A750F capacitor on the dc side provides the D-STATCOMenergy storage capabilities.

where R, Lr and Cr2 are the notch filter resistance,inductance and capacitances respectively.The resonant frequency of the notch filter is tuned to thePWM switching frequency. The capacitor is designed byconsidering its kVA to be 25% of the system kVA.Capacitor value (Ctotal) thus obtained is divided intoCr1and Cr2 equally. The notch filter designed for switchingfrequency resonance condition is capacitive in nature forfrequencies less than its resonance frequency. Hence atfundamental frequency it is capacitive of value Cr2 and is inparallel with Cr1 resulting to Ctotal.

The set of switches shown in Fig. 7 were used to assistdifferent loading scenarios being simulated with ease. To showthe effectiveness of this controller in providing continuousvoltage regulation, simulations were carried out with and withno D-STATCOM connected to the system.

A set of simulations was carried out for the test system shownin Fig. 7. The simulations relate to three main operatingconditions.1) In the simulation the load is increased by closing SwitchBRK3. In this case, the voltage drops by almost 27% withrespect to the reference value2) The switch BRK3 is opened and remains so throughoutthe rest of the simulation. The load voltage is very close tothe reference value, i.e., 1 pu.3) Three phase faults also applied in the study system tostudy the performance of the device.4) In the simulation Switch BRK1 is closed, connecting acapacitor bank to the high voltage side of the network. Therms voltage increases 27% with respect to the referencevoltage



Fig. 7. Test system implemented in PSCAD for D- STATCOM simulation.



Fig. 8. Simulation results for load voltage during voltage sag withoutD-STATCOM

Under normal conditions D-STATCOM continuousmonitors the load voltage and generates the error voltage.The voltage sag can be created by using either loadswitching or by using three phase fault. The load voltagecorresponding to sag is shown in Fig 8.The d-statcom canmitigate the sag as shown in Fig 9.

Fig.9.Simulation results for load voltage during voltage sag with D-STATCOM

Fig.10.Simulation results for load voltage during voltage swell WithoutD-STATCOM.

Voltage swell created by using a capacitor bank switchingduring a period of 0.3s to 0.6s.under this condition voltageswell is experienced. By using D-statcom it can beeliminated. The corresponding wave forms are shown inFigs 10 and Fig 11.

Fig.11.Simulation results for load voltage during voltage swell with D-STATCOM

(B).VOLTAGE SAG/SWELL MITIGATION BY USINGA PWM SWITCHED AUTOTRANSFORMER

Under normal condition, the power flow is through theantiparallel SCRs and the gate pulses are inhibited to IGBT.



The load voltage and current are same as supply voltage andcurrent. When a disturbance occurs, an error voltage whichis the difference between the reference rms voltage and theload rms voltage is generated. The PI controller thus givesthe angle δ. Control voltage at fundamental frequency (50Hz) is generated and compared with the carrier frequencytriangular wave of carrier frequency 1.5 kHz. The PWMpulses now drive the IGBT switch. The simulation modeling. t

of PWM switched autotransformer used as mitigating devicealong with its control circuit is shown in Fig. 12. Theautotransformer rating in each phase is 6.35/6.35 kV (as linevoltage is 11 kV) with 1:1 turns ratio. The effective voltageavailable at the primary of autotransformer is such that theload voltage is maintained at desired rms value (6.35 kV or 1pu).The simulation results of load voltages are shown in fig13-fig 16 during voltage sag and voltage swell disturbances

PWM Switched autotransformer control circuit

Fig. 12.Test implemented in PSCAD for simulation of PWM Switched Autotransformer



Fig.13.simulation result of load voltage during voltage sag withoutcompensator

Fig. 14. Simulation result of load voltage during voltage sagwith compensator

Fig 15. Simulation result of load voltage during voltage swell withoutcompensator

Fig. 16.Simulation result of load voltage during voltage swell withcompensator



Type of disturbance Vs (rms) VLoad (rms) THD (%)

LLLG Fault(sag)(VDC=30kv)

0.9645 3.445

0.9651 5.011

0.9766 6.291

Inductive loadswitching(sag)

(VDC=20kv)0.975 2.456

Capacitive loadswitching(swell) 1.012 3.978

Type of disturbance VLoad (rms) THD (%)

LLLG fault(sag)0.9732

0.9673

0.9622 3.711

Inductive loadswitching(sag) 0.9823

Capacitive loadswitching(swell)

TABLE IITHDSOf Load Voltage For Voltage Sag Of D-Statcom

TABLE IIITHDSOf Load Voltage For Voltage Sag Of Pwm Switched

Autotransformer

Tables II and table III summarizes the simulation results forboth these devices for various sag conditions and swells

V. CONCLUSIONA New Voltage sag mitigation topology called

PWM switched autotransformer is modeled and simulatedwith RMS voltage as a reference. This topology requiresonly one PWM switch per phase as compared toDSTATCOM requires two switches per phase. The PWMswitched autotransformer does not require energy storagedevice for mitigation of voltage sag as compared toDSTATCOM requires energy storage elements. The voltagemitigation capability of D-STATCOM depends on energystorage device. Hence it is shows that PWM switchedautotransformer is more economical than D-statacom.

