Direct-current transformer for direct-current grid interconnection and control and protection method therefor

Abstract

The present invention relates to the technical field of flexible direct-current transmission. Disclosed are a direct-current transformer for direct-current grid interconnection and a control and protection method therefor. The direct-current transformer comprises a positive pole converter, a negative pole converter, and a controller. The positive pole converter and the negative pole converter each comprise a plurality of phase units. Each of the phase units comprises a common bridge arm and a plurality of port bridge arms, wherein a first end of each port bridge arm and a first end of the common bridge arm are connected to a common point, and a second end of each port bridge arm is connected to a second end of the port bridge arm of another phase unit at the same position before an output port is led out; and a direct-current voltage is input into or output from the output port and a second end of the common bridge arm. The present invention uses only the plurality of phase units to form a monopole converter, without the need for including an alternating-current isolation transformer. In addition, fewer power semiconductors are used, resulting in lower manufacturing costs.

Description

A DC transformer for DC grid interconnection and its control and protection method

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This invention claims priority to Chinese patent application No. 202311264395.1 filed with the Patent Office of China on September 27, 2023, entitled “A DC transformer for DC grid interconnection and its control and protection method”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of flexible direct current (DC) transmission, and in particular to a DC transformer for DC grid interconnection and a control and protection method thereof. Background Art

[0004] Compared to traditional AC transmission, DC transmission technology offers advantages such as longer transmission distances and greater capacity. Flexible DC transmission, based on voltage-source converters (VSDCs), offers flexibility, controllability, and adaptability to weak grids, playing a crucial role in integrating renewable energy into the grid.

[0005] Flexible DC transmission has evolved from point-to-point transmission systems to single-voltage, multi-terminal DC grids. To achieve wide-area interconnection between DC grids, leverage the advantages of DC grids at different voltage levels, and accommodate the high proportion of diverse renewable energy access, multi-voltage DC grids are a future development trend. Therefore, a converter capable of interconnecting DC grids at multiple voltage levels is needed.

[0006] Existing technologies often use multi-winding AC isolation transformers to implement multi-port DC transformers. However, this structure requires a large number of components, and the isolation transformer's isolation capacity must be higher than the system voltage level. This results in high cost, severe component losses and heat generation, and a large size, making it difficult to apply to high-voltage systems. Therefore, existing DC transformers are only used in low-voltage distribution networks. Therefore, there is an urgent need for a DC transformer that can be used in high-voltage DC grid interconnection scenarios.

[0007] Summary of the Invention

[0008] In view of this, the present invention provides a DC transformer for DC grid interconnection and a control and protection method thereof to solve the problem that a multi-winding AC isolation transformer requires too many components, resulting in high converter cost and serious device loss and heat generation.

[0009] In a first aspect, the present invention provides a DC transformer for DC grid interconnection, comprising a positive converter, a negative converter, and a controller, wherein the positive converter and the negative converter each comprise: a plurality of phase units, each phase unit comprising: a common bridge arm and a plurality of port bridge arms, wherein the first end of each port bridge arm is connected to the first end of the common bridge arm at a common point, the second end of each port bridge arm is connected to the second end of the port bridge arm at the same position of other phase units, and an output port is led out; the second end of each common bridge arm is interconnected and grounded; each output port is connected to a DC grid; the controller is connected to each port bridge arm and each common bridge arm, and the controller is used to control the operating status of each port bridge arm and each common bridge arm.

[0010] The present invention only uses a plurality of phase units to form a unipolar converter without an AC isolation transformer, and uses a small number of power semiconductors, thereby having a lower manufacturing cost.

[0011] In an optional implementation manner, the common bridge arm and the port bridge arm both include: a plurality of cascade-connected half-bridge sub-modules and / or full-bridge sub-modules.

[0012] In an optional embodiment, the common bridge arm and the port bridge arm further include: a bridge arm reactor; the bridge arm reactors of the port bridge arm and the common bridge arm are configured according to actual needs; some port bridge arms and the common bridge arm may not be configured with bridge arm reactors.

