Subtransient reactance impacts the ability of a generator to work as much like a utility as possible so system loads can work normally when running on a generator set. The objective of an alternator selection process is to get as close to a utility capability as possible. From a generator supplier’s perspective, that means that a machine with relatively low subtransient reactance is needed. Such machines usually cost a bit more, though, and because subtransient reactance is not easy to understand or measure in the field, it’s often overlooked or ignored. It is possible to get a machine with subtransient reactance that’s too low especially in larger paralleling applications, so it’s necessary to understand exactly what you’re doing when you select an alternator.
A generator providing standby power for a data center or critical environment can be exposed to significant levels of total harmonic distortion (THD) from the significant number of nonlinear loads. In these environments, it is critical to specify a generator with a low alternator subtransient reactance. Preferably, the subtransient reactance is in the 8% to 12% range. A generator with a high subtransient reactance in an environment with high THD current will produce a high THD voltage waveform that will affect the entire electrical distribution system.
IEEE standards provide guidance for alternator selection based on THD from the generator and application to load and fault duty. Balance the needs with the THD from the loads, generator, and voltage regulator and with the short-circuit capabilities of the switchgear and other equipment based on the results of a properly analyzed fault and protective relaying study.
