Current limitation control technique for parallel operation of UPS inverters

This paper presents a current limitation control technique for multi-module parallel UPS inverters. All inverter modules are with voltage and current control loops. However, only the voltage loop of the master module is active. The current command of the master module is generated by its voltage controller. Voltage loops of all slave modules are idle running. The slave module is only for sharing load current with its current command generated from the previous module. By limiting the output current command of each module and sending the difference between the limited and unlimited current commands to the next module, each module only has to share the load current not completely provided by all previous modules. Through circular chain connection of the modules, the number of control interconnection can be minimized. The master module also can be switched on-line with negligible disturbance for voltage regulation. Due to only one voltage loop is active at any time, the proposed method is the most stable one as compared with other methods with control interconnection. This paper also proposes a robust controller for maintaining good voltage regulating performance even when the parallel number of modules has a large change. Three 1KVA modules are implemented. The effectiveness of the proposed technique is confirmed with experimental results.