The PWM switched autotransformer and D-STATCOM formitigation of voltage sag/swell could identify thedisturbance and capable of mitigating the disturbance bymaintaining the load voltage at desired magnitude and THDwithin limits.

VI. REFERENCES[1] Electric Power Research Institute (EPRI), “Power quality in

commercial buildings,” Tech. Rep. BR-105018.[2] M. F. Mc Granaghan, D. R. Muller and M. J. Samotyj,

“Voltage sags in industrial systems,” IEEE Trans. Ind. Appl.,vol. 29, no. 2, pp. 397–403,Mar./Apr. 1993.

[3] M. H. J. Bollen, Understanding Power Quality Problems:Voltage Sags and Interruptions. New York: IEEE Press, 2000.

[4] Gareth A. Taylor, “Power quality hardware solutions fordistribution systems: Custom power”, IEE North EasternCentre Power Section Symposium, Durham, UK, 1995, pp.11/1-11/9.

[5] HendriMasid, Norman Moriun, Senan Mahmud, AzahMohamed and Sallehuddin Yusuf, “Design of aprototype D-Statcom for Voltage Sag Mitigation,” in Proc.2004 National Power and Energy. Conf.,Kuallampur, Malaysia,Nov. 2004, pp. 61-66.

[6] J. R. Rostron and D.-M. Lee, “Voltage Sag and OverVoltage Compensation Device With Pulse Width ModulatingSwitch Connected in Series With Autotransformer,” U.S. Patent6 750 563, Jun. 2004.

[7] A. Hernandez, K. E. Chong, G. Gallegos, and E. Acha “Theimplementation of a solid state voltage source inPSCAD/EMTDC,” IEEE Power Eng. Rev., pp. 61-62, Dec1998.

[8] L. Xu, Anaya-Lara, V. G. Agelidis, and E. Acha “Developmentof custom power devices for power quality enhancement,” inProc. 9th ICHQP 2000, Orlando, FL, Oct. 2000, pp. 775-783.

[9] Y. Chen and B. T. Ooi, “STATCOM based on multimodules ofmultilevel converters under multiple regulation feedbackcontrol,” IEEE Trans. Power Electron., vol. 14, pp.959-965,Sept. 1999.

[10] Dong-Myung Lee, Thomas G. Habetler, Ronald G. Harley,Thomas L.Keister and Joseph R. Rostran, “A Voltage sagSupporter Utilizing a PWM-Switched Autotransformer,” IEEETrans. Power Electronics,Vol. 22, No. 2, Mar. 2007, pp. 626-635.

BIBILIOGRAPHY

M. Koteswara Rao was born in july 14th1988.He

graduated in 2011 from Vignan’s Lara Institute Of

Technology And Science,Guntur, India in

Electrical Engineering. He is currently pursing

M.TECH. in GVP college of engineering,

Visakhapatnam. His area of interests are

Renewable Energy and Power Quality.

T.Ganeshkumar was born in April 24th 1987.He

graduated in 2008 from Sri Sai Aditya institute of

science and technology, Rajahmundry, India in

Electrical Engineering. He is currently pursing

M.Tech from GVP college of Engineering,

Visakhapatnam. His area of interest is

Power Quality.

P. PawanPuthra was born in 15thNov 1983. He

graduated in JNTU Hyderabad from St.Theresa

Institute of Engg.& Tech., Garividi India in the year

2006. He obtained his post graduation from Vellore

Institute of Technology (VIT) with Specialization

Power Electronics & Drives in year 2008.

Presently he is working as an Asst. professor in

GVP College of Engineering. His main area of research is Power Electronics

& Drives, FACTS, and HVDC Transmission Systems

Documents

M. Koteswara Rao, T.Ganeshkumar and … 7/Issue1/8...M. Koteswara Rao, T.Ganeshkumar and PappuPawan International Journal of Emerging Trends in Electrical and Electronics (IJETEE –