In all power systems the requirement is that the steady state speed deviation, and hence frequency, is kept small for incremental changes in power demand, even if these power increments are quite large – 20%, for example.
There are two main methods used for speed governing gas turbines,
a) Droop governing.
b) Isochronous governing.
Droop governing requires a steady state error in speed to create the necessary feedback control of the fuel value. ‘Droop’ means that a fall in shaft speed (and hence generator electrical frequency) will occur as load is increased. It is customary that a droop of about 4% should occur when 100% load is applied. Droop governing provides the simplest method of sharing load between groups of generators connected to the same power system
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In control theory terminology this action is called ‘proportional control’. This method of governing is the one most commonly used in power systems because it provides a reasonably accurate load sharing capability between groups of generators.
Isochronous governing causes the steady state speed error to become zero, thereby producing a constant speed at the shaft and a constant frequency for the power system. Isochronous governing is also a form of ‘integral control’. This method is best suited to a power system that is supplied by one generator. This type of power system has very limited application. However, there are situations where one isochronously governed generator can operate in parallel with one or more droop-governed generators. The droop-governed generators will each have a fixed amount of power assigned to them for the particular system frequency. This is achieved by adjusting their set points. As the demand on the whole system changes, positively or negatively, the isochronously governed generator will take up or reject these changes, and the steady state frequency will remain constant. This hybrid type of load sharing is seldom used in the oil industry.
Accurate power sharing and constant speed control can be obtained by using a specially designed controller. This controller incorporates load measurement of each generator, measurement of common system frequency and a sub-system to reduce the power mismatches of each generator to zero. The controller regularly or even continuously trims the speed set points of each gas turbine to maintain zero mismatches. A slowly operating integrator can be superimposed onto these set points to adjust them simultaneously so that the frequency is kept constant. This is a form of ‘proportional integral’ control.
The basic control system of most gas turbine generator systems is shown in Figure.
Where
ω = shaft speed
ωref = reference speed
Pe = electrical power at the generator shaft
Pm = mechanical output power of the gas turbine
Pa = accelerating power
Pf = friction and windage power