Microprocessor-based overcurrent protection for PV powered air-conditioning system

Abstract When photovoltaic (PV) systems are used to drive a.c. loads, d.c./a.c. inverters are necessary to convert the d.c. power to a.c. power. Dynamic a.c. loads, driven by single phase induction motors, require high starting current. It is known that the r.m.s. value of the starting current is about seven times the rated r.m.s. current of the motor. A theoretical and experimental investigation of the instantaneous starting current showed that it reaches values much higher than seven times the rated r.m.s. current. If the A/C is driven by an infinite a.c. source, such a high instantaneous current does not affect the source. However, if the A/C is driven by a PV array through an electronic inverter, such a high starting current can damage the electronic components of the inverter. In this work, a microprocessor-based control system to limit the high starting current is discussed. The proposed technique measures the rate of change of current and, hence, predicts the current at the forthcoming moments. According to this measured value, the microprocessor determines whether the A/C should be disconnected from the inverter or not. The microprocessor is programmed in such a way as to give the disconnect command before the current reaches the maximum allowable limit. The advance interval is equal to that needed to turn off the main electronic switch connecting the A/C to the inverter. Thus, the proposed control system guarantees a secure operation of the inverter. If the rate of change of the current is below a certain level, the current will be within the allowable limits, and the microprocessor system keeps the load connected to the inverter. The proposed microprocessor control system offers protection against the inrush starting current and keeps monitoring the motor current even after the starting period. If the motor current is accidentally increased during normal operation because of any problem, short circuit for example, the control system disconnects the motor from the source. Thus, the system provides continuous protection for the motor and the source.