A flexible loop closing device and its functional quantity control method

Abstract

A flexible loop-closing device and its active power control method are disclosed. The device includes a power electronic transformer and distribution networks connected to both sides. Each phase of the power electronic transformer includes several power modules symmetrically arranged on both sides. Each power module includes an H-bridge section composed of power electronic switching devices T1-T4 and a DAB section composed of power electronic switching devices T5-T8. The power modules on both sides are connected through a resonant inductor Lr and an isolation transformer T. The H-bridge sections of the power modules on the same side are connected in series to the high-voltage AC bus of the distribution network on the same side. The active power control method is as follows: the AC side of the loop-closing device is controlled by a cascaded H-bridge, and the DC side is controlled by a DAB. When there is no active power flow control requirement, the DAB section does not need to work, and the DC bus voltage of the power modules on both sides is controlled by their respective cascaded H-bridges. This method can simplify the operation complexity of the loop-closing device to the greatest extent, ensure the stable operation of the loop itself and the reliability of the system power supply, and greatly reduce the system losses when there is no active power flow control requirement. The active power control strategy proposed in this invention can avoid frequent switching of the cascaded H-bridge control mode, ensure the stable operation of the flexible loop-closing system, and improve the dynamic response speed.

Description

Technical Field

[0001] This invention relates to the field of flexible loop closing device technology, specifically to a flexible loop closing device and its functional quantity control method. Background Technology

[0002] As users' demands for electricity, power quality, and power supply reliability continue to increase, traditional power supply networks are increasingly unable to meet these needs. Existing power distribution networks typically employ a closed-loop design but operate in an open-loop mode. Overloading of any one line can negatively impact the system's power quality. Flexible loop-closing devices can enable closed-loop operation of the distribution network, improving power supply reliability and thus gaining increasing attention.

[0003] Flexible loop connection is a power conversion device based on power electronics technology. It is used to connect two AC power grids, enabling interconnection of grids with different voltage levels and frequencies. Furthermore, it allows for control of power flow direction, achieving load balancing, optimizing grid power supply capacity, providing dynamic reactive power support, and improving distribution network reliability and equipment utilization.

[0004] Flexible loop-connected devices possess flexible power flow control capabilities. Regarding existing flexible control strategies, most domestic and international research schemes draw upon the modular multilevel converter (MMC) scheme with a back-to-back structure in flexible DC transmission. Power regulation is achieved by controlling the active and reactive currents separately in a two-phase stationary coordinate system through phase-locked loop (PLL) grid voltage phase control.

[0005] Flexible loop-connected circuits based on electronic transformer structures are a widely used circuit topology. The AC side employs a cascaded H-bridge structure to connect to the medium-voltage power grid. The cascaded H-bridges on both sides transmit functional quantities through a DAB circuit, while also providing electrical isolation. In terms of control strategy, the traditional method uses the cascaded H-bridges to control the DC bus voltage on the primary side of the DAB, and the DAB to control the DC bus voltage on the secondary side. The cascaded H-bridges on the secondary side of the DAB do not participate in the DC bus voltage control. In this approach, when a fault occurs on the primary side of the DAB grid, the cascaded H-bridges on the secondary side of the DAB need to be switched to DC bus voltage control mode, the DAB needs to be switched to primary side DC voltage control mode, and the cascaded H-bridges on the primary side of the DAB need to be switched to AC voltage control mode. It can be seen that under this traditional control method, changing the operating state of the flexible loop-connected device requires switching the load control mode. Current research in this area is not yet very mature. Summary of the Invention

[0006] In order to overcome the problems existing in the prior art, the purpose of this invention is to propose a flexible loop closing device and its functional quantity control method, which can simplify the complex switching of control mode when the working mode of the flexible loop closing device changes, ensure the stable operation of the flexible loop closing system, improve dynamic response performance, and has important practical significance for promoting the development of flexible loop closing related technologies and industries.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A flexible loop-closing device includes a power electronic transformer and distribution networks connected to both sides. Each phase of the power electronic transformer includes several power modules symmetrically arranged on the left and right. Each power module includes a power electronic switching device T1-T4 forming an H-bridge section and power electronic switching devices T5-T8 forming a DAB section. The power modules on the left and right sides are connected through a resonant inductor Lr and an isolation transformer T. The H-bridge sections of the power modules on the same side are connected in series to the high-voltage AC bus of the distribution network on the same side.

