Discussion
Which of the following statements are true:
1) The frequency of DC current is zero.
2) It is relatively easy to step-up and step-down DC using transformers.
3) Corona loss is relatively less in DC transmission.
4) There is no Ferranti effect in DC transmission system.
Answer: C
Statement 1 is true: direct current does not alternate, so its frequency is zero. Statement 2 is false: a transformer works by changing flux, so it cannot step DC up or down, and DC is changed in voltage with converters. Statement 3 is true: corona loss is lower in DC transmission for the same peak voltage. Statement 4 is true: the Ferranti effect, the voltage rise at the open end of a long AC line caused by line capacitance, does not occur on DC. So the true statements are 1, 3 and 4, option (C), and the other options include statement 2.
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