±500kV张北柔性直流电网工程系统设计

System Design of ±500kV Zhangbei VSC-based DC Grid Project

张北柔性直流电网工程是世界首个直流电网工程。与前期工程相比,张北工程系统设计的主要难点在于:工程故障穿越要求高、故障发展速度快而核心设备的暂态应力水平受限;新能源孤岛接入双极直流电网存在双极协调控制困难、宽频振荡风险高以及暂时性盈余功率问题严重等问题。针对张北工程系统设计的主要难点,提出新型三次谐波注入策略、直流电网电抗器最优化配置方法、基于接地电阻的续流快速衰减方法、子模块动态平均工作电压策略、新型阀控不平衡快速保护策略、阀本体过压保护的动态定值策略等一系列方案,有效提高了核心设备的电流和电压应力水平对直流电网的适应性;提出了双极V/f下垂控制、大规模新能源场站宽频阻抗特性获取方法、基于交流耗能装置的盈余功率解决方案等,解决了大规模新能源孤岛接入直流电网的双极协调控制问题、宽频振荡问题和暂时性盈余功率问题。通过张北柔性直流电网工程实践,验证了上述策略的正确性和有效性。

Zhangbei voltage source converter(VSC)-based DC grid project is the first of its kind over the world.Compared with the previous projects,the difficulties of the Zhangbei project system design lay in the following aspacts:(1)the demand for the fault ride-through was high and the fault developing speed was fast but the transient stress capability of the key equipments was limited;(2)there were serious problems when the islanded renewable energy farms were connected to the DC grid,including the difficulty of bipolar coordination control,the high oscillation risk,and the serious temporary power surplus problem.Aiming at these main difficulties,a series of methods were proposed such as the novel three-time harmonic injection method,the optimization configuration method of the DC grid reactors,the quick damping method of the continuing current based on the grounding resistor,the submodule dynamic average working voltage strategy,the novel unbalanced quick protection valve-based controller and the dynamic setting strategy of the valve overvoltage protection and so on,improving the adaptability of current and voltage stress of the key equipments to the dc grid effectively.Several effective measures were also put forward such as the bipolar V/f droop control,the capture method of the frequency-impedance characteristics of large-scale renewable energy farms and the solution scheme of the power surplus based on the AC chopper,settling the problems like the bipolar coordination control problem,the wide-band oscillation problem and the temporary power surplus problem of the islanded renewable energy farm connected to the DC grid.The proposed strategies are validated with the engineering practice of Zhangbei VSC-based DC grid.