CN115441431A - A Multi-mode Coordinated Control Method for Photovoltaic Medium Voltage DC Series System - Google Patents
A Multi-mode Coordinated Control Method for Photovoltaic Medium Voltage DC Series SystemDownload PDFInfo
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Abstract
Translated fromChinese
Description
Translated fromChinese技术领域technical field
本发明涉及机电领域，具体涉及一种光伏中压直流串联系统多模式协调控制方法。The invention relates to the electromechanical field, in particular to a multi-mode coordinated control method for a photovoltaic medium-voltage DC series system.
背景技术Background technique
在多个分布式光伏阵列应用系统中，常规集中型汇集升压方式存在线路损耗大、发电效率低和升压比不够高等问题，引入串联型光伏直流升压结构可有效解决上述问题。In multiple distributed photovoltaic array application systems, the conventional centralized collection boost method has problems such as large line loss, low power generation efficiency, and insufficient boost ratio. The introduction of a series photovoltaic DC boost structure can effectively solve the above problems.
在光伏直流串联系统中，各台直流变换器输入独立、输出串联，系统电压由各台直流变换器按其输出功率占比分担。浮云遮挡、环境遮挡会导致各台直流变换器输出功率不均衡，输出功率相对较大的直流变换器将分担更高的系统电压，极端条件下可能出现过电压停机。同时，光伏直流串联系统运行受到光伏阵列、并网换流器、光伏直流变换器多方面影响，要考虑正常态和故障态两类状态，实现系统可靠运行和最大化功率输出两个运行目标，运行工况复杂，控制难度极大。因此，如果不采取有效控制措施，串联系统在运行过程中将面临严重问题，不利于工程化及推广应用。In the photovoltaic DC series system, the input of each DC converter is independent and the output is connected in series, and the system voltage is shared by each DC converter according to its output power ratio. Cloud occlusion and environmental occlusion will cause the output power of each DC converter to be unbalanced. The DC converter with relatively large output power will share the higher system voltage, and overvoltage shutdown may occur under extreme conditions. At the same time, the operation of the photovoltaic DC series system is affected by various aspects of the photovoltaic array, grid-connected converter, and photovoltaic DC converter. Two types of states, normal state and fault state, must be considered to achieve the two operational goals of reliable system operation and maximum power output. The operating conditions are complex and the control is extremely difficult. Therefore, if effective control measures are not taken, the series system will face serious problems during operation, which is not conducive to engineering and popularization and application.
现有针对光伏直流串联型系统的控制方法中，在阈值调节方面，仅考虑了变换器内部工作模块数量的影响，而未考虑系统中其他变换器运行状态变化带来的耦合影响，此外，在控制方式上主要采用固定步长调节方式。总结现有控制方法，主要存在以下问题：(1)仅考虑变换器内部运行模块数量因素，该方式仅能确定阈值上限值，而无法准确确定阈值区间的下限值，导致直流变换器在实际工作过程中无法及时进入最大功率跟踪模式，即直流变换器未处在最优运行状态，造成发电量的损失；(2)在固定步长控制方式下，无论变换器运行在单一稳态工作模式状态还是不同模式切换状态，均会降低系统响应及调节速度，导致出现输出过压、最大功率跟踪点追踪不准确等问题；(3)采用固定步长及固定阈值控制方式，在部分工作模式运行及切换过程中，会引起较大的电压/电流变化率及线路传导干扰，同时，变换器难以兼顾实现平滑稳定切换与高动态响应。In the existing control methods for photovoltaic DC series systems, in terms of threshold adjustment, only the influence of the number of working modules inside the converter is considered, and the coupling effects brought about by changes in the operating status of other converters in the system are not considered. In addition, in The control method mainly adopts the fixed step size adjustment method. Summarizing the existing control methods, there are mainly the following problems: (1) Only considering the number of operating modules inside the converter, this method can only determine the upper limit of the threshold, but cannot accurately determine the lower limit of the threshold interval, resulting in In the actual working process, the maximum power tracking mode cannot be entered in time, that is, the DC converter is not in the optimal operating state, resulting in the loss of power generation; (2) In the fixed step control mode, no matter whether the converter is running in a single steady Mode status or different mode switching status will reduce the system response and adjustment speed, resulting in problems such as output overvoltage and inaccurate maximum power tracking point tracking; (3) using fixed step size and fixed threshold control method, in some working modes During the operation and switching process, it will cause a large voltage/current change rate and line conduction interference. At the same time, it is difficult for the converter to achieve smooth and stable switching and high dynamic response.
