Emergency coordinated control of multiple VSC-HVDC for improving power system transient frequency and steady-state frequency

With the penetration of the high proportion of power electronics interfaced energy resources, the frequency stability issue in power systems is becoming increasingly prominent. This paper proposes a coordinated control strategy of multiple voltage source converter based high voltage direct current transmission (VSC-HVDC) to simultaneously improve the transient frequency and steady-state frequency after a severe disturbance. Firstly, a modified average system frequency response (MASFR) model is constructed with reserve power limitation, which takes into account the frequency responses of different types of generators (thermal, hydro and new energy resources). Secondly, according to the frequency stability requirements, the corresponding constraints, which consider the overload capacity of VSC-HVDC, the AC transmission capacity and the frequency of AC grids on both sides of VSC-HVDC, are formulated. Thirdly, for the transient frequency control, the coordinated optimization control model of multiple VSC-HVDC is established, which aims to minimize the control energy considering the transient frequency threshold. For the steady-state frequency control, the coordinated optimization control model is fomulated, which aims to minimize the control cost while considering the steady-state frequency threshold. The two control schemes achieve seamless and smooth switching. Finally, the correctness and effectiveness of the proposed control strategy are verified in the improved IEEE 39 bus system.