Combined Operation Of DSTATCOM With Distributed Generation To Improve Power Quality

Combined Operation Of DSTATCOM With Distributed Generation To Improve Power Quality

Jatin K. Patel, S. P. Gupta and S. P. Singh, Member, IEEE

Abstract This paper proposes a combined operation of the distribution static compensator (DSTATCOM) with distributed generation systems to improve power quality in low voltage grid systems. A distributed generation unit is connected to the dc-link of the DSTATCOM through suitable converter. The dc-link capacitor voltage of the DSTATCOM and the magnitude of three-phase grid voltages are used as feedback signals for two proportional integral controllers. The DSTATCOM controls the active power of the distribution generation system while compensating harmonics, reactive and neutral currents and provides load balancing in the gird currents. Apart from these the DSTATCOM is also capable for voltage regulation at the point of common coupling. In unity power factor mode of operation of DSTATCOM the reactive power supplied by grid is remains zero. But in zero voltage regulation modes sliding leading power factor is maintained. The simulation of the proposed system is carried out in the MATLAB environment using simulink and power system toolboxes.

Index TermsDistribution generation, distribution static compensator, harmonics, neutral current, reactive power.

T

T

1. INTRODUCTION

   HE rapid industrialization and dependence of mankind growths on electricity, the world energy demand has been increasing exponentially. However, the conventional energy resources are exhaustible and limited in use nowadays. Therefore, there is an urgent need to conserve what we have in hand. This has diverted a lot of research and attention of mankind towards alternate energy sources and wider use of energy efficient devices [1]. Also the present generating units and transmission lines are already loaded up to their rated maximum and it is not possible to load them further. As a result, the focus has shifted to generation of electric power locally using alternate source of energy. This has led to use of distributed generation (DG) [2]. Distributed generation encompasses a wide range of prime mover technologies, such as internal combustion (IC) engines, gas turbines, microturbines, photovoltaic cells, fuel cells and wind-power

   [3-5].

   Moreover, the increasingly use of energy efficient devices results in continuous increment in the size and numbers of power electronics converters, adjustable speed drives, etc. for

   Jatinkumar Patel (R/S), Prof. S. P. Gupta and Prof. S. P. Singh are with the Department of Electrical Engineering, Indian Institute of Technology, Roorkee, India-247667.

   commercials, residential and industrial applications [6]. These power electronics devices operate on the sinusoidal supply voltages but at the same time they inject significant harmonics and draw reactive power from grids [7]. The loads responsible for injection of harmonics in the system are termed as nonlinear loads. Nowadays, the ac distribution systems are suffering from severe power quality problems due to continuous increment of nonlinear type of loads. In addition to harmonics and reactive current the large 1-phase nonlinear loads such as computers, lighting ballasts, small rating adjustable speed drives(ASDs), refrigerators and other commercial appliances etc. further complicate the issue by causing excessive neutral current in the three-phase and four- wire (3P4W) distribution supply systems [8]. The neutral conductor will carry a large percentage of each of the three- phase conductors current even under balanced load conditions. The excessive neutral current may cause many adverse effects such as overloading power feeders, overloading distribution transformers, voltage distortion, and common mode noise [9-12].

   It is obvious that DG interconnection should not disturb normal operation of loads and the network itself and the power quality indices should remain in the range required by standards viz. IEEE-519-1992/IEC-61000-3-12, etc. should not interrupt the sources operation. Therefore, DG integration may need some control devices to be applied in the network which will facilitate the integration process and assure the required power quality. The basic objective of the distribution generation system connected with grid is to control the power that the inverter injects into the grid. According to the grid demands, injected power does not only include the control of the reactive power, but also the control of the injected active power [13]. Fig.1 shows a general purpose block diagram of DG-grid with power electronics system for injecting DG power into the grid while improving the system power quality.

   Many compensation methods are developed to mitigate these power quality problems in distribution systems are reported in literature [5]-[10], [13]-[16]. A group of controllers together called custom power devices (CPD), which include active power filters(APF) or more appropriately distribution static compensator (DSTATCOM), dynamic voltage restorer (DVR) and unified power quality conditioners(UPQC) are used for compensating power quality problems in current, voltage and both current and voltage, respectively. The DSTATCOM is a shunt connected device, which take care of the power quality problems in the currents. There are many DSTATCOM topologies reported in the literatures for 3P4W distribution systems such as four-leg voltage source inverter (VSI) [14], a three single-phase VSI

   [15], three-leg VSI with split capacitors [16], three-leg VSI with zigzag transformer [16]. Several studies proposed an interconnection system for DG with the power system through the DSTATCOM and APF because they give versatile solutions for improving the performance of DG [4, 11].

