Hybrid synchronous control-based grid-connected and off-grid seamless switching method and system

Abstract

The invention discloses a hybrid synchronous control-based grid-connected and off-grid seamless switching method and system, and relates to the technical field of microgrid grid-connected and off-grid switching, and the method comprises the steps: obtaining a grid side voltage in a hybrid synchronous control system when the hybrid synchronous control system operates in a grid-connected mode; judging the amplitude and the total harmonic distortion degree of the power grid side voltage by using an island detection strategy, and if a judgment result meets a preset off-grid condition or the hybrid synchronous control system receives an off-grid signal, opening off-grid operation of the hybrid synchronous control system in an off-grid mode; when a grid-connected signal is received during operation in an off-grid mode, a first phase angle of a micro-grid output voltage and a second phase angle of a to-be-accessed power grid voltage are obtained and judged, and if a preset grid-connected condition is met, the hybrid synchronous control system is switched on to be accessed into the power grid and enters the grid-connected mode to operate; the grid-connected and off-grid strategy which has good stability and carries out pre-synchronization by using hybrid synchronization control can effectively ensure the smoothness of the grid-connected and off-grid switching process.

Description

Technical Field

[0001] This invention relates to the field of microgrid parallel-to-offline switching technology, and more specifically, to a seamless parallel-to-offline switching method and system based on hybrid synchronous control. Background Technology

[0002] Microgrids composed of distributed renewable energy sources and energy storage primarily operate in two modes: "grid-connected" and "off-grid." When the main grid experiences a fault or requires independent operation, the connection to the main grid should be quickly disconnected, and the microgrid should enter off-grid mode. When the main grid returns to normal or grid-connected operation is required, the off-grid microgrid should be reconnected to the main grid. During the switching between the two operating modes, to avoid significant current surges, voltage distortions, and other adverse effects that could degrade power quality due to the converter switching back and forth between off-grid and grid-connected modes, appropriate control strategies should be adopted to ensure the smoothness of the switching process.

[0003] Currently, scholars at home and abroad have conducted research on seamless switching technology between grid and off-grid systems and proposed strategies such as PQ-VF and PQ-VSG control. However, problems still exist, such as unsmooth switching due to communication delays and failure to promptly disconnect grid faults, leading to system instability. Summary of the Invention

[0004] The purpose of this invention is to provide a seamless switching method and system for grid connection and disconnection based on hybrid synchronous control, which provides voltage and frequency support for microgrids. The PLL loop of hybrid synchronous control can not only ensure good stability when current limiting occurs during grid connection, but also effectively ensure the smoothness of grid connection and disconnection switching by using hybrid synchronous control for pre-synchronization strategy.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0006] Firstly, this application provides a seamless handover method for grid connection and disconnection based on hybrid synchronization control, including the following specific steps:

[0007] Based on the pre-set hybrid synchronous control system, when the hybrid synchronous control system is running in grid-connected mode, the grid-side voltage in the hybrid synchronous control system is obtained;

[0008] The corresponding amplitude is obtained by performing Fast Fourier Decomposition on the grid-side voltage, and the total harmonic distortion of the grid-side voltage is calculated.

[0009] The islanding detection strategy is used to judge the amplitude of the grid-side voltage and the total harmonic distortion. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives the off-grid signal, the hybrid synchronous control system will disconnect from the grid and enter the off-grid mode.

[0010] When a grid connection signal is received while the microgrid is operating in off-grid mode, the first phase angle of the microgrid output voltage and the second phase angle of the voltage to be connected to the grid are obtained.

[0011] The judgment is made based on the first phase angle and the second phase angle. If the judgment result meets the preset grid connection conditions, the hybrid synchronous control system is closed to connect to the grid and enters grid connection mode operation.

[0012] Based on the above technical solution, the present invention can be further improved as follows.

[0013] Furthermore, the aforementioned preset disconnection conditions are as follows:

[0014] When the amplitude of the grid-side voltage does not exceed the preset range of the preset rated value, and the total harmonic distortion does not exceed the first threshold, or the duration of the total harmonic distortion exceeding the first threshold does not exceed 70ms, the judgment result of maintaining grid-connected operation is obtained.

[0015] When the amplitude of the grid-side voltage exceeds the preset range of the preset rated value, or the total harmonic distortion exceeds the first threshold, and the duration of the total harmonic distortion exceeding the first threshold exceeds 70ms, the judgment result of entering off-grid operation is obtained.

