Comparative Study of Control Strategies of AC Voltage Controller

Transcription

1 Comparative Study of Control Strategies of AC Voltage Controller M. Narayanan 1, R. M. Karthika 2, K. Karthika 3, V. K. Aravind 4 and S. Vignesh 5 1 Assistant Professor, Info Institute of Engineering, Coimbatore Abstract The applications such as speed control of induction motor, induction heating, lighting system, etc., needs a simple and a flexible circuit to convert fixed ac to variable ac. With the advent of power electronic devices like SCR, MOSFET, IGBT, this conversion is made more reliable, flexible and cost effective with a circuit named AC voltage controller. The control strategies and the effect of harmonics during the conversion of ac ac needs to be studied. Hence in this paper, the control strategies available to control the output voltage, the presence of harmonics and percentage Total Harmonic Distortion (THD) are studied. Keywords: AC AC voltage controller, Control Strategies, Total Harmonic Distortion. 1. INTRODUCTION 2 5 UG Students, Info Institute of Engineering, Coimbatore Loads such as induction motors, heating furnaces, pumps, blowers, lights, etc. need variable ac supply. The conversion of fixed ac to variable ac can be done by autotransformers. This system suffers from disadvantages like power loss, heating, reduced system efficiency, high cost and occupying more space. Therefore it is required to go for an effective control technique. The advent of power electronic devices have intensified the area of research and made the study of power electronic converters an interesting topic. The main objective of going for these converters is: High efficiency of conversion, compact size, increased operating time and fast dynamic response. AC voltage controller is a power electronic circuit in which fixed ac is converted to variable ac without changing the frequency. The converter circuit consists of SCR as switches and provides variable ac to the load. Figure 1 AC Voltage Controller The control strategies available to control the output voltage are [1]: a) On Off cycle control b) Phase angle control c) Integral cycle switching control 2. ON OFF CYCLE CONTROL Figure. 2 shows the AC voltage controller circuit diagram. The output voltage is controlled by triggering the SCRs such that the entire cycle appears across the load for n number of cycles and load voltage is zero for m number of cycles. Hence in this method of control the shape of the input waveform is not changed but the input voltage does not appear across the load continuously. Therefore in this method the %THD should be less and the drawback being load has to sustain the voltage variations ie., A full supply voltage during the ON period and zero voltage during the OFF period. Volume 3, Issue 11, November 2015 Page 11

2 Figure 2 Circuit diagram Figure 3 Input and output waveforms The expression for rms value of output voltage for resistive load is given by: Vo( rms) Vi( rms) ton To Vs where, ton controller ON time = n x T toff controller OFF time = m x T T Input cycle time period To Output cycle time period To = t ON + t OFF Vs rms supply voltage The expression for duty cycle (k) is given by: ton k ton To ton toff nt ( m n) T ton To The expression for rms value of load current is given by: where, Z Load impedance in ohms Vo( rms) Io( rms) Z Vo( rms) Load Re sis tan ce Volume 3, Issue 11, November 2015 Page 12

3 3. PHASE ANGLE CONTROL Figure 4 Input and output waveforms In this method, the output voltage is controlled by triggering the SCRs T1 and T2. By varying the firing angle the rms value of output voltage is varied [3]. Since the sine wave pattern is getting changed, harmonics will be introduced in the system and hence %THD will get increased. The expression for rms value of output voltage for resistive load is given by: ( ) (sin 2 ) / 2 Vo ( rms) Vs Vm where, Vs = RMS Value of input supply voltage 2 4. INTEGRAL CYCLE SWITCHING CONTROL (ICSC) Figure 5 Output voltage and current waveform Volume 3, Issue 11, November 2015 Page 13

4 5. TOTAL HARMONICDISTORTION Harmonics are frequencies present in the output waveform other than the fundamental. If fundamental is 50Hz, then its integral multiple ie., 100Hz, 150Hz, 200Hz, 250Hz, and so on are said to be the harmonic frequency. The study of the frequencies other than the fundamental is necessary why because it leads to poor power factor, electromagnetic interference, heating of machines and degrade the performance of the machine. Therefore it is important to gauge the total effect of these harmonics. The summation of all harmonics in a system is known as total harmonic distortion (THD) As per the IEEE standards the %THD limit in the voltage waveform is 5% [7]. So designing a system to control the required parameters along with maintain the %THD level with limits helps in obtaining an efficient control and improves the life time of the machine.. This method of control is the combination of on off control and phase angle control. This technique is carried out to reduce the %THD and to improve the load characteristics. Here firing angle can be kept constant and duty cycle can be varied or vise versa [1]. It reduces the disadvantages of the above two techniques. Figure 6 Harmonic spectrum Total harmonic distortion, or THD, is the summation of all harmonic components of the voltage or current waveform compared against the fundamental component of the voltage or current wave [7]. 6. MATLAB SIMULATION Figure 7 Simulation diagram for ON OFF and Phase angle control Volume 3, Issue 11, November 2015 Page 14

5 Figure 8 FFT analysis for duty cycle = 0.2 with R - load Figure 9 RL load output voltage and current waveform Figure 10 FFT analysis for duty cycle = 0.2 with RL - load Volume 3, Issue 11, November 2015 Page 15

