A Method for Switching the Control Mode of a Converter Follow-up Network Based on a Class Latch Operator

Abstract

The present invention discloses a method for switching the control mode of a converter's grid-following and grid-forming network based on a latch-like operator, belonging to the technical field of new energy grid connection, and comprising the following steps: S1, determining the specific control schemes of the grid-forming control and grid-following control that are expected to achieve the switching function; S2, separately separating the synchronization loop of the grid-forming control and grid-following control from the remaining loops; S3, transforming the synchronization loop according to the proposed method and transforming the remaining loops into the proposed latch-like operator structure; S4, the system has the ability to seamlessly switch between grid-following and grid-forming control, and can flexibly switch between the grid-following operation state and the grid-forming operation state as needed. By adopting the above method for switching the control mode of a converter's grid-following and grid-forming network based on a latch-like operator, the present invention can solve the problem of switching the control mode between grid-following and grid-forming in the absence of the same control loop.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid connection, and in particular to a method for switching the grid-connected control mode of a converter based on a latch-like operator. Background Art

[0002] Currently, in order to address energy and environmental issues, new energy power generation systems including photovoltaic and wind power have developed rapidly. However, with the decrease in the proportion of synchronous machines and the increase in the proportion of grid-following converters in the power grid, the inertia and stability of the power grid have been greatly affected. Therefore, it is necessary to strengthen the stability of the grid-connected converter system through control methods, enabling the converter to also have a certain power grid support ability. Such control methods are usually referred to as grid-forming control.

[0003] It has been found in research that there are dual characteristics between grid-following converters and grid-forming converters. For example, grid-following converters are more stable under strong grid conditions, while grid-forming converters are more stable under weak grid conditions. Therefore, a grid-following and grid-forming control mode switching technology is needed to change the control mode of the converter under specific circumstances to adapt to the real-time changing power grid environment. This has great practical significance for further increasing the new energy penetration rate, realizing large-scale grid connection of new energy systems and becoming the main power source, and alleviating the energy shortage crisis.

[0004] Currently, there are many grid-following and grid-forming switching schemes designed based on the same control loop. Usually, both grid-following control and grid-forming control require a current control loop. However, with the in-depth research, various single-voltage-loop grid-forming controls have been proposed due to their better voltage source characteristics and faster current response speed, but no grid-following and grid-forming switching method applicable to the case without the same loop has been proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for switching the grid-connected control mode of a converter based on a latch-like operator, which can solve the problem of switching the grid-connected control mode in the case of no same control loop.

[0006] To achieve the above purpose, the present invention provides a method for switching the grid-connected control mode of a converter based on a latch-like operator, including the following steps:

[0007] S1. Determine the specific control schemes of the grid-forming control and grid-following control that are expected to achieve the switching function;

[0008] S2. Separate the synchronization loop and the remaining loops of the grid-forming control and grid-following control respectively;

[0009] S3. Transform the synchronization loop according to the proposed method, and transform the remaining loops into the proposed latch-like operator structure;

[0010] S4. The system has the ability to seamlessly switch between following-grid control and forming-grid control, and can flexibly switch between the following-grid operation state and the forming-grid operation state as needed.

[0011] Preferably, in S1, the schemes of forming-grid control and following-grid control are separately determined in advance according to the requirements of system design, which are applicable to various mode switches of following-forming-grid control, including the situation where there is no same control loop.

[0012] Preferably, in S2, the synchronization loop is defined as the relevant loop that generates the phase of the control coordinate system, which is the phase-locked loop in following-grid control and the power synchronization loop in forming-grid control; the remaining loops are defined as the loops that generate control effects on the remaining variables other than the synchronization loop.

[0013] Preferably, in S3, during the transformation of the remaining loops, it is necessary to first screen out all the control loops containing integrators, keep their operation loops unchanged, and add a hot standby loop to pre-synchronize the output value when the mode to which the controller belongs is not working, so as to achieve subsequent seamless mode switching.

[0014] Preferably, in S3, during the transformation of the synchronization loop, it is necessary to turn off the primary frequency regulation function of the forming-grid mode in the following-grid mode, and add a phase error feedback loop, so that the power synchronization loop of the forming-grid control can track the output of the following-grid phase-locked loop without error in the following-grid operation state.