[0013] In an optional embodiment, the output voltage DC components of the port bridge arms and the common bridge arm at the same position of different phase units have the same value, the AC component amplitudes are the same, and the phases are symmetrical; the output voltage of the output port is the sum of the output voltage of the corresponding port bridge arm and the output voltage of the common bridge arm.

[0014] In an optional embodiment, the controller includes: a basic controller, a system operation controller and a system protection controller, wherein the basic controller is connected to each sub-module, and is used to calculate the average value of the capacitor voltage of each bridge arm sub-module and send it to the system operation controller and the system protection controller; receive the bridge arm voltage control instruction issued by the system operation controller, and process it to obtain the output state of each sub-module, and issue it to each sub-module; receive the protection locking instruction issued by the system protection control, and control the corresponding bridge arm sub-module to complete the locking action; the system operation controller is connected to the basic controller; the system protection controller is connected to the basic controller.

[0015] In a second aspect, the present invention provides a control and protection method for a DC transformer for DC grid interconnection. Based on the DC transformer for DC grid interconnection of the first aspect, the method includes: controlling the voltage or current of each port, the overall energy balance of the DC transformer for DC grid interconnection, and the energy balance between each bridge arm by controlling the operating state of the port bridge arm; real-time detection of each port voltage and the average value of the bridge arm sub-module capacitor voltage, and when the real-time detection value is abnormal, executing the corresponding protection mechanism.

[0016] In an optional embodiment, the process of controlling the voltage or current of each port, the overall energy balance of the DC transformer for DC grid interconnection, and the energy balance between the bridge arms by controlling the operating state of the port bridge arm includes: dividing the port bridge arm into a controllable port bridge arm and a balanced port bridge arm according to the characteristics of the DC grid to which each port bridge arm is connected; giving a DC component control value of the common bridge arm; detecting the AC voltage at the common point, subtracting it from the corresponding reference value to obtain a first error value, and the first error value is calculated by the system operation controller to obtain the AC component control value of the common bridge arm; detecting the port voltage or current, subtracting it from the corresponding reference value to obtain a second error value, and the second error value is calculated by the system operation controller to obtain the DC component control value of the controllable port bridge arm. Detect the capacitor voltages of all bridge arm submodules, obtain the overall energy value of the DC transformer through calculation, and subtract the third error value from the corresponding reference value to obtain the DC component control value of the balanced port bridge arm; detect the capacitor voltages of all bridge arm submodules, obtain the energy value of each port bridge arm through calculation, and subtract the fourth error value from the overall energy value of the DC transformer to obtain the AC component control value of each port bridge arm through the system operation controller; add the DC component control value and the AC component control value of each bridge arm to obtain the voltage control value of each bridge arm, and send it to the basic controller. The basic controller controls the switching of the bridge arm submodules according to the voltage control value of each bridge arm for output.

[0017] In an optional implementation, the process of executing the port overvoltage protection mechanism includes: detecting the voltage of each port; and when the port voltage exceeds a first preset protection threshold, locking the port bridge arm connected to the port.

[0018] In an optional embodiment, the process of executing the port submodule overvoltage protection mechanism includes: detecting the average value of the capacitor voltage of each bridge arm submodule; when the average value of the capacitor voltage of the bridge arm submodule exceeds a second preset protection threshold, locking the bridge arm.

[0019] In an optional embodiment, the process of executing the extremely asymmetric protection mechanism includes: dividing the port bridge arms into controllable port bridge arms and balanced port bridge arms according to the parameters of the DC power grid to which each port bridge arm is connected; detecting the voltage of each port and calculating the positive and negative difference of the voltage of each port; when the positive and negative difference of the port voltage corresponding to the balanced port bridge arm exceeds a third preset protection threshold, locking all bridge arms; when the positive and negative difference of the port voltage corresponding to the controllable port bridge arm exceeds a fourth protection threshold, locking the controlled port bridge arm. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG1 is a topological diagram of a DC transformer for interconnecting DC power grids according to an embodiment of the present invention;

