In case of the environment that the scale of renewable energy access to the grid

such as wind power and photovoltaic power

is expanding year by year and the load of large users with different characteristics is increasing progressively year after year as well as in dealing with frequent power imbalance events

the system needs to reserve sufficient spare capacity and

at the same time

ensure economy.In view of this

a kind of method for analysis and evaluation of system reserve capacity demand based on discrete Fourier transform（DFT）is proposed.The time-frequency conversion for system load and new energy output is carried out by adopting DFT.The frequency domain is divided in accordance with the response time.The various reserve demand are obtained by Parceval theorem.This method can comprehensively consider the randomness brought by load fluctuation and the output of the new energy.All levels of reserve demand under multiple time scales are analyzed.And whether all levels of the reserve operated by the present unit can meet requirement can be judged on line so to achieve ultra-short-term real-time reserve monitoring of the units.The simulation results show that the method can quickly calculate the system reserve requirements in any time scale

and achieve the purpose of guiding the quarterly capacity allocation of power plant

making the current plan and performing real-time reserve monitoring.