INTRODUCTION
One of the most interesting structures of energy conditioner is two backtoback
connected DC/AC fully controlled converters. In this case, depending on
the control scheme, the converters may have different compensation functions.
For example, they can function as active series and shunt filters to compensate
simultaneously load current harmonics and supply voltage fluctuations.
In this case, the equipment is called Unified Power Quality Conditioner
(UPQC) (Akagi et al., 2007; Aredes and Watanabe, 1995; Cavalcanti
et al., 2005; Han et al., 2006).
An active shunt filter is a suitable device for currentbased compensation.
It can compensate current harmonics and reactive power. The active series
filter is normally used for voltage harmonics and voltage sags and swells
compensation (Cavalcanti et al., 2005). The UPQC, which has two
inverters that share one DC link capacitor, can compensate the voltage
sag and swell, the harmonic current and voltage and control the power
flow and voltage stability. Nevertheless, UPQC cannot compensate the voltage
interruption due to lack of energy source in its DC link (Han et al.,
2006).
Nowadays, generation of electricity from renewable sources has improved
very much. Utilizing of wind energy as a renewable source to generate
electricity has developed extremely rapidly and many commercial wind generating
units are now available on the market. The cost of generating electricity
from wind has fallen almost 90% since the 1980s (Karrari et al.,
2005).
Wind is a variable and random source of energy. All types of machines,
i.e., DC, synchronous, induction, depending on the size of the system
have been used to convert this form of energy to electrical energy. Induction
generators are more common and more economical by improvement of power
electronics devices and drive methods (Datta and Rangenathan, 2002).
Various forms of systems can be used to have some level of control on
the wind generation unit. In the variable speed constant frequency systems,
power electronic devices are used to allow the rotor speed to be changed
while the grid frequency is constant. In one scheme, as studied in this
research, a Variable Speed Cage Machine (VSCM) system is used with a rectifier
and an inverter connecting the cage induction generator stator to the
grid. The advantage of the variable speed constant frequency system is
that the rotor speed can be controlled. This makes it possible to capture
maximum energy from the wind turbine (Karrari et al., 2005). In
Datta and Rangenathan (2003), a method of tracking the peak power points
for a VSCM system is suggested.
Numerous studies are now available on UPQC and distributed generation.

Fig. 1: 
Configuration of proposed UPQC with WECS 
In a new combined UPQC and synchronous generator is proposed, in which
the synchronous generator is connected to UPQC DC bus through an uncontrolled
rectifier (Han et al., 2006).
In this study, a new configuration of UPQC is proposed that has a Wind
Energy Generation System (WEGS) connected to the DC link through the rectifier
as shown in Fig. 1. The significant advantage of this
configuration in compare with separate operation of UPQC and wind energy
generation system is reduction in using of one inverter and use of shunt
inverter of UPQC as a WEGS`s inverter. The UPQC can compensate the voltage
interruption in the source, while the WEGS supplies power to the source
and load or the load only. There are two operation modes in the proposed
system. The first is interconnected mode, in which the WEGS provides power
to the source and the load. The second is islanding mode, in which the
WEGS provides power to the load only within its power rating when voltage
interruption occurs. The system operation transfers from the islanding
mode to the interconnected mode when the voltage interruption is removed.
The VA rating of series and shunt inverters of UPQC are estimated for
proposed system. The investment cost of proposed system is compared with
investment cost of separated use of UPQC and WECS using the VA rating
calculations and the economic saving due to use of proposed system is
estimated.
PROPOSED SYSTEM
In Fig. 1, there are six main parts in proposed system:
wind turbine, induction generator, maximum power point tracking which
controls induction generator speed, PWM rectifier, shunt inverter and
series inverter of UPQC. The modeling of each section is discussed separately
and then the overall model is investigated.
Wind turbine: The output power from a wind turbine can be expressed
as (Kim and Kim, 2007):
where, λ is tipspeed ratio, V_{WIND} is the wind speed,
R is blade radius, ω_{r} is the rotor speed (rad sec^{1}),
ρ is the air density, C_{P} is the power coefficient, P_{M}
is mechanical output power of wind turbine and T_{M} is the output
torque of wind turbine.
The power coefficient C_{P} depends on the pitch angle β,
the angle at which the rotor blades can rotate along its long axis and
tipspeed ratio λ given by Eq. 4:
where, β is the blade pitch angle. For a fixed pitch type, the value
of β is set to a constant value.
Maximum power point tracking: In this study, the pitch angle is
kept at zero until the nominal power of the induction generator is reached
(Horiuchi and Kawahito, 2001).

