Power Quality Improvement Using Custom Power Devices in Squirrel Cage Induction Generator Wind Farm to Weak-Grid Connection by using Neuro-fuzzy Control

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 5, No. 4, April 2015, pp. 477~485
ISSN: 2088-8694  477
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Power Quality Improvement Using Custom Power Devices in
Squirrel Cage Induction Generator Wind Farm to Weak-Grid
Connection by using Neuro-fuzzy Control
Kopella Sai Teja, R.B. R.prakash
Departement of Electrical and Electronics Engineering, K L University
Article Info ABSTRACT
Article history:
Received Jun 12, 2014
Revised Nov 18, 2014
Accepted Dec 5, 2014
Wind farm is connected to the grid directly.The wind is not constant voltage
fluctuations occur at point of common coupling (PCC) and WF terminal. To
overcome this problem a new compensation strategy is used. By using
Custom power devices (UPQC).It injects reactive power at PCC. The
advantages of UPQC are it consists of both DVR and D-STATCOM. DVR is
connected in series to the line and it injects in phase voltage into the line .D-
STATCOM is connected shunt to the line .The internal control strategy is
based on management of active and reactive power in series and shunt
converters of UPQC. The power exchainge is done by using DC-link.
Keyword:
DClink
Neuro-fuzzy logic control
Simulation
SCIG
UPQC Copyright © 2015 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Kopella sai teja,
Departement of Electrical and Elctronics Engineering,
K L University.
Email: kopellasaiteja@live.com
1. INTRODUCTION
The location of generation facilities for wind energy is determined by wind energy resource
availability, often far from high voltage (HV) power transmission grids and major consumption centres. In
case of facilities with medium power ratings, the Wind Farm is connected through medium voltage (MV)
distribution headlines.
Also, is well known that given the random nature of wind resources, the wind farm generates
fluctuating electric power. These oscillations have a negative impact on stability and power quality in electric
power systems. furthermore, in development of wind resources, turbines utilizing squirrel cage induction
generators (SCIG) have been used since the beginnings. The operation of squirrel cage induction generator
demands reactive power, generally provided from the mains and/or by local generation in capacitor
banks [1].
In the event that changes occur in its mechanical speed, i.e. due to wind disturbances will the WIND
FARM active (reactive) power injected (demanded) into the power grid, leading to variations of wind farm
Terminal voltage because of system impedance. This power disturbances transmit into the power system, and
can produce a phenomenon known as “flicker”, which consists of fluctuations in the illumination level
caused by voltage variations. Also, the normal operation of Wind Farm is impaired due to such disturbances.
In particular for the case of “weak grids”, the impact is even better.
In order to reduce the voltage fluctuations that may cause “flicker”, and improve Wind Farm
terminal voltage regulation, several results have been posed. The most common one is to raise the power
grid, enhancing the short circuit power level at the point of common coupling point of common coupling,
thus reducing the impact of power fluctuations and voltage regulation problems.

 ISSN: 2088-8694
IJPEDS Vol. 5, No. 4, April 2015 : 477 – 485
478
In recent years, the technological development of high power electronics devices has led to
implementation of electronic equipment suited for electric power systems, with very fast response compared
to the line frequency. These active compensators allow groovy flexibility in: I) controlling the power flow in
transmission systems using Flexible AC Transmission System (FACTS) devices, and II) enhancing the power
quality in distribution systems employing Custom Power System (CUPS) devices [2]. The use of these active
compensators to improve integration of wind energy in weak grids is the approach adopted in this work. In
this project we analyse a compensation strategy using an UPQC, for the SCIG–based Wind Farm connected
to a weak distribution power grid. This system is taken from a real case [3].
The UPQC is controlled to regulate the Wind Farm terminal voltage, and to mitigate harmonics at
the point of common coupling (PCC), caused by system load changes in generated power of Wind Farm,
respectively. By using UPQC series converter in wind farm voltage regulation process was done, by voltage
injection “in phase” with PCC voltage.
The shunt converter is used to filter the Wind Farm generated power to forbid voltage flickers in
active and reactive power capability. The sharing of active power between converters, is supervised through
the common DC link.
2. SYSTEM DESCRIPTION AND MODELLING
2.1. System Description
Figure 1 depicts the power system under consideration in this study.
Figure 1. Single line diagram of wind farm connected to week grid system
The Wind Farm is composed by 36 wind turbines using SCIG, adding up to 21.6MW electric power.
Each turbine has given fixed reactive compensation capacitor banks (175kVAr), and is connected to the
power grid via 630KVA 0.69/33kV transformer. This system is Carry out from, and represents a real case.
The ratio between short circuit power and rated WIND FARM power, give us an idea of the
connection weakness. Thus considering that the value of short circuit power in MV6 is SSC ≃ 120MV A this
ratio can be calculated:
5.5
WF
SC
P
S
r
Values of r < 19 are considered as a “weak grid” connection [2].
2.2. Turbine Rotor and Associated Disturbances Model
The power that can be obtained from a wind turbine, is expressed by:
pCVRP 32
2
1

