Connecting wind power plant with weak grid - Challenges and solutions

The size of individual wind power plant is continuously increasing, while sites with good wind conditions often are located far from electrical loads. This often results in wind power plants connecting to weak transmission grids. The short circuit ratios at the point of common coupling of wind power plant can be lower than 3 in many cases, and even lower than 2 in extreme cases. This paper analyzes the problems of connecting wind power plant with a weak AC system through detailed voltage stability analysis, small signal stability analysis and transient stability analysis, using power flow, frequency domain and time domain simulation methods. Among the technical challenges, the voltage stability is identified as most critical to the stable operation of wind power plant within weak grid. If the wind power plant itself cannot provide sufficiently fast and extensive compensation, the typical solution for the voltage stability problem is to install dynamic reactive power compensation with fast voltage control capability, such as STATCOM or even Synchronous Condenser. Such additions heavily increase investment cost. In this paper, a coordinated control method of wind power plant is proposed, to minimize the size of any additional reactive power compensation, and it is compared to de facto voltage controllers. The new method enables wind power plant to be controlled as an integral generation source to fulfill grid code requirements, configuring individual WTGs to work as stiff voltage sources. However, with the increase of bandwidth of voltage controller and decrease of grid short circuit ratio, the system is susceptible to a shunt resonance between voltage controller and grid impedance, and its influence on the proposed method is discussed.