A dc transformer for dc grid interconnection and a control and protection method thereof

Abstract

This invention relates to the field of flexible DC transmission technology, and discloses a DC transformer for DC grid interconnection and its control and protection method. The transformer includes a positive converter, a negative converter, and a controller. Both the positive and negative converters include multiple phase units. Each phase unit includes a common bridge arm and multiple port bridge arms. 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 a port bridge arm at the same position in another phase unit, and then leads out an output port. The output port receives or outputs DC voltage from the second end of the common bridge arm. This invention uses only multiple phase units to form a single-pole converter, without an AC isolation transformer, and uses a small number of power semiconductors, resulting in lower manufacturing costs.

Description

Technical Field

[0001] This invention relates to the field of flexible DC power transmission technology, specifically to a DC transformer for DC grid interconnection and its control and protection method. Background Technology

[0002] Compared to traditional AC transmission, DC transmission technology has advantages such as longer transmission distance and larger transmission capacity. Flexible DC transmission technology, with voltage source converters as its core equipment, is flexible and controllable, and adaptable to weak power grids, playing an important role in renewable energy grid connection and transmission scenarios.

[0003] Flexible DC transmission has evolved from point-to-point transmission systems to multi-terminal DC grids at a single voltage level. To achieve wide-area interconnection and mutual support between DC grids, leverage the advantages of DC grids at different voltage levels, and accommodate the integration of a high proportion of diverse renewable energy sources, multi-voltage-level DC grids are the future development trend. Therefore, a converter capable of interconnecting DC grids at multiple voltage levels is needed.

[0004] Existing technologies commonly use multi-winding AC isolation transformers to implement multi-port DC transformers. However, this structure requires an excessive number of components, and the isolation capacity of the isolation transformer needs to be higher than the system voltage level. This results in high cost, significant component losses and heat generation, and large size, making it difficult to apply to high-voltage systems. Therefore, existing DC transformers are only used in low-voltage distribution networks. Thus, there is an urgent need for a DC transformer suitable for high-voltage DC grid interconnection scenarios. Summary of the Invention

[0005] In view of this, the present invention provides a DC transformer for DC grid interconnection and its control and protection method to solve the problem that multi-winding AC isolation transformers require an excessive number of components, resulting in high converter costs and severe device losses and heat generation.

[0006] 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. Both the positive and negative converters include multiple phase units, each phase unit comprising a common bridge arm and multiple port bridge arms. 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 a port bridge arm at the same position in other phase units, and then leads out an output port. The second ends of each common bridge arm are 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.

[0007] This invention uses only multiple phase units to form a single-pole converter, without an AC isolation transformer, and uses a small number of power semiconductors. It has a lower manufacturing cost.

[0008] In one alternative implementation, both the common bridge arm and the port bridge arm include: multiple cascaded half-bridge sub-modules and / or full-bridge sub-modules.

[0009] In one 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.

[0010] In one optional embodiment, the DC component values ​​of the output voltages of the port arms and the common arm 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 its corresponding port arm and the output voltage of the common arm.

[0011] In one optional embodiment, the controller includes: a basic controller, a system operation controller, and a system protection controller. The basic controller is connected to each submodule and is used to calculate the average capacitor voltage of each bridge arm submodule and send it to the system operation controller and the system protection controller; receive bridge arm voltage control commands from the system operation controller and process them to obtain the output status of each submodule, then issue these commands to each submodule; receive protection interlocking commands from the system protection controller and control the corresponding bridge arm submodule to complete the interlocking action; the system operation controller is connected to the basic controller; and the system protection controller is connected to the basic controller.

[0012] Secondly, the present invention provides a control and protection method for a DC transformer used for DC grid interconnection, based on the DC transformer used for DC grid interconnection in the first aspect. The method includes: controlling the operating status of the control port bridge arm, controlling the voltage or current of each port, the overall energy balance of the DC transformer used for DC grid interconnection, and the energy balance between each bridge arm; detecting the average value of the voltage of each port and the capacitor voltage of the bridge arm submodule in real time, and executing the corresponding protection mechanism when the real-time detected value is abnormal.

