Impact of Reactive Power on Stable Production of Wind Farms

Majlesi Journal of Energy Management
Vol. 4, No. 2, June 2015
Impact of Reactive Power on Stable Production of Wind
Farms
Ashkan Edrisian1, Mohsen Hajian2, Mostafa Kermani3, Mahmoud Ebadian4
1- Department of Electrical Engineering, Islamic Azad University (IAU), Science and Research branch of Birjand, Birjand, Iran
Email: [email protected] (Corresponding author)
2- Department of Electrical Engineering, Islamic Azad University (IAU), Bojnourd branch, Bojnourd, Iran
Khorasan Regional Electric Company (KREC.), Esfarayen, Iran
Email: [email protected]
3- Technical Institute of Ebn-Hesam, Birjand, Iran
Email: [email protected]
4- Assoc. Prof. Department of Electrical Engineering, University of Birjand, Birjand, Iran
Email: [email protected]
Received September 2014
Revised November 2014
Accepted December 2014
ABSTRACT:
Increasing the share of wind energy in supplying load demand of power grid has created new challenges in the field of
power system stability. So that, the stable operation of each units of wind power producer on interaction with grid
could be affected by power system instability. Among them we can mention the phenomenon of voltage instability in
power system which eventually will lead to voltage collapse if stability controls are not applied. In this paper the
impact of reactive power control on improvement of SCIG-based wind farms operation has been studied. With regard
to the asynchronous operation of induction machine, the increment of wind power generation will be possible only by
absorption of more reactive power. Increasing in the power reactive consumption of induction generator leads to
reduction of voltage in point of common connection (PCC) which the wind farm connects to the grid. Ultimately, this
process in lack of proper reactive power control can be lead to instability in whole of power system. The conducted
studies in this paper are in framework of quasi-static time-domain simulations (QSTDS) and continuation power flow
(CPF) algorithm. The results indicate with increment of wind power penetration in power system if the proper support
of reactive power is not used, the stable performance of the wind farm will be threatened and caused to over speed of
SCIGs. Another task done in this paper is the employment of SVC and STATCOM as a practical solution in
improvement of wind farms-based power system stability.
KEYWORDS: Squirrel Cage Induction Generator (SCIG), Reactive Power, Quasi-Static Time-Domain simulation
(QSTDS), Continuation Power Flow (CPF)
1. INTRODUCTION
In recent years, environmental concerns and the rising
cost of fossil fuels and petroleum products has caused a
fundamental change in the mechanism of energy
production. So that the use of cleaner and cheaper
sources of energy have been on the agenda of
manufacturers. Among the policies taken in this context
are the use and development of renewable energies.
Considering the long background of wind energy in the
energy supply as one of the practical aspects of
utilization of renewable resources along with other
energy sources such as solar energy, it has enjoyed
considerable growth.
The production capacity of wind turbines and number
of Installed units in wind farms per year has had
witnessed a growth of 20%. The share of wind power
in 2020 is expected to reach 12 % of total world energy
[1]. Figure (1) shows the increase of at least 4 times in
the annual production capacity of wind turbines.
The assessment of the connecting impact of wind farms
on power system stability issue and conditions of
network-connecting of these units, especially in remote
areas (where the sufficient infrastructures are not
developed enough and in terms of network topology are
poorly known) is one of the facing significant problems
of wind energy expansion [2].
As the penetration level of wind power in the power
system increases, the stable operation of power system
will significantly be affected by characteristics of the
wind turbines.
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Majlesi Journal of Energy Management
Fig. 1. Annual capacity increment of wind turbines
during the years 1980 to 2006 [3]
The stability issue comes from this principle that these
turbines have often used of induction generators to
convert the mechanical torque into electricity which
potentially lead to a voltage drop and a tendency to
absorb the reactive power and voltage instability [2].
Whereas, wind farms are connected to grid via weak
buses (from voltage point of view) and distribution
network, a considerable amount of reactive current
during the instability event ranging from small-signal
or large-signal instability are absorbed [4].
Nowadays utilization of variable speed wind turbine
with power electronic interconnection such as DFIG
and PMSG [5] and [6] have solved the stability
problem of power system, especially voltage stability.
