International Journal of Latest Research in Science and Technology
Volume 4, Issue 4: Page No.157-161, July-August 2015
http://www.mnkjournals.com/ijlrst.htm
ISSN (Online):2278-5299
APPLICATION OF 3-PHASE 4-WIRE
VOLTAGE SOURCE INVERTER AT THE
DISTRIBUTION LEVEL
1
K. VIDYASREE, 2 G. C. PRABHAKAR
PG Student, VNR VJIET, Hyderabad, India
2
Assistant professor, VNR VJIET, Hyderabad, India
1
Email:vidyasreekottapalli@gmail.com 2Email:prabhakar_gc@vnrvjiet.in
1
Abstract- Renewable energy resources (RES) employing the power electronic based converters play a crucial role and has a great demand
in the current trend applications of Distribution systems. This paper presents a novel control strategy for achieving the maximum results
from these grid interfacing inverters when installed in 3-phase 4-wire distribution systems. The inverter is controlled to Perform multi-task
by incorporating Shunt active power filter functionality. The grid Interfacing inverter is mainly utilized to perform the functions like to
inject maximum available Power of renewable energy sources to the ac grid, to provide load active power, to compensate load reactive
power and current harmonics at PCC , to compensate current unbalance and neutral current in case of 3-phase 4-wire system. All of these
functions may be successfully performed either individually or simultaneously. With such a control, the combination of grid interfacing
inverter and the 3-phase 4-wire linear/non-linear unbalanced load at point of common coupling appears as a Balanced linear load to the
grid. This new control concept is explained with extensive MATLAB/SIMULINK simulation studies and the results are validated.
Keywords - Active power filter (APF), distributed generation (DG), distribution system, grid interconnection, power quality (PQ), renewable
energy, Photo Voltaic (PV) System.
I. INTRODUCTION
The increasing use in the industry of non-linear loads[2]
based on the power electronic elements introduced serious
and complex problems in the electric power distribution
grids. Also, symmetrical increase in the harmonics and
unbalance currents in addition to high expenditure of reactive
power can be noticed. The harmonic currents[1]flowing in
the electric grids can also leads to voltage harmonics and
disturbance. These harmonic currents can
act together
adversely with a outrange of power system equipment’s,
control systems, protection circuits, and other harmonic
sensible loads[7]. The energy users like consumers were
referred by enforcing some regulation protection against the
expansion of harmonic problem.In order to face the problem
of harmonics, many solutions have been indicated. These
solutions included modifications on the load side itself for
less harmonic emissions like the case of special structure
single phase[4] and three phase rectifier, and PWM rectifiers
or the connection on the polluted power grids of other
traditional or modern compensation systems.In order to look
up the problem of harmonics, many researches were
encouraged to develop innovative, flexible, and more
effective solutions for power quality problems including
harmonics problem[1]. These innovative solutions have been
given the name of active compensators or active power
filters. The objective of these active power filter abbreviated
mostly APF is to compensate harmonic currents and voltages
in addition to selective reactive power compensation[8].Many
types of APF have been proposed and used in harmonic
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compensation. Series APF is used for voltage harmonics
compensation. Shunt APF was proposed for current
harmonics and reactive power compensation. The Unified
Power Quality Filter [7] or Conditioner combines the two
types Shunt and Series APF in one device responsible for the
simultaneous compensation of voltage, current harmonics and
reactive power. Even though there are different types of APF
are available but the Shunt APF stands ahead among all those
due to its reliability and portability .The key role of Shunt
Active Power Filter is to cancel harmonic currents occurring
in power grids[8]. The principle of SAPF is to generate
harmonic currents equal in magnitude and opposite in phase
to those harmonics that circulate in the grid. The non-linear
loads absorb non-sinusoidal currents from the grid. Whereas,
the SAPF current is generated in a manner that grid current
keeps the sinusoidal form. SAPF is controlled to be seen with
the non-linear load by the grid either as linear resistive
load[5].There are two main structures for the control of Shunt
Active Power Filter; these are the direct control and the
indirect control of APF[5]. In the direct control the main idea
is to generate filter current references using the appropriate
methods. The generated reference currents are then to be
compared with the measured APF currents. The error is then
used to produce control signals of the filter. The indirect
control interests in controlling the grid currents instead of
filter currents. It compares the measured grid currents with
their generated references. The error is then sent to the
control circuit which determines the control signal of the
APF. In this project work based on the above analysis, the
power quality disturbances arises due to current, current
harmonics and load reactivepower etc. are being analyzed
and compensated by using Shunt Active Power Filters[8].
