Phase-locked loop control method for inverter grid-connected and off-grid seamless switching

Abstract

The invention discloses a phase-locked loop control method for grid-connected and off-grid seamless switching of an inverter, and the method comprises the steps: taking the output of a power grid angular frequency calculation module as the input of a grid-connected phase calculation module during the grid-connected operation of the inverter, and calculating the instantaneous phase of the power grid as the reference phase of the inverter; when the inverter is switched from the grid-connected state to the off-grid state, the output of the power grid angular frequency calculation module at the last moment is used as the input of the off-grid phase calculation module, and the off-grid voltage phase is calculated as the reference phase of the inverter; when the inverter is switched from the off-grid state to the grid-connected state, the pre-synchronization module is used for adjusting the off-grid voltage phase to be gradually close to the grid-connected voltage phase, and when the phase difference between the off-grid voltage phase and the grid-connected voltage phase is smaller than a set value, the inverter completes grid connection. According to the invention, the bus voltage phase can be smooth and continuous when the inverter is switched from grid connection to off-grid or from off-grid to grid connection, and seamless switching from grid connection to off-grid and from off-grid to grid connection of the inverter is realized.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a phase-locked loop control method suitable for seamless on-grid and off-grid switching of an inverter. Background Art

[0002] In microgrids, the two operation modes of grid-connected and off-grid are the key to reflecting the technical and economic advantages of microgrids. The seamless and smooth switching technology of inverters is the key to ensuring the smooth transition of microgrids between the two operation modes of grid-connected and off-grid. When the inverter is grid-connected, it outputs electric energy to the grid. When the grid fails or is planned for maintenance, the inverter needs to switch to independent operation mode. The switching process must be fast and accurate to ensure uninterrupted power supply to local critical loads.

[0003] After the fault is cleared or the maintenance is completed, the grid returns to normal and the inverter needs to be reconnected to the grid. At this time, the voltage amplitude, frequency and phase of the inverter output may be inconsistent with the grid voltage, so the inverter must be adjusted before grid connection to reduce the impact on the grid when connected to the grid. In the prior art, the inverter is prone to instability during the on / off grid switching link, the switching time is too long, and there is even a large voltage / current impact, which is extremely unfavorable to the grid, load and inverter. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a phase-locked loop control method suitable for seamless switching of inverters on and off the grid, which can ensure the smoothness and continuity of the bus voltage phase when the inverter switches from on-grid to off-grid or from off-grid to on-grid.

[0005] Technical solution: To achieve the above-mentioned purpose, the present invention provides a phase-locked loop control method for seamless switching of an inverter on and off the grid, comprising: when the inverter is on the grid, the on / off-grid phase selection module switches to the on-grid phase calculation module, the output of the grid angular frequency calculation module is used as the input of the on-grid phase calculation module, and the instantaneous phase θ of the on-grid voltage is calculated. BW , as the reference phase of the inverter;

[0006] When the inverter switches from grid-connected to off-grid, the grid-connected / off-grid phase selection module switches to the off-grid phase calculation module, and the output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module to calculate the off-grid voltage phase θ LM , as the reference phase of the inverter;

[0007] When the inverter switches from off-grid to grid-connected, the grid-connected / off-grid phase selection module switches to the grid-connected phase calculation module, and the pre-synchronization module adjusts the off-grid voltage phase θ LW Gradually approaching the instantaneous phase θ of the grid-connected voltage BW , when the off-grid voltage phase θ LM and the instantaneous phase of the grid voltage θBM When the phase difference is less than the set value, the inverter completes grid connection.

[0008] Among them, the grid angular frequency calculation module includes an abc / dq converter and a phase-locked loop PI regulator.

[0009] Among them, the grid connection phase calculation module includes 2*PI*f0, an integration operator 1 / s, and a remainder module Mod. The output of the grid angular frequency calculation module is added to the angular frequency 2*PI*f0 of the grid voltage. After passing through the integration operator 1 / s, the grid voltage phase is obtained. Then, the remainder module mod and 2*PI are used to take the remainder to generate the instantaneous grid connection voltage phase θ BM .

