Stability and power sharing in microgrids
Abstract:
Microgrids represent a promising concept to facilitate the integration of distributed renewable generation units into the electrical grid. A microgrid gathers a combina- tion of generation units, loads and energy storage elements at distribution level into a locally controllable system. This system can be operated either connected to or completely isolated from the main transmission grid. Three important performance criteria in such networks are frequency stability, voltage stability and power sharing. A widely used, though heuristic, control solution to address these problems is droop- control. First, we review the most commonly used droop-control laws for networks with dominantly inductive power lines. Thereafter, assuming the transfer conductances in the microgrid can be neglected, a port-Hamiltonian description of a droop-controlled microgrid is derived, from which sufficient conditions for local stability are deduced. In addition, we propose a design criterion for the controller gains such that a desired active power distribution is achieved in steady-state. Furthermore, our analysis reveals that droop-control does in general not guarantee a desired reactive (as opposed to ac- tive) power distribution. We therefore propose a distributed consensus-based voltage control scheme, which solves the problem of reactive power sharing. Under the as- sumption of small angle differences, we prove that the choice of the control parameters uniquely determines the corresponding equilibrium point of the closed-loop voltage and reactive power dynamics. Also, a necessary and sufficient condition for local ex- ponential stability of that equilibrium point is given. The analysis is illustrated via a simulation example of a microgrid based on the CIGRE benchmark medium voltage distribution network.
Biography:http://www.control.tu-berlin.de/User:Johannes_Schiffer