Power Electronics Based Voltage and Frequency Controller Feeding Fixed Loads for Application in Stand-Alone Wind Energy Conversion System

Power Electronics Based Voltage and Frequency Controller Feeding Fixed Loads for Application in Stand-Alone Wind Energy Conversion System

Bikram Das1, Naireeta Deb2

1. Electrical Engineering Department, NIT, Agartala.

   Prabir Ranjan Kasari1, Abanishwar Chakraborti1

2. Student M.Tech (Power electronics & Drives) Electrical Engineering Department,

   NIT, Agartala.

   Abstract

   It deals with a power electronic controller which controls the voltage and frequency of an isolated asynchronous generator feeding consumer loads. The circuit has bi-directional power flow capability in order to control active and reactive power thus controlling the voltage and frequency of the system. The terminal voltage, value of excitation capacitor, s peed and generated power of the generator are considered constant under all operating conditions. Proposed controller consists of a 3 leg uncontrolled bridge rectifier, which acts as a low cost voltage regulator and the output of the rectifier is passed through the filter capacitor and is fed to a 3-phase PWM inverter. S PWM signals have been generated by switching pulse generator for the three phase inverter which provides the function of a harmonic eliminator and load balancer. The complete system is modeled and simulated in MATLAB using the S IMULINK AND PS B (Power S ystem Blockset) Toolboxes. The simulated results are presented to demonstrate the capability of an isolated generating system driven by a wind turbine feeding three phase loads.

   Keywords- Isolated asynchronous generator, uncontrolled bridge rectifier, voltage and frequency controller (VFC), switching pulse generator.

   1. Introductions

An increasing rate of depletion of conventional sources of energy and growing power demand has diverted attention of scientists towards nonconventional sources of energy such as wind and solar energy. Induction generators have been found to be very suitable for wind energy conversion. These may be operated in grid- connected or self-e xc ited mode [5]. A Standalone wind energy conversion system (WECS) is useful for powering small villages located far from the grid. It is also well known that an externally driven asynchronous machine can sustain self e xc itation when an appropriate value of capacitor bank known as e xcitation capacitor is connected across its stator terminals. In case of constant speed application, the Isolated Asynchronous Generator (IA G) operates at practically constant speed. In variable speed operation, IAG needs an interface to convert the variable voltage output of the generator to the fixed voltage at the load terminals. Moreover brushless constructions with squirrel cage rotor, reduced size, absence of DC e xcitation reduces the ma intenance and improves the transient performance. Thus asynchronous generator has emerged as ma in candidate to supply energy using non-conventional resources like wind and

hydro power potential. This specific paper emphasizing on

wi nd energy conversion-system.

However even with large number of advantages the main disadvantage of synchronous generator are poor regulation of voltage under varying load condition. This is the ma in barrier in its effective operation. In this project work an attempt has been made to ma ke a voltage controller with power e lectronic equipment. The whole controller feeding from a wind turbine whose torque has been calculated and fed into the asynchronous machine. The induction machine stators are further connected to the rectifier-filter-inverter bridge via a delta connected excitation capacitor bank. This allows the controller to produce a constant power to the grid. All simulations are performed in MATLAB using SIM ULINK toolbox and power system block set.

1. System Configurations and principle of operation

   Fig -1shows the schematic d iagra m of the proposed system with the imag inary wind turbine, the asynchronous machine, the e xcitation capacitor, the proposed power electronic controller and the consumer loads. A delta connected [1] e xcitation capacitor is used to generate the rated voltage at no load. The stator termina ls of the IM are connected [4] to the power e lectronic controller. The controller consists of a three phase diode bridge rectifier connected to an inverter via a filter capacitor of 1mF. The inverter consists of three legs each containing one pair of IGBTs. With the use of diode rect ifier to generate DC voltage we can a im to cut down its cost. However the pulse width modulated switch of the inverter gives a precise switching.

   Figure 1. Complete arrangement of the system.

   W ith this configuration an attempt has been made to simu late the control algorith m of the wind power generator scheme. The proposed controller has bi-direct ional power flow capability of react ive and active powers [1]. So it controls the magnitude of the voltage under various wind speed condition.

2. Wind turbine Characteristics

   In wind parks, many wind turbines are equipped with fixed frequency induction generators. Thus the power generated is not optimized for a ll wind speed conditions [5].To operate a wind turbine in its optimu m condition [3] at diffe rent wind speeds, the wind turbine should be operated at its ma ximu m powe r coeffic ient. To operate around its ma ximu m power coefficient, the wind turbine should be operated at a constant tip-speed ratio, which is proportional to ratio of the rotor speed to the wind speed. As the wind speed increases, the rotor speed should follow the variation of the wind speed. In general, the load to the wind turbine is regulated as a cube function of the rotor RPM to operate the wind turbine at the optimu m e fficiency.