[0016] Furthermore, the aforementioned preset grid connection conditions are as follows:

[0017] The first q-axis component of the first phase angle and the second q-axis component of the second phase angle are calculated respectively.

[0018] Based on the first q-axis component and the second q-axis component, the phase angle difference between the converter output voltage and the grid voltage of the microgrid is calculated, and the angular frequency of the converter output voltage is obtained based on the phase angle difference.

[0019] The angular frequency of the grid voltage is adjusted based on the angular frequency of the converter output voltage until the phase angle difference between the converter output voltage of the microgrid and the grid voltage does not exceed the second threshold, thus obtaining the judgment result for entering grid-connected operation.

[0020] Furthermore, the aforementioned hybrid synchronous control system includes inner-loop control and outer-loop control. The inner-loop control is a dual-loop control of voltage and current, while the outer-loop control is a hybrid synchronous control consisting of a reactive-frequency loop, an active-frequency loop, and a phase-locked loop connected in parallel.

[0021] Furthermore, the aforementioned hybrid synchronization control obtains the difference between the hybrid synchronization control's angular frequency and the rated angular frequency by performing a weighted average calculation on the difference between the angular frequency output by the phase-locked loop and the rated angular frequency, and the difference between the angular frequency output by the active-frequency loop in the hybrid synchronization control and the rated angular frequency. Specifically, the difference between the hybrid synchronization control's angular frequency and the rated angular frequency is as follows:

[0022]

[0023] In the formula, Δω HSC K represents the difference between the angular frequency of hybrid synchronization and the rated angular frequency. P Δω represents the weight of the PLL in the hybrid synchronous control. PLL K represents the difference between the output angular frequency of the phase-locked loop and its rated angular frequency. V Δω1 represents the weight of VSG in the hybrid synchronous control, and Δω1 represents the difference between the output angular frequency of the active-frequency loop and the rated angular frequency.

[0024] Furthermore, under the above-mentioned grid-connected operation mode, the active-frequency loop in the hybrid synchronous control is an inertia and damping loop, and the constraints of the active-frequency loop are satisfied as follows:

[0025]

[0026] In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, and P m K represents the mechanical active power of the inverter. pp This represents the droop factor for primary frequency regulation, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, and P... e The inverter's electromagnetic active power is represented by δ, the inverter's damping parameter is represented by D, and the inverter's virtual power angle is represented by ω. N This represents the angular velocity of the power grid synchronization.

[0027] Furthermore, under grid-connected operation, the specific voltage reference value of the reactive power-voltage loop in the hybrid synchronous control is as follows:

[0028]

[0029] In the formula, U ref U0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp Q represents the droop coefficient of the secondary voltage regulation stage. e K represents the electromagnetic reactive power of the inverter. q Q represents the droop coefficient of the reactive power loop in hybrid synchronous control. ref U represents the rated value of reactive power. pcc This indicates the maximum voltage at the common coupling point.

[0030] Furthermore, under off-grid operation, the above-mentioned constraints on the active-frequency loop in hybrid synchronous control are satisfied:

[0031]

[0032] In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, D represents the damping parameter of the inverter, δ represents the virtual power angle of the inverter, and ω N K represents the angular velocity of the power grid synchronization. pp This represents the droop factor of a single frequency modulation.

[0033] Furthermore, under off-grid operation, the specific voltage reference value for the reactive power-frequency loop in the hybrid synchronous control is as follows:

[0034]

[0035] In the formula, U ref U0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp K represents the droop coefficient of the secondary voltage regulation stage. q U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control, s represents the Laplace operator, and U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control. pcc This indicates the maximum voltage at the common coupling point.

[0036] Secondly, this application provides a seamless handover system for parallel and offline grid connection based on hybrid synchronous control, applied to the seamless handover method for parallel and offline grid connection based on hybrid synchronous control according to any one of the first aspects, comprising:

[0037] The first module is used to obtain the grid-side voltage in the hybrid synchronous control system when the hybrid synchronous control system is running in grid-connected mode, based on a preset hybrid synchronous control system.

[0038] The second module is used to perform fast Fourier decomposition on the grid-side voltage to obtain the corresponding amplitude and calculate the total harmonic distortion of the grid-side voltage.

[0039] The third module is used to judge the amplitude of the grid-side voltage and the total harmonic distortion using the islanding detection strategy. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives the off-grid signal, the hybrid synchronous control system will disconnect from the grid and enter the off-grid mode.

