技术领域Technical Field
本发明涉及低频输电技术领域，具体涉及一种基于M3C的低频输电系统及其不对称故障穿越控制方法。The present invention relates to the technical field of low-frequency power transmission, and in particular to a low-frequency power transmission system based on M3C and an asymmetric fault ride-through control method thereof.
背景技术Background Art
随着全球对气候变化的深切关注和环境保护意识的提升，能源低碳化已成为能源发展的重要方向。近年来，可再生能源的发展一直保持高速度、高比例、高质量的态势，特别是在海上风电领域，因其巨大的未开发风力资源和优越的风况条件，展现出蓬勃的发展势头。With the world's deep concern about climate change and the improvement of environmental protection awareness, low-carbon energy has become an important direction for energy development. In recent years, the development of renewable energy has maintained a high speed, high proportion and high quality, especially in the field of offshore wind power, which has shown a booming development momentum due to its huge undeveloped wind resources and excellent wind conditions.
海上风电的发展不仅是推进能源革命、构建清洁低碳、安全高效现代能源体系的关键举措，还是未来可再生能源并网的重要发展方向。随着风电场站和换流器的不断扩容，海上风电的建设正在逐步向中远海距离、大容量方向拓展，这对风电并网系统和输电系统的容量、稳定性和安全性提出了更高的要求。为满足这些要求，低频交流输电技术(LFAC)应运而生。这一技术兼具交流和直流输电系统的优势，通过降低交流输电系统的主频率，有效降低线路电抗与充电无功，显著提升线路输送容量和输送距离，同时减小电压降落。此外，低频交流输电技术还具备对地电容增大、充电功率降低、传送距离增大、柔性调控能力强、交流灵活组网等特点，尤其在80～200千米距离内输电经济性更优、损耗更小，为海上风电资源的开发提供了重要支持。The development of offshore wind power is not only a key measure to promote the energy revolution and build a clean, low-carbon, safe and efficient modern energy system, but also an important development direction for the future grid connection of renewable energy. With the continuous expansion of wind farms and converters, the construction of offshore wind power is gradually expanding towards medium and long-distance sea distances and large capacity, which puts higher requirements on the capacity, stability and safety of wind power grid connection systems and transmission systems. To meet these requirements, low-frequency AC transmission technology (LFAC) came into being. This technology combines the advantages of AC and DC transmission systems. By reducing the main frequency of the AC transmission system, it effectively reduces the line reactance and charging reactive power, significantly improves the line transmission capacity and transmission distance, and reduces the voltage drop. In addition, low-frequency AC transmission technology also has the characteristics of increased capacitance to the ground, reduced charging power, increased transmission distance, strong flexible regulation capability, and flexible AC networking. In particular, the transmission economy is better and the loss is smaller within a distance of 80 to 200 kilometers, which provides important support for the development of offshore wind power resources.
变频器作为海上低频输电技术的核心组件，其电力电子器件的精选与控制设计的精良性尤为关键。随着电力电子技术的不断进步，交交变频器已经从高损耗、缺乏柔性调控能力的倍频变压器进化为频率高度可调、功能多样化的模块化多电平换流器，其中包括背靠背模块化多电平换流器(BTB-MMC)、Y型模块化多电平换流器(Y-MMC)，以及备受瞩目的模块化多电平矩阵换流器(M3C)。M3C以其卓越的控制灵活性、易集成性、可扩展性、功率因数可控性以及适应高压大功率环境的能力，成为目前研究和应用的热点变频器拓扑结构。As the core component of offshore low-frequency power transmission technology, the selection of power electronic devices and the excellence of control design are particularly critical. With the continuous advancement of power electronics technology, AC-AC converters have evolved from high-loss, frequency-multiplying transformers that lack flexible regulation capabilities to modular multilevel converters with highly adjustable frequencies and diverse functions, including back-to-back modular multilevel converters (BTB-MMC), Y-type modular multilevel converters (Y-MMC), and the highly anticipated modular multilevel matrix converter (M3C). M3C has become a hot inverter topology for current research and application due to its excellent control flexibility, easy integration, scalability, power factor controllability, and ability to adapt to high-voltage and high-power environments.
海上低频输电技术作为未来电网发展的重要方向，其M3C变频器的控制策略设计直接关乎系统传输的效率、安全性和经济效益。在保障低频输电系统安全稳定运行的同时，实现故障特性的精准分析并建立可靠的故障穿越控制策略，是保护系统免受潜在威胁的重要防线。然而，海上低频输电系统是一个高度电力电子化的复杂网络，其中包含了大量的风场换流器和岸上变频器等关键组件，低频输电通道两侧更是连接着密集的换流器设备，这对系统的控制稳定性提出了极高的要求。Offshore low-frequency transmission technology is an important direction for the development of future power grids. The control strategy design of its M3C inverter is directly related to the efficiency, safety and economic benefits of system transmission. While ensuring the safe and stable operation of the low-frequency transmission system, accurate analysis of fault characteristics and establishment of a reliable fault-crossing control strategy are important lines of defense to protect the system from potential threats. However, the offshore low-frequency transmission system is a complex network with a high degree of power electronics, which contains a large number of key components such as wind farm converters and onshore converters. The two sides of the low-frequency transmission channel are connected with dense converter equipment, which places extremely high demands on the control stability of the system.