Fig. 2: 
Power coefficient factor versus tipspeed ratio for
various pitch angles 
At high wind speeds, the pitch angle is
increased to limit the input power (Fig. 2).
Therefore, the optimized rotational speed ω_{opt} for maximum
aerodynamic efficiency for a given wind velocity is given by:
where, λ_{opt} is the optimized tipspeed ratio which β
is zero and C_{P} is maximum. Hence, to fully utilize the wind
energy, λ should be maintained at λ_{opt}, which is
determined from the blade design (AboKhaIil et al., 2004). Then
from Eq. 2:
where, P_{M max} is maximum mechanical output power of wind turbine
at a given wind speed.
Once the wind velocity V_{WIND} is measured, the reference speed
for extracting the maximum point is obtained from Eq. 5.
Induction generator: In this study, a fifth order model for induction
generator simulation is used. To overcome the complexity of the model,
usually Park`s transformation is used. The transformed induction machine
equations are described by Ong (1997). Also, it is shown:
where,
is the number of poles in the induction generator. Equation
8 describes torque equation of an induction generator.
Wind turbine converter: The mechanical output power of wind turbine
and rotor speed for a given wind speed is determined by the intersection
of wind turbine and the induction generator characteristic curves.
Rotor flux reference frame is used for transformation of induction machine
equations. Selecting the daxis aligned with the rotor flux, the qaxis
component of the flux will be zero. This makes the equations easier to
handle. In this frame, the torque and flux Eq. 13 described
in Eq. 8 can be rewritten as:
where, T_{r} is time constant of rotor and equals .
The Eq. 912, are the basis for field
oriented control. This approach simplifies the induction machine control.
The model is very similar to a separately excited DC machine where the
flux depends on the field current and the torque is proportional to the
flux and the armature current. The main problem associated with field
oriented control is the requirement to estimate the flux axis angle. This
is done either by measuring the flux at two different points (with 90
° displacement), or estimating through rotor speed measurement (Ong,
1997). In this study, flux axis angle θ is calculated through rotor
speed measurement.
The wind turbine converter is designed to control the rotational speed
in order to produce the maximum output power, where the indirect vector
control is used.

Fig. 4: 
Shunt inverter control block diagram 
The control part consists of a speed controller and the
dq current controllers. The daxis current component is generally set
to maintain the rated field flux in the whole range of speed, while the
speed loop will generate the qaxis current component through a PI controller
to control the generator torque and speed at different wind speed as shown
in Fig. 3.
The proportional and integral gains for speed controller used in simulation
are K_{P} = 12 and K_{i} = 25, respectively.
Shunt inverter of UPQC: The shunt inverter described in this study has
two major functions. First, to compensate the current harmonics generated by
the nonlinear load and reactive power and to inject the active power of WECS
to grid (normal mode). Second, to supply the power to the load when the voltage
interruption occurs in the source side (interruption mode). Figure
4 shows the controller developed for the shunt compensator.
Normal mode operation: The measured load current is transformed
into synchronous dq reference frame with the sine and cosine functions
calculated using a PLL (phase locked loop) (Hu and Chen, 2000):
With this transformation, the fundamental positive sequence components
are transformed into dc quantities in d and q axes, which can easily be
extracted by LowPass Filter (LPF). All harmonic components are transformed
into ac quantities with a fundamental frequency shift.
which
where, i_{l} is the nonlinear load current, i_{s}
is the source current and i_{c} is compensating current.
The switching loss can cause the DC link capacitor voltage to decrease.
Other disturbances, such as unbalances and sudden variations of loads
can also cause this voltage to fluctuate. In order to avoid this, in Fig.
3a PI controller is used.
To avoid DC voltage fluctuation, a PI controller is used. The input of
the PI controller is the error between the actual capacitor voltage and
the desired value, its output Δi_{dc} is then added to the
reference current component in the daxis to form a new reference current.
When the DC link voltage decreases from desired value, the shunt inverter
draws active power from grid to regulate its voltage. When WECS injects
power to DC link, the bus voltage increases, in this case, the shunt inverter
injects power to grid to maintain DC link voltage at nominal value. The
proportional and integral gains for dc voltage controller used in simulation
are K_{P} = 0.5 and K_{I} = 0.05, respectively.
As shown in Fig. 5, these reference currents in Eq.
18 are then inversely transformed into abc reference frame. And
the output compensatory currents of the shunt compensator are obtained
by a PWM voltage current control.
Interruption mode operation: By estimating the entering voltage
and current from the both ends of SolidState Breaker (SSB) terminals
as input signals, if the interruption exceeds a threshold level, then
the SSB is opened into the islanding mode. Hereafter if the disturbances
are removed, the SSB is closed immediately into normal mode.
When the voltage interruption occurs and SSB opens, the WECS provides
the active power to maintain the load voltage constant. The shunt inverter
starts to perform the voltage and current control using the PI controller.

