Paralleled Inverter Systems

Research project P 13894 Paralleled Inverter Systems Franz ZACH 28.06.1999 Based on the technical results of the FWF-project P12075-TEC/FWF476 and the resulting questions concerning the parallel operation of inverters (stand alone grid and public mains connected inverter groups) as well as based on several interesting discussion sessions about the problematic of paralleled inverters a new project should be initiated to determine the special problems of this topic. Furthermore, possible solutions should be developed and published. The parallel connection of inverters in the low power range (several NOW up to some kilowatts) lead especially in small grid topologies to major problems. Power pulsations can occur due to frequency and/or phase errors in between each inverter and the grid. This will lead to additional losses in the whole system. A further technical aspect is the operational security. Local standards require in case of inverters connected to public mains a reliable detection of grid shutdown in no more than 5sec. This will require a major additional technical equipment to determine the grid impedance to ensure the required standard. implementing this measurement equipment in the inverter will lead to excessive reduction in cost and will rise up the total system reliability. Especially in small local grid topologies it is necessary to analyze the problems of paralleling the inverters; in detail to develop a: good solution. The described problems should be the basic of the new project. In conventional inverter concepts primarily the mains current and the mains voltage are measured. This is the basic to control the produced current in sinusoidal form. Based on the used measurement principles phase errors will occur. Even when complex PLL-algorithms are used, differential phase errors will occur (e.g., as a result of rapid connecting a motor to the grid). The resulting phase errors (especially their tendency) and the parasitic impedance of the grid (as well as the parasitic impedance of the inverter itself) lead to a phaseshift (a voltage distortion) at the mains junction. As a result, power pulsation will occur (in the range of several percent up to 10% of delivered inverter power) in between the inverter and the grid. The resulting losses lower the total efficiency of the system. The described problems will occur especially in small wind-energy and photovoltaic plants. A further point of view is the intelligent power management of inverter groups. This also could be used to improve the overall efficiency extremely. As a side effect, the redundancy is increased and it is more easy to control the output voltage waveform. The planned work will continuously follow the results produced by the current project (P12075TEC/FWF476). They will lead to system designs which are of major interest in case of solar and wind energy plants.