由于故障特征的复杂性和不确定性，针对海上低频输电系统的故障穿越设计变得尤为困难。特别是在低频线路遭遇单相接地、两相短路或两相短路接地等故障时，系统的功率平衡将受到严重影响；而在系统三相电流抑制、三相电压平衡、有功波动抑制以及子模块电容电压平衡等方面的控制效果往往不尽如人意。Due to the complexity and uncertainty of fault characteristics, the fault ride-through design for offshore low-frequency transmission systems has become particularly difficult. In particular, when the low-frequency line encounters single-phase grounding, two-phase short circuit or two-phase short circuit grounding faults, the power balance of the system will be seriously affected; and the control effects in terms of system three-phase current suppression, three-phase voltage balance, active power fluctuation suppression and sub-module capacitor voltage balance are often unsatisfactory.
因此，如何提出一种基于M3C的低频输电系统及其不对称故障穿越控制方法，即便在故障特征尚不明晰的情况下，也能确保低频输电系统的安全稳定运行，实现故障特性的精准分析和建立可靠的故障穿越控制策略，已成为当前本领域技术人员亟需解决的技术难题。Therefore, how to propose a low-frequency transmission system based on M3C and its asymmetric fault ride-through control method, which can ensure the safe and stable operation of the low-frequency transmission system even when the fault characteristics are not clear, realize accurate analysis of fault characteristics and establish a reliable fault ride-through control strategy, has become a technical problem that technical personnel in this field need to solve urgently.
发明内容Summary of the invention
本发明的目的在于提供一种基于M3C的低频输电系统及其不对称故障穿越控制方法，用于解决现有技术中因低频线路发生单相接地、两相短路、两相短路接地等故障而引发的系统功率不平衡问题，并优化系统在三相电流抑制、三相电压平衡、有功波动抑制以及子模块电容电压平衡方面的控制效果。The purpose of the present invention is to provide a low-frequency power transmission system based on M3C and an asymmetric fault ride-through control method thereof, which are used to solve the system power imbalance problem caused by single-phase grounding, two-phase short circuit, two-phase short circuit grounding and other faults in the low-frequency line in the prior art, and optimize the system control effect in three-phase current suppression, three-phase voltage balance, active power fluctuation suppression and sub-module capacitor voltage balance.
本发明通过下述技术方案来解决上述技术问题：The present invention solves the above technical problems through the following technical solutions:
一种基于M3C的低频输电系统的不对称故障穿越控制方法，包括以下步骤：An asymmetric fault ride-through control method for a low-frequency power transmission system based on M3C includes the following steps:
步骤一、实时采集基于M3C的低频输电系统的低频侧电压；Step 1: Real-time collection of the low-frequency side voltage of the M3C-based low-frequency transmission system;
步骤二、根据步骤一采集的低频侧电压，判断所述低频输电系统是否出现不对称故障，若是，进入步骤三，否则返回步骤一；Step 2: According to the low-frequency side voltage collected in step 1, determine whether an asymmetric fault occurs in the low-frequency power transmission system. If so, proceed to step 3; otherwise, return to step 1;
步骤三、对低频输电系统进行有功自适应修正控制，并改进M3C电流内环的PI控制器。Step 3: Perform active adaptive correction control on the low-frequency power transmission system and improve the PI controller of the M3C current inner loop.
进一步地，所述步骤二具体包括：Furthermore, the step 2 specifically includes:
将步骤一采集的低频侧电压输入可编程控制器中，并与可编程控制器中预设的电压额定值进行比较，当低频侧电压小于电压额定值的0.9倍时，判定低频输电系统出现不对称故障。The low-frequency side voltage collected in step 1 is input into the programmable controller and compared with the voltage rated value preset in the programmable controller. When the low-frequency side voltage is less than 0.9 times of the voltage rated value, it is determined that an asymmetric fault occurs in the low-frequency power transmission system.
进一步地，步骤三中，所述有功自适应修正控制具体为：通过设置低频输电系统的有功功率二倍频为0，抑制有功功率二倍频波动；并修正M3C低频侧dq轴正、负序电流参考值。Furthermore, in step three, the active power adaptive correction control is specifically as follows: by setting the active power double frequency of the low-frequency power transmission system to 0, the active power double frequency fluctuation is suppressed; and the dq axis positive and negative sequence current reference values on the low-frequency side of M3C are corrected.
进一步地，所述修正M3C低频侧dq轴正序电流参考值的表达式为：Furthermore, the expression for correcting the reference value of the dq axis positive sequence current at the low frequency side of the M3C is:
所述M3C低频侧dq轴负序电流参考值的表达式为：The expression of the reference value of the dq axis negative sequence current on the low frequency side of the M3C is:
其中，和分别为M3C低频侧dq坐标系下的电压的正、负序分量；为M3C低频侧dq坐标系下的电流的正序分量；和和分别为修正M3C低频侧dq轴正、负序电流参考值。in, and They are respectively the positive and negative sequence components of the voltage in the dq coordinate system at the low frequency side of M3C; is the positive sequence component of the current in the dq coordinate system on the low-frequency side of M3C; and and They are respectively used to correct the reference values of the positive and negative sequence currents of the dq axis on the low-frequency side of M3C.
