Off-grid and grid-connected switching control method, inverter and converter

Abstract

The invention discloses an off-grid and grid-connected switching control method, an inverter and a converter, and relates to the technical field of energy storage. The off-grid and grid-connected switching control method comprises the following steps: in an off-grid operation state, when a first to-grid-connected condition or a second to-grid-connected condition is satisfied, obtaining a phase difference between an inverter capacitor voltage and a power grid voltage; under the condition that the phase difference between the inverter capacitor voltage and the power grid voltage is greater than a first preset threshold value, controlling the frequency of the inverter capacitor voltage to follow the frequency of the power grid voltage; under the condition that the phase difference between the inverter capacitor voltage and the power grid voltage is smaller than or equal to a first preset threshold value, the amplitude and frequency of the inverter capacitor voltage are controlled in a PI control mode to correspondingly track the amplitude and frequency of the power grid voltage; and when the amplitude and the frequency of the capacitor voltage of the inverter correspondingly finish tracking, sending a control command for closing the grid-connected contactor, and switching to a grid-connected operation state. By adopting the off-grid and grid-connected switching control method, rapid and smooth switching of off-grid and grid-connected of the inverter can be realized.

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to an off-grid and on-grid switching control method, an inverter and a converter. Background Art

[0002] The microgrid mainly consists of two parts: the AC bus and the DC bus. On the one hand, when the power generation unit in the microgrid generates a lot of electricity and the power demand is small, the power of the AC side grid can be converted to the DC side through the rectification mode of the energy storage converter (Power Conversion System, PCS) to supply power to the battery energy storage and DC loads. The power is stored in the battery as chemical energy. On the other hand, when the power demand reaches the peak, the power stored in the energy storage system can be connected to the AC bus through the inverter to supply power to the AC load. Among them, the AC grid is the main part of the system and provides the main energy.

[0003] The transition from grid-connected mode to off-grid mode and from off-grid mode to grid-connected mode must be smooth. The fast and smooth switching of the inverter off-grid and on-grid is the focus of current research.

[0004] The above contents are only used to assist in understanding the technical solution of the present application and do not constitute an admission that the above contents are prior art. Summary of the invention

[0005] The main purpose of the present application is to provide an off-grid switching control method, an inverter and a converter, aiming to achieve fast and smooth off-grid switching of the inverter.

[0006] To achieve the above objectives, in a first aspect, the present application proposes an off-grid switching control method, the off-grid switching control method comprising:

[0007] In the off-grid operation state, when the first grid-connected condition or the second grid-connected condition is met, the phase difference between the inverter capacitor voltage and the grid voltage is obtained, wherein the first grid-connected condition is that the grid is normal and a grid-connected conversion instruction is received, and the second grid-connected condition is that the grid is normal and within the off-grid to grid-connected allowable time;

[0008] When a phase difference between the inverter capacitor voltage and the grid voltage is greater than a first preset threshold, controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage;

[0009] When the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to a first preset threshold, the PI control method is used to control the amplitude and frequency of the inverter capacitor voltage to correspondingly track the amplitude and frequency of the grid voltage;

[0010] When the amplitude and frequency of the inverter capacitor voltage correspond to the amplitude and frequency of the grid voltage, a control command to close the grid-connected contactor is issued to switch to the grid-connected operation state.

[0011] In one embodiment, the off-grid switching control method further includes:

[0012] In the grid-connected operation state, judging whether the first off-grid switching condition or the second off-grid switching condition is met, wherein the first off-grid switching condition is receiving an off-grid switching instruction, and the second off-grid switching condition is that the grid fails and unplanned on-grid switching is allowed;

[0013] When the first off-grid switching condition or the second off-grid switching condition is met, the grid-connected control parameter is controlled to follow the off-grid control parameter, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are controlled to correspondingly track the amplitude and frequency of the inverter capacitor voltage;

[0014] adjusting the output power of the inverter according to load demand;

[0015] When the active power and reactive power of the power grid are respectively less than the second preset threshold value, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are tracked, a control command to disconnect the grid-connected contactor is issued to switch to the off-grid operation state.

[0016] In one embodiment, controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage includes:

[0017] Obtaining a phase difference between a positive zero-crossing point of the inverter capacitor voltage and a positive zero-crossing point of the grid voltage;

[0018] When the phase difference is greater than or equal to a third preset threshold and less than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a first compensation parameter, wherein the third preset threshold is greater than or equal to the first preset threshold, and the first compensation parameter is a positive number;

[0019] When the phase difference is greater than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a second compensation parameter, and the second compensation parameter is a negative number.

[0020] In one embodiment, the adopting PI control method to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage includes:

[0021] Controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the grid voltage to obtain a first component of the inverter dq axis under the phase of the grid voltage;

[0022] Acquire a first error value corresponding to the d-axis and a second error value corresponding to the q-axis according to the first component and a second component of the grid voltage dq-axis at the grid voltage phase;

[0023] Performing voltage amplitude difference PI control according to the first error value to synchronize the amplitude of the inverter capacitor voltage with the amplitude of the grid voltage;

[0024] The voltage-frequency difference PI control is performed according to the second error value to synchronize the frequency of the inverter capacitor voltage with the frequency of the grid voltage.

[0025] In one embodiment, controlling the grid-connected control parameter to follow the off-grid control parameter includes:

[0026] The output parameters of the grid-connected power loop and the output control parameters corresponding to the off-grid voltage outer loop are controlled by PI control, so that the output parameters of the grid-connected power loop follow the output control parameters corresponding to the off-grid voltage outer loop.