   Power Electronics and Control

   Power Electronics and Control

   Distributed Energy Resources

   AC-DC or DC-DC

   Converter Module

   DC-AC

   Converter Module

   Output

   I

   Distributed Energy Resources

   AC-DC or DC-DC

   Converter Module

   DC-AC

   Converter Module

   Output

   I

   reference dc-link capacitor voltage should be higher than the PCC voltage VL[13]. During reactive power compensation the operation of switches generate a voltage having fundamental component Vc towards AC side of the DC-AC converter (inverter). For proper reactive current compensation (to maintain source fundamental current Isp in-phase with the VL)

   the DSTATCOM generate a current Icq of equal magnitude and 1800 out of phase with load reactive current as shown in Fig. 2 (b).

   nterfaced

   nterfaced

   Module

   PV, Wind, Microturbine, Fuel Cells, IC Engine

   Module

   PV, Wind, Microturbine, Fuel Cells, IC Engine

   Grid System

   Grid System

   Grid

   PCC

   PCC

   Vss

   Local Loads

   Local Loads

   Battery, Flywheel Energy Storage

   Battery, Flywheel Energy Storage

   Zs Rs jLs

   Monitoring and Control System

   Monitoring and Control System

   AC

   DC

   Distribution Generation

   Vdc

   DC

   AC

   Cdc

   V

   Zc Rc jLc

   VL0

   Fig.1. General purpose block diagram of DG-grid system with power electronic interfaced.

   This paper proposed a simple control algorithm for combined operation four-leg VSI based DSTATCOM with distributed generation. The proposed controller is capable to control the active power of the distributed generated system while compensating load power factor, harmonics, neutral current, and load balancing under unbalanced and distorted

   Icq

   Lp Isp

   c c

   (a)

   VL

   LocalLoads

   900

   jLc Icq

   Icq

   grid voltages.

   I

   Vc

   jLc Icq

2. SYSTEM DESCRIPTION AND DESIGN OF DSTATCOM Lq

   A four-leg, PWM controlled voltage source converter (VSC) is used as a DSTATCOM and it has eight insulated- gate bipolar transistors (IGBTs), three interface inductors, and

   one dc-link capacitor. The single line diagram of the proposed

   L

   IL

   (b)

   DG-grid system interfaced with DSTATCOM is shown in Fig. 2(a) and the complete schematic of the proposed system is shown in Fig. 3 (a). A Three-phase grid of source resistance Rs, and inductance Ls per phase is supplying power to nonlinear load. A current controlled three-phase DSTATCOM with energy storage capacitor Cdc is connected in parallel with load and grid. The DSTATCOM consists of an inductor Lc and a resistance Rc (equivalent resistance of the inverter circuit) per

   Fig.2 (a) Single line diagram of DG-grid interfaced system, (b) Phasor diagram of basic operation of DSTATCOM.

   The vector diagram represents the reactive power flow in which source fundamental current Isp is in-phase with PCC fundamental voltage VL and the reactive component of filter current Icq orthogonal to VL. From this phasor diagram of DSTATCOM it is evident that,

   phase and a three-phase IGBT bridge type current-controlled voltage source inverter (CC-VSI). A variable speed/power

   Vc VL jLc Icq

   From (1) Icq is calculated as:

   (1)

   distributed generation unit is connected to the dc-link of

   I Vc VL

   Vc (1 VL )

   (2)

   DSTATCOM through AC-DC converter. A smoothing inductor of resistance Rsm and inductance Lsm per phase is also connected in series with nonlinear load.

   For designing various parameters of the DSTATCOM the line voltage and the rating of the voltage source inverter are

   cq jL L V

   c c c

   c c c

   Three-phase reactive power delivered by the DSTATCOM is equal to the three-phase reactive power requirement of the nonlinear load, hence from vector diagram.

   considered as 400 V and 10 kVA, respectively. The ac

   Q Q

   3V I

   3V

   Vc

   1 VL

   (3)

   inductor, dc-link capacitor and the ripple filter selection are as

   cq L L cq L L V

   c c

   below.

