Development of parallel operation of UPS with variable virtual resistance and flexible communication

This paper presents a complete paralleling control method to be used for the parallel inverters connection. The algorithm performs a phase/frequency droop, and presents a new method for the voltage control that operates with or without data communication among the inverters. When communication is operating, a new algorithm is used inserting a variable virtual resistance in the inverter output control, along with active power sharing control, obtaining a low THD value and very good power sharing. Without communication a constant virtual resistance is inserted, increasing the output THD with non-linear load, but still allowing a good active power sharing. The advantage is to obtain a good performance operation with communication, but, in case of communication failure, the system still works providing more reliability. The experimental results are obtained with the parallel connection of two inverters with 8.4 kW and 5.6 kW of nominal output power. The control algorithm allows the correct operation with the communication connection or disconnection, as well with the insertion or removal of the parallel inverters during the system operation.