IEEE 421.2-2014 pdf free download – IEEE Guide for Identification, Testing, and Evaluation of the Dynamic Performance of Excitation Control Systems.
4. Dynamic performance classification
4.1 Overall The subclauses below describe methods for tuning and testing excitation system functions as applied to synchronous generators. The techniques and discussions apply directly to analog electronic proportional style controls with feedback from various measured signals and measured with external testing and recording instruments. Actual excitation system equipment and test equipment, however, have evolved from magnetic and rotating devices, through analog electronic implementations to digital control implementations. Both the controls and the test equipment details must be understood and the techniques described below adapted appropriately to achieve the desired tuning goals. Additionally, modern controls may have multiple modes of operation where gains or feedback signals are changed to allow better performance depending on unit conditions such as start-up, on-line at full load, or shut-down. As a result, tests performed in one operating condition, e.g., off-line, may not capture the control gains and time constants which are in service under load. A thorough review of the control gains and time constants settings as well as logic, which may alter these parameters, must be performed prior to embarking on a testing and tuning program.
4.2 Large signal performance Large signal performance is the response of an excitation control system, excitation system, or elements of an excitation system to signals that are large enough that nonlinearities must be included in the analysis of the response to obtain accurate results. The purpose of large signal performance criteria is to provide a means of evaluating excitation system performance for severe transients that may include large variations in synchronous machine stator voltages,synchronous machine stator currents, and induced synchronous machine field currents; that is, for transients affecting power system transient stability. To assess the ability of the excitation system to improve synchronous machine performance, the criteria must reflect the effects of operation under realistic power system disturbances. With respect to performance testing, it is often impractical to adequately duplicate all of these effects. In cases where tests can only be made on individual components and only at partial load or open circuit, analytical means may be used to predict performance under actual operating conditions. Criteria used to assess the large signal performance include quantities such as dynamic responses, ceiling currents and voltages, voltage response times, and nominal responses derived from dynamic responses (see IEEE Std 421.1™).
4.3 Small signal performance Small signal performance is the response of an excitation control system, excitation system, or elements of an excitation system to signals that are small enough that nonlinearities can be disregarded in the analysis of the response, and operation can be considered to be linear. Small signal performance of an excitation control system or its components can be assessed from dynamic responses, frequency responses, or by eigenvalue analysis. (See IEEE Std 421.5, Committee Report, 1973 [B6], IEEE Tutorial Course [B23], Kundur [B29], Kundur, et al. [B30], for examples.) Small signal performance criteria provide a means of evaluating the response of systems for incremental load changes, incremental voltage changes, and the incremental changes in synchronous machine rotor speed associated with the initial stages of oscillatory instability (where oscillations are small enough that nonlinearities are insignificant). Small signal performance data provide a means for determining or verifying excitation system model parameters for system studies (see IEEE Std 421.5, Hurley and Baldwin [B4], and Dandeno, et al. [B41]). The assumption of linearity limits the application of small signal models as noted above.
4.4 Effects of excitation limiters A modern excitation system used on a synchronous machine may have sufficient excess capability to supply excitation under all operating conditions. The voltage regulator utilizes various auxiliary control and limiting functions that are designed to enhance power system performance and reliability while protecting both the excitation system and the synchronous machine. These limiting functions generally have no effect on the excitation output during normal operating conditions, but protect the exciter and machine during severe unit or power system disturbances in which the excitation system is pushed near or beyond defined operating limits (see Carleton, Bobo, and Burt [B40], Eberly and Schaefer [B42], Overexcitation Limiting Devices [B43], Underexcitation Limiter Models [B44], Nagy [B46], Ribeiro [B47], Rubenstein and Temoshok [B48]). Typical excitation limiters include overvoltage, volts/hertz, overexcitation (both instantaneous and timed), underexcitation, and stator current limiters. These limiters may act to either decrease or increase the excitation level of the synchronous machine, depending on the type of limiter and the power system circumstances. Where a power system stabilizer (PSS) is applied, limiters should coordinate with PSS functions, and preferably not block the stabilizing signal. (This is more critical for underexcitation limiters as instabilities develop more readily at reduced levels of machine field current.).IEEE 421.2 pdf download.