发明内容Contents of the invention
为克服现有控制方法的不足，本发明提出一种光伏中压直流串联系统多模式协调控制方法，其为基于分段式变步长与动态阈值调节的光伏中压直流串联系统多模式协调控制方法，该方法依据光伏直流串联系统中各变换器自身输出电压、输入功率实时变化状态，通过分段式变步长与动态阈值调节控制策略实现串联系统中各分布式直流变换器工作模式快速平稳切换，在确保光伏直流变换器安全可靠运行的同时，提高系统发电量，解决光伏直流串联系统中复杂工况变化下直流变换器协调控制难度大、运行可靠性低的问题。In order to overcome the shortcomings of existing control methods, the present invention proposes a multi-mode coordinated control method for photovoltaic medium-voltage DC series systems, which is a multi-mode coordinated control method for photovoltaic medium-voltage DC series systems based on segmental variable step size and dynamic threshold adjustment The method, based on the real-time change state of the output voltage and input power of each converter in the photovoltaic DC series system, realizes the fast and stable working mode of each distributed DC converter in the series system through the segmental variable step size and dynamic threshold adjustment control strategy Switching, while ensuring the safe and reliable operation of the photovoltaic DC converter, increases the power generation of the system, and solves the problems of difficult coordination control and low operational reliability of the DC converter under complex working conditions in the photovoltaic DC series system.
本发明通过以下技术方案实现：The present invention is realized through the following technical solutions:
一种光伏中压直流串联系统多模式协调控制方法基于多台直流变换器输入端相互独立、输出端串联方式构成的光伏直流串联系统，每台直流变换器输入端口连接对应的光伏阵列，所有直流变换器输出端串联之后接入直流电网；所述控制方法基于光伏直流串联系统中各直流变换器自身输出电压、输入功率变化状态及变化区间，通过分段式变步长与动态阈值调节实现系统中各分布式直流变换器工作模式快速稳定切换。A multi-mode coordinated control method for a photovoltaic medium-voltage DC series system is based on a photovoltaic DC series system composed of multiple DC converters with independent input terminals and serial output terminals. The input port of each DC converter is connected to the corresponding photovoltaic array. All DC The output terminals of the converters are connected to the DC power grid after being connected in series; the control method is based on the output voltage, input power change state and change range of each DC converter in the photovoltaic DC series system, and the system is realized through segmental variable step size and dynamic threshold adjustment. The working mode of each distributed DC converter can be switched quickly and stably.
进一步地，每台直流变换器在运行过程中存在两种工作模式，分别为最大功率跟踪模式和输出电压闭环模式，其中最大功率跟踪模式实现光伏阵列最大功率并网运行，输出电压闭环工作模式用于在不同光伏阵列光照不均衡条件下限制直流变换器输出功率；Furthermore, each DC converter has two working modes during operation, which are the maximum power tracking mode and the output voltage closed-loop mode. To limit the output power of the DC converter under the unbalanced illumination conditions of different photovoltaic arrays;
当直流变换器处于上述两种工作模式之一时，根据输入功率变化率动态调节控制步长，以实现在光伏功率变化剧烈时，快速准确追踪最大功率以及控制输出电压。When the DC converter is in one of the above two working modes, the control step size is dynamically adjusted according to the input power change rate, so as to quickly and accurately track the maximum power and control the output voltage when the photovoltaic power changes drastically.
进一步地，当外界状态发生变化，直流变换器切换运行模式时，在模式切换过程中采用变步长调节，实现运行模式快速平稳切换；当直流变换器从输出电压闭环模式切换到最大功率跟踪模式时，根据当前控制步长，在模式切换开始之后的一定数量控制周期内，动态减小该台直流变换器的占空比调节步长，使直流变换器更平稳切换过渡至最大功率跟踪模式，防止切换时刻电压/电流变化率过大产生串扰甚至出现故障；当直流变换器从最大功率跟踪模式切换到输出电压闭环模式时，此时在模式切换开始之后的一定数量控制周期内，动态增大该台直流变换器的占空比调节步长，使直流变换器更快进入输出电压闭环模式，防止发生过压故障。Furthermore, when the external state changes and the DC converter switches the operating mode, variable step adjustment is adopted in the mode switching process to realize fast and smooth switching of the operating mode; when the DC converter switches from the output voltage closed-loop mode to the maximum power tracking mode , according to the current control step size, within a certain number of control cycles after the start of mode switching, the duty cycle adjustment step size of the DC converter is dynamically reduced, so that the DC converter switches to the maximum power tracking mode more smoothly. Prevent crosstalk or even failure caused by excessive voltage/current change rate at the switching moment; when the DC converter switches from the maximum power tracking mode to the output voltage closed-loop mode, at this time, within a certain number of control cycles after the start of the mode switching, the dynamic increase The duty ratio adjustment step of the DC converter enables the DC converter to enter the output voltage closed-loop mode faster to prevent overvoltage faults.