[0040] The fourth module is used to obtain the first phase angle of the microgrid output voltage and the second phase angle of the voltage to be connected to the grid when a grid connection signal is received during off-grid operation;

[0041] The fifth module is used to make a judgment based on the first phase angle and the second phase angle. If the judgment result meets the preset grid connection conditions, the hybrid synchronous control system closes the switch to connect to the grid and enters the grid connection mode.

[0042] Thirdly, this application provides an electronic device including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the method of any one of the first aspects.

[0043] Fourthly, this application provides a non-transitory computer-readable storage medium that stores computer instructions that cause a computer to perform any of the methods in the first aspect.

[0044] Compared with the prior art, the present invention has at least the following beneficial effects:

[0045] In this application, under the control of a hybrid synchronous control system, when operating in grid-connected mode, the grid-side voltage in this mode is acquired to obtain the amplitude and total harmonic distortion (THD) of the grid-side voltage. Based on the amplitude and THD, a decision is made regarding whether to switch operating modes. If the amplitude of the grid-side voltage does not exceed a preset range of the preset rated value, and the THD does not exceed a first threshold, or the duration of the THD exceeding the first threshold does not exceed 70ms, then grid-connected operation is maintained. If the amplitude of the grid-side voltage exceeds a preset range of the preset rated value, or the THD exceeds the first threshold, and... If the total harmonic distortion exceeds the first threshold for more than 70ms, the system needs to enter off-grid operation mode. When the hybrid synchronous control system is in off-grid mode, it determines whether it meets the grid-connected operation mode requirements based on the first phase angle of the microgrid output voltage and the second phase angle of the grid voltage to be connected. When the phase angle difference and the angular frequency of the converter output voltage are calculated using the first q-axis component of the first phase angle and the second q-axis component of the second phase angle, and the phase angle difference between the microgrid converter output voltage and the grid voltage does not exceed the second threshold, the system can enter grid-connected operation mode.

[0046] This application proposes a seamless grid-connected / off-grid switching method based on a hybrid synchronous control system. This method enables smooth switching between grid-connected and off-grid states. By refining the grid-connected / off-grid switching strategy, it ensures the microgrid maintains stability in both grid-connected and off-grid states while exhibiting good dynamic characteristics during the switching process. During grid-connected operation, the hybrid synchronous control grid-connected mode provides good stability for the microgrid during current-limiting periods. During off-grid operation, it establishes voltage and frequency for the microgrid. When switching from off-grid to grid-connected, the hybrid synchronous outer loop of the hybrid synchronous control system achieves pre-synchronization, ensuring the microgrid voltage is synchronized with the main grid voltage before closing the circuit breaker, thus achieving seamless grid-connected / off-grid switching for the microgrid. Attached Figure Description

[0047] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0048] Figure 1 This is a flowchart of the switching method in an embodiment of the present invention;

[0049] Figure 2 This is a structural block diagram of the energy storage system in an embodiment of the present invention;

[0050] Figure 3 This is a connection diagram of the hybrid synchronization control system in an embodiment of the present invention;

[0051] Figure 4 This is a logic diagram for determining the grid connection control strategy in an embodiment of the present invention;

[0052] Figure 5 This is a block diagram illustrating the grid-to-offline switching decision in an embodiment of the present invention.

[0053] Figure 6 This is a vector diagram of the off-grid to grid-connected pre-synchronization process in an embodiment of the present invention;

[0054] Figure 7 This is a block diagram for determining off-grid to on-grid switching in an embodiment of the present invention. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0056] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0057] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0058] Example 1: Due to existing problems such as unsmooth switching caused by communication delays and system instability caused by failure to promptly disconnect grid faults, in order to provide support for microgrid voltage and frequency, the phase-locked loop of the hybrid synchronous control system can ensure good stability even when current limiting occurs during grid connection. Furthermore, the hybrid synchronous control's pre-synchronization grid connection / disconnection strategy effectively guarantees a smooth grid connection / disconnection switching process. Therefore, this example provides a seamless grid connection / disconnection switching method based on hybrid synchronous control, such as... Figure 1 As shown, the specific steps include the following:

[0059] S1, based on a preset hybrid synchronous control system, obtains the grid-side voltage in the hybrid synchronous control system when the hybrid synchronous control system is running in grid-connected mode.

[0060] Firstly, the structure of a hybrid synchronous control system can be built. Commonly used main circuit structures include: Figure 2 As shown, in the main circuit section, U dc This refers to the DC bus voltage; L f C f Form an LC filter; U abc Voltage at the common coupling point (PCC point); I abc U is the output current of the converter; g and Z g Representing the grid voltage and grid line impedance respectively, it can collect the three-phase voltage U output at the point of common coupling under normal / fault operation of the power system. abc Three-phase current I abc and grid voltage U g .

