Page 44 - Handbook of Electrical Engineering

GAS TURBINE DRIVEN GENERATORS 23

removable coupling shaft between the two turbines. Some points to consider with regard to the two
types of driver are:-

a) High speed of rotation tends to improve the compressor and turbine efficiency. Hence, with
two separate shafts, the best thermodynamic performance from both turbines and the compressor
is obtainable.
b) Using aero-derivative machines means that a simple ‘add on’ power turbine can be fitted in the
exhaust streams of the aero engine. This enables many manufacturers to design a simple power
turbine and to use a particular aero engine.
c) Two-shaft machines are often criticised as electrical generators because of their slower response
to power demands in comparison with the single-shaft machines. This can be a problem when a
two-shaft machine may have to operate in synchronism with other single-shaft machines or steam
turbine generators. Sometimes the slower response may affect the power system performance
during the starting period of large motors. A power system computerised stability study should
be carried out to investigate these types of problem.

Some of the recent aero engines could be called ‘three-shaft’ arrangements because within the
gas generator there are two compressor turbines and two compressors.

2.1.5 Fuel for Gas Turbines

The fuels usually consumed in gas turbines are either in liquid or dry gas forms and, in most cases,
are hydrocarbons. In special cases non-hydrocarbon fuels may be used, but the machines may then
need to be specially modified to handle the combustion temperatures and the chemical composition
of the fuel and its combustion products.
Gas turbine internal components such as blades, vanes, combustors, seals and fuel gas valves
are sensitive to corrosive components present in the fuel or its combustion products such as carbon
dioxide, sulphur, sodium or alkali contaminants, see also sub-section 2.2.5.

The fuel can generally be divided into several classifications:-
• Low heating value gas.
• Natural gas.
• High heating value gas.
• Distillate oils.
• Crude oil.
• Residual oil.

2.2 ENERGY OBTAINED FROM A GAS TURBINE

A gas turbine functions as a heat engine using the thermodynamic Joule cycle, as explained in many
textbooks, see for example References 1 to 5. Most gas turbines used in the oil industry use the