进一步地，根据直流变换器输出电压的变化率动态更新切换电压阈值区间，用于快速稳定实现切换模式，提高发电量，并防止输出过压；当某台直流变换器运行在输出电压闭环模式且实时检测到自身输出电压快速下降时，通过控制系统动态调节减小电压阈值区间下限值，使该台直流变换器更快由输出电压闭环模式进入最大功率跟踪模式。Further, the switching voltage threshold interval is dynamically updated according to the rate of change of the output voltage of the DC converter, which is used to quickly and stably realize the switching mode, increase power generation, and prevent output overvoltage; when a DC converter operates in the output voltage closed-loop mode and When the rapid drop of its own output voltage is detected in real time, the lower limit of the voltage threshold interval is reduced through the dynamic adjustment of the control system, so that the DC converter enters the maximum power tracking mode from the output voltage closed-loop mode faster.
进一步地，所述协调控制方法用于降低变换器运行过程中电压/电流变化率及线路传导干扰。Further, the coordinated control method is used to reduce the voltage/current change rate and line conduction interference during the operation of the converter.
本发明的有益效果是：The beneficial effects of the present invention are:
本发明的分段式变步长调节方式，无论当变换器运行在单一稳态工作模式状态还是不同模式切换状态下，均可提高系统动态响应及调节速度，并提高在光伏功率变化剧烈时的最大功率跟踪点追踪准确度；针对不同模式切换过程，采用不同变化方向的动态步长调节方式，可实现工作模式平稳切换，同时又可有效避免发生过压故障；本发明的动态阈值区间调节方式，控制系统根据直流变换器自身工作过程中输出电压变化率、实际运行模块数量参数，实时动态调节电压阈值区间的上限值和下限值，优化直流变换器工作运行状态，提高光照利用率；本发明所述的分段式变步长与动态阈值调节，可减小模式切换过程中以及单一工作模式下的电压/电流变化率及线路传导干扰。所述控制方法在有效防止输出过压的同时，可提高光伏直流串联系统发电量；此系统不需设置专门的集中控制器，可有效减小控制器软硬件成本及实施难度。The segmented variable step length adjustment method of the present invention can improve the dynamic response and adjustment speed of the system no matter when the converter is running in a single steady-state working mode or in different mode switching states, and can improve the performance when the photovoltaic power changes sharply. The tracking accuracy of the maximum power tracking point; for different mode switching processes, the dynamic step size adjustment method with different changing directions can be used to realize the smooth switching of the working mode, and at the same time, it can effectively avoid the occurrence of overvoltage faults; the dynamic threshold interval adjustment method of the present invention , the control system dynamically adjusts the upper limit and lower limit of the voltage threshold range in real time according to the output voltage change rate during the working process of the DC converter itself and the number of actual operating modules, optimizes the working status of the DC converter, and improves the utilization rate of light; The segmented variable step size and dynamic threshold adjustment described in the present invention can reduce the voltage/current change rate and line conduction interference during the mode switching process and in a single working mode. The control method can effectively prevent the output overvoltage while increasing the power generation of the photovoltaic DC series system; this system does not need to be equipped with a special centralized controller, which can effectively reduce the cost of controller software and hardware and the difficulty of implementation.
附图说明Description of drawings
图1是本发明的一种光伏中压直流串联系统多模式协调控制方法原理图。Fig. 1 is a schematic diagram of a multi-mode coordinated control method for a photovoltaic medium-voltage DC series system according to the present invention.
图2是采用本发明控制方法的光伏直流串联系统拓扑图。Fig. 2 is a topological diagram of a photovoltaic DC series system adopting the control method of the present invention.
图3是采用本发明的控制方法下光伏直流串联系统启动场景的工作原理图。Fig. 3 is a working principle diagram of the start-up scene of the photovoltaic DC series system using the control method of the present invention.
