Flc based statcom for a dfig driven wind turbine to enhance voltage stability

FLC Based Statcom For A DFIG Driven Wind Turbine To Enhance Voltage Stability, Mrs.K.Sree
Latha, Dr.M.Vijaya Kumar, Journal Impact Factor (2015): 7.7385 (Calculated by GISI)
www.jifactor.com
www.iaeme.com/ijeet.asp 35 editor@iaeme.com
1
Assoc.Prof, GNITC, HYDERABAD, INDIA
2
Professor, JNTU, ANANTAPUR, INDIA
ABSTRACT
Now-a-days the contribution of wind power in conventional power generation is increasing
rapidly. But with the wind power generation system the inherent problem is the low power quality
because of intermittent power generation which affects the grid connected to it. Power quality
problems are harmonic distortion, voltage instability, reliability are of major concern which needs to
be addressed. This paper mainly investigates the implementation of FLC based STATCOM with
wind farms for stabilizing the grid-side voltage after a sudden change in load.
Key words: Wind farms, STATCOM, DFIG, Voltage Stability, Wind Turbines, Fuzzy Logic
Controller (FLC).
I. INTRODUCTION
Increase in electric power demand and has led to the production of energy from renewable
energy sources such as wind energy, solar energy etc. With the technical advancements and support
from various government and private institutions have led to the increased wind power generation in
recent years. As the wind power penetration has increased enormously in recent years, hence it has
become necessary to address the issues associated with maintaining a stable electric power system
which has different sources of energy including thermal, hydro, nuclear, solar, wind etc., As the wind
power generation has got increased, it has become important for the wind farm to be connected
continuously in order to enable uninterrupted power supply to the load even in case of minor
disturbances. By installing more and bigger wind turbines connected online to the system, increases
the impedance making the system as a weak grid. Voltage stability and an efficient fault ride through
capability are the basic requirements of higher penetration. Grid codes require the wind energy to be
provided continuously even under transient voltage conditions (Qiao and Harley, 2008). Grid codes
and certain standards set by different regulating agencies should be met by the wind power system so
as to be interconnected to grid. Different grid code standards are set by different regulating bodies
but Nordic grid codes are increasingly popular. For the interconnection of the wind farm to the grid
the major problem that needs to be addressed is its dynamic stability. The collapse of grid may occur
FLC BASED STATCOM FOR A DFIG DRIVEN WIND
TURBINE TO ENHANCE VOLTAGE STABILITY
1
Mrs.K.Sree Latha, 2
Dr.M.Vijaya Kumar
Volume 6, Issue 6, June (2015), Pp. 35-43
Article ID: 40220150605004
International Journal of Electrical Engineering & Technology (IJEET)
© IAEME: www.iaeme.com/IJEET.asp
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
IJEET
© I A E M E

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due to voltage instability which is caused by heavy loading and the system not able to meet the
reactive power demand.
An induction generator is a sink of reactive power which needs necessary voltage support at
the terminal where the wind farm is connected. Most of the interconnected system require wind
farms to maintain a power factor (PF) between 0.9 to 0.95. In general the output from the wind farm
varies from time to time due to variation in wind speed as shown in fig.1.Theoritically these reactive
power sources have to be adjusted to maintain power factor within the range to control the terminal
voltage. Practically, fixed capacitor banks will be connected to compensate the reactive power but
this may lead to voltage fluctuation and wear out the turbines gear box.
Fig.1 Uncertainty of the wind Generation
Recent advancements in power electronic converters introduces the use of Flexible AC
Transmission systems (FACTS) which are used extensively in power system. They are capable of
controlling the network in a very fast manner and so can be exploited. The various types of FACTS
are SVC, STATCOM, UPFC are used for power control. STATCOM (Static Synchronous
Compensator) is preferred in wind farms due to its ability to provide bus bar voltage either by
supplying and /or absorbing reactive power in to the system. Authors in [2,3] have proved that the
short-term and transient stability conditions of a generator have been improved when a STATCOM
is introduced into the system as a var/voltage supporter.
One of the major issue related to wind farm is its dynamic stability. Stand alone systems are
easy to simulate when compared to large systems. So, extensive modeling is required to study the
various effects of wind energy system before implementation. A wind farm is usually spread over a
wide area and has many wind generators which produce different amounts of power when they are
exposed to different wind patterns.
II. VOLTAGE STABILITY AND WIND FARM TURBINE TECHNOLOGIES
A) Voltage Stability
Voltage instability will be experienced by a system when there is a progressive or
uncontrollable drop of voltage after a disturbance. Thus voltage stability is defined as the ability of
the power system to regain stable voltage at all buses in the system even after being subjected to a
disturbance. Although voltage instability is essentially a local phenomenon and is related to whole
power system. The main reason is due to variation in load and so it is also called as load stability
problem. Voltage instability leads to a blackout or abnormally low voltages in a significant part of
the power system.

