International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
Enhancement of Voltage Stability in Grid Connected Wind
Farms Using SVC
K. Sree Latha1, Dr. M.Vijaya Kumar2
1
Assoc. Prof (EEE), GNITC, HYDERABAD
2
Professor, JNTU, ANANTAPUR
In the past, the total installed wind power capacity was
a small fraction of the power system and continuous
connection of the wind farm to the grid was not a big
concern. With increasing share from the wind power
sources, it has become important for continuous
connection of the wind farm to the system to enable
uninterrupted power supply to the load even in the case
of some minor disturbances. The capacity of wind farms
are being increased by installing more and bigger wind
turbines connected online which implies that more
impedance is being added to the system, thus making the
connected system as a weak grid [2]. Voltage stability
and an efficient fault ride through capability are the basic
requirements of higher penetration. The wind turbines
have to be able to continue uninterrupted operation under
transient voltage conditions to be in accordance with the
grid codes. Grid codes are certain standards set by
regulating agencies and the wind power systems should
meet these requirements for their interconnection to the
grid. There are different grid code standards established
by different regulating bodies and the Nordic grid codes
are becoming increasingly popular. Flexible AC
Transmission Systems (FACTS) based power electronic
converters like SVC, Static Synchronous Compensator
(STATCOM) and the Unified Power Flow Controller
(UPFC) are being used extensively in power systems
because of their ability to provide flexible power flow
control [3]. The main motivation for choosing SVC in
wind farms is its ability to provide bus bar system
voltage support either by supplying and/or absorbing
reactive power into the system.
One of the major issues concerning a wind farm
interconnection to a power grid is its dynamic stability
considerations on the power system [4]. Stand alone
systems are easy to simulate, analyze and control when
compared to large power systems. A wind farm is usually
spread over a wide area having many wind generators
each producing different power as they are exposed to
different wind patterns.
Abstract -- The wind energy stands out to be one of the
most promising new sources of electrical power in the near
term. The latest technological advancements in wind energy
conversion and an increased support from governmental
and private institutions have led to increased wind power
generation in recent years. Wind power is the fastest
growing renewable source of electrical energy hence it has
become necessary to address problems associated with
maintaining a stable electric power system. Unlike
conventional sources, wind does not provide reactive power,
which is necessary to maintain acceptable voltage conditions
on the network. The use of Flexible AC Transmission
Systems (FACTS) in distribution network to compensate for
vagaries such as production related to wind energies and to
control the voltage is an optimal solution.
In this paper the effect of real power supplied, reactive
power consumed due to variation and fluctuation of load
and wind speed studied, the effect on grid voltage due to
variation is extensively considered. The variation in voltage
of grid is controlled by SVC which is shunt connected
Thyristor control Reactor and Thyristor switched capacitor
TCR-TSC. Results are produced in MATLAB/SIMULINK
and considerable improvement in grid voltage received by
compensating the reactive power.
Keywords-- SVC: Static Var Compensator; TCR:
Thyristor Controlled Reactor.
I. INTRODUCTION
The increase in electric power demand and the
depleting natural resources has led to the increased need
for production of energy from renewable sources such as
wind energy. The latest technological advancements in
wind energy conversion and the increased support from
government and private institutions have led to increased
wind power generation in recent years. wind power is the
fastest growing renewable source of electrical energy.
Total wind power installation in the US is 11,603 MW in
2006 and is expected to increase by 26% in the year 2007
[1]. Wind power penetration has increased multifold in
the past few years; hence it has become necessary to
address the problems associated with maintaining a stable
electric power system which has different sources of
energy including hydro, thermal, coal, nuclear, wind,
solar and many others.
II. VOLTAGE S TABILITY
Voltage Stability is defined as the ability of power
system to maintain steady voltages at all buses in the
system after being subjected to a disturbance from a
given initial operating condition [5].
593
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
Voltage stability is a problem in power networks,
which are heavily load, faulted, or with insufficient
reactive power supply. Although voltage instability is
essentially a local phenomenon, the problem of voltage
stability concerns whole power system, and is essential
for its operation and control. The main reason for voltage
instability is the increased of load, for that reason,
voltage stability is also called load stability problem.
Voltage collapse is the process by which the sequence of
events accompanying voltage instability leads to a
blackout or abnormally low voltages in a significant part
of the power system. Most of problem found in power
system realizes voltage collapse as a static phenomenon.
Static study is appropriate for the bulk power system
study, which involves enormous number of buses and
generators. Static voltage instability is mainly associated
with reactive power imbalance. Slowly developing
changes in the power system occur that eventually lead to
a shortage of reactive power and declining voltage. This
phenomenon can be seen from the plot of the voltage at
receiving end versus the power transferred. The plots are
popularly referred to as P-V curve or “Nose” curve. As
the power transfer increases, the voltage at the receiving
end decreases. Eventually, the critical (nose) point, the
point at which the system reactive power is out of use, is
reached where any further increase in active power
transfer will lead to very rapid decrease in voltage
magnitude. Before reaching the critical point, the large
voltage drop due to heavy reactive power losses can be
observed. The only way to save the system from voltage
collapse is to reduce the reactive power load or add
addition reactive power prior to reaching the point of
voltage collapse.
