PV Based Distributed Generation with Compensation Feature Under Unbalanced andNon-linear Load Conditions for a 3-4, 4 Wire System

Hiren Patel and Vivek Agarwal'Applied Power Electronics LaboratoryDepartment of Electrical Engineering

Indian Institute of Technology, Powai, Mumbai - 400 076, India.

Abstract - With the increased penetration of Distributed improves the power factor. A PV based system has beenResources (DR) into the grid, it is desirable and advantageous to used with a 1-, 3 wire inverter[6] to compensate for thetap additional benefits out of these resources besides their main harmonics and to improve the power factor under non-function of active power generation. This paper presents a . . .Photo Voltaic (PV) fed system which can not only feed active susoidal ie oltage cntions An extensive literaturepower into the grid, but can also compensate for unbalanced, survey carried out by the authors shows that limited literaturenon-linear loads connected to the power grid. The interface with is available related to the compensation of non-linear andthe grid is realized using three, 1-0 inverters. The proposed unbalanced loads, usingDG based active filters.system is capable of compensating the negative and zero Compensation of unbalanced linear loads, using PV basedsequence harmonics of the load currents and to achieve a zero active filter, has been described [7]. However, this method isneutral current. Extensive simulations have been performed to not applicable to non-linear loads as the observer modelstudy the viability of the proposed system, including the becomes highly complex and the system order increasesmaximum power point tracking (MPPT) performance and requiring complex calculations. PV power generation, withresponse of the configuration to step disturbances in linear load, rq xclutionas PV toer generation, witunbalanced operation and sudden application of shading effect. an sat eatreo shuence fite use for

All the results are presented.compensating the negative sequence currents has also beendemonstrated [8]. But the system is unable to compensate the

I. INTRODUCTION zero sequence currents as the proposed topology uses

standard (3-+, 3 wire) 3-leg voltage source inverter. It hasThe present electrical distribution systems are been shown[9] that by removing only the negative sequence

characterized by the presence of a variety of loads - both components from the 4-wire system, the level of imbalancelinear and non-linear. These loads pollute the power system may increase. Hence, it is necessary to remove even the zeroby drawing reactive power and also distort the waveforms sequence components, which may otherwise lead to[1]. Apart from this, the overall system may operate under problems like additional losses in transmission linesbalanced or unbalanced conditions mainly due to the (including neutral lines), voltage drift of neutral points,asymmetrical distribution of large, 1-+ loads and energy protection and interference problems. Another elegantsources. This also depends on the system design which may scheme, which takes care of reactive and harmonicinvolve three-wire or four-wire circuits. Further, different compensation as well as load imbalance has also beentypes of Distributed Generation (DG) sources of different proposed with the compensation circuit fed by a capacitorcapacities, like Fuel Cell, Photo Voltaic (PV) and Micro- acting as a stiff DC source [10]. Yet another scheme [11]turbines etc. may lead to negative impact on voltage control, incorporating similar features has been proposed for non-power quality and unbalance in the power system and may sinusoidal grid voltage conditions.further worsen the situation [2]. This paper describes a PV fed DG source cum active filterThe DG sources interface with the power grid through which, not only feeds active power but also acts as a

inverters. It is reasonable to explore if, under certain compensator for an unbalanced, non-linear load in a 3-+, fourconditions, these can be used to provide compensation wire distribution system. It eliminates the negative and zeroagainst harmonics and reactive power. Further, can these sequence components in the load current and reduces theinterfaces be used to correct the unbalance in the system? In neutral current to zero. It also provides solution for otherline with these thoughts, several researchers have worked on power quality problems. The system configuration is'active filters' fed by DG sources [3-9]. But most of the presented in Section II. The control scheme for unbalancedreported work is limited to only 1-+ circuits [3-6]. Jimnez et compensation of linear and non-linear loads with this PVal., [3] have employed an adaptive filter to generate a based DG, is explained in Section III.reference current which modulates the output current forcompensating the reactive and harmonic currents drawn by II. PROPOSED CONFIGURATIONthe non-linear loads. Kuo et al., [4], have used a 1-, three-leginverter to realize zero neutral line current. A method has Fig. shows the proposed scheme for active powerbeen~~~~~propsdb Wu et a., [5,wt.Vara steD generation and non-linear, unbalanced load compensation. In

source, that takes into account the effect Of non-linear filter adiint hs h ytmprvdsalohrcnetoainutr copnae.o h amnccret n features. The configuration uses a dc-dc converter to track

