IEEE 421.5-2005 pdf free download – IEEE Recommended Practice for Excitation System Models for Power System Stability Studies.
Excitation control elements include both excitation regulating and stabilizing functions. The terms cxci tation system stabilizer and transient gain reduction are used to describe circuits in several of the models encompassed by the excitation control elements shown in Figure 3-I that affect the stability and response of those systems.
Recently, modeling of field current limiters has become increasingly important, resulting in the addition to this recommended practice of Clause 9 and Clause 10 describing overexcitation and underexcitation limiters (OELs and UELs. respectively). The individual excitation system models in this document show how the output signals from such limiters (VOEL and VUEL) would normally be connected.
The output of the UEL may be received as an input to the excitation system (V,,FL) at various locations, either as a summing input or as a gated input, but for any one application of the model, only one of these inputs would he used.
For the OEL sonic models provide a gate through which the output of the overexcitation limiter or tenninal voltage limiter (VOEL) could enter the regulator loop.
In the implementation of all of the models, provision should be made for handling zero values of parameters. For some zero values, it may be appropriate to bypass entire blocks of a model.
The per unit (pu) system used for modeling the excitation system is described in Annex B.
Three distinctive types of excitation systems are identified on the basis of excitation power source, as follows:
a) Tipe DC excitation systems, which utilize a direct current generator with a commutator as the source of excitation system power (see ClauseS)
h) Type .4C exCitation sctenss, which use an alternator and either stationary or rotating rectifiers to produce the direct current needed for the synchronous machine field (see Clause 6)
c) Type STexcilution systems, in which excitation power is supplied through transformers or auxiliary generator windings and rectifiers (see Clause 7)
The following key accessory functions common to most excitation systems are identified and described as follows:
I) Voltage sensing and load compensation (see Clause 4)
2) Power system stabilizer (see Clause 8)
3) Ovcrexcitation limiter (see Clause 9)
4) Underexcitation limiter (see Clause 10)
5) Power factor and var control (see Clause 11)
6) Discontinuous excitation controls (see Clause 12)
In addition, models for some supplementaiy discontinuous excitation controls are provided.
Most excitation systems represented by the Type AC and ST models allow only positive current flow to the field 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 the synchronous machine. Methods of accommodating this in the machine/excitation system interface for special studies are described in Annex G.
4. Synchronous machine terminal voltage transducer and current compensator models
Several types of compensation are available on most excitation systems. Synchronous machine active and reactive current compensation are the most common. Either reactive droop compensation and,or line-drop compensation may be used, simulating an impedance drop and effectively regulating at some point other than the terminals of the machine. The impedance or range of adjustment and type of compensation should he specified.
Droop compensation takes its name from the drooping (declining) voltage profile with increasing reactive power output on the unit. Line-drop compensation. also referred to as irunsfiiriner-drop conipensasion, refers to the act of regulating voltage at a point partway within a generator’s step-up transformer or. less frequently, somewhere along the transmission system. This form of compensation produces a rising voltage profile 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. These model elements arc 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 of voltage sensing and load compensation. The distinction is not recognized in this model, in which only one time constant, TR, is used for the combined voltage sensing and compensation signal. Single-phase voltage and current sensing will, in general. require a longer time constant in the sensing circuitiy to eliminate ripple.
When load compensation is not employed (Rc=Xc=O), the block diagram reduces to a simple sensing circuit. 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 he coniplex, it can usually he reduced, for modeling purposes. to the single time constant TR shown. For many systems. this time constant is very small and provision should be made to set it to zero.IEEE 421.5 pdf download.