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Power Quality Improvement using Four leg gridinterfacing inverter for grid connectedDistributed generation system
CONFERENCE PAPER · APRIL 2012
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2 AUTHORS:
Ilavarasi Veeran
Pondicherry Engineering College
4 PUBLICATIONS 2 CITATIONS
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C. Christober Asir Rajan
Pondicherry Engineering College
124 PUBLICATIONS 437 CITATIONS
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Available from: Ilavarasi VeeranRetrieved on: 22 January 2016
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Abstract — This paper suggests a new method that consists of a
four leg inverter that is capable of simultaneously compensatingproblems like power factor, current imbalance and currentharmonics, and also of injecting the energy generated byrenewable energy power sources. The fourth leg of inverter isused to compensate the neutral current of load. The gridinterfacing inverter can thus be utilized as: 1) power converter toinject power generated from RES to the grid, and 2) shunt APFto compensate current unbalance, load current harmonics andload reactive power demand. The inverter is actively controlled insuch a way that it draws/supplies fundamental active powerfrom/to the grid. All of these functions may be accomplishedeither individually or simultaneously. This new control concept isdemonstrated with extensive MATLAB/Simulink simulationstudies.
Index Terms — Active power lter (APF ), distr ibutedgenerati on (DG) , power qual ity (PQ), renewable energy (RE)and Voltage source I nverter ( VSI ).
I. I NTRODUCTION
The widespread use of non-linear loads is leading to avariety of undesirable phenomena in the operation of powersystems. The harmonic components in current and voltagewaveforms are the most important among these.Conventionally, passive filters have been used to eliminate linecurrent harmonics. However, they introduce resonance in the
power system and tend to be bulky. So active power line
conditioners have become popular than passive filters as itcompensates the harmonics and reactive power simultaneously[1]. The active power filter topology can be connected inseries or shunt and combinations of both. Shunt active filter ismore popular than series active filter because most of theindustrial applications require current harmonicscompensation. Different types of active filters have been
proposed to increase the electric system quality; a generalized block diagram of active power filter is presented in [2]. Theclassification is based on following criteria.
a. Power rating and speed of response required in
compensated system b. System parameters to be compensated (e.g. current
harmonics, power factor , voltage harmonics)c. Technique used for estimating the reference
current/voltage.Current controlled voltage source inverters can be utilized
with appropriate control strategy to perform active filterfunctionality. The electrical grid will include a very largenumber of small producers that use renewable energy sources,like solar panels or wind generators. One of the most common
problems when connecting small renewable energy systems tothe electric grid concerns the interface unit between the powersources and the grid, because it can inject harmoniccomponents that may detoriate the power quality. However,the extensive use of power electronics based equipment andnon-linear loads at PCC generate harmonic currents, whichmay detoriate the quality power [3],[4]. In [5] an inverteroperates as active inductor at a certain frequency to absorb theharmonic current. A similar approach in which a shunt activelter acts as active conductance to damp out the harmonics indistribution network is proposed in [6].
Generally, current controlled voltage source inverters areused to interface the intermittent RES in distributed system.This paper suggests a new method that consists of four leg VSIthat is capable of simultaneously compensating problems like
power factor, current imbalance and current harmonics, andalso of injecting the energy generated by renewable energy power sources with a very low THD. Even when there is noenergy available from the power source the Voltage sourceinverter can still operate, increasing the power quality of theelectric grid. Thus the grid interfacing inverter is effectivelyutilized to perform the following functions
a. Active power injection b. Current harmonics compensation at PCC.c. Current unbalance and neutral current compensation in
3-phase 4-wire system.