[0010] Among them, the off-grid phase calculation module includes 2*PI*f0, an integration operator 1 / s, and a remainder module Mod. The output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module, which is added to the angular frequency 2*PI*f0 of the grid voltage. After passing through the integration operator 1 / s, the grid voltage phase is obtained. Then, the remainder module mod and 2*PI are used to take the remainder to generate the off-grid voltage phase θ LM .

[0011] Among them, the pre-synchronization module includes an error comparator and a PI regulator. The error comparator calculates the difference θ BM -θ LM of the off-grid phase and outputs it to the PI regulator to reduce the difference of the off-grid phase below the set value to meet the grid connection condition of the inverter.

[0012] Among them, the pre-synchronization module is connected to the input ends of the grid connection phase calculation module and the off-grid phase calculation module respectively. Among them, the pre-synchronization module and the grid connection phase calculation module are closed or disconnected through a switch K3. Switches K1 and K2 are provided between the error comparator and the PI regulator and between the PI regulator and the switch K3; when the inverter operates in grid connection, switches K1 and K2 are disconnected and switch K3 is closed; when grid connection is switched to off-grid, switches K1, K2, and K3 are disconnected; when off-grid is switched to grid connection, switches K1 and K2 are closed and switch K3 is disconnected.

[0013] Among them, the grid connection / off-grid phase selection module includes channels 1 and 2. Among them, channel 2 is connected to the grid connection phase calculation module, channel 2 is connected to the off-grid phase calculation module, and the switch SW port selection is controlled by a switching control signal Ctrl. The grid connection / off-grid phase selection module outputs a signal θ ref as the reference phase of the inverter to adjust the real-time phase of the inverter.

[0014] Among them, the seamless switching working process of the inverter between grid connection and off-grid is as follows:

[0015] When the inverter is operating in parallel with the grid, the grid angular frequency calculation module outputs to the grid-connected phase calculation module. The grid-connected phase calculation module outputs the real-time phase of the grid to input channel 2 of the grid-connected and off-grid phase selection module. The grid-connected and off-grid phase selection module outputs the reference phase θ ref to the inverter for grid-connected phase synchronization of the inverter. At this time, switches K1 and K2 are disconnected, and switch K3 is closed;

[0016] When the inverter switches from grid-connected to off-grid, switch K3 is disconnected, and the output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module. The working process of the off-grid phase calculation module is the same as that of the grid-connected phase calculation module. The off-grid phase calculation module outputs θ LW , which is given to input channel 1 of the grid-connected and off-grid phase selection module. At this time, the grid-connected and off-grid phase selection module selects input channel 1 as the reference phase θ of the inverter ref ;

[0017] When the inverter switches from off-grid to grid-connected, K1 and K2 are closed, and the input of the pre-synchronization module is θ BW , and its phase is subtracted from the phase of θ LW . After PI regulation, the grid-connected and off-grid phase difference is less than the set value, so that the current phase of the inverter is consistent with the grid phase. At this time, the grid-connected and off-grid phase selection module can select input channel 2 as the reference phase of the inverter, disconnect switches K1 and K2, and close switch K3 to complete grid connection.

[0018] Among them, the control method for the inverter to switch from grid-connected to off-grid or from off-grid to grid-connected is: a switching control signal Ctrl is input to the switch SW of the grid-connected / off-grid phase selection module. Ctrl = 2 represents grid-connected operation. At this time, the grid-connected / off-grid phase selection module switches to be connected to the grid-connected phase calculation module. Ctrl = 1 represents off-grid operation. At this time, the grid-connected / off-grid phase selection module switches to be connected to the off-grid phase calculation module; when the inverter is off-grid, only when Ctrl = 1 and the phase difference between the grid and the inverter is less than the set value, the grid connection action is performed.