[0027] In one embodiment, the voltage amplitude and voltage frequency of the off-grid setting of the control inverter correspondingly track the amplitude and frequency of the inverter capacitor voltage, including:

[0028] Controlling the output reference voltage of the inverter to perform coordinate transformation under the phase of the capacitor voltage to obtain a third component of the inverter dq axis under the capacitor voltage phase;

[0029] Controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the current grid voltage to obtain a fourth component of the inverter dq axis under the grid voltage phase;

[0030] Acquire a third error value corresponding to the d-axis and a fourth error value corresponding to the q-axis according to the third component and the fourth component;

[0031] Performing voltage amplitude difference PI control according to the third error value to synchronize the amplitude of the inverter output set voltage with the amplitude of the capacitor voltage;

[0032] The voltage-frequency difference PI control is performed according to the fourth error value to synchronize the frequency of the inverter output set voltage with the frequency of the capacitor voltage.

[0033] In one embodiment, the switching to the grid-connected operation state includes:

[0034] Obtaining a first feedback signal of the grid-connected contactor;

[0035] When it is determined according to the first feedback signal that the grid-connected contactor is closed, the control algorithm of the inverter is switched to a PQ control algorithm corresponding to the grid-connected operation state.

[0036] In one embodiment, the switching to the off-grid operation state includes:

[0037] Obtaining a second feedback signal of the grid-connected contactor;

[0038] When it is determined according to the second feedback signal that the grid-connected contactor is disconnected, the control algorithm of the inverter is switched to a VF control algorithm corresponding to the off-grid operation state.

[0039] In a second aspect, the present application further provides an inverter, the inverter comprising: an inverter circuit, a grid-connected contactor and a controller;

[0040] The grid side of the inverter circuit is connected to the grid connection point of the power grid via the grid connection contactor;

[0041] The controller is used to execute the off-grid and on-grid switching control method of the inverter as described in the first aspect.

[0042] In a third aspect, the present application further provides a converter, comprising: a battery and the inverter as described in the second aspect; the battery is connected to an inverter circuit of the inverter.

[0043] The above-mentioned off-grid switching control method, inverter and converter, when the off-grid to grid conditions are met, obtain the phase difference between the inverter capacitor voltage and the grid voltage, and when the phase difference between the inverter capacitor voltage and the grid voltage is greater than the first preset threshold, control the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage, and quickly reduce the phase difference between the inverter capacitor voltage and the grid voltage by frequency following; and when the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to the first preset threshold, use the PI control method to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage, so that the amplitude and frequency of the inverter capacitor voltage track the amplitude and frequency of the grid voltage. When the amplitude and frequency of the inverter capacitor voltage complete the tracking of the amplitude and frequency of the grid voltage, the pre-synchronization from off-grid to grid-connected is completed, and the inverter can be safely and smoothly switched to the grid-connected operation state. Therefore, a control command to close the grid-connected contactor is issued to control the inverter to switch to the grid-connected operation state, thereby realizing the fast and smooth off-grid switching of the inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0045] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0046] Figure 1 This is a flow chart of an off-grid switching control method in an embodiment of the present application;

[0047] Figure 2 A schematic diagram of a topological structure of a converter in an embodiment of the present application;

[0048] Figure 3 This is a flow chart of the process of switching from grid connection to off-grid connection in one embodiment of the present application;

[0049] Figure 4 This is a schematic diagram of the principle of adjusting the output power of the inverter according to the load demand in an embodiment of the present application;

[0050] Figure 5 This is a flow chart of the steps of controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage in one embodiment of the present application;

[0051] Figure 6 A schematic diagram of a capacitor voltage phase tracking a grid voltage phase along a square direction in an embodiment of the present application;

[0052] Figure 7 A schematic diagram of the phase of the capacitor voltage tracking the phase of the grid voltage along an anti-square direction in an embodiment of the present application;

[0053] Figure 8 A schematic diagram of an off-grid to grid-connected process in an embodiment of the present application;

[0054] Fig. 9 This is a control block diagram of the frequency and amplitude of the capacitor voltage during the off-grid to grid-connected process in one embodiment of the present application;

[0055] Fig.10 This is a schematic diagram of the controller state following during the grid-connected to off-grid transition process in one embodiment of the present application;

[0056] Fig.11 A schematic diagram of a process of switching from grid connection to off-grid connection in one embodiment of the present application;

[0057] Fig.12 This is a schematic diagram of the state of on-grid and off-grid switching in an embodiment of the present application;

[0058] Fig.13 This is a structural block diagram of an inverter in one embodiment of the present application;

[0059] Fig.14 1 is a structural block diagram of a converter in one embodiment of the present application.

[0060] The purpose, features and advantages of this application will be further described in conjunction with the embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0061] It should be understood that the specific embodiments described herein are only used to explain the technical solutions of the present application and are not used to limit the present application.

[0062] In order to better understand the technical solution of the present application, a detailed description will be given below in conjunction with the accompanying drawings and specific implementation methods.

[0063] In recent years, with the rapid development of power electronics technology, there are more and more types of power converters, and the topology options for converting AC to DC or DC to AC are becoming more and more abundant, and the efficiency is getting higher and higher. At the same time, since the generated electrical energy cannot be directly stored and needs to be converted into other forms of energy to be fully utilized, efficient power converters are needed to convert various energies.