Nonetheless, still 30% of installed capacity of wind
turbines is dedicated to SCIG-based wind farms. SCIGs
are fixed speed type of generators that are connected to
grid directly [7]. Among the benefits of SCIG which
lead to its attraction, are simplicity, robustness,
reliability [8] and its economic utilities [9], [10]. These
characteristics are caused to needless of SCIGs to
complicated equipment of exciter, voltage regulator
and frequency controllers [10]. With regard to
incapability of SCIG in regulation and controlling of
voltage of system and also the absorption of reactive
power, these generators appear as source of voltage
fluctuations [2]. On the other hand, considering fixed
speed operation of SCIGs, the wind speed fluctuation
appears in format of mechanical torque and finally
output electrical power fluctuations. Consequently, due
to the close relationship between reactive power and
active power in SCIGs, the fluctuation of active power
led to variation of reactive power absorption. The
importance of power system stability issue, especially
in the voltage stability discussion so far as goes that the
most of recent researches have been focused on voltage
stability of the power system in presence of wind
farms. In order to cope with load fluctuating and output
power fluctuation of wind farms that are changed in
influencing by wind variation nature, reference [11] has
introduced combination of energy storage and reactive
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Vol. 4, No. 2, June 2015
power compensator as a proper alternative solution.
And has discussed to how interaction between energy
storage system and compensator. The use of power
electronic converters to interface the generator to grid
is one of another approach which has been addressed in
reference [9].
Connecting the SCIG-based wind farm to grid with
respect to the random nature of wind speed can lead to
instability and voltage fluctuation, especially when the
connected network is weak in voltage point of view.
Hence, providing the local support and as much as
possible close to the level of demand for reactive power
compensation is necessary [11]. Predominantly the
study topics, such as [2, 4, 12, 13, 14], have been
focused on using FACTs devices for stable operation of
wind farms.
In this paper the reactive power control and its impact
on stable operation of SCIG-based wind farms has been
investigated. Another task done in this paper is the
effect of upward increasing the production capacity of
wind farms on voltage stability. The main focus of
studies has been on small signal voltage stability, and
employment of CPF method which is an advanced
numerical technique, coincide with QSTDS have been
led to the plausible and complementary results
concerning the interplay of power system and wind
farms. The results which presented in this study
introduce subject of reactive power control as the
determining factor in the improvement of working
conditions of grid and wind farm, with expression the
reaction of power system for connection of wind farm.
Finally, compensation of reactive power in the use of
compensator devices such as SVC and STATCOM is
modeled.
2. CHARACTERISTIC OF WIND TURBINE AND
INDUCTION GENERATOR
The wind energy through the rotor of wind turbine is
converted into rotational energy and this extracted
rotational energy by either a gearbox or directly with
no mechanical interconnection is connected to the
generator [15]. This section explains the characteristics
of the wind turbine and induction generator, as well as
how these characteristics influence on voltage stability
problem are described.
2.1. Performance characteristics of wind turbine:
Wind power generation is intimately depended to wind
speed so that every change in wind speed would lead to
considerable variations in outlet electrical power. The
extracted power from an air mass that is blowing with
speed v on the turbine swept area (A) is calculated as
follows:
1
Pwind  . air . A.v 3
2
(1)
Where, ρair is the air density, v is the wind speed and Ar
Majlesi Journal of Energy Management
Vol. 4, No. 2, June 2015
is equivalent to the area swept by turbine blades.
Wind energy potential is not fully achievable,
attainable optimal power of the wind for the first time
was discovered by Betz in 1926 [16]. According to
Betz theory, the maximum of extracted power from
wind would be:
PBetz 
1
1
Av 3C Betz  Av 3  0.59
2
2
Fig.3. Equivalent circuit of squirrel cage induction
generator (SCIG)
The fraction of the wind energy which can be achieved
by wind turbine is determined via performance
coefficient of power (CP). In practical cases CP will not
exceed of 48%. Eventually, the output power of turbine
is calculated as follows:
According to equivalent circuit shown, the equations
describing the active and reactive power are as follows:
Pm  C p  ,  
Vr 
A
2
3
v wind
(2)
C
 C 5
C p  ,    C1  2  C3   C4  e  i  C6
 i