[0021] FIG2 is a topological structure diagram of a positive converter or a negative converter according to an embodiment of the present invention;

[0022] FIG3 is a topological diagram of a positive converter and a negative converter according to an embodiment of the present invention;

[0023] FIG4( a ) and FIG4 ( b ) are topological diagrams of a half-bridge submodule and a full-bridge submodule, respectively, according to an embodiment of the present invention;

[0024] 5 is a schematic flow chart of a method for controlling and protecting a DC transformer for DC grid interconnection according to an embodiment of the present invention;

[0025] FIG6 is a schematic diagram of a flow chart of controlling energy balance according to an embodiment of the present invention;

[0026] 7 is a schematic diagram of a flow chart of an implementation of a port overvoltage protection mechanism according to an embodiment of the present invention;

[0027] 8 is a schematic diagram of a flow chart of an execution port submodule overvoltage protection mechanism according to an embodiment of the present invention;

[0028] FIG9 is a schematic diagram of a process for executing an extremely asymmetric protection mechanism according to an embodiment of the present invention;

[0029] FIG10 is a structural diagram illustrating an example of a DC transformer for interconnecting DC power grids according to an embodiment of the present invention. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative efforts shall fall within the scope of protection of the present invention.

[0031] According to an embodiment of the present invention, a DC transformer for DC grid interconnection is provided, as shown in Figure 1, including a positive converter, a negative converter and a controller. As shown in Figure 2, the positive converter and the negative converter both include: multiple phase units 11, each phase unit 11 includes: a common bridge arm 111 and multiple port bridge arms 112. Figure 2 takes two phase units 11, each phase unit 11 including a common bridge arm 111 and three port bridge arms 112 as an example.

[0032] As shown in FIG2 , the first end of each port bridge arm 112 is connected to the first end of the common bridge arm 111 at a common point, and the second end of each port bridge arm 112 is connected to the second end of the port bridge arm 112 at the same position of other phase units 11 to lead out the output port.

[0033] Specifically, the output port and the second end of the common bridge arm 111 input a DC voltage or output a DC voltage. The output voltage of the output port is the sum of the output voltage of the corresponding port bridge arm 112 and the output voltage of the common bridge arm 111. As shown in FIG2 , the input or output DC voltage of the three ports can be a positive voltage or a negative voltage, which is not limited here.

[0034] As shown in Figures 1 and 3, the second ends of each common bridge arm 111 are interconnected and then grounded; each output port is connected to a DC grid. In this embodiment, each port is connected to a DC grid, thereby ensuring that a single grid failure does not affect other grids.

[0035] Specifically, this embodiment can expand the number of controllable port bridge arms 112 based on system needs to connect multiple DC grids. Figure 3 uses one positive converter and one negative converter as an example, with each port connected to a DC grid. However, this embodiment can also include multiple positive converters and one negative converter, with each positive converter connected to a DC grid; or it can include one positive converter and multiple negative converters, with each negative converter connected to a DC grid.

[0036] Specifically, the controller is connected to each port bridge arm 112 and each common bridge arm 111 , and the controller is used to control the operating status of each port bridge arm 112 and each common bridge arm 111 .

[0037] Optionally, the controller of this embodiment includes: a basic controller, a system operation controller, and a system protection controller. The basic controller is connected to each submodule via a communication line, the system operation controller is connected to the basic controller via a communication line, and the system protection controller is connected to the basic controller via a communication line.

[0038] Specifically, the basic controller receives arm voltage control commands from the system operation controller, processes them to obtain the output status of each submodule, and then transmits them to each submodule. The basic controller also receives protection blocking commands from the system protection controller and controls the corresponding arm submodule to complete the blocking action. The basic controller calculates the average capacitor voltage of each arm submodule and sends this value to the system operation controller and the system protection controller.

[0039] Specifically, the output voltages of the port bridge arms 112 and the common bridge arm 111 at the same position of the different phase units 11 have the same DC component value, the same AC component amplitude, and symmetrical phases.