6 Figure 11 RL load output voltage and current waveform for firing angle = 30 deg Figure 12 FFT analysis firing angle = 30 deg with RL - load Figure 13 FFT analysis firing angle = 150 deg with RL load Volume 3, Issue 11, November 2015 Page 16

7 Figure 14 Simulation diagram for Integral cycle switching control Figure 15 FFT analysis firing angle = 30 deg and duty cycle = 0.2 with R - load Figure 16 R load output voltage and current waveform for firing angle = 30 deg and duty cycle 0.4 Volume 3, Issue 11, November 2015 Page 17

8 Figure. 7 shows the circuit diagram of ON OFF control and phase angle control method. The simulation diagram is same for both the control but the values given in the pulse generator for controlling the output voltage and obtaining the waveforms is different. Figure. 8, 9 and 10 shows the output voltage waveform, output current waveform and FFT analysis to obtain %THD with R and RL load for ON OFF cycle method of control. Figure. 11, 12 and 13 shows the output voltage waveform, output current waveform and FFT analysis to obtain %THD with R and RL load for Phase angle method of control. Figure. 14, 15 and 16 shows the MATLAB simulation diagram, FFT analysis to obtain %THD and output voltage waveform, output current waveform with R load respectively for Integral cycle switching control. Table 1 : Comparison of ON OFF control and Phase angle control with R load ON OFF Phase angle S. Output control control N voltage α in o rms (V) δ %THD %THD deg where, α - Firing angle in degrees δ - Duty cycle S. N o S. No Table 2 : ON OFF control with RL load Output δ voltage rms %THD (V) Table 3: Phase angle control with RL load Output S. α in deg voltage rms No (V) %THD Table 4: Comparison of Phase angle control and Integral cycle switching control with RL load Integral cycle Phase angle switching control control δ = 0.8 Output voltage rms (V) α in deg %THD Volume 3, Issue 11, November 2015 Page 18 α in deg %THD In Table 1 the %THD is compared for ON OFF control and phase angle control with R load. It is clear that %THD increases as firing angle increases in phase angle control whereas the %THD is constant for any values of duty cycle in the ON OFF control method since the pattern of the sine wave is not disturbed. From Table 2 and 3 it is clear that with RL load the % THD is higher in phase angle control. Table 4 shows the comparison between comparison of Phase angle control and Integral cycle switching control with RL load. For the same rms output voltage the Integral cycle switching control produces %THD less than the Phase angle

9 control method. Here duty cycle is kept constant (δ = 0.8) and firing angle is varied. As the firing angle increases rms output voltage decreases. 7. CONCLUSION The various control strategies of AC voltage controller used to control the output voltage is discussed in this paper. In the ON OFF cycle control even though the %THD is less, the load has to withstand the voltage variations which is not smooth whereas in phase angle control smooth voltage variation can be achieved but %THD is high. So to eliminate the above two drawbacks, the Integral cycle switching control technique can be used in which by setting duty cycle constant and varying firing angle can lead to get both smooth voltage variation and reduced %THD. References [1]. Dharmesh.V.Khakhkhar, Design and Simulation of Novel Integral Switching Cycle Control for Heating Load, International Journal of Emerging Trends in Electrical and Electronics, Vol. 5, Issue. 1, July [2]. Rohit Gupta, Ruchika Lamba, and Subhransu Padhee, Thyristor Based Speed Control Techniques of DC Motor: A Comparative Analysis, International Journal of Scientific and Research Publications, Vol. 2, Issue 6, June [3]. E.S. Oluwasogo, and I.K. Okakwu, Performance Analysis Of A Single-Phase Ac Voltage Controller Under Induction Motor Load, International Journal of Research in Engineering and Technology, Vol. 03, Issue: 06, Jun [4]. Dr. Jamal A. Mohammed, Speed Control of Single Phase Induction Motor Using Micro-Controller, ICIAC-12-13th April [5]. O. Oladepo and G.A. Adegboyega, MATLAB Simulation of Single-Phase SCR Controller for Single Phase Induction Motor, International Journal of Electronic and Electrical Engineering, Vol. 5, Number 2 (2012). [6]. R. P. Akabari, and Hitarth buch, Modeling of split phase induction motor with single phase cycloconverter, Journal of Information, Knowledge and Research in Electrical Engineering, Vol. 02, Issue - 02, Nov 12 To Oct 13. [7]. Total Harmonic Distortion and Effects in Electrical Power Systems, Associated Power Technologies. [8]. Umar Farooq Siddiqui, Ajit Verma, and Shilpa Soni, Comparative Performance Analysis of Induction Motor Using Semiconductor Devices in Terms of Firing Angle, International Journal of Emerging Technology and Advanced Engineering, Vol. 4, Issue 2, February [9]. Devandra Kumar Shukla and Sudhanshu Tripathi, Thyristor controlled power for induction motor, International Journal of Innovative Research and Studies, Vol. 2, Issue 7, July AUTHOR Mr. M. Narayanan, received his B. E. degree in 2008 in Electrical and Electronics Engineering from Karpagam College of Engineering Coimbatore, Anna University- Chennai and M. E. in Power Electronics and Drives from Karpagam University, Coimbatore, Tamil Nadu, India in Presently he is working as Assistant Professor in Electrical and Electronics Engineering Department at Info Institute of Engineering, Coimbatore. His area of interests includes Inverters, Converters and Electrical drives and control. Volume 3, Issue 11, November 2015 Page 19