[0064] The microgrid mainly consists of two parts: the AC bus and the DC bus. On the one hand, when the power generation unit in the microgrid generates a lot of electricity and the power demand is small, the power of the AC side grid can be converted to the DC side through the rectification mode of the power storage converter (Power Conversion System, PCS) to supply power to the battery energy storage and DC loads. The power is stored in the battery as chemical energy. On the other hand, when the power demand reaches the peak, the power stored in the energy storage system can be connected to the AC bus through the inverter to supply power to the AC load. Among them, the AC grid is the main part of the system and provides the main energy. The stability and reliability of each unit in the grid depends on the stability of the power generation unit and the dynamics of the load, that is, PCS, photovoltaic grid-connected inverters and wind power generation equipment. The main function of the microgrid is to provide users with higher quality and more stable power. Therefore, how to manage and operate the complex and huge system of new energy microgrid has become the current research goal.

[0065] The transition from grid-connected mode to off-grid mode and from off-grid mode to grid-connected mode must be smooth. The fast and smooth off-grid switching of the inverter is the focus of current research.

[0066] Based on this, the embodiment of the present application provides a method for controlling off-grid switching. Figure 1 As shown, the off-grid switching control method includes the following steps S101 to S104.

[0067] S101: In the off-grid operation state, when the first grid-connected condition or the second grid-connected condition is met, the phase difference between the inverter capacitor voltage and the grid voltage is obtained. The first grid-connected condition is that the grid is normal and a grid-connected conversion instruction is received, and the second grid-connected condition is that the grid is normal and within the off-grid to grid-connected allowable time.

[0068] The connection relationship between the inverter and the grid can be as follows: Figure 2 As shown, the capacitor voltage is the voltage across the capacitor Cf in the inverter. It can be found that the capacitor voltage has a corresponding relationship with the grid voltage. In this application, it is a comparison between the corresponding capacitor voltage and the grid voltage.

[0069] It can be understood that the power grid is in normal operation, which means that the voltage, frequency, phase and other parameters of the power grid are stable within the allowable range and can accept the off-grid inverter. On this basis, when the grid-connected conversion instruction issued by the host computer or the cloud is received, the off-grid to grid-connected operation can be performed to achieve planned grid conversion. Similarly, when the power grid is normal and within the off-grid to grid-connected time, the inverter can perform the off-grid to grid-connected operation. It should be noted that the allowed time can be set in advance, and the allowed time can be set after comprehensively considering factors such as the stability of the power grid, the changing law of the power load, and the characteristics of the off-grid system itself.

[0070] S102: When the phase difference between the inverter capacitor voltage and the grid voltage is greater than a first preset threshold, control the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage.

[0071] It can be understood that if the phase difference between the inverter capacitor voltage and the grid voltage is greater than the first preset threshold, it means that the phase difference between the inverter capacitor voltage and the grid voltage is large, and it is difficult for the inverter to switch off the grid quickly and smoothly. By controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage, the phase difference between the inverter capacitor voltage and the grid voltage can be quickly reduced, so as to achieve rapid synchronization of the inverter capacitor voltage phase with the grid voltage phase.

[0072] S103: When the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to a first preset threshold, a PI control method is used to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage.

[0073] Among them, PI control includes proportional control and integral control.

[0074] It can be understood that if the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to the first preset threshold, it means that the phase difference between the inverter capacitor voltage and the grid voltage is small. On this basis, the PI control method is used to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage. The error between the amplitude and frequency of the inverter capacitor voltage and the amplitude and frequency of the grid voltage can be effectively reduced through the combined effect of proportion and integration, thereby further reducing the phase difference between the inverter capacitor voltage and the grid voltage, and achieving the amplitude, frequency and phase of the inverter capacitor voltage corresponding to the amplitude, frequency and phase of the grid voltage, thereby completing the pre-synchronization from off-grid to grid-connected.

[0075] S104: When the amplitude and frequency of the inverter capacitor voltage correspond to the amplitude and frequency of the grid voltage, a control command to close the grid-connected contactor is issued to switch to the grid-connected operation state.

[0076] Among them, the grid-connected contactor is Figure 2 K2 in the figure can control the on / off of the inverter 11 and the grid 2. When a first amplitude difference between the amplitude of the inverter capacitor voltage and the amplitude of the grid voltage is within a first preset range, and a first frequency difference between the frequency of the inverter capacitor voltage and the frequency of the grid voltage is within a second preset range, it can be determined that the amplitude and frequency of the inverter capacitor voltage correspond to complete the tracking of the amplitude and frequency of the grid voltage.

[0077] It can be understood that when the amplitude and frequency of the inverter capacitor voltage correspond to the amplitude and frequency of the grid voltage, the pre-synchronization from off-grid to grid-connected is achieved, and the off-grid to grid-connected operation can be started. Therefore, the grid-connected contactor is controlled to close by issuing a control command to close the grid-connected contactor, so as to connect the inverter and the grid, so that the inverter is switched to the grid-connected operation state, and the smooth switching of the inverter from off-grid to grid-connected is achieved.

[0078] The above-mentioned off-grid switching control method, when the off-grid to grid-connected conditions are met, obtains the phase difference between the inverter capacitor voltage and the grid voltage, and when the phase difference between the inverter capacitor voltage and the grid voltage is greater than the first preset threshold, controls the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage, and quickly reduces the phase difference between the inverter capacitor voltage and the grid voltage by frequency following; and when the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to the first preset threshold, uses the PI control method to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage, so that the amplitude and frequency of the inverter capacitor voltage track the amplitude and frequency of the grid voltage. When the amplitude and frequency of the inverter capacitor voltage complete the tracking of the amplitude and frequency of the grid voltage, the pre-synchronization from off-grid to grid-connected is completed, and the inverter can be safely and smoothly switched to the grid-connected operation state. Therefore, a control command to close the grid-connected contactor is issued to control the inverter to switch to the grid-connected operation state, thereby realizing the fast and smooth off-grid switching of the inverter.

