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) based HVDC link, thyristors are used which only allow current to flow in one direction. Therefore, the current cannot be reversed and must remain in the direction from System 1 to System 2.
To reverse the direction of power flow (), the polarity of the DC voltage must be reversed.
Original state (Power from 1 to 2):
.
Power at System 1: (sending).
Power at System 2: (receiving).
To reverse flow (Power from 2 to 1):
We need System 2 to act as the sender and System 1 as the receiver. Since is fixed in direction from 1 to 2, we reverse the voltage polarities.
Consider Option B: .
Power at System 1: .
Power at System 2: .
In this configuration, System 2 is delivering to the line (negative sign indicates power leaving the terminal), and System 1 is receiving from the line. Thus, power flows from System 2 to System 1.
To reverse the direction of power flow (), the polarity of the DC voltage must be reversed.
Original state (Power from 1 to 2):
.
Power at System 1: (sending).
Power at System 2: (receiving).
To reverse flow (Power from 2 to 1):
We need System 2 to act as the sender and System 1 as the receiver. Since is fixed in direction from 1 to 2, we reverse the voltage polarities.
Consider Option B: .
Power at System 1: .
Power at System 2: .
In this configuration, System 2 is delivering to the line (negative sign indicates power leaving the terminal), and System 1 is receiving from the line. Thus, power flows from System 2 to System 1.
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