LINEAR FEEDBACK CONTROLLER FOR D-STATCOM IN DPG FAULTAPPLICATION

S. Aizam I", Kok. B.C. "', N. Mariun (2), H. Hizam (2) and N. I. Abd Wahab(2)

(1) University College of Technology Tun Hussein Onn, Malaysia, (2) University Putra Malaysia, Malaysia

ABSTRACT

This paper presents the application of linear feedback controller on Distribution Static Synchronous Compensator (D-STATCOM) for mitigating Double Phase to Ground (DPG) fault at the distribution system. The pole placementtechnique is applied by shifting the existing poles to the new poles locations for fast response compare to theconventional controller. This type of inverter control is very useful in D-STATCOM application which controls theinverter to inject the unbalanced current or voltage or both for mitigating the fault condition that occurs at thedistribution system. The controller will give respond to the lines that are affected by the fault and restore the system toits normal conditions. The simulation and design of the controller is done using MATLAB software with PowerBlockset Toolbox and SIMULINK.

INTRODUCTION feedback controller and the proposed controller is it hasbeen design using Signal Flows diagram with feedback

D-STATCOM has been implemented in distribution loops of the gain rather than using transfer functionsystem rather than transmission system because it is which needs more equations.where most of power quality problems occur. D-STATCOM is a shunt device that generates a balanced In this linear feedback controller the combination ofthree phase voltage or current with ability to control the input-output pole placement method as well as DQmagnitude and the phase angle [1]. These parameters transformations are utilized. Pole placement method isare varied by referring to absorbing or generating VAR used to shift the existing poles from the right to the leftat the faulted system that happen in milliseconds. The of the complex diagram, in order to increase the stabilityabsorbing or generating VAR is proportional to the of the system and damping response [1]. Anotherinjected currents from the inverter which is controlled advantage of using the pole placement method is the D-by the controller. Because of the faulted time period, it STATCOM is a MIMO system which is based onneeds a controller that gives fast response which dynamic model rather than phasor diagram [4] and isoperates in milliseconds. The D-STATCOM is a Multi also known as a discrete time control technique withInput and Multi Output (MIMO) system. Due to this close loop poles [3]. The DQ transformation is appliedadvantage, the controller that gives better performance to change the 3 phase system into vector representationand fast response can be designed. The new controller system by reducing the number of equations formust have a capability to give fast response to any designing the controller circuit. In this paper, the D-changes of the operating point in the system. STATCOM with the linear feedback controller is

applied on an llkV distribution system and was testedGenerally, the controller of the D-STATCOM is a in DPG fault situation.Proportional Integration (PI) controller which is notsuitable to operate at all operating point [2] and giveslow response which is about 100 ms [1]. This will cause TRANSFORMATION OF D-STATCOMthe injected currents from the D-STATCOM gives slowresponse to mitigate the faulted lines. One of the In designing the linear feedback controller, the statemethods to solve these problems is by using the pole space equations from the D-STATCOM circuit must beplacement method. This method has been used because introduced. The theory of DQ transformation of currentsit consistence to any changes in the operating points and has been applied in the circuit, which makes the d and qgives fast response. This linear feedback controller is components as independent parameters. Figure 1 showsdesign based on pole placement method and it is not the circuit diagram of the D-STATCOM. It consists ofwidely used as the D-STATCOM controller. This type Voltage Source Converter (VSC), dc energy storageof controller has been used in D-STATCOM application device and a step down transformer connected in shunt[1,3] The different between the existing linear with the ac system.

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Distribution Line _ -RSbIcbkI I@|LO L b cos(a)

IMeasure{ Variables -Rswb (Obkvs Wables | [AV]= -'b sin(a)

Step Down Transformer -3!kCwbcos(a) 3-kCobwsin(a) b02 2 Rp

HARMONIC BLOCKINGREACTOR 1~~~~~~~~~~~~~~~~~~~~~~o kico b1 b-v dcvj I~~~~~~~~~- Ikbd -sin( a)

L L3 PHASE .4 CCONTROL

L- [BV ]= °0 b cos( a)

dc capacitor Note: Vi -inverter output 0 2 kC cob (i d sin( a) - i q cos( a))voltage

Vs - system voltageFigure 1 D-STATCOM Configuration The prime parameters indicate the p.u values. The D-

STATCOM parameters (in p.u.) used in the followingThe configuration of Figure 1 can be simplified to discussion are given as,single phase diagram as shown in Figure 2. a= 0; V

as,

?L= 0;V dc= 1.0; L: =0.1; R', =0.01; k=1.273;Rs L

1. 2 cb=314;C = 2.275; i d = 0.082 ;i q = -0.048v

a /; L /; ) Substituting the above values into Equation 5 produces-J the complete equation as follows,

id --31.4 314 3997.22 id -3140 0Figure 2 Single Phase Diagram d 314 -310 + 0I ~~Iqdt iq =-314 -31.4 0 iq + 0 5396247 bz