[0079] In one embodiment, Figure 3 As shown, the off-grid switching control method also includes the following steps S301 to S304.

[0080] S301: In the grid-connected operation state, determine whether the first off-grid switching condition or the second off-grid switching condition is met. The first off-grid switching condition is receiving an off-grid switching instruction, and the second off-grid switching condition is that the grid fails and unplanned on-grid switching is allowed.

[0081] Among them, when receiving the off-grid conversion command issued by the host computer or the cloud, the inverter will prepare to execute the switching operation to the off-grid mode, so as to realize the planned grid-to-off-grid conversion. When the power grid fails and unplanned grid-to-off-grid conversion is allowed, in the case of a grid failure, if the setting of allowing unplanned grid-to-off-grid conversion is met at the same time, the inverter will also switch to the off-grid operation mode. Grid failure covers a variety of situations, such as voltage drops caused by line short circuits, power supply interruptions caused by natural disasters such as lightning strikes, or power supply abnormalities caused by equipment aging failures. By setting the second off-grid conversion condition, it can be ensured that when the grid cannot supply power normally, the inverter can quickly and autonomously switch to the off-grid operation mode to maintain the continuous operation of important loads and avoid losses caused by power outages.

[0082] S302: When the first off-grid switching condition or the second off-grid switching condition is met, the grid-connected control parameter is controlled to follow the off-grid control parameter, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are controlled to correspond to the amplitude and frequency of the tracking capacitor voltage.

[0083] Among them, controlling the grid-connected control parameters to follow the off-grid control parameters can start the state following of grid-connected to off-grid, so that the controller of the PQ control algorithm will not have a sudden change in controller parameters when it is converted to the controller of the VF control algorithm during the grid-connected to off-grid transition. Therefore, when the control algorithm is changed, the controller parameters will transition smoothly and there will be no sudden change in control.

[0084] By controlling the voltage amplitude and voltage frequency of the inverter off-grid setting to track the amplitude and frequency of the grid voltage, the amplitude and phase of the inverter output voltage setting value can track the amplitude and phase of the capacitor voltage. When the amplitude and frequency of the inverter capacitor voltage correspondingly complete the tracking of the amplitude and frequency of the capacitor voltage, pre-synchronization from grid-connected to off-grid is achieved.

[0085] S303: Adjust the output power of the inverter according to load demand.

[0086] It is understandable that in order to ensure that the AC side contactor or relay is not damaged by excessive load current when disconnected, the power output of the grid needs to be converted into the inverter output. Therefore, after the inverter switches from the grid-connected operation state to the off-grid operation state, the load needs to be powered by the inverter. Therefore, the output power of the inverter needs to be adjusted according to the load demand to match the power supply demand of the load. Figure 4 As shown, the power grid 2 is connected to the load 3, and the inverter 11 is connected via a switch (eg Figure 2 K1) in the figure is connected to the load 3, and can sample and calculate the grid-side power to determine the power demand of the load, which is then compared with the output power of the inverter 11, and the output power of the inverter 11 is adjusted within the allowed rated output power range of the inverter 11.

[0087] It should be noted that before the inverter switches from the grid-connected operation state to the off-grid operation state, it is necessary to stop the power scheduling of the main controller (such as the controller of the host computer). Taking the inverter controller as the execution subject, the inverter controller will stop executing the power scheduling instructions of the main controller to avoid the output power of the inverter not matching the power demand of the load.

[0088] S304: When the active power and reactive power of the power grid are respectively less than the second preset threshold value, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are tracked, a control command to disconnect the grid-connected contactor is issued to switch to the off-grid operation state.

[0089] Among them, when the active power and reactive power of the power grid are respectively less than the second preset threshold value, the active power and reactive power of the power grid are close to zero. The second amplitude difference between the voltage amplitude of the off-grid setting and the amplitude of the capacitor voltage is within the third preset range, and the second frequency difference between the voltage frequency of the off-grid setting and the frequency of the capacitor voltage is within the fourth preset range. The voltage amplitude and voltage frequency of the off-grid setting of the inverter are both tracked. When the voltage frequency tracking is completed, the voltage phase of the off-grid setting of the inverter is also tracked, that is, the voltage phase of the off-grid setting is roughly the same as the phase of the capacitor voltage.

[0090] When the active power and reactive power of the power grid are respectively less than the second preset threshold value, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are tracked, the pre-synchronization from grid-connected to off-grid is completed. At this time, it can be switched to the off-grid operation state safely and smoothly. Therefore, a control command to disconnect the grid-connected contactor is issued to switch to the off-grid operation state, thereby realizing the fast and smooth switching of the inverter from grid-connected to off-grid.

[0091] In one embodiment, Figure 5 As shown, in step S102, controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage includes the following steps S501 to S503.

[0092] S501: Obtain a phase difference between a positive zero-crossing point of the inverter capacitor voltage and a positive zero-crossing point of the grid voltage.

[0093] The positive zero-crossing point of the voltage refers to the moment when the voltage value passes through the zero point when changing from the negative half cycle to the positive half cycle during the sinusoidal wave change of the AC voltage.

[0094] S502: When the phase difference is greater than or equal to a third preset threshold and less than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a first compensation parameter, wherein the third preset threshold is greater than or equal to the first preset threshold and the first compensation parameter is a positive number.