[0015] Preferably, in S4, the system has two states: the following-grid operation state and the forming-grid operation state. In the following-grid operation state, all loops of the following-grid control are in the working state, the forming-grid power synchronization loop turns off the primary frequency regulation function and turns on the phase error feedback to track the phase of the following-grid control, and the remaining loops synchronize the current values of each variable in real time by the latch-like operator, so as to seamlessly switch to the forming-grid mode at any time; in the forming-grid mode, the forming-grid synchronization loop turns off the phase error feedback function and operates normally, and the phase-locked loop of the following-grid control keeps running to achieve error-free tracking of the phase of the forming-grid control, and the remaining loops synchronize the current values of each variable in real time by the latch-like operator, so as to seamlessly switch to the following-grid mode at any time.

[0016] Therefore, the beneficial effects of the present invention adopting the above-mentioned method for switching the following-forming-grid control mode of the converter based on the latch-like operator are as follows:

[0017] (1) It is applicable to the following-forming switching transformation of various following-grid control and forming-grid control, and does not require the two to have the same loop at the same time. All existing other following-forming switching methods require the forming-grid converter to have a current loop to achieve seamless switching, while the present invention realizes seamless switching of all heterogeneous loops through the transformation of the latch-like operator and the synchronization loop. At the same time, the present invention is applicable to various power grid environments, including strong power grids, weak power grids, power grid frequency deviation scenarios, power grid voltage abnormality scenarios, etc.

[0018] (2) The control loop of the grid-connected converter in the present invention is divided into a synchronization loop and the remaining loops. The purpose is to clarify the physical meanings of each loop to achieve a more reasonable mode switching. The purpose of the synchronization loop is to generate the built-in phase information in the controller. Different synchronization loops represent different modes of interaction between the grid-connected converter and the power grid. The remaining loops are mainly responsible for the specific control of each key variable on the premise of synchronization with the power grid. The physical meanings of the two are essentially different. The separate processing of the two types of loops is one of the reasons why the present invention is superior to various existing structure-switching schemes.

[0019] (3) In the processing of the synchronization loop, the phase error feedback loop structure proposed in the present invention is simple but has good effects. It uses the characteristics of the sin function to eliminate the error caused by phase mutation, and at the same time reuses the PI controller of the power synchronization loop, enabling the grid-forming converter to follow the output of the grid-following control phase-locked loop in the grid-following mode. The main purpose of closing the primary frequency modulation loop is to reduce the impact during the switching process, especially in the scenario of power grid frequency deviation. In the grid-forming mode, the original grid-following control loop is fully utilized, and the phase-locked loop of the grid-following control is kept running. Without adding other control loops, the grid-following phase information can track the output of the power synchronization loop in the grid-forming mode without error.

[0020] In the processing of the remaining loops, the present invention proposes a general structure of a latch-like operator. By analogy with the concept of a latch in digital circuits, its core idea is to utilize the integrator structure of the idle loop in dual-mode control to synchronize the corresponding variables in the static-error-free operation loop to achieve seamless mode switching. Due to the generality of the latch-like operator, various grid-forming and grid-following controls can be modified and combined according to this operator structure. Therefore, this method has strong universality. At the same time, since the formation of the latch-like operator mainly utilizes the static-error-free control characteristics of the integrator, only the original control loop needs to have an integrator, and there is no requirement for the control object of the loop. Therefore, this operator can be applied to the mode switching of grid-forming and grid-following modes without the same loops, such as the mode switching between the single-voltage-loop grid-forming control and the single-current-loop grid-following control shown in the example, which is more flexible than the traditional switching method.

