The Influence of Impedance Values and Excitation System Tuning on Synchronous Generator Stability in Distribution Grids

Synchronous machines installed in recently developed small-scale hydro power plants in Norway have relatively high values of the synchronous reactances, as well as the sub-transient and transient reactances. Typical synchronous reactance is 2 – 2.5 pu, but values up to 3.1 pu are experienced, which is 3 times the typical values for larger low-speed synchronous generators (with salient poles). High reactance in synchronous machines is a result of design, and is characteristic for machines with a small airgap, which again gives a cheaper machine. In the present analysis, small-signal stability of a real life single-machine infinite bus system is studied for two different synchronous machine parameter value sets, one reflecting typical machine reactances and one with high reactance values. The analysis is performed in relation to active and reactive power production levels, and parameter settings for the automatic voltage regulator (AVR). The machine having “typical” parameters is small-signal stable for all situations studied. The stability of the machine with high reactance values, however, is strongly dependent of production level and AVR parameter settings. Stability margins and operation limits are given for the studied real life case. It is recommended that detailed power system analyses are performed, when the use of high reactance synchronous machines is considered, to ensure that stable operation can be obtained.