Converter network following control mode switching method based on latch-like operator

Abstract

The invention discloses a converter following network construction control mode switching method based on a class latch operator, which belongs to the technical field of new energy grid connection and comprises the following steps: S1, determining a specific control scheme of network construction type control and network following type control which are expected to realize a switching function; s2, respectively separating a synchronous loop and other loops for constructing network type control and following network type control; s3, transforming a synchronous loop according to the proposed method, and transforming other loops into proposed latch-like operator structures; and S4, the system has a network following and constructing control seamless switching capability, and flexible switching between a network following operation state and a network constructing operation state is carried out according to needs. By the adoption of the converter following network control mode switching method based on the like latch operator, the problem of following network control mode switching under the condition that no same control loop exists can be solved.

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 a converter grid connection control mode based on a latch-like operator. Background Art

[0002] At present, in order to cope with energy and environmental issues, new energy power generation systems including photovoltaic and wind power are developing rapidly. However, with the decrease in the proportion of synchronous machines in the power grid and the increase in the proportion of grid-connected converters, the inertia and stability of the power grid have been greatly affected. Therefore, it is necessary to enhance the stability of the grid-connected converter system through control methods so that the converter also has a certain grid support capability. This type of control method is usually called grid-connected control.

[0003] In the study, it was found that the grid-following converter and the grid-building converter have dual characteristics. For example, the grid-following converter is more stable under a strong grid, while the grid-building converter is more stable under a weak grid. Therefore, it is necessary to use the grid-building control mode switching technology to change the control mode of the converter under specific circumstances to adapt to the real-time changing grid environment. This has great practical significance for further improving the penetration rate of new energy, realizing large-scale grid connection of new energy systems and becoming the main power source, and alleviating the energy shortage crisis.

[0004] At present, there are many follow-up switching schemes based on the same control loop design, which usually require both the follow-up control and the grid-forming control to have current control loops. However, as the research deepens, various single voltage loop grid-forming controls have been proposed due to their better voltage source characteristics and faster current response speed, but no follow-up switching method suitable for non-identical loops has been proposed. Summary of the invention

[0005] The purpose of the present invention is to provide a method for switching the control mode of a converter based on a latch-like operator, which can solve the problem of switching the control mode of the converter without the same control loop.

[0006] To achieve the above object, the present invention provides a method for switching a converter control mode based on a latch-like operator, comprising the following steps: 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, transform the synchronization loop according to the proposed method, and transform the remaining loops into the proposed latch-like operator structure; 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.

[0007] Preferably, in S1, the schemes of network-building control and network-following control are determined separately in advance according to the requirements of system design, which is applicable to mode switching of various types of network-building control, including the case where the same control loop does not exist.

[0008] Preferably, in S2, the synchronization loop is defined as a related loop that generates the phase of the control coordinate system, which is a phase-locked loop in the following network control and a power synchronization loop in the networking control; the remaining loops are defined as loops other than the synchronization loop that generate control effects on the remaining variables.

[0009] Preferably, in S3, in the transformation of the remaining loops, it is necessary to first screen out all control loops containing integrators, and their operating loops remain unchanged. A new hot standby loop is added to pre-synchronize its output value when the mode to which the controller belongs is not working, so as to achieve subsequent seamless mode switching.

[0010] Preferably, in S3, in the modification of the synchronization loop, it is necessary to turn off the primary frequency modulation function of the networking mode in the networking mode, and add a phase error feedback loop so that the power synchronization loop of the networking type control can track the output of the networking type phase-locked loop without error in the networking operation state.

[0011] Preferably, in S4, the system has two states: a grid-following operation state and a grid-building operation state. In the grid-following operation state, all loops of the grid-building control are in working condition, 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-building 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.

[0012] Therefore, the beneficial effects of the present invention using the above-mentioned method for switching the inverter and network control mode based on the latch-like operator are as follows: (1) It is applicable to the switching transformation of various types of grid-following control and grid-forming control, and it does not require the two to have the same loop. Other existing switching methods require the grid-forming converter to have a current loop in order to achieve seamless switching, while the present invention achieves seamless switching of fully heterogeneous loops through the transformation of latch-like operators and synchronous loops. 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.

[0013] (2) The present invention divides the control loop of the grid-connected converter into a synchronous loop and other loops, the purpose of which is to clarify the physical meaning of each loop to achieve more reasonable mode switching. The purpose of the synchronous loop is to generate phase information built into the controller. Different synchronous loops represent different modes of interaction between the grid-connected converter and the power grid, while the other loops are mainly responsible for realizing the specific control of various key variables under the premise of achieving synchronization with the power grid. There is an essential difference in the physical meaning of the two. The separate treatment of the two types of loops is one of the reasons why the present invention is superior to various existing switching schemes.

