A number of situations exist where a generator could be driven as a motor. Anti-motoring protection will more specifically apply in situations where the prime-mover supply is removed for a generator supplying a network at synchronous speed with the field normally excited.

The power system will then drive the generator as a motor.

A motoring condition may develop if a generator is connected improperly to the power system. This will happen if the generator circuit breaker is closed inadvertently at some speed less than synchronous speed.

Typical situations are when the generator is on turning gear, slowing down to a standstill, or has reached standstill. This motoring condition occurs during what is called “generator inadvertent energization.”

The protection schemes that respond to this situation are different and will be addressed later in this article. Motoring will cause adverse effects, particularly in the case of steam turbines.

The basic phenomenon is that the rotation of the turbine rotor and the blades in a steam environment will cause windage losses.

Windage losses are a function of rotor diameter, blade length, and are directly proportional to the density of the enclosed steam. Therefore, in any situation where the steam density is high, harmful windage losses could occur.

From the preceding discussion, one may conclude that the anti-motoring protection is more of a prime mover protection than a generator protection.

The most obvious means of detecting motoring is to monitor the flow of real power into the generator. If that flow becomes negative below a preset level, then a motoring condition is detected.

Sensitivity and setting of the power relay depends upon the energy drawn by the prime mover considered now as a motor. With a gas turbine, the large compressor represents a substantial load that could reach as high as 50% of the unit nameplate rating. Sensitivity of the power relay is not an issue and is definitely not critical.

With a diesel type engine (with no firing in the cylinders), load could reach as high as 25% of the unit rating and sensitivity, once again, is not critical. With hydroturbines, if the blades are below the tail-race level, the motoring energy is high.

If above, the reverse power gets as low as 0.2 to 2% of the rated power and a sensitive reverse power relay is then needed. With steam turbines operating at full vacuum and zero steam input, motoring will draw 0.5 to 3% of unit rating. A sensitive power relay is then required.

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