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International Journal of Latest Research in Science and Technology.
II . SHUNT ACTIVE POWER FILTER
Shunt active power filter remunerates current harmonics by
injecting equal-but-opposite harmonic compensating currents
into the grid[8]. In this case the shunt active power filter
operates as a current source injecting the harmonic
components generated by the load but phase shifted by 180°.
This principle is applicable to any type of load considered as
harmonic source. Moreover, with an appropriate control
scheme, the active power filter can also compensate the load
power factor. In this way, the power distribution system sees
the non-linear load[2] and the active power filter as an ideal
resistor. The current compensation characteristics of the
shunt active power filter is shown in Figure 1
III. MODELING OF THE SYSTEM
System Description
The proposed system consists of RES interconnected to the
grid, in which Grid, Distribution transformer, set of loads
(Linear & Non-Linear), Control Circuit (DSP control &
Hysteresis controller), RES & 4-leg inverter are connected as
shown in Fig 3
Fig 3: Schematic of proposed renewable based distributed
generation system
Fig 1: Compensation characteristic of shunt active power
filter
Modelling of Active Power Filter
The connection of the shunt active power filter to the point
of common coupling of the grid is done mostly by the mean
of a RL low pass filter[6] as shown in figure 2. The voltage
equation for each phase can be given by:
k=a,b,c
(1)
The Voltage is fed from the grid is connected to the DeltaStar distribution transformer in order to circulate to different
loads connected to it i.e., Commercial loads, Industrial loads
& House hold loads. The RES may be a DC source or an
ACsource with rectifier joined to dc-link. Usually, the fuel
celland photovoltaic energy sources yield power at variable
lowdc voltage, though the variable speed wind turbines
yieldpower at variable ac voltage. Thus, the power generated
fromthese renewable sources requires power conditioning
(i.e., dc/dcor ac/dc) infrontconnecting on dc-link.A capacitor
is positioned throughout the renewable energy sources which
decouples the RES from grid and also allows individual
control ofconverters on both side of dc-link.As the voltage
from the RES may not be fixed throughout the day so, in
order to maintain the voltage unremitting a capacitor is used
and acts as voltage regulator[5].
The three phase equations are then given by:
(2)
And for the dc side:
(3)
The equation system defining the SAPF in the three phase
frame is then given by:
IV .DC-Link Voltage and Power Control Operation
Due to the intermittent nature of RES, the generated power
isof variable nature. The dc-link plays an important role in
transferringthis variable power from renewable energy source
to thegrid. RES are represented as current sources connected
to thedc-link of a grid-interfacing inverter [5]. Fig4shows the
systematicrepresentation of power transfer from the
renewable energyresources to the grid via the dc-link. The
current injected by renewable energy sources into the dc-link
can be given as
at voltage level
(4)
(5)
Where
is the power generated from RES.
Fig 2: SAPF connection to the PCC
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International Journal of Latest Research in Science and Technology.
Fig 4: DC-Link equivalent diagram.
The current flow on the other side of dc-link can be
representedas,
(6)
and
are total power available at
Where
grid-interfacinginverter side, active power supplied to the
grid and inverter losses, respectively. If inverter losses are
.
negligible then
VControl of Grid Interfacing Inverter
Fig 6: Extraction of Unit Vector Template
Fig
5:Controller of Grid Interfacing Inverter
The control diagram of grid- interfacing inverter [5]
for a 3-phase4-wire system is shown in Fig 5.The fourth leg
of the inverter isused to compensate the neutral current of
load. The main aimof proposed approach is to regulate the
0; 2) PRES< total load power
power at PCC during:
(PL) ; and 3) PRES >total load power (PL). While performing
the power management operation,the inverter is actively
controlled in such a way that italways draws/ supplies
fundamental active power from/ to thegrid. If the load
connected to the PCC is non-linear or unbalancedor the
combination of both, the given control approachalso
compensates the harmonics, unbalance, and neutral current
[2].
V. Extraction of Unit Vector Templates
The extraction of 3 phases voltage reference signals are
based on Unit Vector Template Generation. A Phased Locked
Loop (PLL) is used to extract the pure sinusoidal signal at
fundamental frequency [3]. The PLL gives signal in terms of
sine and cosine functions. Here only sine terms are
considered. As we know the supply voltage peak amplitude
in advance, we can generate the unity supply voltage signals.
