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Maximum Power and Maximum Power Factor Tracking of Grid Connected Wound Rotor Induction Generators
Haroon Ashfaq, Mohammad Saood Department of Electrical Engineering
Jamia Millia Islamia New Delhi, India [email protected] [email protected]
Ikhlaq Hussain
Department of Electrical Engineering, Indian Institute of Technology Delhi
New Delhi, India [email protected]
M.S.Jamil Asghar
Department of Electrical Engineering, Aligarh Muslim University
Aligarh, U.P, India [email protected]
Abstract—Input voltage control has been used for improvement of power factor in grid-connected induction generators. However, this method reduces the efficiency drastically due to high input current. Moreover the control range is also limited. In the proposed schemes, both voltage control and slip control are employed together to control the efficiency and power factor. Scheme-I is used to tap maximum power from the wind turbines using wound rotor induction generators. It also controls the local bus voltage and maximizes efficiency of the generator. At an optimum voltage, slip control is employed such that the output is maximized at every operating point. The scheme-II is used for maximizing the power factor. An optimum grid voltage is applied to the generator and the effective rotor resistance is varied to operate it at desired slip for the optimum power-factor. Here the power-factor hence the reactive power demand remains nearly constant throughout the range of speed variation.
These schemes are very useful for wind based power generation where wind speed varies abruptly over wide range. For this purpose personal- computer (PC) based ac regulator is employed. The proposed controllers work satisfactorily even at dynamic conditions.
Keywords- Ac regulators; induction generators; peak power point tracking; power factor control; voltage control; voltage rise; wind power __________________________________________________*****_________________________________________________
I. INTRODUCTION
Both deteriorating environmental conditions and depletion of fossil fuels have increased interest in renewable energy generation technologies. A rapid development is taking place in the field of wind energy conversion system (WECS). For this purpose, two types of wind turbines are used, namely fixed speed turbine and variable speed turbines. A fixed speed system is simple and reliable but it is a sub-optimal performance system.
If the fixed speed turbines are directly connected to induction generators, the system suffers from the following disadvantages
1) The wind turbine output power varies as the cube of wind speed, and the wind turbine can deliver maximum output power only at a particular rotor speed as shown in Fig.1.
Therefore a significant amount of power is lost for operation at speeds other than optimum speeds. Thus, it is under-utilization of resources.
2) At different wind speed, the operating point is not generally the maximum efficiency point of the generator.
3) Connection of an embedded generator to a weak grid may result in unacceptably higher magnitude of voltage at the bus to which it is connected.
As compared to fixed speed systems, the variable speed systems improve the dynamic behavior of the turbine, reduce mechanical stresses, improve the power quality and tap a higher amount of power. However, the main disadvantage is the requirement of a costly and complex electrical control system (power electronic controller). For variable speed WECS, the robustness, low maintenance and relative cheapness of induction machines are well known in medium power applications [1-3].For this system a power electronic converter is connected between the stator circuit and the grid
which is designed for the rated power of turbine. However, if the converter is put in the rotor circuit the converter rating is reduced up to half of the system power rating.
Turbine Torque/speed curves for increasing wind speed
Locus of maximum power delivery Lines of constant
power
Turbine rotational speed
Turbine Torque
Fig.1: A family of torque-speed characteristics of generator and wind turbine.
Also with the development in the field of electrical machine designing, slip ring machine ruggedness, the reliability of the wound rotor machine has matched the squirrel cage machine.
Thus, the latter system proves to be more economical in many cases [1].
To extract maximum power from the WECS, various types of power electronic converters have been employed between the generator and the grid. Hansen et al. list some of these topologies [4-6]. A number of schemes are working on these topologies. A comparative study of gear-based schemes was also carried out [1][7-9]. Recently it has been reported that a particular control strategy for variable speed cage machines has enhanced the performance [10-13].
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6 In this work, two control schemes are proposed using both
stator voltage control and rotor slip control methods together.
In the first scheme the output power is maximized by efficiency improvement of induction generator and simultaneous tapping of maximum output power from the prime mover. Also, the voltage rise problem related to weak grids and embedded systems are also dealt with. In the second scheme the power factor of the WECS is optimized using the same converter. A personal computer (PC) based controller has been developed and tested for the system.
This paper consists of six sections. Section 1 describes the brief introduction. Section 2 presents the proposed approach.
Section 3 presents the system description. Section 4 presents the control methods. Section 5 presents the realization and experimental Results. Finally, Section 6 provides some important conclusions that we have drawn from this study.
II. PROPOSED APPROACH
The proposed approach is based on the two improved schemes. Fig. 2 shows the proposed approach implementation steps which uses simulated and experimental results.