[0040] Optionally, the arm reactors of the port bridge arm 112 and the common bridge arm 111 are configured according to actual needs, and some port bridge arms 112 and the common bridge arm 111 may not be configured with arm reactors.

[0041] In some optional embodiments, the common bridge arm 111 and the port bridge arm 112 both include: multiple cascade-connected half-bridge sub-modules HBSM and / or full-bridge sub-modules FBSM, and the topological structures of the half-bridge sub-module HBSM and the full-bridge sub-module FBSM are shown in Figures 4(a) and 4(b).

[0042] Optionally, the common bridge arm 111 and the port bridge arm 112 both include: multiple cascade-connected submodules, which are half-bridge submodules, full-bridge submodules or other types of power semiconductor modules, and can output positive capacitor voltage, zero voltage or negative capacitor voltage.

[0043] Optionally, this embodiment includes a system measurement component that measures, including but not limited to, the common point voltage, the currents of each port bridge arm 112 and the common bridge arm 111, and the output port current and voltage. The system measurement component is connected to the basic controller, the system operation controller, and the system protection controller to transmit measurement information.

[0044] Specifically, the system operation controller generates a bridge arm voltage control instruction based on the measured values ​​of the port voltage and current. The system protection controller determines and identifies the fault based on the measurement results of the system measurement component and generates a protection lockout instruction.

[0045] This embodiment provides a control and protection method for a DC transformer for DC grid interconnection. The method is based on the DC transformer for DC grid interconnection in the above embodiment, as shown in FIG5 , and includes:

[0046] Step S1: By controlling the operating state of the port bridge arm, the voltage or current of each port, the overall energy balance of the DC transformer for DC grid interconnection, and the energy balance between each bridge arm are controlled.

[0047] Specifically, as shown in FIG6 , step S1 of this embodiment is specifically performed by the following steps:

[0048] Step S11: dividing the port bridge arm 112 into a controllable port bridge arm and a balanced port bridge arm according to the characteristics of the DC grid to which each port bridge arm 112 is connected;

[0049] Step S12: setting a common bridge arm DC component control value;

[0050] Step S13: detecting the AC voltage at the common point, subtracting the voltage from the corresponding reference value to obtain a first error value, and calculating the first error value through the system operation controller to obtain the common bridge arm AC component control value;

[0051] Step S14: detecting the port voltage or current, and subtracting the corresponding reference value from the detected port voltage or current to obtain a second error value. The second error value is calculated by the system operation controller to obtain a DC component control value of the controllable port bridge arm.

[0052] Step S15: Detecting the capacitor voltages of all bridge arm submodules, calculating and obtaining the overall energy value of the DC transformer, subtracting the energy value from the corresponding reference value to obtain a third error value, and performing a controller operation on the third error value to obtain a DC component control value of the balanced port bridge arm;

[0053] Step S16: Detecting the capacitor voltages of all bridge arm submodules, calculating and obtaining the energy value of each bridge arm 112, subtracting the energy value from the overall energy value of the DC transformer to obtain a fourth error value, and calculating the fourth error value through the system operation controller to obtain the AC component control value of each bridge arm;

[0054] Step S17: Add the DC component control value and the AC component control value of each bridge arm to obtain the voltage control value of each bridge arm, and send it to the basic controller. The basic controller controls the switching of the bridge arm submodules according to the voltage control value of each bridge arm to output.

[0055] Specifically, taking Figure 10 as an example, if port #1 is connected to DC grid #1, port #2 is connected to DC grid #2, and port #3 is connected to DC grid #3, there are converter #1 and converter #2 in DC grid #1, converter #1 controls the voltage, converter #2 controls its own output power, converter #3 in DC grid #2 controls its own output power, and converter #4 in DC grid #3 controls the voltage. According to the principle: a DC grid has and can only have one converter control the voltage, the port bridge arm corresponding to port #2 is the controllable port bridge arm, which controls the port #2 voltage, the port bridge arm corresponding to one of ports #1 and port #3 is the balanced port bridge arm, and the port bridge arm corresponding to the other port is the controllable port bridge arm, which controls the port current.

