GENERATOR MOTORING PROTECTION

Motoring protection is provided for the steam or hydraulic turbine, or for the power system, and not for the generator. Motoring is not harmful to the generator in any way. The protection is usually considered as part of the generator protective system since it uses electrical quantities, usually in the form of sensitive power relays.

Steam turbines have the tendency to overheat when the steam supply is cut off for some reason and the turbine generator system begins motoring. The overheating is due to the loss of steam, which normally keeps the turbine blading at a stable temperature. When the steam flow is interrupted, the blade temperature rises. Most modem steam turbines will overheat when the steam flow is less than about 10% of full load. The time constant is large and varies from 30 seconds to 30 minutes, depending on the type of turbine and its design. The practice varies regarding the need for protection and the turbine manufacturer should be consulted regarding any requirements. When protection is required, a reverse power relay is often used and is set to trip at about 0.5% reverse power.
Hydraulic turbines sometimes lose their supply of water, for example, when trash blocks the input gates to the penstocks. When this happens, the low flow can cause cavitation and possible turbine runner damage and will often result in motoring of the generator. To protect against these possibilities, hydro-units are often provided with power relays that are set to trip the unit for low power output or for reverse power of a few percent of normal load.

Diesel engine driven generators almost always are provided with anti-motoring protection when the engine fails. There are two reasons for this requirement. First, the failed engine is a relatively large load, and the generator will draw 15% or so of its rated power from the system. This may be more than the system can supply. Second, there is the danger of explosion or fire from unburned fuel in the Diesel engine if driven by the electrical system.

Combustion turbines that are directly connected to their generator will require from 10% to 50% power if driven as a mechanical load by the motoring generator. Such units are usually protected from generator motoring in order to avoid loading the system with this unnecessary load, even though the motoring is not harmful to the generator or to the turbine.