[0061] Optionally, the above-mentioned hybrid synchronous control system includes inner loop control and outer loop control. The inner loop control is a voltage and current dual-loop control, and the outer loop control is a hybrid synchronous control in which reactive power-frequency loop, active power-frequency loop and phase-locked loop are connected in parallel.

[0062] The connection diagram of the hybrid synchronous control system is shown below. Figure 3 As shown, in Figure 3 In the ω, ω0 represents the rated angular frequency, ω HSC Indicates the converter output angular frequency, θ HSC P represents the phase angle of the converter output; m P represents the mechanical power of the inverter. e Δω represents the electromagnetic power of the inverter; D represents the damping parameter of the inverter; J represents the inertia parameter of the inverter; Δω represents the electromagnetic power of the inverter. PLL Δω1 represents the difference between the output angular frequency of the phase-locked loop and the rated angular frequency; k represents the difference between the output angular frequency of the active power loop in the hybrid synchronous control and the rated angular frequency; pp k is the droop factor for primary frequency modulation; qpω represents the droop coefficient of the primary voltage regulation stage; PCC The angular frequency of the voltage at point PCC; u q u d This represents the d-axis and q-axis voltage components output from the PLL loop of the hybrid synchronous control loop via abc to dq0; U ref U0 represents the voltage reference value, and Q represents the maximum value of the converter output voltage. m Q represents the mechanical reactive power of the inverter. e This refers to the electromagnetic reactive power of the inverter; specifically, the inner loop control is a voltage and current dual-loop control (i.e.,...). Figure 3 The control loop consists of a medium voltage and current dual closed loop, with the outer loop being a reactive-frequency loop and an active-frequency loop. Figure 3 The three links on the lower left side; the outer loop control of the hybrid synchronous control system consists of a hybrid synchronous loop with primary frequency regulation and a reactive-voltage loop with primary frequency regulation. The hybrid synchronous loop can be regarded as a parallel control of the active-frequency loop and the phase-locked loop. Its inner loop adopts voltage and current dual closed-loop control. In grid-connected mode, Pm of the hybrid synchronous loop is the rated active power value, the PLL loop input is the converter side voltage Upcc at the PCC point, and the reactive-voltage loop input is the rated reactive power value.

[0063] Optionally, the outer loop control of the above-mentioned hybrid synchronous control uses the difference Δω between the angular frequency output by the phase-locked loop and the rated angular frequency. PLL The difference between the angular frequency of the hybrid synchronous control and the rated angular frequency is calculated by weighted averaging the difference Δω1 between the angular frequency of the active-frequency loop output and the rated angular frequency. Specifically, the difference between the angular frequency of the hybrid synchronous control and the rated angular frequency is as follows:

[0064]

[0065] In the formula, Δω HSC K represents the difference between the angular frequency of hybrid synchronization and the rated angular frequency. P Δω represents the weight of the PLL in the hybrid synchronous control. PLL K represents the difference between the output angular frequency of the phase-locked loop and its rated angular frequency. V Δω1 represents the weight of VSG in the hybrid synchronous control, and Δω1 represents the difference between the output angular frequency of the active-frequency loop and the rated angular frequency.

[0066] Specifically, the difference between the angular frequency of the above-mentioned hybrid synchronous control and the rated angular frequency can also be expressed as:

[0067]

[0068] In the formula, u qThe PLL output of the hybrid synchronous control loop represents the q-axis voltage component, J represents the inverter's inertia parameter, D represents the inverter's damping parameter, and P represents the inverter's damping parameter. ref P represents the rated active power of the converter. e K represents the electromagnetic active power output of the converter. p2 K represents the PLL proportional gain of the equivalent hybrid synchronization loop. i2 J1 represents the PLL integral coefficient of the equivalent hybrid synchronization loop, J2 represents the inertia coefficient of the power-frequency conversion in the hybrid synchronization loop, and D represents the PLL integral coefficient of the equivalent hybrid synchronization loop. p2 K represents the damping coefficient for the power-frequency conversion in the hybrid synchronization loop. i represents the integral coefficient in the PLL of the hybrid synchronous control loop in the original control structure, and s represents the Laplace operator.

