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Fuzzy Logic Control Strategy for Stand-Alone Self-Excited Induction Generator for a Variable Speed Wind Turbine

M. Prabhu Raj1, Dr. K. Ranjith Kumar2
  1. P.G. Scholar Department of EEE, Government College of Technology, Coimbatore-641 013, Tamil Nadu, India
  2. Assistant Professor, Department of EEE, Government College of Technology, Coimbatore-641 013, Tamil Nadu, India
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This paper presents a control strategy for the operation of a Self Excited Induction Generator (SEIG) based stand-alone variable speed wind turbine. To extract maximum energy form the generator and to regulate the terminal voltage of the generator a control strategy is present. In this system there is no grid connection, only stand-alone system is present. The topology of the system consists of a three phase squirrel-cage induction machine connected to a wind turbine through a step-up gear box, Pulse Width Modulation (PWM) Current Controlled Voltage Source Inverter (CC-VSI), Electronic Load Controller (ELC), three fuzzy logic PI controllers and one Hysteresis Current Controller (HCC). A dump-load resistance with IGBT chopper control and the dc link voltage is present to consume the maximum output power from the generator. Dynamic and steady-state performance where simulated using Matlab/Simulink software

Index Terms

Self-Excited Induction Generator (SEIG), maximum power extraction, stand-alone variable speed wind turbine, voltage control.


The conventional energy sources such as thermal power generation, nuclear power generation etc., are limited and pollute the nature. So more attention and interest have been paid to the utilization of renewable energy source such as Wind Energy, Fuel Cell, Solar Energy etc., Wind Energy is the fastest growing and most promising renewable energy source among them as it is economically viable.
Electrical generators are used to produce electricity, which are driven by the wind turbines by using the wind power. Blades are rotated when the wind passes over it, the blades are connected to the shaft so that the speed can be increased with the help of gear box the rotational speed is increased. The wind energy is converted into electrical energy. The output power form the generator is given to a transformer, which step up the electrical voltage from 700V to 33 kV. The energy produced from the Wind is not a constant source. It varies continuously and gives energy in sudden bursts. About 50% of the entire energy is given out in just 15% of the operating time. Wind strengths continuous vary and thus cannot guarantee continuous power.
The wind turbine output power can be calculated by the given formula:
Pw = 0.5ρπ R2V3Cp(λ,β) (1)
Pw = extracted power from the wind,
ρ = air density, (1.225 kg/m3 at 20° C at sea level)
R = blade radius (in m), (it varies between 40-60 m)
V = wind velocity (m/s) (velocity can be controlled between 3 to 30 m/s)
Cp = the power coefficient


The power circuit diagram of the proposed system is shown in the fig 1. Self-Excited Induction Generator (SEIG) is connected to the variable speed wind turbine througt a step up gers box. The terminals of the SEIG is connected in parallel with the fixed capacitor bank and main load resistance to the Current Control Voltage source inverter through a smoothing reactance. The fixed capacitor bank provides two functions one to avoid pre-charging of DC side capacitor Cdc of the CC-VSI for the start up process of the Induction generator and the second function it acts as
Fig 1 Power Circuit Diagram
a second order filter to reduce the higher order harmonics. The Electronics Load Controller(ELC) is Connected in parallel with the CC-VSI to extract to maximum available energy from wind turbine. The ELC and dump load resistance are connected in series. The dump load resistance may be a battery charger or a heater.
The active power and reactive power of the Wind Turbine Induction generator (WTIG) is used to control and extract maximum available energy from the wind turbine and to maintain the generated terminal voltage against wind speed and main load variations using control strategy.
Control strategy consists of two controllers
i. The Voltage controller
ii. The Electronics Load Controller
Fig 2 The Voltage controller
The voltage generated from the SEIG can by adjusting its reactive power (excitation).The output current of the CC-VSI is used to control or regulate the terminal voltage of the SEIG. Fig 2 shows the diagram of a voltage controller. There are two control loops. The Hysteresis Current Controller (HCC) provides the required switching pulses to the inverter which is generated from the two loops, the outer gives the reference current ii*(abc) and the inner loop gives the actual inverter current ii(abc). The reference current ii*(abc) are formed by adding two current component of each phase.
1. The in-phase active current component iiα* (abc)
2. The quadrature reactive current component iiβ* (abc)
The in-phase active current component iiα*(abc) is also known as real power is used to keep the DC side capacitor charged to the specified level and excess real power is given to the dump load resistance as a wastage. The quadrature reactive current component iiβ*(abc) is also known as reactive power is used to regulate the generated voltage. The AC voltage magnitude of the SEIG is sensed and compared against the AC reference voltage magnitude. The AC voltage error output is given to the first fuzzy logic controller (FLC-1). The output of the FLC-1 is iβ* of the AC voltage control loop is multiplied by the quadrature unit vectors uβ (abc) which lead the unit vectors of AC voltages by a phase shift of 90° to give the reference reactive current components iiβ* (abc) that control the amplitude of the reactive power generated in the CC-VSI and the reference reactive current components lead by a phase shift of 90° the corresponding AC voltages for a positive sign of the AC voltage error. The negative sign of the AC voltage error, they lag by a phase shift of 90°. Thus, the CC-VSI operates in capacitive and inductive modes respectively for positive and negative sign of the AC voltage error. Similarly, the in-phase components iiα*(abc) are obtained through the DC voltage control loop. The DC voltage error is given the second fuzzy PI controller (FLC-2).The output of FLC-2 iα* is multiplied by the unit vectors uα(abc) (which in-phase with the corresponding AC voltages) to give the reference active current components iiα*(abc).


The generator speed is controlled, by controlling the electrical load on the induction generator. An active power controller is used as an ELC. ELC circuit is shown in the fig 3 in which the generator speed and generator feedback speed is compared and the speed error is given and processed in the third fuzzy logic PI controller FLC-3. The output of FLC-3 and saw tooth carrier wave is compared to get the required PWM pulses for the IGBT of the ELC.


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