Generator Efficiency

 

Generator Efficiency

Normally, a generator is designed @ 0.85 Lag (inductive) and should be operated at this region in any given MW.

If during low load of operations, always look for the power factor of the generator and adjust accordingly to smooth out generator voltage and operation costs.

Adjust the Generator voltage (increase or decrease) until it satisfies the Generator power factor vs Generator Voltage and Grid Voltage whilst looking at the Grid frequency. The idea is to always keep the reactive power zero MVAR and adjust accordingly. Ideally a power factor of 1 is perfect but impossible to achieve and operator should aim to control the generator at this state.

See the Tap Position for the Corresponding Generator Voltage

GTG Unit # 1 / 2 / 3 / 4 / 5 / 7 / 9 (Have to check Generator Voltage vs Tap Position)

Tap Position	LV	HV
1	11 kV
2
3
4
5
6
7
8
9

 

GTG Unit 11 (Have to check Generator Voltage vs Tap Position)

Tap Position	LV	HV
1	15 kV
2
3
4
5
6
7
8
9

 

 

 

 

 

 

LOWER TAP	Increase reactive power (exporting)
	Increase Generator Voltage
HIGHER TAP	Decrease reactive power (importing)
	Decrease Generator Voltage

 

Operating a generator beyond the design of the Generator will pose a problem within the generator and will eventually high operation cost (Fuel Consumption). Protections are in place to protect the generator from certain heating and to inform the operator for necessary adjustments as per generator efficiency. See Generator capability curve for the limitations of the generator operations.

Rotor Winding Heating – Beyond generator curve (OEL)

Stator Heating – Ideal

Core Winding Heating – Below generator curve (UEL)

Protection System:

·         Overexcitation Limiter (OEL) – Protects the generator filed circuitry from excessive current versus time heating. Its settings should be coordinated with the generator capability curve in the overexcitation region. The setting should be also allowing the exciter to respond to fault conditions where field current is boosted (field forcing) to a high level for a short period of time. Typically, the OEL takes over control to limit field current from the steady state AVR control for close in faults where the induced field current remains high or during sustained system low voltage conditions requiring field current above rated levels.

·         V/Hz Limiter – Limits the generator V/Hz ratio by limiting the generator voltage to a programmed setting. Steady state limit is +/- 5% of rated generator stator terminal voltage at rated frequency. The setting should permit short time excursions during transient conditions. The V/Hz limiter is a limit function to the AVR setpoint and is not a variable.

·         Field Overcurrent Protection – DC overcurrent protection is provided in exciters. Some exciters have a protective inverse time module that calculates the I*t to provide an inverse time curve. It needs to be coordinated with the OEL settings as well as the short time capability of the field. It also should allow field forcing to take place during fault conditions. In some cases, this protection may trip the exciter if OEL initiated runback is unsuccessful.

·         Under excitation Limit (UEL) – prevents the generator from exceeding its core-end heating and/or its stability limit, when it is operating at a leading power factor. It prevents the AVR from reducing excitation to such a low level that the generator is in danger of losing synchronism. Exceeding machine under-excited capability or tripping due to exceeding the loss of excitation protection setting. The UEL must prevent reduction of field current to a level where the generator loss-of-field protection may operate.