Power Quality Improvement in Grid connectedsystem using Four leg VSI
V.Ilavarasi C.Christober Asir RajanPG Student, Department of EEE Associate Professor, Department of EEE
Pondicherry Engineering College Pondicherry Engineering CollegePuducherry, India Puducherry, India
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d. Load reactive power demand support.In three phase application with three leg inverter, if the load
requires a neutral point connection a simple approach is to usetwo capacitor to split the dc link and tie the neutral point to themidpoint of two capacitors. In this case the unbalanced loadswill cause the neutral currents that flow through the fourth wiredistorting the output voltage. Another drawback is the need forexcessively large dc link capacitors. The important parameters
of VSIs are the level of dc link voltage, value of interfaceinductor and hysteresis band. These parameters must becarefully selected to provide satisfactory performance whiletracking reference currents [7], [8]. In [9] a control strategy
based on p-q theory is proposed where load current andinverter current sensing are required to compensate load anharmonics.
II. SYSTEM TOPOLOGY AND OPERATING PRINCIPLE
The proposed system consists of RES connected to the dc-link of a grid-interfacing inverter as shown in Fig. 1. It isassumed that a non-linear unbalanced load consisting of athree phase and single phase diode rectifier is connected to athree-phase balanced source voltages. The voltage sourceinverter is a key element of a DG system as it interfaces therenewable energy source to the grid and delivers the generated
power. The RES may be a DC source or an AC source withrectier coupled to dc -link.
Grid
Reference current
generationCurrent control
External Hyteresis
Va Vb Vc
Ia Ib Ic In
P1 P2 P 3 P4 P 5 P 6 P7
Lsh
A
B
C
N
PCC
Distribution
Transformer
Delta - Star
I i n v a
I i n v b
I i n v c
I i n v n
Unbalanced
Non linear
loads
IaIb
IcIn
Vdc
P8
Four leg Grid Interfacing inverter Renewable Energy
Source
Fig.1 Schematic of Proposed Distributed Generation system
Generally, the power circuit of shunt APF consists of a three phase Voltage Source PWM Inverter (VSI) using IGBTs
coupled at the Point of Common Coupling (PCC) via couplinginductance. The active filter compensates the harmonicsgenerated by nonlinear loads by generating the same harmoniccomponents in the opposite phase. Harmonics are thuscancelled and the result is a non-distorted sinusoidal current.Each leg of VSI consists of two IGBT. The single phaseequivalent circuit of the system is shown in Fig.2. Load currentcan be written asiLa = iLaf + i Lah (1)iLa= i Inva + i S (2)
Fig.2 Single Phase Equivalent Circuit of the system and VSI
In equation (1), i Lf is the fundamental component and i Lh
harmonic component of load current. Since the harmoniccomponent of load current should not be transferred to thesupply side, the inverter has to inject a current whosemagnitude should be equal to harmonic component.We should have,
iLah = iInva (3)
From equation (2) and (3), we get
iS = i Laf (4)
If i Lah > i Inva switch2 should be OFF and switch 1 should
be ON so the current generated by dc capacitor i Inva is equal toiLh.If iLah < i Inva switch2 should be ON and switch 1 should be
OFF so the current i Inva should be transferred to the ground inorder to have i Inva = i Lah. .
III. CONTROL STRATEGY
The main aim of the proposed approach is to inject the power from RES and also to make the voltage source inverterto function as an APF. There are many control approachesavailable for the generation of reference source currents for thecontrol of VSI system in the literature [10], [11]. The blockdiagram of the control scheme is shown in Fig. 3. The controlstrategy applied to the grid side inverter consists mainly of twocascaded loops. Usually there is a fast internal current controlloop, which regulates the grid current and an external voltageloop which controls the DC-link voltage [12]. Conduction andswitching losses of diodes and IGBTs in inverters increasevoltage ripple in DC-link which affects the performance of thefilter. These effects controlled by a feedback loop where PIregulator compares the DC-link voltage with a referencevoltage. The control scheme approach is based on injecting thecurrents into the grid using hysteresis current controller.
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LPF
UNIT
VECTOR
TEMPLATE
VOLTAGE
REGULATOR
Current Controller
NOT
NOT
NOT
P1
P2
P3
P4
P5
P6
+
+
+
-
-
-
Im
Ia
Ib
Ic
Ia*
Ib*
Ic*
VdcVdc*
HysteresisNOT P8
P7+
-In
In*
0
Ua
Ub
Uc
Vdc
Fig.3 Block diagram representation of control scheme
A. Voltage control of DC capacitor
The DC link voltage regulates balanced power flow in thegrid system so the DC link voltage is maintained constantacross the capacitor. A PI controller is used to maintain theDC link voltage at specified value. The DC link voltage issensed and compared with reference value and the error is
passed through a PI controller.
