90 CHAPTER 5 CONTROL SYSTEM DESIGN FOR UPFC 5.1 INTRODUCTION This chapter deals with the performance comparison between a closed loop and open loop UPFC system on the aspects of power quality. The UPFC has been modeled and simulated for closed loop and open loop systems using Matlab/Simulink. To ascertain the performance characteristics of these systems, voltage sag and swell have been simulated. 5.2 OPEN LOOP AND CLOSED LOOP UPFC SYSTEMS The conventional rectifier-inverter based UPFC system discussed in chapter 2, can be implemented as open loop or closed loop system. The control algorithms for these systems are detailed in the following sections. 5.2.1 Control Algorithm of Open Loop UPFC System The control algorithm of open loop system is based on the active power filter reference current calculation method. In the UPFC system without shunt compensation, the line current consists of active and reactive components (neglecting the dc and harmonic components) as in Equation (5.1). (5.1)
91 Where, i p (t)- in phase line active current of the transmission line i q (t)- reactive current of the transmission line To regulate the voltage at bus connected to the shunt converter of the UPFC, the only component that this bus should supply is the active current component. Using Equation (5.1), it can be noted that if the shunt converter of the UPFC supplies the reactive component, then the sending bus needs only to supply the active component. This can easily accomplished by subtracting the active current component from the measured line current. (5.2) In Equation (5.2), I p is the magnitude of the in-phase current (to be estimated) and sin( t) is a sinusoid in phase with the line voltage. The circuit shown in Figure 5.1 can accomplish this operation. Figure 5.1Open-loop system for calculating the UPFC shunt injected current (5.3) After the multiplication, the only dc term in Eqn. (5.3) is proportional to I p. Thus, a low-pass filter whose cut off frequency is below permits to obtain I p which is an estimation of the magnitude of i p (t). Then, this dc value is multiplied by the same in-phase sinusoid, obtaining an estimation of the instantaneous active current i p (t). Finally, this value of i p (t) is
92 subtracted from the measured line current obtaining the reactive current i q (t) injected to the power system. 5.2.2 Drawbacks of Open Loop UPFC System The UPFC has to be switched ON or OFF manually after monitoring the transmission line parameters. Hence it lacks dynamic response. It takes more time for compensation as the observations have to be done and the firing angle has to be varied manually in line with the changes in the transmission line parameters. 5.2.3 Control Algorithm for Closed Loop UPFC System Figure 5.2 Closed-loop modified systems for UPFC shunt injected current The Figure 5.2 shows the closed loop control algorithm for the UPFC system. The reactive component of the current i q (t) is multiplied with instantaneous voltage v(t) and the resultant is passed through the low pass filter to get i p. The output of low pass filter is integrated with an integral constant, so as to get i p (t). The difference of the real component i p (t) and instantaneous current i(t) gives the resultant reactive component i q (t) required for the UPFC system, to inject the shunt current. Similarly for series control the following algorithm, as in Fig 5.3, is used. The Figure 5.2 shows the closed loop control algorithm for the UPFC
93 system. The reactive component of the voltage Vq(t) is multiplied with instantaneous current i(t) and the resultant is passed through the low pass filter to get v p. The output of low pass filter is integrated with an integral constant, so as to get v p (t). The difference of the real component v p (t) and instantaneous voltage v(t) gives the resultant reactive component v q (t) required for the UPFC system, to inject the series voltage. Figure 5.3 Closed-loop system for UPFC series injected voltage 5.3 SYSTEM CONFIGURATION AND CONTROL Figure 5.4 shows the system configuration of a single phase UPFC and its control system. The single phase transmission line model is fed from a single phase AC supply. The voltage and current signals are taken from the mid-point of the transmission line. These signals are the input of the control system, shown in Figure 5.6. The signals from the control system are sent to the PWM subsystem shown in Figure 5.7. The outputs of this PWM subsystem are the gate signals of the eight IGBTs, constructing shunt and series converters. Each signal switches ON two IGBTs at the same time. The output of the shunt converter is the input of shunt transformer, which injects reactive current to the mid-point of the system. The output of series converter is the input of series transformer which adds a series injected voltage to the mid-point voltage.
