Current limitation control for multi-module parallel operation of UPS inverters

A current limitation control technique for multi-module parallel UPS inverters is presented. All inverter modules have 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 idling. 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 interconnections can be minimised. The master module can also be switched online with negligible disturbance for voltage regulation. As only one voltage loop is active at any time, the proposed method is the most stable one when compared with other methods with control interconnection. A robust controller for maintaining good voltage regulating performance even when the parallel number of modules has a large change is also proposed. Three 1 kVA modules have been implemented. The effectiveness of the proposed technique is confirmed with some simulation and experimental results.