[0014] (3) In the processing of the synchronization loop, the phase error feedback loop proposed in the present invention has a simple structure but good effect. It uses the sin function characteristics to eliminate the error caused by the phase mutation, and reuses the PI controller of the power synchronization loop, so that the grid-forming converter can follow the output of the grid-forming control phase-locked loop in the grid-forming mode. The main purpose of closing the primary frequency modulation loop is to reduce the impact of the switching process, especially in the scenario of grid frequency deviation. In the grid-forming mode, the original grid-forming control loop is fully utilized to keep the grid-forming control phase-locked loop running, and there is no need to add other control loops to enable the grid-forming phase information to track the output of the grid-forming mode power synchronization loop without error.

[0015] In the processing of the remaining loops, the present invention proposes a general structure of a latch-like operator, which is analogous to the latch concept in digital circuits. The core idea is to use the corresponding variables in the static-error synchronous operation loop of the integrator structure of the idle loop in the dual-mode control to achieve seamless mode switching. Due to the generality of the latch-like operator, all types of network control can be transformed and combined according to the operator structure, so 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, it is only necessary for the original control loop to have an integrator, and there is no requirement for the control object of the loop, so the operator can be applied to network mode switching that does not contain the same loop, such as the mode switching of the single voltage loop network control and the single current loop network control shown in the example, which is more flexible than the traditional switching method.

[0016] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a flow chart of an embodiment of a method for switching a converter and a network control mode based on a latch-like operator of the present invention; Figure 2 is a structural diagram of a latch-like operator proposed in the present invention; Figure 3 This is a schematic diagram of the operation state of switching the networking mode in an embodiment of the present invention; Figure 4It is a schematic diagram of the synchronous loop mode switching method of the following network control proposed by the present invention; Figure 5 Schematic diagram of a three-phase two-level new energy grid-connected system with an LC filter in an embodiment of the present invention; Figure 6 It is a control block diagram of the single current loop following network type control selected by S1 in the embodiment of the present invention; Figure 7 It is a control block diagram of the single voltage ring network type control selected by S1 in the embodiment of the present invention; Figure 8 It is a system control block diagram of the overall control running in the networking mode after the transformation of the single current loop following network type control and the single voltage loop networking type control in the example of the present invention; Fig. 9 It is a system control block diagram of the overall control running in the grid-following mode after the transformation of the single current loop grid-following control and the single voltage loop grid-building control in the example of the present invention; Fig.10 It is a waveform diagram of the experimental results of switching the grid-following mode to the grid-building mode under a weak grid (grid short-circuit ratio of 1.9 pu) in an example of the present invention; Fig.11 It is a waveform diagram of the experimental results of switching the network construction mode to the network following mode under a weak network (grid short circuit ratio is 1.9 pu) in the example of the present invention; Fig.12 It is a waveform diagram of the experimental results of switching the grid-following mode to the grid-building mode under a strong grid (grid short-circuit ratio of 12.8 pu) in an example of the present invention; Fig.13 It is a waveform diagram of the experimental results of switching the network construction mode to the grid following mode under a strong grid (grid short circuit ratio is 12.8 pu) in an example of the present invention. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0019] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0020] Embodiment 1 like Figure 1 As shown, the present invention provides a method for switching a converter and a network control mode based on a latch-like operator, comprising the following steps: S1. Determine the specific control schemes for the network-building control and network-following control that are expected to achieve the switching function.

[0021] The schemes for network-building control and network-following control are determined separately in advance according to the requirements of system design, and are applicable to mode switching of various types of network-building control, including the case where the same control loop does not exist.

[0022] S2. Separate the synchronization loop of the network-building control and the synchronization loop of the network-following control from the other loops.

[0023] 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 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.

[0024] S3. Transform the synchronization loop according to the proposed method, and transform the remaining loops into the proposed latch-like operator structure.

[0025] In the transformation of the remaining loops, it is necessary to first screen out all control loops containing integrators. The operating loops remain unchanged, and a new hot standby loop is added to pre-synchronize the integrator output value when the mode to which the controller belongs is not working, so as to achieve the subsequent seamless mode switching, such as Figure 2 As shown, control loop 1 and control loop 2 are respectively from two different modes of control loops in this embodiment, and a controller containing an integrator needs to be selected to form this type of latch operator structure.

[0026] In the transformation of the synchronization loop, it is necessary to turn off the primary frequency modulation function of the networking mode in the networking mode, and add a phase error feedback loop so that the power synchronization loop of the networking control can track the output of the networking phase-locked loop without error in the networking operation state. Since the phase is a periodic variable, the sin function is used to eliminate the sudden changes that may be caused by the periodic change of the phase, such as Figure 4 shown.

[0027] S4. The system has the ability to seamlessly switch between network control and network operation, and can flexibly switch between network operation and network operation according to needs, such as Figure 3 shown.

[0028] 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 other 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 other 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.

[0029] like Figure 4 As shown, in the network following mode, the switch is selected in the lower position, the phase error feedback loop is selected, and the primary frequency modulation function is turned off, so that the network building control loop can track the output phase of the network following phase-locked loop in the network building mode; in the network building mode, the switch is selected in the upper position, the phase error feedback is turned off, the primary frequency modulation function is turned on, the network following phase-locked loop keeps running, and naturally follows the network building phase output.

