A new advanced over excitation limiter for enhancing the voltage stability of power systems

Mitsubishi Electric has developed an advanced over excitation limiter (OEL) that not only protects the generator field winding from overheating, but also allows the generator to supply reactive power up to its maximum limitation. This in turn allows for the enhancement of voltage stability of a power system by allowing the full range of reactive power supplying ability of a generator to be utilized. Different from the conventional OEL, the limiting margin of the advanced OEL is estimated from online computations based on the field current and voltage measurements of the generator. Hence, the "surplus" and "deficiency" that exists with the conventional OEL margin are omitted in the advanced OEL. The advanced OEL is implemented as a function in a digital automatic voltage regulator (D-AVR). In this paper, the principles and features of the advanced OEL are described. A comparison between the advanced OEL and conventional OEL based on numerical simulations is given. Also, a power system application example of the advanced OEL applied to a synchronous condenser is given to show its effectiveness.