CHAPTER 5 MITIGATION OF VOLTAGE SAG AND SWELL USING DIRECT CONVERTERS WITH MINIMUM SWITCH COUNT

Transcription

1 75 CHAPTER 5 MITIGATION OF VOLTAGE SAG AND SWELL USING DIRECT CONVERTERS WITH MINIMUM SWITCH COUNT 5.1 INTRODUCTION Though many DVR topologies have been proposed based on direct converters, in the literature survey it is found that minimum number of switches used in each phase for compensation is four. In order to reduce the number of switches a new simplified topology with a series centre tapped transformer at the output side of the direct converter is presented. The direct converter is fabricated using only three bi-directional controlled switches. The DVR can properly compensate long-duration, balanced and unbalanced voltage sag and swell by taking power from the grid. The switches are driven by Pulse Width Modulation (PWM) signals. The simulation and the hardware results validate the idea that the proposed topology can mitigate sag and swell effectively. 5.2 PROPOSED TOPOLOGY WITH MINIMUM SWITCH COUNT Each DVR consists of a direct converter with a LC filter and a series centre tapped transformer with the turns-ratio of 1:1as shown in the Figure 5.1.

2 76 Figure 5.1 Proposed topology with minimum switches Figure 5.2 Bidirectional switch topology In this arrangement, the output voltage of the compensator can be either in-phase with the grid voltage or out of phase with the grid voltage. In the proposed topology, for the a-phase, when the switch Saa is put-on, supplies an in-phase voltage to the series transformer. When the switch Saa is put-on, the topology supplies the out-of-phase voltage to the series transformer. The switch Sga is connected across the series transformer. When the a-phase voltage is at the rated level, the switch Sga is kept closed, shortcircuiting the primary side of the series transformer. If the a-phase experiences sag, in order to generate the compensating voltage in phase with

3 77 the grid voltage, rnatively modulated. If it has swell, then in order to generate the compensating voltage out of phase with the grid voltage, the switches Saa and Sga are alternatively modulated. The compensating voltage is added to the grid voltage through the series transformer. Each DVR is structured by three bidirectional power switches. The topology of the bidirectional switch used is shown in the Figure 5.2. The switches are controlled by a simple PWM technique. Considering Figure 1, the following equation can be obtained: (5.1) In (5.1), l, g and con subscripts are used for the load, grid, and compensating quantities, respectively; the second subscript refers to the corresponding phases, respectively. Assuming sinusoidal waveforms, and considering only the a-phase, the voltages can be expressed as follows: (5.2) In the aforementioned equations, and are the peak values of load, grid, and injected voltages, respectively., is the phase angle of the injected voltage and is defined as follows: (5.3)

4 CONTROL PROCEDURE As mentioned in the first chapter, 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 Voltage Sag Mitigation In order to compensate the voltage sag, it is necessary to inject voltage in-phase with the grid voltage. As the centre tapped series transformer gives output voltages of different polarity, the voltage which is in-phase with the grid voltage is chosen. This voltage is modulated using bi-directional switches and added in-phase with the grid voltage through a series transformer. Figure 5.3 Block diagram of switching pulse generation So if the voltage sag has occurred in a-phase, then the bidirectional A detailed block diagram for switching pulse generation to mitigate the voltage sag is shown in Figure 5.3. U gmax has been obtained from single phase d-q transform as explained in the first chapter. U ref is the desired terminal voltage, which is a user specified constant value set in the micro controller program.

5 79 The difference between the reference voltage 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 compared with the triangular carrier signal to generate the switching pulses. The pwm pulses are given to the corresponding switches through the logic gates as shown in the Figure Voltage Swell Mitigation In order to compensate the voltage swell, it is necessary to inject voltage out of phase with the grid voltage. So if there is voltage swell in phase modulated to mitigate the swell. Switching pulse generation is same as that of sag. 5.4 COMPENSATION RANGES OF THE DVR In this section, the voltage sag and swell compensation ranges for the proposed topology are calculated. It is assumed that the phase shift from V g to V con as well as the phase shift from V con to V l are negligible. The relationship between V g and V con (the filtered output voltage of the converter) can be expressed as (5.4) In (5.4 The above equation is valid because the transformation ratio of the series transformer is 1: Voltage Sag Compensation Range According to Figure 1, for voltage sag condition: (5.5)

6 80 Considering (5.4), (5.5) can be rewritten as follows: (5.6) The voltage sag percentage is defined by (5.7) Considering (5.6) and (5.7), the voltage sag percentage can be simplified to (5.8) 50% for D=1. It is seen that the maximum value of sag that can be compensated is Voltage Swell Compensation Range According to Figure 1, for voltage swell condition: (5.9) Considering (5.4), (5.9) can be simplified to (5.10) The voltage swell percentage is defined as follows: (5.11) Considering (5.10) and (5.11), the voltage swell percentage is simplified as follows: (5.12)