[0009] A method for controlling the functional quantity of a flexible loop closing device, specifically including:

[0010] 1) The cascaded H-bridge of the left power module controls the reactive component on the AC side in a closed loop, and at the same time controls the DC bus voltage Va1-Va9 of the power module in a closed loop.

[0011] 2) The cascaded H-bridge of the right-side power module controls the reactive component of the AC side in a closed loop, and at the same time controls the DC bus voltage Vb1-Vb9 of the power module in a closed loop.

[0012] 3) Control the phase shift angle between the left and right DABs to achieve closed-loop control of the magnitude and direction of the active power transmitted by the DABs. Specifically, the difference between the command value and the sampled value of the active power is used to obtain the phase shift angle between the primary and secondary sides of the DABs through the PI controller, thereby achieving closed-loop control of the active power.

[0013] The specific method for step 1) is as follows:

[0014] 1.1) Obtaining instructions: The difference between the DC capacitor voltage control instruction value and the sampled value of the power module is used to obtain the d-axis current instruction value through the PI controller. The difference between the d-axis current instruction value and the d-axis current sampled value is used to obtain the d-axis modulation wave through the PI controller. The reactive power instruction value generates the q-axis current instruction value. The difference between the q-axis current instruction value and the q-axis current sampled value is used to obtain the q-axis modulation wave through the PI controller. The modulation waves of the d-axis and q-axis are transformed from a 2-phase rotation to a 3-phase stationary coordinate system to obtain the modulation signal Da of phase A, the modulation signal Db of phase B, and the modulation signal Dc of phase C on the left.

[0015] 1.2) Execute instructions: Based on the modulation signals Da, Db, Dc obtained in step 1.1) and the port current, control the output voltage of the cascaded H-bridge. When the port voltage and current are in the same direction, the submodule is charged. When the port voltage and current are in opposite directions, the submodule is discharged. When the port voltage is zero, the submodule is neither charged nor discharged.

[0016] The specific method for step 2) is as follows:

[0017] 2.1) Command Acquisition: The difference between the DC capacitor voltage control command value and the sampled value of the power module is used to obtain the d-axis current command value through the PI controller. The difference between the d-axis current command value and the d-axis current sampled value is used to obtain the d-axis modulation wave through the PI controller. The reactive power command value generates the q-axis current command value. The difference between the q-axis current command value and the q-axis current sampled value is used to obtain the q-axis modulation wave through the PI controller. The modulation waves of the d-axis and q-axis are transformed from a 2-phase rotation to a 3-phase stationary coordinate system to obtain the modulation signal Da of phase A, the modulation signal Db of phase B, and the modulation signal Dc of phase C on the right.

[0018] 2.2) Execution instructions: Based on the modulation signals Da, Db, Dc obtained in step 2.1) and the port current, control the output voltage of the cascaded H-bridge. When the port voltage and current directions are in the same direction, the submodule is charged. When the port voltage and current directions are in opposite directions, the submodule is discharged. When the port voltage is zero, the submodule is neither charged nor discharged.

[0019] The control methods for steps 1) and 2) are the same and run in parallel.

[0020] Step 3) states that the DAB section does not operate when there is no active power flow control requirement.

[0021] The flexible loop-closing device of this invention is used in two AC power grids. By controlling the direction and magnitude of power flow, it can achieve load balancing, optimize power grid power supply capacity, provide dynamic reactive power support, and improve the reliability of distribution networks and equipment utilization.

[0022] The active power control method proposed in this invention is based on the power balance principle. It utilizes the phase shift angle of the flexible loop-closing device (DAB) to control the direction and magnitude of active power flow, while the DAB does not control the output voltage. Both cascaded H-bridges operate in Q-Vdc control mode, with completely identical control methods. This avoids frequent switching of the cascaded H-bridge control modes, ensuring the stable operation of the flexible loop-closing system and improving dynamic response speed.

[0023] The active power control method proposed in this invention eliminates the need for the DAB section to operate when there is no active power flow control requirement. The DC bus voltages of the power modules on both sides are controlled by their respective cascaded H-bridges. This approach simplifies the operation of the loop-closing device to the greatest extent possible, ensures the stable operation of the loop itself and the reliability of the system power supply, and significantly reduces system losses when there is no active power flow control requirement. Attached Figure Description

[0024] Figure 1 This is a circuit topology diagram of the flexible loop closing device of the present invention.

