Iaetsd power-quality improvement of grid interconnected

POWER-QUALITY IMPROVEMENT OF GRID INTERCONNECTED
WIND ENERGY SOURCE AT THE DISTRIBUTION LEVEL
BHARTH KUMAR.D Dr.R.VIJAYA SANTHI
PG Scholar Asst.Professor
DEPARTMENT OF ELECTRICAL ENGINEERING
Andhra University,
Visakhapatnam,
Andhra Pradesh.
ABSTARCT: Utility of power electronic
converters are increasingly connected in
distribution systems using renewable energy
resources. But due to their intermittent nature,
they may pose a threat to network in terms of
stability, voltage regulation and power quality
issues. The present work explains the concept of
grid interconnection of Wind Farm using grid
interfacing inverter along with the facility of
power quality improvement. In this paper it
presents control strategy for achieving
maximum benefits from grid-interfacing
inverter when interconnected in 3-phase 4-wire
distribution system is installed. For controlling
the performance as a multi-function device
which is active power filter functionality
inverters are used, using hysteresis current
control technique. With such a control, the
combination of grid-interfacing inverter and the
3-phase 4-wirelinear/non-linear unbalanced load
at point of common coupling appears as
balanced linear load to the grid. This new
control concept is demonstrated with extensive
MATLAB/Simulink simulation study.
Index Terms—Active power filter (APF),
distributed generation (DG), distribution
system, grid interconnection, power quality
(PQ), renewable energy.
I INTRODUCTION
Electric utilities and end users of
electric power are becoming increasingly
concerned about meeting the growing
energy demand. Seventy five percent of
total global energy demand is supplied by
the burning of fossil fuels. But increasing
air pollution, global warming concerns,
diminishing fossil fuels and their
increasing cost have made it necessary to
look towards renewable sources as a future
energy solution. Since the past decade,
there has been an enormous interest in
many countries on renewable energy for
power generation. The market
liberalization and government’s incentives
have further accelerated the renewable
energy sector growth.
A renewable resource is a natural
resource which can replenish with the
passage of time, either through biological
reproduction or other naturally recurring
processes. Renewable resources are a part
of Earth's natural environment and the
largest components of its ecosphere.
Renewable resources may be the source
of power for renewable energy.
Renewable energy is generally
defined as energy that comes from
resources which are naturally replenished
on a human timescale such as sunlight,
wind, rain, tides, waves and heat.
Renewable energy replaces conventional
fuels such as coal, nuclear, natural gas etc.
in three distinct areas. The extensive use of
power electronics based equipment and
non-linear loads at PCC generate harmonic
currents, which may deteriorate the quality
of power [1], [2]. In [3] an inverter
operates as active inductor at a certain
frequency to absorb the harmonic current.
A similar approach in which a shunt active
filter acts as active conductance to damp
out the harmonics in distribution network
is proposed in [4]. A [5] control strategy
264
INTERNATIONAL CONFERENCE ON CURRENT INNOVATIONS IN ENGINEERING AND TECHNOLOGY
INTERNATIONAL ASSOCIATION OF ENGINEERING & TECHNOLOGY FOR SKILL DEVELOPMENT
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

for renewable interfacing inverter based on
p-q theory is proposed.
Power generation: Renewable energy
provides 19% of electricity generation
worldwide. Renewable power generators
are spread across many countries, and
wind power alone already provides a
significant share of electricity in some
areas: for example, 14% in the U.S. state
of Iowa, 40% in the northern German state
of Schleswig-Holstein, and 49% in
Denmark. Some countries get most of their
power from renewable, including Iceland
(100%), Norway (98%), Brazil (86%),
Austria (62%) and Sweden (54%).
Heating: Solar hot water makes an
important contribution to renewable
heat in many countries, most notably in
China, which now has 70% of the global
total (180 GWth). Most of these systems
are installed on multi-family apartment
buildings and meet a portion of the hot
water needs of an estimated 50–60 million
households in China. Worldwide, total
installed solar water heating systems meet
a portion of the water heating needs of
over 70 million households. The use of
biomass for heating continues to grow as
well. In Sweden, national use of biomass
energy has surpassed that of oil.
Transport fuels: Renewable bio-
fuels have contributed to a significant
decline in oil consumption in the United
States since 2006. The 93 billion liters of
bio-fuels produced worldwide in 2009
displaced the equivalent of an estimated 68
billion liters of gasoline, equal to about 5%
of world gasoline production.
About 16% of global final energy
consumption presently comes
from renewable resources, with 10% of all
energy from traditional biomass, mainly
used for heating, and 3.4%
from hydroelectricity. New renewable
(small hydro, modern biomass, wind,
solar, geothermal, and bio-fuels) account
for another 3% and are growing
rapidly. At the national level, at least 30
nations around the world already have
renewable energy contributing more than
20% of energy supply. National renewable
energy markets are projected to continue
to grow strongly in the coming decade and
beyond. Wind power, for example, is
growing at the rate of 30% annually, with
a worldwide installed capacity of 282,482
megawatts (MW) at the end of 2012.
Renewable energy resources
mainly can be used for power generation,
heating and transportation purposes. In
case of power generation, the generation
can take place either as a separate unit to
feed a particular area or the renewable
energy resources can be interconnected to
the grid at the transmission level, sub-
transmission level and distribution level to
enhance load supplying capability of the
grid. Large wind farms, concentrated solar
power photovoltaic system, bio-power,
hydro power, geothermal power are
interconnected at the transmission and sub-
transmission levels. Photovoltaic system,
small wind farm, hydro power and fuel
cells are interconnected at the distribution
level. The resources are connected to grid
using grid interfacing inverter by suitable
controlling of the inverter switches. But
their highly intermittent nature may result
in instability and power quality problems;
hence an appropriate control circuitry is
required.
II SYSTEM DESCRIPTION:
The proposed system consists of RES
connected to the dc-link of a grid-
interfacing inverter as shown in Fig. 1. The
voltage source inverter is a key element of
a DG system as it interfaces the renewable
energy source to the grid and delivers the
265
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

