Journal of Electrical Engineering and Electronic TechnologyISSN: 2325-9833

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Research Article, J Electr Eng Electron Technol Vol: 4 Issue: 1

Active and Reactive Power Control of Distributed Energy Generation System with DC Supply Source Using PI Controller

Asmita Poddar1 and Pragya Nema2*
1Netaji Subhash Engineering College, WBUT West Bengal India
2Lakshmi Narain College of Technology, Indore, M.P.India
Corresponding author : Pragya Nema
Lakshmi Narain College of Technology, Indore, M.P., India
E-mail: [email protected]
Received: July 07, 2013 Accepted: November 03, 2014 Published: November 05, 2014
Citation: Poddar A, Nema P (2015) Active and Reactive Power Control of Distributed Energy Generation System with DC Supply Source Using PI Controller. J Electr Eng Electron Technol 4:1. doi:10.4172/2325-9833.1000113

Abstract

Alternative energy sources are seen as reliable substitutions to conventional energy sources in modern days because of their renewable and pollution-free properties. In this regard Distributed Generation (DG) has to play a vital role in the energy supplydemand realm. Consequently inverter technology has gained extreme importance and the study of the control structures of the inverter has become an important area of research. In this paper, a theoretical analysis, control and simulation of a grid connected three phase inverter has been given. The investigated control strategy is based on the feed forward current control method using conventional Proportional Integral (PI) controller in the synchronous rotating reference frame.

Keywords: Distributed generation; Grid connected inverter; Current control

Keywords

Distributed generation; Grid connected inverter; Current control

Introduction

Worldwide economy mainly runs on fossil fuels, resultingin saturation of resources and ultimately leads to environmentaldestruction. In this context, renewable energy sources are gainingextreme importance as those free the nature from the hazardous effectsof fossil fuels. This leads to increase in the penetration of distributedgeneration (DG) [1] in the system significantly. Interconnecting DGto an existing distribution system provides various benefits to theowner, utility and the final user by enhancing system security andsystem adequacy. Apart from delivering pollution free environmentdistributed energy offers solutions to most of the nation’s pressingenergy and electric power problems by providing enhanced powerquality and higher reliability to the distribution system.
Alternative energy sources generally supply DC, with exceptionsin specific cases. Therefore common alternative energy sourcessupplied systems necessitate the introduction of an inverter todevelop suitable form of power for the utility. There are two mainconnection modes of the inverter in DG systems, grid connected andislanded mode.In islanded mode the DG system supplies a singlesystem without any connection with the traditional system, thus normally installed in remote areas.DG system is supposed to supplya stable frequency and voltage in the presence of arbitrarily varyingloads in this mode. Besides, grid-connected inverter system uses theexisting utility grid and therefore should work in synchronizationwith it. The inverter output parameters need to be same as that of thegrid parameters to avoid malfunction. Different methodologies existas synchronization algorithm, Phase Lock Loop (PLL) being the mostpopular one because of its several advantages [2].
Literature survey proposes parallel operation has shownsatisfactory performance to increase reliability and power rating ofthe system. For parallel operation synchronization between unitsbecomes a serious issue as even a small phase difference will producecirculating currents between modules. Even for single operation inisland mode, stable frequency and voltage can’t be delivered withoutthe help of control methods. Various control methods are availablefor satisfactory operation of the DG system in both the modes ofoperation depending on the presence of communication link betweenDG unit and the network [3].
This paper aims to present a comprehensive study of a threephase grid connected inverter with grid side control technique. Themethod of feed forward current control of the grid side inverter inthe synchronous rotating frame is an effective solution for this [4].In this work a pulse width modulated (PWM) three phase voltagesource inverter (VSI) with LC filter has been considered. In [5], theavailable current control techniques for grid connected inverter aredetailed. In this study the most common PI controller is considered.The modeling and simulation of the whole inverter system is carriedout in MATLAB-SIMULINK environment.