1Corresponding author. email: agarwal@ee.iitb.ac.in

1-4244-0726-5/06/$20.OO '2006 IEEE 322

the maximum power from the PV array and three, 1-+, VSI and is computed as under.bridges with a common dc bus [12] to enable an independent Duty cycle for this dc-dc converter can be expressed ascontrol of all the three phases. For this work a combination D 1-1 Vin (1)of linear and non-linear loads is considered. Linear loads Vconsist of a 3-+ resistive load. While the non-linear load is In conventional controller working with P&O method, D*formed of an un-balanced, 3-+ load, formed by three l1-A (given by (2))is generated to track the MPP.rectifier (with a large filter capacitor) supplying a resistive *

load. Effect of partial shading of the PV array has also been D* = 1 -__v (2)investigated. O

To linear loads It is observed that such a controller is slow to respond. The(resistive) and proposed controller (Fig. 2.) is fast to respond and can

77 Non-linear loads quickly track the MPP. Here, VO (same as VdJ) can be

V 1t + R replaced by Vdc ref to feed-forward reference DC link voltagelaf

for the computation of D*. The principle involved in thiscontroller can be mathematically explained as under.

D+AD=_ Vn +AVn (3)V?o +AV?O

Pv DC-DC Vdc As the DC link voltage is maintained constant by drawing theArray converter LdC T power from grid, A Vo can be neglected. Hence, from (1) and

(3)I ~~~~~~~~AVAD-=_ (4)

MPPT and Reference VOBoost controller Current

Generator The function of the inverter controller is to generate the 3-(hreference currents and to drive the switches of the three, 1-Fig. 1 PV-based system configuration for power generation and T r

compensation. ), full-bridge inverters using the hysteresis control to trackthe generated reference currents. The process of reference

III. CONTROL SCHEME current generation is depicted in Fig. 3.

This section details the control principle involved in Vdcjref Peak Supply Currentoperating the dc-dc converter and the three, 1-, VSI bridges Vdc Calculator for maintaining gCem ILto act as an unbalanced compensator. ] constant voltage acrossThe objective of the control scheme for the DC/DC Vg dc link capacitor

converter block[Fig. 1] is to track the maximum power from rithe PV source to ensure an effective utilization of the array. VL Estimation of peak of +fundamental component +Also, as the system iS grid connected, the excess power can /IL of current required by the /9/mlbe fed into the grid with unity power factor, without load +imposing any requirement for batteries. P&O (Hill climbing)method is applied for tracking the peak power point. The Unit current vector lafcontrol signal for dc-dc converter is obtained in a feed- g generator for unity powerforward manner as shown in Fig. 2. L factor at the gridr ~~~~~~~~~~+

+ D 0* vg Calculation of current tosend power generated by

< / \>laf + PV to the grid after /Ph_PVconsidering the losses in

V AvD the inverterin AL; __V

o I: from MPP

Iv* l 0'Fig. 3 Reference current generator block.

Reference current is formed of three current components:(i) Current required to maintain constant voltage across the

Fig. 2 Control scheme used for the dc-dc converter operation. DC link capacitor (ii) Current required to satisfy the active

ViiiandV0 ae te inut nd otpu volageof d-dc power demand of the load, in a case when the PV power isconveterespecivel. L*. .th reeec volag (otane not available (iii) Current for feeding the maximum powerfromMPPT algorithm) for operating point to track the available firom the pv to the grid.maximumpower poin. Dn * ar th aculad,eeec It iS assumed that the grid iS strong and provides a

duty cycle. AD is the perturbation to track the required MPP balanced supply represented by (5), where Vga. VgTb and Vgc

323

are phase to neutral voltages and Vgm is the amplitude of the Hence, the peak of the current component from the grid togrid voltage. the load and DC link capacitor (IgC&L m) can be obtained from

Vgd=VgnSinot (10) and (13).Vgb=Vgj,,sin(t -2zT/3) (5) IgC&L m =gC'm + IgLml (14)

Vgc=Vg,jin(o + 2T/3) The instantaneous values of this current in the three phasesare expressed by (15).