generated power. The RES may be a DC
source or an AC source with rectifier
coupled to dc-link. Usually, the fuel cell
and photovoltaic energy sources generate
power at variable low dc voltage, while the
variable speed wind turbines generate
power at variable ac voltage. Thus, the
power generated from these renewable
sources needs power conditioning (i.e.,
dc/dc or ac/dc) before connecting on dc-
link [6]-[8]. The dc-capacitor decouples
the RES from grid and also allows
independent control of converters on either
side of dc-link.
Fig.1. Schematic of proposed renewable
based distributed generation system
A. DC-Link Voltage and Power Control
Operation:
Due to the intermittent nature of RES,
the generated power is of variable nature.
The dc-link plays an important role in
transferring this variable power from
renewable energy source to the grid. RES
are represented as current sources
connected to the dc-link of a grid-
interfacing inverter. Fig. 2 shows the
systematic representation of power transfer
from the renewable energy resources to the
grid via the dc-link. The current injected
by renewable into dc-link at voltage level
can be given as
Fig. 2. DC-Link equivalent diagram.
1 = …………………. (1)
Where is the power generated from
RES.
The current flow on the other side of dc-
link can be represented as,
= ………. (2)
Where , and are total power
available at grid-interfacing inverter side,
active power supplied to the grid and
inverter losses, respectively. If inverter
losses are negligible then =
B. Control of Grid Interfacing Inverter:
Fig.3. Block diagram representation of
grid-interfacing inverter control.
266
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

The control diagram of grid- interfacing
inverter for a 3-phase 4-wire system is
shown in Fig. 3. The fourth leg of inverter
is used to compensate the neutral current
of load. The main aim of proposed
approach is to regulate the power at PCC
during: 1) = 0; 2) PRES< total load
power (PL); and 3) PRES > PL. While
performing the power management
operation, the inverter is actively
controlled in such a way that it always
draws/ supplies fundamental active power
from/ to the grid. If the load connected to
the PCC is non-linear or unbalanced or the
combination of both, the given control
approach also compensates the harmonics,
unbalance, and neutral current. The duty
ratio of inverter switches are varied in a
power cycle such that the combination of
load and inverter injected power appears
as balanced resistive load to the grid. The
regulation of dc-link voltage carries the
information regarding the exchange of
active power in between renewable source
and grid. Thus the output of dc-link
voltage regulator results in an active
current Im. The multiplication of active
current component (Im) with unity grid
voltage vector templates (Ua, Ub and Uc)
generates the reference grid currents (Ia
*
,
Ib
*
and Ic
*
). The reference grid neutral
current (In
*
) is set to zero, being the
instantaneous sum of balanced grid
currents. The grid synchronizing angle (θ)
obtained from phase locked loop (PLL) is
used to generate unity vector template.
= sin(Ө)………………. (3)
= sin Ө − …………. (4)
= sin Ө + …………. (5)
The actual dc-link voltage (Vdc) is sensed
and passed through a second-order low
pass filter (LPF) to eliminate the presence
of switching ripples on the dc-link voltage
and in the generated reference current
signals. The difference of this filtered dc-
link voltage and reference dc-link voltage
(Vdc
*
) is given to a discrete- PI regulator to
maintain a constant dc-link voltage under
varying generation and load conditions.
The dc-link voltage error (Vdcerr (n)) at nth
sampling instant is given as:
( ) = ∗
( ) − ( )….. (6)
The output of discrete-PI regulator at nth
sampling instant is expressed as
………….… (7)
Where and
are proportional and integral gains of dc-
voltage regulator. The instantaneous
values of reference three phase grid
currents are computed as
……………. (8)
………….. (9)
…………. (10)
The neutral current, present if any, due to
the loads connected to the neutral
conductor should be compensated by forth
leg of grid-interfacing inverter and thus
should not be drawn from the grid. In other
words, the reference current for the grid
neutral current is considered as zero and
can be expressed as
…………………..… (11)
The reference grid currents (Ia
*
, Ib
*
,Ic
*
and In
*
)are compared with actual grid
currents
(Ia
*
, Ib
*
,Ic
*
and In
*
) to compute the
current errors as
267
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

