Hydrogen-cooled Generators

Generators up to approximately 300 MW may be air-cooled. Air-cooled generators with asphalt-insulated windings have been provided with carbon dioxide extinguishing system protection. Carbon dioxide can be used on energized equipment without damage to windings. A single supply of extinguishing agent, with automatic directional valves to discharge the gas into the generator, may be used to protect two or more generators, thus distributing the cost of protection over several machines. Fires in open, air-cooled generators are usually controlled with portable extinguishers. Where enerators have a Class E or F type insulation, the fire can be expected to self-extinguish when the generator is de-energized.

Hydrogen cooling is often used for larger machines. The windings in a conventional hydrogen-cooled generator are cooled by the gas coming in contact with the outer surface of the insulation on the conductors.

The gas may be at a pressure of 75 psig (5.1 bar).

Hydrogen-cooled generators have control equipment to maintain the proper gas pressure and purity in the machine. The use of hydrogen in a generator also calls for oil-pressure gland and labyrinth seals to prevent gas leakage where the shaft extends through the generator housing. Hydrogen seal-oil equipment is provided in order to maintain these oil seals.

A concentration of 95% hydrogen is used. This concentration is monitored and maintained during operation. It may also be maintained for short outages. However, for work on the generator or for extended outages, the generator is purged with carbon dioxide.

An explosion hazard exists in the oil system associated with hydrogen-cooled generators. The hydrogen may leak through the oil seals of the machine and accumulate as an explosive mixture in the oil reservoir. Several explosions with localized damage have occurred in older installations. Most installations now include a hydrogen-etraining system that separates air and hydrogen gas from oil and vents it to the outside. In large machines, stators and/or rotors are cooled internally. Hydrogen gas may be circulated internally through the stator and/or rotor coils. The gas is at varying pressures up to 75 psig (5 bar) and is in direct contact with the conductors. Another cooling method consists of circulating hydrogen through the rotor coils and liquids internally through the stator conductors. Water is also used for cooling. Older generators sometimes used oil. Hydrogen is maintained inside the generator casings of these liquid-cooled machines. Like the conventional hydrogen-cooled type, the gas-conductor-cooled and the liquid-conductor-cooled generators require no special internal fire protection.

Hydrogen may increase the hazard of a turbine generator incident if released in the event of mechanical failure or severe vibration. Fire is the most common result of hydrogen leakage. Incidents have occurred where escaping hydrogen has mixed with air and exploded.

 

When oil is used as the stator winding cooling liquid, the liquid cooling system, pumps, and associated piping present a fire hazard external to the generator.