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When considering the voltage stability, the P-V curve or "Nose" curve is of interest. The
process involves the transfer of P from one region to another and monitoring the effects of V. As
Power P increases Voltage V decreases. Eventually, the nose point is reached where any further
increase in active power transfer will lead to reduced voltages as shown in fig.2. Load flow solutions
do not converge beyond this point and is called as critical point. Generally, operating points above
the critical point signifies a stable system else unstable condition [4].
B) Squirrel-Cage Induction Generator
These are fixed - speed machines that are directly grid connected. As the variations in rotor
speed are so small, the SCIG's are called as fixed- speed induction generators. They are widely
employed, however their inability to generate reactive power is a limiting factor in future use.
Fig.2 P-V Characteristics
In this the turbine blades are so designed to operate at lower efficiency during high wind
speed using stall control or the angle of the blades can be actively adjusted according to the wind
speed using pitch angle control. Most conventional wind farms use this type of induction
generator.Fig.3 shows the fixed speed induction generator.
Fig.3 Wind turbine system with SCIG
C) Doubly-Fed Induction Generator
These are variable speed machine and are used widely. These are connected to the rotor through a
gear box and a back-back voltage source converter. The converter enables variable speed operation
of the wind turbine by controlling the voltage and current.Fig.4 shows the DFIG which is wound
rotor induction generator. As the stator is connected directly to the grid, the magnetic field rotation
speed is fixed in the stator winding.

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Fig.4 Doubly- Fed induction generator Block Diagram
The rotor is made to rotate with speed of the stator magnetic field. At synchronous speeds
there is no slip, whereas at sub-synchronous speeds, the drive will adjust the frequency fed to the
rotor to make it running faster at synchronous speed.
III. STATCOM
It is a shunt connected device that is capable of absorbing or generating reactive power and
can provide reactive power compensation. The reactive output power of the compensator is varied to
control the voltage at given terminals to maintain the desired power flow under any system
disturbances and contingencies. A STATCOM improves power oscillation damping, transient
stability and provides dynamic voltage control. By controlling angle Φ, the flow of current either
from the converter to the AC system or vice-versa can be controlled. The maximum compensation
current in a STATCOM is independent of the system voltage .Fig.5 shows the base model of the
STATCOM. The exact ratings of the device are derived based on many parameters. One of the
parameter is the reactive power demanded by the system to recover and ride through typical faults on
the system and reduce the equipment from going out of the synchronism with the grid. In the case of
this test system, STATCOM rating chosen is ---------- to maintain the voltage of the load bus to 1.0
pu.
Fig.5 Basic block diagram of STATCOM
Also, a STATCOM connected in a transmission system is mostly used to support the grid
voltage at severe disturbances and to control the reactive power. This also can be used in distribution
networks for controlling the power factor, active power, flicker mitigation and power quality
improvement.

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IV. FUZZY LOGIC CONTROLLER (FLC)
Fuzzy Logic Controller makes use of uncertainty, tolerance ,impression and fuzziness in the
decision making process offers a very satisfactory performance just by incorporating the experts
knowledge into fuzzy rules without using any detailed mathematical modes of the system. In
addition to this it has the ability to deal with impartial and noisy data so that it can be extended to
control capability even to those operating conditions where linear control techniques fail.
Conventional PI controller has its inability to react to abrupt changes in the error signal as it
is inefficient during non linear variations. Thus FLC is much efficient to with stand for abrupt error
signal changes Δε. The output control signal can be determined by using an interface engine using If-
Then rules. With this the value of the output can be changed according to the values of the error
signal ε and the rate-of-error Δε. The determination and structure of the rule base is done using trial -
and - error methods and is done through experimentation.
Fig.6 Basic block diagram of FLC
The member functions and rule viewer set is as shown in figure 7& 8
Fig.7 Member ship functions of FLC

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Fig.8 Rule viewer set FLC
V. SIMULATION RESULTS
The system under study is as shown in figure 9.
Fig.9 Block Diagram of the system under study
The system under study consists of a six units each of 0.5MW capacity thus generating 3MW
capacity connected to a 25KV distribution system which is connected to a 120 KV grid located
30KM from the wind farm. The wind turbine uses a Doubly fed Induction Generator with its stator
directly connected to the grid. In order to study the working of windfarm a sudden load of 10MW is
added at around 0.2 secs and because of which the voltage of the wind farm and voltages at all buses
will fall from 1p.u. At around 0.4secs statcom with fuzzy is added so as to enhance the voltage
profile. Figure 1 shows clearly the enhancement in voltage after adding statcom at time t=0.4 secs

