Assignment #2: Line frequency AC-DC converters

Concordia University – ECE Department ELEC-433/6411 – Power Electronics (I) Assignment #2: Line frequency AC-DC converters Fall 2023 Due date: October 5, 2023 at 11:59 PM on Moodle 1) A resistive load of 4 Ω is to be supplied with 200 VDC. This can be done with a single-phase or a three-phase diode bridge rectifier supplied by 60 Hz sources, single-phase and three-phase. For these topologies, compute a. The input voltages (rms) of the rectifiers. Specify if it is phase or line voltage. b. The minimum value of a series inductor that will lead to a “harmonic free” load current. c. The peak voltage and the average and rms currents of the diodes. d. The total kVA of the converter. This is done by multiplying the number of diodes by the catalogue values of voltage and rms currents. Including the derating factors. e. The converter power losses, assuming that the same diode was used in both topologies. Assume that at the on state, the diode can be represented by a threshold voltage of 0.65 V in series with a series resistor of 4mΩ. (VS40HF(R) Series.) Neglect the switching losses and the off-state losses. 2) Consider a thyristor-based grid interface for a PV array as shown below. The rated values of the PV array are 85 V/13.2 A and the grid is 220VLL/60 Hz. a. Calculate the required value for the firing angle of the thyristors in steady-state if a Δ:Y transformer with turns ratio (NP:NS) equal to 5.5:1 is used. b. How much reactive power does the PV grid interface absorb under rated condition? c. Compute the reactance of the input capacitor bank to bring the PF of the system to 0.92 lagging. d. Calculate the value of the DC side inductor so that IDC_6 = 0.1*IDC_0, (average value) under rated conditions. 3) Consider the semi-controlled (thyristor-diode hybrid) AC-DC converter shown below. Assume that the thyristors are fired with the same firing angle (α) and that the load is “highly inductive”. The grid voltage is 120 V/60 Hz and the load resistance is 20 Ω. a. How large should be the DC side inductance for the load current to be considered ripple free? b. Similar to the discussion presented in class for diode and thyristor AC-DC converters, explain how the converter operates, that is, when each semiconductor turns ON and OFF. Recall that diodes are turned ON when forward biased while thyristors need to be forward biased and fired. c. Draw 2 line cycles of the following waveforms for α = 45°: Source voltage, load voltage, load current and source current. d. Derive an expression for the average voltage across the load e. By inspection, one can determine how the displacement power factor of this converter varies with α. Present an expression that represents this relation. 4) A thyristor presents a minimum reverse recovery time (tq) of 25 μs. It is to be used in an AC-DC converter operating as an inverter, connected to a 60 Hz power system. What would be the maximum firing angle of the thyristor (αmax) to prevent commutation failure. Recall that this issue occurs when the thyristor is NOT reverse biased long enough, before being subject to forward voltages.