[0021] The technical solution of the present invention will be further described in detail below through the accompanying drawings and embodiments. Description of the Drawings

[0022] Figure 1 is a flowchart of an embodiment of a method for switching the grid-forming and grid-following control modes of a converter based on a latch-like operator according to the present invention;

[0023] Figure 2 is a structure diagram of the latch-like operator proposed by the present invention;

[0024] Figure 3It is a schematic diagram of the operating state of the structure network mode switching in the embodiment of the present invention;

[0025] Figure 4 It is a schematic diagram of the method for switching the structure network control synchronization loop mode proposed by the present invention;

[0026] Figure 5 It is a schematic diagram of a three-phase two-level new energy grid-connected system with an LC filter in the embodiment of the present invention;

[0027] Figure 6 It is a control block diagram of the single-current-loop network-following control selected by S1 in the embodiment of the present invention;

[0028] Figure 7 It is a control block diagram of the single-voltage-loop network-forming control selected by S1 in the embodiment of the present invention;

[0029] Figure 8 It is a system control block diagram of the overall control operation in the network-forming mode after the transformation of the single-current-loop network-following control and the single-voltage-loop network-forming control in the example of the present invention;

[0030] Figure 9 It is a system control block diagram of the overall control operation in the network-following mode after the transformation of the single-current-loop network-following control and the single-voltage-loop network-forming control in the example of the present invention;

[0031] Figure 10 It is a waveform diagram of the experimental results of switching from the network-following mode to the network-forming mode under a weak grid (grid short-circuit ratio is 1.9 p.u.) in the example of the present invention;

[0032] Figure 11 It is a waveform diagram of the experimental results of switching from the network-forming mode to the network-following mode under a weak grid (grid short-circuit ratio is 1.9 p.u.) in the example of the present invention;

[0033] Figure 12 It is a waveform diagram of the experimental results of switching from the network-following mode to the network-forming mode under a strong grid (grid short-circuit ratio is 12.8 p.u.) in the example of the present invention;

[0034] Figure 13 It is a waveform diagram of the experimental results of switching from the network-forming mode to the network-following mode under a strong grid (grid short-circuit ratio is 12.8 p.u.) in the example of the present invention. Detailed implementation manners

[0035] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings as understood by those of ordinary skill in the field to which the present invention pertains. The "first", "second" and similar terms used in the present invention do not denote any order, quantity or importance, but are only used to distinguish different components. Words such as "comprising" or "including" mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects. Words such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Upper", "lower", "left", "right", etc. are only used to represent relative position relationships, and when the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0037] Embodiment 1

[0038] As Figure 1 shown, the present invention provides a method for switching the control mode of a converter following network based on a latch-like operator, including the following steps:

[0039] S1. Determine the specific control schemes of the network-forming control and the network-following control that are expected to achieve the switching function.

[0040] The schemes of the network-forming control and the network-following control are separately determined in advance according to the requirements of system design, which is applicable to the mode switching of various following network-forming controls, including the case where there is no identical control loop.

[0041] S2. Separate the synchronization loop and the remaining loops of the network-forming control and the network-following control respectively.

[0042] The synchronization loop is defined as the relevant loop that generates the phase of the control coordinate system, which is the phase-locked loop in the network-following control and the power synchronization loop in the network-forming control; the remaining loops are defined as the loops that have a control effect on other variables except the synchronization loop.

[0043] S3. Transform the synchronization loop according to the proposed method, and transform the remaining loops into the proposed latch-like operator structure.

[0044] In the transformation of the remaining loops, it is necessary to first screen out all the control loops containing integrators. Their operating loops remain unchanged, and a hot standby loop is added to pre-synchronize the output value of its integrator when the mode to which the controller belongs is not working, so as to achieve seamless mode switching, as Figure 2 shown. Among them, control loop 1 and control loop 2 come from the control loops of two different modes in this embodiment, and it is necessary to select the controllers containing integrators to form this latch-like operator structure.

[0045] In the transformation of the synchronization loop, it is necessary to turn off the primary frequency regulation function of the grid-forming mode in the grid-following mode and add a phase error feedback loop, so that the power synchronization loop of the grid-forming control can track the output of the grid-following PLL without error in the grid-following operation state. Since the phase is a periodic variable, the sin function is used to eliminate the possible mutations caused by the periodic change of the phase, such as Figure 4 shown.

[0046] S4. The system has the ability to seamlessly switch between grid-following and grid-forming controls, and can flexibly switch between the grid-following operation state and the grid-forming operation state as needed, such as Figure 3 shown.

