Michael Bernhard Buhl

Voltage source operation of parallel PV/wind inverters-stabilized hybrid power plants in “diesel off-mode”- an experimental verification

Microgrids are emerging as an important component of large-scale smart grids. It is a very effective way of integrating various distributed generation resources into the AC power system. Microgrids or off grid hybrid power plants utilize renewable generation to offset fuel cost in remote areas. Maximizing the economic benefit requires maximizing the utilization of the renewable generation and therefore conventional generation to be reduced whenever possible. However conventional generators typically require a certain minimum power set point, and therefore consume fuel, even if there is enough renewable energy. Consequently it is desirable to switch off the conventional generation completely (“diesel-off mode”). During this mode storage systems or synchronous condensers may be used for grid stabilization tasks, which are normally performed by the generators. However only in rare cases the high cost for such extra installations are financially reasonable. This paper presents a novel renewable inverter control method to enable using existing renewable inverters for grid stabilization in diesel-off, and also in diesel-parallel mode. The approach enables parallel connected inverters to run in voltage source mode, compensate load fluctuations and run in parallel to conventional generators. The concept is derived for photovoltaic inverters and can be applied to wind inverters even more easily. By using short-term renewable forecasts the amount of spinning reserve needed to compensate unforeseen renewable fluctuations can be minimized. The control algorithm is presented and its stability is discussed and demonstrated in a laboratory experiment.

Voltage control for Distributed Energy Resources - The value of coordination

Due to the large development and integration of renewable energy generators, voltage control in power distribution grids have become a very important topic for the energy automation systems. In this paper we present aspects regarding the design of local controllers and the overall stability of the control system. The results stem from analytical investigations and from measurements and experiments performed on a low voltage distribution grid. In the paper we will show that voltage asymmetry in the distribution grids cannot be neglected and consequently, the control of the voltage needs to be performed specifically for each phase. Decentralized local controllers can perform the control and, if designed correctly, they are highly efficient with respect to the use of reactive power.