   1. DC-Link Capacitor Voltage

      The reference value of dc-link capacitor voltage of DSTATCOM is mainly depends upon the reactive power compensation capability [14]. For reactive power compensation the primary condition is that the magnitude of

      From (4) it is evident that the DSTATCOM can compensate the load reactive current only when Vc >VL and for this case value Qcq is positive. For the case when Vc < VL, the Qcq is negative and DSTATCOM will draw the reactive power from the utility. The upper limit of Vc is calculated on the basis of maximum compensation capacity of the DSTATCOM, calculated as follows:

      d 3V V

      dQc1 0

      dVL

      3V 2

      d. Voltage Ripple Filter

      A first order low-pass filter tuned at half the switching frequency is used to filter the high frequency noise of the PCC

      voltages. For considering a low impedance of 10 for the

      L c L 0 ; V 2V

      (4)

      dV L L c L

      harmonic voltage at a frequency of 5 kHz, the ripple filter

      L c c

      Thus the maximum capacity of DSTATCOM can be obtained as,

      3V 2

      capacitor is designed as Cf = 3.6 F when a series resistance Rf of 5 is included in series with the capacitor Cf. The impedance at fundamental frequency is found to be 884 ,

      Qcq max

      L

      Lc

      which is sufficiently large and hence the ripple filter draws negligible fundamental current.

      Hence, the value of reference dc-link capacitor voltage of the DSTATCOM must be according to the reactive power requirement of the system. From above relations the range of Vc1 will be,

      VL Vc 2VL

      If it is assumed that the PWM converter operates in the linear modulation mode 0 ma 1 [10] then,

3. OPERATION PRINCIPLE OF PROPOSED DSTATCOM

   The Power as well as control circuit diagrams of the proposed DG-grid interconnected system with DSTATCOM are shown in Fig. 3 (a) and (b), respectively. The system works in interconnected mode when both the DG as well as the grid supplies power to the load. But it works in islanding mode

   2 V 2 2 V

   2 2 V

   when the voltage interruption on grid occurs. The control of

   m c L , for m

   1, V

   L

   (5)

   DSTATCOM involved the control of active power supplied by

   a V 3V

   a dc 3

   dc dc

   Thus Vdcref is obtained as 655.88 V for VL of 400V (line voltage) and it is selected as 650 V.

   1. DC-Link Capacitor

      The energy Edc required by the dc-link capacitor to charge the capacitor from actual Vdc to the reference voltage Vdcref can be expressed as:

      1

      DG and reactive power requirement of load. The ac-side voltages of the DSTATCOM interfaced inverter are controlled both in magnitude and phase to control the active and reactive power. The load currents, PCC voltages, and dc-link capacitor voltage of the DSTATCOM are sensed as feedback signals for the controller. The load currents are converted into the d-q-0 frame using parks transformation. The three-phase source voltages are applied to three-phase Phase Locked Loops (PLL) to synchronize the three-phase voltages with the voltages at

      PCC. The d-q components of load currents are then passed

      E C

      (V 2

      V 2 )

      (6)

      through low pass filters to extract the dc components.

      2

      2

      dc dc dcref dc

      On the other hand, the total energy Eac delivered by the source will be as:

      Rs Ls

      isa

      vLa

      PCC

      iLa Za

      E

      E

      VL

      3KL

      ac

      ac

      Isp

      .t

      (7)

      isb

      isc

      isn

      vLb

      v

      Lc

      iLb

      iLc

      iLn

      Zb n

      Zc

      3 3

      where, VL is the line value of source voltage, Isp is the line value of active component of source current, t is the time for which source supplies power and KL is the over loading factor.

      3-phase Grid System

      Distribution

      Distributed Generation control

      CC-VSI

      S7 S5 S3

      C

      ica icb icc icn

      S1 Lc

      Rc

      Linear Unbalanced Load

      The principle of energy conservation is applied as:

      Generation

      dc

      LL RL

      1 C

      (V 2 V 2 )

      (V )( I ).

      (8)

      Controller

      Induction

      Transformer

      Rectifier

      Vdc

      Nonlinear Load

      2

      dc dcref dc

      L KL sp t

      generator

      S8 S2 S6 S4

      Considering, a 5% (32.50 V) reduction in dc-link capacitor voltage during transients, Vdcref = 650 V, Vdc= 617.5 V, VL=

      (a)

      i

      i

      Idg

      Idg

      LPF

      LPF

      Idg (acdc )

      Idg (acdc )

      La

      La

      ica

      ica

      i*

      i*

      iLd (acdc)

      iLd (acdc)

      iLd (dc )

      iLd (dc )

      400 V, Isp= 3 *48.5 A, t= 1000 s, KL= 1.2, the calculated

      + sa

      + sa

      sd

      sd

      i*

      i*

      i

      i

      La

      La

      +

      +

      LPF

      LPF

      PWM Controllers

      PWM Controllers

      a-b-c

      a-b-c

      d-q

      d-q

      value of Cdc is 1957 F and it is selected as 2000 F.