[0069] Optionally, in the above-mentioned grid-connected mode operation, the active-frequency loop in the hybrid synchronous control is an inertia and damping loop, and the constraints of the active-frequency loop satisfy:

[0070]

[0071] In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, and P m K represents the mechanical active power of the inverter. pp This represents the droop factor for primary frequency regulation, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, and P... e The inverter's electromagnetic active power is represented by δ, the inverter's damping parameter is represented by D, and the inverter's virtual power angle is represented by ω. N This represents the angular velocity of the power grid synchronization.

[0072] Specifically, in the hybrid synchronous control loop, the active-frequency loop Pm represents the rated active power P. ref The phase-locked loop input is 0, and the reactive-voltage loop input is the rated reactive power Q. ref .

[0073] Optionally, in the above-mentioned grid-connected operation mode, the PLL (phase-locked loop) in the hybrid synchronous loop outputs the converter-side voltage at the PCC point. During the current limiting period, the PLL loop plays a major control role, giving it better stability during the current limiting period. The voltage amplitude and phase angle obtained from the outer loop control of the hybrid synchronous control are sent to the voltage-current inner loop for further control and adjustment. Then, after inverse transformation and PWM modulation, the switching signal of the converter switching transistor can be obtained. The specific voltage reference value of the reactive power-voltage loop in the hybrid synchronous control is as follows:

[0074]

[0075] In the formula, U refU0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp Q represents the droop coefficient of the secondary voltage regulation stage. e K represents the electromagnetic reactive power of the inverter. q Q represents the droop coefficient of the reactive power loop in hybrid synchronous control. ref U represents the rated value of reactive power. pcc This indicates the maximum voltage at the common coupling point.

[0076] S2, perform Fast Fourier Decomposition on the grid-side voltage to obtain the corresponding amplitude, and calculate the total harmonic distortion of the grid-side voltage.

[0077] Among them, the grid-side voltage U g Perform fast Fourier decomposition on it to calculate the total harmonic distortion (THD) of the voltage; when U g When the amplitude does not exceed ±10% of the rated value and the THD does not exceed the threshold, or the duration of exceeding the threshold does not exceed 70ms, the grid connection mode of hybrid synchronous control is adopted.

[0078] S3 uses an islanding detection strategy to judge the amplitude of the grid-side voltage and the total harmonic distortion. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives an off-grid signal, the hybrid synchronous control system trips the circuit breaker, disconnects from the grid, and enters off-grid mode.

[0079] Optionally, the above-mentioned preset offline conditions are as follows:

[0080] S31, when the amplitude of the grid-side voltage does not exceed the preset range of the preset rated value, and the total harmonic distortion does not exceed the first threshold, or the duration of the total harmonic distortion exceeding the first threshold does not exceed 70ms, the judgment result of maintaining grid-connected operation is obtained.

[0081] S32, when the amplitude of the grid-side voltage exceeds the preset range of the preset rated value, or the total harmonic distortion exceeds the first threshold, and the duration of the total harmonic distortion exceeding the first threshold exceeds 70ms, the judgment result of entering off-grid operation is obtained.

[0082] Among them, such as Figure 4 As shown in a), when planning to disconnect from the grid, the system will operate directly off-grid; or when operating in grid-connected mode at a voltage range of ±10% of the rated voltage, the system will operate off-grid. Let Soncmd = 0 (this can be achieved using S...). plan S oncmd S swc S state and S pre_sny These respectively represent grid connection plan, grid connection control, grid connection switch, islanding / grid connection status, and pre-synchronization signal; such as Figure 4As shown in b), when Soncmd = 0, the grid-connected switch trips directly; when Soncmd = 1, the grid-connected switch is closed based on the phase difference of the voltage phasors on both sides of the switch. Simultaneously, islanding and grid-connected status are detected based on the phase difference of the voltage phasors on both sides of the switch; as shown in b), when Soncmd = 0, the grid-connected switch trips directly; when Soncmd = 1, the grid-connected switch is closed based on the phase difference of the voltage phasors on both sides of the switch. Figure 4 As shown in c), the pre-synchronization signal for switching from grid connection to off-grid can be obtained based on the grid connection control signal Soncmd and the islanded grid connection state Sstate; specifically, the process for switching from grid connection to off-grid is as follows: Figure 5 In the grid-connected state, the control mode of the hybrid synchronous control is in the grid-connected state. When the off-grid signal is detected, the control mode is switched to hybrid synchronous control, and the circuit breaker is disconnected from the grid, entering the off-grid operation mode.

[0083] S4, when receiving a grid connection signal while operating in off-grid mode, obtain the first phase angle of the microgrid output voltage under off-grid mode operation and the second phase angle of the voltage to be connected to the grid.

