Intermediate circuit capacitor-current in inverters for uninterruptible power supplies without transformer

In recent years the undisturbed electrical power supply became very important, because of the increasing automation (Schmidt, et al., 1995). A protected electrical power supply is possible by using a static uninterruptible power supply (UPS), because its electrical energy is stored in a DC-link intermediate circuit, connected with a battery. An important cost factor and power loss producer is the usually inserted line frequency output transformer. The elimination of the transformer causes a changing of the circuit topology and a rising of the DC-link voltage. It is possible to omit the transformer by connecting the ground wire of the load to the centre point of the DC-link capacitor. The DC-link voltage has to be higher than the sum of the peak value of the sinusoidal output voltage and the voltage drop of the output filter inductance at maximum load (e.g. U_d= 800 V) (Patt, M, 2002). A second cost factor is the DC-link capacitor. Its reactive power capacity becomes bigger compared to conventional UPS because the low-frequency load current flows through the centre point of the DC-link capacitors. This is an essential and an expenditure property (F. Renken et al., 2002). The centre of the paper is the analysis of the DC-link capacitor current in different topologies. In the first part of the analysis the current-loading of a two and a three level voltage source inverter will be described. In the second part of the paper a three phase two-level and a three phase three level-inverter will be compared.