进一步地，步骤三中，所述改进M3C电流内环的PI控制器具体为：将M3C电流内环的PI控制器改为ADRC控制器；Further, in step 3, the PI controller of the M3C current inner loop is improved by: changing the PI controller of the M3C current inner loop into an ADRC controller;
所述ADRC控制器包括跟踪微分器TD、扩张状态观测器ESO及非线性反馈控制律NFC。The ADRC controller includes a tracking differentiator TD, an extended state observer ESO and a nonlinear feedback control law NFC.
进一步地，所述跟踪微分器TD的表达式为：Furthermore, the expression of the tracking differentiator TD is:
其中，为输入信号，分别为对的跟踪信号，r为调节参数，为低频侧d轴电流，为低频侧q轴电流。in, is the input signal, Respectively for The tracking signal, r is the adjustment parameter, is the d-axis current on the low-frequency side, is the q-axis current on the low-frequency side.
进一步地，所述扩张状态观测器ESO的表达式为：Furthermore, the expression of the extended state observer ESO is:
其中，为电流跟踪值，为总扰动观测值，为估计误差，为估计电流，为电流内环综合干扰，βld0、βlq0和βlq1、βld1是ESO的增益，α0、α1为阶数关联量，δ为线性尺度，bl1、bl2为反馈补偿参数，分别为低频侧d、q轴电压。in, is the current tracking value, is the total disturbance observation value, is the estimation error, To estimate the current, is the integrated disturbance of the current inner loop, βld0 , βlq0 and βlq1 , βld1 are the gains of the ESO, α0 , α1 are the order correlation quantities, δ is the linear scale, bl1 , bl2 are the feedback compensation parameters, They are the d-axis and q-axis voltages on the low-frequency side respectively.
进一步地，所述非线性反馈控制律NFC的表达式为：Furthermore, the expression of the nonlinear feedback control law NFC is:
式中，kd0、kd1、kd2、kq0、kq1和kq2为估计误差的反馈增益，α为阶数关联量，δ为线性尺度，和为估计误差，其表达式分别为：Where kd0 , kd1 , kd2 , kq0 , kq1 and kq2 are the feedback gains of the estimation error, α is the order correlation, δ is the linear scale, and is the estimation error, and its expressions are:
一种基于M3C的低频输电系统，当低频输电系统出现不对称故障时，通过上述基于M3C的低频输电系统的不对称故障穿越控制方法，解决系统在不对称故障时的不平衡功率。A low-frequency power transmission system based on M3C, when an asymmetric fault occurs in the low-frequency power transmission system, the unbalanced power of the system during the asymmetric fault is solved by the asymmetric fault ride-through control method of the low-frequency power transmission system based on M3C.
进一步地，基于M3C的低频输电系统包括风力发电机模块、M3C变频器模块和可编程处理器，所述风力发电机模块通过低频输电线路连接M3C变频器模块；Further, the M3C-based low-frequency power transmission system includes a wind turbine module, an M3C inverter module and a programmable processor, wherein the wind turbine module is connected to the M3C inverter module via a low-frequency power transmission line;
风机模块由风力发电机的输出端依次连接换流站和升压变压器；M3C变频器模块由第一变压器依次连接M3C变频器、第二变压器及工频电网；The wind turbine module is connected to the converter station and the step-up transformer in sequence from the output end of the wind turbine generator; the M3C inverter module is connected to the M3C inverter, the second transformer and the power frequency grid in sequence from the first transformer;
其中，低频输电线路上设置有电压互感器，用于采集M3C变频器的低频侧电压；所述电压互感器连接可编程处理器。Among them, a voltage transformer is arranged on the low-frequency transmission line for collecting the low-frequency side voltage of the M3C inverter; the voltage transformer is connected to the programmable processor.
与现有技术相比，本发明的积极进步效果在于：Compared with the prior art, the positive and progressive effects of the present invention are:
本发明提供的基于M3C的低频输电系统的不对称故障穿越控制方法，通过电压互感器采集的M3C低频侧电压值，判断低频输电系统的运行状态；当系统出现不对称故障，如单相接地、两相短路或两相短路接地时，对低频输电系统进行有功自适应修正控制，同时对M3C电流内环进行基于自抗扰控制的优化改进。采用本方法能够解决系统在不对称故障时的不平衡功率问题，提高系统的有功自适应性和不对称故障的应对能力，同时优化了系统在三相电流抑制、三相电压平衡、有功波动抑制以及子模块电容电压平衡等方面的控制效果。The asymmetric fault ride-through control method of the low-frequency power transmission system based on M3C provided by the present invention determines the operating state of the low-frequency power transmission system through the voltage value of the M3C low-frequency side collected by the voltage transformer; when an asymmetric fault occurs in the system, such as single-phase grounding, two-phase short circuit or two-phase short circuit grounding, the low-frequency power transmission system is subjected to active adaptive correction control, and the M3C current inner loop is optimized and improved based on the self-disturbance rejection control. The method can solve the unbalanced power problem of the system during asymmetric faults, improve the active adaptability of the system and the ability to cope with asymmetric faults, and optimize the control effects of the system in terms of three-phase current suppression, three-phase voltage balance, active fluctuation suppression and submodule capacitor voltage balance.