[0095] Wherein, when the phase difference is greater than or equal to the third preset threshold and less than the fourth preset threshold, the frequency of the inverter capacitor voltage is compensated by using the first compensation parameter, so that the phase of the capacitor voltage can quickly track the phase of the grid voltage in a positive direction, and the difference between the phase of the inverter capacitor voltage and the phase of the grid voltage can be quickly reduced. For example, Figure 6 As shown, curve a is the waveform corresponding to the capacitor voltage, and curve b is the waveform corresponding to the grid voltage. Figure 6 It means that the phase of the capacitor voltage quickly tracks the phase of the grid voltage in the positive direction.

[0096] S503: When the phase difference is greater than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a second compensation parameter, where the second compensation parameter is a negative number.

[0097] When the phase difference is greater than the fourth preset threshold, the second compensation parameter is used to compensate the frequency of the inverter capacitor voltage, so that the phase of the capacitor voltage can quickly track the phase of the grid voltage in the opposite direction, and the difference between the phase of the inverter capacitor voltage and the phase of the grid voltage can be quickly reduced. Figure 7 As shown, curve a is the waveform corresponding to the capacitor voltage, and curve b is the waveform corresponding to the grid voltage. Figure 7 It means that the phase of the capacitor voltage quickly tracks the phase of the grid voltage in the opposite direction.

[0098] Exemplarily, the fourth preset threshold value can be 180°. Then, when the phase difference is less than 180°, the phase of the relative capacitor voltage tracks the phase of the grid voltage in the reverse direction, and the phase of the capacitor voltage tracks the phase of the grid voltage in the positive direction faster. When the phase difference is greater than 180°, the phase of the capacitor voltage tracks the phase of the grid voltage in the reverse direction faster. On this basis, further, the compensation parameter can be calculated according to the following formula: F_compensate=K*F_trace, F_trace can be preset, for example, F_trace=0.5Hz, corresponding to the first compensation parameter, K=1, corresponding to the second compensation parameter, K=-1.

[0099] In one example, the third preset threshold is equal to the first preset threshold, and the third preset threshold is 9°.

[0100] In one embodiment, in step S103, the PI control method is used to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage, including: controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the grid voltage to obtain the first component of the inverter dq axis under the phase of the grid voltage; obtaining the first error value corresponding to the d axis and the second error value corresponding to the q axis according to the first component and the second component of the grid voltage dq axis under the phase of the grid voltage; performing voltage amplitude difference PI control according to the first error value to synchronize the amplitude of the inverter capacitor voltage with the amplitude of the grid voltage; performing voltage frequency difference PI control according to the second error value to synchronize the frequency of the inverter capacitor voltage with the frequency of the grid voltage.

[0101] like Figure 8 and Fig. 9 As shown, Ucap_A, Ucap_B, Ucap_C are the three-phase capacitor voltages, Ugrid_A, Ugrid_B, Ugrid_C are the three-phase grid voltages, grid_cos_sin is the phase information of the grid voltage, offgrid_to_ongrid is the off-grid to grid-connected signal, and Wref_give and Uref_give are the given values ​​of the inverter phase and amplitude.

[0102] In the application, the output capacitor voltage of the inverter is transformed by the phase of the grid voltage to obtain the first component Ucap_D_By_theta_grid (d-axis component) and Ucap_Q_By_theta_grid (q-axis component) of the inverter dq axis under the grid voltage phase. The grid voltage is transformed by using its own phase information to obtain the second component Ugird_D_By_theta_grid (d-axis component) and Ugrid_Q_By_theta_grid (q-axis component) on the dq axis. The two quantities Ucap_D_By_theta_grid and Ugird_D_By_theta_grid are calculated by the formula 1-f(u)=sqrt(u(1)*u(1)+u(2)*u(2)) to obtain the first error value deta_Uinv_set for voltage amplitude difference PI control. It should be noted that in the formula, f(u) represents the error value, u(1) and u(2) are two input parameters, and sqrt() represents the square root function. The result output by the voltage amplitude difference PI controller is superimposed with the given amplitude and acts on the amplitude control of the inverter.

[0103] The two quantities Ucap_Q_By_theta_grid and Ugird_Q_By_theta_grid are calculated by the formula 1-f(u)=sqrt(u(1)*u(1)+u(2)*u(2)) to obtain the second error value deta_Finv_set for voltage-frequency difference PI control. The result of the voltage-frequency difference PI controller output is superimposed with the given frequency and applied to the frequency control of the inverter. The amplitude and frequency work together to complete the pre-synchronization from off-grid to grid-connected, so that the amplitude and phase of the inverter output capacitor voltage track the amplitude and phase of the grid voltage.

[0104] In one embodiment, in step S302, controlling the grid-connected control parameters to follow the off-grid control parameters includes: using a PI control method to control the output parameters of the grid-connected power loop and the output control parameters corresponding to the off-grid voltage outer loop, so that the output parameters of the grid-connected power loop follow the output control parameters corresponding to the off-grid voltage outer loop.

[0105] In applications, such as Fig.10 As shown, the output parameters of the PQ power loop and the output parameters of the VF voltage outer loop controller are controlled by a PI controller, so that the grid-connected control parameters follow the off-grid control parameters, and the state following of the controller is achieved when the grid is switched to the off-grid. Therefore, when the controller of the PQ control algorithm is switched to the VF control algorithm, there will be no sudden change in the controller parameters, and the controller can also transition smoothly. In this way, when the control algorithm is changed, the controller parameters will transition smoothly, and there will be no sudden change in the control.