[0019] Beneficial effects: The present invention has the following advantages: 1. In the process of the inverter switching from grid-connected to off-grid, through the pre-synchronization link, the frequency and phase before disconnection are locked, so that the load voltage will not have any flicker, and seamless switching of the inverter from grid-connected to off-grid can be realized;

[0020] 2. In the process of the inverter switching from off-grid to grid-connected, through the pre-synchronization link, the phase of the off-grid inverter gradually approaches the grid phase, and there will be no impact on the inverter and the grid, and seamless switching of the inverter from off-grid to grid-connected can be realized;

[0021] 3. The grid-connected / off-grid seamless switching control algorithm of the present invention is simple and reliable, easy to be implemented by a digital system, and improves the stability and safety of the inverter operation process. Description of the Drawings

[0022] Figure 1 is the overall control block diagram of the microgrid inverter;

[0023] Figure 2 is the calculation logic diagram of each module of the present invention;

[0024] Figure 3 is the on / off-grid control logic flow chart. Detailed Implementation Manner

[0025] The technical solution of the present invention will be described in detail below in conjunction with the embodiments and the drawings.

[0026] As Figure 1 shown, in the control block diagram of the microgrid master inverter system, L1 is the filter inductor on the inverter side, C1 is the filter capacitor, L2 is the filter inductor on the grid side, and C in is the input capacitor of the inverter, and PCC is the connection point between the grid and the inverter. When operating in parallel grid, the microgrid master inverter adopts single current loop control. When operating in island mode, the inverter adopts a double loop control strategy of voltage outer loop and current inner loop.

[0027] Based on the above microgrid inverter control strategy, the present invention proposes a novel phase-locked loop system for realizing seamless switching between on-grid and off-grid of the inverter, including a grid angular frequency calculation module, a grid-connected phase calculation module, a pre-synchronization module, an off-grid phase calculation module, and an on / off-grid phase selection module. Voltage amplitude and phase signals are collected from the grid, and after calculation and analysis by these five modules, control signals are sent to realize the switching between on-grid and off-grid modes of the inverter.

[0028] The grid angular frequency calculation module includes an abc / dq converter for abc to dq transformation and a phase-locked loop PI regulator. The abc / dq converter converts the grid voltages u a 、u b 、u c to the voltages u d 、u q in the dq coordinate system (for further simplification, through PI regulation, the amplitude of u q is controlled to 0, and u d is not processed, then the amplitude of u d is the amplitude of the grid voltage, which is used as the reference voltage amplitude for the inverter to connect to the grid).

[0029] The output of the grid angular frequency calculation module is connected to the input of the grid connection phase calculation module. The grid connection phase calculation module includes 2*PI*f0 (f0 is the grid frequency), an integration operator 1 / s, and a modulo operation module Mod. The working process of the grid connection phase calculation module is as follows: Add the output of the grid angular frequency calculation module to the angular frequency 2*PI*f0 of the grid voltage, and obtain the grid voltage phase through the integration operator 1 / s. Since the integration operator will continuously accumulate, the modulo operation module mod and 2*PI are used for modulo operation to form a periodic carrier θ BM , and output it to the input channel 2 of the grid-connected / off-grid phase selection module.

[0030] The grid-connected / off-grid phase selection module includes channel 1 and channel 2. Among them, channel 2 is connected to the grid connection phase calculation module, channel 2 is connected to the off-grid phase calculation module, and the switch SW port selection is controlled by the switching control signal Ctrl. The grid-connected / off-grid phase selection module outputs a signal θ ref as the reference phase of the inverter, which is used to adjust the real-time phase of the inverter.

[0031] As Figure 2 shown, the specific working principles of each module are as follows:

[0032] (1) When the inverter is operating in grid-connected mode, the grid angular frequency calculation module outputs to the grid connection phase calculation module, and the grid connection phase calculation module outputs the real-time grid phase to the input channel 2 of the grid-connected / off-grid phase selection module. The grid-connected / off-grid phase selection module outputs the reference phase θ ref to the inverter for grid connection phase synchronization. At this time, switches K1 and K2 are open, and switch K3 is closed;

[0033] (2) When switching from grid-connected to off-grid, disconnect switch K3, and use the output of the grid angular frequency calculation module at the previous moment as the input of the off-grid phase calculation module. The working process of the off-grid phase calculation module is the same as that of the grid connection phase calculation module. The off-grid phase calculation module outputs θ LW , and gives it to the input channel 1 of the grid-connected / off-grid phase selection module. At this time, the grid-connected / off-grid phase selection module selects the input channel 1 as the inverter reference phase θ ref ;

[0034] (3) When switching from off-grid to grid-connected, close K1 and K2. The input of the pre-synchronization module is θ BW , subtract its phase from the phase of θ LW , and through PI regulation, make the grid-connected / off-grid phase difference less than the set value, so that the current phase of the inverter is consistent with the grid phase. At this time, the grid-connected / off-grid phase selection module can select the input channel 2 as the inverter reference phase, disconnect switches K1 and K2, and close switch K3 to complete grid connection.