[0030] like Figure 5 As shown in the figure, it is a three-phase two-level new energy grid-connected system with LC filter applied in this embodiment, wherein the DC side is powered by a new energy system or an energy storage system, and the grid-connected converter controller can select any one of the following grid type control, the forming grid type control or the following forming control mode switching controller mentioned in the present invention. is the DC side capacitance, , are the DC side voltage and current values ​​respectively, S ABC,H and S ABC,L It constitutes a three-phase two-level converter, which is controlled by six switch signals given by the grid-connected converter controller. and For LC filter, , They are the capacitor voltage and inductor current of the three-phase filter respectively, which are input into the grid-connected converter controller after sampling. PCC represents the grid-connected point of the converter. L g is the grid-side equivalent impedance.

[0031] like Figure 6 As shown in FIG. 1 , it is a control block diagram of a single current loop and grid type control selected by S1 in the embodiment. , are the capacitor voltage and inductor current of the three-phase filter obtained by sampling, V dq_GFL and I Ldq_GFL They are the phases generated by the grid-type control phase-locked loop. θ GFL The dq-axis capacitor voltage and inductor current are obtained after the coordinate transformation. 1 / s represents the integral link. ω N Indicates the grid angular frequency reference value. is the active and reactive output reference of the converter, The current active and reactive power output of the system is calculated from the capacitor voltage and inductor current. The grid-following power loop obtains the current reference value through the PI controller. , and then obtain the modulation wave through the PI controller of the current loop , and finally the switching signal is obtained through modulation.

[0032] like Figure 7 As shown in FIG. 1 , it is a control block diagram of a single voltage ring network type control selected by S1 in the embodiment. , are the capacitor voltage and inductor current of the three-phase filter obtained by sampling, and They are the phases generated by the grid-type control power synchronization loop. θ GFM The dq-axis capacitor voltage and inductor current obtained after the coordinate transformation is implemented are: D p is the droop coefficient of the primary frequency modulation function. is the active and reactive output reference of the converter, It is the active and reactive power currently output by the system, which is calculated by the capacitor voltage and inductor current. D q is the reactive power-voltage droop coefficient of the reactive power loop, U N is the nominal value of the grid voltage, |V| is the output voltage amplitude, and the reactive loop obtains the grid voltage reference value through the integrator The network voltage loop obtains the modulation wave through the PI controller , and finally the switching signal is obtained through modulation.

[0033] like Figure 8 As shown, in this embodiment, after S1-S3 Figure 6 The single current loop and grid type control shown Figure 7 The system control block diagram of the overall control after the transformation of the single voltage loop networking type control is shown in the networking mode. The solid line indicates that the loop is enabled, the dotted line indicates that the loop is not enabled, and all switches are selected in the upper position, indicating that the system is in the networking mode. All loops in the original networking mode operate normally, and the PI controllers of the power loop and current loop in the original following network mode operate as hot standby loops, synchronizing the current control variables in real time so that they can switch to the following network mode at any time.

[0034] like Fig. 9 As shown, in this embodiment, after S1-S3 Figure 6 The single current loop and grid type control shown Figure 7 The system control block diagram of the overall control after the transformation of the single voltage loop grid-forming control is shown in the following grid mode. The solid line indicates that the loop is enabled, the dotted line indicates that the loop is not enabled, and all switches are in the lower position, indicating that the system is in the following grid mode. All loops in the original following grid mode operate normally, and the integrators and PI controllers of the reactive loop and voltage loop belonging to the original grid-forming mode operate as hot standby loops, synchronizing the current control variables in real time so that they can be switched to the grid-forming mode at any time.

[0035] In this example, the experimental result waveform of the switching from the grid-following mode to the grid-building mode under weak grid conditions (grid short-circuit ratio is 1.9 pu) is as follows: Fig.10 The experimental waveform of the switching from the grid-building mode to the grid-following mode under weak grid conditions (grid short-circuit ratio is 1.9 pu) is shown in Figure 2. Fig.11 The experimental result waveform of the switching from the grid-following mode to the grid-building mode under strong grid conditions (grid short-circuit ratio is 12.8 pu) is shown in Figure 2. Fig.12 The experimental result waveform of the switching from the grid construction mode to the grid following mode under strong grid (grid short circuit ratio is 12.8 pu) is shown in Figure 2. Fig.13 shown.

[0036] Therefore, the present invention adopts the above-mentioned method for switching the inverter network control mode based on the latch-like operator to solve the problem of switching the network control mode without the same control loop.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution 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, transform the synchronization loop according to the proposed method, and transform the remaining loops into the proposed latch-like operator structure; 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 S3, when modifying the remaining loops, it is necessary to first screen out all control loops containing integrators. The operating loops remain unchanged, and a new hot standby loop is added to pre-synchronize its output value when the mode to which the controller belongs is not working, so as to achieve subsequent seamless mode switching.

5. 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 S3, in the modification of the synchronization loop, it is necessary to turn off the primary frequency modulation function of the networking mode in the networking mode, and add a phase error feedback loop so that the power synchronization loop of the networking control can track the output of the networking phase-locked loop without error in the networking operation state.

6. 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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