[0106] In one embodiment, in step S302, controlling the voltage amplitude and voltage frequency of the off-grid setting of the inverter to correspond to the amplitude and frequency of the capacitor voltage includes: controlling the output reference voltage of the inverter to perform coordinate transformation under the phase of the capacitor voltage to obtain the third component of the inverter dq axis under the phase of the capacitor voltage; controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the current grid voltage to obtain the fourth component of the inverter dq axis under the phase of the grid voltage; obtaining a third error value corresponding to the d axis and a fourth error value corresponding to the q axis according to the third component and the fourth component; performing voltage amplitude difference PI control according to the third error value to synchronize the amplitude of the inverter output set voltage with the amplitude of the capacitor voltage; performing voltage frequency difference PI control according to the fourth error value to synchronize the amplitude of the inverter output set voltage with the frequency of the capacitor voltage.

[0107] In this embodiment, Fig.11 As shown, the reference voltage of the inverter output setting is transformed by the phase of the capacitor voltage to obtain the third components Uset_D_By_theta_cap and Uset_Q_By_theta_cap of the inverter dq axis under the capacitor voltage phase, and the capacitor voltage is transformed by the phase information of the current power grid to obtain the fourth components Ucap_D_By_theta_grid (d-axis component) and Ucap_Q_By_theta_grid (q-axis component) of the inverter dq axis under the grid voltage phase. The two quantities Uset_D_By_theta_cap and Ucap_D_By_theta_grid are calculated by the formula 1-f(u)=sqrt(u(1)*u(1)+u(2)*u(2)) to obtain the third error value deta_Uinv_set for voltage amplitude difference PI control. The result of the voltage amplitude difference PI controller output is superimposed with the given amplitude to act on the amplitude control of the inverter.

[0108] The two quantities Uset_Q_By_theta_cap and Ucap_Q_By_theta_grid are calculated by the formula 1-f(u)=sqrt(u(1)*u(1)+u(2)*u(2)) to obtain the fourth error value deta_Finv_set for voltage-frequency difference PI control. The result of the voltage-frequency difference PI controller output is superimposed with the given frequency and applied to the frequency control of the inverter. The amplitude and frequency work together to complete the pre-synchronization from grid-connected to off-grid. Let the amplitude and phase of the inverter output set voltage track the amplitude and phase of the capacitor voltage.

[0109] In one embodiment, in step S104, switching to the grid-connected operation state includes: obtaining a first feedback signal of the grid-connected contactor; and when it is determined that the grid-connected contactor is closed according to the first feedback signal, switching the control algorithm of the inverter to a PQ control algorithm corresponding to the grid-connected operation state.

[0110] The first feedback signal of the grid-connected contactor can be obtained through a sensor or an auxiliary contact of the contactor. These sensors or auxiliary contacts monitor the mechanical action state of the contactor to reflect whether the contactor is closed. For example, when an auxiliary normally open contact is used, when the contactor is closed, the normally open contact will close, thereby outputting a high level or low level signal (depending on the specific circuit design), which is the first feedback signal.

[0111] It can be understood that when it is determined that the grid-connected contactor is closed based on the first feedback signal, it can be determined that the grid-connected contactor is actually in a closed state, the inverter is connected to the power grid, and the inverter turns to the grid-connected operation state. At this time, the control algorithm of the inverter can be safely switched to the PQ control algorithm corresponding to the grid-connected operation state, so that the inverter operates in the grid-connected operation state.

[0112] In one embodiment, in step S204, switching to an off-grid operation state includes: obtaining a second feedback signal of a grid-connected contactor; and when it is determined according to the second feedback signal that the grid-connected contactor is disconnected, switching the control algorithm of the inverter to a VF control algorithm corresponding to the off-grid operation state.

[0113] It can be understood that when it is determined that the grid-connected contactor is disconnected according to the second feedback signal, it can be determined that the grid-connected contactor is actually in the disconnected state, the inverter is disconnected from the power grid, and the inverter switches to the off-grid operation state. At this time, the control algorithm of the inverter can be safely switched to the VF control algorithm corresponding to the off-grid operation state, so that the inverter operates in the off-grid operation state.

[0114] Based on the above embodiment, in one example, the off-grid switching process of the inverter can be as follows: Fig.12 As shown, it includes: in the off-grid operation state, if the grid is normal and the grid-connected conversion instruction is received or the grid is normal and within the off-grid to grid-connected time allowed, the inverter is controlled to perform off-grid to grid-connected pre-synchronization and enter the off-grid to grid-connected pre-synchronization state. Then the off-grid to grid-connected operation is performed to enter the grid-connected operation state. In the grid-connected operation state, if the off-grid conversion instruction is received or the grid fails and unplanned grid-connected to off-grid is allowed, the inverter is controlled to perform grid-connected to off-grid pre-synchronization and enter the grid-connected to off-grid pre-synchronization state. Then the grid-connected to off-grid operation is performed to enter the off-grid operation state.

[0115] It should be noted that the above examples are only used to understand the present application and do not constitute a limitation on the off-grid switching control method of the present application. More simple transformations based on this technical concept are all within the protection scope of the present application.

[0116] The present application also provides an inverter, such as Fig.13 As shown, the inverter 11 includes: an inverter circuit 111 , a grid-connected contactor 113 and a controller 112 .

[0117] The grid side of the inverter circuit 111 is connected to the grid connection point of the power grid via the grid connection contactor 113. The controller is used to execute the off-grid switching control method of the inverter in any of the above schemes.

[0118] The controller 112 can control the on and off of the grid-connected contactor 113. For example, the topology of the inverter can be found in Figure 2 shown.

