Market power refers to conditions where the providers of a service can consistently charge prices above those that would be established by a competitive market. There are many well known definitions of market power, including indices intended to quantify the degree of market concentration of energy supplies. This paper explores a method by which one of the standard indices in market power analysis (the Herfindahl-Hirschman Index) can be used to measure reactive power market concentration. The proposed method is illustrated by means of a simple example.
