Grid-connected and off-grid switching control method for energy storage converter

Abstract

The invention relates to the technical field of energy storage converters, and discloses a grid-connected and off-grid switching control method for an energy storage converter, which comprises a method for switching the energy storage converter from off-grid to grid-connected and a method for switching the energy storage converter from grid-connected to off-grid. Phase angle pre-synchronization and amplitude pre-synchronization are subjected to time-sharing processing, and the problem of amplitude phase coupling existing in traditional pre-synchronization is avoided; phase angle pre-synchronization can be realized by the shortest path through phase angle processing; a fixed frequency is selected to adjust a phase angle, so that over-frequency and under-frequency protection of a load can be prevented from being triggered due to relatively large frequency change, and instantaneous voltage jump can be avoided by updating a reference voltage amplitude at a zero crossing point; according to the invention, by adding the auxiliary voltage loop, conversion from grid connection to grid disconnection can be realized under the condition of not sampling power grid side current; smooth switching under the condition of three-phase imbalance can be realized through split-phase control, and influence on normal work of the system due to voltage outer ring misjudgment caused by voltage fluctuation can be avoided through a PR regulator with errors.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage converters, and in particular to a method for controlling on-grid and off-grid switching of energy storage converters. Background Art

[0002] As energy crisis and environmental pollution become increasingly serious, countries around the world have turned their attention to more environmentally friendly and reliable energy. Energy storage inverters have also received more and more attention and are playing an indispensable role in microgrids, distributed power generation and other fields. The smooth switching between grid-connected and off-grid modes of energy storage inverters can not only flexibly respond to grid failures and load fluctuations, but also improve the stability of microgrid operation and the reliability of power supply. In addition, energy storage inverters can absorb and store excess electricity during the peak period of renewable energy generation, and release the stored electricity during the low period of power generation or the peak of grid demand, so as to achieve "peak shaving and valley filling" and improve energy utilization efficiency.

[0003] The stable operating states of the energy storage inverter include grid-connected operation and island operation. In the grid-connected operation mode, the inverter uses the PQ control method with the voltage and frequency reference provided by the grid, which is equivalent to a current source, and provides power to the load together with the large grid. In the island operation mode, V / F control is used to maintain the stability of the system voltage and frequency, which is equivalent to a voltage source that provides power to the load alone. The on-grid and off-grid switching mainly includes two processes: off-grid to grid-connected and grid-connected to off-grid. The on-grid to off-grid switching is divided into planned islanding and unplanned islanding. In the process of switching between the grid-connected / islanding modes, the energy storage inverter has voltage oscillation and current shock problems, which affect the stability of the system.

[0004] The existing off-grid to grid-connected solutions mainly achieve phase and amplitude pre-synchronization through the zero-static adjustment of the PI controller. However, this technology has some problems. First, both the grid voltage phase angle and the output phase angle of the energy storage converter are piecewise functions. Using the PI regulator near the segmentation point may cause overshoot and oscillation in the system, resulting in a longer response time for the system. Secondly, the two variables of voltage phase angle and amplitude are coupled. Pre-synchronizing the voltage amplitude and phase angle at the same time will cause mutual interference, and the system may find it difficult to achieve the precise setting values ​​of amplitude and phase at the same time. In addition, through the pre-synchronization automatic adjustment of the PI regulator, excessive frequency changes may trigger the over-frequency and under-frequency protection of the load, causing the system to shut down.

[0005] The existing grid-connected to off-grid scheme is mainly implemented through the state follower of PQ control and V / F control or droop control. Before switching, the state of V / F controller or droop controller is kept to follow the output state of PQ controller, and the control output is kept unchanged after the mode is switched, so as to ensure that the system can achieve smooth switching. At present, the most common solution is power adjustment. When the grid-connected to off-grid command is received, the current flowing through the grid side is adjusted to 0 to ensure that no large current shock will be generated during the switching process. However, sampling the grid-side current requires adding sensors, which is costly, and when the distance between the converter and the grid connection point is far, the grid-side current may be difficult to obtain. In addition, there is a method of adding an auxiliary voltage outer loop. At the moment when the grid-side switch is disconnected, the voltage at the common connection point is kept unchanged to achieve smooth switching. Although the sampling of the grid-side current is avoided, the imbalance of the three-phase voltage is not taken into account. Summary of the invention

