Load Variation

Most power plant loads vary in an irregular manner. They can usually anticipates the hour to hour changes but not their exact values. The fuel and air feed to boiler furnace must be varied accordingly or steam pressure and turbine shaft speed will get out of hand.

At steady load, energy to plant input exactly balances to plant output and losses. At the same time a bank of light is switched on the system, this reduce the over-all electric resistance of the system and a greater current flows through the generator armature and connected circuit. Higher amperage raises the armature magnetic field strength, increase the turning resistance offered the field rotor, this slows the generator and turbine shaft speed. By giving up some of its rotating kinetic energy the shaft momentarily support the added load.


FREE GOVERNOR MODE OF OPERATION
When frequency changes in the grid every TG unit reacts and adjusts its generation as dictated by power frequency or droop characteristic. For instance when frequency falls by 0.1 %,

generation has to be increased by 20 % with droop of 5 %. In Indian situation most of the generating units operate at their peak values and no additional generation is possible. With the result many units do not increase their generation and load shedding is resorted to. In some cases, due to various operational reasons generating companies do not like to their machines to respond, even though spare capacity is available. The governing is bypassed. If most of the generating stations in a grid do not respond naturally, there is a danger of grid becoming unstable also.

In the recently approved Grid code it has been made mandatory for each generator to be provided with capability to allow up to 105 % MCR( maximum continuous rating capacity) generation whenever situation demands. This is called Free Governor Mode of operation (FGMO).

It has been reported that after introduction of FGMO, frequency profile has improved considerably.
Valve Opening Dead band or insensitive zone Speed frequency



Turbine Speed Droop

s = [(No - Npr)/Nr ]x 100%

where
s = speed droop
Nr = rated speed
Npr = speed at full load
No = speed at no load

for example
Nr = 3000 RPM
Npr = 3000 RPM
No = 3150 RPM
s = 5%