[0013] In one optional embodiment, the process of controlling the voltage or current of each port arm, the overall energy balance of the DC transformer used for DC grid interconnection, and the energy balance between each arm by controlling the operating state of the port arm includes: dividing the port arm into controllable port arms and balanced port arms according to the characteristics of the DC grid to which each port arm is connected; giving a control value for the DC component of the common arm; detecting the AC voltage at the common point and subtracting it from its corresponding reference value to obtain a first error value, the first error value being calculated by the system operation controller to obtain the control value for the AC component of the common arm; detecting the port voltage or current and subtracting it from its corresponding reference value to obtain a second error value, the second error value being calculated by the system operation controller to obtain the controllable port arm DC component control value. The system calculates the DC component control value for each bridge arm by detecting the capacitor voltage of all bridge arm submodules and calculating the overall energy value of the DC transformer. This value is then subtracted from the corresponding reference value to obtain the third error value. The system controller calculates the third error value to obtain the DC component control value for the balanced port bridge arm. The system also calculates the AC component control value for each bridge arm by detecting the capacitor voltage of all bridge arm submodules and calculating the energy value of each port bridge arm. This fourth error value is then subtracted from the overall energy value of the DC transformer to obtain the fourth error value. The system controller calculates the AC component control value for each port bridge arm by using the fourth error value. Finally, the system adds the DC component control value and the AC component control value of each bridge arm to obtain the voltage control value for each bridge arm. This value is then sent to the basic controller, which controls the switching of the bridge arm submodules according to the voltage control value for each bridge arm for output.

[0014] In one optional implementation, the process of implementing the port overvoltage protection mechanism includes: detecting the voltage of each port; and when the port voltage exceeds a first preset protection threshold, blocking the port bridge arm connected to that port.

[0015] In one optional implementation, the process of implementing the port submodule overvoltage protection mechanism includes: detecting the average value of the capacitor voltage of each bridge arm submodule; and locking the bridge arm when the average value of the capacitor voltage of the bridge arm submodule exceeds a second preset protection threshold.

[0016] In one optional implementation, the process of implementing the asymmetric protection mechanism includes: dividing the port arms into controllable port arms and balanced port arms based on the parameters of the DC grid to which each port arm is connected; detecting the voltage of each port and calculating the positive and negative voltage difference of each port; blocking all arms when the positive and negative voltage difference of the port corresponding to the balanced port arm exceeds a third preset protection threshold; and blocking the controllable port arm when the positive and negative voltage difference of the port corresponding to the controllable port arm exceeds a fourth protection threshold. Attached Figure Description

[0017] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a topology diagram of a DC transformer for DC grid interconnection according to an embodiment of the present invention;

[0019] Figure 2 This is a topology diagram of a positive or negative converter according to an embodiment of the present invention;

[0020] Figure 3 This is a topology diagram of the positive and negative converters according to an embodiment of the present invention;

[0021] Figures 4(a) and 4(b) are topology diagrams of the half-bridge submodule and the full-bridge submodule according to embodiments of the present invention, respectively.

[0022] Figure 5 This is a flowchart illustrating the control and protection method for a DC transformer used in DC grid interconnection according to an embodiment of the present invention.

[0023] Figure 6 This is a schematic diagram of the process for controlling energy balance according to an embodiment of the present invention;

[0024] Figure 7 This is a schematic flowchart of the execution port overvoltage protection mechanism according to an embodiment of the present invention;

[0025] Figure 8 This is a flowchart illustrating the overvoltage protection mechanism of the execution port submodule according to an embodiment of the present invention;

[0026] Figure 9 This is a flowchart illustrating the execution of an extremely asymmetric protection mechanism according to an embodiment of the present invention.