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Fig.1: Voltage at bus 4 showing enhancement in the voltage.
Table1: Voltage recovery time at the point of common coupling
With facts device With out facts device Statcom with FLC (10MVAR)
Voltage recovery time(secs) Collapse 0.45secs
Fig 2. shows the comparison between the voltages with and without statcom
The next parameters which were monitored during the study is its reactive power and rotor
speed. The variations of these parameters are studied with and with out the presence of statcom
.Fig.3 and fig.4 gives the variation in reactive power without and with support of the devices at bus
B4 respectively

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Fig.3 Variation in PQ with out statcom
Fig.4 Variation in PQ with statcom.
Table.2: Variation of Q at PCC
With facts device Statcom with FLC
Reactive power recovery time (secs) 0.44secs
VI. CONCLUSIONS
The behavior of the Fuzzy Logic based STATCOM under steady state condition is studied. In
this a simulated model is designed in which when the load is connected at the load is connected at
the point of common coupling there is a voltage variation which then enhanced by going for the
implementation of fuzzy based STATCOM. Results of the study show that
1. During a disturbance near IG based windfarm, generators absorbs large amount of reactive
power which leads to instability of the system.
2. .During such disturbance as the system cannot provide required reactive power especially
when the wind generator is connected to weak grid. Hence Facts devices need to be provided
in order to enhance dynamic reactive power support leading to power system stability.
3. It can be seen that STATCOM with implemented FLC based controller will enhance the
system performance well and is the fastest one in improving the stability.
4. So, it can be explained that STATCOM with better controller like FLC can be used effectively
in a system with wind energy as a source for enhancing the dynamic stability of the system and
can be helpful in improving the grid code requirements.

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VII. PARAMETERS
Wind Generator: Power 3MW
Stator resistance: 0.0071 pu, Rotor resistance: 0.005pu, Magnetising Inductance: 2.9pu, reactive
power of STATCOM: 10MVAR. Vbase = 120KV,Pbase = 3MW , Fbase = 50 Hz.
VII. APPENDIX
1. 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.
2. L. Chun, J. Qirong, X. Jianxin, “Investigation of Voltage Regulation Stability of Static
Synchronous Compensator in Power System,” IEEE Power Engineering Society Winter
Meeting, vol. 4, 2642-47, 23-27 Jan. 2000
3. M.Chinchilla, S. Arnalte, J.C. Burgos, and J.L.
Rodríguez:"Power limits of grid connected modern wind energy systems", Renewable
Energy, September 2005,pp.14551470.
4. M.Davidson, Interaction of a wind farm with the distribution network and its effect on
voltage quality, Proc. IEE Colloquium on the impact of Embedded Generation on
Distribution Networks (Digest 1996/194), 1996, 9/1–9/5.
5. Tascikaraoglu A, Uzunoglu M, Vural B, Erdinc O. “Power quality assessment of wind
turbines and comparison with conventional legal regulations: a case study in Turkey”.
Applied Energy, 2011; 88: 1864–72.
6. M. Tsili and S. Papathanassiou, “A review of grid code technical requirements for wind
farms,” IET Renewable Power Generation., September 2009, vol. 3, no. 3, pp. 308–332
7. A. Petersson, T. Petru, and T. Thiringer, ―Grid Disturbance Response of Wind Turbines
Equipped with Induction Generator and Doubly-Fed Induction Generator‖, in Proceedings of
2003 IEEE PES General Meeting, Toronto, Canada, July 13-17, 2003.
8. J. Feltes, Y. Kazachkov, R. Zavadil, ―Modeling Wind Farms for Power System Stability
Studies‖, Procedings of 2003 IEEE Power Engineering Society General Meeting, Toronto,
Canada, 13-17 July 2003.
9. Mustafa Jawad Kadhim and Prof.D.S.Chavan, “Overview LVRT Capability of DFIG
Techniques” International Journal of Electrical Engineering & Technology (IJEET), Volume
4, Issue 3, 2013, pp. 75 - 81, ISSN Print: 0976-6545, ISSN Online: 0976-6553.
10. Nadiya G. Mohammed, “Application of Crowbar Protection on DFIG-Based Wind Turbine
Connected To Grid” International Journal of Electrical Engineering & Technology (IJEET),
Volume 4, Issue 2, 2013, pp. 81 - 92, ISSN Print: 0976-6545, ISSN Online: 0976-6553.
11. Dr Naarisetti Srinivasa Rao and Dasam Srinivas, “An Experimental Design To Improve
Power Quality on Statcom” International Journal of Electrical Engineering & Technology
(IJEET), Volume 4, Issue 5, 2013, pp. 96 - 103, ISSN Print: 0976-6545, ISSN Online: 0976-
6553.
12. G.Kumar, P.S.Raju, “Study of Dstatcom In Improved Custom Power Park For Power Quality
Improvement” International Journal of Electrical Engineering & Technology (IJEET),
Volume 2, Issue 2, 2011, pp. 12 - 20, ISSN Print: 0976-6545, ISSN Online: 0976-6553.

Flc based statcom for a dfig driven wind turbine to enhance voltage stability

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