Speed Control of Doubly-Fed Induction Generator Using PI-Controller

ISSN 2320 – 6020

IJBSTR REVIEW PAPER VOL 1 [ISSUE 8] AUGUST 2013

Speed Control of Doubly-Fed Induction Generator Using PIController Devesh Singh and A.N. Tiwari* ABSTRACT- The success of wind energy is, it's running cost, environmental clean, safety and most important, it is a renewable energy. The Non-linear controllers are used to maximize the system output and optimize the control of wind turbine speed. This paper proposed wind speed estimation based PI-control to extract power from the wind electrical power generating system. Fully controlled wind turbine which contains of induction generator and back-to-back converter. The IG is worked in the vector control mode, where the speed of the Doubly-fed Induction Generator (DFIG) is controlled with respect to the variation of the wind speed in order to produce the enormous output power. The PI- controller is more reliable to control the power extraction, especially in case of regularly varying wind conditions. The simulated system with the PI-control unit of wind turbine keeps the system stability and conforms to the active power to protect the DFIG over speeding and keeps the output power to the maximum power point.

KEYWORDS: power extraction, Doubly fed induction generator (DFIG), PI- Controller (FLC), alternative source of energy, Renewable energy, power generation, wind power generation,

1. INTRODUCTION A consistence effort to increase the generation of electrical power to meet the continuously increasing power demand in world, is a research area. The existing conventional power generation sources have considerable limitations and continually diminishing reserves and adverse impact on the environment. Due to the unfavorable challenges such as conventional energy sources consumption, pollution, global climate change and surety of energy supply, significant efforts have been made to develop renewable energy sources such as wind, solar energy and ocean energy. In modern days’ wind energy is proving itself as a low cost and reliable energy resource around the world. Wind power growth with a 25% yearly rate has experienced the fastest growth among all nonconventional energy sources science five years ago. It is expected that by 2030 up to 15% of the world's electricity will have been supplied by wind power [8].

To obtain the maximum wind power capacity, its decisionmaking variables such as protection of the wind turbine, wind speed modeling, power generation units’ restrictions on the smallest effort, the system load level, the daily running plans should be considered, but also taken into account of the system voltage levels and safety factors after the wind farm connected to the grid. At the same time, wind power as renewable energy, we must consider the random fluctuations and intermittent in wind.

Author: Devesh Singh is currently pursuing master of technology program in in Power Electronics and Drives in MMM. Engg. College , Gorakhpur India.

E-mail: *Co-Author: A. N. Tiwariis currently Associate Professor in MMM. Engg. College , Gorakhpur India.

E-mail:

The electrical power generation system relies on the engineer's ability to provide reliable and continuous service to the consumer’s requirement. The reliability of the power supply implies much higher than merely being available. Ideally, the loads must be fed at constant voltage and frequency at all times. In practical terms this means that both voltage and frequency must be held within close tolerances so that the induction generator is controlled according to the variation of the wind speed in order to obtain the output power [2].

Grid connected, constant speed wind energy systems employing induction generators are popular and they extract optimum power from the wind for a single wind speed.

Variable speed wind energy systems integrated with power electronic interfaces are becoming popular because they can extract optimum power, alleviate the load peak in drive train, smoothly the electrical power output and supply reactive power on demand [12].

In terms of wind power generation technique, as a result of numerous technical profits (higher energy yield, decreasing power oscillations and improving VAR supply) the modern MW-size wind turbines always use variable speed maneuver which is attained by power converters [6] .These converters are usually linked with separable generators and they give considerable to the costs of wind turbines. As a result of largescale wind power generation, interrelating large wind farms to power grids and the relevant effects on the host grids need to be prudently investigated. Wind farms are now required to fulfill with severe connection requirement including [1]: reactive power support, transient recovery, system stability and voltage & frequency regulation.

In this paper we are presented a PI-controlled power point tracking system which is suitable for the induction generator

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 8] AUGUST 2013 operating at varying speeds. The projected system uses the generator angular speed and power output amounts to search for the optimal speed at which the turbine should run for producing maximum power. But, the key shortcomings of wind power are it has not a certain speed. It depends upon the environmental circumstance. Due to variation of wind speed, the generation is changed and the production of wind power is changing according to climatic condition [3]. The PIcontroller is used to improve efficiency and enhance performance.

