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WIND ENERGY CONVERSION SYSTEMS USING FUZZY CONTROLLED STATCOM FOR POWER QUA

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WIND ENERGY CONVERSION SYSTEMS USING FUZZY CONTROLLED STATCOM FOR POWER QUA Powered By Docstoc
					INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME
                             TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)                                                        IJEET
Volume 4, Issue 4, July-August (2013), pp. 108-117
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2013): 5.5028 (Calculated by GISI)
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  WIND ENERGY CONVERSION SYSTEMS USING FUZZY CONTROLLED
         STATCOM FOR POWER QUALITY IMPROVEMENT

            1
                S.MUNISEKHAR, 2O.HEMAKESAVULU, 3Dr.M.PADMALALITHA
                          1
                             PG Student, Dept of EEE, AITS, Rajampet, India
                      2
                         Associate Professor, Dept of EEE, AITS, Rajampet, India
                  3
                    Professor & Head of the Dept, Dept of EEE, AITS, Rajampet, India

ABSTRACT

        This paper investigates the power quality issues due to installation of wind energy conversion
system(WECS) with the distribution system.When the wind energy conversion system is connected
to distribution system the power quality issues like variation of voltage,current and harmonics at
source side and load side will be penerated into the distribution system. To mitigate the harmonics
produced at source side and load side, a fuzzy controlled static compensator(F-STATCOM) is
connected at point of common coupling. The F-Statcom controller and STATCOM for distribution
system connected wind energy generating(WGS) to mitigate power quality issues is simulated by
MATLAB/SIMULINK software.

Keywords: Fuzzystatcom (F-STATCOM), Powerquality (PQ), Wind Generating System (WGS),
Wind Energy Conversion System(WECS).

I. INTRODUCTION

        The generation of wind energy has been increasing rapidly and has become cost competitive
with other means of generation. The power generated from wind turbine is always fluctuating due to
environmental conditions. The wind power generated from wind turbine is expected to be a
promising alternative energy source which can bring new challenges[1]. The kinetic energy of the
wind is being absorbed by the rotor which constitutes blades which are mechanically coupled to the
alternator. There are three types of alternator technologies to interface with wind turbine.
    1. Conventional wound rotor or squirrel cage induction machines. These are supplemented by
        capacitors to supply reactive power needs.
    2. Doubly fed wound rotor induction machines which employ power converters to control the
        rotor current to provide reactive power support and control.
    3. Non-power frequency generation that requires an inverter or converter interface.
        The issue of power quality is of great importance to the wind energy conversions systems.
The main power quality that arises when wind turbine connected to distribution system are sustained

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interruptions, voltage regulation, harmonics and voltage sags. The power quality issues that arise
when wind turbine is connected to distribution system is minimized by fuzzy controlled statcom. The
fuzzy controlled statcom works with a set of fuzzy rules which are implemented in the matlab
program. The way to represent inexact data and knowledge, closer to humanlike thinking, is to use
fuzzy rules instead of exact rules when representing knowledge. Fuzzy systems are rule-based expert
systems based on fuzzy rules and fuzzy inference. Fuzzy rules represent in a straightforward way
"commonsense" knowledge and skills, or knowledge that is vague, or contradictory. This knowledge
might have come from many different sources. Commonsense knowledge may have been acquired
from long-term experience, from the experience of many people, over many years.
        The FUZZY-STATCOM control scheme for Distribution line connected wind energy
generation for power quality improvement has following objectives.
    • Sustained interruptions are minimized in the distribution system.
    • Reactive power support only from F-STATCOM to wind Generator and Load.
    • Minimization of Harmonics at source and load sides i.e reduction in THD values.

II. FUZZY-STATIC COMPENSATOR (STATCOM)

A. Principle of STATCOM
         A STATCOM is a voltage source converter (VSC), with the voltage source behind a reactor.
The voltage source is created from a DC capacitor and therefore a STATCOM has very little active
power storage. However, its active power capacity can be increased if a suitable energy storage
device is connected across the DC capacitor. The reactive power at the terminals of the STATCOM
depends on the amplitude of the voltage source. For example, if the terminal voltage of the VSC is
higher than the AC voltage at the point of connection, the STATCOM generates reactive current; on
the other hand, when the amplitude of the voltage source is lower than the AC voltage, it absorbs
reactive power. The time respone of a STATCOM is shorter than that of an SVC, mainly due to the
fast switching times provided by the IGBTs of the voltage source converter[3]. The STATCOM also
provides better reactive power support at low AC voltages than an SVC, since the reactive power
from a STATCOM decreases linearly with the AC voltage. Reactive power can be altered by
modifying the voltage amplitude of the VSC. For this purpose, a transformer with a turns-ratio of 1:1
or a reactor is assumed. In addition, constant distribution voltage is assumed. STATCOMs have the
ability to address transient events at a faster rate and with better performance at lower voltages than a
Static Voltage Compensator (SVC). The maximum compensation current in a STATCOM is
independent of the system voltage. A STATCOM provides dynamic voltage control and power
oscillation damping, and improves the system’s transient stability. By controlling the phase angle,
the flow of current between the converter and the ac system are controlled[5].




