摘要
多端柔性直流输电技术更适合风电联网系统的潮流优化控制,然而直流系统内不具备交流同步电网的机械惯性支持,因此风电机组和换流站需具备快速的有功协同调节能力,以提高其稳定性。本文首先提出变速风电机组的虚拟惯性和一次调频综合控制策略,使风电机组具有较完备的动态有功-频率调节能力。然后建立风电端换流站的交流侧频率与直流侧电压的联动调节关系,进而提出含风电的多端直流系统的有功协调控制策略,充分利用各端电网间的相互支持能力及风电机组的机械储能,确保风电直流联网系统稳定运行及增强其故障穿越能力。本文的主要研究内容如下：
1.研究了变速风电机组及直流输电系统的动态模型及控制方法,分析了直流电网的组网方式,提出一种适用于风电联网的多端直流系统拓扑结构及其运行模式,搭建了基于直流联网的风力发电系统的仿真平台,为风电直流联网系统的功率协调控制策略研究奠定了基础。
2.分析了变速风电机组虚拟惯性控制及调频控制方法,提出了风电机组虚拟惯性与一次调频相结合的综合频率控制策略。通过引入减载水平和桨距静调差系数的定义,并利用变桨技术,改进了风电机组的减载运行方法和可整定静调差系数的一次调频控制策略,并最终实现了与虚拟惯性控制的有机结合,使风电场不仅具备惯性频率响应,并可满足系统的一次调频要求,进而对电网有功扰动具有较为完备的快速有功调节能力。
3.分析了交流电网故障扰动对两端柔性高压直流输电系统稳定运行的影响,提出了变速风电机组及换流站协同控制的两端柔性直流输电系统的故障穿越方法。在网侧换流站因电网故障而限流期间,改变风电侧换流站的恒频控制方式,将直流侧电压波动与交流侧频率调节建立有效联系,从而可通过变速风电机组的虚拟惯性控制快速调节电磁功率,利用风电机组的机械储能为直流系统的故障穿越提供有效支持。该方法成本低,不依赖于通讯,可拓展到多端直流系统之中。
4.含风电的直流系统内惯性小、功率波动大,为提高其稳定性,本文提出了多端系统之间的分散协同控制及源网协调控制策略,使互联系统具备了快速的有功协同调节能力。首先,设计了各端换流站的分散协同控制特性,确保直流电网在不同运行模式下均可对直流电压进行有效控制。然后分别设计了受端换流站的直流电压-有功功率控制和风电端换流站的变频控制环节,使分散在各端电网内的常规电源以及在综合频率控制下的风电均可充分发挥其有功调节能力,实现源网协调控制。本文所提出的风电直流联网系统的协调控制策略,充分利用各端电网间的相互支持能力及风电机组的机械储能,从而增强了高风电渗透率下直流互联电网的稳定运行能力,并且该控制策略无需通讯,易于扩展。
The voltage source converter multi-terminal direct current (VSC-MTDC) transmission technology is suitable for power flow optimal control of the wind power interconnection system, however, DC network cannot provide effective mechanical inertia for dynamic support as AC network, thus the quick collaboration capability of active power regulation for both wind turbines and converters should be achieved to enhance the system stability. In this dissertation, an integrated control scheme of variable speed wind turbines for virtual inertia and primary frequency regulation is proposed to achieve the complete active power-frequency control ability. Following that, the linkage adjustment relationship between the system frequency and the dc voltage is established, and then the power coordinated control strategy of MTDC system with high wind penetration level is further presented, so as to fully utilize the mutual support ability of each network and the mechanical energy storage of wind turbines. In addition, the operation stability and the fault ride through (FRT) capabilty of the MTDC system connected with wind turbines can be enhanced as well. The main research results are as follows:
1. The dynamic model of variable speed wind turbines and DC transmission system are established, the different topologies of MTDC system are analyzed, and then the suitable topology and operating mode of MTDC system for wind power interconnection is proposed, and the simulation platform of the wind power system is also built, which are used to provide the basis for designing the coordinated power control of wind turbines and each terminal converters.
2. The virtual inertia control and the frequency regulation method of variable speed wind turbines are analyzed, and the integrated frequency control strategy for virtual inertia and primary frequency regulation is proposed. According to the definition of de-loading level and the pitch static difference coefficient, an optimized de-loading control scheme and a primary frequency regulation scheme with settable static difference coefficient are proposed respectively by variable pitch technology, which resolve the issue of the combination between virtual inertia and primary frequency regulation. With the proposed strategies, wind turbines achieve not only the inertia frequency response but also the system frequency regulation requirement, and thus the response ability to the grid active disturbances is enhanced.
3. According to the analysis of the impact of grid faults on the safe operation of two terminal voltage source converter high voltage direct current (VSC-HVDC) system, an active power coordinated control strategy of wind turbines for fault ride through capability enhancement of DC system is proposed. During the current limit process of the grid side converter, the constant frequency control mode of the wind farm side converter is changed, and the relationship between the DC voltage fluctuation and the system frequency regulation is established, then the electromagnetic power of variable speed wind turbines can be regulated by the virtual inertia control, so as to provide the effectively support for DC system FRT using the mechanical energy storage. Moreover, the proposed scheme at low cost can be easily applied in MTDC system and without any communication.
4. In the DC system connected with wind turbines, the inertia is low while power fluctuation is high. In order to improve the system stability, the decentralized cooperative control of each terminal network and the source network coordination control strategy are proposed respectively, and then the coordinate regulation ability of the interconnection systems is achieved. The decentralized cooperative control characteristics is explored firstly to regulate the dc network voltage in different operating modes effectively. And then, the dc voltage-active power control of the receiving end converters and the variable frequency control loop of the wind turbine side converter are designed respectively. Both the conventional power and the wind turbines under the integrated frequency control, which are dispersed in each terminal network, can develop their active power regulation ability to achieve the the source network coordination control. With the proposed coordinated control strategy, the mutual support ability of each network and the mechanical energy storage of wind turbines can be fully utilized, and thus the operation stability of the MTDC system with high wind penetration level is further enhanced. Moreover, the proposed scheme is easier to extended and without communication.
引文
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