The power coefficient (Cp) describes the efficiency of
a turbine that converts the energy in the wind to
rotational power. Therefore power output of the turbine is
given by Pο = 0.5ρAV3Cp ------------ (3.1)
The tip speed ratio of the wind turbine is defined as λ
=ωR/V ------------ (3.2)
Where R = radius of the swept area in meters
ω= angular speed in radians per second.
Cp varies with change in λ. Cp-λ characteristics of a
turbine is essential to develop the turbine model. Most
wind turbines in the world use three-phase asynchronous
(squirrel cage or wound rotor) generator to connect to the
grid. One reason for choosing this type of generator is
that it is very reliable, and comparatively inexpensive.
The power from the wind turbine rotor is transferred to
the generator through a power train (i.e. through the low
speed turbine shaft, the gearbox and the high-speed
generator shaft). Power curve of a WEG is a graph that
indicates the electrical power output at different wind
speeds. Power curves are available along with
commercial models of WEGs.
IV. STATIC V AR C OMPENSATOR
The SVC is a widely used FACTS controller, it is a
shunt connected absorber or generator which exchange
capacitive or inductive current to maintain/control
specific parameter of power system. Figure 1 shows SVC
having controllable variable inductor with switchable
capacitance.
III. W IND E LECTRIC GENERATOR
The kinetic energy of moving air molecules are
converted into rotational energy by the rotor of wind
turbine. This rotational energy in turn is converted into
electrical energy by wind electric generator. The amount
of power, which the wind transfers to the rotor, depends
on the density of air, the rotor area, and the wind speed.
The power contained by wind is given by [7],
P =0.5*(air mass flow rate)*(wind velocity)2
= 0.5* (ρ*A*V) * (V)2
=0.5ρAV3
where, P = power contained in the wind (W)
ρ = air density (kg/m3)
A = rotor area (m2)
V=wind velocity before rotor interference (m/s)
Fig.1 SVC having variable inductor with switchable capacitance
594
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
SVC may have:
(a) Thyristor control Reactor (TCR)
(b) Thyristor Switched Capacitor
(c) Combination of (a) and (b)
(d) Fixed capacitor-TCR and
(e) TCR-Mechanically switched Capacitor(TCR-MCR)
The high voltage at system bus is measured, filtered
and compared with reference voltage and the error
voltage is processed through gain time constant controller
to provide a desired susceptance for SVC. This
susceptance is now implemented by logic control to
select number of TSCs or to determine firing angle for
the TCR.
The modeling and simulation of TSC based SVC and
TCR based SVC are investigated using MATLAB. Effect
of both Thyristor switched Capacitor and Thyristor
Controlled Reactor VAR compensator on load voltage in
a single machine infinite bus system are analyzed.
The three modeling of SVC generator fixed
susceptance model, total susceptance model and firing
model are compared . The dimension under which
voltage comparisons done at regulated bus are equivalent
susceptance of SVC at the fundamental frequency and
load flow convergence rate when SVC is operating both
within and on the limit. Two modified models are also
proposed to improve SVC regulated voltage under static
condition and better convergence rate has been achieved.
The current is essentially reactive, lagging behind the
voltage by nearly 90º. It contains a small in phase
component due to power loss in the reactor, which may
be of the order of 0.5-2% of the reactive power. Full
conduction is obtained with a gating angle of 90º. Partial
conduction is obtained with gating angle between 90º and
180º. The effect of increasing the gating angle is to
reduce the fundamental harmonic component of the
current. This is equivalent to an increase in the
inductance of the reactor, reducing its reactive power as
well as its current. So far as the fundamental component
of current is concerned, the thyristor-controlled reactor
has a control label susceptance and can therefore be
applied as a static compensator. The instantaneous
current is given by
i=
for α ≤ wt ≤(α+σ)
i = 0 for (α + σ) < wt < (α +π)
Where,
V is the rms voltage;
XL = wL is the fundamental frequency reactance of the
reactor;
w = 2πf; and α is the gating delay angle.
The time origin is chosen to coincide with a positivegoing zero crossing of the voltage. The fundamental
component is found by Fourier analysis and is given by:
4.1 Thyristor Controlledreactor
Figure 2 shows the TCR having a shunt connected
inductor whose effective reactance is varied continuously
with partial conduction control of thyristor.
I1= BL(σ).V,
Where,BL(σ) is an adjustable fundamental frequency
susceptance controlled by the conduction angle. The
maximum value of BL is 1/XL , obtained with ó = 0
or180°, that is, full conduction in the thyristor controller.