The computation of first component is made by computing & L Vgathe charge required to restore the voltage across DC link g _f Vgmcapacitor (Cdc).The actual energy stored, Edc, on the capacitor Vgb (15)is 'gb(C&L ref =gC&L m X

2 _fVgmEdC = Cdc x Vdc (6) vgc2 'gcC&L ref - gC&Lm X

while that required to have a reference DC link voltage, VgmVdc ref iS The final component of the reference current, Iph ,, is

Edc ref = Cdcc ref (7) computed from the information regarding the maximumpower available (Ppv) from the PV array. After meeting the

Hence, the amplitude (Igcm) of the current component from inverter losses, rest of the PV power Ppv G&L iS supplied tothe grid to the capacitor (Igc), to restore this deficit energy the grid and load.can be computed from the energy balance given by (8). It is ( 2 2 1seen from (8), that the current in the respective phases are in Ppv (G&L =Ppv 3ph pv gC iv _phphase with the line to neutral voltages, thereby maintaining Here riv_,h is the resistance of the inverter per phase. Henceunity power factor. the current components for the three phases are given by

Edc ref - EdcT g,Z, sin cot x gc.. x sin +ot + la_lov=- &_LXV

Vg,msin(ot - 27c/ 3) x lg(. x sin(wot- 2z / 3) (8) x gmV G Vgb(g17)

T is the half period of the input supply voltage cycle. +/i x Pv (&L Vgc

Edcredc=VgmXIg mXF (9) 3X(Vgm /V) VgmHence, from (15) and (17) the reference currents (shown as

( 2 2 Iaj* in Fig. 3.), which the inverter has to track forgCm C dc _ Vdc ref - Vdc / (1) compensating the unbalanced, non-linear load and to feed

XT x Vgm active power into the grid, are given by (18).

The second component of reference current is generated, I - I - I. . * * . . .* . r^1 inva ref ~~~~~~~~-apv gaC&L ref+Las under, by monitoring the load power requirement. The f + La

instantaneous active load power demand, Pload, is given by: iinvb ref = Ib_ pv - IgbC&L ref + 'Lb (18)Pload= VgaILa+VgbILb + VgcLc (11) iinvc re C + LC

V ' invc re~~~~~~~~~fcpv_P gcC&L ref L

where ILa, ILb and ILC are the three unbalanced load currents ofthe three phases. IV. SIMULATION RESULTSThe average power, PgL, which the grid must supply to

meet this active power demand of the load is expressed by This section contains the simulation results for proposed(12) below: controller for the dc-dc converter to track the MPPT. It also

PgL =i 5Vgm'gLml cos (12) shows the simulation results for the scheme employed to

Here, IgLm] is peak current from the grid to non-linear load to compensate unbalanced components and to solve the othersupply only the fundamental component of current for power quality issues using this PV based DG. For this work,matching the active power demand of the load. X1 is the angle a 3-+, linear resistive load and an un-balanced, non-linear, 3-between fundamental component of the load current and grid + load, formed using a three 1-+ rectifiers and capacitors, arevoltage. But as unity cos¢b1 is desired, 'gLm, can be obtained as considered.

