Power current loop control method for seamless switching of grid-connected and off-grid modes of energy storage PCS

Abstract

The invention discloses a power current loop control method for seamless switching of grid-connected and off-grid modes of an energy storage PCS, and the method achieves the seamless switching of the two control modes through the dynamic adjustment of a power reference value of an inverter, the merging of a PQ-controlled current loop and a Vf-controlled current loop, and the introduction of a PQ-controlled reference current into a Vf control loop. The method specifically comprises the following steps: detecting voltage, current and phase parameters in grid-connected and off-grid states, and calculating corresponding power; when the switching signal is received, the grid-connected active power is adjusted to an off-grid preset value through a PI controller; switching an inverter control mode within a power error allowable range, and introducing a PQ reference current to a Vf voltage loop; and control mode switching and switch state change are completed in sequence. According to the method, the problems of voltage fluctuation and current sudden change during switching in a traditional method are effectively solved, bidirectional seamless switching can be realized without reducing zero output power, dependence on an additional energy storage system is reduced, and the operation stability of the micro-grid is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of seamless switching of on-grid and off-grid modes of energy storage power conversion systems, and in particular to the technical field of a comprehensive seamless switching method based on an inverter power reference value and a current loop. Background Art

[0002] As the penetration rate of renewable energy in the power system continues to increase, the stability and reliability of the Power Conversion System (PCS), the core equipment for interconnecting microgrids with the main grid, are crucial to power quality and system safety. During microgrid operation, the PCS needs to frequently switch between on-grid and off-grid modes: when the main grid fails or requires maintenance, the PCS needs to quickly switch to an off-grid state to maintain power supply to local loads; after the fault is restored, it needs to resynchronize and connect to the grid. However, there are significant technical challenges in the on-grid and off-grid mode switching process, specifically: 1. Voltage and current fluctuations In grid-connected mode, the PCS relies on the main grid for voltage and frequency support, while in off-grid mode, it must independently establish voltage and frequency. Due to differences in inverter control strategies (such as PQ (Power-Quality Control) and Vf (Voltage-Frequency Control)) between the two modes, sudden changes in reference values ​​during switching can easily lead to mismatches in voltage amplitude, phase, and current amplitude, causing voltage fluctuations (with traditional switching methods, voltage fluctuations can exceed 20% of the rated value). In severe cases, this can even cause the microgrid to collapse.

[0003] 2. Limitations of existing solutions Grid-type inverter control: Suppresses switching disturbances by increasing system inertia and damping. However, due to the limitations of existing PCS, which mostly adopts grid-following control strategies, its applicable scenarios are limited.

[0004] Pre-tracking technology: By adjusting the inverter output voltage phase in advance to match the main grid, it is only applicable to one-way switching from off-grid to grid-connected, and has strict requirements on the real-time performance of grid parameters.

[0005] Output power reduction to zero method: The inverter output power is reduced to zero to reduce the switching impact, but this will cause a power shortage and require reliance on additional energy storage compensation, increasing system costs, and the switching response delay is significant.

[0006] 3. Timing contradiction between control mode switching and grid disconnection Traditional approaches typically treat inverter control mode switching and grid disconnection as instantaneous actions, ignoring the timing relationship between the two. For example, if disconnection occurs before switching control modes, the indirect voltage regulation capability of PQ control is insufficient, leading to voltage instability during the initial disconnection phase. Alternatively, if the control mode is switched directly without pre-adjusting the power reference, a sudden change in the current loop reference can trigger transient shocks.

[0007] These issues make it difficult to achieve a truly seamless on-grid and off-grid switching process for existing PCSs, hindering the reliable operation of microgrids. Therefore, a comprehensive control method is urgently needed that can dynamically coordinate the power reference value with the current loop control, optimize the switching sequence, and significantly reduce voltage and current fluctuations. Summary of the Invention

[0008] The purpose of the present invention is to solve the problems in the prior art and propose a power current loop control method for seamless switching between grid-connected and off-grid modes for energy storage PCS. By adjusting the current loop structure, current reference value, and reference power value in the inverter PQ control and Vf control, the voltage and current mutations during switching are reduced, thereby improving system stability. This method is based on the PQ and Vf control strategies in the inverter grid-connected control mode, merges the current loops of the two control modes, and corrects the parameters of the Vf current loop reference current value based on the reference power value. At the same time, the PCS off-grid process is analyzed and a specific device switching sequence is proposed to achieve the goal of maintaining the same voltage amplitude, phase, and current amplitude during switching, thereby achieving seamless switching of the PCS.

