Wind Farm’s Participating In Grid Frequency Regulation Through Active Power Control

In recent years, significant increase of wind power generation has been emerged for the purpose of releasing the pressure of global energy shortage and environmental pollution. However, with the increase of wind power penetration, daily operation of conventional power system can be inevitably affected that there must be enough reserve for frequency regulation and peak shaving. Gradually, wind power’s frequency regulation and active power control capability are requested and can be found in grid codes of representative countries. Active power control capability of wind power means that wind turbine can adjust its output power according to current wind condition and system operation state in order to keep reasonable reserve for emergency response. The total system reserve will decrease if wind power equipping with active power control ability. Accordingly, the system economic efficiency will be greatly enhanced. To explore wind power’s potentiality on system frequency regulation and reserve, the thesis mainly focus on the following contents:1) The state of art of wind power’s participating in frequency regulation has been reviewed in way of following a hierarchical organization framework including the wind turbine level, the plant level and the system level. The responsibilities and the dilemma of individual level are briefly introduced. Moreover, the grid codes of representative countries are summarized and compared with each other aiming at learning the industry status. The comparison results shows that participating in system frequency regulation and equipping with power control ability are inevitable tendency of wind power.2) On turbine level, typical control strategies are investigated in detail, including inertial emulation, short term over-production, de-loading control and droop control and hybrid control which is formed by several independent strategy aforementioned. Specifically, the concept of de-loading ratio and long term reserve ability of de-loading control are qualitatively analyzed.3) On plant level, a frequency regulation capacity evaluation method considering multiple control strategies for wind plant is proposed for the propose of determining the relationship between plant frequency regulation capacity and wind velocity prediction value. Firstly, the distribution of wind velocity in same plant is modeled. Then, the frequency regulation capacities of wind turbines are differentially evaluated according to the division of wind intervals:1) under low wind speed interval, the energy conservation theory is adopted to calculate the capacity transformed from kinetic energy; 2) under middle wind speed interval, an optimization model is established to search an optimal de-loading point so that the maximum frequency regulation capacity is ensured; 3) under high wind speed interval, the frequency regulation capacity is equal to the deviation between aerodynamic power captured and rated power until the maximum overload capacity of the converter reaches. Finally, the plant frequency regulation capacity is defined as the probability weighted summation of all wind intervals.4) On system level, a bi-level optimization model of two-stage reserve scheduling problem considering WTG’s de-loading control is proposed to evaluate the benefits of WTG de-loading control on system reserve scheduling. On the upper level, a reserve scheduling model is developed to minimize the total system scheduling cost, including the energy and reserve capacity costs of thermal units and WTGs; on the lower level, the deviation of power generation between the real-time stage and the pre-scheduling stage is minimized. By mathematical KKT application, the proposed bi-level model can be transformed into a single-level optimization model for purpose of easy resolution. Finally, the effectiveness of the proposed model is validated using a modified IEEE 6-geneartor and 30-buse system. The results indicate that the WTG de-loading control can help to reduce the scheduled reserve capacities of conventional units, suppress wind power fluctuation and enhance the system overall economy performance.