[0006] In view of the shortcomings and defects of the prior art, the present invention provides a control method for on-grid and off-grid switching of an energy storage inverter, which can avoid the amplitude-phase coupling problem existing in traditional pre-synchronization and the overshoot and oscillation caused by the piecewise function properties in the phase adjustment process; subsequently, a control method for an auxiliary voltage outer loop is provided, which can ensure the smoothness and continuity of the voltage at the common link point during the mode switching process without sampling the grid-side current during the on-grid to off-grid conversion process, and can achieve seamless switching even under the condition of unbalanced three-phase voltage.

[0007] The purpose of the present invention can be achieved through the following technical solutions.

[0008] A method for controlling on-grid and off-grid switching of an energy storage converter includes a method for switching an off-grid energy storage converter to a grid-connected method and a method for switching an energy storage converter from a grid-connected method to an off-grid method.

[0009] The method for switching an energy storage converter from an off-grid to a grid-connected state comprises the following steps.

[0010] S1, receiving the instruction to switch from off-grid state to grid-connected state, calculates the phase angle difference between the three-phase voltage of the grid and the output voltage of the corresponding phase of the energy storage converter.

[0011] S2, determining the fixed adjustment value of the frequency of each phase according to the phase angle difference of each phase.

[0012] S3, superimposing the fixed adjustment value of each phase frequency of the energy storage converter with the original frequency to obtain a new fixed frequency value, and adjusting the output voltage phase angle with the new fixed frequency value.

[0013] S4, respectively calculating the phase angle differences between the three-phase voltage of the power grid and the output voltage of the corresponding phase of the energy storage converter, and switching the corresponding energy storage converter reference voltage phase angle whose phase angle difference is less than or equal to the threshold to the corresponding phase voltage phase angle of the power grid.

[0014] S5, when the three-phase reference voltage phase angles of the energy storage converter are switched to the corresponding phase voltage phase angles of the grid, the reference voltage amplitude of the energy storage converter is updated to the corresponding phase voltage amplitude of the grid at the zero-crossing point of each phase voltage of the grid; otherwise, jump to S1.

[0015] S6, based on the updated result of S5, controls the grid-connected and off-grid switch to close, and switches the V / f control to PQ control after detecting that the grid-connected and off-grid switch is in the closed state.

[0016] The method for switching an energy storage converter from grid-connected to off-grid includes the following steps.

[0017] T1, receiving the instruction to switch from the grid-connected state to the off-grid state, records the amplitude, phase and frequency of the three-phase output voltage of the energy storage converter respectively.

[0018] T2, generates the three-phase reference voltage of the energy storage converter according to the recorded amplitude, phase and frequency.

[0019] T3, the three-phase reference voltage passes through the PR controller with error, and the output power of the energy storage converter is adjusted accordingly to maintain the voltage unchanged.

[0020] T4, control and disconnect the grid switch.

[0021] T5, after detecting that the on-grid and off-grid switch is in the open state, the PQ control is switched to V / f control.

[0022] Preferably, the frequency fixed adjustment value is adjusted according to actual conditions.

[0023] Preferably, the specific process of determining the fixed frequency adjustment value is as follows.

[0024] Determine whether the phase angle difference is greater than 0: if so, no processing is performed; otherwise, the phase angle difference is rounded up and added with 2π.

[0025] Determine whether the phase angle difference is greater than or equal to π: if yes, the frequency fixed adjustment value Δf=-Delta_freq; otherwise, the frequency fixed adjustment value Δf=Delta_freq.

[0026] Where Delta_freq is the preset value.

[0027] Preferably, the three-phase reference voltage U refX =U pastX ·cos(θ pastX +2·π·f pastX ·T).