………...…. (12)
…….……… (13)
……….……. (14)
……………. (15)
These current errors are given to hysteresis
current controller. The hysteresis
controller then generates the switching
pulses (P1 to Pg ) for the gate drives of
grid-interfacing inverter. The average
model of 4-leg inverter can be obtained by
the following state space equations
………... (16)
…….….. (17)
………….. (18)
…………. (19)
(20)
Where , and are
the three-phase ac switching voltages
generated on the output terminal of
inverter. These inverter output voltages
can be modeled in terms of instantaneous
dc bus voltage and switching pulses of the
inverter as
……….. (21)
………… (22)
……………(23)
………….. (24)
Similarly the charging currents
, and on dc
bus due to the each leg of inverter can be
expressed as
………….. (25)
………….. (26)
…………… (27)
…………... (28)
The switching pattern of each IGBT inside
inverter can be formulated On the basis of
error between actual and reference current
of inverter, which can be explained as:
If , then upper
switch will be OFF and
lower switch will be ON in the
phase “a” leg of inverter. If
, then upper switch
will be ON and lower switch
will be OFF in the phase
“a” leg of inverter.
Where hb is the width of hysteresis band.
On the same principle, the switching
pulses for the other remaining three legs
can be derived.
268
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

III SIMULATION RESULTS:
Fig:4 Simulation result of Grid Voltages
Fig:5 Simulation result of Grid Current
Fig:6 Simulation result of unbalanced
load currents
Fig:7 Simulation result of PQ-Grid
Fig:8 Simulation result of PQ-Load
Fig:9 Simulation result of Grid Voltages
Fig:10 Simulation result of dc-link
Voltages
269
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

IV CONCLUSION
Copious literature survey has been
done with fruitful discussions about
controlling of grid interfacing inverter to
improve power quality. From literature
survey it is found that hysteresis current
control technique is well suited to meet the
requirements. The proposed scheme is
validated by connecting the controller to
the 3-phase 4-wire system. This approach
eliminates the need for additional power
conditioning equipment to improve the
quality of power at PCC.
Simulations have been done to
validate the performance of the proposed
system in MATLAB/Simulink
environment. The THD in grid current
with grid interfacing inverter comes out to
be 1.64% for unbalance non-linear load
and 1.67% for unbalanced linear load
which is well within of 5 percent limits
laid down in IEEE std.
V REFERENCES
[1] M. Singh , K. vinod , A. Chandra, and
Rajiv K.varma. “ Grid interconnection of
renewable
energy sources at the distribution level
with power-quality improvement
features,” IEEE Transactions on power
delivery, vol. 26, no. 1, pp. 307-315,
January 2011
[2] J. M. Guerrero, L. G. de Vicuna, J.
Matas, M. Castilla, and J. Miret, “A
wireless controller to enhance dynamic
performance of parallel inverters in
distributed generation systems,” IEEE
Trans. Power Electron., vol. 19, no. 5, pp.
1205–1213, Sep. 2004.
[3] J. H. R. Enslin and P. J. M. Heskes,
“Harmonic interaction between a large
number of distributed power inverters and
the distribution network,” IEEE Trans.
Power Electron., vol. 19, no. 6, pp. 1586–
1593, Nov. 2004.
[4] 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.
[5] P. Jintakosonwit, H. Fujita, H. Akagi,
and S. Ogasawara, “Implementation and
performance of cooperative control of
shunt active filters for harmonic damping
throughout a power distribution system,”
IEEE Trans. Ind. Appl., vol. 39, no. 2, pp.
556–564, Mar./Apr. 2003.
[6] J. P. Pinto, R. Pregitzer, L. F. C.
Monteiro, and J. L. Afonso, “3-phase 4-
wire shunt active power filter with
renewable energy interface,” presented at
the Conf. IEEE Renewable Energy &
Power Quality, Seville, Spain, 2007.
[7] 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.
[8] 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, “Power electronic systems for the
grid integration of renewable energy
sources: A survey,” IEEE Trans. Ind.
Electron., vol. 53, no. 4, pp. 1002–1016,
Aug. 2006.
[9] B. Renders, K. De Gusseme, W. R.
Ryckaert, K. Stockman, L. Vandevelde,
and M. H. J. Bollen, “Distributed
generation for mitigating voltage dips in
low-voltage distribution grids,” IEEE
Trans. Power. Del., vol. 23, no. 3, pp.
1581–1588, Jul. 2008.
270
ISBN: 378 - 26 - 138420 - 5
www.iaetsd.in

Iaetsd power-quality improvement of grid interconnected

More Related Content

What's hot (18)

Viewers also liked (8)

Similar to Iaetsd power-quality improvement of grid interconnected (20)

More from Iaetsd Iaetsd (20)

Recently uploaded (20)

Iaetsd power-quality improvement of grid interconnected