Design of SVPWM Based Inverter for Mitigation of Harmonics in Power System

Design of SVPWM Based Inverter for Mitigation of Harmonics in Power System 1 Leena N C, 2 B. Rajesh Kamath, 3 Shri Harsha 1,2,3 Department of EEE, Sri Siddhartha Institute of Technology, Tumkur-572105, Karnataka, India Abstract-In order to improve the quality of the active power filter different control algorithms have been reported such as proportional integral (PI) control, deadbeat control, and hysteresis control. Due to the restriction of the control bandwidth, the PI controller is not an appropriate solution for the APF applications as the current controller should deal with harmonic currents, which are high frequency signals. In contrast, the deadbeat controller is capable of giving fast control response, but the control performance depends extensively on knowledge of the APF parameters. In spite of the simple and robust feature of the hysteresis control, this method also has an intrinsic drawback of switching frequency variation, which causes a difficulty in design of ripple filter for the APF and results in redundant resonance problems with the system. Additionally, in order to attain superior current control, the hysteresis band limit has to be set as small as possible. It results in a major increase of the switching frequency and as a result introduces huge switching loss on the APF.Space vector pulse width modulation (SVPWM) has been extensively utilized in the three-phase voltage source inverters (VSI) for the benefit of fixed switching frequency, full utilization of DC bus voltage and superior control. In recent times, SVPWM technique was applied for active power filter (APF) control application, as the APF is nothing but of a current controlled VSI. Index terms- Voltage source inverter, SVPWM, active power filter I. INTRODUCTION The exponential growth in nonlinear loads has generated a prime concern in the power supply systems. Power electronics based applications draw non-sinusoidal currents, although the applied voltage being sinusoidal. Because of the non-ideal characteristics of voltage source, harmonic currents create voltage distortion. Various nonlinear loads such as arc furnaces, cyclo converters, rectifiers, variable speed drives and other asymmetrical loads can cause huge disturbances in the power supply system. In order to retain harmonic disturbances at reasonable levels, to comply with present standards, we can go through various solutions applicable to supply systems and to harmonics sources. Conventional solutions like passive filters (PF) for mitigating the harmonic pollution are ineffective due to fixed compensation, large size, and resonance. Active Power Filter (APF) is used to improve the power quality. The SVPWM technique is also gaining importance in APF control. However, the computational burden involved due to complex trigonometric calculations and sector identification limits the application of SVPWM technique for APF application. An improved SVPWM technique with effective time concept has been developed to overcome the above drawback in induction motor drive applications. This effective time concept in the improved SVPWM technique is able to overcome the disadvantages of complex trigonometric calculations and sector identification and it finds a useful application in APF control. II. BASIC PRINCIPLE OF POWER COMPENSATION IN TRANSMISSION SYSTEM Figure 2.1(a) shows the simplified model of a power transmission system. Two power grids are connected by a transmission line which is assumed lossless and represented by the reactance X L. V1 δ and V2 δ represent the voltage phasors of the two power grid buses with angle δ= δ 1 - δ 2 between the two. The corresponding phasor diagram is shown in Figure 2.1(b). Fig 2 : Power transmission system: (a) simplified model; (b) phase diagram The magnitude of the current in the transmission line is given by: 1.1 The active and reactive components of the current flow at bus 1 are given by: 1.2 63

