FREQUENCY DROOP CHARACTERISTIC FOR GRID FORMING BATTERY INVERTERS - OPERATION IN ISLANDED GRIDS WITH THE INFEED OF DISTRIBUTED GENERATION SYSTEMS

Mobile generators are used for the auxiliary supply of low voltage grids during maintenance work of distribution system operators. In the islanded operation of conventional mobile generators, distributed generation systems have to be disconnected, by increasing the grid frequency, as they would destabilize the islanded grid. In order to avoid this disconnection, a hybrid mobile generator consisting of a grid forming inverter and a battery storage system is under development. Tests in the real low voltage grid showed that limiting and decreasing the active power output of distributed generation systems by increasing the islanded grid frequency can lead to a higher system stability. To assure a stable operation of the hybrid mobile generator a frequency droop characteristic was developed. The development considers the statistical analysis of publicly available and distribution system operator data, as well as the lessons learned from several islanded grid field tests.