                         Figure 1: STATCOM connected to distribution line
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B. Fuzzy logic controller
        In a fuzzy logic controller (FLC), the dynamic behaviour of a fuzzy system is characterised
by a set of linguistic description rules based on expert knowledge. The expert knowledge is usually
of the form IF (a set of conditions are satisfied) THEN (a set of consequences can be inferred). Since
the antecedents and the consequents of these IF-THEN rules are associated with fuzzy concepts
(Linguistic terms), there are often called fuzzy conditional statements. A fuzzy control rule is a fuzzy
conditional statement in which the antecedent is a condition in its application domain and the
consequent is a control action for the system under control. Basically, fuzzy control rules provide a
convenient way for expressing control policy and domain knowledge. Furthermore several linguistic
variables might be involved in the antecedents and the conclusion of these rules. When this is the
case the system will be referred as multiple input multiple output fuzzy systems.




                                   Figure 2: Fuzzy logic controller

III. DISTRIBUTION CO-ORDINATION RULE

       The Distribution quality characteristics and limits are given for references that the customer
and the utility may expect.

A. Voltage Rise (u):
                The voltage rise at the point of common coupling can be approximated as a function
of maximum apparent power Smax of the turbine, the grid impedances R and X at the point of
common coupling and the phase angle ∅ given in Eq.1

               u = Smax(Rcos φ– Xsinφ )/U2                   (1)

Where        u = Voltage Rise
          Smax = Maximum Apparent power
            U = Nominal Voltage of the grid
          Φ = Phase difference
   The limiting voltage rise value is < 2%.

B. Voltage Dips (d):
The voltage dips is due to startup of wind turbine and it causes a sudden reduction of voltage. It is
the relative % voltage change due to switching operation of wind turbine. The decrease of nominal
voltage change is given in Eq. 2.

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                     d=KuSn/Sk                               (2)

        Where, d is relative voltage change.
                Sn is rated apparent power.
                Sk is short circuit apparent power.
                Ku is sudden voltage reduction factor.
     The acceptance voltage dips limiting value is ≤ 3%.

C. Flicker:
       The long term flicker is given by

         Plt = C (   k)   Sn/Sk                              (3)

Where, Plt = long term flicker.
   C ( k) =flicker coefficient calculated from
      Rayleigh distribution of the wind speed . The Limiting Value for flicker coefficient is about ≤
0.4.

D. Harmonics:
The harmonic distortion is assessed for variable speed turbine with an electronic power converter at
the point of common connection. The total harmonic voltage distortion of voltage is given as in Eq. 4

           V                                                (4)

        Where, Vn is the nth harmonic voltage.
              V1 is the fundamental frequency(50) Hz.
        The THD limit for 132 KV is 3%.
        THD of current ITHD is given as in Eq. 5

                                                            (5)

         Where, In is the nth harmonic current.
              I1 is the fundamental frequency (50) Hz.
     The THD of current and limit for 132 KV is <2.5%.


IV. WIND ENERGY GENERATING SYSTEM

A.      Wind Turbine Generating System
        Wind generations are based on constant speed topologies with pitch control turbine. The
induction generator is used in the proposed scheme because of its simplicity, it does not require a
separate field circuit, it can accept constant and variable loads, and has natural protection against
short circuit. The available power of wind energy system is presented as below. The kinetic energy in
air of mass “m” moving with speed V is given by the following in SI units:
         Kinetic Energy = 1/2. m . V2 joules.
The power in moving air is the flow rate of kinetic energy per second wind Power = ½. (ρA V3).


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               Figure 3: wind energy conversion system connected to distribution system

cut‐in wind speed (in the order of 3‐5 m/s) and nominal wind speed or rated wind speed: wind speed
at which the nominal power of the turbine is reached (between 11 m/s and 16 m/s). cut‐out wind
speed : When the wind speed becomes very high, the energy contained in the airflow and the
structural loads on the turbine become too high and the turbine is taken out of operation. Depending
on whether the wind turbine is optimized for low or high wind speeds, (between 17 and 30 m/s).

B. Generator Model equations
Vds = - Rsids - ωs ψqs + D(ψds)                              (6)

Vqs = - Rsiqs + ωs ψds + D(ψqs)                              (7)

Vdr =0= - Rridr -s ωs ψqr + D(ψdr)                           (8)

Vqr =0= - Rriqr +s ωs ψdr + D(ψqr)                           (9)

         Where D=d/dt. All quantities are in per unit. Indices d and q indicate the direct and
quadrature axis components and s and r indicate stator and rotor quantities. The d‐q reference frame
is rotating at the synchronous speed with the q‐axis leading the d‐axis by 90°.
C. Torque Balance Equation

Te = ψds iqs – ψqs ids                                      (10)

D(ωm) = 1/2Hm(Tm – Te)                                     (11)

Where D=d/dt.
      Hm = mechanical inertia constant.
      Tm = Mechanical torque.
     Te = Electromagnetic torque.