本发明提供的一种基于M3C的低频输电系统，能够对低频输电系统进行有功自适应修正控制，同时对M3C电流内环进行基于自抗扰控制的优化改进。不仅跟踪和控制性能得到了提升，波形质量也有提高，同时谐波污染得到了有效控制，为低频输电系统的稳定运行和故障应对提供了强有力的技术支撑。The present invention provides a low-frequency power transmission system based on M3C, which can perform active adaptive correction control on the low-frequency power transmission system, and optimize and improve the M3C current inner loop based on active anti-disturbance control. Not only the tracking and control performance are improved, the waveform quality is also improved, and the harmonic pollution is effectively controlled, providing strong technical support for the stable operation and fault response of the low-frequency power transmission system.
附图说明BRIEF DESCRIPTION OF THE DRAWINGS
说明书附图用来提供对本发明的进一步理解，构成本发明的一部分，本发明的示意性实施例及其说明用于解释本发明，并不构成对本发明的不当限定。The drawings in the specification are used to provide further understanding of the present invention and constitute a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.
图1是本发明基于M3C的低频输电系统图；FIG1 is a diagram of a low-frequency power transmission system based on M3C according to the present invention;
图2是本发明M3C低频侧有功自适应修正控制方法设计框图；FIG2 is a block diagram of the design of the active adaptive correction control method of the M3C low-frequency side of the present invention;
图3是本发明M3C电流内环基于自抗扰控制的改进设计框图；FIG3 is a block diagram of an improved design of the M3C current inner loop based on active disturbance rejection control of the present invention;
图4是本发明基于M3C的低频输电系统及其不对称故障穿越控制框图；FIG4 is a block diagram of a low-frequency power transmission system based on M3C and its asymmetric fault ride-through control according to the present invention;
图5是本发明单相接地故障电气量仿真波形图；5 is a waveform diagram of electrical quantity simulation of a single-phase grounding fault according to the present invention;
图6是本发明两相短路故障电气量仿真波形图；6 is a two-phase short circuit fault electrical quantity simulation waveform diagram of the present invention;
图7是本发明两相短路接地故障电气量仿真波形图；7 is a two-phase short-circuit grounding fault electrical quantity simulation waveform diagram of the present invention;
图8是本发明ADRC控制与PI控制单相接地故障电流效果对比图；FIG8 is a comparison diagram of the effects of ADRC control and PI control on single-phase ground fault current of the present invention;
图9是本发明ADRC控制与PI控制单相接地故障谐波含量对比图。FIG9 is a comparison diagram of harmonic content of single-phase ground fault under ADRC control and PI control of the present invention.
其中，1为风力发电机、2为换流站、3为升压变压器、4为低频输电线路、5为电压互感器、6为第一变压器、7为M3C变频器、8为第二变压器、9为工频电网。Among them, 1 is a wind turbine, 2 is a converter station, 3 is a step-up transformer, 4 is a low-frequency transmission line, 5 is a voltage transformer, 6 is a first transformer, 7 is an M3C inverter, 8 is a second transformer, and 9 is an industrial frequency power grid.
具体实施方式DETAILED DESCRIPTION
为使本发明实施例的目的、技术方案和优点更加清楚，下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例是本发明一部分实施例，而不是全部的实施例。通常在此处附图中描述和展示出的本发明实施例的组件可以以各种不同的配置来布置和设计。In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.
因此，以下对在附图中提供的本发明的实施例的详细描述并非旨在限制要求保护的本发明的范围，而是仅仅表示本发明的选定实施例。基于本发明中的实施例，本领域普通技术人员在没有作出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.
应注意到：相似的标号和字母在下面的附图中表示类似项，因此，一旦某一项在一个附图中被定义，则在随后的附图中不需要对其进行进一步定义和解释。It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.
在本发明实施例的描述中，需要说明的是，若出现术语“上”、“下”、“水平”、“内”等指示的方位或位置关系为基于附图所示的方位或位置关系，或者是该发明产品使用时惯常摆放的方位或位置关系，仅是为了便于描述本发明和简化描述，而不是指示或暗示所指的装置或元件必须具有特定的方位、以特定的方位构造和操作，因此不能理解为对本发明的限制。此外，术语“第一”、“第二”等仅用于区分描述，而不能理解为指示或暗示相对重要性。In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.
此外，若出现术语“水平”，并不表示要求部件绝对水平，而是可以稍微倾斜。如“水平”仅仅是指其方向相对“竖直”而言更加水平，并不是表示该结构一定要完全水平，而是可以稍微倾斜。In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.
在本发明实施例的描述中，还需要说明的是，除非另有明确的规定和限定，若出现术语“设置”、“安装”、“相连”、“连接”应做广义理解，例如，可以是固定连接，也可以是可拆卸连接，或一体地连接；可以是机械连接，也可以是电连接；可以是直接相连，也可以通过中间媒介间接相连，可以是两个元件内部的连通。对于本领域的普通技术人员而言，可以根据具体情况理解上述术语在本发明中的具体含义。In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.
以下结合实施例对本发明做进一步的详细说明，所述是对本发明的解释而不是限定。The present invention is further described in detail below in conjunction with the embodiments, which are intended to explain the present invention rather than to limit it.
