Testing of three-phase equipment under voltage sags

This paper provides insight into the testing of three-phase equipment exposed to voltage sags caused by faults. The voltage sag recovers at the fault-current zeros, leading to a `discrete´ voltage recovery, that is, the fault is cleared in different steps. In the literature, the most widespread classification divides `discrete´ sags into 14 types. The authors study shows that it is generally sufficient to consider only five sag types for three-phase equipment, here called `time-invariant (TI)´ equipment. As the remaining nine sag types cause identical equipment behaviour in Park or Ku variables, the number of laboratory tests (or of extensive simulations) on equipment under sags is reduced by a ratio of 14/5. The study is validated by simulation of a three-phase induction generator and a three-phase inverter, which are `TI´, and a three-phase diode bridge rectifier, which is not `TI´. Both analytical study and simulation results are validated by testing a three-phase induction motor and a three-phase diode bridge rectifier.