2. WIND POWER PRODUCTION SYSTEM

power generated is a non-linear function of the turbine angular speed and the wind speed, for a fixed blade pitch angle. With large and abrupt variations in wind speed, it is therefore necessary to extract maximum power from the wind under normal operating conditions. As a result, the generating system is also required to operate at variable speed, giving significant improvements in power efficiency, compared to fixed speed operation [9].

As above, variable speed induction generators are widely used, due to their rugged construction and relatively low maintenance costs. The generator output is interfaced to the power grid through a dual converter system, consisting of a current controlled rectifier and dc to ac inverter. Such a dual converter allows power transfer from the variable speed, variable frequency generator to the constant frequency grid side [14].

This system consists of a wind turbine with doubly-fed induction generator. This is mainly due to the fact that the power electronic converter only has to handle a fraction of the total power. Therefore, the losses in the power electronic converter can be reduced, compared to a system where the converter has to handle the total power. In addition, the cost of the converter is reduced. There exists a variant of the DFIG method that uses controllable external rotor resistances (compare to slip power recovery).

2.1 Converter System

Figure 1: Wind-turbine’s parameter system In the simulation diagram of the wind power production system given in Figure 1, the output of the wind speed model is defined as the kinetic energy or speed of wind. The wind speed is converted to electric power or moment by the wind turbine model. The obtained mechanic power or moment is the first input of the mechanic system (of drive system). The other second input of the mechanic system is the proportional speed of the doubly-fed induction generator.

The inputs of the DFIG are: mechanical energy obtained from the wind, voltage and frequency magnitudes at ends of the network or load. Outputs of the doubly-fed induction generators (DFIG) are the active and reactive power values required for the network or load. In the wind power production systems that operate in isolated manner from electric networks, voltage and frequency of the doubly-fed induction generator can be expressed as output magnitudes.

Mostly, wind energy propels a wind turbine, which in turn drives a generator to generate electrical power. However, in a wind power generation system, the proportion of available wind power converted to mechanical power of the turbine is largely dependent on the blade-tip speed and blade pitch-angle to the wind speed ratio of the turbine [7]. Accordingly, the

A wind power conversion system consists of the constituents wind turbine, gearbox, generator, power-conditioner, transformer and grid connection. The gearbox becomes obsolete for the case of direct drive generators, featuring a large number of poles and a high diameter of the rotor in order to allow for suitable electrical power generation, even for the comparatively low rotation speed of a large wind turbine.

Wind power converters (WPC) can be divided in two main groups defined in terms of rotation speed control: [6] WPCs at constant rotation speed and regulation by stall and WPC with variable rotation speed with adaptable blades of the turbine (pitch controlled WPC). The electrical power from generator is fed into the grid .Variable speed WPCs allow a higher produce but every time require a converter. For that case the rotation speed is determined by the frequency of the generator that is generally accustomed to maximum power output of the WPC. 2.2 Doubly Fed Induction Generator Doubly- fed induction generator where the rotor windings are not short circuited and are connected a back-to-back electronics converter to the machine terminals. To make a variable speed constant frequency system, an induction generator is deliberated attractive due to its flexible rotor speed characteristic with respect to the constant stator frequency. One solution to increase the speed range and

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 8] AUGUST 2013 reduce the slip power losses instantaneously is to doubly excite the stator and rotor windings. The power converters in the rotor circuit regenerate the majority of the slip power. If it is runs the above the stator speed, it will act as the induction generator whereas rotor Speed (Nrotor) and stator speed (Nstator), where, Nstator = Nrotor for the Synchronous speed, Nrotor>Nstator for the Induction Generator and Nrotor<Nstator for the Induction Motor.

must be considered that the pitch angle can be changed up to a limited rate. [3] This rate may be completely low because of rotor blade dimension.