图4是采用本发明的控制方法下光伏直流串联系统停机场景的工作原理图。Fig. 4 is a working principle diagram of the downtime scenario of the photovoltaic DC series system under the control method of the present invention.
图5是采用本发明的控制方法下光伏直流串联系统功率不均衡场景的工作原理图。Fig. 5 is a working principle diagram of a power unbalanced scene of a photovoltaic DC series system using the control method of the present invention.
图3、图4和图5中，颜色较浅的部分，表示该部分电路在该工况下处于不导通状态；而颜色较深的部分表示该部分电路在该工况下处于导通状态。In Figure 3, Figure 4 and Figure 5, the part with a lighter color indicates that the part of the circuit is in a non-conducting state under this working condition; while the part with a darker color indicates that this part of the circuit is in a conducting state under this working condition .
具体实施方式detailed description
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、完整的描述，显然，所描述的实施例仅为本发明的一部分实施例，而不是全部的实施例，基于本发明中的实施例，本领域的普通技术人员在不付出创造性劳动的前提下所获得的所有其他实施例，都属于本发明的保护范围。The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.
本发明的光伏直流串联系统多模式协调控制方法基于分段式变步长与动态阈值调节，基于多台直流变换器输入端相互独立、输出端串联并接入直流电网的光伏直流串联系统，每台直流变换器存在最大功率跟踪模式和输出电压闭环两种工作模式。The multi-mode coordinated control method of the photovoltaic DC series system of the present invention is based on segmental variable step size and dynamic threshold adjustment, based on a photovoltaic DC series system in which the input terminals of multiple DC converters are independent from each other, the output terminals are connected in series and connected to the DC power grid, each There are two working modes of maximum power tracking mode and output voltage closed-loop in a DC converter.
在实际工作过程中，如图1所示，每个控制周期内实时检测直流变换器输出电压是否有越过阈值，从而判断当前是否需要进行模式切换，模式切换方案为：设置输出电压阈值区间为[Vα,Vβ]；当检测到输出电压大于上限值Vβ时，如果直流变换器当前工作在最大功率跟踪模式，则切换至输出电压闭环模式，如果当前工作在输出电压闭环模式，则继续维持在输出电压闭环模式；当检测到输出电压小于下限值时Vα时，如果直流变换器当前工作在输出电压闭环模式，则切换至最大功率跟踪模式，如果当前工作在最大功率跟踪模式，则继续维持在最大功率跟踪模式；In the actual working process, as shown in Figure 1, it is detected in real time whether the output voltage of the DC converter has crossed the threshold in each control cycle, so as to judge whether the current mode switching is required. The mode switching scheme is: set the output voltage threshold range to [ Vα , Vβ ]; when it is detected that the output voltage is greater than the upper limit Vβ , if the DC converter is currently working in the maximum power tracking mode, it will switch to the output voltage closed-loop mode, if it is currently working in the output voltage closed-loop mode, then Continue to maintain the output voltage closed-loop mode; when it is detected that the output voltage is less than the lower limit Vα , if the DC converter is currently working in the output voltage closed-loop mode, then switch to the maximum power tracking mode, if the current working in the maximum power tracking mode , then continue to maintain the maximum power tracking mode;
进一步的，为提高直流变换器应对复杂工况的动态响应性能，提高发电量，采用动态阈值调节的控制方式，即当某台直流变换器运行在输出电压闭环模式时，如果实时检测到该台直流变换器自身输出电压出现快速下降，意味着系统中其他直流变换器的输出电压正在快速上升，此时，该台直流变换器需要通过动态变阈值方式使其更快地从输出电压闭环模式进入最大功率跟踪模式，从而充分利用光照条件，提高日发电量；具体操作是：在每个控制周期内，实时检测输出电压变化率
其中Vα0为阈值电压下限值的初始值，k1为控制系统中设定的系数。Among them, Vα0 is the initial value of the lower limit of the threshold voltage, and k1 is the coefficient set in the control system.