Components of Grid Connected Distributed Generation system

Three phase VSI
VSI is the heart of the system as it delivers suitable form ofpower to meet the demand. In order to apply to a broad range ofDG, the input power nature is ignored, the inverter being poweredby a dc power source. The VSI contains power electronics switches asswitching losses across power electronic devices are less. Here IGBThas been used as the component along with free-wheeling diodes tofacilitate bi-directional current flow. The switches are turned ON andOFF by a control trigger signal that is developed by a PWM.
LC filter
PWM inverter output waveform contains ripple which needsto be eliminated for proper synchronization. The output must bepassed through a filter to sieve the undesired components from theoutput current spectrum. LC filter plays a very important role in thissituation as the inductor eliminates the current ripple and to eliminateswitching frequency components capacitor has been the best choiceamongst the shunt impedances. The design details of L and C dependon the system conditions [6].
Allowing 10% of ripple current the inductor value is chosen as,
Where, Vdc = DC source voltage, fs = Switching frequency, Δ iLmax= ripple current
Design of capacitor C is based on the reactive power suppliedby the capacitor at fundamental frequency. Here reactive power ischosen as 15% of the rated power and value of C is given by:
Where, f = grid frequency, Prated= rated power, Vrated = ratedvoltage
Phase Lock Loop (PLL)
The most serious condition of the system is achieved by usingsynchronous reference frame based PLL block as the synchronizationalgorithm. The module determines the phase angle θ of the grid andlock is realized when the error between the phase angles of inverterand grid is nil. The block diagram of the used PLL is further shownin Figure 1 [2].
Figure 1: PLL block diagram.
PI controller
In this work, synchronous PI controller is chosen as the controlstructure of the grid-connected inverter.PI compensators reduce theerrors between the desired current id* and iq* and actual currents id andiq to zero. To simulate the operation of the current control, referenceinput active current (id*) and reactive current (iq*) need to be generatedfirst. For that reference active and reactive load power demand ischosen and using suitable mathematical equations reference currentsare generated then. The useful equations are given here [7].
The mathematical calculation becomes complex in abc frame dueto its sinusoidal components and time varying nature. The outputcurrents in the three-phase form are thus transformed onto thed-qaxis current by means of Park-transformation.
Pulse Width Modulator
The inverter gate pulse is delivered by the pulse width modulator.The duty cycle for the PWM is achieved by adding the decouplingterms with the PI controller output. The decoupling gain depends onthe system inductance and grid voltages [8]. The voltage output of theinverter in dq reference frame is given by
The complete current structure shown in Figure 2 has been usedin this study.
Figure 2: Block diagram of current control structure.