[0119] The inverter includes an inverter circuit, a grid-connected contactor and a controller, and the grid side of the inverter circuit is connected to the grid connection point of the grid via the grid-connected contactor. When the off-grid to grid-connected condition is met, the controller obtains the phase difference between the inverter capacitor voltage and the grid voltage, and when the phase difference between the inverter capacitor voltage and the grid voltage is greater than a first preset threshold, controls the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage, and quickly reduces the phase difference between the inverter capacitor voltage and the grid voltage by frequency following; and when the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to the first preset threshold, uses a PI control method to control the amplitude and frequency of the inverter capacitor voltage to track the amplitude and frequency of the grid voltage, so that the amplitude and frequency of the inverter capacitor voltage track the amplitude and frequency of the grid voltage. When the amplitude and frequency of the inverter capacitor voltage correspond to the amplitude and frequency of the grid voltage, the pre-synchronization from off-grid to grid-connected is completed. At this time, the inverter can switch to the grid-connected operation state safely and smoothly. Therefore, a control command to close the grid-connected contactor is issued to control the inverter to switch to the grid-connected operation state, thereby realizing fast and smooth switching of the inverter from off-grid to grid-connected.

[0120] like Fig.14 As shown, the embodiment of the present application further provides a converter 1, comprising: a battery 12 and an inverter 11 as described above; the battery 12 is connected to an inverter circuit 111 of the inverter 11. For example, the topological structure of the converter 1 can be referred to Figure 2 shown.

[0121] The beneficial effects of the converter of this embodiment relative to the related technology are the same as the beneficial effects of the above-mentioned inverter relative to the related technology, which will not be repeated here.

[0122] In particular, according to the embodiments disclosed in the present application, the process described above with reference to the flowchart can be implemented as a computer software program. For example, the embodiments disclosed in the present application include a computer program product, which includes a computer program carried on a computer-readable medium, and the computer program includes a program code for executing the method shown in the flowchart. In such an embodiment, the computer program can be downloaded and installed from a network through a communication device, or installed from a storage device, or installed from a ROM. When the computer program is executed by a processing device, the above-mentioned functions defined in the method of the embodiment disclosed in the present application are executed.

[0123] It should be understood that the various parts disclosed in this application can be implemented by hardware, software, firmware or a combination thereof. In the description of the above embodiments, specific features, structures, materials or characteristics can be combined in any one or more embodiments or examples in a suitable manner.

[0124] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

[0125] The present application provides a computer-readable storage medium having computer-readable program instructions (ie, computer programs) stored thereon, and the computer-readable program instructions are used to execute the off-grid and on-grid switching control method in the above-mentioned embodiment.

[0126] The computer-readable storage medium provided in the present application may be, for example, a USB flash drive, but is not limited to electrical, magnetic, optical, electromagnetic, infrared, or semiconductor systems, systems or devices, or any combination of the above. More specific examples of computer-readable storage media may include, but are not limited to: an electrical connection with one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above. In this embodiment, the computer-readable storage medium may be any tangible medium containing or storing a program that can be used by or in combination with an instruction execution system, system or device. The program code contained on the computer-readable storage medium may be transmitted using any appropriate medium, including but not limited to: wires, optical cables, RF (Radio Frequency), etc., or any suitable combination of the above.

[0127] The computer-readable storage medium may be included in the controller of the inverter circuit; or may exist independently without being assembled into the controller of the inverter circuit.

[0128] The computer-readable storage medium carries one or more programs. When the one or more programs are executed by the controller of the inverter circuit, the controller executes the off-grid and on-grid switching control method of the inverter as described in any of the above schemes.

[0129] Computer program code for performing the operations of the present application may be written in one or more programming languages ​​or a combination thereof, including object-oriented programming languages ​​such as Java, Smalltalk, C++, and conventional procedural programming languages ​​such as "C" or similar programming languages. The program code may be executed entirely on the user's computer, partially on the user's computer, as a separate software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., via the Internet using an Internet service provider).

[0130] The flow chart and block diagram in the accompanying drawings illustrate the possible architecture, function and operation of the system, method and computer program product according to various embodiments of the present application. In this regard, each square box in the flow chart or block diagram can represent a module, a program segment or a part of a code, and the module, the program segment or a part of the code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the square box can also occur in a sequence different from that marked in the accompanying drawings. For example, two square boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each square box in the block diagram and / or flow chart, and the combination of the square boxes in the block diagram and / or flow chart can be implemented with a dedicated hardware-based system that performs a specified function or operation, or can be implemented with a combination of dedicated hardware and computer instructions.

[0131] The modules involved in the embodiments described in this application may be implemented by software or hardware, wherein the name of the module does not constitute a limitation on the unit itself in some cases.

[0132] The readable storage medium provided in the present application is a computer-readable storage medium, which stores computer-readable program instructions (i.e., computer programs) for executing the above-mentioned off-grid switching control method, and can realize fast and smooth off-grid switching of the inverter. Compared with the related art, the beneficial effects of the computer-readable storage medium provided in the present application are the same as the beneficial effects of the off-grid switching control method provided in the above-mentioned embodiment, and will not be repeated here.

[0133] The present application also provides a computer program product, including a computer program, which implements the steps of the above-mentioned off-grid switching control method when executed by a processor.

[0134] The computer program product provided in this application can realize fast and smooth off-grid switching of the inverter. Compared with the related art, the beneficial effects of the computer program product provided in this application are the same as the beneficial effects of the off-grid switching control method provided in the above embodiment, which will not be repeated here.

[0135] The above descriptions are only some embodiments of the present application, and are not intended to limit the patent scope of the present application. All equivalent structural changes made using the contents of the present application specification and drawings under the technical concept of the present application, or direct / indirect applications in other related technical fields are included in the patent protection scope of the present application.