[0056] Step S2: Detect the voltage of each port and the average voltage of the bridge arm submodule capacitor in real time. When the real-time detection value is abnormal, execute the corresponding protection mechanism.

[0057] Optionally, the hardware configuration method of the protection mechanism of this embodiment is: the controllable port bridge arm submodule is configured as a full-bridge submodule or a mixed series connection of a full-bridge submodule and a half-bridge submodule; an inductor is installed at the port; the common connection point between the positive and negative converters is connected to the ground through a small resistor; the bridge arm inductance value is increased; and a controllable DC energy consumption device is configured at the corresponding port of the controllable port bridge arm.

[0058] Specifically, the protection mechanisms of this embodiment include a port overvoltage protection mechanism, a port submodule overvoltage protection mechanism, and an extremely asymmetric protection mechanism. The execution steps of the three protection mechanisms are as follows:

[0059] As shown in Figure 7, the process of implementing the port overvoltage protection mechanism includes:

[0060] Step S211: detecting the voltage of each port;

[0061] Step S212: When the port voltage exceeds a first preset protection threshold, the port bridge arm 112 connected to the port is locked.

[0062] As shown in FIG8 , the process of executing the overvoltage protection mechanism of the port submodule includes:

[0063] Step S221: detecting the average value of the capacitor voltage of each bridge arm submodule;

[0064] Step S222: When the average value of the capacitor voltage of the bridge arm submodule exceeds a second preset protection threshold, the bridge arm is locked.

[0065] As shown in Figure 9, the process of implementing the extremely asymmetric protection mechanism includes:

[0066] Step S231: dividing the port bridge arm 112 into a controllable port bridge arm and a balanced port bridge arm according to the parameters of the DC grid to which each port bridge arm 112 is connected;

[0067] Step S232: Detect the voltage of each port and calculate the positive and negative voltage differences of each port;

[0068] Step S233: When the positive and negative voltage difference of the balanced port bridge arm exceeds the third preset protection threshold, all bridge arms are locked; when the positive and negative voltage difference of the controllable port bridge arm exceeds the fourth protection threshold, the controllable port bridge arm 112 is locked.

[0069] Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention. Such modifications and variations are all within the scope defined by the appended claims. Industrial Applicability

[0070] This invention proposes a DC transformer for DC grid interconnection and its control and protection method. This design utilizes only multiple phase units to form a unipolar converter, without an AC isolation transformer, and uses a small number of power semiconductors. This design offers a low cost and addresses the issue of multi-winding AC isolation transformers requiring a large number of components, resulting in high converter costs and severe device losses and heat generation.

Claims

1. A DC transformer for DC grid interconnection, comprising a positive converter, a negative converter and a controller, wherein the positive converter and the negative converter both comprise: Multiple phase units, each phase unit includes: a common bridge arm and multiple port bridge arms, wherein: The first end of each port bridge arm is connected to the first end of the common bridge arm at a common point, and the second end of each port bridge arm is connected to the second end of the port bridge arm at the same position of other phase units to lead out an output port; The second end of each common bridge arm is interconnected and then grounded; Connection of each output port to a DC grid; The controller is connected to each port bridge arm and each common bridge arm, and is used to control the operating state of each port bridge arm and each common bridge arm.

2. The DC transformer for DC grid interconnection according to claim 1, wherein: The common bridge arm and the port bridge arm both include: a plurality of half-bridge sub-modules and / or full-bridge sub-modules connected in cascade.

3. The DC transformer for DC grid interconnection according to claim 1, wherein: The common bridge arm and the port bridge arm further include: a bridge arm reactor; The bridge arm reactors of the port bridge arm and the common bridge arm are configured according to actual needs; Some port bridge arms and common bridge arms may not be equipped with bridge arm reactors.

4. The DC transformer for interconnecting a DC power grid according to any one of claims 1 to 3, wherein: The output voltage DC component values ​​of the port bridge arms and the common bridge arms at the same position of different phase units are the same, and the AC component amplitudes are the same and the phases are symmetrical; The output voltage of the output port is the sum of the output voltage of the bridge arm of the corresponding port and the output voltage of the common bridge arm.