Fig. 4 shows the response of dc-dc converter with thePgL (13) conventional controller expressed by (2). Perturbation of 2 V

'gLml =iS considered in the simulations. Initially, uniform shading*gm with solar insolation of )=1 kW/m2 is considered. MPP is

324

reached at t=0.97 s. At t=1.21 s partial shading is applied on -I( 20C

the PV array. At t=1 .36 s once again array receives uniform _minsolation (X=l kW/ .PV is disconnected at t=1.46 s and >> (a)reconnected at t=1.56 s. Wvhen the performance of this Jo0 o.i 0.2 0.3 0.4 0.5 0.6 0.7 0.8 01.9controller was tested with a perturbation of 0.1 V, the time ~40Ctaken to track the MPP was found to be more than 3 s. 2 b

CL L) 200 >0(._1~~~~~~~~_ 0) ~~~~~~~~~~0-

100

cu

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91

00.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 (a) (c)

=3 1400- ~~~ ~~ ~ ~~~~~~~~~~(b) - ~ - C00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91

>0

01402 0.4 0.6 0.8 1.2 1.4 1.6 1.82 4

4 \C3: > o~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~0.1 0.20.30.4 0.5 0.6 0.7 0.8 0.90Q02 Time (s)

>0CL002 0 6 08 12 14 16 18 2Fig. 5. Response of dc-dc converter with proposed controller.

0 600

> 400 ~~~~~~~~~~~~Fig.6 shows that grid is free from the harmonic currentsa) ~~~~~~~~~~~~~~andit is the inverter which acts as an active filter. Also it

0.O.4 06 0.=d supplies the power generated from PV to the grid. Fig. 7>00. 0.4 06 0.8 1.2 1.4 1.6 1.8 2 shows that the proposed controller for dc-dc converter

Time (s) quickly tracks the maximum power point. It attains the MPPFig. 4. Response of dc-dc converter with conventional controller. PV array's within 0.1I s. There is no effect of load change on the tracking(a) output voltage (b) output current and (c) output power. (d) shows dc link performance. However, at t=0.8 s when the partial shadingvoltage Vd,. occurs, the operating point shifts quickly to a new maximum

Fig. 5 shows the response of dc-dc converter with the power point. But this is the local maximum point. Sincepropsed ontrller Her, peturbtionof 01 V is partially shaded PV array shows multiple peaks in the power-proposieed.controller Here persturbastio ro 0.1h Vage voltage characteristics, an appropriate algorithm is required

consrbaidere. Responsey canif be staillfatewith larger2i to track the global maximum point. This is currently under

considered. MPP is reached at t=0.09 s, much earlier than investigations and will be presented in future.the previous case when MPP was reached at t=0.97 s. This is Removal of linear

deto the modulation of existing duty cycle with JD given I.ncrease in loads from phases adue l~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Ine?r load aby (4). At t=0.285 s partial shading is applied to the PV 200a~array. At t=0.45 1 s once again array receives uniform

=

insolation (X=l kW/ .PV is disconnected at t=0.555 s and -Co -100-reconnected at t=0.65 1 s. - 206 0. 0. 0. 0. 0. 0. 0. 0.

The satisfactory performance of the proposed PV based 01 02 0 04 0 0 07 0a 09

system in trackingthe MPP, supplying active power togrid,30compensating the harmonics and in balancing the unbalanced 20(b)non-linear load is evident from the results shown in Figs. 6 «II

through 9. The step changes (disturbances) are applied at t )-0

0.3 s, t=0.5 s and t=0.8 s respectively. At t=0.3 s, the linear 0 0.1 0.4 0.5 0.6 0. 0.6 0.load is increased by a factor of 4. At t=0.5 s, the linear load ~ 200 Ilntroductionis completely removed from two of the phases. At t =0.8 s, a ~ 0ofSadingsudden step change in the power output ofDG (reduction by -

about 34 kW) due to the partial shading of the PV array panel _ (C)is introduced. The shading effect is obtained using a special 0.1 0004 0 6 0simulation tool developed for studying the response of PV 0iearray under different (including -partially shaded) conditions. Tm s

initially, but within 20 ms (one ac cycle period), the system connected, grid has to supply the non-linear unbalanced loadenforces the grid current to regain sinusoidal conditions. current. At t=0.203 s inverter is connected with the condition

Introduction of that PV is still not able to generate any power (i.e. PV is stillshadina not connected). Under this case, there is no active power+ a ~~~supplied to grid. Actually the inverter draws the active power

