Handbook of Electrical Engineering For Practitioners in the Oil, Gas ...

2Gas Turbine Driven Generators2.1 CLASSIFICATION OF GAS TURBINE ENGINESFor an individual generator that is rated above 1000 kW, and is to be used in the oil industry, itis usual practice to use a gas turbine as the driving machine (also called the prime mover). Below1000 kW a diesel engine is normally preferred, usually because it is an emergency generator runningon diesel oil fuel.Gas turbines can be classified in several ways, common forms are:-• Aero-derivative gas turbines.• Light industrial gas turbines.• Heavy industrial gas turbines.2.1.1 Aero-derivative Gas TurbinesAircraft engines are used as ‘gas generators’, i.e. as a source of hot, high velocity gas. This gas isthen directed into a power turbine, which is placed close up to the exhaust of the gas generator. Thepower turbine drives the generator. The benefits of this arrangement are:-• Easy maintenance since the gas generator can be removed as a single, simple module. This can beachieved very quickly when compared with other systems.• High power-to-weight ratio, which is very beneficial in an offshore situation.• Can be easily designed for single lift modular installations.• Easy to operate.• They use the minimum of floor area.The main disadvantages are:-• Relatively high costs of maintenance due to short running times between overhauls.• Fuel economy is usually lower than other types of gas turbines.• The gas generators are expensive to replace.Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry.© 2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6Alan L. Sheldrake