   The wind turbine output power is given by,

   Rotating shaft: Horizontal

   Stress way of blades: Resistance Rotor-blade -d ia meter:8.0m Startup-wind-speed:3.0m/s Rated-wind speed:8.0m/s

   Rated-output-power:10000W Maximu m output =15000W Pole Height:14m

   Generator we ight:150kg Pole dia meter: 360mm

   Generator specifications:

   Three phase Power= 15000kW

   Pm=½ R2 V 3C

   (1)

   AC voltage =400V

   Where

   w p Frequency =50Hz

   Pm- Mechanical output power of the turbine (W)

   Cp- Pe rformance coefficient of the turbine – Air density (kg/ m3)

   R – Turb ine rotor rad ius (m)

   Vw – wind speed (m/s)

   The tip speed ratio of the turbine blades is given by

   = R (2)

   Where

   -rotational speed of the wind turbine. The wind turbine model used here is represented as a fa mily of turbine powe r- speed curves are shown in fig .2.

   Figure 2. Wind turbine speed versus turbine output power characteristics.

3. Selection of wind turbine and generator parameters

   For this project work a 15kW wind energy conversion system with the following specifications is used

   Turbine specifications:

   Nu mber of b lades: 3

   Pole pairs= 2

   Type= squirrel cage induction machine Speed =1460 rp m

   Parameters:

   • Stator

     Resistance (Rs) = 0.2147 Inductance (Ls) = 0.000991H

   • Rotor

     Resistance (Rr) =0.2205 Inductance (L r) =0.000991H

     Mutual inductance (Lm) =0.06419H Loses, Inertia (J) = 0.102Kg m2 Friction = 0.009541 N-m-s

4. Calculations

   Parameter of the machine

   Slip= (Ns-Nr)/ Ns

   For a 4 pole mach ine Ns = (120*50)/4=1500rp m Slip= (1500-1460)/1500 = 2.67%

   Leakage reactance, Xr*= 2fLr=0.311H

   Stator current (Is)= (SXEr*)/(((Rr)2+(SXE r*)2)) Where

   Er*= e mf induced/phase when the rotr is stand-still

   Is=(0.0267* 400)/ (3*((0.2205)2+(0.0267*0.0311)2))

   =27.95A

   Power (P) = 3VsIscos1

   =>15000= 3*400* 27.95*cos1

   => Cos1= 0.774

   =>1= 39.22

   =>tan1=0.816

   Power/phase Pp =15000/ 3 =5000W Desired p.f =0.8 =cos2

   =>2=36.86

   =>tan 2=0.75

   Qcp=Pp(tan 1-tan 2) [6]

   =5000(0.816-0.75) KVa r

   =330KVar/phase

   Calculations of the value of capacitance ( Connection) [6]

   Vp = VL=400V Qc=VPIC

   IC=QC/ VP=(330* 1000)/400 =825A/phase

   Capacitance /phase

   C= IC/ VP=825/(2*50* 400)F=6.5mF

   Speed of wind turbine rotor:

   R=8/2m= 4m

   Speed of wind =8m/s (standard value) No. of b lades= 3

   Tip speed ratio for optimu m output 0 = 4/n =4.188

   =>4.188 = (R)/0

   =>= 8.376 (at standard temp)

   Torque calculation [8]

   At 25C speed of wind = 19m/s Therefore,

   Pmax= 1/2(A)Vi3

   =1/2* 1.225*(50.265)* 193

   =211170W

   As, CTmax =CPmax/=0.593/4.188=0.1415

   Tm = (Pmax/0)*R

   = (211170/ 12)*4

   =70390N

   TSmax= Tm * CTmax

   = (70390* 0.1415)

   =9960N

   =9.96KN

5. Control Strategy

   The induction machine driven by wind turbine is controlled to get fixed output powers under varying wind speed conditions. The stator voltage [4] is fed to the power electronics converter. The stator is however connected in paralle l to delta connected capacitor bank. The value of capacitor bank is calculated so as to generate power at no – load. These are called e xc itation capacitor.

   However a source inductance of small value (1mH) is placed at the input of the uncontrolled rectifie r [7]. This will thus act as source inductance of the uncontrolled rectifier. The presence of source inductance thus has significant effect on the performance of the converter. With the source inductance present the output voltage of a converter does not remain constant for a given firing angle . Instead it d rops with load current. When there would have been no source inductance the diode pair stops conducting. But with the source inductance present the four diodes /two legs continue to conduct for some interval known as overlap interval [2].

   The use of diode rectifier significantly reduces the cost and is ideal for low / med iu m voltage applicat ion.

   The ripple DC voltage is fed through 1mF capacitor and we get stiff DC. This is fed to the inverter input. The switching pulse required for the diffe rent devices of the inverter has been shown by using pulse generator. Co mp lete MATLAB simu lation circuit for the switching pulse generation is shown in figure 3 belo w.

