This study focuses on investigating the interactions between an energy supplier and a grid operator under the renewable portfolio standard (RPS) policy. The energy supplier produces both green power and thermal power for the grid operator, who in turn is obliged to provide each type of power to the end consumers. The cost of generating green power is faced with a learning-by-doing effect. Several interesting results are obtained. First, the change trend of the electricity supplier's and grid operator's profits with the quota obligation is non-monotonic. Second, interestingly, with the increase of quota obligation, green power consumption may decrease and thereby increase carbon emissions. Third, both the grid operator's profit and the green power consumption levels can be increased through a higher operation cost. Our findings reveal the importance of considering the grid operator and learning-by-doing effect when improving the utilization rate of renewable energy.