90 CHAPTER 6 MITIGATION OF VOLTAGE SAG, SWELL AND SINGLE PHASE OUTAGE USING MULTI WINDING TRANSFORMER 6.1 INTRODUCTION From the literature survey it is observed that the DVRs based on direct converters are not able to mitigate single phase outage. In this chapter a DVR topology based on direct converter is proposed which is able to compensate single phase outage. The DVR consists of a direct converter, a multi winding transformer at the input side and a series transformer at the output side of the converter. The direct converter is fabricated using three bidirectional controlled switches. The DVR can properly compensate long duration, balanced and unbalanced voltage sag and swell by taking power from the grid. To compensate the voltage sag in any one phase, the other two phase voltages are added using a multi winding transformer. The added voltage is modulated using controlled switches to compensate the sag. To compensate the swell, power from the same phase is taken and the corresponding phase voltage is attenuated. The switches are controlled by ordinary pulse width modulation (PWM) technique. The simulation and the hardware results validate the idea that the proposed topology can mitigate balanced sag, balanced swell, unbalanced swell and single phase outage effectively.
91 6.2 PROPOSED TOPOLOGY USING MULTI WINDING TRANSFORMER Each DVR consists of a direct converter with LC filter, a multi winding transformer with a turns ratio of 1:1 and a series transformer as shown in the Figure 6.1. Each three-phase-to-single-phase converter is structured by three bidirectional power switches. The topology of the bidirectional switch used is shown in the Figure 6.2. Figure 6.1 Proposed topology using multi winding transformer Figure 6.2 Bidirectional switch topology
92 The switches are controlled by ordinary pulse width modulation (PWM) technique. The bypass switches are connected across the series transformers which remain closed when the DVR is not activated. However when compensation takes place, the bypass switches are modulated by the PWM signals. 6.3 CONTROL PROCEDURE As mentioned earlier, the voltage sag and swell is identified using single phase d-q theory. In this section switching pulse generation for the mitigation of sag and swell is explained in detail. 6.3.1 Voltage Sag Mitigating Scheme In order to compensate the voltage sag, power from the alternate phases is utilized by the converter. So if there is voltage sag in a-phase then are added using a multi winding transformer such that resultant voltage will be out of phase with a-phase voltage. We know that under balanced condition (6.1) So ( (6.2) Where v a, v b and v c are three phase voltages. This added voltage is modulated using bi-directional switches S, S ga and added to the a-phase voltage using the series transformer which is connected in appropriate polarity. The block diagram of switching pulse generation for the mitigation of voltage sag is shown in the Figure 6.3. U gmax has been obtained from single phase d-q transform as explained.
93 Figure 6.3 Block diagram of switching pulse generation U ref is the desired terminal voltage, which is a user specified constant value set in the micro controller program. The difference between the reference voltage value U ref and the peak value of the grid voltage U gmax gives the amount of voltage sag or swell in the grid. The error signal is used for generating the PWM signals as shown in the Figure 6.3. 6.3.2 Voltage Swell Mitigating Scheme In order to compensate the voltage swell, power from the same phase is utilized by the converter due to the availability of a large voltage in the phase itself where the swell occurs. Whenever there is voltage swell in aa and S ga will be alternatively modulated to mitigate the voltage swell. Switching pulse generation is similar to that of sag. 6.4 SIMULATION RESULTS The MATLAB/SIMULINK software has been used for simulation. RL loads of 0.8 power factor lag, 240VA per phase were connected to the lines. The desired terminal voltage has been set at 60 V rms (1p.u), 50 Hz. The switching frequency of the converters is 8 khz. Passive LC filters were used on the output of the converters (1.732mH and 15µF) to filter the
94 harmonics in the compensating voltage. The turns ratio of the injection transformers is 1:1. The ability of the DVR to mitigate balanced voltage sag of 50% in all the phases is shown in Figure 6.4 and the single phase outage compensation is shown in the Figure 6.5. The compensation of balanced swell of 100% is illustrated in Figure 6.6. The ability of the DVR to mitigate unbalanced voltage swell of 100% in a-phase, 50% in b-phase, and 25% in c-phase can be observed in Figure 6.7. Figure 6.4 Mitigation of balanced (50%) sag (a) Grid voltage (b) Load voltage (c) DVR compensation voltage
95 Figure 6.5 Mitigation of single phase outage (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR Figure 6.6 Mitigation of balanced voltage swell (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR
96 Figure 6.7 Mitigation of unbalanced voltage swell (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR 6.5 SUMMARY The three-phase DVR presented is based on direct converters which do not require the dc link as in the conventional DVRs. The absence of the dc link reduces the cost, weight, and volume of the DVRs and also avoids the maintenance of energy storage devices. The DVR in each of the three phase lines is constructed using only three bidirectional switches. During the occurrence of voltage swell, as more voltage is available in the line itself, for compensating the voltage swell power is fed from the same line. So it enables unlimited swell compensation. As this topology aims at mitigating single phase outage, for the voltage sag compensation in any one phase, power derived from the other two phases. The other two phase voltages are added with the help of a multi-winding transformer and then the compensating voltage is synthesised using the converter. During the unbalanced sag, if other two phase voltages are balanced then the sag compensation range could be
97 more than 50%. The control of DVR is done by using a very simple PWM procedure. The DVR is able to mitigate 50% of balanced voltage sag, unlimited voltage swell and single phase outage effectively. The topology presented uses an additional multi-winding transformer for each phase with only three switches, thereby effectively compensating single phase outages with simple control logic.