Simulation Results

In the work different scenarios have been studied in order tosupport a simulation based design of an inverter system capable tooperate in grid-connected mode using Matlab/Simulink platform.First, the synchronization of the grid-connected inverter with the utility grid is checked to achieve a stable voltage profile. Next, thereal and reactive output power of the inverter is verified whether theyfollow the fixed and dynamic load demand.
Simulation of Grid Connected Inverter
With switching frequency of 5 kHz the inverter input is taken as600V DC source and the grid filter parameter values are inductanceof 15 mH and capacitance of 350 μF in order to achieve the desiredvoltage and current ripple characteristics. Finally, to emulate theutility grid a three-phase 415V, 50Hz AC source is used.
Figure 3 shows the inverter output voltage before the LC filteris connected. The desired voltage waveform and current ripplecharacteristics after getting passed through the filter is shown inFigure 4.
Figure 3: Inverter output (before filtering).
Figure 4: Inverter output (after filtering).
Simulation of PLL Synchronization circuit
The simulation results of PLL, shown in Figure 5, depicts thatthe PLL can successfully extract, without errors, the phase angle ofthe grid voltages, which allows for synchronization with the grid.Consequently, synchronization between output of inverter phase andgrid phase angle is achieved by locking PLL for every instant of timebetween 0 to 2π.
Figure 5: PLL output
Simulation of grid Synchronization conditions
In this study a switch is used to discriminate between the statesbefore and after the inverter is connected to the utility grid. Inthis section the inverter output and grid output have been shownseparately for both the cases.
Case I: Before switching on the circuit breaker
Case I shows the system conditions when the inverter is supplyingthe load demand separately without any connection with the grid.In Figure 6 and 7 the inverter side voltage and current outputs areshown respectively. The parameters are measured after the LC filtersection. It can be seen that the waveforms are smooth sine wave withdesired magnitudes, the voltage output being 415 V (rms) and thecurrent magnitude being 4.5 A.
Figure 6: Inverter output voltage.
Figure 7: Inverter output current.
Figure 8 and 9 shows the grid side output voltage and outputcurrent before switching on the circuit breaker. The voltage waveformsare smooth sine wave and show the magnitude of 415 V (rms) and thecurrent magnitude is 7.5 A. From the analysis of the waveforms it canbe seen that the voltages of both the sides are exactly same but thecurrent magnitude differs. The difference in values of current outputis because of different values of load in each side.
Figure 8: Grid voltage output.
Figure 9: Grid current output.
Case II: After switching on the circuit breaker
After the transition time of the breaker the DG system is connected to the grid. Figure 10 shows the synchronized voltage profile ofinverter and grid for a single phase. It can be observed that the voltagemagnitude is same as the previous case. Again, from Figure 5 it canbe seen that there is no change in parameters of the PLL block andthe waveforms are distortion less even after the switch is closed. Thisimplies that the condition of synchronization is established.
Figure 10: Grid and Inverter output voltage.
Simulation of current control structure for grid-connectedinverter
When the renewable source power plant is connected to thegrid, the real and reactive output power of the inverter should becontrolled based on the load demand changes. In this section casestudies have been considered for active power injection and mixed power injection to the grid. All those cases have been thoroughlysimulated in order to observe system behavior and performance andto support the work objective.
Case III: Active power injection
For this case, the grid and inverter are connected together witha step change in active power value and the reactive power value isignored. The study shows that the output follows the load demand ofthe network.
The circuit breaker transition time is set to 0.5 s. The active powerinput function is imposed a step change at 0.5 s from 3 kW to 4 kWand the reactive power function is taken as zero in order to ignore thereactive power demand.
The simulated inverter current output of the model is shown inFigure 11 and the figure shows that after a small transient time, theoutput inverter current reaches its steady state value which is exactlyequal to the reference value.
Figure 11: Three phase inverter output current waveform.
Case IV: Step change in Load with lagging power factor
For this case, the grid and inverter are connected together witha step change in active and reactive power values. The study showsthat these variable values of active and reactive power follow the loaddemand of the network.
The circuit breaker transition time is set to 0.5 s. The active powerinput function is imposed a step change at 0.5 s from 3 kW to 4 kWand the reactive power function changes its value from 2.5 kVAR to3 kVAR.
The simulated inverter current output of the model is shown inFigure 12 and the figure shows that after a small transient time, theoutput inverter current reaches its steady state value which is exactlyequal to the reference value.
Figure 12: Active power output.
The Figure 13 and 14 show the changes in input functionsfollowed by the load values. The figures illustrate that the powercontrol by feed forward current control loop method implementationis successful to maintain the desired power.This proves that thecurrent loop controller is effective such that measured currents tracktheir references. In addition, its dynamic behavior is satisfactory.
Figure 13: Active power output.
Figure 14: Reactive power output.
The complete simulink model is given in Figure 15.
Figure 15: Simulink model.

Conclusion

Though renewable energy sources are seen as reliable alternativesfor conventional energy sources, still their performance and efficiency are under development. Consequently the control structureof the grid-connected inverter is an important sector to deal with.This paper demonstrated the design and modeling of a powerconditioning system for the renewable-source power plant. Twomain tasks for the proposed grid-connected DG system, which areproper synchronization of the inverter with the utility grid and powerflow control of the system, have been done.Simulation results ofdifferent cases are taken into consideration to validate the proposedscheme.A switch has been introduced in the circuit in order to testthe effectiveness of the synchronization algorithm and from the casestudies it can be seen that the system works in exact synchronism.Again, load tests have also been done and the system performance hasbeen observed and the results have indicated the proposed currentcontrol method can effectively control the power flow to follow theactive and reactive power reference.

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