参见图1，一种基于M3C的低频输电系统，包括风力发电机模块、M3C变频器模块和可编程处理器，所述风力发电机模块通过低频输电线路4连接M3C变频器模块；Referring to FIG1 , a low-frequency power transmission system based on M3C includes a wind turbine module, an M3C inverter module and a programmable processor, wherein the wind turbine module is connected to the M3C inverter module via a low-frequency power transmission line 4;
风机模块由风力发电机1的输出端依次连接换流站2和升压变压器3；M3C变频器模块由第一变压器6依次连接M3C变频器7、第二变压器8及工频电网9；The wind turbine module is connected to the converter station 2 and the step-up transformer 3 in sequence from the output end of the wind turbine 1; the M3C inverter module is connected to the M3C inverter 7, the second transformer 8 and the power frequency grid 9 in sequence from the first transformer 6;
其中，低频输电线路4上设置有电压互感器5，用于采集M3C变频器的低频侧电压；所述电压互感器5连接可编程处理器。Among them, a voltage transformer 5 is provided on the low-frequency transmission line 4 for collecting the low-frequency side voltage of the M3C inverter; the voltage transformer 5 is connected to the programmable processor.
一种基于M3C的低频输电系统的不对称故障穿越控制方法，包括以下步骤：An asymmetric fault ride-through control method for a low-frequency power transmission system based on M3C includes the following steps:
步骤一、通过电压互感器采集低频侧电压；Step 1: Collect the low-frequency side voltage through a voltage transformer;
步骤二、根据步骤一采集的电压，判断低频输电系统是否出现不对称故障，具体包括：将步骤一采集的低频侧电压即系统电压输入到可编程控制器中，并与可编程控制器中预设的电压额定值进行比较，当系统电压大于等于电压额定值的0.9倍时，判定系统为稳态运行；当系统电压小于电压额定值的0.9倍时，判定系统出现不对称故障；Step 2: judging whether an asymmetric fault occurs in the low-frequency power transmission system according to the voltage collected in step 1, specifically comprising: inputting the low-frequency side voltage collected in step 1, i.e., the system voltage, into the programmable controller, and comparing it with the voltage rating preset in the programmable controller; when the system voltage is greater than or equal to 0.9 times the voltage rating, judging that the system is in steady-state operation; when the system voltage is less than 0.9 times the voltage rating, judging that an asymmetric fault occurs in the system;
步骤三、在低频输电系统出现不对称故障的情况下，结合有功自适应修正控制方法与M3C电流内环基于自抗扰控制的改进设计，完成基于自抗扰控制的有功自适应M3C不对称故障穿越控制。其中，有功自适应修正控制为：通过设置低频输电系统的有功功率二倍频为0，抑制有功功率二倍频波动；并修正M3C低频侧dq轴正、负序电流参考值；改进M3C电流内环的PI控制器为：将M3C电流内环的PI控制器改为自抗扰控制器即ADRC控制器；ADRC控制器包括跟踪微分器TD、扩张状态观测器ESO及非线性反馈控制律NFC。Step 3: In the case of an asymmetric fault in the low-frequency transmission system, the active adaptive correction control method is combined with the improved design of the M3C current inner loop based on the anti-disturbance control to complete the active adaptive M3C asymmetric fault ride-through control based on the anti-disturbance control. Among them, the active adaptive correction control is: by setting the active power double frequency of the low-frequency transmission system to 0, the active power double frequency fluctuation is suppressed; and the dq axis positive and negative sequence current reference values of the M3C low-frequency side are corrected; the PI controller of the M3C current inner loop is improved: the PI controller of the M3C current inner loop is changed to an anti-disturbance control controller, namely, an ADRC controller; the ADRC controller includes a tracking differentiator TD, an extended state observer ESO, and a nonlinear feedback control law NFC.
参考图2-4，图2虚线框部分为有功自适应修正控制，可以看作内环拓扑的输入参考值，图3为M3C电流内环拓扑的整体改进，图4为将这两者结合起来的整体改进图。Refer to Figure 2-4. The dotted box part in Figure 2 is the active adaptive correction control, which can be regarded as the input reference value of the inner loop topology. Figure 3 is the overall improvement of the M3C current inner loop topology, and Figure 4 is the overall improvement diagram combining the two.
上述步骤三中，有功自适应修正控制具体为：In the above step 3, the active adaptive correction control is specifically as follows:
参见图2，对基于M3C的低频输电系统进行基于有功功率波动抑制的控制设计，抑制有功功率二倍频波动，即减弱由于不对称故障导致的有功二倍频产生。因此，控制目标设定有功功率二倍频为0，即：Referring to Figure 2, a control design based on active power fluctuation suppression is performed on the low-frequency transmission system based on M3C to suppress the fluctuation of active power double frequency, that is, to weaken the generation of active power double frequency caused by asymmetric faults. Therefore, the control target sets the active power double frequency to 0, that is:
其中，Pp,和Pp,c2为有功二倍频分量，为低频侧dq坐标系下的电压正序分量。Among them, Pp, and Pp,c2 are active double frequency components, It is the voltage positive sequence component in the dq coordinate system on the low-frequency side.
