Question

1-What are normal allowed voltage variation, and frequency variation

2-What is purpose of islanding?

Answer 1

1. The National Electrical Manufacturers Association (NEMA) standards recommend that motors should be designed to operate satisfactorily at a voltage variation of plus or minus 10 percent. For 230 volts, that range is 207 to 253 volts; for 460 volts, it is 414 to 506 volts

2. Under/over voltage

A system that periodically samples voltage and looks for sudden changes can be used to detect a fault condition. Under/over voltage detection is normally trivial to implement in grid-interactive inverters, because the basic function of the inverter is to match the grid conditions, including voltage.