7 81 In this method the swell has been compensated by feeding the voltage from the same phase. From (5.12) it is observed that for D = 0.5, 100% voltage swell can be compensated. As the value of D is increased the topology can mitigate a greater amount of swell. It is not required to choose D =1 to mitigate large swells since sufficient voltage is available in the phase where swell is occurred. 5.5 SIMULATION RESULTS The MATLAB/SIMULINK software has been used for simulation. Three phase RL load (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. The filter is designed for a cut off frequency of 1000Hz with the value of inductance 1.732mH and the capacitance of 15uF according to the formula f = 1/ The ability of the DVR to mitigate balanced voltage sag of 50% in all the phases is shown in Figure 5.4. Mitigation of unbalanced voltage sag Figure 5.5. Figure 5.6. The compensation of balanced swell of 100% is illustrated in 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 5.7.

8 82 Figure 5.4 Mitigation of balanced voltage sag (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR Figure 5.5 Mitigation of unbalanced voltage sag (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR

9 83 Figure 5.6 Mitigation of balanced voltage swell. (a) Grid voltage (b) load Voltage (c) compensation voltage Figure 5.7 Mitigation of unbalanced voltage swell (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR

10 EXPERIMENTAL RESULTS A three-phase DVR described in this chapter has been fabricated in order to verify the design procedure. A photograph of the experimental prototype is shown in the Figure 5.8. The PIC16F877A microcontroller has been used to generate the switching pulses. The rating of the centre tapped series transformer is 720VA, 120V with the transformation ratio of 1:1. IRFP460 power MOSFET switches of rating 500V, 20A were used to synthesize the direct converter. The hardware prototype has been designed for a rated voltage of 60V. Figure 5.8 Hardware prototype

11 85 The ability of the DVR to mitigate balanced voltage sag of 50% in all the phases is shown in Figure 5.9. Figure 5.9 Balanced Sag Compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR

12 86 The compensation of unbalanced voltage sag of 50% in a-phase, 25% in b- phase, and 10% in c-phase can be seen in Figure Figure 5.10 Unbalanced sag compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR

13 87 The compensation of voltage swell of 90% in a-phase, 100% in b-phase, and 115% in c-phase can be seen in Figure Figure 5.11 Swell compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR 5.7 PERFORMANCE ANALYSIS Babai et al (2010) proposed a topology based on direct converter, with 5 switches per phase in which switches are controlled based on sampling process and it needs computations throughout a cycle. Using the control

14 88 algorithm explained, the compensation range of voltage sag is 0-33% and of the range for the voltage swell is 0-100%. Perez et al (2006) presented a topology for a single-phase DVR based on a single phase matrix converter with 4 switches per phase and the compensation range is 25% for voltage sags and 50% for swells. In this topology, during compensation 3 switches are modulated. So generation of switching pulses is complicated. Wang & Venkataramanan (2009) designed a DVR based on an indirect matrix converter for balanced voltage sags compensation of 60%. This topology needs flywheel energy storage element and the capability of the topology in voltage swell compensation has not been investigated. In this work, a centre tapped series transformer is used only with 3 bi-directional switches such that the only 2 switches are modulated during compensation. So the switching loss is less and switching pulse generation is also easier. In this topology switches are controlled by ordinary PWM. As a result, computation is avoided, control is simpler and 50% of voltage sag and unlimited quantity of voltage swell is compensated as mentioned in Table 5.1.

15 89 Table 5.1 Comparison of various DVR topologies S. No Converter Topology Compensation Range Sag Swell PWM Technique Used No of Switches 1 Direct Converter topology by Babai et al (2010) 33% 100% Pulse Width is computed during each switching period 5 2 Matrix Converter topology by perez et al (2006) 25% 50% Ordinary PWM 4 3 Indirect matrix Converter with Flywheel energy Storage by Wang & Venkataramanan (2009) 60% Nil Ordinary PWM 5 4 Proposed converter 50% Unlimit ed Ordinary PWM SUMMARY A three-phase DVR based on direct converters has been presented 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 DVR 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. The control of DVR is done by using a very simple PWM procedure. The DVR is able to mitigate 50% of balanced and unbalanced voltage sag, unlimited quantity of balanced and unbalanced voltage swell effectively. The topology presented uses only three switches, with one centre tapped series transformer for each phase for effective compensation with simple control logic.