Coordinated control of wind farm and VSC–HVDC system using capacitor energy and kinetic energy to improve inertia level of power systems

• We propose a new coordinated control strategy.
• Energy in the DC capacitors and wind turbine rotors provides inertial support.
• Frequency of the two-side AC systems is coupled without remote communication.
• A wide range of inertia time constant for inertial support is provided.
• Inertia time constant is influenced by the power controllers and control parameters.

For large-scale offshore wind power integration to main grids over a long distance, the VSC–HVDC transmission is a typical way. However, the asynchronous characteristic of HVDC link leads to the frequency decouple of the offshore grid and the main grid, i.e., the offshore grid has little or no inertia support for the main grid. The high level penetration of wind energy makes the main grid an “inertia-less” system and impairs the overall stability of the system. This paper proposes a new coordinated control strategy which uses the electrical energy stored in the DC capacitors and the kinetic energy stored in wind turbine rotors to emulate the inertia of synchronous generators. By this control strategy, the DC link capacitors release or absorb energy following the droop DC voltage control of the grid side VSC (GSVSC), and the wind farm VSC (WFVSC) changes its output frequency according to the DC voltage. Thus, an artificial coupling of the frequencies of the two-side AC systems is obtained without remote communication. According to the WFVSC’s output frequency, the wind turbine power controller alters its power reference, and the wind turbine speed changes. Thus, the kinetic energy stored in wind turbine rotors is absorbed or released. As a result, the wind turbine is utilized to keep the main grid frequency stable. Based on the doubly fed induction generator (DFIG) wind turbine, this paper analyzes the influence of different additional power controllers and different control parameters of the proposed control strategy on the inertia time constant. Within the permissible range of the DC voltage variation, the proposed control strategy can provide a wide range of inertia time constant, which improves the overall stability of the main grid system. Simulation results of three operation conditions, i.e., sudden load changes, variation of the wind speed, and AC system faults, validated the effectiveness of the proposed coordinated control strategy.