Where:  is air density
R the radius of the swept area
v the wind speed
CP the power coefficient

IJPEDS ISSN: 2088-8694 
Power Quality Improvement Using Custom Power Devices in Squirrel Cage Induction… (Kopella sai teja)
479
For the assumed turbines (600kW) the values are R = 31.2 m ,  = 1.225 kg/m3 and CP calculation
is taken from [4].
A complete model of the wind farm is obtained by turbine aggregation; this means that the whole
wind farm can be modelled by only a equivalent wind turbine, whose power generated by the arithmetic sum
of each turbine according to the equation given below:


36.........1i
iT PP
Wind speed v in eqn (1) can differ around its average value due to variation in the wind flow. Such
variations can be classified as random and deterministic. The first are caused by the symmetry in the wind
flow observed by the turbine blades due to tower shadow and due to the atmospheric boundary layer, while
the latter are random changes known as turbulence. For our analysis, wind flow variation due to support
structure is considered, and modeled by a sinusoidal modulation superimposed to the mean value of v. The
frequency for this modulation is rotorN.3 for the three bladed wind turbine, while its distance depends on the
geometry of the tower. In our case we have considered a mean wind speed of 12m/s and the amplitude
modulation of 15%.
The effect of the boundary layer can be ignored compared to those produced by the shadow effect of
the tower in almost cases [3]. It should be noted that while the arithmetic sum of perturbations occurs when
all turbines function synchronously and in phase, this is the case that has the great impact on the power grid,
since the power pulsation has high amplitude. So, turbine aggregation method is valid.
The effect of the boundary layer can be ignored compared to those produced by the shadow effect of
the tower in almost cases [3]. It should be noted that while the arithmetic sum of perturbations occurs when
all turbines function synchonously and in phase, this is the case that has the great impact on the power grid,
since the power pulsation has high amplitude. So, turbine aggregation method is valid.
3. MODEL OF INDUCTION GENERATOR
The model available in Matlab/Simulink Sim Power Systems library the squirrel cage induction
generator is used. It consists of a second–order mechanical model and a fourth–order state–space electrical
model [5].
4. DYNAMIC COMPENSATOR MODEL
The dynamic compensation of voltage changes is performed by injecting voltage in series and active
& reactive power into the MV6 (PCC) busbar; this is accomplished by using an UPQC [1]. In Figure 2 we
can see the basic single line diagram of this compensator; the impedances and busbars numbering is referred
to Figure 1.
Figure 2(a). Block diagram of UPQC Figure 2(b). Phasor diagram of UPQC
The operation is based on the generation of three phase voltages, using power electronic converters
either current source type Current Source Inverter or voltage source type Voltage Source Inverter. Voltage
Source converters are preferred. Faster response in the system than CSI [1] and It has lower DC link losses.
The shunt converter of UPQC injecting current at PCC, hear the series converter generates voltages between
U1 and PCC, illustrated in the phasor diagram of Figure 3. An important feature of this compensator is the
operation of both VSI converters sharing the same DC–bus, it enables the active power exchange between
them.