将上式代入M3C有功功率表达式(1)，可得：Substituting the above formula into the M3C active power expression (1), we can get:
整合公式(1)、(2)，可得未修正的M3C低频侧正负序电流参考指令值为：By integrating formulas (1) and (2), the uncorrected M3C low-frequency side positive and negative sequence current reference command values can be obtained as follows:
其中，P1,和Q1,分别为有功和无功功率中的直流分量；和分别为M3C低频侧dq坐标系下的电压的正、负序分量；和和分别为M3C低频侧dq坐标系下的电流的正、负序分量；和分别为M3C低频侧dq轴正、负序电流参考值。Among them, P1, and Q1, are the DC components in active and reactive power, respectively; and They are respectively the positive and negative sequence components of the voltage in the dq coordinate system at the low frequency side of M3C; and and They are respectively the positive and negative sequence components of the current in the dq coordinate system at the low frequency side of M3C; and They are the reference values of the positive and negative sequence currents of the dq axis at the low frequency side of M3C respectively.
又不对称故障主要导致M3C低频侧功率不平衡，从而对系统造成的影响，进一步威胁M3C器件安全。Asymmetric faults mainly lead to power imbalance on the low-frequency side of M3C, which affects the system and further threatens the safety of M3C devices.
M3C上能量与子模块电容电压的关系为：The relationship between the energy on M3C and the submodule capacitor voltage is:
其中，Esm_M3C为M3C储存能量，Ce为M3C整体等值电容，Uce为M3C整体等值电压。Wherein,Esm_M3C is the energy stored in M3C,Ce is the overall equivalent capacitance of M3C, andUce is the overall equivalent voltage of M3C.
由上式可以看出，低频侧不对称故障时，有功功率的变化将导致M3C能量不平衡，不平衡能量的变化会影响子模块电容电压。考虑到抑制M3C低频侧功率脉动，可以更好的保护M3C器件，所以采取公式(3)和(4)作为低频侧VF控制的正负序电流参考值。It can be seen from the above formula that when an asymmetric fault occurs on the low-frequency side, the change in active power will cause M3C energy imbalance, and the change in unbalanced energy will affect the submodule capacitor voltage. Considering that suppressing the power pulsation on the low-frequency side of M3C can better protect the M3C device, formulas (3) and (4) are used as the positive and negative sequence current reference values for VF control on the low-frequency side.
针对M3C电流正序电流参考值往往由外环VF控制所确定，且与低频系统的功率传输具有固定的正向关系，无法直接改变，可以根据公式(3)和(4)对低频侧内环电流控制加入电流修正环节，以对修正后的系统进行跟踪，以实现有功功率支撑，抑制系统功率波动。The positive sequence current reference value of M3C current is often determined by the outer loop VF control and has a fixed positive relationship with the power transmission of the low-frequency system and cannot be changed directly. According to formulas (3) and (4), a current correction link can be added to the inner loop current control on the low-frequency side to track the corrected system, so as to achieve active power support and suppress system power fluctuations.
设定修正电流参考值如下所示：The correction current reference value is set as follows:
其中，和分别为M3C低频侧dq轴正负序电流修正参考值。in, and They are respectively the reference values for correction of the positive and negative sequence current of dq axis on the low frequency side of M3C.
上述步骤三中，M3C电流内环基于自抗扰控制的改进设计具体为：In the above step 3, the improved design of the M3C current inner loop based on the active disturbance rejection control is as follows:
参见图3，从M3C低频侧展开自抗扰控制器(ADRC控制器)的设计，相当于将低频输电系统中M3C变频器控制环节的传统内环PI控制器改为了ADRC控制器，相较于传统的PI控制器具备更优异的控制效果，能更好、更快的实现控制目的。由于正、负序分量的控制设计相同，故以低频侧正序分量为例展开ADRC控制器设计分析。As shown in Figure 3, the design of the ADRC controller from the M3C low-frequency side is equivalent to changing the traditional inner loop PI controller of the M3C inverter control link in the low-frequency transmission system to an ADRC controller. Compared with the traditional PI controller, it has a better control effect and can achieve the control purpose better and faster. Since the control design of the positive and negative sequence components is the same, the ADRC controller design analysis is carried out using the positive sequence component on the low-frequency side as an example.
第一、非线性跟踪微分器(TD)设计：First, nonlinear tracking differentiator (TD) design:
利用TD使系统快速稳定，并通过改变r来调节过渡时间以获得期望的跟踪效果。TD is used to make the system stabilize quickly, and the transition time is adjusted by changing r to obtain the desired tracking effect.
非线性跟踪微分器表达式：Nonlinear tracking differentiator expression:
第二、扩张状态观测器(ESO)设计：Second, Extended State Observer (ESO) design:
ESO是ADRC的核心，其将所有干扰值取做一个整体视为综合干扰扩张为新的状态变量，并进行补偿，将控制系统简化为一个积分串联的系统。ESO is the core of ADRC. It takes all disturbance values as a whole, regards them as comprehensive disturbances, expands them into new state variables, and compensates them, thus simplifying the control system into an integral series system.