The equations for single phase diagram can be written vdc -136405 0 0 v'dc 0 65.474as,

R i+Ldi + (1) This equation will give the existing poles to have highlyR s + L d + v = e (1) damp and high frequency oscillation at the operatingdi . (2) points which are located at -15.9±2346j and -30.8.L dt= e - v - R s These point locations are not suitable in designing the

di e-v (3) D-STATCOM controller due to the existing polesdt L L

The loop equation for the circuit may be written invector as, ~~~~~LINEAR FEEDBACK CONTROLLER DESIGNvector as,

d Rs 1In designing the linear feedback controller, the newabc Labc+ L(eabc-vabc) (4) poles must satisfy a few conditions stated below,dtj'abc L ac bc * the oscillations in id, iq and Vd, must respond

less then one period of cycle due to systemEquation 4 represents the D-STATCOM circuit frequency which is 50 Hz L4],without DQ transformation. After the DQ theqoershoot of 5dan ms potransformation and linearization process to Equation 4, M the ovoltag Of th capacitor (e shouldbthe state space for D-STATCOM is shown in Equation kthe voltage of the capacitor (Vdo should be5 [5], kept constant [4, 6] in the controller,

* id must give zero output because it representsF-d j F-d - the active power [1].

dIq' = [A ] Iq ++[By] (5) The new poles locations are then selected according todt! V ac- [3] where one of the pole locations is needed to be at the

v dc 'V'dc origin to fulfill the requirements stated above. Indesigning the controller block diagram using

where ~~~~~~~~~~SIMULINK-MATLAB application is done after thenew poles locations have determined based to Figure 3

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concept. This figure shows the block Equation 5 with a diagram based on Figure 4 and the complete signal flowKp feedback block which is used to stabilize and is given in Figure.5.increase the response system where the gains aredetermined from the new poles locations. Vdc

Output __

(id,t __ dd, i, Vdc) id q iq

Inu d . N.-

Ref d'_ji ___ '

Vdcf

Kp x(t)

Figure 3 Control Block Diagram iK

The new poles locations and the Kp values can bedetermined by writing the program in MATLAB M-File Figure 5.Signal Flows Diagram of Linear Feedbackreferring to Figure 3 flow diagram. Figure 4, shows the Controllerresponse of the new poles locations which are at (-5000± 2346j, 0) to fulfill the conditions in designing the The outputs of this controller which are id and iq havelinear feedback controller and the values of Kp gains, been used as reference signals to generate the triggering

signals to the power electronic devices and the Vdc is theinput to the inverter. The concept of the sinusoidal pulsewidth modulation (SPWM) has been applied bycomparing the references signal with the carrier

id 0.06- - frequency. The magnitude of the reference sine0.04 waveform must be in the range of [-1 to 1] for switching0,02 application. The generated pulses from the linear

feedback controller are shown in Figure 6. Figure 6shows the firing pulse from SPWM. There are 3 pairs ofpulses which are pulse 1 and 2, 3 and 4, and 5 and 6.

0.3 - The firing pulses for 1 and 2 must have the oppositeiq 0.2 - direction because, inverter 1 and 2 are located at the

same arm which means it must not operate at the same0.1 time.

irin ls

ffi oLLLIL L LL LLL I ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~II0.05 I

0. 0.020.0Z. 0.03 0.03. 0.04 0.045 0.050.0550.06

VdCO.60o020o025 0 03 0o035 0.04 0.045 0o05 0o0550o06

repne the V1 is kept contan duin the0irllaio,A1St0 0 00]t ]0jtFigure 4 Response of The New Poles to The System

0.020.0250.03 0.035 0.04 0.045 0.050.0550.06

From Figure it shows that the id, iq andVdc give fastresponse, the Vdc is kept constant during the simulation 0Figure 6 S Tand the id iS maintain zero before designing thecontroller. From this simulation it is indicated that the 19STATCOMNIar 11;IVapplicationl C'1- rforDPGfalt 1+ 1-apliatin.h ki '~BH1UUH HI IID[JHH[HJ

D-STATCOM CONFIGURATION FOR DPG DPG period, the current in lines A and B are less thanFAULT SIMULATION the current in line C.

Load VoltgThe system shown in Figure 7 has been simulated using 2 II

A ~~~~~~~~~~~~~~~~~A,MATLAB software. The system parameters and the i;rtransformer data for the simulation are given in Table 1.