The active power and reactive power at bus 1 are given by: 1.3 Similarly, the active and reactive components of the current flow at bus 2 can be given by: 1.4 The active power and reactive power at bus 2 are given by: Equations (1-1) through (1-5) indicate that the active and reactive power/current flow can be regulated by controlling the voltages, phase angles and line impedance of the transmission system. From the power angle curve shown in Figure 3.1(c), the active power flow will reach the maximum when the phase angle δ is 90º. In practice, a small angle is used to keep the system stable from the transient and dynamic oscillations. Generally, the compensation of transmission systems can be divided into two main groups: shunt and series compensation. 2.1 SERIES COMPENSATION 1.5 Series compensation aims to directly control the overall series line impedance of the transmission line. Tracking back to Equations (1-1) through (1-5), the AC power transmission is primarily limited by the series reactive impedance of the transmission line. A series-connected can add a voltage in opposition to the transmission line voltage drop, therefore reducing the series line impedance. A simplified model of a transmission system with series compensation is shown in Figure 2.1(a). The voltage magnitudes of the two buses are assumed equal as V, and the phase angle between them is δ. The transmission line is assumed lossless and represented by the reactance X L. A controlled capacitor is series-connected in the transmission line with voltage addition V inj. The phase diagram is shown in Figure 2.1(b) Fig 2.1 Transmission system with series compensation: (a) simplified model; (b) phase diagram; (c) power-angle curve Defining the capacitance of C as a portion of the line reactance, X C = k X L The overall series inductance of the transmission line is, X = X L - X C = (1 K)X L The active power transmitted is, The reactive power supplied by the capacitor is calculated as: In Figure 3.1(c) shows the power angle curve from which it can be seen that the transmitted active power increases with k. 2.2 SHUNT COMPENSATION Shunt compensation, especially shunt reactive compensation has been widely used in transmission system to regulate the voltage magnitude, improve the voltage quality, and enhance the system stability [5]. Shunt-connected reactors are used to reduce the line over-voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levels by compensating the reactive power to transmission line. 64

Fig 2.2 Transmission system with shunt compensation: (a) simplified model; (b) phase diagram; (c) power-angle curve A simplified model of a transmission system with shunt compensation is shown in Figure 3.2(a). The voltage magnitudes of the two buses are assumed equal as V, and the phase angle between them is δ. The transmission line is assumed lossless and represented by the reactance X L. At the midpoint of the transmission line, a controlled capacitor C is shunt-connected. The voltage magnitude at the connection point is maintained as V. As discussed previously, the active powers at bus 1 and bus 2 are equal. The injected reactive power by the capacitor to regulate the voltage at the mid-point of the transmission line is calculated as: From the power angle curve shown in Figure 3.2(c), the transmitted power can be significantly increased, and the peak point shifts from δ=90º to δ=180º. The operation margin and the system stability are increased by the shunt compensation. The voltage support function of the midpoint compensation can easily be extended to the voltage support at the end of the radial transmission. The reactive power compensation at the end of the radial line is especially effective in enhancing voltage stability. III. SIMULATION RESULTS 3.1 THREE PHASE INVERTER WITH PWM Fig 3.2 Pulse generated in PWM three phase inverter Three phase inverter with PWM is and simulated using MATLAB/SIMULINK as shown in above figure 3.4.2. Here the input voltage used in DC source of voltage Vs=100v. The load resistance R=1Ω. The simulation time used is 50μs. The design of gating pulse is as given below Carrier frequency=1080hz The output frequency of output voltage=50hz Time period corresponds to the frequency=1/50=0.02sec The gate pulses for switches Q1 and Q4 is considering Phase delay=0 Similarly the gate pulses for switches Q3 and Q6 is Phase delay=120 (0.02/3) Similarly the gate pulses for switches Q5 and Q2 is Phase delay=-120 (-0.02/3) Fig 3.1 Three Phase inverter with PWM Fig 3.3 simulation result of three phase inverter with PWM 65