改写低频侧正序表达式，可得：Rewriting the low-frequency side positive sequence expression, we can get:
其中，in,
其中，将fl1与fl2视为综合干扰，包括电流跟踪误差、电阻、电感变化干扰。bl1与bl2为反馈补偿参数。基于此，ESO设计如下：Among them,fl1 andfl2 are regarded as comprehensive interference, including current tracking error, resistance, and inductance change interference.b l1 and bl2 are feedback compensation parameters. Based on this, the ESO is designed as follows:
其中，为电流跟踪值，为总扰动观测值，为估计误差，为估计电流，为电流内环综合干扰，βl0和βl1是ESO的增益，α为阶数关联量，δ为线性尺度。in, is the current tracking value, is the total disturbance observation value, is the estimation error, To estimate the current, is the integrated interference of the current inner loop, βl0 and βl1 are the gains of the ESO, α is the order correlation quantity, and δ is the linear scale.
第三、非线性反馈控制律(NFC)设计：Third, nonlinear feedback control law (NFC) design:
NFC是通过读取TD与ESO产生的状态变量与估计值之间的误差，有效地改善系统的补偿性能，其与系统总补偿组成：NFC effectively improves the compensation performance of the system by reading the error between the state variables and the estimated values generated by TD and ESO. It is composed of the total compensation of the system:
其中，k0、k1、k2为各误差的反馈增益，Among them, k0 , k1 , k2 are the feedback gains of each error,
对基于M3C的低频输电系统的不对称故障穿越控制方法进行验证：通过PSCAD仿真实验模拟基于M3C的海上低频输电系统运行状态，得到发生不同类型的故障信息，进行系统故障穿越控制策略的切换，故障期间采取基于M3C的低频输电系统及其不对称故障穿越控制方法，恢复稳态后切换稳态控制策略。The asymmetric fault ride-through control method of the M3C-based low-frequency transmission system is verified: the operating state of the M3C-based offshore low-frequency transmission system is simulated through a PSCAD simulation experiment, and information on different types of faults is obtained. The system fault ride-through control strategy is switched. During the fault period, the M3C-based low-frequency transmission system and its asymmetric fault ride-through control method are adopted, and the steady-state control strategy is switched after the steady state is restored.
当系统稳态运行时，稳态控制策略为低频侧外环采用VF控制，工频侧外环采用子模块电容电压控制与无功控制，M3C电流内环采用传统电流内环控制及PI控制器。通过PSCAD仿真实验模拟基于M3C的海上低频输电系统运行状态，得到发生不同类型的故障信息。When the system is in steady-state operation, the steady-state control strategy is that the outer loop of the low-frequency side adopts VF control, the outer loop of the power frequency side adopts submodule capacitor voltage control and reactive power control, and the M3C current inner loop adopts traditional current inner loop control and PI controller. The PSCAD simulation experiment simulates the operating state of the offshore low-frequency transmission system based on M3C, and obtains different types of fault information.
当系统发生不对称故障时，不同类型的故障包括单相接地故障、单相接地故障及两相短路接地故障，下面对不同故障类型，进行故障穿越控制效果验证。When an asymmetric fault occurs in the system, different types of faults include single-phase grounding fault, single-phase grounding fault and two-phase short-circuit grounding fault. The fault ride-through control effect is verified for different fault types.
(1)在低频线路中点处设置单相接地故障(A相)，选取1.9s到2.4s区间展示，故障发生时刻为2s，对比本章控制与传统控制的效果。参见图5，相对于传统控制，本章控制在故障期间的电流抑制效果明显，将A相传统控制下故障初期的10kA过电流稳定抑制到1.5kA，大大降低了过流风险，也同时减小了三相电流的不平衡；相对于传统控制，本章控制在故障期间的电压平衡效果明显，传统控制下的三相电压不平衡，存在三相偏移差别，本章控制抬升了A相过低电压，减小了三相电压的不平衡度，提升了系统的稳定性。(1) A single-phase ground fault (phase A) is set at the midpoint of the low-frequency line, and the interval from 1.9s to 2.4s is selected for display. The fault occurs at 2s, and the effects of this chapter's control are compared with those of traditional control. See Figure 5. Compared with traditional control, this chapter's control has a significant current suppression effect during the fault period, and the 10kA overcurrent at the initial stage of the fault under traditional control of phase A is stably suppressed to 1.5kA, greatly reducing the overcurrent risk and also reducing the imbalance of the three-phase current; compared with traditional control, this chapter's control has a significant voltage balance effect during the fault period. The three-phase voltage under traditional control is unbalanced, and there is a difference in three-phase offset. This chapter's control raises the A phase's low voltage, reduces the imbalance of the three-phase voltage, and improves the stability of the system.
(2)在低频线路中点处设置单相接地故障(AB相)，选取1.9s到2.4s区间展示，故障发生时刻为2s，对比本章控制与传统控制的效果。参见图6，相对于传统控制，本章控制在故障期间的电流抑制效果明显，将AB相传统控制下故障初期的11kA过电流稳定抑制到3.8kA，大大降低了过流风险，也同时减小了三相电流的不平衡；相对于传统控制，本章控制在故障期间的电压平衡效果明显，传统控制下的三相电压不平衡，存在三相偏移差别，本章控制抬升了AB相过低电压、降低了C相过高电压，减小了三相电压的不平衡度，提升了系统的稳定性。(2) A single-phase ground fault (AB phase) is set at the midpoint of the low-frequency line, and the interval from 1.9s to 2.4s is selected for display. The fault occurs at 2s, and the effects of this chapter's control are compared with those of traditional control. See Figure 6. Compared with traditional control, this chapter's control has a significant current suppression effect during the fault period, and the 11kA overcurrent at the initial stage of the fault under the traditional control of the AB phase is stably suppressed to 3.8kA, greatly reducing the overcurrent risk and reducing the imbalance of the three-phase current; compared with traditional control, this chapter's control has a significant voltage balance effect during the fault period. The three-phase voltage under traditional control is unbalanced, and there is a difference in three-phase offset. This chapter's control raises the AB phase's low voltage and reduces the C phase's high voltage, reducing the imbalance of the three-phase voltage and improving the stability of the system.