进一步的，计算每台直流变换器内实际运行的模块数量n，假设直流变换器内部每个模块输出电压额定值为Vmodule，且考虑模块输出电压具有一定裕量，则更新电压阈值区间的上限值Vβ，Further, calculate the number n of modules actually running in each DC converter, assuming that the output voltage rating of each module in the DC converter is Vmodule , and considering that the module output voltage has a certain margin, update the upper voltage threshold interval Limit Vβ ,
Vβ＝nVmodule+1 (2)Vβ =nVmodule +1 (2)
根据式(1)和式(2)得出的最新阈值数值，更新动态阈值区间为[Vα,Vβ]，此后读取直流变换器当前工作模式，并检测输出电压Vo实际大小是否处于更新后的阈值区间[Vα,Vβ]内，来判断运行模式是否需要切换。According to the latest threshold value obtained from formula (1) and formula (2), update the dynamic threshold interval to [Vα , Vβ ], then read the current working mode of the DC converter and check whether the actual value of the output voltage Vo is in the In the updated threshold interval [Vα , Vβ ], it is judged whether the operation mode needs to be switched.
进一步的，如果不需要进行模式切换，继续维持在原工作模式工作运行，为确保在光伏功率变化剧烈时，直流变换器能快速准确追踪最大功率，此时采用变步长调节方式，通过检测输入功率变化率
其中，k2为控制系统中设置的动态步长调节系数，D为变换器中功率器件当前控制周期内的占空比，D0为上一个控制周期内的占空比。Among them, k2 is the dynamic step adjustment coefficient set in the control system, D is the duty cycle of the power device in the converter in the current control cycle, and D0 is the duty cycle in the previous control cycle.
进一步的，如果需要切换模式，进一步判断需要进行哪一种切换过程，如果需要由最大功率跟踪模式切换至输出电压闭环模式，为使输出电压快速下降防止发生过压故障，此时在模式切换时刻之后的m个控制周期内，按照式(4)动态增大占空比步长；如果需要由输出电压闭环模式切换至最大功率跟踪模式，为使输出功率平稳增大实现运行模式平滑切换，此时在模式切换时刻之后的m个控制周期内，按照式(5)动态减小占空比步长；Further, if it is necessary to switch modes, further determine which switching process is required, if it is necessary to switch from the maximum power tracking mode to the output voltage closed-loop mode, in order to make the output voltage drop rapidly and prevent the occurrence of overvoltage faults, at this time, at the moment of mode switching In the following m control cycles, the duty cycle step size is dynamically increased according to formula (4); if it is necessary to switch from the output voltage closed-loop mode to the maximum power tracking mode, in order to make the output power increase steadily and realize the smooth switching of the operating mode, this In the m control cycles after the mode switching moment, dynamically reduce the duty cycle step according to formula (5);
ΔDb-new≥k3ΔDb (4)ΔDb-new ≥k3 ΔDb (4)
ΔDb-new≤k4ΔDb (5)ΔDb-new ≤ k4 ΔDb (5)
其中，ΔDb为调节前的占空比步长，ΔDb-new为动态调节后的占空比步长，k3、k4控制系统中设置的动态步长调节系数。Among them, ΔDb is the duty cycle step size before adjustment, ΔDb-new is the duty cycle step size after dynamic adjustment, and k3 and k4 control the dynamic step size adjustment coefficient set in the system.
根据上述分段式变步长与动态阈值调节的控制方式更新控制步长及电压阈值区间，执行程序指令，则此时一个控制周期结束，下一个周期内执行与上述控制方式相同的动态调节与控制。Update the control step size and the voltage threshold interval according to the control method of segmental variable step size and dynamic threshold value adjustment, and execute the program instructions. At this time, one control cycle ends, and the same dynamic adjustment and control method as the above control method will be executed in the next cycle. control.
具体地，本发明采用由3台额定输出电压及输出功率为20kV/500kW的直流变换器通过输入相互独立、输出串联方式构成的光伏直流串联系统作为实例，系统结构图如图2所示，来分析本发明的基于分段式变步长与动态阈值调节的光伏直流串联系统多模式协调控制方法工作原理。其中，每台直流变换器输入端各自连接一个500kW的光伏阵列，3台直流变换器的输出端串联后接入60kV直流电网。以下分别给出在本发明的控制方法下，光伏直流串联系统在启动、停机和功率不均衡三种典型场景下的控制原理。Specifically, the present invention uses a photovoltaic DC series system composed of three DC converters with rated output voltage and output power of 20kV/500kW as an example. The system structure diagram is shown in Figure 2. Analyze the working principle of the multi-mode coordinated control method for photovoltaic DC series system based on segmental variable step size and dynamic threshold value adjustment of the present invention. Among them, the input end of each DC converter is connected to a 500kW photovoltaic array, and the output ends of the three DC converters are connected in series to a 60kV DC grid. Under the control method of the present invention, the control principles of the photovoltaic DC series system under three typical scenarios of startup, shutdown and power unbalance are respectively given below.