[0025] Figure 2 This is a control diagram of the flexible closed-loop cascaded H-bridge on the left side of the present invention.

[0026] Figure 3 This is a control diagram of the right-side cascaded H-bridge of the flexible closed loop according to the present invention.

[0027] Figure 4 This is a power control diagram for the flexible closed-loop DAB of the present invention. Detailed Implementation

[0028] The present invention will now be described in further detail with reference to the accompanying drawings.

[0029] See Figure 1 A flexible loop-closing device includes a power electronic transformer and distribution networks connected to both sides. Each phase of the power electronic transformer includes several power modules symmetrically arranged on the left and right. Each power module includes an H-bridge section composed of power electronic switching devices T1-T4 and a DAB section composed of power electronic switching devices T5-T8. The power modules on the left and right sides are connected through a resonant inductor Lr and an isolation transformer T. The H-bridge sections of the power modules on the same side are connected in series to the high-voltage AC bus of the distribution network on the same side.

[0030] A method for controlling the functional quantity of a flexible loop closing device, specifically including:

[0031] 1) The cascaded H-bridge of the left-side power module provides closed-loop control of the AC reactive component, while simultaneously controlling the DC bus voltages Va1-Va9 of the power module; see [link / reference]. Figure 2 .

[0032] 1.1) Obtaining instructions: The difference between the DC capacitor voltage control instruction value and the sampled value of the power module is used to obtain the d-axis current instruction value through the PI controller. The difference between the d-axis current instruction value and the d-axis current sampled value is used to obtain the d-axis modulation wave through the PI controller. The reactive power instruction value generates the q-axis current instruction value. The difference between the q-axis current instruction value and the q-axis current sampled value is used to obtain the q-axis modulation wave through the PI controller. The modulation waves of the d-axis and q-axis are transformed from a 2-phase rotation to a 3-phase stationary coordinate system to obtain the modulation signal Da of phase A, the modulation signal Db of phase B, and the modulation signal Dc of phase C on the left.

[0033] 1.2) Execution instructions: Based on the modulation signals Da, Db, Dc obtained in step 1.1) and the port current, control the output voltage of the cascaded H-bridge. When the port voltage and current directions are in the same direction, the submodule is charged; when the port voltage and current directions are in opposite directions, the submodule is discharged; when the port voltage is zero, the submodule is neither charged nor discharged.

[0034] 2) The cascaded H-bridge of the right-side power module provides closed-loop control of the AC reactive component, while simultaneously controlling the DC bus voltages Vb1-Vb9 of the power module; see [link / reference]. Figure 3 .

[0035] 2.1) Command Acquisition: The difference between the DC capacitor voltage control command value and the sampled value of the power module is used to obtain the d-axis current command value through the PI controller. The difference between the d-axis current command value and the d-axis current sampled value is used to obtain the d-axis modulation wave through the PI controller. The reactive power command value generates the q-axis current command value. The difference between the q-axis current command value and the q-axis current sampled value is used to obtain the q-axis modulation wave through the PI controller. The modulation waves of the d-axis and q-axis are transformed from a 2-phase rotation to a 3-phase stationary coordinate system to obtain the modulation signal Da of phase A, the modulation signal Db of phase B, and the modulation signal Dc of phase C on the right.

[0036] 2.2) Execution instructions: Based on the modulation signals Da, Db, Dc obtained in step 2.1) and the port current, control the output voltage of the cascaded H-bridge. When the port voltage and current directions are in the same direction, the submodule is charged; when the port voltage and current directions are in opposite directions, the submodule is discharged; when the port voltage is zero, the submodule is neither charged nor discharged.

[0037] 3) Controlling the phase shift angle between the left and right DABs achieves closed-loop control of the magnitude and direction of active power transmitted through the DABs. Specifically, the difference between the commanded active power value and the sampled active power value is used by a PI controller to obtain the phase shift angle between the primary and secondary sides of the DABs, thereby achieving closed-loop control of the active power. See [link to relevant documentation] Figure 4 .

[0038] The control methods for steps 1) and 2) are the same and run in parallel.

[0039] Step 3) states that the DAB section does not operate when there is no active power flow control requirement.

