Frequency stability is concerned with the ability of synchronous generators to supply their loads within an acceptable frequency range after large generation or load unbalances. The frequency stability is governed by the inertia of the rotating masses of the generators and the response of their primary frequency regulation. Frequency stability is a key aspect of the operation of power systems. In large interconnected power systems like the European one, the large rotating kinetic energy and the quality of the primary frequency regulation has prevented frequency stability to affect both quality and security of the supply of electrical energy. However, the incident of the European system of November 4, 2006 has proved that severe disturbances that lead to large frequency excursions can occur. Such frequency excursions led to the tripping of large amounts wind power generation in a number of countries because of the settings of the frequency relays of the wind power generators. This paper contains a fundamental study on the impact of wind power generators in the frequency stability of synchronous generators. Wind power generators of different technologies (squirrel cage and doubly fed induction generators and on multi-pole synchronous generators) have been considered. Two simple test systems have been developed for this purpose. The first test system contains a synchronous generator feeding a load. The second test system comprises of a synchronous and a wind power generator feeding a load. It has been found that when the system includes wind power generators, the steady-state value of the frequency after of a generation-load unbalance (i.e. a generator tripping) is determined by primary frequency regulation capability of both the synchronous and the wind power generators. If wind power generators do not provide primary regulation, the transient variation of the frequency depends on the inertia of the synchronous generators and the speed of primary frequency regulation of the synchronous generators. It means that systems with high wind power penetration will experience larger transient frequency excursions in case of generation trip due to the lack of inertia of the wind power generators.
Keywords: Frequency stability, synchronous generators, wind power generators, squirrel cage induction generators, doubly fed induction generators
Cigré Session 2008, Paper no. A1-203. Paris, France. 24-29 August 2008
Published: August 2008.