  The high penetration of converter-based renewable energy sources has brought
challenges to the power system frequency control. It is essential to consider
the frequency security constraints and frequency control reserve requirements
in unit commitment (UC). Considering that the risk of frequency insecurity
varies under the changeable operational condition, we propose to optimize the
PFC droop gains and reserve capacities in the UC model to provide diverse
control efforts in different risk levels adaptively. Copula theory is used to
establish the joint distribution model among frequency control performance,
secondary frequency control (SFC) reserve capacities, and power fluctuations.
Then the distributionally robust optimization technique is utilized in the SFC
reserve requirement determination to handle the possible error in the
probability model. The UC simulation is conducted on IEEE 118-bus system to
test the proposed optimal PFC droop gain strategy and SFC reserve requirement
quantification method. Simulation results show that the proposed optimal PFC
droop gain strategy is better than the traditional fixed PFC droop gain setting
on economic efficiency and operational flexibility. Besides, the SFC reserve
requirement calculated by the proposed method is more appropriate than the
actual SFC reserve capacity in the historical operation.