The minimum value of BL is zero obtained with σ = 0° (σ
=180°). This control principle is called phase control.
V. S IMULATION RESULTS
The system under study is as shown in figure2
Fig 2: Thyristor controlled reactor
TCR is also a subset of SVC in which conduction time
and hence the current in a shunt reactance is controlled
by a thyristor based AC switch using firing angle control.
For three phase networks three inductors can be
connected in star with each anti parallel thyristor.
There are two thyristors connected in anti-parallel
which conduct during alternate half cycles of the supply
voltage, if the thyristors are gated into conduction.
Precisely at the peak of the supply voltage, a full
conduction results in the reactor and the current is the
same as though the thyristor controller were short
circuited.
595
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
In this study Matlab/Simulink is used to create the
experimental setup. The set up consists of SIX 0.5MW
wind turbine capacity is connected to a 16 kV
distribution system. The distribution system is connected
to a 735 kV grid located 30 km from the wind farm . In
this model Wind turbines using a doubly-fed induction
generator (DFIG) consist of a wound rotor induction
generator and an AC/DC/AC IGBT-based PWM
converter modeled by voltage sources. The stator
winding is connected directly to the 50 Hz grid while the
rotor is fed at variable frequency through the AC/DC/AC
converter. The DFIG technology allows extracting
maximum energy from the wind for low wind speeds by
optimizing the turbine speed, while minimizing
mechanical stresses on the turbine during gusts of wind.
The simulation model is as shown in figure using
SVC(TCR-TSC) model
The next graph shows the voltage at bus_735.
Fig.4 shows the variations in wind speed and upto its maximum
limit of 11m/s
In this demo the wind speed is maintained constant at
15 m/s. The control system uses a torque controller in
order to maintain the speed at 1.2 pu. The reactive power
produced by the wind turbine is regulated at 0 Mvar. The
sample time used to discretize the model (Ts= 50
microseconds).
In this demo you will observe the steady-state
operation of the DFIG and its dynamic response to
voltage sag resulting from a sudden change in load.
Initially the DFIG wind farm produces 3 MW. The
corresponding turbine speed is 1.2 pu of generator
synchronous speed. The DC voltage is regulated at 1150
V and reactive power is kept at 0 Mvar. At time t=0.1s a
load of 35Mw is switched by means of the breaker
arrangement near the turbine because of this sudden
change the voltage drops to 0.8pu when the SVC is
switched at 0.2 sec after some transition time of
t=0.24sec, the voltage recovers in approximately 4
cycles. Fig3 shows the voltage waveform at bus_575
Fig5.shows the wind turbine characteristics for various pitch angles
VI. CONCLUSIONS
The steady state behaviour of an interconnected DFIG
based wind farm with SVC is studied. A case was
developed in which when the load is connected at the
point of common coupling there is a voltage dip which if
not timely corrected then it will eventually leads to
voltage stability and voltage collapse of the
interconnected power system.
596
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
REFERENCES
The response of the system is analyzed and is found
that SVC have made the system to be stable.
[1]
[2]
VII. APPENDIX
Rated power
Stator Voltage
Rs (stator resistance)
Rr (rotor resistance)
Ls (stator inductace)
Lr (rotor inductance)
Lm(magnetizing
inductance)
Number of pole pairs
Inertia constant
3 MW
575 V
0.0071 pu
0.005 pu (ref to
stator)
0.171 pu
0.156 (ref to stator)
2.9 pu
[3]
[4]
[5]
3
5.04
[6]
[7]
[8]
597
http://www.awea.org/newsroom/releases/Wind_Power_Capacity_
012307.html, November 2007.
M. Molinas, S. Vazquez, T. Takaku, J.M. Carrasco, R. Shimada,
T.Undeland, “Improvement of transient stability margin in power
systems with integrated wind generation using a STATCOM: an
experimental verification,” International Conference on Future
Power Systems, 16-18 Nov. 2005.
S.M. Muyeen, M.A. Mannan, M.H.Ali, R. Takahashi, T. Murata,
J.Tamura, “Stabilization of Grid Connected Wind Generator by
STATCOM,” IEEE Power Electronics and Drives Systems, Vol.
2, 28-01 Nov. 2005, 1584 – 1589.
E. Muljadi, C.P. Butterfield, “Wind Farm Power System Model
Development,” World Renewable Energy Congress VIII,
Colorado, Aug-Sept 2004
N.Mithulananthan,
A.Sode-Yome,
N.Acharya,
S.Phichaisawat,“Application of FACTS Controllers in Thailand
Power Systems,” RTG Budget-Joint Research Project, Fiscal-Year
2003, January 2005.
AWEA Electrical guide to utility scale wind turbines, March
2005.
P.Kundur
Power
System
Stability
and
Control,
NewYork:McGrawHill,1994.
Trinh Trong Chuong; Voltage stability analysis of grid connected
wind generators; International Conference on Electrical
Engineering; Japan, 2008.