IEEE 421.5:2005 pdf free download – IEEE Recommended Practice for Excitation System Models for Power System Stability Studies
Most excitation systems represented by the Type AC and ST models allow only positive current flow to thefield of the machine, although some systems allow negative voltage forcing until the current decays to zero.Special provisions are made to allow the flow of negative field current when it is induced by thesynchronous machine.Methods of accommodating this in the machine/excitation system interface forspecial studies are described in Annex G.
4.Synchronous machine terminal voltage transducer and currentcompensator models
Several types of compensation are available on most excitation systems. Synchronous machine active andreactive current compensation are the most common. Either reactive droop compensation andor line-dropcompensation may be used, simulating an impedance drop and effectively regulating at some point otherthan the terminals of the machine. The impedance or range of adjustment and type of compensation shouldbe specified.
Droop compensation takes its name from the drooping (declining) voltage profile with increasing reactivepower output on the unit. Line-drop compensation， also referred to as transformer-drop compensation,refers to the act of regulating voltage at a point partway within a generator ‘s step-up transformer or, lessfrequently, somewhere along the transmission system. This form of compensation produces a rising voltageprofile at the generator terminals for increases in reactive output power.
A block diagram of the terminal voltage transducer and the load compensator is shown in Figure 4-1.Thesemodel elements are common to all excitation system models described in this document. It is realized that,for some systems，there may be separate and different time constants associated with the functions ofvoltage sensing and load compensation. The distinction is not recognized in this model, in which only onetime constant, Tp, is used for the combined voltage sensing and compensation signal. Single-phase voltageand current sensing will, in general, require a longer time constant in the sensing circuitry to eliminateripple.
When load compensation is not employed(Rc=Xc=0), the block diagram reduces to a simple sensingcircuit. The terminal voltage of the synchronous machine is sensed and is usually reduced to a dc quantity.While the filtering associated with the voltage transducer may be complex, it can usually be reduced, formodeling purposes, to the single time constant Tp shown.For many systems, this time constant is very smalland provision should be made to set it to zero.
5.1 Type DC1A excitation system model This model, described by the block diagram of Figure 5-1， is used to represent field- controlled dc commutator exciters with continuously acting voltage regulators (especially the direct-acting rheostatic, rotating amplifier, and magnetic amplifier types).5 Because this model has been widely implemented by the industry, it is sometimes used to represent other types of systems when detailed data for them are not available or when a simplified model is required.
The principal input to this model is the output, VC, from the terminal voltage transducer and load compensator model previously described. At the summing junction, terminal voltage transducer output, VC, is subtracted from the set point reference, VREF: The stabilizing feedback, VF, is subtracted and the power system stabilizing signal, Vs, is added to produce an error voltage. In the steady state, these last two signals are zero, leaving only the terminal voltage error signal. The resulting signal is amplified in the regulator. The major time constant, TA, and gain, KA, associated with the voltage regulator are shown incorporating non- windup limits typical of saturation or amplifier power supply limitations. A discussion of windup and non- windup limits is provided in Annex E. These voltage regulators utilize power sources that are essentially unaffected by brief transients on the synchronous machine or auxiliary buses. The time constants, Tp and Tc. may be used to model equivalent time constants inherent in the voltage regulator, but these time constants are frequently small enough to be neglected and provision should be made for zero input data.