During the construction of electricity spot market at this stage in China, unbalanced costs have appeared during the settlement of spot market trial operation in all pilot areas. The specific subject bearing responsibility for unbalanced cost is not clear. Besides, unbalanced costs are not inevitable in the operation of spot market in China during transition period. Therefore, the calculation and allocation methods of unbalanced costs are the key and difficult problem in the design of spot market rules in the transition period. Based on this background, this paper first analyzes two double-track modes of China's spot market reform under parallel mechanism of planning and market in the transition period, which are decoupling operation mode and coupling operation mode. Based on the problems that the decoupling mode will bring about, such as influencing the degree of market opening and going against the optimal allocation of resources, the coupling mode is chosen as the double-track mode under the parallel mechanism of planning and market. Secondly, this paper analyzes the settlement mode of medium-long-term contracts and spot market under the coupling double-track mode, which is suitable for the current stage of electricity spot market construction under the node pricing mechanism in China. Finally, based on the selected double-track mode and settlement mode, this paper analyzes the calculation methods and allocation methods of some major unbalanced costs (i.e. congestion cost, double-track deviation cost and deviation profit on the user side) in the current market that account for a large proportion and have a high degree of concern, which can provide a reference for the formulation of spot market settlement rules in China.

spot market, transition period, unbalanced cost, double-track, calculation method, allocation method