1
1
0.035

 3
i   0.08   1
(4)
(5)
Figure 2 indicates the generated mechanical power of
turbine as a function of wind speed and generator shaft
speed for pitch angle of β=0 (regardless of pitch control
system).
1.2
output power [p.u.]
1.5
1
1
0.8
0.5
0.6
0
0.4
-0.5
14
0.2
10
8
6
4
wind speed [m/s]
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0
-0.2
rotor speed [p.u.]
Fig. 2. The mechanical power of wind turbine versus
changes in wind speed and rotor speed.
As can be seen in Fig.3 by varying the wind speed in
range of 5m/s to 25m/s, the active power production and
subsequently the absorption of reactive power has
changed.
2.2. Characteristics of Squirrel Cage Induction
Generator (SCIG)
In this section, using the steady-state equivalent circuit
of a SCIG indicated in figure (3), the equations
describing the reactive power and active power are
extracted.
Pm 
(6)
2
 1  s 

2
 s  R2  Re   X e




21 s 
Vs 
 R2
 s 
(3)
R

v
12
1 s 
Vs 
 R2
 s 
2
(7)
 1  s 

2
 s  R2  Re   X e



Pm2
Pe  Pm  Re 2
Vr
(8)
Pm2 Vs2
Qe   X e 2 
Vr
Xm
(9)
With placement the equations (6) and (7) in equations
(8) and (9) the final form of the active and reactive
power would be as follows:
2  1 s

