GATE EE 2015 Set 1 — Question 26
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Power Electronics → AC-DC Converters → AC & DC Transmission
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Question
Consider a HVDC link which uses thyristor based line-commutated converters as shown in the figure. For a power flow of from System 1 to System 2, the voltages at the two ends, and the current, are given by: and . If the direction of power flow is to be reversed (that is, from System 2 to System 1) without changing the electrical connections, then which one of the following combinations is feasible?
Correct answer
(B) V₁ = -485 kV, V₂ = -500 kV and I = 1.5 kA
Solution
In a line-commutated converter (LCC) HVDC link the thyristors conduct in one direction only, so the DC current cannot reverse. It must stay positive, flowing from System 1 to System 2 as marked.Line resistance from the given operating pointAny feasible operating point must satisfy , i.e. (algebraically).Reversing the powerWith fixed in direction, power can be reversed only by reversing the polarity of the DC voltages: System 1 then works as an inverter and System 2 as a rectifier.Checking the options
- (A) , : , which would need . Not consistent with .
- (C), (D) need , which the thyristors cannot carry.
- (B) , : , giving . Consistent.
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