Vdcerr = V dc* - V dc (5)
Thus the output of dc link voltage regulator results in currentIm .
B. Current Control of VSI
Unit vector templates are generated as
aU Sin (6)
2
3b
U Sin (7)
2
3c
U Sin (8)
The multiplication of current Im with unit vector template( U a ,U b ,U c) generates reference grid currents (I a
*, I b*, Ic
*).The instantaneous values of reference grid currents arecomputed as
Ia * = I m . U a (9)
I b* = I m . U b (10)
Ic* = I m. . Uc (11)
The neutral currents present if any due to the loadsconnected to the neutral conductor should not be drawn fromthe grid. Thus reference grid neutral current is considered aszero and can be expressed as
In* = 0 (12)
Current errors are obtained by comparing reference gridcurrents ( I a
*, I b*, Ic
*) with actual grid currents ( I a, ,I b , I c ).These current errors are given to the hysteresis currentcontroller.
Iaerr = I a* - I a (13)
I berr = I b* - I b (14)
Icerr = I c* - I c (15)
Inerr = I n* - I n (16)
C. Switching Control of IGBTs
Switching pulses are generated using hysteresis currentcontroller. There are various current control methods for active
power filter configurations but hysteresis method is preferredamong other current control methods because of quick currentcontrollability, easy implementation and unconditionedstability [13].The conventional current control scheme is thehysteresis method where the actual filter currents are comparedwith their reference currents with a predefined hysteresis bandin their respective phases. Thus the actual currents track thereference currents generated by current control loop. Theswitching pattern of each IGBT is formulated as,
If (I a* - I a ) = +hb then the upper switch S 1 will be ON in the phase a leg of inverter.If (I a* - I a ) = -hb then the lower switch S 4 will be ON in the
phase a leg of inverter.
Where, hb width of hysteresis band. Similarly switching pulsesare derived for other three legs.
IV. SIMULATION RESULTS
To verify the proposed control approach to achieve the multi
function of four leg inverter simulation study is carried outusing MATLAB/Simulink. The block diagram of the designedsystem is shown in Fig.4. A Four leg current controlled voltagesource inverter is actively controlled to achieve balancedsinusoidal grid currents with unity power factor despite ofunbalanced non linear loads at PCC. The supply voltages areassumed to be a balanced three-phase voltage sources with themagnitude of 30V. The System Parameter is given in Table Ishown .
TABLE I SYSTEM PARAMETER
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Fig.4 Block diagram representation of designed system
A DC voltage equivalent to RES is connected across the dclink of the grid interfacing inverter The inverter is effectivelycontrolled to inject compensation current that cancel outsource current harmonics. This utilizes the control circuitshown in Fig.5 In this control circuit the Proportional andIntegral gains of PI controller are K PVdc = 0.28 and K IVdc =0.01
Fig.5 Control circuit
An unbalanced 3 phase 4 wire non linear load whoseunbalance, harmonics and reactive power need to becompensated is connected at PCC. The waveforms of gridvoltages (V a,V b,V c), grid currents (I a, I b, Ic), load currents (I la,Ilb, I lc) and inverter currents (I inva , I invb , I invc) are shown in Fig.6.
Fig.6 Grid Voltages, Grid Currents, Unbalanced load Currents and InverterCurrents
Initially the grid interfacing inverter is not connected to thenetwork; therefore the grid current is identical to the loadcurrent profiles shown in Fig.6. The load power demand isentirely supplied by grid alone. At t=0.32s the grid interfacinginverter is connected to the network. At this instant inverter ismade to inject compensating current as a result the source side
harmonics is reduced. The grid currents starts changing fromunbalanced non linear to balanced sinusoidal currents Thusonce the inverter is in operation the grid only suppliesfundamental current. At t=0.52s the active power from RES isincreased to evaluate the system under variable powergeneration from RES. This can be clearly noticed by increasedmagnitude of inverter current. At t=0.72s the power from RESis reduced. The corresponding change in the inverter and gridcurrents can be seen from the Fig.6.