V. TOPOLOGY FOR POWER QUALITY IMPROVEMENT

        The F-STATCOM based current control voltage source inverter injects the current into the
grid in such a way that the source current are harmonic free and their phase-angle with respect to
source voltage has a desired value. The injected current will cancel out the reactive part and
harmonic part of the load and induction generator current, thus it improves the power factor and the
power quality. To accomplish these goals, the grid voltages are sensed and are synchronized in
generating the current command for the inverter. The proposed distribution connected system in Fig.
4 consists of wind energy generation system and battery energy storage system with F-STATCOM.


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              Figure 4: Line Diagram for Proposed system (with FUZZY STATCOM)

VI. SIMULATION CIRCUITS AND RESULTS

A. System Performance
        The Simulink model library includes the model of Conventional Source, Asynchronous
Generator, FUZZY STATCOM, Non-Linear Load, Inverter, Distribution Voltage, Line Series
Inductance and others that has been constructed for simulation. The simulation parameter values for
the given system are given in Table 1.

                 S. No     Parameters                           Rating
                 1         Source voltage                       3-phase 11KV 50Hz

                 2         Asynchronous generator               3.35KVA, 415V
                 3         Distribution line                    R=2ohms,L=0.1H
                 4         Non Linear Load                      Diode Bridge
                                      Table 1: System parameters

        The STATCOM is designed by IGBT’S as shown in the figure 5.The gating pulses to the
IGBT is given through fuzzy logic controller. A capacitor of 100 micro farads is connected in
parallel to the circuit and it acts as storage element or battery in the circuit. The reactive power is fed
to the power system network by the capacitor.




                       Figure 5: Simulink model of Statcom Simulation Circuit


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B. Control Scheme
       The control scheme approach is based on injecting the currents into the distribution system
using ―Fuzzy controller. Using such technique, the controller keeps the control system variable
between boundaries of hysteresis area and gives correct switching signals for STATCOM operation.
The control system scheme for generating the switching signals to the F-STATCOM is shown in Fig.
6.




                   Figure 6: Simulink model of Fuzzy Statcom Control Circuit

       The output of Load Voltage , Load Current, Source current and Source Voltage without fuzzy
Logic Controller is shown in figure 7 as shown below. The percentage of harmonics are more in the
output waveforms and the waveforms are distorted in their position.




   Figure 7: Output Waveforms of load voltage, load current, Source current and Source voltage
                                     Without F-Statcom

       The output of Load Voltage ,Load Current, Source current and Source Voltage with fuzzy
Logic Controller is shown in figure 8. as shown below. The percentage of harmonics are less in the
output waveforms.

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    Figure 8: Output Waveforms of load voltage,load current,Source current and Source voltage
                                       With F-Statcom

      The output waveforms of stator current, Rotor current, Electromagnetic torque and speed in
RPM are shown in the figure 9.




                Figure 9: Output waveforms of Generator and Wind Turbine

       The Total Harmonic distortion i.e THD analysis of Load current and load voltage without and
with Fuzzy Stacom are shown in fig10 ,11,12 &13.




                 Figure 10: FFT Analysis (Load Current) Without F-STATCOM


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                    Figure 11: FFT Analysis (Load Current) With F-STATCOM




                  Figure 12: FFT Analysis (Load Voltage) Without F-STATC0M




                    Figure 13: FFT Analysis (Load Voltage) With F-STATCOM

                                    Table 2: Comparison Results
    S.No.        Parameters                   Without                         With
                                      F-STATCOM (THD %)                F-STATCOM (THD %)
      1        Source voltage                  20.00                          0.00
      2        Source Current                  27.55                          0.00
      3         Load voltage                   43.49                          0.07
      4         Load Current                   27.55                          0.00

VII. CONCLUSION

       The paper presents the FUZZY STATCOM-based control scheme for power quality
improvement in Distribution line connected wind generating system and with non linear load. The
power quality issues and its consequences on the consumer and electric utility are presented. The
operation of the control system developed for the FUZZY STATCOM in MATLAB/SIMULINK for
maintaining the power quality is simulated. It has a capability to cancel out the harmonic parts of the
load current. It maintains the source voltage and current in-phase and support the reactive power
demand for the wind generator and load at PCC in the Distribution system, thus it gives an
opportunity to enhance the utilization factor of Distribution line. The integrated wind generation and
STATCOM with FUZZY Controller have shown the outstanding performance. Thus the proposed
scheme in the Distribution connected system fulfills the power quality norms as per the IEC standard
61400-21[8].

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