 ISSN: 2088-8694
480
Eshuc(t)
Ishuc(t)
Lshuc(t)
Rshuc(t)
I_grid(t)
Vpcc(t)
Vserc(t)
I_wind_farm(t)
U1(t)
Figure 3. Power stage compensation model AC side
Figure 4. Series compensator controller
We have build up a simulation model for the UPQC based on the ideas chosen from [6]. Since
switching control of converters is complete different of this work, and considering that higher order
harmonics generated by VSI converters are outside the bandwidth of significance in the simulation study, the
converters are modelled using ideal controlled voltage sources. Figure 4 shows the adopted model of power
side of UPQC.
The control of the UPQC, will be enforced in a rotating frame dq0 using Park’s transformation as
given in Equation (3&4).





















2
1
2
1
2
1
)
3
2
cos()
3
2
cos()cos(
)
3
2
sin()
3
2
sin()sin(
3
2 







T





















c
b
a
q
d
f
f
f
T
f
f
f
.
0
Where: cbafi ,, represents phase voltage or currents
0,,qdfi  represents magnitudes transformed to the dqo space.
This transformation admits the alignment of a rotating reference frame with the positive sequence of
the PCC voltages space vector. To attain this, a reference angle_synchronized with the PCC positive
sequence fundamental voltage space vector is calculated using a Phase Locked Loop (PLL) system. In this
work, an “instantaneous power theory” based PLL has been enforced [7].
Under balance steady-state conditions, voltage and currents vectors in this synchronous reference
frame are constant quantities. This strategy is useful for analysis and decoupled control.

481
5. UPQC CONTROL STRATEGY
In this paper we have used the neuro – fuzzy logic controlling strategy which is very advanced
strategy now a days.
Figure 5. Shunt compensator controller using neuro – fuzzy
The powers shucP and shucQ are calculated in the rotating reference frame, as follow:
)(.)(.
2
3
)(
)(.)(.
2
3
)(
tItVtQ
tItVtP
shuc
q
pcc
dshuc
shuc
d
pcc
dshuc


We Ignore PCC voltage variation, the above equations can be written as follows:
.
)(.)(
)(.)(
_
'
_
'
tIktQ
tIktP
shucqqshuc
shucdpshuc


Taking in consideration that the shunt converter is based on a VSI, we need to generate adecuate
voltages to obtain the currents in equation. This is attained using the VSI model proposed in [6], leading to a
linear relationship between the controller voltages and generated power. The resultant equations are:
)(.)(
)(.)(
*
_
''
_
*''
tEktQ
tEktP
shucqqshuc
shucdpshuc


P and Q control loops comprise a PI controller, while DC–bus loop hear we use a neuro – fuzzy
controller, In generally, in the proposed scheme the UPQC can be seen as a power buffer, leveling the power
injected into the power system grid. The Figure 7 illustrates a conceptual diagram of this mode of operation.
It must be observed that the absence of an external DC source in the UPQC bus, forces to maintain
zero–average power in the storage element installed in that bus. This is accomplished by a proper design of
DC voltage controller.
Also, it is necessary to note that the proposed scheme cannot be implemented using other CUPS
devices like DVR or D–Statcom. The power buffer concept may be implemented using a DStatcom, but not
using a DVR. On the other side, voltage regulation during relatively large disturbances, cannot be easily
using reactive power only from DStatcom; in this work, a DVR device is more suitable.

 ISSN: 2088-8694
482
Figure 6. Power buffer concept
Figure 7. Active and reactive power demand at power grid
The model of the power system strategy illustrated in Figure 1, including the controllers with the
control scheme detailed in section III, was implemented using Matlab/Simulink ®
software. Numerical
simulations were performed to determine and then compensate voltage fluctuation due to wind power
variation, and voltage regulation problems due to a sudden load connection. The simulation was conducted
with the following chronology.
a) At t = 0.0’’ the simulation starts with the series converter and the DC-bus voltage controllers in
operation.
b) At t = 0.5’’ the tower shadow effect starts
c) At t = 3.0” Q and P control loops
d) At t = 6.0” L3 load is connected.
e) At t = 6.0 “ L3 load is disconnected[8].
6. COMPENSATION OF VOLTAGE FLUCTUATION
Simulation results for 0 < t < 6 are shown in Fig.8. At t = 0.5′′ begins the cyclical power pulsation
produced by the tower shadow effect. As was mentioned, the tower shadow produces variation in torque, and
hence in the active and reactive wind farm generated power. For nominal wind speed condition, the power
fluctuation frequency is f = 3.4Hz, and the amplitude of the resulting voltage variation at PCC, expressed as a
percentage is:
%50.1