[0040] This invention relates to a flexible loop-closing device with AC-side cascaded H-bridge control and DC-side DAB control. When there is no active power flow control requirement, the DAB section does not need to operate, and the DC bus voltage of the power modules on both sides is controlled by their respective cascaded H-bridges. This approach simplifies the operation complexity of the loop-closing device to the greatest extent, ensures the stable operation of the loop itself and the reliability of the system power supply, and greatly reduces system losses when there is no active power flow control requirement. The method proposed in this invention is an innovative approach to the operation control of flexible loop-closing devices.

Claims

1. A method for functional energy control of a flexible ring closure device, characterized by: The flexible loop device comprises a power electronic transformer and power distribution networks connected to both sides of the power electronic transformer, each phase of the power electronic transformer comprises a plurality of left-right symmetrical power modules, each power module comprises a H-bridge part composed of power electronic switching devices T1-T4 and a DAB part composed of power electronic switching devices T5-T8, and the left and right power modules are connected through a resonant inductor Lr and an isolation transformer T; the H-bridge parts of the power modules on the same side are connected in series to the high-voltage alternating current bus of the same side power distribution network. The active power control method based on the flexible loop device comprises the following steps: 1) closed-loop control of the reactive power component on the alternating current side of the left power module, and closed-loop control of the direct current bus voltage Va1-Va9 of the power module; 2) closed-loop control of the reactive power component on the alternating current side of the right power module, and closed-loop control of the direct current bus voltage Vb1-Vb9 of the power module; 3) control of the phase shift angle between the left DAB and the right DAB to realize closed-loop control of the size and direction of the active power transmitted by the DAB, specifically: the difference between the command value of the active power and the sampling value of the active power is subjected to PI control to obtain the phase shift angle of the primary side and the secondary side of the DAB, thereby realizing closed-loop control of the active power.

2. A method of functional capacity control of a flexible ring closure device according to claim 1, characterized in that: The specific method of step 1) is as follows: 1.1) obtaining the command: the difference between the command value and the sampling value of the direct current capacitor voltage control of the power module is subjected to PI control to obtain the d-axis current command value, the difference between the d-axis current command value and the d-axis current sampling value is subjected to PI control to obtain the d-axis modulation wave; the q-axis current command value is generated by the command value of the reactive power, the difference between the q-axis current command value and the q-axis current sampling value is subjected to PI control to obtain the q-axis modulation wave; the modulation waves of the d-axis and the q-axis are subjected to 2-phase rotation to 3-phase stationary coordinate system transformation to obtain the modulation signal Da of the left A-phase, the modulation signal Db of the left B-phase and the modulation signal Dc of the left C-phase; 1.2) executing the command: according to the modulation signals Da, Db and Dc obtained in step 1.1) and the current of the port, the output voltage of the cascaded H-bridge is controlled, when the directions of the port voltage and the current are the same, the sub-module is charged, when the directions of the port voltage and the current are opposite, the sub-module is discharged, and when the port voltage is zero, the sub-module is neither charged nor discharged.

3. A method of functional capacity control of a flexible ring closure device according to claim 1, characterized in that: The specific method of step 2) is as follows: 2.1) obtaining the command: the difference between the command value and the sampling value of the direct current capacitor voltage control of the power module is subjected to PI control to obtain the d-axis current command value, the difference between the d-axis current command value and the d-axis current sampling value is subjected to PI control to obtain the d-axis modulation wave; the q-axis current command value is generated by the command value of the reactive power, the difference between the q-axis current command value and the q-axis current sampling value is subjected to PI control to obtain the q-axis modulation wave; the modulation waves of the d-axis and the q-axis are subjected to 2-phase rotation to 3-phase stationary coordinate system transformation to obtain the modulation signal Da of the right A-phase, the modulation signal Db of the right B-phase and the modulation signal Dc of the right C-phase; 2.2) Execute instruction: according to the modulation signal Da, Db, Dc and the current of the port obtained in step 2.1), control the output voltage of the cascade H-bridge, when the port voltage direction and the current direction are in the same direction, the sub-module charges, when the port voltage and the current direction are reversed, the sub-module discharges, when the port voltage is zero, the sub-module neither charges nor discharges.

4. A method of functional capacity control of a flexible ring closure device according to claim 1, characterized in that: The control method of the step 1) and the step 2) is consistent, and the operation is parallel.

5. A method of functional capacity control of a flexible ring closure device according to claim 1, characterized in that: The step 3) does not work when there is no active power flow control requirement.

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