基于直流串联的海上风电场及其控制
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摘要
目前，人们对海上风能的关注已超过陆上风能，大型海上风电场建设已经进入很多国家和地区的计划。由于以前在这方面的工程非常少，因此还有很多问题需要研究和解决。从国际研究趋势来看，深入分析直流型风电场并提出创新性的解决方法已成为海上风电场研究的主要方向。
本文以直流串联型风电场和模块化多电平并网端为研究对象，以风电机组、风电场和并网变流器的控制作为主题，从原理分析、模型建立和仿真验证三个方面入手，全面、深入、细致的开展了研究工作，其研究内容和创新成果包括：
⑴根据变速风力发电系统的工作原理设计了直流串联型风机的控制策略，使风机按照最佳功率控制和输出电压限幅控制，满足了直流串联型风电场的运行要求。以永磁同步发电机为对象，建立了各控制目标的控制框图，按照“最优对称法”的形式完成了控制器的设计，通过仿真验证，说明控制系统能够实现串联型风机的控制要求，且效果良好。
⑵根据风力发电机和变流器的工作原理和控制策略，设计了风机的简化控制模型，并结合其简化物理模型，形成直流串联型风力发电系统的简化模型。通过和详细模型的比较分析，以及组建风电场进行风场特性分析，说明该模型能够反映风机的各项性能，其稳态响应和动态响应能力和详细模型接近。
⑶直流串联型风电场在受端电网发生电压跌落故障时，需要减少风能吸收以保持直流输电电压稳定。针对串联风机的运行特性，本文提出根据风机输出功率实时调整出口电压限幅值的方法，使串联机群的出口电压和直流输电电压的保护值相对应，仿真证明了该控制方法可以使风电场成功实现低电压穿越。
⑷以模块化多电平换流站作为并网端，结合电网要求和风场特性，叙述了换流站参数设计的原则和方法。在换流站的阶梯波调制中采用了基于拉格朗日乘子的算法，解决了电平数较多时计算负担过大的问题；同时采用了阶梯波和多重载波混合调制的方法，解决了阶梯波调制范围较窄的问题，提高了并网端应对电网故障的能力。
⑸由于并网端采用阶梯波调制，存在采样周期较长的缺点，为提高换流站的动态响应能力，在对电流的控制上采用了基于平坦系统的前馈控制；另外，根据变流器最大输出电压、电流和电网电压的测量值，对参考电流的工作范围进行实时预测。两者结合可以减小控制环之间的冲突，仿真证明该控制策略可以显著提高并网端的动态响应速度。
At present, offshore wind energy has been taken care more than onshore wind energy,so that large-scale offshore wind farm is being planned by many countries. Since fewengineering of such kind have been implemented, there is many issues needed to beresearched. By the conclusion of its research achievements globally, it become a maintrend to analysis of DC wind farm in detail and propose innovative solution in offshorewind farm research.
The paper takes the wind farm based on series DC and its integration by use ofModular Multilevel Converter (MMC) as subjects, which is intended to the control of windpower generating system, wind farm and its integration converter. Comprehensive,thorough and detailed studies with respect to theoretical analysis，model design andsimulation verification, are carried out. Some important conclusions andindependent-innovative achievements are made and obtained.
⑴Based on the operation principle of variable speed wind power generating system,the control strategy is designed for series-connected wind turbine. It consists of MaximumPower Point Tracking (MPPT) and the voltage limitation control, which can grantee thewind turbine running normally in the wind farm based on series-connected DC. In case ofpermanent magnetic synchronous generator, control diagram have been made up for eachtarget and the controller have been designed according to the Optimum Symmetric Method(OSM). The simulation verifies the control system can deal with each status and showsgood performance.
⑵In accordance with the theory of wind generator and converter and their controlstrategy, the simplify control model for DC series-connected wind turbine is get. It,combined with the physical simplify model, can be kept as simplify model of wind powergenerating system. By simulation, it has been compared with detailed model and used toconstruct wind farm for testing its characteristics. The results show it can response thewind turbine closely and has likely static and dynamic performance.
⑶If a grid voltage sag happens in the receiving grid, the wind farm must reduce the wind energy absorbed to keep the DC transmission voltage stable. Based on thecharacteristic of series-connection, the paper proposes a method to adjust the voltagelimitation in real time according the wind power generating system’s output power. It canmake the clsters’ voltage limitation equal to the limitation of DC transmission voltage. Thesimulation verifies the method can make wind farm ride through low voltage successfully.
⑷The integration side of wind farm is of MMC. By the requirement of the grid andwind farm, the principle and method are demonstrated for the parameters calculating ofMMC. For its step modulation, an algorithm based on Langrage has been provided whichwouldn’t increase the calculate burden even as the number of levels increases. Furthermore,the hybrid modulation with step and multi carrier has been proposed which can enlarge themodulating range. Such modulation would improve the ability to handle with the fault ofgrid.
⑸MMC, with step modulation, has to take long sampling period and is lack ofdynamic response speed. To solve it, flatness based control is applied with the convertercurrent. Simultaneously, the range of current reference is predicted in real time accordingto the converter maximum output voltage, the grid voltage and the current measurement.The two methods decrease the interaction between the control loops, and the simulationverifies they can enhance the dynamic performance of converter obviously.