Vs (
) R2  Re 
 s

Pe 
2
 1  s 

2
 s  R2  Re   X e



2
Qe  
(10)
2
Vs
Vs
 Xe
2
Xm
 1  s 

2
 s  R2  Re   X e



(11)
The close relationship of voltage and slip of speed with
active power, and also same relationship on reactive
power assesses that the reactive power has direct effect
on the active power generation of induction machine,
clearly. And even can play a role in controlling or
limiting.
The figures (4)-a and (4)-b indicate the characteristics
of the active and reactive powers of induction generator
versus the variation of rotor slip at different voltage
levels. In other word these tow figure are the
graphically description of above equations.
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Majlesi Journal of Energy Management
4
Electrical Power [p.u.]
3
2
Vol. 4, No. 2, June 2015
Vs=1 p.u.
Vs=0.75 p.u.
Vs=0.5 p.u.
1
0
-1
-2
-3
-4
-5
-6
-0.2
-0.15
-0.1
-0.05
0
slip
0.05
0.1
0.15
0.2
Fig. 4. a. The power-slip characteristics of induction
machine.
0
Electrical Reactive Power [p.u.]
-1
Vs = 1 p.u.
Vs = 0.75 p.u.
Vs = 0.5 p.u
-2
-3
-4
3. THE EFFECT OF INCREASED
PRODUCTION
In order to modeling the increase of generation and
penetration of wind power in power system and also
studying how characteristic of wind turbines impact on
stable operation of power system the quasi-static timedomain simulation (QSTDS) has been used. Also, the
necessity and the role of reactive power in providing
the optimal and stable performance of wind turbine by
using the continuation power flow (CPF) algorithm
have been studied.
3.1. Continuation Power Flow (CPF)
Among the most common tools to study the voltage
stability is CPF which the most salient feature of CPF
is that it remains well-conditioned at near of the point
of voltage collapse and divergence caused by the
singularity of the Jacobian matrix at the critical point
would not happen [18]. The procedure stages of the
CPF have been shown in figure 5.
-5
Start
-6
-7
-8
-9
-10
-0.2
-0.15
-0.1
-0.05
0
slip
0.05
0.1
0.15
0.2
Fig. 4. b. The reactive power-slip characteristics of
induction machine.
According to these tow figures, the increase of active
power production is associated with rise of slip and it
leads to increasing in reactive power consumption.
Consuming the reactive power as a criterion of voltage
stability leads to voltage sag in the bus connecting the
wind farm to grid which This phenomenon in turn
reduces the active power output of the wind farm.
In a constant mechanical torque due to constant wind
speed with regard to the inability of SCIG to control the
input mechanical power [4], reduction of output power
resulted of voltage drop leads to an imbalance between
the input powers of generator. The difference due input
powers imbalance appears in form of kinetic energy
and resulted in an increase in rotor speed of SCIG.
Under these conditions, due to the raise of rotor slip
and the reactive power increment, the voltage of
terminal and output active power reduce continuously
[4]. In the lack of rapid and timely recovery of voltage
the process increasing the rotor speed (slip) will
continued [17]. This increase in speed to over of a
critical value steers the generator to instability region.
Hence, the turbine work must be prevented by using the
high-speed protection equipment.
Run conventional
power flow on base
case
Specify continuation
parameters
Calculate target
vector
Check to see critical point
has been passed
Yes
Stop
No
Choose continuation
parameter for next step
Predict
solution
Preform
correction
Fig. 5. The stages of CPF
In implementing this method, after each stage of the
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Majlesi Journal of Energy Management
Vol. 4, No. 2, June 2015
implementation of load flow, the load-demand would
be affected by load parameter λ and increased.
To simulate a load change, the active and reactive
power (PLi and QLi) must be broken into tow
components. One component corresponds to the
original load at bus i and the other component will
represent a load change brought about by a change in
the loading parameter λ [18].
Thus:
PLi  PLio   k Li S base cos i 
(12)
QLi  QLio   k Li S base sin i 
(13)
The load parameter λ must be chosen in bellow range
where λ=0 corresponds to base load and λ= λCritical
corresponds to the point of voltage collapse or (Critical
Load):
0    Critical
(14)
In addition, the term of active power generation can be
reformed to:
PGi  PGio 1  kGi 
(15)
Where PGio is the active generation at bus i, kGi and kLi
are constant coefficient to specify the rate of change in
generation and load at bus i as changes, respectively.
The value of kGi , kLi and Ѱi can be specified for every
bus of power system [18].
3.2. Quasi-Static Time-Domain Simulation
(QSTDS)
In this approach the wind power production will be
gently increased as a function of time. The salient
distinction of this approach in comparison with
conventional continuation methods is that the power
generation and consumption in whole grid is fixed and
only the increase of wind power production is
observable. This increase in production will be in
format of wind speed increment and will be simulate as
a time- dependent function [13, 19]. The rotor slip and
consequently the associated reactive power increase
with wind speed increasing, synchronism of these
incidents would lead to voltage drop at the point of
common connection (PCC) of wind farm to grid which
will result in the deterioration of voltage stability.
Equation (16) which is available by some of the
calculations in equations (2) and (7), demonstrates the
relationship between voltage, wind speed and induction
generator slip:
2
3
1  s R2 Vs  AC Pv R2 Re


s
ACPv3 R22
(16)

4.1. Static Var Compensator
This type of compensators, as the one of effective and
simple ways to compensate the reactive power [11],
includes controlled thyristor switches, capacitor banks
and parallel reactors placed among the FACTs devices
with parallel connection. Among the common type of
SVCs, the thyristor controlled reactor with fixed
capacitor can be pointed out, which also are recognized
as TCR/FC. The figure (6) shows the single-line
diagram of the TCR/FC.