[0028] Where X is A, B or C, representing A, B or C phase; U pastX The output voltage amplitude of the energy storage converter recorded at the moment of receiving the grid-connected state to off-grid state instruction; θpastX f is the output voltage phase angle of the energy storage converter at the moment of receiving the grid-connected state to off-grid state instruction; pastX The output voltage frequency of the energy storage converter recorded at the moment when the grid-connected state to off-grid state instruction is received.

[0029] The beneficial technical effects of the present invention are as follows: the phase angle pre-synchronization and amplitude pre-synchronization are processed in a time-sharing manner to avoid the amplitude-phase coupling problem existing in the traditional pre-synchronization; the phase angle pre-synchronization can be realized in the shortest path through the phase angle processing; the over-frequency and under-frequency protection of the load can be avoided from being triggered due to large frequency changes, and the reference voltage amplitude can be updated at the zero crossing point to avoid instantaneous voltage jumps.

[0030] By adding an auxiliary voltage loop, the grid-connected to off-grid transition can be achieved without sampling the grid-side current; phase-splitting control can achieve smooth switching in the case of three-phase imbalance; and a PR regulator with error can avoid voltage fluctuations that may cause misjudgment of the voltage outer loop and affect the normal operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 4 is a system structure diagram of an embodiment of the present invention.

[0032] Figure 2 The figure is an overall flow chart of the method for switching from off-grid to grid-connected in the present invention.

[0033] Figure 3 Schematic diagram of phase angle presynchronization control according to an embodiment of the present invention.

[0034] Figure 4 The figure is an overall flow chart of the method for switching from grid-connected to off-grid in the present invention.

[0035] Figure 5 This is a schematic diagram of the grid-connected to off-grid control principle of an embodiment of the present invention.

[0036] Figure 6 This is a schematic diagram of the grid-connected to off-grid control principle under non-ideal conditions of an embodiment of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not limit the present invention.

[0038] Embodiment: A method for controlling on-grid and off-grid switching of an energy storage converter includes a method for switching an off-grid energy storage converter to a grid-connected method and a method for switching an energy storage converter from a grid-connected method to an off-grid method.

[0039] like Figures 1 to 3As shown, the method for switching an off-grid energy storage converter to a grid-connected one includes the following steps.

[0040] S1, receiving the instruction to switch from off-grid state to grid-connected state, calculates the phase angle difference between the three-phase voltage of the grid and the output voltage of the corresponding phase of the energy storage converter.

[0041] S2, determining the fixed adjustment value of the frequency of each phase according to the phase angle difference of each phase.

[0042] like Figure 2 As shown, it is determined whether the phase angle difference Delta_θ is greater than 0. If it is greater than 0, no processing is performed; otherwise, the phase angle difference is rounded up and 2π is added to restore the phase angle difference to ensure that the difference is always positive.

[0043] Determine whether the phase angle difference Delta_θ is greater than or equal to π: if yes, the frequency fixed adjustment value Δf=-Delta_freq; otherwise, the frequency fixed adjustment value Δf=Delta_freq. The size of the frequency fixed adjustment value can be adjusted according to the actual situation, and generally takes a smaller value.

[0044] S3, superimposing the fixed adjustment value of each phase frequency of the energy storage converter with the original frequency to obtain a new fixed frequency value, and adjusting the output voltage phase angle with the new fixed frequency value.

[0045] Δf is superimposed on the output voltage frequency reference value f of the energy storage converter, thereby adjusting the converter output phase to be closer to the distribution network.

[0046] S4, respectively calculating the phase angle differences between the three-phase voltage of the power grid and the output voltage of the corresponding phase of the energy storage converter, and switching the corresponding energy storage converter voltage phase angle whose phase angle difference is less than or equal to the threshold to the corresponding phase voltage phase angle of the power grid.

[0047] S5, when the three-phase reference voltage phase angles of the energy storage converter are switched to the corresponding phase voltage phase angles of the grid, the reference voltage amplitude of the energy storage converter is updated to the corresponding phase voltage amplitude of the grid at the zero-crossing point of each phase voltage of the grid; otherwise, jump to S1.

