混合多端直流输电运行特性研究

发布时间：2018-08-07 17:45

【摘要】：传统直流(LCC-HVDC)与柔性直流(VSC-HVDC)具有各自的优势与劣势,二者有着不同的适用场合。传统直流输送容量大、损耗低,适合大容量远距离输电,而柔性直流控制灵活、便于构成多端直流,适合新能源接入与向弱交流系统供电。混合多端直流能够结合传统直流和柔性直流各自的优势,实现多电源供电、多落点受电,提供一种更为灵活、快捷的输电方式,以获得最大的经济和技术效益。目前电力工业正朝着加快利用清洁能源以及大规模电网互联的方向发展,混合多端直流输电将在电力系统发展中起到十分重要的作用,而且现有的一些工程已经有混合多端直流的雏形和设想,因此有必要对混合多端直流的运行特性做深入的研究,为混合多端直流的实际应用做好准备。本文的研究内容及相应的成果主要有:(1)研究了基于一致性算法的混合多端直流自律分散控制策略。在这种控制方法中,各端子仅通过与相邻端子的通信即可获取全局信息,降低了对通信系统性能和可靠性的要求。各端子既能相对独立地运行,又能相互协作完成系统的优化控制目标;既能实现各端子运行点的精细调节,又能根据实际情况对各端子进行分组控制。相比于集中控制,这种控制方法无需中心控制单元,将在规模不断增大的直流电网中具有广阔的应用前景。(2)研究了影响混合多端直流输电系统损耗的关键因素。本文经过分析指出混合多端直流损耗优化需要在两个相互矛盾的目标中寻求最优:一方面,为使损耗最小需要使直流电压尽可能高且功率尽量经LCC送出;另一方面,受端LCC在电流较大时将转为定γ控制,此时需要将VSC的直流电压相应降低,以避免LCC逆变端流过更多的直流电流,进而导致换相失败,而直流电压的降低将导致损耗增加。在此基础上,本文将基于一致性算法的自律分散控制应用到混合多端直流的损耗优化中,使各个端子能相互协作达到优化目标。针对不同的运行模式,本文还比较了全局信息完整程度不同以及参与优化控制的端子数目变化时的优化结果,得出端子获得的全局信息完整程度越高、参与优化控制的端子越多,系统损耗越小的结论。(3)研究了适用于混合多端直流中连接风电孤岛的VSC端子的控制策略。由于风能的随机性和波动性,风电基地并网宜采取孤岛运行方式并由VSC提供交流电压,因此本文设计了适用于混合多端直流中VSC端子的模型预测控制。该控制策略对VSC的交直流侧均建立了预测模型,且不含PI环节,可以直接求取几个采样周期内调制波的最优解,避免了与双馈风机控制器中PI环节的相互影响,且在风速波动、交直流故障时有较好的抗扰动能力及故障恢复性能。(4)研究了交流背景谐波通过MMC的传递及其引起的直流网络谐振问题,并设计了相应的谐波抑制控制策略。本文首先从理论上分析了交流系统背景谐波电压对MMC桥臂电压的影响,揭示了背景谐波通过MMC向直流侧以及远端交流系统的传递机理。然后以MMC构成的4端直流输电系统为例,分析了直流网络的频率阻抗特性,得出直流网络谐振引起谐波放大现象的原因。在此基础上,设计了利用MMC子模块电容储能能力的谐波抑制控制策略,抑制了由交流背景谐波引发的直流网络谐振及谐波传递。在PSCAD/EMTDC中的仿真表明,本文所提出的控制策略能有效抑制背景谐波向直流网络的传播,避免直流网络由于背景谐波产生谐振现象。在由MMC和LCC构成的混合多端直流中,直流网络依然会存在若干谐振频率,本文提出的利用MMC子模块电容储能能力的谐波抑制控制策略也可用于抑制混合多端直流的谐波传递。
[Abstract]:Traditional DC (LCC-HVDC) and flexible DC (VSC-HVDC) have their own advantages and disadvantages, the two have different applications. The traditional DC transmission capacity is large, low loss, suitable for large capacity long distance transmission, flexible DC control flexible, easy to form multi terminal DC, suitable for new energy access and power supply to weak AC system. Hybrid multi end direct Combined with the advantages of traditional DC and flexible DC, the flow can supply power supply with multi power supply, and provide a more flexible and fast transmission mode to obtain the greatest economic and technical benefits. At present, the electric power industry is developing to speed up the use of clean energy and large scale power grid interconnection, and mixed multi terminal direct current transmission. Electricity will play a very important role in the development of the power system, and some of the existing projects already have the prototype and assumption of mixed multi terminal DC. Therefore, it is necessary to do a thorough study on the running characteristics of the mixed multi terminal DC, and prepare for the practical application of the Mixed Multi terminal DC. The main contents of this paper are as follows: (1) the hybrid multi terminal DC self-control decentralized control strategy based on the conformance algorithm is studied. In this control method, the terminals can obtain the global information only through communication with adjacent terminals, and reduce the requirements for the performance and reliability of the communication system. The terminals can run in relative independence and cooperate with each other to complete the optimization of the system. The control target can not only realize the fine adjustment of the terminal operation points, but also can control the terminals according to the actual conditions. Compared with the centralized control, this control method does not need the central control unit, and it will have a wide application foreground in the increasing scale of the DC power grid. (2) the influence of mixed multiterminal HVDC system is studied. It is pointed out that the optimization