Excitation System and PSS

Excitation System and PSS

EXCITATION SYSTEM AND REACTIVE CAPABILITY CURVE

EXCITATION SYSTEM REQUIREMENTThe performance requirement of the excitation system are determined by considerations of the synchronous generator as well as power system.GENERATOR CONSIDERATION The basic requirement is that the excitation system supply and automatically adjust the field current of the synchronous generator to maintain the terminal voltage as the output varies with the continuous capability of generator.

In addition, the excitation system must be able to respond to transient disturbances with field forcing consistent with the generator instantaneous and short term capabilities.POWER SYSTEM CONSIDERATIONFrom the power system viewpoint, the excitation system should contribute to effective control of voltage and enhancement of system stability. It should be capable of responding rapidly to disturbance so as to enhance transient stability, and of modulating the generator field so as to enhance small-signal stability

FUNCTIONAL BLOCK DIAGRAM OF A SYNCH. GENERATOR EXCITAION CONTROL SYSTEMREFTO POWER SYSTEM

ELEMENTS OF AN EXCITATION SYSTEMEXCITER: Provides dc power to the synchronous machines field winding, constituting the power stage of the excitation systemREGULATOR: Processes and amplifies input control signals to a level and form appropriate for control of the exciter.

TERMINAL VOLATGE TRANSDUCER AND LOAD COMPENSATOR: Senses generator terminal voltage, rectifies and filters it to dc quantity and compares it with a reference, which represents the desired terminal voltage. In addition, load compensator may be provided, if it is desired to hold constant voltage at some point electrically remote from the generator terminal.POWER SYSTEM STABILIZER: Provides an additional input signal to the regulator to damp power system oscillation.LIMITERS AND PROTECTIVE CIRCUITS: These include a wide array of control and protective functions which ensures that capability limits of exciter and synchronous generator are not exceeded.

TYPES OF ECITATION SYSTEMDC excitation systemAC excitation systemStatic excitation system

STATIC EXCITATION SYSTEM(a). POTENTIAL SOURCE CONTROLLED RECTIFIER SYSTEM GENERALIn this system, the excitation power is supplied through a transformer from the generator terminal or the station auxiliary bus and is regulated by a controlled rectifierLIMITATIONThe maximum excitation output voltage is dependent on the input ac voltage. Hence during system fault conditions causing depressed generator terminal voltage , the available exciter voltage is reduced.

POTENTIAL SOURCE CONTROLLED RECTIFIER SYSTEM

(b). COMPOUND SOURCE RECTIFIER SYSTEMGENERALThe power to the excitation system in this case is formed by utilizing the current as well as the voltage of the main generator. This may be achieved by means of a power potential transformer and a saturable current transformer.Alternatively, the voltage and current sources may be combined by utilizing a single excitation transformer, referred to as saturable current potential transformer.

COMPOUND SOURCE RECTIFIER SYSTEM

(c). COMPOUND CONTROLLED RECTIFIER EXCITATION SYSTEMGENERAL This system utilizes controlled rectifiers in the exciter output circuits and the compounding of voltage and current derived source within the generator stator to provide excitation power

COMPOUND CONTROLLED RECTIFIER EXCITATION SYSTEM

DYNAMIC PERFORMANCE MEASUREEXCITATION CONTROL SYSTEM IN CLASSICAL FEEDBACK FORM

LARGE SIGNAL PERFORMANCE MEASURELarge signal performance measure provide a means of assessing the excitation system performance for severe transients such as those encountered in consideration of transient mid term and large term stability of power system. Such measures are:Excitation system ceiling voltageExcitation system ceiling currentExcitation system voltage time responseHigh initial-response excitation systemExcitation system nominal response

SMALL SIGNAL PERFORMANCE MEASURESmall signal performance measure provide a means of evaluating the response of the closed-loop excitation control system to incremental changes in system conditions. Small signal performance may be expressed in terms of performance indicesIndices associated with time responseIndices associated with frequency response

TYPICAL TIME REPONSE TO STEP INPUT

EXCIATTION SYSTEM CONTROL AND PROTECTIVE CIRCUITS

REACTIVE CAPABILITY LIMITS The continuous reactive power output capability is limited by three consideration:Armature current limitField current limitEnd region heating limit

(a). ARMATURE CURRENT LIMITThe armature current results in an RI power loss and the energy associated with this loss must be removed so as to limit the increase in temperature of the conductor and its immediate environmentPer unit complex output is S=P+Qi=EtIt* =|Et||It|(cos+isin)

(b). FIELD CURRENT LIMITBecause of the heat resulting from Rfdifd power loss, the field current imposes a second limit on the operation of the generator.Constant field current locus may be developed by steady state equivalent circuit shown in figure a aside. With Xd=Xq=Xs, the equvalent circuit gives relationship between Et and Eq(=Xadifd).The corresponding phasor diagram is shown in figure b

(a)(b)

P=EtItcos=(Xad/Xs)Etifdsini Q=EtItsin=(Xad/Xs)Etifdcosi-(Et /Xs)The relationship between the active and reactive powers for a given field current is a circle at (-Et /Xs)on the Q axis and with (Xad/Xs)Etifd as the radius as shown in figure c

(c)

(c). END REGION HEATING LIMITThe 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 conditionSECTION VIEW OF END REGION OF A GENERATOR

END REGION HEATING LIMITREACTIVE CAPABILITY CURVES OF A HYDROGEN- COOLED GENERATOR AT RATED VOLTAGE