I was reading about shunt faults in power systems in the book Fundamentals of Power System Protection by Y.G. Paithankar and S.R. Bhide and the following sentence confused me:

"Further, in an interconnected system, there is another dimension to the effect of faults. The generators in an interconnected power system must operate in synchronism at all instants. The electrical power output from an alternator near the fault drops sharply." Blockquote

Why should the power of the generator decrease?

Is it because the voltage across the generator will decrease by a greater amount than the current increase? If so, why should the voltage decrease?

Here's is the how I understand it:

The rotor, which produces the back EMF, and, consequently, the current, of the said generator, will continue to rotate at the same speed, so with the current increase from the short-circuit fault the overall current of the generator should be very high and the power should drastically increase.

  • \$\begingroup\$ Consider this: Under what conditions do you get the most power from a generator? When the load matches the output from the generator. No load, no power. Too much load, low power (or no power when it bogs down completely and stops or burns out.) \$\endgroup\$
    – JRE
    Mar 29 '18 at 12:30
  • 2
    \$\begingroup\$ Or, another way. Power is the product of voltage and current. In a short circuit, the current goes way up, but voltage drops to near zero. The product of the two is therefore lower. \$\endgroup\$
    – JRE
    Mar 29 '18 at 12:32
  • \$\begingroup\$ The rotor produces forward EMF in a generator, not back EMF. \$\endgroup\$
    – Transistor
    Feb 15 '20 at 9:45

The wording is confusing. It might be better to say, "The electrical power output from an alternator near the fault must be reduced sharply."

The alternator remains synchronous to the grid, and its terminal voltage is related to its speed, as you say. Therefore, in order to keep its current within limits, its power output must be reduced, which means throttling back its prime mover to reduce the power input.

In other words, the alternator can remain connected to the grid, but the amount of power that it contributes to the grid is way less than it can do under normal circumstances.

Alternatively, if the current rises too quickly, circuit breakers will open in order to protect the alternator, disconnecting it from the grid and reducing its contribution to zero.


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