[0009] To achieve the above objectives, the present invention proposes a power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS. The control method comprises the following steps: When the PCS is off-grid, detect the output voltage , current , and the phase angle , and the active power is obtained by the power calculation formula and reactive power ; After the PCS is in grid-connected working mode, the voltage of the grid-connected point is detected in real time. , current , and the phase angle , and use the method in step 1 to obtain the active power and reactive power ; When the Micro-grid Central Controller (MGCC) or Micro-grid Scheduling Controller (MSC) detects a switching signal (active switching or fault signal), it issues a pre-switching instruction and starts reference power preprocessing. After receiving the pre-processing instruction, the inverter starts the power feedback loop and converts the actual grid-connected active power into the power of the inverter through the PI (Proportional-Integral Controller) controller. Smoothly adjust to the preset value ,in ; When − When ∣<ε (ε is the allowable error), MGCC and MSC send current control instructions to the inverter, and Vf control voltage loop is introduced variable; The inverter control mode is switched from PQ control to Vf control; MGCC and MSC at specified time intervals Back, switch and A switching command is issued, the switch status changes, and the PCS enters the off-grid working state; When the fault is recovered or the active switching signal is issued, the MSC and MGCC issue the pre-switching command again to control the inverter to perform pre-processing; After receiving the pre-processed signal, the inverter starts the voltage phase feedback loop and frequency feedback loop to track the phase and frequency of the main grid; The inverter, switch, and PCS status are switched, and the grid connection is successful.

[0010] Beneficial effects of the present invention: 1. By dynamically adjusting the power reference value, the power mutation during switching is effectively reduced and the voltage fluctuation is reduced.

[0011] 2. Introduce in Vf voltage loop , which eliminates the current mutation problem when the inverter control mode is switched and improves the stability of the inverter when it is disconnected from the main grid.

[0012] 3. No need to reduce the output power to zero, power compensation can be provided during the switching process, reducing dependence on additional energy storage systems.

[0013] 4. Applicable to bidirectional switching from grid-connected to off-grid and from off-grid to grid-connected, making up for the limitations of pre-tracking technology.

[0014] The features and advantages of the present invention will be described in detail through embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a control strategy flow chart of the power current loop control method for seamless switching of energy storage PCS on-grid and off-grid modes of the present invention; Figure 2This is a Vf control voltage loop diagram used in the power current loop control method for seamless switching of energy storage PCS on-grid and off-grid modes of the present invention; Figure 3 This is the PQ control reference power loop diagram used in the power current loop control method for seamless switching between on-grid and off-grid modes of the energy storage PCS of the present invention; Figure 4 This is the voltage waveform when switching from grid-connected to off-grid under the conventional control strategy; Figure 5 This is the current waveform when switching from grid-connected to off-grid under the conventional control strategy; Figure 6 This is a voltage waveform diagram when switching from grid-connected to off-grid under the control strategy of the power current loop control method for seamless switching of the energy storage PCS on-grid and off-grid modes of the present invention; Figure 7 This is a current waveform diagram when switching from grid-connected to off-grid under the control strategy of the power current loop control method for seamless switching of energy storage PCS on-grid and off-grid modes of the present invention; Figure 8 This is an active power waveform diagram under the control strategy of the power current loop control method for seamless switching between on-grid and off-grid modes of the energy storage PCS of the present invention. DETAILED DESCRIPTION