Fig. 5: 
Voltage control of shunt inverter of UPQC in interruption
mode 

Fig. 6: 
Control block diagram of the shunt converter of the
UPQC 
Figure 5 shows the voltage control block diagram of
shunt inverter in interruption mode (Han et al., 2006).
Series inverter of UPQC: The function of series inverter is to
compensate the voltage disturbance in the source side, which is due to
the fault in the distribution line. The series inverter control calculates
the reference value to be injected by the series inverter as shown in
Fig. 6.
The system voltages are detected and then transformed into synchronous
dq0 reference frame using Eq. 19 (Basu et al.,
2007):
The load bus voltage should be kept sinusoidal with constant amplitude
even if the voltage on system side is disturbed. So, the expected load
bus voltage in dqo reference frame has only one value:
Where:
where, V_{m} is peak value of desired load voltage and θ
is phase angle of load voltage which is determined by PLL (phase lockedloop).
This means daxis of load reference voltage equals V_{m} while
qaxis and zero axis of load reference voltage equals zero.
The compensation reference voltage is:
The compensation reference voltage in Eq. 22 is then
inversely transformed into abc reference frame. Comparing the compensation
reference voltage with a triangular wave, the output compensation voltage
of the series compensator can be obtained by PWM voltage control.
PHASOR DIAGRAM OF UPQC
Figure 7 shows phasor diagram of shunt inverter of
UPQC for fundamental power frequency when the supply voltage equals the
desired load voltage (Basu et al., 2007).
When the supply voltage has no deficiency; V_{S }= V_{L1}
= V_{S1} = V_{o} (a constant) and the series injected
voltage V_{inj} requirement is zero. This state is represented
by adding suffix 1 to all the voltage and current quantities of interest.
The load current is I_{L1}(I_{L1} = I_{L}) and
the shunt inverter compensates the reactive component I_{C1} of
the load, resulting in unity input power factor. Thus, the current drawn
by the shunt inverter is –I_{C1}, which is opposite to the
load reactive current I_{C1}. As a result, the load always draws
the inphase component I_{S1} from the supply. For nonlinear
loads, the shunt inverter not only supplies the reactive current, but
also the harmonic currents required for the load. Thus, after the compensation
action of the shunt inverter, only the fundamental active component of
the current is required to be supplied from the utility.
Figure 8 shows phasor diagram of shunt and series inverter
of UPQC for fundamental power frequency when the supply voltage sags and
UPQC injects V_{inj} to maintain the load voltage at its desired
level (Basu et al., 2007).
As soon as the load voltage V_{L} sags, the UPQC is required
to take action to compensate for the sag, so that V_{L} is restored
to its desired magnitude.
This condition is represented by adding suffix 2 to the parameters. Consequently
the load current changes to I_{L2}.