Figure 5.4 Power system with UPFC closed loop configuration 94
Figure 5.5Converter and Inverter circuit 95
96 Figure 5.6 Control Algorithms for UPFC Figure5.7 PWM sub system
97 5.4 SIMULATION RESULTS AND ANALYSIS (a)voltage across load 1 and Load 2 (b)real and Reactive Power Figure 5.8 Uncompensated System
98 (a)voltage across Load 1 and Load 2 (b) Real and Reactive Power Figure 5.9 Compensated System
99 From the Figure 5.8 (a) it is learnt that load1 is initially in ON condition. At time t=0.3 sec Load 2 is switched ON and at this instant the voltage dips across the load. The real and reactive powers of the uncompensated system during sag are depicted in Figure 5.8 (b). In the compensated system, it is observed that as soon as the voltage dips UPFC comes into action automatically and the voltage dip is overcome by series voltage injection through the series inverter as shown in Figure 5.9 (a). Reduction in reactive power and increase in real power after the occurrence of sag in the compensated system are observed as shown in Figure 5.9 (b) unlike the uncompensated system. In open loop based UPFC system, the sag sustains for 0.1 sec whereas in the closed loop system the duration of sag is 0.03 sec only as observed in Figures 5.10(a) and 5.10(b). (a)open loop system (b) Closed loop system Figure 5.10 Voltage Sag
100 5.5 SUMMARY From the Figure 5.9, it can be clearly seen that the closed loop operation of UPFC has better characteristics when compared to open loop operation. The voltage sag caused by the addition of load is easily overcome by voltage injection that takes place almost instantly. The control algorithm used is simple and therefore the response is faster and more accurate. It can be inferred that when closed loop configuration is used, the stability of the system remains the same even upon addition of different loads. 5.5.1 Advantages of Closed Loop Configuration Voltage injection is automatically controlled by computing the line parameters. Response is faster and for any change in line voltage the system responds almost instantly and provides voltage support. Improves transient and dynamic stability of the system. 5.5.2 Disadvantages of Closed Loop Configuration The circuit arrangement becomes more complex as more blocks are added. The system responds even for very small changes especially when no change is required, which may result in low frequency oscillations. The cost of the circuit also becomes higher.
101 5.6 EXPERIMENTAL STUDY 5.6.1 Hardware Implementation The Unified Power Flow Controller mainly consist of two back to back converters - one connected in shunt to the transmission line through a shunt transformer and the other in series to the transmission line through the series transformer respectively. The experimental setup of converter-inverter circuit is shown in Figure 5.11. 1-PHASE AC SOURCE DC LINK RECTIFIER H-BRIDGE IGBT INVERTER AC IGBT DRIVER POWER SUPPLY dspic30f20 10 OPTO ISOLATOR Figure 5.11Experimental Setup of UPFC
102 The experimental setup of UPFC is fed by a single phase, 230 V, 50 Hz AC supply source. Using a diode bridge rectifier this supply is rectified. The DC output from the rectifier is given as input to the H-Bridge inverter shown in Figure 5.12. Figure 5.12 H-Bridge Inverter The High Voltage capacitor provides the DC link between the rectifier and inverter. IGBTs (IHW25N120R2) are employed as the switching devices for the inverter which are driven by IR2110 IGBT Driver. The firing pulses are generated by the high performance digital signal controller - dspic30f2010. Appendix 5 in this Thesis deals with the pin details, the internal architecture and the features of this digital signal controller. The power and control circuitry is isolated by using an Opto Isolator PC187. The details of the driver IR2110, opto isolator PC187 and IGBT IHW25N120R2 are furnished in appendix 6.
103 Figure 5.13IGBT Driver Circuit The IR2110 driver used in the control circuit of the experimental setup has independent high and low side referenced output cannel. The high voltage input to this driver drives IGBT 1 and 3 and low voltage input to the driver drives IGBT 2 and 4 shown in Figure 5.13. it may be noted that when the high voltage input drives IGBT 1, the low voltage input triggers the IGBT 4. The conduction of IGBT 1 and 4 provides positive half cycle of the output. The negative half cycle of the output is obtained during the conduction of IGBTS 3 and 2 which are driven by high and low voltage input to the IR2110 respectively. The typical experimental results of output voltage and triggering pulses are shown in Figure 5.14 and Figure 5.15 respectively.
104 (a) Firing angle = 0 0 (b) Firing Angle = 30 0 Figure 5.14 Inverter output voltage to be injected in series Figure 5.15 Driving pulses for Inverter 5.7 CONCULSION The UPFC has been modeled and simulated for closed loop and open loop system using matlab / simulink. The performance comparison has been made between the open and closed loop systems on the aspects of power quality. From the simulation results, it is clear that the response of the closed loop configuration is faster than the open loop system as far as time taken to restore the normal voltage and the required amount of real and reactive powers for the given load demand. The open loop system based UPFC have been physically realized using the digital signal controller and their output voltage waveforms are measured.