52 HANDBOOK OF ELECTRICAL ENGINEERING
Block 1 in Figure 2.15 contains most of the main control and turbine functions, such as,a) Governor gain.b) Governor lead and lag compensating dynamics.
c) Derivative damping term for the speed signal.d) Fuel valve gain, limits and dynamic terms.
e) Combustion system lag dynamic term.
f) Combustion system limits.g) Power turbine dynamics.
h) Compressor dynamics.
i) Compressor protection system.
j) Turbine temperature measurement dynamics and limit or reference level.The functions h), i) and j) are used when a two-shaft drive system needs to be simulated.
When applied they usually require a special signal selection block to be incorporated just before the
fuel valve or governor. The purpose of this signal selector is to automatically choose the lowest value
or its two input signals, so that the least fuel is passed to the combustion system. This contributes to
the slower response of a two-shaft machine.
The data supplied by the manufacturer is often given in physical units such as, the position of
the fuel valve in angular degrees, shaft speed in revolutions per minute, power output in kilowatts,
combustion temperature in degrees Kelvin. In most power system computer programs these data
need to be converted into a compatible per-unit form. This can be a little difficult to achieve and a
source of numerical errors, which can lead to incorrect results from the program. Manufacturers may
also provide a per-unit form of the block diagram, if requested to do so. The time constants used in
these diagrams vary significantly from one type and rating of gas turbine to another. It is difficult to
generalise their values. The rotor inertia of the turbine should include the inertia of the gearbox and
the rotor of the generator. The speed measurement block usually contains the governing lead and lag
compensation time constants. These time constants and the derivative damping gain have a strong
influence on the speed response to a change in electrical power, and should therefore be chosen or
calculated carefully.
2.6 Mathematical Modelling of Gas Turbine Speed Governing Systems
2.6.1 Modern practice
Control systems used for the speed governing of gas turbines have become highly involved in
electronic circuitry. Electromechanical fuel value control has largely replaced methods based on
hydraulic control. The reliability of electronic and electrical devices has improved to such a level
that they are generally preferred to hydraulic and mechanical devices, where their use is appropriate.
Most computer programs used for dynamic studies of power systems are capable of represent-
ing control systems and machinery dynamics to a reasonably high level of detail. Manufacturers of