Fig shows the three phase voltage source components V ab, V bc V ca and I ab in which X axis represents time and Y axis represents voltage and current respectively. IV. THREE PHASE INVERTER WITH SVPWM The RMS output voltage obtained during simulation is 66.02V Fig 3.4: THD of the line voltage for nonlinear load Fig 4.1 Three phase inverter with SVPWM Fig 3.5: THD of the line voltage for nonlinear load Fig 3.6: THD of the line voltage for nonlinear load Fig 3.7: THD of the line current for nonlinear load Fig 4.2 Pulses of Three phase inverter with SVPWM Three phase inverter with spwm is and simulated using MATLAB/SIMULINK as shown in above figure 3.4.2. Here the input voltage used in DC source of voltage Vs=100v. The load resistance R=1Ω. The simulation time used is 50μs. The design of gating pulse is as given below Carrier frequency=1080 Hz The output frequency of output voltage=50 Hz Time period corresponds to the frequency=1/50=0.02sec The gate pulses for switches Q1 and Q4 is considering Phase delay=0 Similarly the gate pulses for switches Q3 and Q6 is 66

Phase delay=120 (0.02/3) Similarly the gate pulses for switches Q5 and Q2 is Phase delay=-120 (-0.02/3) Fig 5.6: THD of the line voltage for nonlinear load Fig 4.3 Simulation of three phase inverter with SVPWM Fig 4.4: THD of the line voltage for nonlinear load Fig 4.7: THD of the line current for nonlinear load V. AC SYSTEM MODELING AC system is modeled as a simple three phase AC source with internal resistance and inductance that is calculated from short circuit level MVA calculations. (MVA) B = 2000MVA (kv) B = 400 kv (Phase to Phase rms) X R = 10; f=50hz. Source Inductance L = V base 2 MVA base 2πf or V base 2 SC power in VA X 2πf 400 L = 400 X 2000 X 2π X 50 = 0.2546H X=0.2546 X 2π X 50 = 80Ω R = X 10 = 8Ω Fig 4.5: THD of the line voltage for nonlinear load 5.1 TRANSFORMER DESIGN Y grounded Δ is used to permit the optimal voltage transformation. It also blocks the triplen harmonics produced by the converter. The following data for the transformer is considered: Nominal Power =315MVA (total for three phases) Nominal frequency=50hz. 67

Winding1 specifications: Y connected, nominal voltage = 400kV rms (Line to Line) X 0.915 (to simulate a fixed tap ratio) =366kV Resistance = 0.0025pu, Leakage reactance = 0.0075pu Winding 2 specifications: Δ connected, nominal voltage = 225kV rms(line to Line), Resistance = 0.0025pu, Leakage reactance = 0.075pu Magnetizing losses at nominal voltage in % of nominal current: Resistive 5%(=500pu), Inductive 5%(500pu) Calculations for winding1: Base power=315mva/3 = 105.33 MVA/phase. Base voltage = 366k = 211.3kV rms 3 Base MVA Base current = = 105.33X10^6 V base 211.3X10 3 = 498.46A (rms) Base impedance = V base I base = 211.3kV 498.46A = 423.92Ω Base Resistance = 211.3kV 498.46A = 423.92Ω Base impedance Base Inductance = 2πf = 1.349H = 423.92 2πX50 Hence, winding1 resistance R1= 0.0025*423.92=1.059 Winding1 inductance = 0.075X1.349 = 0.2546H Calculations for winding2: Base power=315mva/3 = 105.33 MVA/phase. Base voltage = 225kV rms Base MVA Base current = = 105.3X10^6 V base 225X10 3 = 468.139A (rms) Base impedance = V base I base = 480.6Ω Base resistance=base reactance=base impedance=480.6ω Base impedance Base Inductance = 2πf = 1.5297H = 480.6 2πX50 Hence, winding2 resistance R1=0.0025 X 480.6=1.2015Ω Winding2 inductance L 2 = 0.075X0.1.529= 0.1147H Magnetising Resistance(Rm) and Inductance(Lm): Rm=500pu X R base on winding1 =500 X 480.6 = 211.96e3Ω Lm = 500pu X L base on winding1 =500 X 1.349 = 674.5Hz VI. TRANSMISSION LINE PARAMETER The line considered was of double circuit with standard vertical configuration, both circuits on the same tower. The same line configuration was used irrespective of the type of conductor for line loading performance comparison purpose. Table shows the line resistance, inductive reactance per km of the line length. SI NO Conductor configurati on 1 Twin 2 Twin AAAC 3 Quad 4 Quad AAAC 5 Triple 6 Quad Bersimis Resistance (Ω/km) Inductive reactance (Ω/km) 0.028713 0.31337 3.698 0.029490 0.31319 3.7 0.014655 0.25107 4.625 0.015048 0.25090 4.627 0.01934 0.2753 4.211 Susceptance (micromho/km) 0.011415 0.24960 4.6546 VII. SIMULATION MODEL OF SHUNT ACTIVE FILTER Fig 7: Model of shunt active filter Based on a voltage-sourced converter, the shunt active filter regulates system voltage by absorbing or generating reactive power. Shunt active filter output current (inductive or capacitive) can be controlled independent of the AC system voltage. The power grid consists of two 400-kV equivalents (respectively 2000 MVA and 2000 MVA) connected by a 300-km transmission line. When the shunt active filter is not in operation, the "natural" power flow on the transmission line is 930 MW from bus B1 to B3. In our model, the shunt active filter is located at the midpoint of the line (bus B2) and has a rating of +/- 100MVA. Shunt active filter having a DC link nominal voltage of 68

