This article builds a simulation model to analyse the effect of a change in the main policy variables of the British Renewable Obligation System, i.e. how the increase in the minimum requirement of green electricity can affect the prices and quantities in the electricity market under oligopoly market structure. The results show that the increasing quota obligation increases the price of electricity. The outcomes of this computational model emphasize the importance of the capacity limit which can constrain the market power and increase the competition in the market. In addition, the simulation shows that the integration of fossil-fuel and renewable producers has very small effect on production and prices because of the capacity limits.