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(a) without Inverter
(b) with Inverter
Fig.7 Grid voltages and current
Fig.7 shows gird voltages and current with and without Fourleg grid interfacing inverter. From Fig.7a it can be observedthat the grid voltage and current are in phase which shows thatthe unity power factor is maintained. Fig.8 shows the dc linkvoltage across the grid interfacing inverter maintained atconstant level in order to facilitate active and reactive powerflow.
Fig.8 DC link Voltage
Fig.9 shows the simulation results of neutral currents forgrid, load and inverter. It can be noticed that as the invertersupplies the load neutral current demand the grid neutralcurrent becomes zero after t=0.32s. The load neutral currentdue to single phase loads is effectively compensated by thefourth leg of the inverter such that the current in grid sideneutral conductor is reduced to zero.
Fig.9 Grid, Load and Inverter Neutral Currents
Active and reactive powers of grid (P grid , Q grid), load (P load ,Q load) and inverter (P inv, Q inv) are shown in Fig.10. Where
positive values of grid active-reactive powers and inverteractive-reactive powers imply that these powers flow from gridside towards PCC and from inverter towards PCC respectively.
Negative values of load active-reactive powers imply thatthese powers are drawn by the load. The active and reactive
power flows between the inverter, load and grid duringincrease and decrease of energy generation from RES can benoticed from Fig.10. Initially the grid interfacing inverter isnot connected to the network so the entire load power demandis supplied by the grid alone. At t=0.52s the inverter startsinjecting active power generated from RES. Since thegenerated power is sufficient to meet the load demand theactive power supplied by the grid becomes zero. Moreover the
grid interfacing inverter also supplies the load reactive powerdemand locally.At t = 0.72s the power available from RES is reduced. The
corresponding change in the inverter and grid real and reactive power can be seen from Fig.10.The active and reactive powerflows between the inverter, load and grid during increase anddecrease of energy generation from RES can be noticed fromFig.10. When there is no power generation from RES the gridinterfacing inverter acts as an Active power filter enhancingthe quality of power. During this period the inverter consumesa small amount of active power to maintain the dc-link voltageand to overcome the losses associated with inverter while mostof the load reactive power need is supported by inverter
effectively. When RES power is sufficient to supply thedemand the grid interfacing inverter can simultaneously beutilized to inject power generates from RES to PCC and toimprove the quality of power (current unbalancecompensation, current harmonics compensation , load reactive
power support, neutral current compensation) at PCC. Thusfrom the simulation results it is evident that the three phasethree leg current controlled voltage source inverter can beeffectively utilized to compensate current harmonics and alsoenables the grid to supply sinusoidal power at UPF.
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Fig.10 Active and Reac tive Power flow
The total harmonic distortions (THDs) of phase a, b and cload currents are noticed as 13.47%, 29.21% and 14.81%respectively. After compensation the grid current THDs arereduced to 1.16%, 1.08% and 1.01% for a, b and c phasesrespectively which is shown in Fig.11.
Fig.11 Grid Current Harmonic Spectrum
V. CONCLUSIONS
This paper presented a control of an Three phase Four leggrid interfacing inverter improve the quality of power at PCCfor a 3 phase 4 wire system. It has been shown that the gridinterfacing inverter can simultaneously be utilized to inject
power generated from RES to PCC and to improve the qualityof power at PCC. Thus the proposed controller preciselymanages any variation in real power at dc link and effectivelyfeeds it to the main grid. The current harmonics caused by nonlinear load connected at PCC are compensated effectively suchthat the grid currents are always maintained sinusoidal at unity
power factor. This approach thus eliminates the need foradditional power conditioning equipment to improve the
quality of power at PCC. Thus the load neutral current is prevented from flowing into the grid side by compensating itlocally from the fourth leg of the inverter.
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