[0035] In the above process, θ ref is equal to θ BW or θLW is the reference phase of the inverter, which is provided to the control algorithm of the inverter and used to control the turning on and off of the switching tubes inside the inverter.

[0036] When a grid fault occurs or planned islanding is carried out, it should be ensured that during the dynamic switching process of the inverter system from grid-connected to islanding, the voltage and frequency output by the inverter will not have any flicker. The pre-synchronization module has locked the phase of the grid during grid connection; when the pre-synchronization module receives the grid-connected to islanding switching control signal, K3 is disconnected, and at the same time, the parallel / islanding phase selection switch SW in the parallel / islanding phase selection module is switched to 1, and the phase θ output by the parallel / islanding phase selection module ref from the grid-connected instantaneous phase θ BW is converted to the islanding voltage phase θ LW . Due to the cumulative calculation of the islanding phase calculation module, after the grid is disconnected, the output phase will continue to increase at the frequency f along the grid phase at the moment of grid disconnection, and there will be no phase jump.

[0037] After the grid returns to normal, the inverter needs to switch from the islanding mode to the grid-connected mode again. When the microgrid master inverter system is operating in islanding, the islanding phase calculation module independently maintains the system frequency and phase. After the grid returns to normal, there will inevitably be a certain phase difference between the phase output by the islanding phase calculation module and the grid. At this time, the pre-synchronization module can adjust the islanding voltage phase θ LW to gradually approach the grid-connected voltage phase θ BW . The specific process is as follows: when the pre-synchronization module receives the islanding to grid-connected switching control signal, switches K1 and K2 are closed, and the difference between the grid phase (the instantaneous phase θ of the grid output after the grid recovers) BW ) and the inverter phase (the islanding voltage phase θ after the inverter is islanded) LW ) is PI-regulated and then superimposed with the current inverter phase (the islanding voltage phase θ after the inverter is islanded) LW ). The islanding phase θ LW and the frequency gradually approach the grid phase θ BW . When the phase difference between the two is less than the set value, the pre-synchronization process ends, and a grid-connected switch closing signal is issued to complete grid connection.

[0038] As Figure 3 shown, it is the parallel / islanding control logic flow. Ctrl is the parallel / islanding action signal. Ctrl = 2 represents grid-connected operation, and Ctrl = 1 represents islanding operation. Only when Ctrl = 1 and the phase difference between the grid and the inverter is less than the set value, the grid-connected action is carried out, so as to avoid improper overvoltage and overcurrent caused by the phase difference at the moment of switching between grid connection and islanding of the inverter.

Claims

1. A phase-locked loop control method for seamless on-grid and off-grid switching of an inverter, characterized in that: include: When the inverter is connected to the grid, the on-grid / off-grid phase selection module switches to the on-grid phase calculation module. The output of the grid angular frequency calculation module is used as the input of the on-grid phase calculation module to calculate the instantaneous phase θ of the grid voltage. BW , as the reference phase of the inverter; When the inverter switches from grid-connected to off-grid, the grid-connected / off-grid phase selection module switches to the off-grid phase calculation module, and the output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module to calculate the off-grid voltage phase θ LM , as the reference phase of the inverter; When the inverter switches from off-grid to grid-connected, the grid-connected / off-grid phase selection module switches to the grid-connected phase calculation module, and the pre-synchronization module adjusts the off-grid voltage phase θ LW Gradually approaching the instantaneous phase θ of the grid-connected voltage BW , when the off-grid voltage phase θ LM and the instantaneous phase of the grid voltage θ BM When the phase difference is less than the set value, the inverter completes the grid connection.

2. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 1, characterized in that: The grid angular frequency calculation module comprises an abc / dq converter and a phase-locked loop PI regulator.

3. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 1, characterized in that: The grid-connected phase calculation module includes 2*PI*f0, an integral operator 1 / s, and a remainder module Mod. The output of the grid angular frequency calculation module is added to the angular frequency 2*PI*f0 of the grid voltage, and the grid voltage phase is obtained by the integral operator 1 / s. Then, the remainder module mod and 2*PI are used to obtain the remainder to generate the grid voltage instantaneous phase θ BM .

4. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 1, characterized in that: The off-grid phase calculation module includes 2*PI*f0, an integral operator 1 / s, and a remainder module Mod. The output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module, added to the grid voltage angular frequency 2*PI*f0, and the grid voltage phase is obtained through the integral operator 1 / s. The remainder module mod and 2*PI are used to obtain the remainder to generate the off-grid voltage phase θ LM .

5. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 1, characterized in that: The pre-synchronization module includes an error comparator and a PI regulator. The error comparator calculates the difference θ of the off-grid phase. BM -θ LM , and output it to the PI regulator to reduce the off-grid phase difference to below the set value to achieve the grid-connected condition of the inverter.

6. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 5, characterized in that: The pre-synchronization module is connected to the input ends of the grid-connected phase calculation module and the off-grid phase calculation module respectively, wherein the pre-synchronization module and the grid-connected phase calculation module are closed or opened by a switch K3, and switches K1 and K2 are provided between the error comparator and the PI regulator, and between the PI regulator and the switch K3; when the inverter is grid-connected, the switches K1 and K2 are opened, and the switch K3 is closed; when the grid-connected inverter is switched to the off-grid inverter, the switches K1, K2, and K3 are opened; when the off-grid inverter is switched to the grid-connected inverter, the switches K1 and K2 are closed, and the switch K3 is opened.

7. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 6, characterized in that: The on-grid / off-grid phase selection module includes channels 1 and 2, wherein channel 2 is connected to the on-grid phase calculation module, and channel 2 is connected to the off-grid phase calculation module. The on-grid / off-grid phase selection module outputs a signal θ ref It is the reference phase of the inverter and is used to adjust the real-time phase of the inverter.

8. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 7, characterized in that: The inverter on-grid and off-grid seamless switching working process is: When the inverter is connected to the grid, the grid angular frequency calculation module outputs to the grid phase calculation module, the grid phase calculation module outputs the real-time grid phase to the input channel 2 of the grid-connected and off-grid phase selection module, and the grid-connected and off-grid phase selection module outputs the reference phase θ ref To the inverter, for inverter grid-connected phase synchronization, at this time switches K1 and K2 are disconnected, and switch K3 is closed; When the inverter is connected to the grid and switched off-grid, the switch K3 is disconnected, and the output of the grid angular frequency calculation module at the previous moment is used as the input of the off-grid phase calculation module. The working process of the off-grid phase calculation module is the same as that of the grid-connected phase calculation module. The output of the off-grid phase calculation module is θ LW , is given to the input channel 1 of the on-grid and off-grid phase selection module. At this time, the on-grid and off-grid phase selection module selects input channel 1 as the inverter reference phase θ ref ; When the inverter is off-grid and connected to the grid, K1 and K2 are closed, and the input of the pre-synchronization module is θ BW , and θ LM The phase difference between the on-grid and off-grid phases is made smaller than the set value through PI adjustment, so that the current phase of the inverter is consistent with the grid phase. At this time, the on-grid and off-grid phase selection module can select input channel 2 as the reference phase of the inverter, disconnect switches K1 and K2, close switch K3, and the grid connection is completed.

9. The phase-locked loop control method for seamless on-grid and off-grid switching of an inverter according to claim 1, characterized in that: The control method for switching the inverter from grid-connected to off-grid or from off-grid to grid-connected is as follows: a switching control signal Ctrl is input to a switch SW of a grid-connected / off-grid phase selection module, Ctrl=2 represents grid-connected operation, at which time the grid-connected / off-grid phase selection module is switched to be connected to a grid-connected phase calculation module, Ctrl=1 represents off-grid operation, at which time the grid-connected / off-grid phase selection module is switched to be connected to an off-grid phase calculation module; after the inverter is off-grid, Ctrl=1 and the phase difference between the grid and the inverter is less than a set value, the grid-connected action is performed.

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