Normally, a generator produces power that flows out of its terminals and into the connected system. Reverse power occurs when the direction of power flow reverses, and power flows into the generator.

The reverse power situation that is of greatest concern occurs when something causes the prime mover that rotates the generator to stop producing torque. Because the generator remains connected to the grid, it will act as an electric motor, drawing power from the grid (ie, power reversal). There are two potentially serious consequences that can result from this.

The first is the impact on the grid. For the same of simplicity, assume that the generator normally produces an amount of power equal to P, and that when operating as a motor, it will absorb a similar amount, P. Therefore, the net effect on the grid will be the same as if it experienced a step increase in load equal to 2P. Depending on the size of the grid and the strength of the remaining generation, a step increase of 2P could result in a significant frequency change on the grid.

The second is the impact on the prime mover associated with the generator. If the prime move is a steam turbine, the event that is most likely to result in that loss of prime mover torque is some kind of disruption to steam flow. When the generator becomes a motor, the turbine will become its load. In the absence of steam flow, the turbine blades will be spinning in air, and can overheat, potentially causing damage to the turbine.

In the case of a diesel or gas turbine driven generator, motoring (another term for a power reversal) is not likely to cause damage. In fact, some gas turbines are designed to intentionally motor for a few seconds as part of their shutdown sequence. Gas turbines operate at much higher internal temperatures than steam turbines (fuel combustion actually takes place inside gas turbines), and in that case a few seconds of motoring will help eliminate internal heat and prevent a temperature spike that could damage the turbine.

Motoring can also occur dynamically as a result of severe disturbances on the grid. Dynamic motoring occurs when the system frequency is greater than the rotational speed of the turbine generator and results in angular acceleration.