To get the unity terminal voltage vector UB4-abc ,the
terminal voltages are sensed and multiplied by a gain equal
to 1/Vmwheremis the peak amplitude of fundamental
terminal voltage. These unity voltage vectors are then taken
to PLL. Thus the output of the PLL is equal to unity terminal
voltage at fundamental frequency only. The output of the
(unit vector for phase A) which can be
PLL gives
expressed mathematically as,
(7)
The unit vectors for phase B and phase C can be obtained by
proper phase shifting. They can be expressed as,
(8)
(9)
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Reference Current Signal Generation
The shunt APF is used to compensate for current
harmonics as well as to maintain the dc link voltage at
constant level. To achieve the aforementioned task the dc
link voltage is sensed and passed through the first-order low
pass filter to eliminate the ripples present on the dc-link
voltage, , and then it is compared with the reference dc
link voltage. This gives the dc-link voltage error at the
nthsampling instant is given as,
(10)
The error is then processed by a discrete PI controller. The
output of the PI controller then will be the peak amplitude of
fundamental input current, ,which must be drawn from the
supply in order to maintain dc link voltage at constant level
and to supply losses. The output of this discrete PI-controller
at nthsampling instant can be expressed as,
(11)
The instantaneous values of reference three-phase grid
currents are computed as,
The neutral current, present if any, due to the loads
connectedto the neutral conductor should be compensated by
forth leg ofgrid-interfacing inverter and thus should not be
drawn from thegrid. In other words, the reference current for
the grid neutral current is considered as zero and can be
expressed as
VI. Gating Signal Generation
After extracting the reference current signals from the
shunt active power Filter, the next step is to force the
inverters to follow these signals. This can be done by
switching the inverter IGBTs in a proper manner. To have the
required gating signals, the hysteresis controller is used. The
hysteresis current control is a method of controlling voltage
source inverter so that an output current is generated which
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International Journal of Latest Research in Science and Technology.
follows a reference current waveform [8]. This method
controls the switches in an inverter asynchronously to ramp
the current through an inductor up and down so that it
follows a reference.This method requires a current feedback.
The sensed current is compared to the hysteresis limits and
the results of the comparison are used to control the
switching sequence in the inverter. The advantage of this
method is low switching losses and very high speed.
At t=0.72 sec, the grid interfacing inverter is interfaced with
the system. It is noticed that before t=0.72s the currents
which is non-sinusoidal. After t=0.72s the grid current are
found to be sinusoidal due to the action of Active Shunt
power Filter.
20
15
10
and )are
5
Ilo ad (A )
The reference grid currents (
comparedwith actual grid currents
and ) to compute the currenterrors as
(
0
-5
-10
-15
(16)
(17)
(18)
(19)
-20
0.7
0.75
0.8
0.85
0.9
0.95
1.0
Time (s)
Fig 8: Load Current when SAPF
The load current shows that even after interfacing the inverter
at t=o.72s, the distortion in the current due to non-linear load
is maintained at the same level as before interfacing it
50
Iin v (A )
These current errors are given to hysteresis current
controller.The hysteresiscontroller then generates the
switching pulses(P1to P8) for the gate drives of gridinterfacing inverter as shown in Fig 7.
0
-50
0.7
0.75
0.8
0.85
0.9
0.95
1
Time (s)
Fig Fig 9: Inverter Current when SAPF connected at t=0.72s
40
30
20
Ipcc (A)
10
Fig7: Hysteresis current controller
0
-10
-20
VII. Inverter Design
The average model of 4-leg inverter can be obtained by
thefollowing state space equations
-30
-40
0.7
o.75
0.8
0.85
0.9
0.95
1
Time (s)
Fig 10: Current at PCC when SAPF connected
Simulation Results based on PV Energy:
The Grid Current, Load current and inverter current are as
shown in Figs.
