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Question 5 Discuss (using diagrams where appropriate) possible generators modes of operation and how this effects the generator speed. Total 10 marks.

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IMAGE LOAD G1 Possible generators modes of operation:- 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. (0) 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. Feature:- >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) Total Plant Load Generator with (ΣΜW, ΣΜVAr) Speed Governor Voltage Regulator Stand-alone Operation - the governor and voltage regulators determine frequency and voltage.

UTILITY GRID (infinite bus) (i) Parallel 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. Feature:- >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: PO bus (also Load Bus), PV bus (also Generator Bus) LOAD G1 Total Plant Load IΣMW. ΣΜΥΑ) Generator with Speed Governor Voltage Regulator Paralleled With Utility Operation - if considered as an infinite bus, the utility grid will determine the generator frequency, speed and voltage.

LOAD G1 G2 GC (iii) 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. Feature: >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) >Eengine 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 Total Plant Load Generator with (ΣΜΝ, ΣΜVAr) Speed Governor Voltage Regulator Island Operation - all generators together determine frequency, speed and voltage. The total load (MW, MVAr) should be shared.

Effect on generator speed:- speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a synchronous generator connected to an electrical grid. It works by controlling the rate of power produced by the prime mover according to the grid frequency. This mode allows synchronous generators to run in parallel, so that loads are shared among generators with the same droop curve in proportion to their power rating.