ratedU
U
7. RESULTS AND ANALYSIS
Figure 8 is the pcc voltage is behaviour is shown the upper curve shows the voltage at pcc when
UPQC is not existing. The middle curve shows the when the UPQC is connected to the grid by using PI
controller. The last curve in Figure 8 shows when UPQC is connected to the grid and hear we are using neuro
–fuzzy logic controller. There is a variation in the wave forms.
0 1 2 3 4 5 6
-3
-2
-1
0
1
x 10
7
Pgrid[W]
Qgrid[VAr]
Active and reactive power demand at power grid side with out UPQC
0 1 2 3 4 5 6
-3
-2
-1
0
1
x 10
7 Active and reactive power demand at power grid side with UPQC
Pgrid[W]
Qgrid[VAr]
0 1 2 3 4 5 6
-2
-1
0
1
x 10
7 Active and reactive power demand at power grid side with UPQC using neuro fuzzy
Pgrid[W]
Qgrid[VAr]

483
Figure 8. Pcc voltage
In the Figure 9 the shows the behaviour of wind farm terminal voltage. The upper wave form shows
the behaviour when wind farm is connected to the grid when UPQC is not connected. Middle wave form is
when UPQC is connected to the grid and the control strategy used is PI controller. Final wave form of Figure
9 shows the behaviour of wind farm voltage connected to the grid and the control strategy used is neuro –
fuzzy logic controller.
Figure 9. WF terminal voltage
This voltage fluctuation is seen in Figure 9 for 0.5 < t < 3.
Table 1. Neuro Fuzzy Rule Base
E(K)
E
NB NM NS ZE PS PM PB
NB NB NB NB NB NM NS ZE
NM NB NB NB NM NS ZE PS
NS NB NB NM NS ZE PS PM
ZE NB NM NS ZE PS PM PB
PS NM NS ZE PS PM PB PB
PM NS ZE PS PM PB PB PB
PB ZE PS PM PB PB PB PB
In this paper the above rules are taken for [9]. In the above table PB=positive big, PM = positive medium, PS
= positive small ZE = zero, NS = negative small, NM = negative medium, NB = negative big
In the Figure 10 the shows the behaviour of wind farm terminal voltage and Pcc voltage. The upper
wave form shows the behaviour when wind farm is connected to the grid when UPQC is not connected.
0 5 10 15
-1
0
1
Vpcc[V]
pcc voltage with out UPQC
0 1 2 3 4 5 6
2.6
2.65
2.7
x 10
4 Pcc voltage with UPQC
Vpcc[V]
0 1 2 3 4 5 6
2.64
2.66
2.68
x 10
4 PCC voltage with UPQC using nuro - fuzzy
Time
Vpcc[V]
0 1 2 3 4 5 6
-1
-0.5
0
0.5
1
Vpcc[v]
WF terminal voltage
0 1 2 3 4 5 6
2.655
2.66
2.665
x 10
4 WF terminal voltage UPQC
Vpcc[v]
0 1 2 3 4 5 6
2.655
2.66
2.665
x 10
4 WF terminal voltage UPQC using nero-fuzzy
Time
Vpcc[v]