   Figure 3. Simulink model for switching pulse generation for 3 phase inverter

6. Simulation results

   The schematic diagra m shown here in fig. 4 is done in MATLAB using SIM ULINK toolbo xes for vary ing wind speed conditions. It consists of the power electronic controller which controls the voltage of an isolated asynchronous generator feeding consumer loads. Resistive, inductive and capacitive loads are connected across the output.

   Figure 4. Simulink model for power electronics based voltage and frequency controller feeding fixed loads for application in WECS.

   In fig.4 the co mplete wind turbine is rep laced by the actual torque of the turbine which is 9.96Kilo Newton as calculated for a standard wind turbine.

   The three phase uncontrolled (d iode) rect ifier configuration can handle reasonably high power and has

   acceptable input and output harmonic distortion. The configuration also lends itself to easy series and parallel connection for increasing voltage and current rating or improve ment in harmonic behavior. The fig. 5 shows the input voltage and fig.6 & fig.7 shows the output voltage waveform of the rectifie r without filter e le ment and with filtering ele ment respectively.

   Figure 5. Rectifier input voltage waveform.

   Figure 6. Rectifier output voltage without filter capacitor.

   Figure 7. Rectifier output voltage after filtering.

   Fro m fig.7 we can see that the dc voltage has a bit of ripple at starting that becomes almost stiff with a ripple of 30volts wh ich is acceptable in this case.

   The output PWM signals obtained from the pulse generator is fed to the control terminal of the IGBTs so that the devices

   are turned on at the desired instant to get suitable output from the inverter. The six switching pulses obtained for six devices of the inverter are as shown in figure 8. and the simulink model was as shown in fig.3.

   Figure 8. Six switching pulses for six devices of the inverter

   The inverter consists of 3 IGBT pa irs whose output is controlled by switching pulses. The output current for A- phase and voltage for each phases are as shown in fig.9 and fig.10 respectively.

   Figure 9. Output current waveform for phase A

   Figure 10. Output voltage waveforms for the 3 phases

7. Observation of the waveforms

   Here we have seen the input voltage waveform, rect ifier output dc voltage, switching pulse wave forms, controlled ac voltage and current waveforms of the comp lete system. The result shows that the input voltage is near to 550 volts which is far mo re than prescribed limit. At the output of the inverter we

   can see from the simulated results as in fig.10 that the output voltage is approximate ly 380volts that is very much clos e to 400volts which was the rated output. The time duration of each cycle seems to be 0.1second. Thus the frequency becomes 10Hz (f=1/T). It is fa r be low than the prescribed limit . Also fro m the simu lated results of the voltage as in fig.10 we can see that the time durat ion for each cycle is nearly 0.02 sec. Thus the frequency becomes 50Hz. So this controller controls the reactive power in order to control the frequency.

8. Conclusions

   In this paper controlling the terminal voltage and frequency of an isolated wind turbine generator has been presented. The power e lectronic controller here p roduces a new approach towards the controlling of power generated by IAG (Isolated asynchronous generator). The proposed bi- directional controller controls the active as well as reactive power thus controlling the voltage and frequency of the system. Use of uncontrolled rectifier has reduced the cost significantly. The simulation results demonstrate the capability of VFC (voltage and frequency control) fo r powe r quality improve ment.

9. References

1. Voltage and Frequency Control of Isolated Asynchronous Generator with Reduced Switch Integrated Voltage Source Converter in Isolated Wind Power Generation (Sharma &

   B. Singh, Electrical Engineering Dept. IIT, De lhi).

2. Design and Imple mentation of Power Converters for Wind Energy Conversion System(A. Chih-Ch iang Hua, B. Chien- Hung Cheng,Depart ment of Electrical Engineering National Yun lin Un iversity of Science & Technology, Douliou, Yunlin, Taiwan, ROC).

3. Modeling and Simu lation of Optima l Wind Turb ine Configurat ions in Wind Farms (Feng Wang, Deyou Liu, LihuaZeng College of Water Conservancy and Hydropower Engineering. Hohai Un iversity, Nan jing, Ch ina).

4. Output Vo ltage Control of PWM Inverters for Stand – Alone Wind Power Generation Systems Using Feedback Linearizat ion (Dong-Choon Lee Dept. of Electrical Engineering Yeungnam Un iversity 214-1, Daedong, Gyeongsan, Gyeongbuk, Korea ,Jeong-Ik Jang Dept. of Electrical Engineering Yeungnam Unive rsity, Daedong, Gyeongsan,Gyeongbuk, Korea).

5. Voltage and Frequency Control of Independent Wind Power Generating System (K.Pre ma latha, Da liya.V V, and S.Vasantharathna, Member IEEE).

6. Electrica l power system- Ashfaq Hussain

7. Powe r Electronics Ned Mohan (University of Minnesota), Tore M. Undeland (Norweg ian Institute Of technology), Willia m P.Robbins .

8. Electrica l Machines -S.K Sen.