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EE 220

Lecture 5: Introduction to Power System Control

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Power System Control

Power system control involves the use of varioustechnology and methodology in maintaining certaindynamic performance of a power systemPower system control is closely related to power

system stability

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Power System Stability

Main purpose: Ability to maintain equilibriumDivided into two main types

Angle StabilityTorque Balance of synchronous machine

Voltage Stability Ability to maintain steady voltage

Disturbance disrupts equilibriumi.e. Faults

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Power System Stability

Practical power system network is highly meshedwith different interconnected equipment, loads andgeneratorsUnder ideal condition, the power system operates at

desired frequency i.e. 60 HzHowever power system is imperfect, load variationoccurs and generators vary its load as wellElectrical power output of generator is dependent onthe rotational performance of synchronous machineMachine must maintain its synchronous speed fornormal operation

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Transient Stability

Ability of the power system to maintain synchronismwhen subjected to a severe transient disturbance,such as a short circuit on transmission line

A power system is transiently stable for a particular

steady-state operating condition and for a particular(large) disturbance or sequence of disturbances if,following that (or sequence of) disturbance(s) itreaches an acceptable steady-state operating

condition.

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stabl

first swinginstabilit

t

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Voltage stability ability of a power system tomaintain steady acceptable voltages at all buses ofthe system after being subjected to a disturbancefrom a given initial operating condition.

Voltage collapse the process by which thesequence of events accompanying voltage instabilityleads to a blackout or abnormally low voltages in asignificant part of the power system.

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Frequency stability the ability of a power systemto maintain steady frequency following a severesystem upset resulting in a significant imbalancebetween generation and load.

Philippines60 HzOverfrequency = excess generationUnderfrequency = deficiency in generation

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Voltage Control

Voltage on power system is primarily dependent onreactive power or MVARSources

Generator

Capacitor/Reactor

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Generator Control

Synchronous Generators are the common generatorin a power systemThe reactive power of a generator is dependent onits excitation system

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Generator Excitation system

Main functionProvide DC current to field windingThe field winding is primary responsible in maintainingproper voltage at generator terminal

Generator exciter can be quickly modified for voltageregulationHowever MVAR production of generators depends on itscapability and availabiliy

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Elements of an Excitation System

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Elements of an Excitation System

EE 220

Exciter provides DCpower to generator field

winding

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Elements of an Excitation System

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Process or amplifies inputcontrol signal for exciter

control (feedbackamplifier)

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Elements of an Excitation System

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Ensure that the capabilitylimits of the exciter is

met and protected

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Frequency Control

Frequency is controlled by balancing supply anddemandGovernor system in generators senses any changein rotational speed to adjust its mechanical input

power

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Governor Modes of Operation

Droop ModeDecrease in speed due to increase in loadIf the frequency decreases (droops), the governor willrespond by increasing fuel output until reference speed isattainedDroop mode ensures that the increase in fuel position isenough to prevent over-speeding when the systemreturns to normalIn a meshed network with several generators need toadjust its speed at a single reference to ensure that eachgenerator will adjust to demand proportional to itscapacity

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Governor Modes of Operation

Isochronous ModeFixed frequency regardless of loadGovernor will maintain its frequency setting of thegenerator despite load variation

Usually used on isolated system

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Generator Power Generation andControl Blocks

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Reporting

FACTS DevicesStatic VAR Compensator (SVC)Static Compensator (STATCOM)Universal Power Flow Controllers (UPFC)

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