High Voltage DC Transmission Prof. S.N. Singh Department of Electrical Engineering Indian Institute of Technology, Kanpur Module No # 02 Lecture No # 01 Analysis of Simple Rectifier Circuits Welcome to module two, lecture one, in this module I will discussed about the analysis of convertor circuit and in this convertor circuit, then we will go for the modeling of equivalent electrical circuit, so that we can analyze the complete HVDC link. So, before go for this analysis of convertor circuit it is important, let us see the simple analysis of rectifier circuit, and then we will see what are the desired features or requirement for the convertor circuits that can be used for HVDC application. (Refer Slide Time: 00:57) So, in this rectifier circuit, let us see the various assumptions and their justification that which I am going to make. So, first assumption is this AC source has no impedance, it is believed that AC source, which we are using it is a perfect sinusoidal and also the frequency is constant. Normally, you know the generation is a balance circuit and it is perfectly giving
sinusoidal and also the frequency is constant. So, for all our analysis we will take the voltage of the source is a constant voltage as well as the constant frequency. If you are taking the poly phase circuit source means, we will go for three phase circuit analysis for the convertors then we will assume that it is a balance voltage means, all three phases are balanced and you know the balance means you all the three phases magnitudes are equal and they are displaced by one twenty degree and it is in clockwise, if they are displaced by 120 degree means here. (Refer Slide Time: 01:57) If I want to balance circuit means, we are having e a phase a phase, here it is eb, here it is ec. So, they are displaced by one twenty degree and it is your, if you draw the phasor diagram it is your ea here, it is eb and here it is ec means, it is in clockwise direction. Another assumption regarding the transformer it is believed that it is a ideal transformer means, it has a no leakage impedance means there is no drop across the transformer and its exciting admittance is also negligible or 0 means, it is just like a ideal transformer and even they will also not considered the resistance of the transformer which will assume this is the ideal transformer; however, if you are going to take the your practical transformer it will simply drop some impedance means, it will due to the current flow there will be some loss as well as some drop in the inductance will be there, but for our analysis purpose we will take this transformer is ideal one.
The third assumption is the your DC load has the infinite inductance means, you are the DC current is constant means, it is a ripple free. But the voltage will have pulses, it is not ripple free. (Refer Slide Time: 03:17) Means, I want to say that here if your current is perfect DC in the output, here the this is your DC current the DC or I d, it is a constant, it is a ripple free; however, your output voltage it is not ripple free and we will see later on. The last assumption that is I am going to make, we are assuming the valve is ideal means, it will offer the 0 resistance during the conduction phase, once it is conducting it will offer the 0 resistance and once it is in off condition, then it will offer the infinite resistance. 0 resistance means, it is also loss less means, there is no loss in the valve as well.
(Refer Slide Time: 04:02) Let us see some definitions, which will be used even though for the further analysis; first one is the rating of valve, the valve rating you know at the valve here I mean it may be a simple thyristor, it may be a GTOs or it may be combination of thyristor or GTOs so it is a complete valve and normal rating here. (Refer Slide Time: 04:18) Normally, we say that the thyristor or valve rating is your voltage current and it is a power rating here this voltage rating is nothing but it is the peak inverse rating, peak inverse voltage that is appearing across the valve this power is nothing but it is your the peak inverse voltage