      *

      +

      i

      *

      +

      i

      i

      i

      i*

      i*

      sb

      sb

      i

      i

      S S

      S S

      cb

      cb

      +

      +

      sq

      sq

      +

      +

      LPF

      LPF

      1 6

      1 6

      +

      +

      i

      i

      i

      i

      i

      i

      Lb

      iLc

      Lb

      iL

      Lq(dc)

      Lq(dc)

      i

      i

      Lq(acdc)

      Lq(acdc)

      a-b-c

      a-b-c

   2. Filter Inductor

      cc

      cc

      Lb

      Lb

      Gate Pulses DC-AC

      Converter

      Gate Pulses DC-AC

      Converter

      +

      +

      i

      i

      *

      sc

      *

      sc

      t

      t

      PLL

      PLL

      i

      i

      I

      I

      i '

      i '

      V

      V

      v

      v

      d-q

      d-q

      The selection of the filter inductance (Lc) per phase depends

      on the current ripple icrp-p

      , switching frequency fs

      and dc-link

      Loss

      Loss

      Lc

      Lc

      q

      q

      Lm

      Lm

      La

      La

      i*

      i*

      PI

      Controller

      PI

      Controller

      Amplitude vLb

      Amplitude vLb

      capacitor voltage (Vdc) as [14]:

      sa

      +

      sa

      +

      S7 S8

      S7 S8

      i

      i

      cn

      cn

      i*

      i*

      sn

      sn

      v

      v

      Lc

      Lc

      1

      1

      i*

      i*

      sb

      sb

      c

      c

      L ( 3maVdcref ) (9)

      i

      i

      PI

      Controller

      PI

      Controller

      +

      +

      V

      V

      sn

      sn

      i*

      i*

      dcref

      dcref

      sc

      sc

      V

      V

      dc

      dc

      (12KL fsicrp p )

      where, ma

      is the modulation index and KL

      is the over-load

      factor. Considering, icrp-p = 5%, fs= 10 kHz, ma = 1, Vdcref = 650 V, KL = 1.2, the Lc value is calculated to be 1.86 mH. The round off value of Lc of 2.0 mH is selected in this investigation.

      (b)

      Fig. 3. Proposed DG-Grid interfaced system with DSTATCOM (a) Power circuit (b) Control circuit.

      The error between the actual dc capacitor voltage and reference capacitor voltage of the DSTATCOM is sensed and is given to dc-bus proportional-integral (PI) controller whose output is added with the direct axis component iLd of the load currents. The active current supplied by the DG unit is now subtracted from the direct axis component of load currents to obtain the direct axis component of source current. Similarly, another PI controller is used to regulate the PCC voltages. The amplitude of the PCC voltages and their reference value are fed to another PI controller and whose output is added with the quadrature axis component iLq of the load currents. The resultant direct and quadrature axis currents are converted into the reference currents using Reverse Parks transformation. The desired source currents are now compared with the actual

      load currents to obtain the compensating currents which

      respectively. Fig. 4 (b) shows the phasor diagrams of active power flow with zero voltage regulation in forward interconnected modes. Similar diagrams at unity power factor and zero voltage regulation can be drawn from reverse mode of operations.

4. PROPOSED CONTROL ALGORITHM

   In order to examine the compensation mechanism lets assume that distribution generation uses a variable power generation system. The grid voltages at PCC of vector vL (t) and load

   currents of vector iL (t) consists a set of harmonic components H for h H are expressed in (10) and (11) respectively, where h is the order of harmonics.

   H

   include both active current supplied by the DG unit and reactive current requirement of the load. The compensating

   vLa

   (t)

   VLah sin(h t ah )

   hH H

   (10)

   currents are now applied to the hysteresis controller to obtain

   the switching pulses to switch the devices used in DSTATCOM configuration.

   vL (t) vLb (t) VLbh sin(h( t 2 / 3) bh )

   Lc H

   Lc H

   v (t) hH

   VLch sin(h( t 2 / 3) ch )

   Icq

   Vc

   jL

   jL

   c

   c

   I

   I

   c

   c

   Ic Vs

   i (t)

   hH

   H

   H

   ILah sin(h t ah ah )

   I

   I

   c

   c

   R

   R

   c

   c

   c

   I I s

   jLs Is

   La hH

   H

   (11)

   sp Lp

   iL (t) iLb (t) ILbh sin(h( t 2 / 3) bh bh )

   L I

   hH

   cp VL

   Is Rs

   i (t) H

   I Lc

   I sin(h( t 2 / 3)

   )

   I

   I

   Lq

   L

   ILp Icp , Isp ILp Icp , Isp Is , ILq Icq ,Isq 0

   ILp Icp , Isp ILp Icp , Isp Is , ILq Icq ,Isq 0

   where,

   hH

   Lch ch ch

   (V ,V ,V ) = Peak values of the voltages at PCC

   Lah Lbh Lch

   corresponding to hth order harmonics for phase-a, b and c.