AC v R R  R V   2AC v  R X
2 2
3
P
power in order to achieve stable production in wind
farms, providing reactive power demand from local
sources as close as possible to the place of use is
essential [11]. One of simple approaches to achieve
sustainable production and providing the needs of
induction generator in wind farms is the use of fixed
capacitors which is considered as corrective and
economical solution [10], but with regard to the fixed
impedance nature of this circuit element and direct
relationship of reactive power injected by capacitor
with voltage quadrate (QC α V 2), voltage drop for any
reasons influences reactive power injected and leads to
lack of it [14]. Consequently, the network is forced to
compensate the reactive power shortage, while it leads
to reduction of stability margin of voltage. On the other
hand, reactive power absorption of SCIG, which
influences by changes in wind speed and power
generated, varies and applying fixed capacitors with
static switching because of their transient conditions
and step changes doesn’t seem plausible.
Hence, the use of more flexible devices such as SVC
and STATCOM, which are able to control the reactive
power dynamically and consistently, have been put on
the agenda [4]. The use of such these equipment due to
the high economic costs and internal consumption leads
to a slight decrease in output power of generators and
wind farms [20].
2
e
2
3 2
s
P
2
2
2
e
 Re2

ACPv R
3
2
2
4. REACTIVE POWER CONTROL
As be noted, considering to the importance of reactive
Fig. 6. The single-line diagram of the TCR/FC
4.2. Static Synchronous Compensator (STATCOM)
This compensator is placed among the parallel
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Majlesi Journal of Energy Management
compensators similar to SVC and a voltage source
converter has been used instead of parallel capacitors
and reactors. The STATCOM is also known with alias
names like GTO-SVC or advanced SVC. The figure (7)
shows the STATCOM single-line diagram.
Vol. 4, No. 2, June 2015
5. THE SIMULATION RESULTS
The simulations presented in this paper, have been
conducted by PSAT toolbox of MATLAB software.
For more scientific matching of results the IEEE 9- bus
system as the case study has been used. The desired
wind farm is connected to 5th bus of case study system
through a transformer with capacity of 100 MVA and a
transmission line with line impedance of 5.25×10 -7 + j
9.23×10-3. The wind farm simulated consist of 43 units
of wind turbine equipped SCIGs with total capacity of
28.38Mw. The figure 8 shows the topology of IEEE 9bus system in presence of wind farm.
With CPF Implementing on the base state of system
(without the wind farm connection), the bus number 5
is identified as the weakest bus from voltage stability
point of view and also selected to connect the wind
farm (Figure 9). The main reason for this choice is that
by connecting the wind farm to weakest bus, the worst
case of voltage collapse occurrence will be simulated
[4], [21].
Fig. 7. The single line diagram of STATCOM
Fig. 8. IEEE 9-bus system connected to wind farm
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Vol. 4, No. 2, June 2015
1.4
1
1.2
0.8
VBus W
PBus W
QBus W
1
0.6
0.8
[p.u.]
Voltage amplitude [p.u]
Majlesi Journal of Energy Management
0.6
0.4
0.2
0.4
0
0.2
-0.2
0
1
2
3
4
5
6
7
Bus Number
8
9
-0.4
0
Fig. 9. Voltage amplitude achieved by CPF
implementation for IEEE 9- bus system in absence of
wind farm
6
8
10
12
14
16
Fig. 10-b. The voltage, active power and reactive
power of wind farm with 43 wind turbine.
1.4
VBus W
PBus W
1.2
QBus W
1
0.8
0.6
0.4
0.2
0
-0.2
0
5
10
15
20
25
30
35
40
time [s]
Fig. 10-c. The voltage, active power and reactive
power of wind farm with 25 wind turbine.
0.6
0.4
0.2
0
wind speed
Cp
45
4
time (s)
[p.u.]
The 43- unit wind farm studied, along with the QSTDS
implementation according to figure 10- a to 10- c, can
be seen the raise of wind speed of its nominal speed (12
m/s
) as a rate function (figure 10-a) leads to lose the