[0047] The system has two states: the grid-following operation state and the grid-forming operation state. In the grid-following operation state, all loops of the grid-following control are in the working state. The grid-forming power synchronization loop turns off the primary frequency regulation function and turns on the phase error feedback to track the phase of the grid-following control. The remaining loops synchronize the current values of each variable in real time by the latch-like operator so as to seamlessly switch to the grid-forming mode at any time; in the grid-forming mode, the grid-forming synchronization loop turns off the phase error feedback function and operates normally. The PLL of the grid-following control keeps running to achieve error-free tracking of the phase of the grid-forming control. The remaining loops synchronize the current values of each variable in real time by the latch-like operator so as to seamlessly switch to the grid-following mode at any time.

[0048] such as Figure 4 shown. In the grid-following mode of this implementation, the switch selects the lower position, selects the phase error feedback loop, and turns off the primary frequency regulation function, so that the grid-forming control loop can track the output phase of the grid-following PLL in the grid-following mode; in the grid-forming mode, the switch selects the upper position, turns off the phase error feedback, turns on the primary frequency regulation function, and the grid-following PLL keeps running and naturally follows the output of the grid-forming phase.

[0049] such as Figure 5 shown, which is a three-phase two-level new energy grid-connected system with an LC filter applied in this embodiment. The DC side is powered by a new energy system or an energy storage system. The grid-connected converter controller can select any one of the grid-following control, grid-forming control, or the grid-following and grid-forming control mode switching controller proposed by the present invention. In the figure is the DC side capacitor, , are the DC side voltage and current values respectively, S ABC,H and S ABC,L constitute a three-phase two-level converter, which is controlled by six-way switch signals given by the grid-connected converter controller, and are the LC filter, , are the capacitor voltage and inductor current of the three-phase filter respectively. After sampling, they are input into the grid-connected converter controller. PCC represents the connection point of the converter to the grid. L g is the equivalent impedance on the grid side.

[0050] Such as Figure 6 shown, it is the control block diagram of the single current loop grid-following control selected in S1 of the embodiment. Among them 、 are the capacitor voltage and inductor current of the three-phase filter obtained by sampling respectively, V dq_GFL and I Ldq_GFL are the phases generated by the grid-following control phase-locked loop respectively θ GFL The dq-axis capacitor voltage and inductor current obtained after the coordinate transformation implemented, 1 / s represents the integration link, ω N represents the grid angular frequency reference value. are the active and reactive power output references of the converter, then are the active and reactive power currently output by the system, calculated from the capacitor voltage and inductor current. The grid-following power loop obtains the current reference value , and then obtains the modulation wave through the PI controller of the current loop, and finally obtains the switching signal through modulation.

[0051] Such as Figure 7 shown, it is the control block diagram of the single voltage loop grid-forming control selected in S1 of the embodiment. Among them 、 are the capacitor voltage and inductor current of the three-phase filter obtained by sampling respectively, and are the phases generated by the grid-forming control power synchronization loop respectively θ GFM The dq-axis capacitor voltage and inductor current obtained after the coordinate transformation implemented, D p is the droop coefficient of the primary frequency regulation function. are the active and reactive power output references of the converter, then are the active and reactive power currently output by the system, calculated from the capacitor voltage and inductor current. D q is the reactive power-voltage droop coefficient of the grid-forming reactive power loop, U N is the nominal value of the grid voltage, |V| is the amplitude of the output voltage. The reactive power loop obtains the grid-forming voltage reference value through the integrator. The grid-forming voltage loop obtains the modulation wave , and finally a switching signal is obtained through modulation.

[0052] As Figure 8 shown, this is the system control block diagram of the overall control running in the grid-forming mode after the single-current-loop grid-following control shown in Figure 6 and the single-voltage-loop grid-forming control shown in Figure 7 are transformed through S1 - S3. Among them, the solid line represents the enabled loop, and the dashed line represents the disabled loop. All switches are selected to the upper position, indicating that the system is in the grid-forming mode. All loops in the original grid-forming mode operate normally. The PI controllers belonging to the power loop and current loop of the original grid-following mode operate as standby loops, and the current control variables are synchronized in real time so as to switch to the grid-following mode at any time.