启动场景：Start the scene:
光伏直流串联系统中，第一直流变换器1、第二直流变换器2、第三直流变换器3的输入端相互独立，理想情况下，当3台直流变换器对应光伏阵列的光照强度在启动之前同时达到相同强度时，3台变换器可以实现同步启动，且启动后3台变换器均工作在最大功率跟踪模式，此种场景比较理想，实际环境中很少满足这一场景的条件。在实际系统中，由于3个光伏阵列所处位置不同导致光照强度会有所不同，因此3台变换器通常是不同步启动，其中，由于各光伏阵列光照条件不同而导致3台变换器依次先后启动的工况，是设备启动中最复杂的场景，下面将以此种工况为典型启动场景描述控制方法。In the photovoltaic DC series system, the input ends of the
如图3所示，第三直流变换器3对应的光伏阵列最先达到启动条件，因而第三直流变换器3最先启动，第三直流变换器3启动后首先将进入最大功率跟踪模式，剩余第一直流变换器1、第二直流变换器2未启动。在此工况下，第三直流变换器3的输出电压将持续上升，直至达到输出电压阈值的上限值后切换模式进入输出电压闭环模式。当第三直流变换器3的输出电压持续上升后达到其阈值上限值时，为提高变换器动态响应速度并有效避免发生输出过压故障，在模式切换发生后的m(根据实际系统要求，m可取值10～20)个控制周期内，根据式(4)，动态增大控制系统中的占空比调节步长，使第三直流变换器3的输出电压快速稳定在控制系统设定的目标值31kV，而在另外两台直流变换器均未启动时，这两台直流变换器输出电容上的电压均为14.5kV。As shown in Figure 3, the photovoltaic array corresponding to the
第一直流变换器1其次启动，启动后同样首先工作最大功率跟踪模式，随着其输出电压升高，第三直流变换器3的输出电压将下降。在每个控制周期内，直流变换器实时检测自身输出电压变化率，当自身输出电压变化率大于控制系统中设置的参考值时，根据式(1)，动态增大电压阈值区间的下限值，使阈值区间的下限值与变换器输出电压变化率成正比，并实时更新阈值电压，此时，随着输出电压的降低，第三直流变换器3将更快通过模式切换进入到最大功率跟踪模式。而当第三直流变换器3开始模式切换后，为使变换器输出功率平滑变化、工作模式平稳过渡，进一步在模式切换开始后的m个控制周期内，如式(5)所述，控制系统中动态减小占空比步长，使第三直流变换器3从输出电压闭环模式平稳切换至最大功率跟踪模式。第一直流变换器1启动后，第一直流变换器1和第三直流变换器3均运行在最大功率跟踪模式，如果光伏阵列光照均衡，则第一直流变换器1和第三直流变换器3的输出电压均为30kV。而第二直流变换器2的高压侧整流二极管导通，其输出电容电压被二极管钳位为零。The
第二直流变换器2最后启动，启动后其工作在最大功率跟踪模式，随着其输出电压升高，第一直流变换器1和第三直流变换器3的输出电压相应降低，如果3个光伏阵列光照均衡，则最终3台直流变换器稳态下的输出电压均为20kV。The
停机场景：Downtime scenario:
如图4所示，假设停机之前，3台直流变换器都处在最大功率跟踪模式的正常运行状态，每台输出电压均为20kV。此时，第三直流变换器3最先停机，停机后其输出电压下降，变换器#1和#2输出电压开始上升且均运行在最大功率跟踪模式，稳定后第一直流变换器1、第二直流变换器2的输出电压均为30kV。第三直流变换器3输出侧整流二极管导通钳位，其输出电容两端电压为0。As shown in Figure 4, it is assumed that before the shutdown, the three DC converters are in the normal operation state of the maximum power tracking mode, and the output voltage of each is 20kV. At this time, the
随后第二直流变换器2停机，此时第一直流变换器1的输出电压继续上升，直到达到阈值电压上限值31kV后，切换模式到输出电压闭环模式。与启动过程中的控制相似，在第一直流变换器1切换模式开始后的m个控制周期内，采用变步长调节方式，动态增大占空比长，使得变换器#1模式快速切换并使其输出电压稳定在目标参考值，防止发生过压故障。之后第一直流变换器1、第二直流变换器2、第三直流变换器3的输出电压为31kV、14.5kV和14.5kV，第一直流变换器1运行在输出电压闭环模式，直至第一直流变换器1最后停机。Then the
功率不平衡场景：Power imbalance scenario:
如图5所示，假设稳态下3台直流变换器都运行在最大功率跟踪模式，每台变换器输出电压均为20kV。如果此时3个光伏阵列所处环境条件发生变化，如所处位置的光照强度变化，则此时3台直流变换器将处于功率不均衡的工况。在光伏功率变化剧烈时，为实现快速准确跟踪最大功率，控制系统中采用变步长调节方式，设置动态步长与输入功率变化率成正比，即占空比步长随输入功率变化的增大而增大。通过动态调节后，稳态下3台变换器重新运行至当前光照强度对应的最大功率点，其中第一直流变换器1的输出功率最大，其输出电压为29kV；第二直流变换器2的输出功率最小，其输出电压为5kV；第三直流变换器3的输出功率介于第一直流变换器1、第二直流变换器2之间，输出电压为26kV。此时，3台变换器的输出电压均未超过阈值电压，因而不需要进行模式切换。