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GATE 2015 EE – Question 26

Power Systems · Basic concepts of electrical power generation, AC and DC transmission concepts, Models and performance of transmission lines and cables · 1 mark · Multiple choice

Consider a HVDC link which uses thyristor based line-commutated converters as shown in the figure. For a power flow of 750 MW from System 1 to System 2, the voltages at the two ends, and the current, are given by: $V_1 = 500$ kV, $V_2 = 485$ kV and $I = 1.5$ kA. 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?

Two thyristor converters connected through a DC line. System 1 on the left has voltage $V_1$ across its DC terminals, System 2 on the right has $V_2$, and the current $I$ flows from System 1 to System 2.
  1. $V_1 = -500$ kV, $V_2 = -485$ kV and $I = 1.5$ kA
  2. $V_1 = -485$ kV, $V_2 = -500$ kV and $I = 1.5$ kA
  3. $V_1 = 500$ kV, $V_2 = 485$ kV and $I = -1.5$ kA
  4. $V_1 = -500$ kV, $V_2 = -485$ kV and $I = -1.5$ kA

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Correct answer: (B) $V_1 = -485$ kV, $V_2 = -500$ kV and $I = 1.5$ kA

Explanation

Thyristors conduct current in one direction only, so the current $I$ cannot reverse, which rules out C and D. Power can be reversed by reversing the voltage polarities. For power to flow from System 2 to System 1, the sending end (System 2) must be at the higher voltage magnitude with the line current still positive. That gives $V_1 = -485$ kV and $V_2 = -500$ kV, so System 2 delivers $500 \times 1.5 = 750$ MW and System 1 absorbs $485 \times 1.5$ MW, with the difference lost in the line.