Grid Integration of Offshore Wind Farms via VSC-HVDC – Dynamic Stability Study

Offshore wind farms tend towards larger capacity to make good use of the stronger winds and allow improved fixed cost allocation. An offshore wind farm could be sized at hundreds of MWs, which is competitive with conventional power plants. Consequently, grid integration of such size offshore wind farms could seriously impact the operation and stability of their interconnected power system. To assist in maintaining the power system stability when large disturbances occur in the grid, modern offshore wind farms consisting of variable-speed wind turbines are required to provide ancillary services such as voltage and frequency control.

The greater distance to shore makes commonly used high voltage AC (HVAC) connection unsuitable economically and technically for large offshore wind farms. Alternatively, voltage source converter (VSC)-based high voltage DC (HVDC) transmission becomes more attractive and practical to integrate large-scale offshore wind farms into the onshore power grid, owing to its high capacity, advanced controllability and stabilization potential for AC networks etc.

In this dissertation, some of the key technical issues with grid integration of large-scale offshore wind farms via VSC-HVDC transmission are addressed. The main objectives have been to study the dynamic interactions between offshore wind farms and interconnected power systems, pinpoint the impact on the electrical grid while integrating large-scale offshore wind farms via VSC-HVDC link and propose potential solutions to improve the dynamic stability of the network.

This research work starts with the modelling of full converter wind turbine and VSC-HVDC transmission system. Then, based on those models, the impact of integration of a large offshore wind farm into the power system through VSC-HVDC transmission is investigated. In addition to steady-state voltage profile analysis, dynamic voltage stability and transient angle stability simulations are also conducted at different wind penetration levels. The comprehensive analysis of the results helps to understand the stability concerns of the offshore wind power integration.

VSC-HVDC link is characterized by independent active and reactive power control and hence is able to provide reactive power / voltage support during grid voltage disturbances. However, for a given VSC capacity, the more the active power output, the less the available reactive power output. Therefore, the voltage support capability of VSC-HVDC is limited by both its VSC capacity and the control scheme. In reference to the control strategies of VSC-HVDC used in this research project, a trajectory sensitivity analysis (TSA)-based approximation is proposed as a method to identify the minimum onshore VSC capacity with which the VSC-HVDC link is able to provide effective support for stabilizing the grid voltage following a grid disturbance. The developed TSA-based method implements a two-stage approximation strategy to improve the accuracy of the linear approximation. Both reactive power-based and voltage-based trajectory sensitivities are used to verify the effectiveness of the proposed method. This proposed method avoids running large amount of repeated time-domain simulations.

Due to the decoupling of VSC-HVDC and communication delay, offshore wind farms that run in a de-loading mode to provide frequency reserve, may not be able to respond to the onshore grid frequency excursion in time. Consequently, the stability and security of power system will be put at risk, especially for those with high wind penetration. A coordinated frequency control scheme is developed not only to reduce the responding latency of offshore wind farms effectively but also to allow VSC-HVDC to contribute to system frequency regulation by adjusting its DC-link voltage. The adaptive control of the DC-link voltage enables the DC capacitors of VSC-HVDC to release/absorb energy to regulate the frequency deviation. To further enhance the system frequency response, the frequency support from VSC-HVDC is also finely coordinated with that from offshore wind farm according to the latency of offshore wind turbines responding to onshore grid frequency disturbances.

Deploying energy storage systems is considered an effective way of dealing with the challenges brought by high wind power penetration. Therefore, a PWM converter-interfaced battery energy storage system (BESS) is applied to the power system integrated with a large offshore wind farm via VSC-HVDC link. The BESS is used to provide primary frequency control in cooperation with wind power fluctuation mitigation. A new converter rating evaluation approach as well as a new power management strategy is proposed to enable the BESS to enhance the system frequency response on the basis of wind power fluctuation mitigation.