Second , this paper analyzes the relation of reactive support service with voltage , power limit curve , transaction amounts . according to the power limit curve , this paper analyzes the source of the reactive cost to get an interzone cost function . using marginal cost theory , it concludes that the reactive spot price is the derivative of the cost function in generator buses 另外，本文还从发电机角度上分析了无功服务与电压、功率限制曲线、发电机向负荷传输电能大小的关系；根据功率限制曲线对发电机无功成本的来源进行详尽剖析，从理论上确定一个分区间的成本函数；并依据短期边际成本理论可推出，计算发电机节点的电价可采用无功成本函数的偏导数的方法。