[0027] Figure 10 This is a structural example diagram of a DC transformer for DC grid interconnection according to an embodiment of the present invention. Detailed Implementation

[0028] 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, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] According to an embodiment of the present invention, a DC transformer for interconnecting DC power grids is provided, such as... Figure 1 As shown, it includes a positive converter, a negative converter, and a controller, such as... Figure 2 As shown, both the positive and negative converters include multiple phase units 11, and each phase unit 11 includes a common bridge arm 111 and multiple port bridge arms 112. Figure 2 Taking two phase units 11 as an example, each phase unit 11 includes a common bridge arm 111 and three port bridge arms 112.

[0030] like Figure 2 As shown, 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. After 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, the output port is led out.

[0031] Specifically, the output port receives or outputs a DC voltage at the second terminal of the common bridge arm 111, and the output voltage of the output port is the sum of the output voltage of its corresponding bridge arm 112 and the output voltage of the common bridge arm 111. For example... Figure 2 As shown, the DC voltage at the input or output of the three ports can be either positive or negative, and no restrictions are imposed here.

[0032] like Figure 1 , 3 As shown, the second end of each common bridge arm 111 is interconnected and grounded; each output port is connected to a DC power grid. In this embodiment, each port is connected to a DC power grid, thereby ensuring that a single power grid failure does not affect other power grids.

[0033] Specifically, this embodiment can expand the number of controllable port bridge arms 112 according to system needs to connect multiple DC power grids. Figure 3 Taking a positive converter and a negative converter as an example, each port is connected to a DC power grid. However, it may also include multiple positive converters and a negative converter, with each positive converter and the negative converter connected to a DC power grid; or it may include a positive converter and multiple negative converters, with each negative converter and the positive converter connected to a DC power grid.

[0034] 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.

[0035] Optionally, the controller in 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.

[0036] Specifically, the basic controller receives bridge arm voltage control commands from the system operation controller, processes and obtains the output status of each submodule, and issues commands to each submodule. The basic controller also receives protection interlocking commands from the system protection controller and controls the corresponding bridge arm submodule to complete the interlocking action. Finally, the basic controller calculates the average capacitor voltage of each bridge arm submodule and sends it to the system operation controller and the system protection controller.

[0037] Specifically, the DC component output voltage of the port arms 112 and the common arm 111 at the same position of different phase units 11 has the same value, and the AC component amplitude is the same and the phase is symmetrical.

[0038] Optionally, the bridge arm reactors of port arm 112 and common bridge arm 111 may be configured according to actual needs, and some port arms 112 and common bridge arms 111 may not be configured with bridge arm reactors.

[0039] In some alternative implementations, both the common bridge arm 111 and the port bridge arm 112 include multiple cascaded half-bridge submodules HBSN and / or full-bridge submodules FBSM, the topology of which is shown in Figures 4(a) and 4(b).

[0040] Optionally, both the common bridge arm 111 and the port bridge arm 112 include multiple cascaded sub-modules, which are half-bridge sub-modules, full-bridge sub-modules, or other types of power semiconductor modules, capable of outputting positive capacitor voltage, zero voltage, or negative capacitor voltage.

[0041] Optionally, this embodiment includes a system measurement component to measure, but is not limited to, the voltage at the common point, the current in each port arm 112 and the common arm 111, and the current and voltage at the output ports. The system measurement component is connected to the basic controller, the system operation controller, and the system protection controller to transmit the measurement information.

[0042] Specifically, the system operation controller generates bridge arm voltage control commands based on the measured values ​​of port voltage and current. The system protection controller identifies faults based on the measurement results of the system measurement components and generates protection lockout commands.

[0043] This embodiment provides a control and protection method for a DC transformer used in DC grid interconnection. Based on the DC transformer used in the above embodiment for DC grid interconnection, such as... Figure 5 As shown, the method includes:

[0044] Step S1: By controlling the operating status of the control port arm, control the voltage or current of each port, the overall energy balance of the DC transformer used for DC grid interconnection, and the energy balance between each arm.