0 ~~~~~~~~~~~~fromthe grid to meet its losses. However, inverter canK ~~~~provide the compensating current (referred as inverter current

__ ~~~~~~~~~~~~~infigures) to compensate the harmonic currents of the load.__ ~~~ ~ ~~~~~~(b)Hence the grid currents now behave as a set of balanced

sinusoidal currents. At t=0.4 s, PV is connected. BesidesI compensating the load harmonic currents, PV now slowly

and smoothly increases the current (and hence power)(c) supplied to the grid. This also shows that there is no such

0o >0- ~~~~~~~~~~~~suddenpower excursion due to the PV connection.

Presence of Linear

(d~~~~~~~~~~~~~~~~~~hPhasea

U)

<~~~~~~~~~~Time (s) -

Fig. 7 PV array voltage, current, power output and dc link voltage....................I......

Increase { (bin linear Phase a Phase b2

Ph se b loadsPhscoPhasea\~ ~ ~~~~~~PhaeI311~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cu )

W ~~~~~~~~(

CD~~ ~ ~~~i / Time (s)

31]~~~~~~~~~~~~ lFig. 9. 3-4 Current in case of sudden removal of linear loads at t=0.5 s from

DS- 2 DE GM: a 22 015 m oi c :>35 the two phases (with unbalanced, non-linear load still present across each2m ~~~~~~~~~~~~~~~~~phase)and change in the shading pattern at t=0.8 s.

UD 100-

Time (s) o-0

Fig. 8. Effect on 3-~currents due to a step change (increase) in linear load 01 01 . .50303 . .50505att=O.3 s. ---.200

Fig. 9 shows the systems performance as an unbalanced 0

compensator when the array is receiving uniform insolationand also when the array is working under partial shading20conditions. An unbalanced non-linear load is present in the 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

three phases. Also the linear load is absent in the two phases ~~0(a and b), thus creating a large unbalance in the 3-~system.

oBut the system is still able to compensate the unbalances, 0thereby giving the harmonic free grid current in the three -)7

200phases. 0.1~~~~~~~~~~~~~~~~~~~~~ 4 ) )0.150.2 0.2 0.3 C0.3 0. 0.4 0.5 0.55AE

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In this paper a scheme for PV based DG is proposed to 1, 22-28 Oct. 2000, pp. 729 - 734.provide active power as well as to compensate the [8] S. Segui, F. Gimeno, V. Leon, and J. Montanana,unbalanced linear and non-linear loads, including harmonic "Unbalanced power active compensator with unitand reactive power. A dc-dc controller with feed-forward efficiency control for the improvement of the electricfeature is proposed for the MPPT tracking The quality in four wire systems with photovoltaic panels inMATLAB/SIMULINK based simulation results show that the DC bus," in the Proceedings of IEEE Conference onthe proposed scheme can compensate for load imbalance in Industrial Electronics, vol. 1, Nov. 2002, pp. 656 - 661.the 3-* system over a wide range. As the negative sequence [9] D. Hongfa and D. Xianzhong, "A novel compensator forand zero sequence components of the unbalanced currents unbalanced three phase distribution feeders," IEEEare supplied by the PV based DG, the grid currents represent Power Engineering Society Winter Meeting, vol. 4, Jana sinusoidal and balanced set. Hence, the current in the 2000, pp. 2379-2384.neutral conductor of the grid can be made zero. Though the [10] B. Singh, K. Al-Haddad and A. Chandra, "Harmoniceffect of partial shading has been considered, the maximum elimination, reactive power compensation and load

power point tracking scheme used needs to be refined to balancing in three-phase, four-wire electric distributionhandle multiple power peaks in the power-voltage systems supplying non-linear loads," Electric Powercharacteristics due to non-uniform insolation. This work is Systems Research, vol. 44, no. 2, Feb. 1998, pp. 93-100.under investigations and will be presented in a future paper. [11] S. George and V. Agarwal, "A Novel Technique for

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