[0016] See Figures 1-8 , the present invention comprises the following steps: (1) Detect the output voltage of the PCS in the off-grid state , current and phase angle , calculate the off-grid active power and reactive power ; (2) Detect the voltage of the grid connection point in the grid-connected state , current and phase angle , calculate the grid-connected active power and reactive power ; (3) When the switching signal is received, the power feedback loop is started and the actual grid-connected active power is converted to Adjust to preset value ,in ; (4) When the grid-connected active power and The difference is less than the allowable error When the inverter control mode is switched, the PQ control is switched to the Vf control mode, and the reference current in the PQ control mode is introduced into the Vf control voltage loop. ; Allowable error The value range is 1%-5% of the rated power; the PI controller parameters of the Vf control voltage loop are dynamically adjusted according to the off-grid load characteristics to suppress voltage fluctuations when disconnected from the main grid; (5) At preset time intervals After that, the switch is controlled to switch, so that the PCS is disconnected from the main grid and enters the off-grid state; the switch switching sequence is: first complete the inverter control mode switching, and then control the switch to disconnect from the main grid; the preset time interval Set according to off-grid load inertia and system response speed; (6) When the grid connection is restored, the voltage phase feedback loop and the frequency feedback loop are started to track the phase and frequency of the main grid; the voltage phase feedback loop and the frequency feedback loop track the phase and frequency of the main grid voltage in real time and generate a synchronization signal; (7) Synchronously switch the inverter control mode, switch status and PCS working status to complete the grid-connected operation.

[0017] Working process of the present invention: The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to the present invention is described in conjunction with the accompanying drawings during operation.

[0018] When a PCS switches operating states, voltage and current fluctuations primarily arise from two sources: fluctuations caused by the differing output parameters between the two inverter control modes when switching, and fluctuations caused by disconnection from the main grid. Therefore, this method requires different control methods for these two aspects. Furthermore, during the PCS switching process, the switch and inverter control modes do not switch instantaneously, making the order in which these components are switched crucial. Based on analysis, this method employs a sequence that first switches the inverter from PQ control to Vf control, followed by disconnection from the main grid. This is because PQ control indirectly controls voltage and current. When disconnected from the main grid, if PQ control is still used, the inverter cannot quickly adjust to voltage fluctuations. Vf control, on the other hand, directly affects voltage, thus better suppressing voltage fluctuations and improving stability during disconnection. The specific control method is described using this switching sequence.

[0019] First, the mode switching aspect is analyzed. In order to achieve seamless switching of modes, it is necessary to ensure that the output voltage and current of the two control modes are the same at the transient state of switching. The principle block diagram of the inverter PQ control and the principle block diagram of the inverter Vf control in the prior art are analyzed. The PQ control of the traditional inverter realizes the control of the power factor and voltage by controlling the active power and reactive power, while the Vf control controls the inverter by directly controlling the output voltage. By analyzing the control block diagrams of the two, it can be found that the current loops of the two have very similar structures. Therefore, this method integrates the current loops of the two into a unified current loop. The advantage of this is that the final value of the current reference value of the PQ control during switching will be used as the initial value of the current reference value of the Vf current loop, thereby achieving the same amplitude requirement of the output voltage. In addition, in order to achieve the voltage in-phase requirement, the two control modes can use the same phase acquisition signal to work, thereby eliminating the phase correction step.