[0045] Specifically, such as Figure 6 As shown, step S1 in this embodiment is specifically performed by the following steps:

[0046] Step S11: Based on the characteristics of the DC power grid connected to each port arm 112, the port arm 112 is divided into controllable port arms and balanced port arms.

[0047] Step S12: Given the control value of the DC component of the common arm;

[0048] Step S13: Detect the AC voltage at the common point and subtract it from the corresponding reference value to obtain the first error value. The first error value is then processed by the system operation controller to obtain the AC component control value of the common arm.

[0049] Step S14: Detect the port voltage or current, subtract it from the corresponding reference value to obtain the second error value, and use the second error value to calculate the controllable port arm DC component control value through the system operation controller;

[0050] Step S15: Detect the capacitor voltage of all bridge arm submodules, calculate the overall energy value of the DC transformer, subtract it from the corresponding reference value to obtain the third error value, and use the third error value to calculate the DC component control value of the balanced port bridge arm.

[0051] Step S16: Detect the capacitor voltage of all bridge arm submodules, calculate the energy value of each port bridge arm 112, subtract the overall energy value of the DC transformer to obtain the fourth error value, and use the fourth error value to calculate the AC component control value of each port bridge arm through the system operation controller.

[0052] Step S17: Add the DC component control value and 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 sub-modules for output according to the voltage control value of each bridge arm.

[0053] Specifically, with Figure 10For 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, DC grid #1 contains converter #1 and converter #2. Converter #1 controls the voltage, and 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 that a DC grid has one and only one converter controlling the voltage, the port arm corresponding to port #2 is a controllable port arm, controlling the voltage of port #2. The port arm corresponding to one of the ports, port #1 and port #3, is a balanced port arm, and the port arm corresponding to the other port is a controllable port arm, controlling the port current.

[0054] Step S2: Real-time detection of the average voltage of each port and the capacitor voltage of the bridge arm submodule. When the real-time detection value is abnormal, the corresponding protection mechanism is executed.

[0055] Optionally, the hardware configuration method of the protection mechanism in this embodiment is as follows: the controllable port bridge arm submodule is configured as a full-bridge submodule or a mixture of a full-bridge submodule and a half-bridge submodule connected in series; a reactor 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 inductance value of the bridge arm is increased; and a controllable DC power consumption device is configured at the corresponding port of the controllable port bridge arm.

[0056] Specifically, the protection mechanisms in this embodiment include a port overvoltage protection mechanism, a port submodule overvoltage protection mechanism, and a polarity protection mechanism. The execution steps of the three protection mechanisms are as follows:

[0057] like Figure 7 As shown, the process of implementing the port overvoltage protection mechanism includes:

[0058] Step S211: Detect the voltage at each port;

[0059] Step S212: When the port voltage exceeds the first preset protection threshold, the port bridge arm 112 connected to the port is locked.

[0060] like Figure 8 As shown, the process of executing the overvoltage protection mechanism of the port submodule includes:

[0061] Step S221: Detect the average capacitor voltage of each bridge arm submodule;

[0062] Step S222: When the average voltage of the bridge arm submodule capacitor exceeds the second preset protection threshold, the bridge arm is locked.

[0063] like Figure 9 As shown, the process of executing a highly asymmetric protection mechanism includes:

[0064] Step S231: Based on the parameters of the DC power grid connected to each port arm 112, divide the port arm 112 into controllable port arms and balanced port arms.

[0065] Step S232: Detect the voltage at each port and calculate the positive and negative voltage range of each port;

[0066] Step S233: When the positive and negative voltage difference of the corresponding port arm of the balanced port exceeds the third preset protection threshold, all ports 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 arm 112 is locked.

