Three simplified models for adjustable-speed drives are compared with measurements on an actual drive where the inverter control strategy remains unaltered during the event (also known as continue mode of operation) to assess drive behaviour prediction during balanced and unbalanced sags. In particular, the models calculate the dc-link voltage and ac line current dynamic evolution, assuming that neither the drive nor the system protection will trip. The comparison reveals minor differences in behaviour between the three models and the actual drive, although the constant current model exhibits slightly a better agreement. To verify whether differences in the simplified models remain small, extensive ranges of sags are simulated, and drive sensitivity to sag type and depth is illustrated. As the straightforward models studied predict the measured drive dynamic behaviour in severe network conditions with similar accuracy to other unnecessarily complicated models, they are very useful to reduce computational burden and overall complexity for sag immunity as well as dynamic and steady-state network studies. On the other hand, laboratory measurements on the actual drive when the inertia ride-through mode of operation is active illustrate that the simplified models are not valid when the inverter control strategy is modified during the sag.

Peer Reviewed