 ISSN: 2088-8694
484
middle wave form is when UPQC is connected to the grid and the control strategy used is PI controller. Final
wave form of Figure 10 shows the behaviour of wind farm voltage connected to the grid and the control
strategy used is neuro – fuzzy logic controller.
Figure 10. Voltage at Pcc and WF
Figure 11. Power of capacitor in DC bus
The above Figure 11 is DC bus voltage at UPQC, in upper wave form there is no UPQC is
connected to the grid. There is no UPQC no DC bus voltage so it is 0. The middle wave form is UPQC is
connected to the grid there is variation. We have taken variation from time interval 3, so the variation starts
from 3. The final wave form is also same hear we use neuro fuzzy.
Figure 12. Voltage of the capacitor in the dc bus
0 1 2 3 4 5 6 7 8
-1
-0.5
0
0.5
1
V[v]
voltage at WF and PCC UPQC
0 1 2 3 4 5 6 7 8
2.5
2.55
2.6
2.65
2.7
x 10
4 voltage at WF and PCC UPQC
V[v]
0 1 2 3 4 5 6 7 8
2.5
2.6
2.7
x 10
4 voltage at WF and PCC UPQC using nero-fuzzy
Time
V[v]
0 1 2 3 4 5 6
-1
0
1
Pdc[W]
Power of the capasitor in the DC-bus without UPQC
0 1 2 3 4 5 6
x 10
5
-5
0
5
x 10
6 Power of the capasitor in the DC-bus with UPQC
Pdc[W]
0 1 2 3 4 5 6
x 10
5
-2
0
2
x 10
6 Power of the capasitor in the DC-bus with UPQC neuro fuzzy
TIME (sec)
Pdc[W]
0 1 2 3 4 5 6
-1
0
1
Vdc[V]
voltage of the capacitor in the DC bus without UPQC
0 1 2 3 4 5 6
x 10
5
3800
4000
4200
voltage of the capacitor in the DC bus with UPQC
Vdc[V]
0 1 2 3 4 5 6
x 10
5
3800
4000
4200
voltage of the capacitor in the DC bus with UPQC using neuro fuzzy
Time (sec)
Vdc[V]

485
Figure 13. Shunt and series converter active power
8. CONCLUSION
In this paper, a new compensation strategy was employed using UPQC compensator. When SCIG
based wind farms connected to week grid this compensation strategy is used. This compensation strategy
enhances system power quality. The simulation results show the good performance in mitigating the power
fluctuations due to tower shadow effect and voltage regulation in sudden load conditions.
REFERENCES
[1] A Ghosh, G Ledwich. Power Quality Enhancement Using Custom Power Devices. Kluwer Academic Publisher.
[2] Z Saad-Saoud, ML Lisboa, JB Ekanayake, N Jenkins and G Strbac. Application of STATCOM’s to wind farms. IEE
Proc. Gen. Trans. Distrib. 1998; 145(5).
[3] P Rosas. Dynamic influences of wind power on the power system. Technical report RISØR-1408. Ørsted Institute.
March 2003.
[4] T Burton, D Sharpe, N Jenkins, E Bossanyi. Wind Energy Handbook. John Wiley & Sons, 2001. ISBN 0-471-
48997-2.
[5] P Kundur. Power System Stability and Control. McGraw-Hill, 1994. ISBN 0-07-035958-X
[6] C Schauder, H Mehta. Vector analysis and control of advanced static VAR compensators. IEE PROCEEDINGS-C.
1993; 140(4).
[7] MF Farias, PE Battaiotto, MG Cendoya. Wind Farm to Weak-Grid Connection using UPQC Custom Power Device,
IEEE. 2010
[8] M Vishnu vardhan, Dr P Sangameswararaju.Using NFC and modified CS Algorithm based unified power flow
conditioner for compensating power quality problem. International Journal of Scientific & Engineering Research.
2013; 4(9).
0 5 10 15
-5
0
5
x 10
6 shunt and series converter active - power
P[W]
0 5 10 15
-5
0
5
x 10
6 shunt and series converter active - power UPQC
P[W]
0 5 10 15
-5
0
5
x 10
6 shunt and series converter active - power UPQC using nero - fuzzy
Time
P[W]

Power Quality Improvement Using Custom Power Devices in Squirrel Cage Induction Generator Wind Farm to Weak-Grid Connection by using Neuro-fuzzy Control

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