[0084] S5, make a judgment based on the first phase angle and the second phase angle. If the judgment result meets the preset grid connection conditions, the hybrid synchronous control system closes the switch to connect to the grid and enters the grid connection mode.

[0085] Optionally, the above-mentioned preset grid connection conditions are as follows:

[0086] S51, calculate the first q-axis component of the first phase angle and the second q-axis component of the second phase angle respectively.

[0087] S52, based on the first q-axis component and the second q-axis component, calculate the phase angle difference between the converter output voltage of the microgrid and the grid voltage, and obtain the angular frequency of the converter output voltage based on the phase angle difference;

[0088] S53 adjusts the angular frequency of the grid voltage based on the angular frequency of the converter output voltage until the phase angle difference between the microgrid converter output voltage and the grid voltage does not exceed the second threshold, thus obtaining the judgment result of entering grid-connected operation.

[0089] Optionally, in the off-grid mode, the primary frequency regulation and primary voltage regulation stages play a major role and will operate at a new stable point after a transient process.

[0090] Among them, the active power P output by the hybrid synchronous control active-frequency loop input converter is... e When no pre-synchronization signal is received, the PLL loop input is 0, and the primary frequency modulation loop plays a major control role; the constraints of the active-frequency loop in the hybrid synchronization control are satisfied as follows:

[0091]

[0092] In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, D represents the damping parameter of the inverter, δ represents the virtual power angle of the inverter, and ω N K represents the angular velocity of the power grid synchronization. pp This represents the droop factor of a single frequency modulation.

[0093] Optionally, in the off-grid mode operation described above, the reactive power-voltage loop input converter outputs reactive power, so the primary voltage regulation loop plays a major control role. The specific voltage reference value for the reactive power-frequency loop in the hybrid synchronous control is as follows:

[0094]

[0095] In the formula, U ref U0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp K represents the droop coefficient of the secondary voltage regulation stage. q U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control, s represents the Laplace operator, and U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control. pcc This indicates the maximum voltage at the common coupling point.

[0096] Among them, during the off-grid to on-grid transition, the process of the hybrid synchronous control output voltage tracking the grid is as follows: Figure 6 As shown; Figure 6 In the diagram, U represents the hybrid synchronous output voltage vector, and ω represents the rotational angular velocity of the hybrid synchronous converter output voltage. g Represents the grid voltage vector, ω g This represents the angular velocity of the grid voltage rotation; specifically, to achieve synchronization between the off-grid hybrid synchronous converter and the grid voltage, during the pre-synchronization period, the grid voltage vector is used as the d-axis to establish a rotational velocity ω. g For the rotated dq coordinate system, see [link / reference]. Figure 6 By projecting the converter's output voltage onto this coordinate system, synchronization of the two voltages can be achieved as long as the q-axis component is 0 and the d-axis component is the grid voltage amplitude. Therefore, the hybrid synchronization loop of the hybrid synchronization control receives the grid connection signal and acquires the three-phase voltage U from the grid side. gabc The three-phase input voltage U is obtained through the PLL in the hybrid synchronization loop. abc with U g The phase angle can be used to obtain the phase angle difference between the grid voltage phasor and the PCC point voltage phasor. This component is then compared with the VSG frequency component ω output by the active power loop. vsg By combining the results, we obtain the output frequency ω. HSC Thus, adjusting U pcc The angular velocity ω enables error-free regulation of the grid voltage and the converter output voltage.

[0097] The process for switching from off-grid to on-grid is as follows: Figure 7 As shown, during the off-grid to grid-connected transition, if the microgrid and grid voltages are out of sync at the moment of grid connection, a large grid connection inrush current will be generated, potentially leading to grid connection failure. Even if successful, the voltage waveform will be distorted, and power quality will deteriorate. Therefore, pre-synchronization should be performed before connecting to the grid. When in off-grid mode and a grid connection control command is input, i.e., S... state =0, S oncmd When = 1, S pre_sny Output 1 is used for pre-synchronization; the phase-locked loop in the hybrid synchronization loop collects the grid-side voltage U. g with U abc The three-phase voltage U is acquired using the phase-locked loop synchronization method. abc with U g The phase angle is obtained and fed into the abc to dq0 converter. The q-axis component of the obtained voltage is then fed into the PI controller to obtain the difference between the converter output phase angle and the grid voltage phase angle. This component is then compared with the frequency component ω output by the active power loop. vsg By combining the results, we obtain the output frequency ω. HSC Thus, adjusting U pcc The angular velocity ω, when U abc with U g When the phase angle difference between the two phases meets the grid connection requirements, the circuit breaker is closed to connect the microgrid to the grid and convert it to the grid connection mode of hybrid synchronous control. That is, after receiving the grid connection signal, the PLL in the hybrid synchronous loop collects the amplitude and phase information of the grid voltage and performs pre-synchronization adjustment. When the amplitude and phase angle difference of the converter output voltage meet the grid connection conditions, the circuit breaker is closed to connect to the grid and convert it to the grid connection control mode of hybrid synchronous control.