stable performance of wind farm in a wind speed lower
than 15m/s. This instable operation is due to the inability
of system to provide the reactive power demand of
induction generators sets in wind farm. As the figure
10- b reveals, continuity of increasing in wind speed for
over 15m/s leads to a coercive voltage collapse in the
whole network. By reducing the number of units from
43 to 25 for same wind farm, according to figure 10-c,
as the wind speed increases, even close to 37 m/s, the
sufficient reactive power is supplied by case study
system. In figure 10- c, the decrease in output power of
wind farm is due to the characteristic of wind turbine,
so that in a constant voltage the CP factor of turbine
will be decreased by increasing in wind speed till above
nominal wind speed (Figure 11). In this simulation, to
achieve higher speeds the cut-out limit of wind turbine
has been ignored.
2
-0.2
-0.4
40
wind speed [m/s]
-0.6
35
-0.8
30
-1
25
-1.2
5
10
15
20
25
30
35
wind speed
20
Fig. 11. The CP characteristic of wind turbine
versus wind speed
15
10
5
0
5
10
15
20
25
time (s)
30
35
40
45
Fig. 10- a. The rise of wind speed as a rate function
In order to study of the margin of voltage stability, the
CPF algorithm in presence of wind farm (with 43 wind
turbine) in case study system has been implemented.
The analysis of results and P-V curves achieved by
CPF, reveals that the connection of reactive power
compensators caused to stability improvement of power
system connected to wind farm. As can be seen in
figure 12, fixed capacitor despite of its satisfactory
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Majlesi Journal of Energy Management
Vol. 4, No. 2, June 2015
performance in terms of stable operation, is unable to
adapt its injected reactive power with consumption of
induction generators when increasing reduction of
voltage occurs. And eventually causes the instability of
system and wind turbine connected to it. While the use
of SVC and STATCOM, with respect to their flexible
capability in reactive power generation, create more
reliable stability margin and more stable voltage.
1
Voltage Amplitude [p.u.]
0.95
0.9
0.85
0.8
Capacitor
No Comp.
SVC
STATCOM
0.75
0.7
1
1.1
1.2
1.3
1.4
1.5
Loading Parameter (Lambda) [p.u.]
1.6
REFERENCES
1.7
1.8
Fig. 12. The P-V curves of the connecting bus of
wind farm to power system in presence of
compensator devices
The figure 13 indicates the amount of reactive power
transmitted from wind farm to the network via
transmission line.
Transmitted Reactive Power \ From: Bus 15 \ To: Bus 5
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
Capacitor
SVC
STATCOM
1
1.1
1.2
1.3
1.4
1.5
1.6
Loading Parameter (Lambda) [p.u.]
1.7
1.8
Fig. 13. Reactive power transmitted from bus 15 (the
secondary bus of wind farm transformer) versus
loading parameter (λ) increasing.
According to this picture, due to constant impedance
nature of fixed capacitors, the induction generators
absorb reactive power from network. While SVC and
STATCOM, in addition to providing the wind farm
demand, by injecting their surplus reactive power are
able to meet the needs of the network.
8
6. CONCLUSION
In this paper, the importance of reactive power control
to achieve the stable operation of wind farms in
connection of power system was studied. The results
demonstrated that an increase in the turbine numbers of
wind farm despite of desirable impact on power quality
context will be led to reduction of voltage stability
margin of power system. This is due to rise of reactive
power absorption of wind farm. Thus the development
and increasing the capacity of wind farms requires
accurate and detailed studies on the ability of power
system to meet the reactive power needs of induction
generators of wind farms.
The P-V curves analysis clearly illustrated the
advantages of using reactive power compensators.
Among all simulated compensators the STATCOM,
with respect to the use of electronic converters,
displays better performance.
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