1.17Ti. V-~~Tim

Figur8 Wih DPGFaulta) VotageLoadand~nlnl ~ he> h -TTO sapidi thDPfalan

SourceVoltage1

Source~~~~i pedane 18 0i esrda h -TTOinverter110 005i1u05r0

0 0.05 0.10.15 0.2 ~~~~~0.25 0.3 0.350.4

Fodigur ane7.63.11jD9TACO indDitibto SytmFgr ith DPGFesentaut a)e VoltfagetL tthoadan

ToePablmen 1Sse aab)CretLa

Sourt impedanc e Volta gea1oVlWnthe ad-STATCO is aplited.TheD amutandFaulttimeht sut is ru ansetheoalultp are ishownui

Systemtrnforeqen 501HzV Fiur9.8 ansedit mesredvatuthe load siden whileAFigur

Transfource impedance |0.0l+3l.4jQ 10is mea IthD-stA m inverter. In Fie

Load impedance72.6+35.1618j ~be9auansb it canobfesee tha the DPG fault. ato theioa

because it ihos nlot affec tesonthe DPGe faulest.aFromths.The simulation of the D-STATCOM in fault conditionis done by using DPG faults. The duration of the fault isstfor 0.2 sec and the total simulation time is 0.4 sec. n14Slala

SIMULATION RESULT ~~~~~0IN~ F

InDGfauLlt siultion thefulvB:tis introuce aitn dlines

for 0.4 sec with fault occurring from 0.1 sec to 0.3 sec. 0 0.05 01 015 0.2 0.25 0.3 .S 0.4The results for the simulation are shown in Figure 8 that

measuredatloadside. ______ Currentatload~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ClrenalaFigsuredat oandb show theloadvoltageandtheload 5074

current at the distribution system. Figure 8a, it shows l°

the voltage sag appeared during the DP fault and the _____________ 71.ltVySTalyValyVabUre16%. Fgur 8bA showsthe mneasured values for line I0W ltl ll

curetsIa=8AJb=9AadI~=lS uigte.0

fault period and the percentage of unbalanced current is 0 0.05 01 0.15 0.2 0.25 0.3 0.35 0.4

Tine

about 11%. The unbalanced current occurred is high and Figure 9 Mitigation of DPG Fault a) Voltage Loadneeds to be reduced to a level below 2%. During the b) Current Load

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InjectedCunentatD-STATCOM are affected to the fault and during no fault condition itl00 -A will inject the currents to make the system meet the

normal value and the response time is about 20ms fasterfrom the PI controller that response in lOOms.

REFERENCES

1. Rao, P, Crow, M.L & Yang, Z, STATCOM Controlfor Power System Voltage Control Applications, IEEETransactions on Power Delivery, Volume 5, 1311-1317,

Figure10 Mitigation ofDPGFaultInjectedCu n Oct2000.

2. Sahoo, N.C, Panigrahi, BoK, Dash, P.K, & Panda, G,Application of a Multivariable Feedback LinearizationScheme for STATCOM Control, International Journalof Electric Power Research, Volume 62, 81-91, June

3. Ghosh, A, Jindal, AK, & Joshi, A, Inverter Controlresponse to adsqunc.heinetecrrnt Using Output Feedback for Power Compensating

0 005 0a10t15 0f20l250a3 0.35 0.4 Devices, TENCON 2003 Conf. on Convergent

theeferncenputto haenrleisfxdt1.Ts Dstechnoloion, folumAsia Paciic-306o, 48uly 2003

Figure 10 Mitigation of DPG Fault Injected Current Tcat Load 4. Xing, L, A Comparison of Pole Assignment and LQR

maintain theshowsthat to 1ip.ujsysted cuets fm tes s 6Design Methods For Multivariable Control for

tat, F igure10 shows i te chos from the D- STATCOM Florida State University, 2003

current compare to phase A and C because this phase isn.Schauder, C, & Metha, H, Vector Analysis and

response to Vab andeVb sequence. The injected current Control of Advanced Static Var Compensators, IEEis almost constant during fault and without fault because Proceedings on Generation, Transmission andthe reference input to the controller is fixed to 1. This Distribution, Volume 140, 299-306, July 1993will causes the D-STATCOM to inject the currents tomaintain the system toi p.u system. It can be seems 6. Gonzalez, P.G, & Cerrada, A.G, Control System forthat, from the DPG fault analysis, it shows that the D- PWM-Based STATCOM, IEEE Power EngineeringSTATCOM with linear feedback controller is able in Society Summer Meeting, 1252-1257, 2000controlling the line currents that need to be injected forthe DPG mitigation.

AUTHOR'S ADDRESS

CONCLUSION SHAMSUL AIZAM

The modeling of linear feedback controller for the D- Department of Electrical Power EngineeringSTATCOM is presented in this paper. The linear Faculty of Electrical and Electronic Engineeringfeedback controller is capable to control the injected University College of Technology Tun Hussein Onncurrent from the inverter for DPG faults correction. The 86400 Panit Raja, Batu Pahatefficiency of the controller is studied by simulation on a Johore, Malaysiadistribution system with the DPG fault. The D- email: aizam@kuittho.edu.mySTATCOM will inject more currents to the lines that