多端柔性直流电网新型保护技术研究

发布时间：2018-07-31 15:42

【摘要】：可再生能源的大规模接入以及能源资源和消费分布不均衡问题对电网的资源配置能力提出了紧迫要求,可实现多电源供电及多落点受电的多端直流输电(Multi-terminal Direct Current,MTDC)系统在新能源并网及输送领域应用前景广阔。相对传统直流输电而言,柔性直流输电(Voltage Source Converter based High Voltage Direct Current,VSC-HVDC)技术具有有功和无功独立控制、可以向无源负荷供电、不需要装设无功补偿设备以及滤波装置、潮流翻转时保持电压极性不变等优势,因此更适于构建MTDC系统,即建设多端柔性直流输电(Voltage Source Converter based Multi-terminal Direct Current,VSC-MTDC)系统。目前,全球至少已投运24条VSC-HVDC工程,其中大多数是两端直流输电工程,VSC-MTDC系统和直流电网正处于起始发展阶段,直流侧故障处理能力是限制VSC-MTDC电网发展的重要因素之一。针对VSC-MTDC电网,本文开展直流输电线路和直流母线保护的研究,所做的工作和取得的成果如下:其一,提出基于电流行波极性比较的VSC-MTDC电网直流线路保护方法。论文首先分析了直流电网中直流线路区内外故障时线路两端的初始电流行波极性特征:1)当某正极线路故障时,包括正极接地和正负极间故障,故障的正极线路两端的初始电流行波极性相同,并且同为正极性;其他非故障正极线路两端的极性相反或者同为负极性。2)当某负极线路故障时,包括负极接地和正负极间故障,故障的负极线路两端的初始电流行波极性相同,并且同为负极性;其他非故障负极线路两端的极性相反或者同为正极性。然后,利用上述区内外故障时的极性特征差异,构造了直流输电线路保护判据,并根据小波变换的相关理论对初始电流行波的极性进行提取。此外,论文分析了在电磁耦合作用下故障极对非故障极的影响,提出了基于电流行波小波变换模极大值的幅值关系识别直流线路故障类型的方法。其二,提出基于采样值差动电流的VSC-MTDC电网直流母线保护方法。论文分析了直流电网中直流母线区内外故障时与母线相连各支路的电流采样值之和(采样值差动电流)的幅值特征:外部故障或正常运行状态下,母线的采样值差动电流是零,内部短路故障时,采样值差动电流等于各直流线路的短路电流瞬时值的总和,其值非常大。利用母线采样值差动电流在区内外故障时的幅值差异,论文构造了 VSC-MTDC电网直流母线保护判据。其三,通过在每个换流站直流侧出口处的正极和负极线路上串联电阻型超导故障限流器,将直流侧故障电流限制在一定范围内,与所提保护方法相配合,用开断容量较小的直流断路器开断故障电流,实现故障母线或线路的隔离。最后,利用PSCAD电磁暂态仿真软件搭建了采用两电平换流器的±200kV四端柔性直流电网模型,并利用该模型对所提保护方案进行了大量仿真。仿真结果表明,对于不同故障初始条件下的VSC-MTDC电网直流侧短路故障,保护均能做出正确判断,动作速度快,可靠性高,可以耐受一定过渡电阻。
[Abstract]:The large-scale access of renewable energy and the unbalanced distribution of energy resources and consumption have put forward urgent requirements for the power distribution capability of the power grid. The Multi-terminal Direct Current (MTDC) system with multi power supply and multi drop point receiving is widely used in the field of new energy source grid connection and transportation. For HVDC, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) technology has active and reactive power independent control, which can supply power to passive load, do not install reactive power compensation equipment and filter device, and keep the voltage polarity constant when the power flow is turned over, so it is more suitable for construction. MTDC system, namely, the construction of Voltage Source Converter based Multi-terminal Direct Current, VSC-MTDC system. At present, at least 24 VSC-HVDC projects have been delivered in the world, most of which are two terminal DC transmission projects, VSC-MTDC system and direct current network are in the initial stage of development, DC side fault handling capacity It is one of the important factors to restrict the development of VSC-MTDC power grid. Aiming at VSC-MTDC power grid, the research on DC transmission line and DC bus protection is carried out in this paper. The work and achievements are as follows: first, a DC line protection method based on the comparison of current traveling wave polarity is proposed in the VSC-MTDC power grid. The initial electric wave polarity characteristics of the two ends of the line at both ends of the flow line area: 1) when a positive pole line fault, including the positive pole grounding and the positive and negative pole fault, the initial electric wave polarity of the two ends of the fault positive pole line is the same, and it is positive polarity; the polarity of the two ends of the other non fault positive line is opposite to the polarity or the negative polarity.2). When a negative electrode fault, including the negative electrode grounding and the positive and negative pole fault, the initial electric wave polarity of the two ends of the negative electrode line is the same, and it is negative polarity. The polarity of the other two ends of the non fault negative circuit is opposite or positive polarity. Then, the DC transmission is constructed by using the difference of the polarity characteristic of the fault in the above area and the fault. According to the correlation theory of the wavelet transform, the polarity of the initial current traveling wave is extracted. In addition, the paper analyzes the influence of the fault pole to the non fault pole under the electromagnetic coupling, and puts forward a method to identify the fault type of the DC line based on the amplitude relation of the maximum value of the current traveling wave wavelet transform. A DC bus protection method for VSC-MTDC power grid based on sampled value differential current is presented. The paper analyses the amplitude characteristics of the sum of current sampling values of each branch connected to the bus line connected to the busbar when the internal and external faults of the DC power grid are inside and outside the DC bus zone. The differential current of the sample value of the bus is zero under the external fault or the normal state of operation. In the case of short circuit fault, the differential current of sampling value equals the sum of the short-circuit current of each DC line, and its value is very large. Using the difference of the amplitude of the differential current of the bus line sampling value, the DC bus protection criterion of the VSC-MTDC power grid is constructed. The series resistance superconducting fault current limiter on the pole line limits the fault current of the DC side to a certain range, cooperates with the proposed protection method, disconnects the fault current with the DC circuit breaker with small open capacity, and realizes the isolation of the fault bus or line. Finally, the two level converter is set up by using the PSCAD electromagnetically transient simulation software. The model of 200kV four terminal flexible DC power grid is used to simulate the proposed protection scheme with this model. The simulation results show that the protection can make the correct judgment for the short circuit fault of the VSC-MTDC power grid under the initial conditions of different faults. The action speed is fast, the reliability is high, and a certain transition resistance can be tolerated.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM721.1

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