   1. (ILah , ILbh , ILch )

      = Peak values of the load currents

      corresponding to hth order harmonics for phase-a, b and c.

      I Vc (ah ,bh ,ch ) = Arbitrary angles of the PCC voltages and

      c

      Icq

      jLc Ic

      V jLs Is

      load currents corresponding to hth order harmonics.

      s (

      , , )

      = Phase angles of the load currents

      I s

      Ic Rc

      ah bh ch

      corresponding to hth order harmonics.

      s c

      The load currents are converted into the d-q-0 frame using

      L Icp Isp

      ILp

      VL Is Rs

      parks transformation as:

      ILq

      2

      2

      IL sin t

      sin t 3 sin t 3

      iLd

      iLa

      (12)

      ILp Icp , Isp ILp Icp , Isp Is , Icq ILq , Isq 0

      ILp Icp , Isp ILp Icp , Isp Is , Icq ILq , Isq 0

      i 2 cost

      cos t 2 cos t 2

      i

      Lq 3

      iL0 1

      3

      3

      3

      3

      1 1

      Lb

      iLc

   2. 2 2 2

      Fig. 4.Phasor diagram for forward interconnected mode (a) at unity power factor (b) at zero voltage regulation

      Fig. 4 shows the vector diagrams of active power flow at unity power factor and zero voltage regulation for forward interconnected mode of operation. In which Isp, ILp, and Icp are the active fundamental currents of the grid (source), load and DG (ac-side of the inverter) respectively. And Vs, VL, and Vc are the voltages at grid (source), load (PCC), and ac side of the DC-AC converter. The L is the load power factor angle and s is the power angle between grid and load voltage and c is the power angle between inverter and load voltage. ILq and Icq are the reactive current component of load and DSTATCOM,

      The three-phase PCC voltages are applied to three-phase phase locked loops (PLL) for obtaining the unit vectors and synchronizing the current signals with the voltages at PCC. The d-q component of load current consists with fundamental and harmonic components. The d-q components of load currents are then passed through a 5th order low pass filters (LPF) to extract the dc components and block the harmonic component of load currents. The loss component of source current is calculated using dc-bus voltage PI controller. Actual dc-link and reference dc-link capacitor voltages are compared and difference is given to PI controller as shown in Fig. 3(b). The output of PI controller is considered as loss components of source current which is added with the direct axis

      component iLd of the load currents. The output of PI controller is represented mathematically as:

      the study (DG active power transfer, load harmonics, reactive and neutral current compensation and PCC voltage regulation)

      iLoss(n) iLoss(n1 Kpd (Vdce(n) Vdce(n1) ) KidVdc(n)

      (13)

      the DSTATCOM is switched on at t = 0.1 s for all cases.

      where, V

      V V

      and V

      V V

      are

      Various simulation parameters used for this study are given in

      dce(n)

      dcref dc(n)

      dce(n1)

      dcref dc(n1)

      Table1.

      loss cp

      loss cp

      Parameters	Values	Parameters	Values
      Vs ( Line ) RMS	400 V, 50Hz	Lsm : Rsm per phase	0.5 mH : 0.05
      Rs : Ls per phase	0.01 : 0.1 mH	Kpq : Kiq for voltage controller	0.3 : 0.75
      R c : Lc per phase	0.1 : 2.0 mH	Kpd : Kid for dc-bus controller	0.15 : 0.5
      Cdc	2000 µF	Vdcref	650 V

      Parameters	Values	Parameters	Values
      Vs ( Line ) RMS	400 V, 50Hz	Lsm : Rsm per phase	0.5 mH : 0.05
      Rs : Ls per phase	0.01 : 0.1 mH	Kpq : Kiq for voltage controller	0.3 : 0.75
      R c : Lc per phase	0.1 : 2.0 mH	Kpd : Kid for dc-bus controller	0.15 : 0.5
      Cdc	2000 µF	Vdcref	650 V

      the error between reference dc-link capacitor voltage and actual voltage at nth and (n-1)th sampling instant and Kpd and Kid are the proportional and integral constant for dc-bus PI controller. The output of this dc-bus PI controller is added with the dc component of load currents. Finally the fundamental value of DG current is subtracted to obtain the dc reference value of source currents as:

