Power Plant Engineering

GAS TURBINE POWER PLANT 287

(1) The frequency of an a.c. generator,

(2) The speed of a boat or ship,

(3) The speed of an aircraft,

(4) The capacity or head of a pump or compressor,

(5) The road speed of a vehicle.

Power turbine speed

97% 100%

100%

0

Fuel

flow

Main fuel

control

Other inputs

Fuel

supply

Net

fuel

Fuel

valve

Link

age

amp

lifie

d

erro

r^ sig

nal

Relay

Governor

H.P.

oil

Error signal

Speed sensor

Load

Intakeair Exhaust

C

C C

Requiredfuel

through

distributer

Ped

Fig. 9.22. Prime Control (Hydro-mechanical).
On a gas turbine a.c. generator set the prime objective of the control is to maintain the constant
electrical frequency irrespective of the load. This is achieved by selecting the primary controller as a
speed governor (a speed sensing device), which maintains the constant electrical loads. On a gas turbine
driven pipeline compressor, the prime objective of the control is to maintain a constant pipeline pressure
downstream of the driven compressor. In this case, the pipeline pressure is sensed and turbine power is
varied to maintain a constant pipeline pressure for varying flow conditions. There is relatively constant
relationship between turbine power and fuel flow, so in the prime control, the position of fuel valve is
controlled.

In Hydro-mechanical Speed Governing System (Prime Control), the governing loop is similar to
that discussed in the case of steam turbine. The system uses a centrifugal governor (mechanical) to sense
the speed of the turbine and the reference is set for rated speed of the turbine (Fig. 9.22). The governor
senses the speed through the accessory drive shaft and flyweights. Internally, the governor hydraulically
amplifies the error signal and provides an actuator output position that can be linked to a fuel valve. The
fuel valve either throttles or by passes the metered fuel from the main fuel control. The speed-sensing
device may be electrical or hydraulic.

9.7.2 Protective Controls

The objective of the protective control is to ensure adequate protection for the turbine in prevent-
ing its operation under adverse conditions. Whenever, unsafe operating conditions are approached, the
prime control is overtaken by the protective control to protect the turbine or driven equipment.