(3)在低频线路中点处设置两相短路接地故障(AB相)，选取1.9s到2.4s区间展示，故障发生时刻为2s，对比本章控制与传统控制的效果。参见图7，相对于传统控制，本章控制在故障期间的电流抑制效果明显，将A传统控制下故障初期的10kA与B相传统控制下故障初期的13kA过电流稳定抑制到2kA，大大降低了过流风险，也同时减小了三相电流的不平衡；相对于传统控制，本章控制在故障期间的电压平衡效果明显，传统控制下的三相电压不平衡，存在三相偏移差别，本章控制抬升了AB相过低电压、降低了C相过高电压，减小了三相电压的不平衡度，提升了系统的稳定性。(3) A two-phase short-circuit grounding fault (AB phase) is set at the midpoint of the low-frequency line, and the interval from 1.9s to 2.4s is selected for display. The fault occurs at 2s, and the effects of this chapter's control are compared with those of traditional control. See Figure 7. Compared with traditional control, this chapter's control has a significant current suppression effect during the fault period, and the overcurrent of 10kA at the initial stage of the fault under traditional control of phase A and 13kA at the initial stage of the fault under traditional control of phase B are stably suppressed to 2kA, greatly reducing the overcurrent risk and reducing the imbalance of the three-phase current; compared with traditional control, this chapter's control has a significant voltage balancing effect during the fault period. The three-phase voltage under traditional control is unbalanced and there is a difference in three-phase offset. This chapter's control raises the over-low voltage of phase AB and reduces the over-high voltage of phase C, reducing the imbalance of the three-phase voltage and improving the stability of the system.
(4)为验证ADRC控制的优越性，对比ADRC控制器与PI控制器的效果，以低频侧三相电流为例，当单相接地故障时，采用快速傅里叶变换(FFT)分析，并分别计算ADRC与PI控制下的系统总谐波畸变率(THD)，选取2.1s到2.3s范围内低频侧输出电流进行总谐波畸变率计算。参见图8的电流波形，单相接地故障期间，ADRC控制相较于PI控制对三相电流控制性更好，可快速稳定至1.5kA，系统波动更小。参见图9的电流总谐波畸变率，单相接地故障期间，ADRC控制THD值为6.29％，相较于PI控制THD值7.09％电流谐波含有率更低，电能质量更好。(4) To verify the superiority of ADRC control, the effects of ADRC controller and PI controller are compared. Taking the three-phase current on the low-frequency side as an example, when a single-phase grounding fault occurs, fast Fourier transform (FFT) analysis is used, and the total harmonic distortion (THD) of the system under ADRC and PI control is calculated respectively. The low-frequency side output current in the range of 2.1s to 2.3s is selected for total harmonic distortion calculation. Referring to the current waveform in Figure 8, during a single-phase grounding fault, ADRC control has better control over the three-phase current than PI control, can quickly stabilize to 1.5kA, and has smaller system fluctuations. Referring to the current total harmonic distortion rate in Figure 9, during a single-phase grounding fault, the ADRC control THD value is 6.29%, which is lower than the PI control THD value of 7.09%, and the current harmonic content is lower, and the power quality is better.
综上所述，本发明基于M3C的低频输电系统及其不对称故障穿越控制方法，是针对基于M3C的低频输电系统设计了基于M3C的低频输电系统及其不对称故障穿越控制方法。本发明所提出的不对称故障穿越控制方法可以适应单相接地、两相短路、两相短路接地故障情况，提高系统的有功自适应性和不对称故障的应对能力，同时优化了系统在三相电流抑制、三相电压平衡、有功波动抑制以及子模块电容电压平衡等方面的控制效果。并且，相对于PI控制，采取ADRC控制的系统跟踪、控制性能较好、谐波污染小、波形质量高。In summary, the low-frequency power transmission system based on M3C and the asymmetric fault ride-through control method thereof of the present invention are designed for the low-frequency power transmission system based on M3C. The asymmetric fault ride-through control method proposed in the present invention can adapt to the single-phase grounding, two-phase short circuit, two-phase short circuit grounding fault conditions, improve the active adaptability of the system and the ability to cope with asymmetric faults, and optimize the control effect of the system in terms of three-phase current suppression, three-phase voltage balance, active power fluctuation suppression, and sub-module capacitor voltage balance. Moreover, compared with PI control, the system tracking and control performance of ADRC control are better, the harmonic pollution is small, and the waveform quality is high.
以上内容仅为说明本发明的技术思想，不能以此限定本发明的保护范围，凡是按照本发明提出的技术思想，在技术方案基础上所做的任何改动，均落入本发明权利要求书的保护范围之内。The above contents are only for explaining the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.
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