Fig. 7: 
Phasor diagram of shunt inverter of UPQC 

Fig. 8: 
Phasor diagram of shunt and series inverter of UPQC 
The shunt inverter ijects I_{C2} in such a way that the active power requirement of the load is only drawn form the utility. Therefore, from the utility side the load power factor is always unity. It can be observed form the phasor diagram that the utility current is I_{S2} and is in phase with V_{S2}.
If the active power demand is constant,
which can be written as:
VA RATING CALCULATION OF SHUNT AND SERIES INVERTER
Volt Ampere (VA) rating of series and shunt inverters of UPQC determines
the size of the UPQC. The power loss is also related to the VA loading
of the UPQC. Here, the loading calculation of shunt and series inverters
of UPQC with presence of DG at its DC link has been carried out on the
basis of linear load for fundamental frequency.
The load voltage is to be kept constant at V_{o} p.u. irrespective
of the supply voltage variation:
The load current is assumed to be constant at the rated value:
Assuming the UPQC to be lossless, the active power demand in the load
remains constant and is drawn from the source:
In case of a sag when V_{S2}<_{S1}, where x denotes
the p.u. sag:
to maintain constant active power under the voltage sag condition as
explained in (1):
therefore series inverter VA rating equals to:
Injected current through shunt inverter is:
therefore shunt inverter VA rating equals to:
Figure 9 and 10 show VA loading
of series and shunt inverters of UPQC for a wide range of power factor
and supply voltage sag variations. The VA loading of inverters is calculated
for occurrence of supply voltage sag from 10 to 50% and power factor variations
from 0.6 lagging to unity power factor, with Z_{sh} = 1 p.u in
all cases. The range of supply voltage sag has been chosen such that most
practical cases are observed to be in this range as available from power
quality survey reports.
Effect of connecting DG to UPQC on UPQC converters VA: By connecting
DG to the DC link of UPQC, the active power required by series inverter
can be supplied from DG. Hence, the freed capacity created on the shunt
inverter can be used for active power flow (transmission) to grid. These
capacities of inverters are detailed as follows.

Fig. 9: 
VA loading of series inverter of UPQC for different
power factor and p.u voltage sag values 

Fig. 10: 
VA loading of shunt inverter of UPQC for different power
factor and p.u voltage sag values 
From Fig. 8, one can deduce that the current in shunt
inverter which compensates voltage drop in DC link due to operation of
series inverter, is parallel with I_{S1} whose vector sum with
I_{C1} will yield I_{C2}. That would be active current
in the direction of I_{S1} and its rms value is I_{SE_C2}
which is equal to vector difference of IC_{1} and I_{C2}.
From Fig. 8, it can be found that:
By substituting I_{C1} and I_{C2} from Eq.
34 and 31, respectively, I_{SE_C2} is equals
to:
implifying Eq. 35 results in:
Assuming I_{O} = 1 p.u, the value of I_{SE_C2} will be
given for different values of load power factor. Table 1
shows the perunit active current in the shunt inverter for series compensation
for different power factors and voltage sag values. By installing DG on
DC link of UPQC and supplying active power required for series compensation
by DG, the active power of DG can flow to grid through shunt inverter.
Assuming shunt inverter design of 1 p.u capacity for complete current
compensation and voltage sag compensation up to 0.5 p.u, by connecting
DG to the UPQC DC link, 1 p.u shunt inverter will be able to flow 1 p.u
current of DG to grid. Table 2 shows the possible capacity
of shunt inverter for carrying DG active current for different power factor
values. Therefore the proposed configuration is capable of DG active power
transmission of 1 p.u to thr grid without any change in the capacity of
series and shunt inverters.
Table 1: 
p.u active current flow in shunt inverter for series
compensation for various and voltage sags 

Table 2: 
Capability of shunt inverter to transmission of active
and reactive current for different power factors 

ECONOMIC ANALYSIS OF LINKING DG WITH UPQC
Here, the economic analysis of separate linking of UPQC and DG to distribution
network by combined scheme (as discussed in this study) is carried out
and compared. In the combined operation of UPQC and DG, an inverter is
used less compared to the separate operation of them. On the other hand,
there is no need for DG converter and its duty is done by shunt inverter.
The shunt inverter transmits the active power of DG to grid besides compensating
the reactive power and harmonics of load current without increase of shunt
inverter rating.
The investment cost of inverter IC_{elec} can be expressed as
(Kaldellis and Kavadias, 2007):
which N_{P} is the rated power of inverter. Since, UPQC has two
inverters, the investment cost of UPQC is same as two inverters. The investment
cost of wind turbine by rated power of N_{WT}, can be demonstrated
by (Kaldellis and Kavadias, 2007):
a 
= 
8.7x105($/kW) 
b 
= 
621 
c 
= 
700($/kW) 
x 
= 
2.05 
Using Eq. 37 and 38, the investment
cost of separate UPQC and WECS and also combined UPQC and WECS can be
estimated. Table 3 shows investment costs of separate
and combined configurations for three different ratings. Economic savings
due to using combined configuration compared to separate UPQC and WECS
can be shown in Table 3.
Table 3: 
Comparison of Investment cost and economic saving of
separate and combined UPQC and wind energy system 