[0098] Example 2: This application provides a seamless handover system for parallel and offline operations based on hybrid synchronous control, applied to any of the seamless handover methods for parallel and offline operations based on hybrid synchronous control in Example 1, including:

[0099] The first module is used to obtain the grid-side voltage in the hybrid synchronous control system when the hybrid synchronous control system is running in grid-connected mode, based on a preset hybrid synchronous control system.

[0100] The second module is used to perform fast Fourier decomposition on the grid-side voltage to obtain the corresponding amplitude and calculate the total harmonic distortion of the grid-side voltage.

[0101] The third module is used to judge the amplitude of the grid-side voltage and the total harmonic distortion using the islanding detection strategy. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives the off-grid signal, the hybrid synchronous control system will trip and disconnect from the grid and enter the off-grid mode.

[0102] The fourth module is used to obtain the first phase angle of the microgrid output voltage and the second phase angle of the grid voltage to be connected when a grid connection signal is received during off-grid operation.

[0103] The fifth module is used to make a judgment based on the first phase angle and the second phase angle. If the judgment result meets the preset grid connection conditions, the hybrid synchronous control system closes the switch to connect to the grid and enters the grid connection mode.

[0104] Example 3: This application provides a non-transitory computer-readable storage medium that stores computer instructions that cause a computer to perform any of the methods in Example 1.

[0105] Example 4: This application provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the method of any one of Examples 1.

[0106] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A seamless handover method for parallel and offline operations based on hybrid synchronous control, characterized in that, The specific steps include the following: Based on a pre-set hybrid synchronous control system, when the hybrid synchronous control system is running in grid-connected mode, the grid-side voltage in the hybrid synchronous control system is obtained; The corresponding amplitude is obtained by performing Fast Fourier Decomposition on the grid-side voltage, and the total harmonic distortion of the grid-side voltage is calculated. The amplitude and total harmonic distortion of the grid-side voltage are judged by the islanding detection strategy. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives the off-grid signal, the hybrid synchronous control system disconnects from the grid and enters off-grid mode. When a grid connection signal is received while the microgrid is operating in off-grid mode, the first phase angle of the microgrid output voltage and the second phase angle of the voltage to be connected to the grid are obtained. Based on the first phase angle and the second phase angle, if the judgment result meets the preset grid connection conditions, the hybrid synchronous control system closes the switch to connect to the power grid and enters the grid connection mode for operation.

2. The seamless switching method for parallel and offline operations based on hybrid synchronous control according to claim 1, characterized in that, The preset disconnection conditions are as follows: When the amplitude of the grid-side voltage does not exceed the preset range of the preset rated value, and the total harmonic distortion does not exceed the first threshold, or the duration of the total harmonic distortion exceeding the first threshold does not exceed 70ms, the judgment result of maintaining grid-connected operation is obtained. When the amplitude of the grid-side voltage exceeds the preset range of the preset rated value, or the total harmonic distortion exceeds the first threshold, and the duration of the total harmonic distortion exceeding the first threshold exceeds 70ms, the judgment result of entering off-grid operation is obtained.

3. The seamless switching method for parallel and offline operations based on hybrid synchronous control according to claim 1, characterized in that, The preset grid connection conditions are as follows: The first q-axis component of the first phase angle and the second q-axis component of the second phase angle are calculated respectively. Based on the first q-axis component and the second q-axis component, the phase angle difference between the converter output voltage and the grid voltage of the microgrid is calculated, and the angular frequency of the converter output voltage is obtained based on the phase angle difference. The angular frequency of the grid voltage is adjusted based on the angular frequency of the converter output voltage until the phase angle difference between the microgrid converter output voltage and the grid voltage does not exceed the second threshold, thus obtaining the judgment result of entering grid-connected operation.

4. The seamless switching method for parallel and offline operations based on hybrid synchronous control according to claim 1, characterized in that, The hybrid synchronous control system includes inner loop control and outer loop control. The inner loop control is a voltage and current dual-loop control, and the outer loop control is a hybrid synchronous control consisting of a reactive-frequency loop, an active-frequency loop, and a phase-locked loop connected in parallel.

