The Study on Hybrid Multi-Infeed HVDC System Connecting with Offshore Wind Farm

Over the last decade, the High Voltage Direct Current (HVDC) technology has been widely applied for the long-distance, bulk power delivery in modern power systems. With the increasing use of Line-Commutated Converter based HVDC (LCC-HVDC) links, two or more HVDC links tend to feed into an AC system with the short electrical distance. And consequently, a so-called Multi-Infeed Direct Current (MIDC) transmission system is formed. On the other hand, as the fast growing of wind power in electrical grids, the Voltage Source Converter based HVDC (VSC-HVDC) links have becoming a favorable choice for connecting the large offshore wind farms, thanks to their flexibility on controlling active and reactive power. Hence, together with the existing LCC-HVDC links, a new power system structure, also named as the Hybrid Multi-Infeed HVDC system, can be envisioned, where both the LCC- and VSC-HVDC links terminate into the same AC grid.

In contrast to the traditional MIDC systems with only LCC-HVDC links, the flexible power control of VSC-HVDC links bring more possibilities for the stability enhancement of the HMIDC system. Accordingly, there is a need to explore the operation and control aspects of the HMIDC system. Hence, this thesis aims to develop advanced control strategies for the HMIDC system connecting with an offshore wind farm, in order to achieve the maximum use of VSC-HVDC link for enhancing the AC system stability.

In light of the main objective, two research tasks are divided in this project: 1) Modeling the HMIDC system connecting with an offshore wind farm, and investigating the main factors that affect the system stability under the different disturbances. 2) On the basis of the stability analysis, developing the appropriate control strategies for the studied HMIDC system in order to enhance the stability and power quality of the system.

The research work starts with establishing a basic HMIDC system model based on the power system of western Denmark, where the LCC-HVDC link and the VSC-HVDC link feed into one AC grid via two buses, respectively, and are interconnected through a tie-line. Under the built HMIDC system model, the influences of system parameters and the power control methods of the VSC HVDC link on the system voltage stability are evaluated. In light of this, a new calculation method for the Effective Short Circuit Ratio (ESCR) is formulated for the HMIDC system, which provides a quantitative tool for assessing the contributions of the system parameters and the VSC-HVDC link to the system voltage stability.

From the ESCR analysis, it is found that the power control strategies of the VSC-HVDC have an important effect on the system voltage stability. Hence, a flexible power control method of the VSCHVDC link is proposed. The approach employs an adaptive current limiter to dynamically adjust the active current reference based on the output of the AC voltage controller, such that the maximum use of reactive power support capability of the VSC-HVDC link can be achieved.

Following the stability analysis of the basic HMIDC system, the research work moves forward to the control of the HMIDC system comprising the offshore wind farm at the sending end of the VSCHVDC link. In this case, the Low Voltage Ride Through (LVRT) ability of the VSC-HVDC connected wind farm is one of the most important operation requirement. It is demanded that the wind turbines remain connected and actively contribute to the system stability during and after the onshore grid fault, otherwise the loss of the large power transmission will result in serious stability problem. To meet this requirement, a cooperative control of the VSC-HVDC system and a variable speed Squirrel Cage Induction Generator (SCIG)-based offshore wind farm is proposed. In the approach, an active power-frequency droop control is developed to achieve an autonomous reduction of the generated active powers from wind turbines, which thus furnish the VSC-HVDC connected wind farm with a fast and reliable LVRT ability.

The voltage fluctuation caused by the intermittent wind power is another important challenge to the system stability. As in the case of the LCC-HVDC links, their stable operations are highly dependent on the AC side voltage, and thus any voltage fluctuation at the infeed bus of the LCCHVDC link may cause voltage instability of the HMIDC system. To address such a challenge, a voltage sensitivity-based reactive power control scheme for the VSC-HVDC link is developed, where a reactive power increment derived from the sensitivity factors is introduced into the power control loop in order to regulate the voltage of the target bus in the HMIDC system.

In this thesis, the HMIDC system models are built in the EMTDC/PSCAD environment. All of the proposed control strategies are evaluated via a series of simulation case studies. Simulation results have shown that the HMIDC system stability can be effectively improved by the proposed approaches.