of mixed multiterminal DC loss needs to seek the best of two conflicting targets: on the one hand, to make the minimum loss of the DC voltage as high as possible and the power as far as possible through LCC; on the other hand, the end LCC will turn to the fixed gamma control when the current is large, and VSC is required at this time. The DC voltage is reduced so as to avoid more DC current flow through the LCC inverter end, which will lead to the failure of the commutation, and the decrease of the DC voltage will lead to the increase of loss. On this basis, this paper applies the self disciplined decentralized control based on the consistency algorithm to the loss loss optimization of mixed multi terminal DC, so that each terminal can cooperate with each other to reach the advantage of each other. Aiming at different operation modes, this paper also compares the optimization results when the global information integrity is different and the number of terminals participating in the optimization control. The conclusion is that the higher the global information integrity is, the more terminals involved in the optimization control, the smaller the loss of the system loss. (3) the study is applicable to the mixed multi terminal. The control strategy of the VSC terminal connected to the wind power isolated island in DC. Due to the randomness and volatility of wind energy, the wind power base should take the island operation mode and provide the AC voltage with the VSC. Therefore, this paper designs the model predictive control for the VSC terminal in the mixed multi terminal DC. The control strategy has set up the preview of the AC and DC side of the VSC. The test model, without PI link, can obtain the optimal solution of modulation wave in several sampling periods, avoid the interaction with the PI link in the doubly fed fan controller, and have good anti disturbance ability and fault recovery performance when the wind speed fluctuates and the AC and DC faults. (4) studying the transmission of the AC background harmonics through the MMC and its cause. The harmonic suppression control strategy of DC network is designed and the corresponding harmonic suppression control strategy is designed. Firstly, the influence of the background harmonic voltage on the MMC bridge arm voltage is analyzed theoretically, and the transmission mechanism of the background harmonics through the MMC to the DC side and the remote AC system is revealed. Then the 4 terminal DC transmission system composed of MMC is used as an example. The frequency impedance characteristic of DC network is analyzed and the reason of harmonic amplification caused by DC network resonance is obtained. On this basis, a harmonic suppression control strategy using MMC sub module capacitance energy storage capacity is designed to suppress DC network resonance and harmonic transmission caused by the AC background harmonics. The simulation in PSCAD/EMTDC shows that this paper is used in this paper. The proposed control strategy can effectively suppress the propagation of the background harmonics to the DC network and avoid the resonant phenomenon of the DC network due to the background harmonics. In the mixed multi terminal DC system composed of MMC and LCC, the DC network still has several resonant frequencies. This paper proposes a harmonic suppression control strategy using the capacitive energy storage capacity of the MMC sub module. It can also be used to suppress the harmonic transmission of hybrid multiterminal DC.
【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM721.1

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