20
(21)
15
10
Igrid(A )
5
0
-5
(23)
-10
-15
-20
0.7
0.75
0.8
0.85
Time (s)
0.9
0.95
1.0
Fig 11: Grid Current when SAPF connected at t=0.72s
30
20
10
Iload (A)
and
are the three-phase
Where
acswitching voltages generated on the output terminal of
inverter.These inverter output voltages can be modelled in
terms of instantaneousdc bus voltage and switching pulses of
the inverteras
(25)
0
-10
-20
(26)
(27)
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-30
0.7
0.75
0.8
0.85
Time (s)
0.9
0.95
1.0
Fig 12:Load Current when SAPF not connected
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International Journal of Latest Research in Science and Technology.
always maintained as balanced and sinusoidal at unity power
factor.
60
40
Ilinv (A)
20
REFERENCES
0
1.
-20
-40
-60
0.7
0.75
0.8
0.85
Time (s)
0.9
0.95
1.0
Fig13: Inverter Current when SAPF connected at t=0.72s
simulation results Unbalanced Nonlinear Load
The FFT analysis is done for wind energy and PV cell on
grid side current to calculate the THD before and after the
interfacing of the SAPF. The THD of the grid currents before
and after the interfacing of the SAPF is given in Table .1 It is
seen that the THD is high before interfacing the SAPF. After
interfacing the SAPF at t=0.72s, the THD is maintained at the
same level for both the cases.
S.NO
Time
Source current
Source current
(sec)
THD %(WIND)
THD% (PV)
1
0
13.27
17.61
2
0.2
13.44
18.18
3
0.4
13.44
18.18
4
0.6
13.44
18.18
5
0.7
13.44
18.18
6
0.73
6.02
6.40
7
0.75
3.52
3.01
8
0.8
2.76
2.39
9
0.9
2.43
2.32
10
1.0
2.39
2.32
TABLE.1PercentageTHD for Grid currents with Inverter
2.
3.
4.
5.
6.
7.
8.
J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction betweena
large number of distributed power inverters and the distribution
network,” IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1586–
1593, Nov. 2004.
U. Borup, F. Blaabjerg, and P. N. Enjeti, “Sharing of nonlinear load
in parallel-connected three-phase converters,” IEEE Trans. Ind.
Appl., vol. 37, no. 6, pp. 1817–1823, Nov./Dec. 2001.
P. Rodríguez, J. Pou, J. Bergas, J. I. Candela, R. P. Burgos, and
D.Boroyevich, “Decoupled double synchronous reference frame
PLL for power converters control,” IEEE Trans. Power Electron,
vol. 22, no. 2, pp. 584–592, Mar. 2007.
S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, ¯A review of singlephase grid-connected inverters for photovoltaic modules,‖ IEEE
Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.
F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus,
“Overview of control and grid synchronization for distributed power
generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp.
1398–1409, Oct. 2006.
J. M. Carrasco, L. G. Franquelo, J. T.Bialasiewicz, E. Galván, R. C.
P. Guisado, M. Á. M. Prats, J. I. León, and N. M. Alfonso,
“Powerelectronicsystems for the grid integration of renewable
energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no.
4, pp. 1002–1016, Aug. 2006.
V. Khadkikar, A. Chandra, A. O. Barry, and T. D. Nguyen,
“Application of UPQC to protect a sensitive load on a polluted
distribution network,” in Proc. Annu. Conf. IEEE Power Eng. Soc.
Gen. Meeting, 2006, pp. 867–872.
J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3phase4-wire shunt active power filter with renewable energy
interface,” presented at the Conf. IEEE Rnewable Energy & Power
Quality, Seville, Spain, 2007.
VIII. CONCLUSION
In this paper, a dc-coupled System has been studied with
the two kinds of energy sources:
1) A wind energy generation system
2)Photo voltaic system
To improve the power quality at point of common
coupling with 3-phase4-wire distributedgeneration . It has
been shown that the grid interfacing inverter can be
effectively utilized for power conditioning without affecting
its normal operating of real power transfer.The gridinterfacing inverter with the proposed approach can be
utilized to:
I) Inject real power generated from RES to the grid,
II) Operate as a shunt Active Power Filter .This approach
thus eliminates the need for additional power conditioning
equipment to improve the quality of power at PCC.
The MATLAB/SIMULINK simulation model of the
proposed system with the connection of renewable energy
sources is shown and validated. The control circuit is
operated with phase lock loop, proportional integral
controller and hysteresis controller which is used to generate
the gating pulses for the 4-leg inverter and is carried out at
load side with non-linear unbalanced load.Thus the current
unbalance, current harmonics and load reactive power, due to
unbalanced and non-linear load connected to the PCC, are
compensated effectively such that the grid side currents are
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