Inertia Demand Assessment Of Power System Considering Frequency Stability

Under the background of Dual-carbon,the proportion of new energy sources such as photovoltaic and wind power in the power system increases rapidly,the proportion of synchronous generators decreases gradually,and the rotational inertia also decreases synchronously.Due to the limited role of new energy generation connected to the power system through a large number of power electronic devices in providing inertia support,the level of inertia in the power system is significantly reduced.However,the inertia of the power system is sufficient to maintain frequency stability.The anti-disturbance ability and self-recovery ability of the low inertia power system are seriously reduced.The initial frequency stability of the system after disturbance is particularly prominent.The excessive rate of change of frequency(ROCOF)will significantly increase the frequency offset amplitude and seriously affect the frequency stability.Restricting the frequency change rate and frequency offset amplitude of the disturbed power system to a safe range requires sufficient inertia support.Therefore,it is of great significance to accurately evaluate the equivalent inertia and inertia demand of the power system and reasonably allocate the output between new energy and traditional energy to ensure the frequency stability of the power system.Firstly,based on the energy conversion process of synchronous generator rotor,the relationship between power and speed in time domain is analyzed,and the frequency response model of the system is obtained.Taking the rotational inertia of a single synchronous machine as the research object,considering the start-stop and rated capacity of the system synchronous unit and the new energy unit,the equivalent inertia expression of the power system synchronous unit is obtained,and the system frequency response model considering the new energy virtual inertia is given.According to the frequency change and inertia response process of each stage after the system is disturbed,the mechanism of inertia affecting the frequency change is analyzed.The simulation results show that the inertia has the effect of suppressing the system frequency fluctuation too fast.Then,according to the mechanism of inertia affecting frequency change,the frequency dynamic response equation of the system is simplified.The system inertia partition evaluation method is adopted,and the Kalman filter identification model is constructed according to the system frequency dynamic response.The square root cubature Kalman filter method is used to identify the two different disturbance scenarios of load impact and generator tripping.The method has high calculation accuracy and is less affected by noise,and the required data window is short.The identification results show that the square root cubature Kalman filter method has high identification accuracy.Finally,based on the two core frequency constraints of initial frequency change rate and maximum frequency offset and the relationship between unbalanced power and system inertia demand,the inertia demand assessment of power system is transformed into an optimization problem for solution.After setting the initial frequency change rate and maximum frequency offset standard,the inertia demand under different disturbances at 24 time is evaluated by genetic algorithm.The method has strong robustness and fast convergence speed.Then,the inertia demand value after evaluation is introduced into the lower-level system optimization operation model as the inertia demand constraint condition.By calling the CPLEX solver in MATLAB,the unit start-stop and virtual inertia compensation amount under four scenarios,such as inertia constraint and different new energy virtual inertia compensation cost,are solved respectively.The comparison of different results shows that the inertia demand assessment plays an important role in optimizing the operation of power system and enhancing frequency stability.