[0048] After all three phases have completed phase pre-synchronization, a voltage amplitude pre-synchronization instruction is issued. Since the output voltage phase of the energy storage converter is synchronized with the grid voltage phase at this time, the reference voltage amplitude can be updated to the grid voltage amplitude at the zero crossing point of each phase voltage to achieve seamless synchronization of the voltage amplitude.

[0049] S6, based on the updated result of S5, controls the grid-connected and off-grid switch to close, and switches the V / f control to PQ control after detecting that the grid-connected and off-grid switch is in the closed state.

[0050] like Figure 1 , Figure 4 , Figure 6 As shown, the method for switching the energy storage converter from grid-connected to off-grid includes the following steps.

[0051] S1, receiving the instruction to switch from the grid-connected state to the off-grid state, records the amplitude, phase and frequency of the three-phase output voltage of the energy storage converter respectively.

[0052] S2, generates the three-phase reference voltage U of the energy storage converter according to the recorded amplitude, phase and frequency refX =U pastX ·cos(θ pastX +2·π·f pastX ·T).

[0053] Where X is A, B or C, representing A, B or C phase; U pastX The output voltage amplitude of the energy storage converter recorded at the moment of receiving the grid-connected state to off-grid state instruction; θ pastX f is the output voltage phase angle of the energy storage converter at the moment of receiving the grid-connected state to off-grid state instruction; pastX The output voltage frequency of the energy storage converter recorded at the moment when the grid-connected state to off-grid state instruction is received.

[0054] S3, the three-phase reference voltage passes through the PR controller with error, and the output power of the energy storage converter is adjusted accordingly to maintain the voltage unchanged.

[0055] The output voltage u of the energy storage converter is adjusted by the PR regulator CX Controlled as reference voltage U refX , the output current regulation Δi LX_ref and the original current inner loop reference value i L_ref0 Add them together and use them as reference current to adjust the output current of the energy storage inverter in real time, thereby ensuring that the output power of the inverter matches the power required by the load. Before the on-grid switch is opened, ideally, the voltage amplitude and frequency of the common connection point are completely consistent with the locked voltage amplitude and frequency, so the input of the voltage outer loop is zero, which means that the voltage outer loop does not affect the output power of the energy storage inverter. At this time, the energy storage inverter still outputs as a current source. When the on-grid switch is opened, the voltage and frequency changes at the common connection point cause the voltage outer loop to be activated, and the reference value of the current inner loop of the energy storage inverter is smoothly adjusted to adjust the output power, so that the voltage at the common connection point remains consistent with the voltage before the switch is opened, and the energy storage inverter outputs as a voltage source.

[0056] In practical applications, the above ideal situation does not exist. It is considered normal for the actual grid voltage and frequency to fluctuate within a certain range. Once the grid voltage fluctuates, even if the actual value is very small compared to the reference value, the voltage outer loop will change the output power of the converter, and even cause the outer loop controller to saturate, resulting in control system instability and even system failure. In order to avoid the above problems, this paper adopts a PR regulator with error. The voltage outer loop does not participate in the regulation within the allowable error range. However, when the voltage fluctuation of the common connection point exceeds the allowable error after the switching switch is opened, the voltage outer loop controller quickly starts to work and adjusts the output power of the energy storage converter in real time.

[0057] S4, controls the off-grid switch to open.

[0058] S5, after detecting that the on-grid and off-grid switch is in the open state, the PQ control is switched to V / f control.

[0059] To further illustrate this method, a simulation model was built, and its system simulation diagram is shown in Figure 1 shown.

[0060] Figure 3 The control principle diagram of phase pre-synchronization is shown. The frequency adjustment value is obtained by judging the phase angle difference, and the reference phase angle of the energy storage converter is obtained by superimposing it with the original frequency.

[0061] Figure 5 The block diagram of the grid-connected to off-grid control principle is shown. The reference voltage in the grid-connected to off-grid process is generated by recording the voltage amplitude, phase and frequency at the previous moment, and the reference value of the current conversion is adjusted in real time to ensure that the power output of the energy storage inverter matches the power consumed by the load during the transient switching process.