[0053] As Figure 9 shown, this is the system control block diagram of the overall control running in the grid-following mode after the single-current-loop grid-following control shown in Figure 6 and the single-voltage-loop grid-forming control shown in Figure 7 are transformed through S1 - S3. Among them, the solid line represents the enabled loop, and the dashed line represents the disabled loop. All switches are selected to the lower position, indicating that the system is in the grid-following mode. All loops in the original grid-following mode operate normally. The integrators and PI controllers belonging to the reactive power loop and voltage loop of the original grid-forming mode operate as standby loops, and the current control variables are synchronized in real time so as to switch to the grid-forming mode at any time.

[0054] In this example, the waveform of the experimental results of switching from the grid-following mode to the grid-forming mode under a weak grid (grid short-circuit ratio is 1.9 p.u.) is as Figure 10 shown. The waveform of the experimental results of switching from the grid-forming mode to the grid-following mode under a weak grid (grid short-circuit ratio is 1.9 p.u.) is as Figure 11 shown. The waveform of the experimental results of switching from the grid-following mode to the grid-forming mode under a strong grid (grid short-circuit ratio is 12.8 p.u.) is as Figure 12 shown. The waveform of the experimental results of switching from the grid-forming mode to the grid-following mode under a strong grid (grid short-circuit ratio is 12.8 p.u.) is as Figure 13 shown.

[0055] Therefore, the present invention adopts the above-mentioned method for switching the grid-following and grid-forming control modes of a converter based on a latch-like operator, which can solve the problem of switching the grid-following and grid-forming control modes without the same control loop.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the present invention or make equivalent replacements, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for switching a converter and a network control mode based on a latch-like operator, characterized in that: The following steps are involved: S1. Determine the specific control schemes for the network-building control and network-following control that are expected to achieve the switching function; S2, separating the synchronization loop of the network-building control and the synchronization loop of the network-following control from the other loops; S3. The synchronization loop is modified. Specifically, in the modification of the synchronization loop, the primary frequency modulation function of the network-building mode needs to be turned off in the network-following mode, and a phase error feedback loop is added to enable the power synchronization loop of the network-building control to track the output of the network-following phase-locked loop without error in the network-following operation state; The remaining loops are transformed into the proposed latch-like operator structure. Specifically, in the transformation of the remaining loops, all control loops containing integrators need to be screened out first, and their operating loops remain unchanged. A new hot standby loop is added to pre-synchronize the output value of the controller when the mode it belongs to is not working, so as to achieve the subsequent seamless mode switching; S4. The system has the ability to switch seamlessly with network control and can flexibly switch between network-following operation state and network-building operation state as needed.

2. A method for switching a converter and a network control mode based on a latch-like operator according to claim 1, characterized in that: In S1, the schemes of network-building control and network-following control are determined separately according to the requirements of system design, which is applicable to mode switching of various types of network-following control, including the case where there is no identical control loop.

3. The method for switching the control mode of a converter and a network based on a latch-like operator according to claim 1, characterized in that: In S2, the synchronization loop is defined as a related loop that generates the phase of the control coordinate system. In the following network control, it is a phase-locked loop, and in the building network control, it is a power synchronization loop; the remaining loops are defined as loops other than the synchronization loop that have a control effect on the remaining variables.

4. The method for switching the control mode of a converter and a network based on a latch-like operator according to claim 1, characterized in that: In S4, the system has two states: grid-following operation state and grid-building operation state. In the grid-following operation state, all loops of grid-following control are in working state, the grid-building power synchronization loop turns off the primary frequency modulation function and turns on the phase error feedback to track the phase of the grid-following control, and the remaining loops use latch-like operators to synchronize the current values ​​of various variables in real time, so as to seamlessly switch to the grid-building mode at any time; in the grid-building mode, the networking synchronization loop turns off the phase error feedback function and operates normally, and the phase-locked loop of the grid-following control keeps running to achieve error-free tracking of the grid-building control phase, and the remaining loops use latch-like operators to synchronize the current values ​​of various variables in real time, so as to seamlessly switch to the grid-following mode at any time.

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