There are three modes of operation for generators: stand-alone, island and parallel with the utility. Each operation mode requires specific turbine fuel and generator excitation controls. In this post I describe features of each mode of operation.
Stand-alone Operation
In stand-alone operation a generator is not connected with other generators or the utility. As an isolated unit it supplies power to all connected load. Examples are emergency generators, aggregates or portable diesel generator sets.
Stand-alone Operation – the governor and voltage regulators determine frequency and voltage.
Features:
- more / less fuel will raise / lower the frequency (Hz)
- more / less excitation current will raise / lower the voltage (kV)
- the total load determines the generator output power (MW, MVAr)
- engine fuel control options: isochronous speed control, droop speed control
- excitation current control options: constant voltage control, droop voltage control
- power management system could keep bus frequency and voltage constant if droop control modes are active (else not necessary)
- load flow analysis model: Slack Bus (also Reference Bus, Swing Bus)
Paralleled With The Utility Operation
If a generator is paralleled with the utility grid and it is considered as an infinite bus, the utility grid will determine the generator frequency, speed and voltage. An example is an industrial generator used for cogeneration. In practice such an engine generator set can not change the frequency, but it can slightly change the generator terminal voltage.
Paralleled With Utility Operation – if considered as an infinite bus, the utility grid will determine the generator frequency, speed and voltage.
Features:
- more / less fuel will raise / lower the generator active power (MW)
- more / less excitation current will raise / lower the generator reactive power (MVAr)
- the utility determines the frequency (Hz), speed (rpm) and voltage (kV)
- the difference between the total plant load and the generator output power will be imported or exported (MW, MVAr)
- engine fuel control options: droop speed control, base load (MW) control
- excitation current control options: droop voltage control, VAR control, PF control
- power management system could keep the imported or exported power (MW, MVAR) or power factor constant
- load flow analysis model: PQ bus (also Load Bus), PV bus (also Generator Bus)
Island Operation
In island operation a generator is connected with other generators, but not with the utility. As an isolated system the generators supply all power to the connected load. Examples are power systems on ships, on offshore platforms or in the desert.
Island Operation – all generators together determine frequency, speed and voltage. The total load (MW, MVAr) should be shared.
Features:
- more / less fuel will raise / lower generator active power and the bus frequency (MW, Hz)
- more / less excitation current will raise / lower the generator reactive power and the bus voltage (MVAr, kV)
- the total plant load determines the sum of the power of all generators (MW, MVAr)
- engine fuel control options: droop speed control
- excitation current control options: droop voltage control
- power management system keeps the bus frequency and voltage constant, while sharing the load (MW, MVAr) proportional or cost optimised over all engine generator sets
- load flow analysis model: at least one generator (or a fictive untility) must be slack bus, the other generators may be PQ or PV buses
Note that cross-current compensation is sometimes used to share reactive power, while maintaining the bus voltage constant. I prefer a power management system or VAr Load Sharing devices.
Mode Of Operation Changes
All modes of operation are possible after closing or tripping breakers in the configuration in the image below. During the power system engineering, this should be taken into consideration. I highly recommend to use a power management system in this configuration.
Mode Of Operation Changes – all modes are possible after closing or tripping breakers.
See also
References
- Woodward Application Note 01302 – Speed Droop and Power Generation, 1991
- Woodward Manual 26260 – Governing Fundamentals and Power Management, 2004
- Basler Electric Technical Paper – Parallel Operation with a Network System, 2002
- Basler Electric Technical Paper – Voltage Regulator and Parallel Operation, 2002