[0067] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A DC transformer for interconnecting DC power grids, characterized in that, The system includes a positive converter, a negative converter, and a controller. Both the positive and negative converters comprise multiple phase units, and each phase unit includes a common bridge arm and multiple port bridge arms. 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, and then the output port is led out. The second end of each common bridge arm is interconnected and then grounded; Each output port is connected to a DC power grid; The controller is connected to each port arm and each common arm, and is used to control the operating status of each port arm and each common arm; The common bridge arm and the port bridge arm further include: a bridge arm reactor; The DC component values ​​of the output voltages of the port arms and the common arm 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 its corresponding port arm and the output voltage of the common arm. The controller includes: a basic controller, a system operation controller, and a system protection controller.

2. The DC transformer for DC grid interconnection according to claim 1, characterized in that, Both the common bridge arm and the port bridge arm include: multiple cascaded half-bridge sub-modules and / or full-bridge modules.

3. The DC transformer for DC grid interconnection according to claim 2, characterized in that, The basic controller, connected to each submodule, is used to calculate the average capacitor voltage of each bridge arm submodule and send it to the system operation controller and system protection controller; it receives the bridge arm voltage control command issued by the system operation controller, processes and obtains the output status of each submodule, and issues it to each submodule. Receive protection interlocking commands from the system protection control and control the corresponding bridge arm submodule to complete the interlocking action; The system operation controller is connected to the basic controller; The system protection controller is connected to the basic controller.

4. A control and protection method for a DC transformer used in DC grid interconnection, characterized in that, Based on the DC transformer for DC grid interconnection as described in claim 3, the method includes: By controlling the operating status of the control port arms, the voltage or current of each port, the overall energy balance of the DC transformer used for DC grid interconnection, and the energy balance between each arm are controlled. The system monitors the average voltage of each port and the capacitor voltage of the bridge arm submodule in real time. When the real-time detected value is abnormal, the corresponding protection mechanism is executed.

5. The control and protection method for DC transformers used in DC grid interconnection according to claim 4, characterized in that, The process of controlling the operating status of the control port arms, controlling the voltage or current of each port, the overall energy balance of the DC transformer used for DC grid interconnection, and the energy balance between each arm includes: Based on the characteristics of the DC power grid to which each port arm is connected, the port arms are divided into controllable port arms and balanced port arms. Given the control value for the DC component of the common arm; The AC voltage at the common point is detected and subtracted from its corresponding reference value to obtain the first error value. The first error value is then processed by the system operation controller to obtain the control value of the AC component of the common arm. The voltage or current at the detected port is subtracted from its corresponding reference value to obtain a second error value. The second error value is then processed by the system operation controller to obtain the DC component control value of the controllable port arm. The capacitor voltages of all bridge arm submodules are detected, and the overall energy value of the DC transformer is calculated. The third error value is obtained by subtracting the corresponding reference value from the third error value. The system operation controller calculates the DC component control value of the balanced port bridge arm based on the third error value. The capacitor voltage of all bridge arm submodules is detected, and the energy value of each bridge arm is calculated. The fourth error value is obtained by subtracting the overall energy value of the DC transformer from the fourth error value. The AC component control value of each bridge arm is obtained by the system operation controller. 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 then sent to the basic controller. The basic controller controls the switching of the bridge arm sub-modules for output according to the voltage control value of each bridge arm.

6. The control and protection method for a DC transformer used in DC grid interconnection according to claim 4, characterized in that, The process of implementing the port overvoltage protection mechanism includes: Detect the voltage at each port; When the port voltage exceeds the first preset protection threshold, the port bridge arm connected to that port is locked.

7. The control and protection method for a DC transformer used in DC grid interconnection according to claim 4, characterized in that, The process of implementing the overvoltage protection mechanism of the port submodule includes: Detect the average capacitor voltage of each bridge arm submodule; When the average voltage of the bridge arm submodule capacitor exceeds the second preset protection threshold, the bridge arm is locked.

8. The control and protection method for a DC transformer used in DC grid interconnection according to claim 4, characterized in that, The process of implementing a highly asymmetric protection mechanism includes: Based on the parameters of the DC grid to which each port arm is connected, the port arms are divided into controllable port arms and balanced port arms. Detect the voltage at each port and calculate the positive and negative voltage range 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 ports 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 arm is locked.

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