Introduction  In order to solve the problem of the short-term heavy load of main transformers in substations caused by the high peak load of the power grid with the relatively reasonable average-load-rate and increasing utilization hours of the substations, delay the construction investment of the power grid and ensure the economic operation of the power grid, a capacity sizing method of energy storage in substations is proposed in this paper, which meets the power supply requirements with N-1 main transformers.

  Method  Firstly, a capacity sizing mathematical model of energy storage was built for peak load shaving of the load operation curve and reducing the maximum load rate of the transformer. Then, the capacity sizing economic objective function of lithium ion electrochemical energy storage was constructed to compare the construction investment of lithium ion electrochemical energy storage and main transformer expansion and analyze the economy of energy storage capacity sizing. Finally, in combination with the actual condition of substations, the energy storage capacity and its feasible construction cost in economy is proposed.

  Result  Through analysis, with the decreasing of unit cost of lithium ion electrochemical energy storage in the future, the energy storage power can be considered in accordance with the substation capacity of 15% and charging and discharging time of 2 hours. When the unit cost of the energy storage system is decreased to about RMB 1 200/ kWh, the economic advantage of energy storage construction is obvious.

  Conclusion  The capacity sizing method of energy storage proposed can solve the problem of short-term heavy load in substations effectively, and has better economy than conventional power transmission and transformation projects.