Restoration issues in large metropolitan power systems: An example in the Berlin distribution grid

This paper presents results of a restoration field test carried out by the Distribution System Operator (DSO) in the city of Berlin. Based on a previous study, the company decided to use a gas-fired power plant with black start capability at the border of its service territory to start the restoration process in coordination with the Transmission System Operator (TSO). In a subsequent stage a larger combined cycle gas power plant located in the downtown area is intended to be brought into service. The test carried out included the start-up of the smaller unit, its connection to the larger power plant via a 110-kV line, synchronization of both generators, a short parallel operation, and identification of the minimum load on the smaller unit so that it would not trip by the reverse power protection upon synchronization with the larger unit. This paper reports on the experimental set-up and measurement results as well as the simulation study carried out afterward. The purpose of the simulation was to identify the corresponding dynamic models of the involved gas turbines and to verify the first crucial steps during the restoration process. Based on the experience with the Berlin distribution system the authors will provide some general recommendations concerning the autonomous restoration of supply to large metropolitan areas.