[0020] Secondly, analyze the output current problem during mode switching. In PQ control, there are two ways to design the power loop. In the prior art, the power loop of PQ control usually uses a dual PI controller to achieve power decoupling and regulation, which generates a current reference value by connecting the active power PI controller and the reactive power PI controller in parallel. However, since this control method uses multiple PI controllers, the control is more complicated, and the control parameters are difficult to determine, and the stability is poor. Therefore, by decoupling the power loop, another reference current calculation method can be obtained, which is also called the chronological power calculation method. The calculation formula is as follows: Where, 、 are the reference current values ​​of the current loop, namely the d-axis reference current of PQ control and the q-axis reference current of PQ control; 、 is the reference active power and reactive power, 、 is the parameter value of the output voltage in the dq (direct-quadrature) coordinate system. This method can omit a PI controller, making the control process faster and more accurate. However, this power calculation method will lead to a sudden change in the current reference value of the two control modes when switching. This is because when the PCS system is stable, 、 Basically, they are fixed values, the former is generally 311V, and the latter is 0V. At this point, it can be found that 、 Will maintain a constant value to maintain the stability of the inverter output. Since Vf control uses the voltage outer loop, = , is the d-axis reference current of Vfcontrol. In the steady state, , that is, the reference value of the current in the d direction is approximately 0, which is quite different from the current reference value of the PQ control. When switching, the current reference value will suddenly change, and cause a sudden change in the output current. Therefore, this patent proposes a dynamic reference current value control method under the Vf control mode. Figure 2 As shown in the figure, in order to prevent the current reference value from changing when the inverter control mode is switched, this method introduces the reference current in PQ mode at the moment of control mode switching. , by By adding Vf into the voltage loop, a smooth current transition can be achieved during the instantaneous switching process. This method affects the original voltage loop to a certain extent. Since the PCS is still in the process of grid connection, , the Vf voltage loop cannot be eliminated by itself , which will lead to the accumulation of errors in the integral link of the PI controller and seriously affect the voltage stability when the system is disconnected from the main grid. However, when the system is disconnected from the main grid, due to the loss of support from the main grid, There will be a sudden drop, but the accumulated error of the PI link can help the voltage loop to quickly pull back the voltage, making the system more stable. This method can not only eliminate the current mutation when the inverter mode is switched, but also improve the voltage stability when it is disconnected from the main grid.

[0021] Finally, the process of the system disconnecting from the main grid is analyzed. Improving the stability of the inverter when it is disconnected from the main grid can be achieved in two ways. One is to add compensation at the moment of inverter disconnection to quickly level the voltage and suppress the voltage drop, as mentioned above. Another method is to reduce the energy transmission between the inverter and the main grid, so as to reduce the disturbance when the support is lost. In order to achieve the purpose of reducing energy transmission, it is necessary to make the inverter side self-sufficient as much as possible, so that the inverter almost does not provide power to the grid and does not need power compensation from the grid. This method proposes a dynamic adjustment method for the PQ control power reference value, such as Figure 3 As shown (Fig. It is the reference power in the power control module when connected to the grid. In the feedback loop, and The difference After the PI controller, The final output voltage is changed by superposition. Slowly reach (The voltage remains stable during the control process, and the PCS load is known.) Knowing the PCS load during off-grid operation allows us to infer the required active and reactive power output values ​​of the inverter. Therefore, before switching the inverter mode, the PQ power reference value can be adjusted to approach the post-off-grid active and reactive power values. In this case, after the inverter mode switch, Vf control will maintain the same output active and reactive power as during PQ control. When the inverter is disconnected from the main grid, since its own output power just meets the load requirements, disconnection from the main grid has a relatively minimal impact on the inverter's stability, thereby improving system stability.

[0022] The block diagram of the patent control method is as follows Figure 1 As shown in the figure, the detection device detects the various circuit data of the PCC point in real time and sends it to the MSC and MGCC (hereinafter referred to as the central control unit). When receiving an active switching signal or a circuit fault signal, the central control unit sends a preparation signal to the inverter and starts the power feedback loop at the same time, changing the active power reference value to , so that the output power smoothly transitions to the switchable range. − When ∣<ε (ε is the allowable error), the main control unit sends a signal to the inverter Vf control mode, changes the Vf control voltage loop, and converts the inverter control mode to Vf control. After the main control unit waits for the specified time t according to calculation, the voltage loop is cut off. Input and control the switch to change state, and the PCS enters the off-grid working state.

[0023] In order to verify the correctness and effectiveness of the present invention, this application conducted experimental verification. Figures 4 to 8 The waveform is obtained through simulation in Matlab / Simulink. The parameter data used in the simulation are shown in the following table: Table 1 Main circuit parameters parameter value DC voltage source voltage 800V Main grid voltage 380V Main grid voltage and frequency 50hz Off-grid mode load power 30000W Grid-connected mode reference power 40000W Figure 4 (voltage waveform) and Figure 5(Current waveforms) show the voltage and current waveforms during switching under the conventional control strategy. The inverter switches from grid-connected mode to off-grid mode at 0.25s, and disconnects from the main grid at 0.26s. Conventional mode reduces active power to zero during switching, minimizing current and voltage fluctuations and achieving seamless switching. However, the voltage waveform shows that while the inverter control mode switching is relatively effective, the output voltage drops significantly when disconnected from the main grid. The measured drop is 58V, approximately 20% of the rated voltage. This can seriously impact the normal operation of the PCS system and components during off-grid operation, potentially leading to shutdown.

