【作者】 张进；

【导师】 李可军；

【作者基本信息】 山东大学 ， 电气工程， 2021， 硕士

【摘要】 在碳达峰碳中和目标背景下,可再生能源发电发展迅速,风力发电作为可再生能源的代表更是得到长足发展。随着风力发电渗透率的提高,不稳定风功率导致的频率波动和电压闪变对系统电能质量和稳定性提出了新的挑战。同时,我国陆上风电资源多分布在西北与东北,就地消纳难度较高。能源与负荷逆向分布的状态使得规模化风电在大范围输送和消纳成为必要。柔性直流输电(VSC-HVDC)因其没有换相失败问题,无需无功补偿,可连接弱电网等优点,是跨区域远距离输电的优选技术方案之一。VSC-HVDC具有控制策略灵活多样、功率解耦输出的特点,在参与电力系统网络支持(Grid Support)方面颇具前景。本文针对用于风电并网的柔性直流系统,及其无功支援能力影响因素,单机、多机无功支援控制策略展开研究:(1)通过分析电压源型换流器的双解耦控制,提取内部与外部对无功功率输出的影响因素,量化影响程度。考虑电流互优先级、风电并网换流器工作场景以及电网支持能力的需求,建立考虑电网电压支持的柔直系统网侧换流站工作状态模型。基于Matlab/Simulink和PSCAD/EMTDC仿真平台搭建海上风电并网系统模型,结合案例定量验证不同因素对换流器功率输出的限值。(2)为适应复杂场景和电网支持能力的要求,提出用于风电并网的柔直系统网侧换流站无功支持协调控制策略。对电压跌落的时间进行区分,提出一种卸荷电阻协调参与的无功支持策略,可减少风电场频繁响应电压跌落造成的机械疲劳;对于存在的持续性电压跌落,根据功率输出受限的量化模型,提出风电场参与的无功协调控制,避免不必要的功率浪费和设备损耗;设计了模式切换控制以实现各无功支持任务平滑切换和快速响应。(3)针对电网中多网侧换流站参与无功支持的场景,提出动态无功下垂控制。在传统定系数下垂控制的基础上,考虑无功裕度受限因素与剩余容量,设计动态无功下垂函数,动态无功下垂控制能够更合理地分配无功功率,有效利用换流器容量进行快速无功支援;为提高功率分配精准程度,提出基于模型预测控制的无功支持策略。设计全局无功支持环节,并为优化环节确定等式约束,建立计及容量利用率的非线性规划对换流器无功功率分配进行滚动优化。基于SimuLink仿真平台对上述模型进行仿真验证,结果表明,该模型可以实现无功功率的精准分配,实现重载机组的无功支援任务零负担,提高机群无功支援能力上限。更多还原

【Abstract】 Under the background of carbon peak and neutralization target,many kinds of energy utilization and transmission project such as power transmission from west to east,large-scale offshore wind power,distributed generation are booming.As a result,power electronic devices such as grid commutation converter(LCC)and voltage source converter(VSC)are more and more involved in modern power system.Although the power electronic based devices have the advantage of control flexibility,their poor over-current ability results to sensitivity to external voltage.Therefore,voltage stability is attached more and more significance for the safe and stable operation of power system.And as the key equipment in renewable energy integration and power transmission,VSC is considered as a promising solution to provide grid support in the future power system owe to its advantages such as control flexibility,independent output of reactive power ability and capability of participating power system control.In view of the lack of research on the wind farm integration inverter participating in reactive power support,this paper studies the influencing factors of the reactive power support capability and the coordinated reactive power support control strategy of single machine and multi machines:(1)Based on the existing voltage source converter basic model,the double decoupling control commonly used in corresponding literature is introduced,the internal and external influence factors on the reactive power output is extracted and its sensitivity is quantified.With the possible working scenarios and grid support capability of integration inverter considered,the current priority is selected and the working state model of windfarm integration inverter participating at reactive power support is established.The model of windfarm integration system is built by in Matlab/Simulink platform and PSCAD/EMTDC.The inverter power output limiting factors mentioned in this paper is quantitatively verified by the simulation case.(2)In order to meet the requirements of complicated scenarios and grid support task,a coordinated control strategy of reactive power support for grid side inverter of VSC-HVDC used for windfarm integration is designed.Firstly,in order to decrease the impact of frequent response to voltage drop on wind farm,a reactive power support strategy with coordinated participation of unloading resistance is proposed with the time of voltage drop distinguished.Secondly,for the sake of avoiding unnecessary power waste and equipment loss in case of possible persistent voltage sag,the reactive power coordination control of wind farm is established according to the quantitative model of limited power output.Then,for the possible reactive power support requirements of grid connected inverter,the mode switching control is designed,which focused on the switching smoothness and response speed.Finally,the PSCAD/EMTDC simulation is established to verify that the above model can alleviate the voltage sag,reduce unnecessary actions of wind farms and avoid the equipment over-current which endow the inverter grid voltage support capability in case of necessary.(3)Aiming at the scenario of multi grid side converter stations participating in reactive power support in power grid,dynamic reactive power droop control is proposed.On the basis of the traditional constant coefficient droop control,the dynamic droop function is designed considering the reactive power margin limiting factors and the residual capacity.The dynamic droop control can distribute reactive power more reasonably and make effective use of the converter capacity for fast reactive power support.Besides,In order to improve the accuracy of power allocation,a reactive power support strategy based on model predictive control is proposed.The global reactive power support link is designed and the equality constraints are determined for the optimization link.The nonlinear programming considering the capacity utilization is established to optimize the reactive power allocation of the converter.Based on the Simulink simulation platform,the results show that the model can realize the accurate distribution of reactive power,realize the zero load of reactive power support task of heavy-duty units,and improve the upper limit of reactive power support capacity of the cluster.更多还原