These results show the proposed
configuration has 17.6 up to 20.7% economic saving depending on different
ratings.
RESULTS AND DISCUSSION
In order to inspect different variables, analysis will be carried out
of different parameters in three steps. First, the wind speed variation
is considered and the way to inject power to the grid is studied. Second,
voltage sag of 100 V in peak at a constant wind speed is applied and the
results are studied. Finally, interruption is applied and the corresponding
power injection to load is observed.
Wind speed variations: The wind model used in the simulation consists
of base component that is a constant speed and a gust component for fast
variations in wind speed (Kim and Kim, 2007). The simulated wind model
is shown in Fig. 11.
The applied load is an RC load with uncontrolled diode rectifier with
a THD of more than 40%. The load current is shown in Fig.
12. In Fig. 13, the active and reactive power consumed
by the load are demonstrated.
As was described earlier, the optimal rotor speed to maximize the output
power of wind turbine in different wind speeds, is obtained by adjusting
tip speed ratio (λ) at its optimal value. This is applied as the
generator`s reference speed so that the wind turbine rotates with such
speeds that maximize the output power for different wind speeds. Figure
14 shows the reference and actual rotor speed (rad sec^{1}).
As is evident, the generator tracks the reference speed (or the optimal
speed to gain the highest output power) after getting past the starting
stage.
In Fig. 15 the injected active power by series and
shunt inverters of UPQC, source side and power consumption of load are
shown. The load power consumption should equal the sum of injected power
by source, series and shunt inverters at each moment. As is obvious in
the Fig. 15, until t = 1 sec, the wind speed is at
its rated value and the injected active power by shunt converter is 8.5
kW. Since the power required by the load is, the surplus wind system power
is injected into the grid and the power flow direction in grid is reversed.
Therefore, the source power is negative within this interval. By the wind
speed decreasing from t = 1 sec till t = 1.3 sec, the active power injected
by the shunt inverter into the grid is decreased and part of the load
power is supplied from the source. At time t = 1.3 sec, the wind speed
exceeds the nominal value and the injected power through shunt inverter
exceeds 10 kW. In Fig. 15, it is seen that the change
in wind speed has no effect on the performance of series inverter and
it does not inject or receive any power. The series inverter begins to
operate when any voltage disturbances occur.
The reactive power injected by the source, series and shunt inverters
of UPQC is shown in Fig. 16.

Fig. 11: 
Simulated wind model (m sec^{1}) for wind speed
variations step 

Fig. 12: 
Load current (A) 

Fig. 13: 
Active and reactive power consumed by load 
The reactive power injected
by shunt inverter is consumed by the load. Hence the grid capacity is
not dedicated to reactive power support and the source reactive power
became nearly zero. Since the source voltage has no disturbance, the reactive
power of series inverter also remains zero.
The current direction is reversed at the instances t = 1 sec and t =
1.3 sec. In other words, within the time interval t = 1 to 1.3 sec, the
source receives power from the wind system (Fig. 17).

Fig. 14: 
Reference and actual rotor speed (rad sec^{1}) 

Fig. 15: 
Injected active power by source and series and shunt
inverters of UPQC and consumed power of load 

Fig. 16: 
Injected reactive power by source and series and shunt
inverters of UPQC 
Figure 18 shows DC link voltage of UPQC. At the initial
moments, the startup of induction generator necessitates reactive power
absorption which is supplied via DC link capacitor and this causes DC
link capacitor`s voltage drop.

Fig. 17: 
Current drawn from the source 

Fig. 18: 
DC link voltage of UPQC 
After startup of generator, the DC voltage
remains so close to the reference voltage (780 V). An abrupt change in
wind speed, causes a change in the DC link voltage to some extent and
gets back to the desired value after a short time.
Voltage sag: Here, voltage sag at a given wind speed, 11 m sec^{1},
is applied and the results are studied. A voltage sag with peak amplitude
of 100 V is applied from t = 1 to 1.3 sec. The source and load voltage
are shown in Fig. 19. It is seen in this figure that
the UPQC series inverter has modified load voltage correctly. In Fig.
20, the injected power by source and UPQC`s inverters and also the
consumed power by load are shown. The consumed power of load should be
equal to the power injected by source and UPQC. As is shown, when the
voltage sag occurs, the injected active power by shunt inverter is slightly
decreased and the power injected by series inverter is increased by the
same amount The sourceinjected power does not undergo any changes by
the voltage sag.
Figure 21 shows reactive power injection by source
and series and shunt inverters of UPQC. The reactive power required by
load is supplied from the shunt inverter.