Read input data ωt, v, V, R, Im, IR
Applying improvement schemes
Scheme-II Scheme-I
Voltage control
Reduce voltage such that the current become rated for all the wind conditions
Tracked maximum output power
Tracked maximum power factor
Both Maximum power output and maximum power factor tracking of grid connected induction generator
are achieved
Is I≈rated for all wind
Is Generator characteristics shifted to
required speed for the particular
wind
Yes No No
MOPT MPFT
Is Torque requirement Sufficient (such that generator is operated at a torque at maximum power point such that machine
losses are minimum
Resistance control (add rotor resistance)
Yes No
Yes
Fig. 2: Schematic diagram of proposed scheme
Read the input parameters and apply the improvement schemes. In the first scheme, on torque speed curve first match the torque required using voltage control such that the generator is operated at a torque at maximum power point such that the losses are minimized. Then add rotor resistance (Resistance control) such that the generator characteristics are shifted to required speed for the particular wind. In the second scheme, stator voltage is varied such that the current become
rated for all the wind conditions thereby maximum power factor is tracked. Both maximum output power and maximum power factor tracking of grid connected induction generator are achieved.
III. SYSTEM DESCRIPTION
Simple block diagram of the proposed system is shown in Fig.
3. The generator torque mg varies as the square of the angular velocity t of the turbine shaft. It is expressed as
mg = K . t2 (1) For optimum operation, the generator torque is controlled. The optimum generator torque is given by
mgOPT = KOPT . t2 (2) The parameter KOPT is given by
KOPT = 0.5 CpMAX . . A . (R / MAX)3 (3) Cp is power coefficient of the turbine, is air density, R is the radius of the rotor of the turbine, A is the rotor swept area of the turbine and , the tip speed ratio with wind speed is defined as
= (t . R / ) (4) CpMAX and MAX and turbine parameters are generally provided by the manufacturers.
For a particular wind speed, the operating point of the wind turbine i.e., mechanical output power and rotor speed is determined by the intersection point of the prime-mover (turbine) characteristic and the load (induction generator) characteristic. At a particular wind speed, a personal computer based feedback control system is developed to determine the optimum grid voltage to be applied and the rotor resistance to be added. The sampling period of the feedback is so chosen that it is more than the response time of the speed loop (the sampling period is taken five times the speed loop time constant of the system).
Fig. 3: A simple block diagram of the proposed control scheme
IV. CONTROL METHODS
Three control methods are adopted here for controlling the output power, power factor and voltage rise. In this section each of these is described in detail.
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A. Maximum output power tracking (MOPT) system (Scheme-I)
If the stator voltage of an induction motor is controlled, the input power/efficiency of the motor varies [14]. It was reported earlier by the authors that there is always an optimum voltage for every torque, at which the efficiency is maximum [15-17].The same principle is extended here for induction generators to achieve the maximum efficiency at different torques. Hence, there is a minimum power loss point for every speed of the wind turbine as shown in Fig. 1. For an induction machine it can be explained with the help of the vector diagram as shown in Fig.4. The maximum efficiency (minimum losses) of an induction-generator for a given wind torque are found by the following proposed function of the stator current as given by
r 2
122 m
s
(aI ) I a
I
(5) Where,Is = stator current
a = Ratio of the reduced voltage to the rated voltage, Im = magnetizing current at the rated voltage and Ir = reflected rotor current at a given wind torque
From the above equation, the speed-optimum voltage characteristic of the induction generator is simulated. An optimum voltage is found for each wind speed (torque) such that the efficiency becomes maximum. Now, for any wind speed, a pre-calculated optimum input voltage is applied to the generator using an ac regulator such that the efficiency becomes maximum. A pre-calculated external resistance is also added in the rotor circuit corresponding to the speed. Thus the wind turbine delivers maximum output power. Also the input power factor improves throughout the useful speed.
Ir
Is Im
Im
Im
Is
Is
Ir
Ir
(a)
(b)
(c)
a1V
a3V a2V
Fig.4. The stator current variations with decreasing stator voltage
B. Maximum power factor tracking (MPFT) system (Scheme-II)
It was found earlier that there is always an optimum voltage at which the input power factor remains high irrespective of speed variations [18]. It was achieved from the modified characteristics of wound rotor induction motor by applying both stator voltage control and slip control. Again this
principle is extended here for induction generators to achieve the maximum power factor at any speed. Thus an optimum voltage is applied and the external rotor resistance is varied such that the power-factor remains maximum throughout the speed range of the wind turbine. At this reduced voltage the current limits are also not violated heavily. The reduced stator or grid voltage is so chosen that the power factor is made as close to unity as possible. However it is ensured that the stator and the rotor current of induction-generator do not increase beyond a certain limit for every wind speed (i.e.120% of rated current even for highly inductive loads). It is observed that the power-factor remains almost constant throughout the operating range. A constant, low-value capacitive compensation (kVAR) is enough for any wind speed to keep the power factor close to unity. However, the cut-in speed has to be increased marginally due to the reduced input voltage from the grid.
