Snubber Chapter 5 Summary

Chapter 5

Analysis of Results and Comparison with other compensation methods

5.1 Experimental setup at BHEL Engineers training laboratory The Laboratory setup for the PWM Switching auto-transformer for Voltage sag mitigation, the laboratory setup validates the PSCAD results obtained in chapter 4 with lower range approximately meets the higher range. Figure 5.1 shows the schematic of the test setup. The actual test setup is shown in fig 5.2. The test system is built for a voltage of single phase 230V, while IGBT and power diodes are of 600V. The experimental setup consists of an PWM switch of an IGBT and four power diodes connected in such a way that the switch acts as bidirectional switch, a two winding transformer of 1:1 ratio is connected

The normal current of the system is 5 A RMS, and its maximum value is chosen as 10 A RMS. the peak of phase voltage is about 325 V. The Snubber should be designed such that the voltage across the IGBT (IRG4PC50F) should not increase above its breakdown voltage i.e. 600V
To determine the Snubber resistor and capacitor components, the value of the resistor is examined at first. The resistor has two roles. One is to dampen the energy transfer to the capacitor when the IGBT turns off, and the other is to limit the capacitor discharge current when the IGBT turns on. From the IGBT datasheet, it can be found that the maximum peak repetitive current is 120A. From the second role for limiting the initial discharging current of the capacitor, the resistor value is selected by from 600V/120A = 5 .So that the resistor value is chosen to be 5 .
The initial voltage drop across the Snubber resistance is given by 230

The approximate capacitance value of the Snubber is determined by two conditions. The first condition is that the peak voltage during the IGBT off-state should be lower than 5000 V, and the second condition is that the capacitor voltage needs to discharge lower than 10% of its charged value during the IGBT turn-on. When the minimum IGBT on-state time is greater than 2.5 RC, which means 500 s > 2.5 RC C < 45 F, the second condition is satisfied, which results in a 40 F capacitor Snubber capacitor. The Snubber values for the IGBT are R=5 and C=40 F.
5.3.2 Design of ripple filter Ripple filer is connected at the load to remove the higher order harmonics. The design of the ripple filter for this prototype is mentioned in section 5.2.1. The Values of ripple filter are Cf =0.22 , Lf = 1.26mH. The Inductor filter value is not readily available in market and it is designed by using Ferrite core and copper wire. The copper wire is wound on a ferrite core and the inductor value is measured using LCR meter.
5.4 Experimental Results
Test case I: Mitigation of voltage sag using PWM switched transformer