      Table:1 Simulation parameters used for the proposed DSTATCOM controller

      sd Ld (dc)

      sd Ld (dc)

      i* i

      • i i

      (14)

      Similarly, another PI controller is used to regulate the PCC voltages as shown in Fig.3 (b). The amplitude of the PCC voltages and their reference value are fed to a second PI controller whose output is represented mathematically as:

      i

      i

      '

      q(n)

      iLoss(n1)

      • Kpq (Vsme(n)

        Vsme(n1)

        ) KiqVsme(n)

        (15)

        where, Vsme(n) Vsmref Vsm(n) and Vsme(n1) Vsmref Vsm(n1) are

        q

        q

        the error between peak value of reference grid voltage and actual grid voltage at nth and (n-1)th sampling instant and Kpq and Kiq are the proportional and integral constant for the grid PI controller. The output of this PI controller is added with the quadrature axis component iLq of the load currents to obtain the reference quadrature component of the source currents as:

        The discussion on the perofrmance of the DSTATCOM for different mode of operation are as:

        a. Forward Interconnected Mode with Variable Load and DG Power at Unity Power Factor

        In this case, the total active power requirement of the load PL is more than DG active power capacity Pdg thus, the grid also supplies the active power of difference (PL – Pdg) to the

        i i

        i i

        *

        sq Lq(dc)

      • i'

      (16)

      load. The DSTATCOM is switched on at t = 0.1 s and the load

      sd sq

      sd sq

      The resultant current components (i* , i* ) are converted

      into the reference currents using the reverse parks transformation as:

      is changed from 17.72 kW to 23.98 kW at t = 0.25 s. The DG power is changed from 10 kW to 18.94 kW at t = 0.4 s and from 18.94 kW to 10 kW at t = 0.6 s. During t < 0.1s the total active power requirement of the loads PL is supplied by the

      i*

      sin t

      cost

      1

      i*

      (17)

      grid. Hence, the active power demand of the load is same as

      sa

      i*

      sb

      sb

      sin

      t

      2 cost

      3

      2

      3

      sd

      1 i*

      sq

      sq

      the active power supplied by the grid during t < 0.1 s. The

      sb

      sb

      i*

      sq

      sq

      i*

      three-phase grid voltage, load current, grid current and the

      sc

      sin 2 2

      s 0

      DSATCOM currents are shown in Fig. 5. After t < 0.1 s the

      t 3

      cos t 3 1

      The desired source currents are now compared with the actual load currents to obtain the compensating currents which include both active current supplied by the DG unit and reactive current requirement of the load.

      grid currents are in-phase with the respective grid voltages ensures the unity power factor compensation capabilities of the proposed DSTATCOM controller. Fig. 6 shows the phase- wise grid voltages (scaled by a factor of 0.2) and grid currents wherein the gird currents are in-phase with the respective

      i

      ca

      ca

      i*

      i

      La

      La

      phase voltages proves that reactive power is compensated

      sa

      c cb sb Lb

      c cb sb Lb

      i (t) i i* i

      (18)

      completely. Also proves that the grid is supported to meet out

      icc

      *

      i

      i

      sc

      iLc

      the load active power demand. Apart from these the grid currents are balanced after t < 0.1 s. The values of the load,

      The compensating currents are now applied to the hysteresis

      controllers to obtain the switching pulses to switch the devices used in DSTATCOM inverter (DC-AC converter) configuration.

5. RESULTS AND DISCUSSION

   The proposed controller of the DSTATCOM for DG-Grid interfaced system as shown in Fig. 3(a) is simulated under MATLAB/Simulink environment to show the performance characterestics of the controller. The performance of the DSTATCOM is simulated for forward and reverse interconnected mode of operation with variable load and DG power. Three single-phase uncontrolled rectifires with common neutral wire and R-L on there dc-side are used for unbalanced nonlinear loads. To fullfil the basic objectives of

   grid, and DG active powers for t < 0.1 s, 0.1 s < t < 0.25 s,

   0.25 s < t < 0.4 s, 0.4 s < t < 0.6 s and t > 0.6 s are given in Table2 and the variations are shown in Fig. 7. The DSTATCOM energy storage dc-link capacitor supplies extra power during transient. The load and grid neutral currents are shown in Fig. 8 wherein, the grid neutral current is almost zero after t = 0.1 s. The FFT of the load and grid currents of all the phases for all durations are also carried out and the results are given in Table 2. A sample of load and grid currents waveform with harmonic spectrum for phase-a are shown in Fig. 9. The total harmonic distortion (THD) of the grid currents after compensation is well within the recommended limits shows the harmonic compensation capabilities of the controller.