As shown in Figure 5, it is assumed that the three DC converters are all operating in the maximum power tracking mode in the steady state, and the output voltage of each converter is 20kV. If the environmental conditions of the three photovoltaic arrays change at this time, such as the light intensity of the location changes, the three DC converters will be in a power unbalanced working condition at this time. When the photovoltaic power changes drastically, in order to achieve fast and accurate tracking of the maximum power, the control system adopts a variable step size adjustment method, and the dynamic step size is set to be proportional to the input power change rate, that is, the duty cycle step increases with the input power change And increase. After dynamic adjustment, the three converters re-operate to the maximum power point corresponding to the current light intensity in the steady state, in which the output power of the
如果运行一段时间后，出现更极端的工况，即其中一个光伏阵列所处位置由于浮云遮挡严重导致光伏输出功率下降为零，则其对应的第二直流变换器2输出电压也将下降为零，此时剩余2台直流变换器继续运行，这两台变换器输出端共同分担直流电网电压，而第二直流变换器2输出侧整流二极管导通后其输出电容电压被钳位为零。此时剩余两台直流变换器由于各自光伏阵列的光照强度不同，导致这两台变换器输出电压亦不相同。第一直流变换器1由于光伏功率更大其输出电压达到阈值上限值31kV，因而第一直流变换器1将由最大功率跟踪模式切换进入输出电压闭环模式。当第一直流变换器1发生切换模式时，在切换开始后的m个控制周期内采用变步长调节，动态增大占空比步长使第一直流变换器1快速进入输出电压闭环模式；另一台第三直流变换器3由于光伏输出功率相对第一直流变换器1更小，其输出电压为29kV，仍运行在最大功率跟踪模式。If after running for a period of time, a more extreme working condition occurs, that is, the position of one of the photovoltaic arrays is seriously blocked by floating clouds and the photovoltaic output power drops to zero, then the output voltage of the corresponding
从以上分析可知，多模式协调控制在光伏直流串联系统实际运行中极为关键，本发明在剖析典型场景和工况的基础上，提出基于分段式变步长与动态阈值调节的光伏直流串联系统多模式协调控制方法，在确保光伏直流变换器可靠运行的同时，可提高系统发电量。针对不同工况下的模式切换过程，采用不同的动态调节方式，实现工作模式快速、平稳以及安全可靠切换。From the above analysis, it can be seen that multi-mode coordinated control is extremely critical in the actual operation of photovoltaic DC series systems. On the basis of analyzing typical scenarios and working conditions, this invention proposes a photovoltaic DC series system based on segmental variable step size and dynamic threshold adjustment The multi-mode coordinated control method can increase the power generation of the system while ensuring the reliable operation of the photovoltaic DC converter. For the mode switching process under different working conditions, different dynamic adjustment methods are adopted to realize fast, stable, safe and reliable switching of working modes.
以上所述仅为本发明的优选实施例，并非因此限制本发明的范围，凡是利用本发明说明书及附图内容所作的等效结构或流程变换，或直接或间接运用在其它相关的技术领域，均同理包括在本发明的保护范围内。The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Any equivalent structure or process transformation made by using the description of the present invention and the contents of the accompanying drawings, or directly or indirectly used in other related technical fields, All are equally included in the scope of protection of the present invention.
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