本文以直流串联型风电场和模块化多电平并网端为研究对象，以风电机组、风电场和并网变流器的控制作为主题，从原理分析、模型建立和仿真验证三个方面入手，全面、深入、细致的开展了研究工作，其研究内容和创新成果包括：
⑴根据变速风力发电系统的工作原理设计了直流串联型风机的控制策略，使风机按照最佳功率控制和输出电压限幅控制，满足了直流串联型风电场的运行要求。以永磁同步发电机为对象，建立了各控制目标的控制框图，按照“最优对称法”的形式完成了控制器的设计，通过仿真验证，说明控制系统能够实现串联型风机的控制要求，且效果良好。
⑵根据风力发电机和变流器的工作原理和控制策略，设计了风机的简化控制模型，并结合其简化物理模型，形成直流串联型风力发电系统的简化模型。通过和详细模型的比较分析，以及组建风电场进行风场特性分析，说明该模型能够反映风机的各项性能，其稳态响应和动态响应能力和详细模型接近。
⑶直流串联型风电场在受端电网发生电压跌落故障时，需要减少风能吸收以保持直流输电电压稳定。针对串联风机的运行特性，本文提出根据风机输出功率实时调整出口电压限幅值的方法，使串联机群的出口电压和直流输电电压的保护值相对应，仿真证明了该控制方法可以使风电场成功实现低电压穿越。
⑷以模块化多电平换流站作为并网端，结合电网要求和风场特性，叙述了换流站参数设计的原则和方法。在换流站的阶梯波调制中采用了基于拉格朗日乘子的算法，解决了电平数较多时计算负担过大的问题；同时采用了阶梯波和多重载波混合调制的方法，解决了阶梯波调制范围较窄的问题，提高了并网端应对电网故障的能力。
⑸由于并网端采用阶梯波调制，存在采样周期较长的缺点，为提高换流站的动态响应能力，在对电流的控制上采用了基于平坦系统的前馈控制；另外，根据变流器最大输出电压、电流和电网电压的测量值，对参考电流的工作范围进行实时预测。两者结合可以减小控制环之间的冲突，仿真证明该控制策略可以显著提高并网端的动态响应速度。
At present, offshore wind energy has been taken care more than onshore wind energy,so that large-scale offshore wind farm is being planned by many countries. Since fewengineering of such kind have been implemented, there is many issues needed to beresearched. By the conclusion of its research achievements globally, it become a maintrend to analysis of DC wind farm in detail and propose innovative solution in offshorewind farm research.
The paper takes the wind farm based on series DC and its integration by use ofModular Multilevel Converter (MMC) as subjects, which is intended to the control of windpower generating system, wind farm and its integration converter. Comprehensive,thorough and detailed studies with respect to theoretical analysis，model design andsimulation verification, are carried out. Some important conclusions andindependent-innovative achievements are made and obtained.
⑴Based on the operation principle of variable speed wind power generating system,the control strategy is designed for series-connected wind turbine. It consists of MaximumPower Point Tracking (MPPT) and the voltage limitation control, which can grantee thewind turbine running normally in the wind farm based on series-connected DC. In case ofpermanent magnetic synchronous generator, control diagram have been made up for eachtarget and the controller have been designed according to the Optimum Symmetric Method(OSM). The simulation verifies the control system can deal with each status and showsgood performance.
⑵In accordance with the theory of wind generator and converter and their controlstrategy, the simplify control model for DC series-connected wind turbine is get. It,combined with the physical simplify model, can be kept as simplify model of wind powergenerating system. By simulation, it has been compared with detailed model and used toconstruct wind farm for testing its characteristics. The results show it can response thewind turbine closely and has likely static and dynamic performance.
⑶If a grid voltage sag happens in the receiving grid, the wind farm must reduce the wind energy absorbed to keep the DC transmission voltage stable. Based on thecharacteristic of series-connection, the paper proposes a method to adjust the voltagelimitation in real time according the wind power generating system’s output power. It canmake the clsters’ voltage limitation equal to the limitation of DC transmission voltage. Thesimulation verifies the method can make wind farm ride through low voltage successfully.
⑷The integration side of wind farm is of MMC. By the requirement of the grid andwind farm, the principle and method are demonstrated for the parameters calculating ofMMC. For its step modulation, an algorithm based on Langrage has been provided whichwouldn’t increase the calculate burden even as the number of levels increases. Furthermore,the hybrid modulation with step and multi carrier has been proposed which can enlarge themodulating range. Such modulation would improve the ability to handle with the fault ofgrid.
⑸MMC, with step modulation, has to take long sampling period and is lack ofdynamic response speed. To solve it, flatness based control is applied with the convertercurrent. Simultaneously, the range of current reference is predicted in real time accordingto the converter maximum output voltage, the grid voltage and the current measurement.The two methods decrease the interaction between the control loops, and the simulationverifies they can enhance the dynamic performance of converter obviously.
引文
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