C. Voltage Rise
Connecting an embedded generator to an existing distribution network may lead to several local problems such as steady- state voltage rise, voltage dip at the generator cut-in, etc. This is due to the fact that active voltage regulation is generally not provided beyond the last HV sub-station. A number of methods are employed to limit the steady-state voltage rise.
1) Reduction of Line Impedance:
Line impedance may be reduced either by physically moving the connection point closer to the source substation or increasing the size of existing distribution feeder.
(2) Load management can be done either by connecting a large load, sufficient to cover the rise in the voltage due to increase in the output power of the embedded generator or by properly fast switching a number of small loads. [19][20].
(3) Reduction of active power of the turbine:
In order to maintain satisfactory voltages, the generation (output) may be reduced at critical times, by adjusting the wind turbine set point or disconnecting some wind turbines as desired [21].
(4) Regulating the reactive power of the local bus bar. This can be obtained by use of static VAr compensators, switched capacitor banks etc.
(5) Power factor control of the wind generator; this is achieved here by the use of an ac regulator. In the proposed work (scheme-II) an alternative method is employed. Here the power factor of the induction generator is controlled to its maximum.
V. REALIZATION AND EXPERIMENTAL RESULTS A personal computer based feedback control system has been developed and Fig. 5 shows the complete control scheme. It consisted of the following parts:
1) Synchronizing circuit: It generates positive pulses at zero crossing instants of the grid voltage. A zero crossing detector is used for this purpose.
2) Interfacing system: A suitable interfacing system is connected to receive the input (wind speed) and send output (triggering signals). An A to D converter is used to receive the output voltage of the tachogenerator corresponding to the wind speed.
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8 3) Driver and buffer circuit: To isolate the control circuit from
the power circuit (thyristor pairs of ac regulator), driver and buffer circuits are used.
4) Software: Both assembly language and C language based programming are carried out to generate triggering pulses with proper control strategy.
AC regulator generatorTacho- ZCD
Personal computer ZCD
P.T P.T
3-phase ac supply
α
IG Control
δ
δ
N f
Fig. 5: Complete Control Scheme
Waveforms in Fig.6 show the change of voltage and current with reducing wind speed. Speed –efficiency and Speed – power factor experimental values of proposed system are shown in table-1 and table-2 respectively.
(a) Voltage Waveform
(b) Current waveform
Fig. 6: Waveforms show the change of (a) voltage and (b) current with reducing wind speed. Lower trace shows the change in tacho-generator
voltage due to wind speed.
Both simulated and experimental results are shown together for conventional as well as proposed techniques (Fig. 7 and Fig. 8). The first curve and the second curve are related with the conventional stator voltage control and conventional rotor resistance control respectively. The third curve is related with the proposed method (scheme-I), where both stator voltage and rotor resistance/slip are controlled. It is evident that the efficiency and output power remains high throughout the range of speed variations. Thus maximum output power tracking (MOPT) is achieved. The forth (MPFT) curve clearly shows a high power factor operation throughout the operating range. Also, as the controller is very fast it works satisfactorily in dynamic conditions.
Speed(rpm)
Efficiency (%)
Fig. 7: Speed - Efficiency variations for conventional voltage control, MOPT
& MPFT.
Speed (rpm)
Power factor
Fig. 8: Speed – Power factor variations for conventional voltage control, MOPT & MPFT
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VI. CONCLUSION
Here both stator voltage control and slip control are employed simultaneously to improve the efficiency as well as the power factor of the wind turbine based power generation.
The efficiency and power factor of a grid connected induction generator are greatly enhanced. Two schemes are proposed and verified satisfactorily with practical results. Scheme-I offers better efficiency and better input (line) current with an appreciable improvement in power factor. Scheme-II offers better power factor throughout the operating range of the speed.
In scheme II, the efficiency is better as compared to simple voltage reduction method. Moreover, it is comparable to the efficiency of the conventional rotor resistance control method.
The control schemes are found to be working satisfactorily even under dynamic conditions of wind speed variation. The proposed schemes are useful for small and medium power embedded applications because of their simplicity and low cost.
APPENDIX Wound Rotor Induction Machine Data:
1.5kW, 415V, 50Hz, six poles, three phases;
Stator: 415V, star connected, 3.5A;
Rotor: 120V, star connected, 7.2A;
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