5. The seamless handover method for parallel and offline operation based on hybrid synchronous control according to claim 4, characterized in that, The hybrid synchronization control obtains the difference between the hybrid synchronization control angular frequency and the rated angular frequency by performing a weighted average calculation on the difference between the angular frequency output by the phase-locked loop and the rated angular frequency, and the difference between the angular frequency output by the active-frequency loop in the hybrid synchronization control and the rated angular frequency. Specifically, the difference between the hybrid synchronization control angular frequency and the rated angular frequency is as follows: In the formula, Δω HSC K represents the difference between the angular frequency of hybrid synchronization and the rated angular frequency. P Δω represents the weight of the PLL in the hybrid synchronous control. PLL K represents the difference between the output angular frequency of the phase-locked loop and its rated angular frequency. V Δω1 represents the weight of VSG in the hybrid synchronous control, and Δω1 represents the difference between the output angular frequency of the active-frequency loop and the rated angular frequency.

6. The seamless handover method for parallel and offline operations based on hybrid synchronous control according to claim 5, characterized in that, Under the grid-connected mode, the active-frequency loop in the hybrid synchronous control is an inertia and damping loop, and the constraints of the active-frequency loop are satisfied as follows: In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, and P m K represents the mechanical active power of the inverter. pp This represents the droop factor for primary frequency regulation, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, and P... e The inverter's electromagnetic active power is represented by δ, the inverter's damping parameter is represented by D, and the inverter's virtual power angle is represented by ω. N This represents the angular velocity of the power grid synchronization.

7. The seamless handover method for parallel and offline operations based on hybrid synchronous control according to claim 4, characterized in that, Under the grid-connected mode operation, the voltage reference value of the reactive power-voltage loop in the hybrid synchronous control is specifically as follows: In the formula, U ref U0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp Q represents the droop coefficient of the secondary voltage regulation stage. e K represents the electromagnetic reactive power of the inverter. q Q represents the droop coefficient of the reactive power loop in hybrid synchronous control. ref U represents the rated value of reactive power. pcc This indicates the maximum voltage at the common coupling point.

8. The seamless handover method for parallel and offline operations based on hybrid synchronous control according to claim 4, characterized in that, Under the off-grid mode operation, the active-frequency loop constraints in the hybrid synchronous control are satisfied as follows: In the formula, J represents the inertia parameter of the inverter, Δω represents the change in angular frequency, ω0 represents the rated angular frequency, ω represents the angular velocity of the inverter, D represents the damping parameter of the inverter, δ represents the virtual power angle of the inverter, and ω N K represents the angular velocity of the power grid synchronization. pp This represents the droop factor of a single frequency modulation.

9. A seamless handover method for parallel and offline operations based on hybrid synchronous control according to claim 4, characterized in that, Under the off-grid mode operation, the voltage reference value of the reactive power-frequency loop in the hybrid synchronous control is specifically as follows: In the formula, U ref U0 represents the voltage reference value, and K represents the maximum output voltage of the converter. qp K represents the droop coefficient of the secondary voltage regulation stage. q U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control, s represents the Laplace operator, and U represents the integral coefficient of the reactive-frequency loop in the hybrid synchronous control. pcc This indicates the maximum voltage at the common coupling point.

10. A seamless handover system for parallel and offline operations based on hybrid synchronous control, applied to the seamless handover method for parallel and offline operations based on hybrid synchronous control according to any one of claims 1-9, characterized in that, include: The first module is used to acquire the grid-side voltage in the hybrid synchronous control system when the hybrid synchronous control system is running in grid-connected mode, based on a preset hybrid synchronous control system. The second module is used to perform fast Fourier decomposition on the grid-side voltage to obtain the corresponding amplitude, and to calculate the total harmonic distortion of the grid-side voltage. The third module is used to judge the amplitude and total harmonic distortion of the grid-side voltage using an islanding detection strategy. If the judgment result meets the preset off-grid conditions or the hybrid synchronous control system receives an off-grid signal, the hybrid synchronous control system will disconnect from the grid and enter off-grid mode. The fourth module is used to obtain the first phase angle of the microgrid output voltage and the second phase angle of the voltage to be connected to the grid when a grid connection signal is received during off-grid operation; The fifth module is used to make a judgment based on the first phase angle and the second phase angle. If the judgment result meets the preset grid connection conditions, the hybrid synchronous control system is connected to the power grid and enters the grid connection mode.

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