40 kv with an equivalent capacitance of 375 uf. On the AC side, its total equivalent impedance is 0.22 pu on 100 MVA. This impedance represents the transformer leakage reactance and the phase reactor of the IGBT Bridge of an actual PWM shunt active filter. 7.1 SIMULATION RESULTS AND DISCUSSION 7.1(a) Dynamic response of the shunt active filter Fig 7.1(a): Dynamic Response of the Shunt Active Filter Voltage Fig 7.1(b): Change in Reactive Power For Voltage regulation with External control of reference voltage Vref is used. Also, the droop parameter should be set to 0.03 and the Vac Regulator Gains to 5 (proportional gain Kp) and 1000 (integral gain Ki). In the Step Vref block (the red timer block connected to the "Vref" input of the shunt active filter). This block should be programmed to modify the reference voltage Vref as follows: Initially Vref is set to 1 pu; at t=0.2 s, Vref is decreased to 0.98 pu; then at t=0.4 s, Vref is increased to 1.02; and finally at 0.6 s, Vref is set back to 1 pu. Also, make sure that the fault breakers at bus B1 will not operate during the simulation. VIII. CONCLUSION The work presents a novel full IGBT based voltage source converter of shunt active filter. An important aspect considered in the design is the control system. The control strategy for the shunt active filter is the reactive power control. The simulation of shunt active filter in this project has verified that it can be efficiently applied in power transmission systems to solve the problems of poor dynamic performance and voltage regulation in the power transmission system. REFERENCES [1] H. Akagi, E. H. Watanabe, and M. Aredes, Instantaneous Power Theory and Applications to Power Conditioning, M. E. El-Hawari, Ed. New York: Wiley, 2007. [2] Recommended Practice for Harmonic Control in Electric Power Systems, IEEE Std. 519-1992, 1992. [3] Limits for Harmonic Current Emission, IEC 61000-3-2, 2001. [4] H. Akagi, New trends in active filters for power conditioning, IEEE Trans. Ind. Appl., vol. 32, no. 2, pp. 1312 1332, Nov./Dec. 1996. [5] F. Z. Peng, Application issues of active power filters, IEEE Ind. Appl.Mag., vol. 4, no. 5, pp. 21---30, Sep./Oct. 1998. [6] Moleykutty George and Kartik Prasad Basu, Three-Phase Shunt Active Power Filter American Journal of Applied Sciences 5 (8): 909-916, 2008 [7] J. Chelladurai, G. SaravanaIlango, C. Nagamani, and S. Senthil Kumar, Investigation of Various PWM Techniques for Shunt Active Filter, International Journal of Electrical Systems Science and Engineering Volume 1 Number 2. 69