   Volts

   Volts

   500

   0

   -500

   Amps

   Amps

   100

   Three-phase grid voltages (vsa, vsb, vsc)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Three-phase load currents (La, iLb, iLc)

   50

   Amps

   Amps

   0

   -50

   0.12 0.13 0.14 0.15 0.16 0.17

   Time (s)

   0

   -100

   Amps

   Amps

   50

   0

   -50

   Amps

   Amps

   75

   0

   -75

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Three-phase grid currents(isa, isb, isc)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Three-phase DSTATCOM currents (ica,icb,icc)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Time (s)

   100

   Mag (% of Fundamental)

   Mag (% of Fundamental)

   80

   60

   40

   20

   0

   20

   THD= 22.57%

   0 5 10 15 20

   Harmonic order

   (a)

   Amps

   Amps

   Fig. 5. Three-phase grid voltages, load currents, grid currents, 0

   and DSTATCOM currents in forward interconnected mode.

   Volt & Amps

   Volt & Amps

   100

   0

   Phase-a voltage (vsa

   x 0.2) and current (isa)

   -20

   0.12 0.13 0.14 0.15 0.16 0.17

   Time (s)

   Mag (% of Fundamental)

   Mag (% of Fundamental)

   100

   80

   -100

   Volts & Amps

   Volts & Amps

   100

   0

   -100

   Volts & Amps

   Volts & Amps

   100

   0

   -100

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Phase-b voltage (vsb x 0.2) and current (isb)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Phase-c voltage (vsc x 0.2) and current (isc)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Time(s)

   60 THD= 0.72%

   40

   20

   0

   0 5 10 15 20

   Harmonic order

   (b)

   Fig. 9. THD of current waveforms for phase-a in forward interconnected mode. (a) Load current (b) Grid current.

   5.3 Zero Voltage Regulation Mode of Operation

   Fig.10 shows the voltage compensation capabilities of the

   Fig.6. Phase-wise grid voltages (scaled by a factor of 0.2) and currents in forward interconnected mode.

   	Load Powe	r PL
   			Grid Power ps
   	DG Po	wer Pdg

   	Load Powe	r PL
   			Grid Power ps
   	DG Po	wer Pdg

   2.5

   2

   proposed DSTATCOM controller. When a three-phase heavy load is connected at PCC without compensation the PCC voltage will reduced. A case of connecting such a load on PCC during 0.2 s to 0.4 s, results in reduction (sag) in magnitude of PCC voltages as shown in Fig. 10 (a). The DSTATCOM voltage PI controller senses the reduced voltage and

   i

   i

   kW

   kW

   '

   '

   1.5 accordingly generates extra reactive power q

   to compensate

   1

   0.5

   the voltages at PCC. The compensated PCC voltages are shown in Fig. 10 (b). The other parameters are kept same as the previous case.

   0

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Time(s)

   400

   200

   Three-phase PCC voltage before compensation

   Volts

   Volts

   Fig. 7. Grid, load, and DG active powers

   Load neutral current (iLn) 0

   50

   Amps

   Amps

   0

   -50

   50

   Amps

   Amps

   0

   -50

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Grid neutral current (isn)

   0.1 0.2 0.3 0.4 0.5 0.6 0.7

   Time(s)

   -200

   -400

   0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

   Time(s)

   (a)

   Three-phase PCC voltages during compensation

   400

   Volts

   Volts

   200

   0

   -200

   -400

   0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

   Time(s)

   (b)

   Fig.8. Load and grid neutral currents, in forward interconnected mode.

   Fig.10. PCC voltages during application of heavy load (a) Without compensation (b) With compensation.

   Table2: Simulated response for forward interconnected mode of operation with varying load and DG power

   Parameters	Phase	t < 0.1s	0.1s<t<0.25s	0.25s<t<0.4 s	0.4s<t<0.6s	t > 0.6 s
   RMS Load Currents(Amps)	A	40.68	40.68	48.49	48.49	48.49
   	B	28.57	28.57	37.48	37.48	37.48
   	C	20.17	20.17	34.17	34.17	34.17
   	N	04.02	04.02	03.29	03.29	03.29
   RMS Grid Currents(Amps)	A	40.68	12.46	22.10	7.984	22.10
   	B	28.57	12.46	22.05	7.975	22.05
   	C	20.17	12.45	22.05	7.975	22.05
   	N	04.02	0.005	0.003	0.003	0.003
   Load Current % THD	A	16.08	16.08	13.58	13.58	13.58
   	B	19.76	19.76	15.46	15.46	15.46
   	C	22.57	22.57	17.82	17.82	17.82
   Grid Current % THD	A	16.08	00.70	0.44	0.88	0.44
   	B	19.76	00.74	0.47	0.83	0.47
   	C	22.57	00.72	0.47	0.86	0.47
   Three-phase Load Power (kW)		17.72	17.72	23.98	23.98	23.98
   Three-phase DG Power (kW)		10.00	10.00	10.00	18.94	10.00
   Three-phase Grid Power (kW)		17.72	07.71	13.95	05.00	13.95

