Generation Of Electricity: UNDERSTANDING GENERATOR CAPABILITY CURVE

BASICS OF GENERATOR CAPABILITY CURVE

Each & Every Synchronous Generator are running on Capability Curve. Capability curve of Generator is provided by the manufacturer & the curve has been decided during the type test of the Machine during manufacturing process.

— A Synchronous Generators are rated in terms of the maximum MVA output at a specified voltage & power factor (Usually 0.8 to 0.9 Lagging) which then carry continuously without overheating.—The active power output is limited by the prime movers capability to a value within the MVA rating.The continuous reactive power output capability is limited by three consideration ( - Armature Current Limit , - Field Current Limit, - End Region Heating Limit).

Armature Current Limit: The armature current results in an I2R Power Loss and the energy associated with this loss must be removed so as to limit the increase in temperature of the conductor & its immediate environment. There for one of the limitation on generator rating is the maximum current that can be carried out by armature without exceeding the heating limitations. Therefore in P-Q plane the armature current limit appears as a circle with centre at the origin & radius equal to the MVA rating. The per unit complex power output is

S = P + JQ

Armature Current Limit

Field Current Limit: Because of the heat resulting from the Ifd2Rfd power loss, the field current imposes a second limit on the operation of the generator.

Field Current Limit

End Region Heating Limit:The localized heating in the end region of the armature imposes a third limit on the operation of a synchronous machine. This limit affects the capability of the machine in the under excited condition. When Generator running at Over excited region then high current induced in rotor field, that's why rotor retaining remains in saturated condition. However in the under excited region the field current is low & the retaining ring is not saturated; this permits an increase in armature end flux. Also in the under excited condition, the flux produced by the armature currents add to the flux produced by the field current; there fore, the end turn flux enhances the axial flux in the end region & the resulting heating effect may severely limit the Generator output, particularly in the case of a cylindrical rotor machine.