Fig. 19: 
Voltage sag compensation, (a) source voltage and (b)
load voltage 

Fig. 20: 
Injected active power by source and series and shunt
inverters of UPQC and consumed power of load 
Hence, the reactive power drawn
from source will be zero. In normal operation, the series inverter does
not inject or absorb any reactive power but it absorbs reactive power
when the voltage is distored. Considering UPQC control circuit and values
of dq components of voltage, it is indicated that before voltage sag
or disturbance, the q component of voltage is zero, while by the voltage
sag; it will be changed and must be compensated.

Fig. 21: 
Injected reactive power by source and series and shunt
inverters of UPQC 

Fig. 22: 
Shunt inverter injected current during occurrence of
voltage sag 
In order to compensate
the load voltage when voltage sag occurs, the series inverter should inject
active power and absorb reactive power.
The current in the shunt branch of UPQC is shown in Fig.
22. A significant percentage of load current is supplied through the
shunt branch. In other words, the power generated by wind system is consumed
by load. Hence, the current drawn from the source is negligible. Consequently,
if the power generated by wind system is in excess of the load, the surplus
power would flow into the grid (source). At the time t = 1 sec, the current
is slightly decreased. This is because of voltage sag and allocation of
a part of wind system power to injection through series inverter to improve
voltage sag.
The current drawn from the source is shown in Fig. 23.
It is shown in Fig. 23 that the source current is sinusoidal
and its amplitude is nearly 4A. The reason is the wind system function
and active power injection through the shunt inverter. The harmonic current
of load is supplied by shunt inverter and the source current remains sinusoidal.
The DC link voltage is shown in Fig. 24. After startup
of induction generator, the DC voltage value remains very close to the
reference value.

Fig. 23: 
Source current during occurrence of voltage sag 

Fig. 24: 
DC link voltage of UPQC during occurrence of voltage
sag 
The voltage sag has no effect on DC link voltage, because the required
power is supplied from the wind system. Hence the dc voltage will remain
constant.
Voltage interruption: Here, voltage interruption occurs from t
= 1 to 1.3 sec at wind speed of 11 m sec^{1}. As soon as the
interruption occurs, SSB is opened and the shunt inverter control switches
from gridconnected into islanding mode. Therefore the load and shunt
inverter are isolated from grid. It is assumed that there is enough wind
power to supply the load. In case the generated power of wind system does
not meet the required power of load, a part of load that has less importance,
will not be supplied correspondingly.
Figure 25 shows the source and load voltage, respectively.
It is seen that after voltage interruption, load voltage is remained at
its desired value due to shunt inverter operation.
The injected power of source, series and shunt inverters as well as consumed
power of load is shown in Fig. 26. At each instant,
the power consumed by load should be equal to the sum of injected powers.

Fig. 25: 
Voltage interruption compensation, (a) source voltage
and (b) load voltage 

Fig. 26: 
Injected active power by source and series and shunt
inverters of UPQC and consumed power of load during occurrence of
voltage interruption 
As is evident in the Fig. 26, as the outage occurs,
the injected power by source and series inverter becomes zero and the
wind system generated power is fed to the load through shunt inverter.
As the outage is removed, SSB is closed and series inverter gets back
to the circuit and also the shunt inverter control returns to gridconnected
mod.
The voltage control loop is functioning properly keeping DC link voltage
level close to its reference value (Fig. 27).

Fig. 27: 
DC link voltage of UPQC during occurrence of voltage
interruption 
CONCLUSION
This study describes a combined operation of the unified power quality
conditioner with wind power generation system considering investment cost.
The proposed system can compensate voltage sag and swell, voltage interruption,
harmonics and reactive power in both interconnected mode and islanding
mode. The speed of the induction generator is controlled according to
the variation of the wind speed in order to produce the maximum output
power. The VA rating of series and shunt inverters of UPQC are estimated
for proposed system. The investment cost of proposed system is compared
with investment cost of separated use of UPQC and WECS using the VA rating
calculations and the economic saving due to use of proposed system is
estimated nearly 20%. The performance of the proposed system was verified
using computer simulations.