A simple method for parallel operation of inverters

Telecom applications require uninterruptible power supplies for different power levels. If the number of different converter systems should be kept low, parallel operation of inverters is of interest. Thus, a master/slave architecture of an inverter system is presented which allows parallel operation of an arbitrary number of inverter modules by a simple control method. All inverter modules can be equal and no extra overall control unit is needed. The decision, which of the modules becomes master, is made via a single common status line automatically. The output current of the master is used as a command reference current for all slaves. It is shown that the master inverter module is able to stabilise the common output voltage independent of the number of slaves. Furthermore it is investigated how the load current and the reactive filter current is shared by the modules. Apart from results of a simulation, experimental work is presented which confirms the theory.