Figure 3: Pitch -angle control To draw maximum power the blade angle must be tuned with the wind straightforward using pitch angle control of wind turbine blades. The maximum change rate for blade gap angle is about 10 degree/s. By means of blade pitch angle control, in speeds of rotor above slow and very low values, no problem may occur with respect to the structure of the wind turbine.

The pitch angle is made constant at zero degree until the speed reaches point D (maximum rate of change of gap angle).

Beyond point D the pitch angle is proportional to the speed deviation from point D speed. The selection of wind speed is based on the rotational speed which is less than the speed at point D.

Fig. 2: Schematic diagram of wind power generation According to [4], the principle of DFIG is to extract the wind energy from the low speed wind by regulating the turbine speed, while reducing mechanical stresses on the turbine during wind gusts. The rotor is running at sub synchronous speed for wind speeds lower than 10 m/s. For the high wind speed it is running at higher than synchronous speed. The second important advantage of the DFIG is the ability for power electronic converters to produce or absorb reactive power, thus it removes the necessity of installing capacitor banks in squirrel-cage induction generator. The DFIG is able to provide a considerable input to grid voltage support during short circuit periods.

3. PI-CONTROLLER: In the above block diagram of the PI-controlled wind-powered electrical power generating system is assumed that the turbine blades have a fixed pitch angle, so that the power output, Pm, varies non-linearly with the turbine's angular speed (ωm) and the wind speed (v). Hence maximum power is extracted at a particular angular speed, for the received wind speed. The induction generator output is a variable voltage, variable frequency ac, which is converted to dc by the current regulated ac to dc converter.

For a variable-speed wind turbine the generator is controlled by power electronic equipment, which makes it possible to control the generator’s speed. In this manner the power oscillations caused by wind variations can be more or less absorbed by changing the generator speed [9] and thus power variations originating from the wind conversion can be reduced.

A field-oriented control scheme is used to regulate the generator speed to the optimum value at which maximum power is obtained. The proposed PI-controller generates the optimum speed command, which is used to regulate the input current of the ac-dc converter. The output of the converter is inverted back to a constant frequency, constant voltage to supply ac loads

2.3 Pitch Angle Control

3.1 The design procedure to obtain optimal PI controllers is an iterative scheme involving the following steps: 1. The objective function may be evaluated for each one individual of the actual speed. In doing so, the PI controller gains may be decoded for each individual and the DFIG model is simulated to obtain the value of the objective function.;

The pitch angle control is made to control wind flow around the turbine blades by controlling the moment spent on the turbine shaft. If the wind speed is lower than the rated speed of wind turbine, pitch angle is constant in its normal value. It

2. The PI operators, namely selection, are used to produce the next PI-step. These two steps are repeated from generation to generation

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 8] AUGUST 2013 until a final speed is obtained. It is represented the computer procedure to be carried out to obtain the optimal PI controller.

Figure 6: Wind speed per vs. time (in second)

Flow chart of PI controller design 4. SIMULATION AND RESULTS PI operation is a process of transferring the control variables to corresponding variables. Selection of the control variables relies on the nature of the system and its desired output. The PI controller input is wind machine speed error and output signals pitch angle are interpreted into a two variables identifying optimum parameters.

Figure 7: Active power ( in yellow colure) and reactive power (in red colure).

In above figure, we see that when speed start to go high then active power (P-pu) goes low and reactive power (Q-pu) goes high which increase the generator speed. We get more power than constant reactive power and active power.

CONCLUSIONS In this paper work, the system development uses MATLAB software for the simulation of wind energy generation system at varying wind speed conditions by using different PI control.

It has been shown that the turbine output depends nonlinearly on its rotor speed and the wind speed. PI control is well suited

Figure 5: Simulation of wind turbine

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 8] AUGUST 2013 9. for the most suited speed at which the turbine should drive under varying wind conditions.

The performance of the proposed scheme is simulated under various changes in wind velocities. It is practical that the wind speed of 15 m/s (Maximum), for which the system produces the effective output power. It is shown that the PI-controller adjusts the angular speed so that the turbine power coefficient converges to its maximum value in the steady state. The algorithm used is simple and shows step by step all the adjustments and calculations necessary for a satisfactory operation of the system.

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