Simulation

Three applets are presented in this section. The first applet animates the circuit. The only purpose is to illustrate the sequence of operation of this circuit. It does not take in any parameter. To run it, click on the RUN button. The SINGLE STEP button allows the user to step through, the PAUSE button allows the user to stop the program and the RESET button allows the user to view the simulation once more. To view the simulation once more, click on the RESET button and then click on the RUN button. During the simulation, the user can ask the program to pause for a while, then step through for a while and allow it to run through to the end of its cycle.

click here to open the applet

The second applet takes in these parameters:

a. the firing angle in degrees,

b. the ratio of load reactance to load resistance (load reactance evaluated at line frequency),

c. the ratio of source reactance to load resistance(preferably below 0.1 p.u), and

d. the value of dc link source.

The program allows you to view either the waveforms or the statistics. The values of load and line reactance are to be entered in per unit. For example, if the line reactance is entered as 0.05 p.u., rated load current would cause a drop of 4% of phase voltage across the line reactance.

An example is presented now to explain how the per unit values can be set. Let a 3 phase, 415 V, 50 Hz source supply power to the converter. Then the maximum average voltage that can be obtained is obtained as shown below. Let the nominal rated dc link current be 100 A. Then the nominal load resistance or the base impedance for the system is computed as shown below. It is also shown how the line reactance can be obtained, if its value in p.u. is known. Given that the current through the load is free of ripple, the rated RMS line current is obtained as illustrated below. From the total rms line current, inclusive of both the fundamental component and the harmonic components, the fundamental rms component is obtained as illustrated below.

Given that the line inductance is 1 mH, its p.u. value is obtained as shown below.
Given that the dc link inductance is 10 mH, its p.u. value is computed as illustrated below.
Usually the line inductance is called as the 4% reactor, implying that when the line current is at its rated value, the rms value of the fundamental component of voltage across the line reactor is 4% of the phase voltage. For example, if the rms phase voltage is 240 V, the drop across 4% reactor at rated current would be 9.6 V. When the line voltage is 415 V, the phase voltage is calculated as shown below.

The drop across the line inductor can now be stated as a fraction of the phase voltage as given below.

This means that if the drop across the line inductor is to be 4% of phase voltage, the inductance should be 0.4 mH and not 1 mH.

click here to open the applet

The third applet takes in these parameters:

a. the ratio of load reactance to load resistance (load reactance evaluated at line frequency),
b. the ratio of source reactance to load resistance(preferably below 0.1 p.u), and
c. the value of dc link source.

Within the program the firing angle is varied and the plots of rms bridge output voltage, the average bridge output voltage, its RF, the rms line current, the fundamental rms component of line current, the THD in line current, the average output current, its RF and the overlap angle as a function of firing angle are displayed. The value of dc source in the dc link is assigned to be zero in this applet.

click here to open the applet