[0062] Figure 6 The control block diagram of grid-connected to off-grid switching under non-ideal conditions is shown. Figure 5 Compared with the addition of an error judgment module, the output of the voltage outer loop does not act on the current inner loop within the allowable error range. Only when the grid-connected and off-grid switching switch is truly disconnected, the voltage outer loop will work normally and quickly adjust the output power of the converter to match the power consumed by the load, ensuring that the system switches smoothly to the off-grid state.

[0063] The above embodiments are descriptions of specific implementation methods of the present invention rather than limitations of the present invention. Technical personnel in the relevant technical field may make various changes and modifications to obtain corresponding equivalent technical solutions without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A method for controlling on-grid and off-grid switching of an energy storage converter, characterized in that: Including a method for switching an energy storage converter from off-grid to grid-connected and a method for switching an energy storage converter from grid-connected to off-grid; The method for switching an energy storage converter from off-grid to on-grid includes the following steps: S1, after receiving the instruction to switch from off-grid state to grid-connected state, calculates the phase angle difference between the three-phase voltage of the grid and the output voltage of the corresponding phase of the energy storage converter; S2, determining the fixed adjustment value of the frequency of each phase according to the phase angle difference of each phase; S3, adding the fixed adjustment value of each phase frequency of the energy storage converter to the original frequency to obtain a new fixed frequency value, and adjusting the output voltage phase angle with the new fixed frequency value; S4, respectively calculating the phase angle differences between the three-phase voltage of the power grid and the output voltage of the corresponding phase of the energy storage converter, and switching the voltage phase angle of the corresponding energy storage converter whose phase angle difference is less than or equal to the threshold to the voltage phase angle of the corresponding phase of the power grid; S5, when the three-phase reference voltage phase angles of the energy storage converter are all switched to the corresponding phase voltage phase angles of the grid, the reference voltage amplitude of the energy storage converter is updated to the corresponding phase voltage amplitude of the grid at the zero-crossing point of each phase voltage of the grid; otherwise, jump to S1; S6, based on the update result of S5, controls the grid-connected and off-grid switches to close, and switches the V / f control to the PQ control after detecting that the grid-connected and off-grid switches are in the closed state; The method for switching the energy storage converter from grid-connected to off-grid includes the following steps: T1, receiving the grid-connected state to off-grid state instruction, records the amplitude, phase and frequency of the three-phase output voltage of the energy storage converter respectively; T2, generating three-phase reference voltages for the energy storage converter according to the recorded amplitude, phase and frequency; T3, the three-phase reference voltage passes through the PR controller with error, and the output power of the energy storage converter is adjusted accordingly to maintain the voltage unchanged; T4, control and disconnect the grid switch; T5, after detecting that the on-grid and off-grid switch is in the open state, the PQ control is switched to V / f control.

2. A method for controlling on-grid and off-grid switching of an energy storage converter according to claim 1, characterized in that: Adjust the frequency fixed adjustment value according to actual conditions.

3. A method for controlling on-grid and off-grid switching of an energy storage converter according to claim 1, characterized in that: The specific process of determining the fixed frequency adjustment value is as follows: Determine whether the phase angle difference is greater than 0: if yes, no processing is done; otherwise, the phase angle difference is rounded up and 2π is added: Determine whether the phase angle difference is greater than or equal to π: if yes, the frequency fixed adjustment value Δf=-Delta_freq; otherwise, the frequency fixed adjustment value Δf=Delta_freq; Where Delta_freq is the preset value.

4. A method for controlling on-grid and off-grid switching of an energy storage converter according to claim 1, characterized in that: Three-phase reference voltage U of energy storage converter refX : U refX =U pastX ·cos(θ pastX +2·π·f pastX ·T); Where X is A, B or C, representing A, B or C phase; U pastX The output voltage amplitude of the energy storage converter recorded at the moment of receiving the grid-connected state to off-grid state instruction; θ pastX f is the output voltage phase angle of the energy storage converter at the moment of receiving the grid-connected state to off-grid state instruction; pastX The output voltage frequency of the energy storage converter recorded at the moment when the grid-connected state to off-grid state instruction is received.

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