Claims

1. A method for controlling off-grid switching, characterized in that: The off-grid switching control method comprises: In the off-grid operation state, when the first grid-connected condition or the second grid-connected condition is met, the phase difference between the inverter capacitor voltage and the grid voltage is obtained, wherein the first grid-connected condition is that the grid is normal and a grid-connected conversion instruction is received, and the second grid-connected condition is that the grid is normal and within the off-grid to grid-connected allowable time; When a phase difference between the inverter capacitor voltage and the grid voltage is greater than a first preset threshold, controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage; When the phase difference between the inverter capacitor voltage and the grid voltage is less than or equal to a first preset threshold, the PI control method is used to control the amplitude and frequency of the inverter capacitor voltage to correspondingly track the amplitude and frequency of the grid voltage; When the amplitude and frequency of the inverter capacitor voltage correspond to the amplitude and frequency of the grid voltage, a control command to close the grid-connected contactor is issued to switch to the grid-connected operation state.

2. The off-grid switching control method according to claim 1, characterized in that: The off-grid switching control method further includes: In the grid-connected operation state, judging whether the first off-grid switching condition or the second off-grid switching condition is met, wherein the first off-grid switching condition is receiving an off-grid switching instruction, and the second off-grid switching condition is that the grid fails and unplanned on-grid switching is allowed; When the first off-grid switching condition or the second off-grid switching condition is met, the grid-connected control parameter is controlled to follow the off-grid control parameter, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are controlled to correspondingly track the amplitude and frequency of the inverter capacitor voltage; adjusting the output power of the inverter according to load demand; When the active power and reactive power of the power grid are respectively less than the second preset threshold value, and the voltage amplitude and voltage frequency of the off-grid setting of the inverter are tracked, a control command to disconnect the grid-connected contactor is issued to switch to the off-grid operation state.

3. The off-grid switching control method according to claim 1, characterized in that: The step of controlling the frequency of the inverter capacitor voltage to follow the frequency of the grid voltage comprises: Obtaining a phase difference between a positive zero-crossing point of the inverter capacitor voltage and a positive zero-crossing point of the grid voltage; When the phase difference is greater than or equal to a third preset threshold and less than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a first compensation parameter, wherein the third preset threshold is greater than or equal to the first preset threshold, and the first compensation parameter is a positive number; When the phase difference is greater than a fourth preset threshold, the frequency of the inverter capacitor voltage is compensated with a second compensation parameter, and the second compensation parameter is a negative number.

4. The off-grid switching control method according to claim 1, characterized in that: The adopting PI control method to control the amplitude and frequency of the inverter capacitor voltage to correspond to tracking the amplitude and frequency of the grid voltage includes: Controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the grid voltage to obtain a first component of the inverter dq axis under the phase of the grid voltage; Acquire a first error value corresponding to the d-axis and a second error value corresponding to the q-axis according to the first component and a second component of the grid voltage dq-axis at the grid voltage phase; Performing voltage amplitude difference PI control according to the first error value to synchronize the amplitude of the inverter capacitor voltage with the amplitude of the grid voltage; The voltage-frequency difference PI control is performed according to the second error value to synchronize the frequency of the inverter capacitor voltage with the frequency of the grid voltage.

5. The off-grid switching control method according to claim 2, characterized in that: The controlling the grid-connected control parameter to follow the off-grid control parameter comprises: The output parameters of the grid-connected power loop and the output control parameters corresponding to the off-grid voltage outer loop are controlled by PI control so that the output parameters of the grid-connected power loop follow the output control parameters corresponding to the off-grid voltage outer loop.

6. The off-grid switching control method according to claim 2, characterized in that: The voltage amplitude and voltage frequency of the off-grid setting of the control inverter are correspondingly tracked to the amplitude and frequency of the capacitor voltage of the inverter, including: Controlling the output reference voltage of the inverter to perform coordinate transformation under the phase of the capacitor voltage to obtain a third component of the inverter dq axis under the capacitor voltage phase; Controlling the inverter capacitor voltage to perform coordinate transformation under the phase of the current grid voltage to obtain a fourth component of the inverter dq axis under the grid voltage phase; Acquire a third error value corresponding to the d-axis and a fourth error value corresponding to the q-axis according to the third component and the fourth component; Performing voltage amplitude difference PI control according to the third error value to synchronize the amplitude of the inverter output set voltage with the amplitude of the capacitor voltage; The voltage-frequency difference PI control is performed according to the fourth error value to synchronize the frequency of the inverter output set voltage with the frequency of the capacitor voltage.

7. The off-grid switching control method according to claim 1, characterized in that: The switching to the grid-connected operation state includes: Obtaining a first feedback signal of the grid-connected contactor; When it is determined according to the first feedback signal that the grid-connected contactor is closed, the control algorithm of the inverter is switched to a PQ control algorithm corresponding to the grid-connected operation state.

8. The off-grid switching control method according to claim 2, characterized in that: The switching to the off-grid operation state includes: Obtaining a second feedback signal of the grid-connected contactor; When it is determined according to the second feedback signal that the grid-connected contactor is disconnected, the control algorithm of the inverter is switched to a VF control algorithm corresponding to the off-grid operation state.

9. An inverter, characterized in that: The inverter comprises: an inverter circuit, a grid-connected contactor and a controller; The grid side of the inverter circuit is connected to the grid connection point of the power grid via the grid connection contactor; The controller is used to execute the off-grid and on-grid switching control method of the inverter as described in any one of claims 1 to 8.

10. A converter, characterized in that: include: A battery and an inverter as claimed in claim 9; The battery is connected to the inverter circuit of the inverter.

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