 Open Access
 Total Downloads : 942
 Authors : Jatin K. Patel, S. P. Gupta, S. P. Singh
 Paper ID : IJERTV2IS4153
 Volume & Issue : Volume 02, Issue 04 (April 2013)
 Published (First Online): 05042013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
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 dclink of the DSTATCOM through suitable converter. The dclink capacitor voltage of the DSTATCOM and the magnitude of threephase 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

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 windpower
[35].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, India247667.
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 1phase 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 threephase 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 [912].
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. IEEE5191992/IEC61000312, 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 DGgrid 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 fourleg voltage source inverter (VSI) [14], a three singlephase VSI
[15], threeleg VSI with split capacitors [16], threeleg 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
ACDC or DCDC
Converter Module
DCAC
Converter Module
Output
I
Distributed Energy Resources
ACDC or DCDC
Converter Module
DCAC
Converter Module
Output
I
reference dclink 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 DCAC converter (inverter). For proper reactive current compensation (to maintain source fundamental current Isp inphase 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 DGgrid system with power electronic interfaced.
This paper proposed a simple control algorithm for combined operation fourleg 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

SYSTEM DESCRIPTION AND DESIGN OF DSTATCOM Lq
A fourleg, 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 dclink capacitor. The single line diagram of the proposed
L
IL
(b)
DGgrid 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 Threephase grid of source resistance Rs, and inductance Ls per phase is supplying power to nonlinear load. A current controlled threephase 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 DGgrid 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 inphase 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 threephase IGBT bridge type currentcontrolled voltage source inverter (CCVSI). A variable speed/power
Vc VL jLc Icq
From (1) Icq is calculated as:
(1)
distributed generation unit is connected to the dclink of
I Vc VL
Vc (1 VL )
(2)
DSTATCOM through ACDC 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
Threephase reactive power delivered by the DSTATCOM is equal to the threephase 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, dclink capacitor and the ripple filter selection are as
cq L L cq L L V
c c
below.

DCLink Capacitor Voltage
The reference value of dclink 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 lowpass 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 dclink 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,


OPERATION PRINCIPLE OF PROPOSED DSTATCOM
The Power as well as control circuit diagrams of the proposed DGgrid 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.

DCLink Capacitor
The energy Edc required by the dclink 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 acside 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 dclink capacitor voltage of the DSTATCOM are sensed as feedback signals for the controller. The load currents are converted into the dq0 frame using parks transformation. The threephase source voltages are applied to threephase Phase Locked Loops (PLL) to synchronize the threephase voltages with the voltages at
PCC. The dq 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.
3phase Grid System
Distribution
Distributed Generation control
CCVSI
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 dclink 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
abc
abc
dq
dq
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)
abc
abc

Filter Inductor
cc
cc
Lb
Lb
Gate Pulses DCAC
Converter
Gate Pulses DCAC
Converter
+
+
i
i
*
sc
*
sc
t
t
PLL
PLL
i
i
I
I
i '
i '
V
V
v
v
dq
dq
The selection of the filter inductance (Lc) per phase depends
on the current ripple icrpp
, switching frequency fs
and dclink
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 overload
factor. Considering, icrpp = 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 DGGrid 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 dcbus proportionalintegral (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.


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 phasea, b and c.

(ILah , ILbh , ILch )
= Peak values of the load currents
corresponding to hth order harmonics for phasea, 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 dq0 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
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 (acside of the inverter) respectively. And Vs, VL, and Vc are the voltages at grid (source), load (PCC), and ac side of the DCAC 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 threephase PCC voltages are applied to threephase phase locked loops (PLL) for obtaining the unit vectors and synchronizing the current signals with the voltages at PCC. The dq component of load current consists with fundamental and harmonic components. The dq 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 dcbus voltage PI controller. Actual dclink and reference dclink 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 dcbus controller
0.15 : 0.5
Cdc
2000 ÂµF
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 dcbus controller
0.15 : 0.5
Cdc
2000 ÂµF
Vdcref
650 V
the error between reference dclink capacitor voltage and actual voltage at nth and (n1)th sampling instant and Kpd and Kid are the proportional and integral constant for dcbus PI controller. The output of this dcbus 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 (n1)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*
threephase 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 inphase 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 inphase 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 (DCAC converter) configuration.



RESULTS AND DISCUSSION
The proposed controller of the DSTATCOM for DGGrid 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 singlephase uncontrolled rectifires with common neutral wire and RL on there dcside 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 dclink 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 phasea 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
Threephase grid voltages (vsa, vsb, vsc)
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Threephase 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
Threephase grid currents(isa, isb, isc)
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Threephase 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. Threephase grid voltages, load currents, grid currents, 0
and DSTATCOM currents in forward interconnected mode.
Volt & Amps
Volt & Amps
100
0
Phasea 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
Phaseb voltage (vsb x 0.2) and current (isb)
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Phasec 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 phasea 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. Phasewise 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 threephase 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
Threephase 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)
Threephase 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
Threephase Load Power (kW)
17.72
17.72
23.98
23.98
23.98
Threephase DG Power (kW)
10.00
10.00
10.00
18.94
10.00
Threephase Grid Power (kW)
17.72
07.71
13.95
05.00
13.95

CONCLUSION
This paper proposed the application of DSTATCOM as in interface between distributed generation and utilities three phase fourwired 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 inphase with their respective voltages. The grid current THD after compensation are well within the IEEE 5191992 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]T. Ackerman, G. Anderson and L. Soder, "Distributed generation: a definition," Electric Power System Research, vol. 57, pp. 195204, 4/20, 2001.
G. Venkataramanan, M. Illindala, Microgrids and sensitive loads, IEEE Power Engineering Society Winter Meeting, vol. 1, pp. 315322, 2002.

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. 13981409, October 2006.

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/TP58142672, March 2008.

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. 11841194, September 2004.

C. C. Chen, Y. Y. Hsu, A novel approach to the design of a shunt active filter for an unbalanced threephase fourwire system under
nonsinusoidal conditions, IEEE Transactions on Power Delivery, vol. 15 no. 4, pp. 12581264, 2000.

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. 406413, January 2006.

M. C. Benhabib and S. Saadate, New control approach for fourwire active power filter based on the use of synchronous reference frame, Electric Power Systems Research, vol. 73, no. 3, pp. 353362, March 2005.

M. I. Milanes, E. R. Cadaval and F. B. GonzÃ¡lez, Comparison of Control Strategies for shunt active power filters in threephase fourwire Systems, IEEE Transactions on Power Electronics, vol. 22, no. 1, pp. 229236, January 2007.

B. Singh, S. Gairola, B. N. Singh, A. Chandra and K. AlHaddad, "Multipulse ACDC converters for improving power quality: a review," IEEE Trans. on Power Electronics, vol. 23, no. 1, pp. 260281, January 2008.

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. 259264, 2001.

Electromagnetic compatibility part 3:2: Limits Limits of harmonic current produced by equipment to public low voltage systems with input current >16A and 75A perphase, IEC 61000312, April 2005.

H. Fugita and H. Akagi, Voltageregulation performance of a shunt active filter intended for installation on a power distribution system, IEEE Trans. on Power Electronics, vol. 22, no. 1, pp. 10461053, May 2007.

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. 467476, July 2004.

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

S. Choi and M. Jang, Analysis and control of a singlephase inverter zigzag transformer hybrid neutralcurrent suppressor in threephase four wire systems, IEEE Transactions on Industrials Electronics, vol. 54, no. 4, pp. 2201 2208, August 2007.