6. CONCLUSION

This paper proposed the application of DSTATCOM as in interface between distributed generation and utilities three- phase four-wired distribution system. The proposed DSTATCOM system is capable for injecting energy to electric grid while compensating load power factor, harmonics and neutral currents, load balancing and provide voltage regulation at PCC. The grid system currents are sinusoidal and in-phase with their respective voltages. The grid current THD after compensation are well within the IEEE 519-1992 recommended limits. The proposed controller is suitable for islanding as well as integrated connected (both forward and reverse) mode of operation under unbalanced and distorted grid voltages conditions. The regulation in PCC voltages during any change is also quite satisfactory. The simulation results show that the proposed DSTATCOM control system is suitable for islanding as well as integrated mode of operation to improve power quality.

REFERENCES

1. G. Venkataramanan, M. Illindala, Microgrids and sensitive loads, IEEE Power Engineering Society Winter Meeting, vol. 1, pp. 315-322, 2002.

2. F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, "Overview of control and grid synchronization for distributed power generation systems, IEEE Transactions on Industrial Electronics, vol. 53, no. 5, pp. 1398-1409, October 2006.

3. W. Kramer, S. Chakraborty, B. Kroposki, and H. Thomas, Advanced power electronic interfaces for distributed energy systems, Part 1: systems and topologies, Technical Report NREL/TP-581-42672, March 2008.

4. F. Blaabjerg, Z. Chen, and S. B. Kjaer, "Power electronics as efficient interface in dispersed power generation systems. IEEE Transactions on Power Electronics, vol. 19, no.5, pp. 1184-1194, September 2004.

5. C. C. Chen, Y. Y. Hsu, A novel approach to the design of a shunt active filter for an unbalanced three-phase four-wire system under

   nonsinusoidal conditions, IEEE Transactions on Power Delivery, vol. 15 no. 4, pp. 12581264, 2000.

6. G. W. Chang, A new approach for optimal shunt active power filter control considering alternative performance indices, IEEE Transactions on Power Delivery, vol. 21, no. 1, pp. 406-413, January 2006.

7. M. C. Benhabib and S. Saadate, New control approach for four-wire active power filter based on the use of synchronous reference frame, Electric Power Systems Research, vol. 73, no. 3, pp. 353-362, March 2005.

8. M. I. Milanes, E. R. Cadaval and F. B. González, Comparison of Control Strategies for shunt active power filters in three-phase four-wire Systems, IEEE Transactions on Power Electronics, vol. 22, no. 1, pp. 229-236, January 2007.

9. B. Singh, S. Gairola, B. N. Singh, A. Chandra and K. Al-Haddad, "Multipulse AC-DC converters for improving power quality: a review," IEEE Trans. on Power Electronics, vol. 23, no. 1, pp. 260-281, January 2008.

10. P. S. Sensarma, K. R. Padiyar, V. Ramanarayanan, Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations, IEEE Transaction on Power Delivery, vol. 16, no. 2, pp. 259-264, 2001.

11. Electromagnetic compatibility part 3:2: Limits Limits of harmonic current produced by equipment to public low voltage systems with input current >16A and 75A per-phase, IEC 61000-3-12, April 2005.

12. H. Fugita and H. Akagi, Voltage-regulation performance of a shunt active filter intended for installation on a power distribution system, IEEE Trans. on Power Electronics, vol. 22, no. 1, pp. 1046-1053, May 2007.

13. B. N. Singh, P. Rastgoufard, B. Singh, A. Chandra and K. Al. Haddad,

    Design, simulation and implementation of three pole/ four pole topologies for active filters, IEE Electric Power Applications, vol. 151, no. 4, pp. 467-476, July 2004.

14. E. Acha, V. G. Agelids, O. Anaya-Lara, T. J. E. Miller, Power electronic control in electric systems, Newness Power engineering series, 1st Edition, Oxford, 2002.

15. S. Choi and M. Jang, Analysis and control of a single-phase- inverter- zigzag transformer hybrid neutral-current suppressor in three-phase four wire systems, IEEE Transactions on Industrials Electronics, vol. 54, no. 4, pp. 2201- 2208, August 2007.