[0024] Figure 6 (voltage waveform) and Figure 7 (Current waveform) is the voltage and current waveform diagram when switching under the control strategy of this method. Figure 4 、 5 The inverter switches operating modes at 0.25s. As can be seen, at 0.25s, due to the introduction of the PQ control current reference value parameter, the output current does not experience a significant sudden change, and the output voltage does not fluctuate, allowing the inverter to achieve seamless control mode switching. When disconnected from the main grid at 0.26s, the output voltage fluctuates, but the change is approximately 5V, or approximately 1.6% of the rated voltage. This effectively prevents the system and components from operating normally due to voltage fluctuations during switching, achieving seamless switching of the PCS operating mode.

[0025] Figure 8 The following is a waveform diagram of the active power output during the entire workflow under this control strategy. It can be seen that the initial active power is 40,000 W. At 0.1 seconds, the active power loop is activated, steadily reducing the active power to the off-grid active power value of 30,000 W. During the inverter mode switch at 0.25 seconds, the active power does not change suddenly and remains stable. During the 0.26 second separation from the main grid, the active power fluctuates slightly, but quickly recovers and remains stable.

[0026] In summary, the control method used in this invention effectively improves PCS stability during the switchover process from grid-connected to off-grid mode, reduces voltage fluctuations, shortens regulation time, and enables seamless switching of PCS operating modes. This demonstrates the effectiveness and feasibility of a seamless on-grid to off-grid PCS switching method using integrated energy storage based on the inverter power reference value and current loop.

[0027] The above embodiments are intended to illustrate the present invention, not to limit the present invention. Any solution that is a simple transformation of the present invention falls within the protection scope of the present invention.

Claims

1. A power current loop control method for seamless switching between on-grid and off-grid modes of energy storage PCS, characterized by: The following steps are involved: (1) Detect the output voltage of the PCS in the off-grid state , current and phase angle , calculate the off-grid active power and reactive power ; (2) Detect the voltage of the grid connection point in the grid connection state , current and phase angle , calculate the grid-connected active power and reactive power ; (3) When the switching signal is received, the power feedback loop is started and the actual grid-connected active power is converted to Adjust to preset value ,in ; (4) When the grid-connected active power and The difference is less than the allowable error When the inverter control mode is switched, the PQ control is switched to the Vf control mode, and the reference current in the PQ control mode is introduced into the Vf control voltage loop. ; (5) At the preset time interval After that, the control switch is switched to disconnect the PCS from the main grid and enter the off-grid state; (6) When the grid is restored, the voltage phase feedback loop and the frequency feedback loop are started to track the phase and frequency of the main grid; (7) Synchronously switch the inverter control mode, switch status and PCS working status to complete the grid-connected operation.

2. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The active power and reactive power in steps (1) and (2) are calculated using the following formulas: , 。 3. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The step (4) introduced Determined by: In PQ control mode, based on the reference active power and reactive power , combined with the dq axis components of the output voltage and , we can calculate: ， and will 、 is the reference current value of the current loop.

4. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The allowable error The value range is 1%-5% of the rated power.

5. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The switch switching sequence in step (5) is: first complete the inverter control mode switching, and then control the switch to disconnect from the main power grid; the preset time interval Set according to off-grid load inertia and system response speed.

6. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The voltage phase feedback loop and frequency feedback loop in step (6) track the phase and frequency of the main grid voltage in real time and generate a synchronization signal.

7. The power current loop control method for seamless switching between on-grid and off-grid modes of an energy storage PCS according to claim 1, characterized in that: The PI controller parameters of the Vf control voltage loop are dynamically adjusted according to the off-grid load characteristics to suppress voltage fluctuations when disconnected from the main grid.

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