5. The DC transformer for interconnecting a DC power grid according to any one of claims 1 to 4, wherein: The controller includes: a basic controller, a system operation controller and a system protection controller, wherein: The basic controller is connected to each submodule and is used to calculate the average value of the capacitor voltage of each bridge arm submodule. Send to the system operation controller and system protection controller; receive the bridge arm voltage control command issued by the system operation controller, process and obtain the output state of each submodule, and issue it to each submodule; receive the protection locking command issued by the system protection control, and control the corresponding bridge arm submodule to complete the locking action; A system operation controller connected to the basic controller; The system protection controller is connected with the basic controller.

6. A control and protection method for a DC transformer for DC grid interconnection, based on the DC transformer for DC grid interconnection according to claim 5, the method comprising: By controlling the operating state of the port bridge arm, the voltage or current of each port, the overall energy balance of the DC transformer for DC grid interconnection, and the energy balance between each bridge arm are controlled; The voltage of each port and the average voltage of the bridge arm submodule capacitor are detected in real time. When the real-time detection value is abnormal, the corresponding protection mechanism is executed.

7. The control and protection method for a DC transformer for DC grid interconnection according to claim 6, wherein: The process of controlling the voltage or current of each port, the overall energy balance of the DC transformer for DC grid interconnection, and the energy balance between the bridge arms by controlling the operating state of the port bridge arm includes: According to the characteristics of the DC power grid to which each port bridge arm is connected, the port bridge arm is divided into a controllable port bridge arm and a balanced port bridge arm; Given a common bridge arm DC component control value; Detecting the AC voltage at the common point, subtracting the voltage from the corresponding reference value to obtain a first error value, and calculating the first error value through a system operation controller to obtain a common bridge arm AC component control value; Detect the port voltage or current, subtract the corresponding reference value from it to obtain a second error value, and calculate the second error value through the system operation controller to obtain the control value of the DC component of the controllable port bridge arm; Detecting the capacitor voltages of all bridge arm submodules, obtaining the overall energy value of the DC transformer through calculation, subtracting the energy value from the corresponding reference value to obtain a third error value, and the system operation controller of the third error value calculates the DC component control value of the balanced port bridge arm; Detect the capacitor voltage of all bridge arm submodules, calculate the bridge arm energy value of each port, subtract it from the overall energy value of the DC transformer to obtain a fourth error value, and the fourth error value is controlled by the system operation. The controller calculates and obtains the AC component control value of each port bridge arm; The DC component control value and AC component control value of each bridge arm are added together to obtain the voltage control value of each bridge arm, which is sent to the basic controller. The basic controller controls the switching of the bridge arm submodules according to the voltage control value of each bridge arm for output.

8. The control and protection method for a DC transformer for DC grid interconnection according to claim 6 or 7, wherein: The process of implementing the port overvoltage protection mechanism includes: Detect the voltage of each port; When the port voltage exceeds a first preset protection threshold, the port bridge arm connected to the port is locked.

9. The control and protection method for a DC transformer for DC grid interconnection according to any one of claims 6 to 8, wherein: The process of executing the overvoltage protection mechanism of the port submodule includes: Detect the average value of the capacitor voltage of each bridge arm submodule; When the average value of the capacitor voltage of the bridge arm submodule exceeds the second preset protection threshold, the bridge arm is locked.

10. The control and protection method for a DC transformer for DC grid interconnection according to any one of claims 6 to 9, wherein: The process of implementing the extremely asymmetric protection mechanism includes: According to the parameters of the DC power grid to which each port bridge arm is connected, the port bridge arm is divided into a controllable port bridge arm and a balanced port bridge arm; Detect the voltage of each port and calculate the positive and negative voltage difference of each port; When the positive and negative voltage difference of the corresponding port arm of the balanced port exceeds the third preset protection threshold, all bridge arms are locked; when the positive and negative voltage difference of the corresponding port arm of the controllable port exceeds the fourth protection threshold, the controllable port bridge arm is locked.

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