A submodule bridge arm suitable for series connection in a DC transmission line and a startup control method thereof

Abstract

A submodule bridge arm suitable for being connected in series in a DC transmission line and a startup control method thereof, relating to the field of DC power flow controllers. The submodule bridge arm is connected in series in a DC transmission line, and the line contains an equivalent series resistor R1, line 1 includes a node bus, a mechanical switch BS1, a bridge arm A1, a mechanical switch BSU, a mechanical switch BSD and a bridge arm series inductor L, the bridge arm A1 is composed of N H-bridge monopole modules connected in series, the H-bridge monopole module includes a capacitor C and two IBGT branches connected in parallel; each IGBT branch includes two IGBT units connected in series, and each IGBT unit is composed of an IBGT chip and a diode connected in reverse parallel; the output end of the i-th H-bridge submodule is connected to the input end of the i+1-th H-bridge submodule; the input end of the H-bridge monopole module SM1 is one end of the bridge arm A1, and the H-bridge monopole module SM N The output end of is the other end of the bridge arm A1. The startup control method includes three stages, which meets the current demand for reliable and stable operation of the high-voltage direct current transmission system.

Description

Technical Field

[0001] The invention relates to the field of direct current power flow controllers. Background Art

[0002] With the rapid development of high-voltage direct current transmission technology, multi-terminal direct current transmission systems with the characteristics of multi-power supply and multi-point power receiving have been widely used. The current flow controller commonly used in direct current grids is a voltage-type direct current flow controller, which has the advantages of low voltage, small capacity, and small loss. The patent application publication number CN104009463A mentions a series flow controller suitable for direct current transmission; the flow controller adopts a topological structure of a cascade of a bidirectional DC / DC converter and an H-bridge converter; the flow controller is connected in series in the positive and negative lines respectively, and mainly adjusts the value and direction of the voltage difference ΔV1 between the A1 and B1 terminals and the voltage difference ΔV2 between the A2 and B2 terminals to adjust the flow in the line; although this flow controller can meet the flow control function of the direct current transmission network, it has the following shortcomings:

[0003] 1) The voltage on the input side of the power flow controller is too high, which makes the insulation level of the whole machine higher and the engineering cost of the equipment too high;

[0004] 2) The DC / DC conversion unit of the power flow controller uses a multi-stage converter cascade topology. The efficiency of each stage of the converter will affect the overall efficiency of the whole machine, which will eventually cause excessive losses in the whole machine.

[0005] 3) The DC / DC conversion unit of the power flow controller adopts a bidirectional DC / DC converter cascade H-bridge unit structure, which makes the structure of the DC / DC conversion unit complex and also increases the complexity of debugging and installation.

[0006] Publication No. CN 107039980 A mentions a high-voltage DC power flow controller, which solves the above-mentioned problems, but does not propose a corresponding control method, and the controller cannot operate quickly and stably when in use.

[0007] Therefore, how to ensure the control of the high-voltage direct current transmission system so that it can operate reliably and stably is an urgent problem that needs to be solved. Summary of the invention

[0008] The present invention is to meet the current demand for reliable and stable operation of a high-voltage direct current transmission system, and provides a submodule bridge arm suitable for being connected in series in a direct current transmission line and a startup control method thereof.

[0009] The technical solution adopted by the present invention is:

[0010] A submodule bridge arm suitable for being connected in series in a DC transmission line, the submodule bridge arm is connected in series in the DC transmission line, and the DC transmission line includes an equivalent series resistance R1. Assuming that the DC transmission line is line 1, line 1 includes a node bus, a mechanical switch BS1, a bridge arm A1, a mechanical switch BSU, a mechanical switch BSD and a bridge arm series inductor L,

[0011] The output end of the equivalent series resistor R1 is simultaneously connected to one end of the mechanical switch BS1 and one end of the mechanical switch BSU; the other end of the mechanical switch BS1 is simultaneously connected to the node bus and one end of the mechanical switch BSD;

[0012] Two ends of the bridge arm A1 are connected to the other end of the mechanical switch BSU and the other end of the mechanical switch BSD respectively;

[0013] The bridge arm A1 is composed of N H bridge submodules connected in series, where N is an integer greater than or equal to 1; the N H bridge submodules are H bridge submodules SM1, SM2, SM3, SM4, SM5, SM6, SM7, SM8, SM9, SM10, SM11, SM12, SM13, SM14, SM15, SM16, SM17, SM18, SM N ;

[0014] The H-bridge submodule includes a capacitor C and two IBGT branches connected in parallel; each IGBT branch includes two IGBT units connected in series, and each IGBT unit is composed of an IBGT chip and a diode connected in reverse parallel;

[0015] The emitter of the IBGT chip (S1) and the collector of the IBGT chip (S2) in one IGBT branch are connected as the input end of the H-bridge submodule;

[0016] The emitter of another IGBT branch IBGT chip (S3) and the collector connection of the IBGT chip (S4) serve as the output end of the H-bridge submodule; the output end of the i-th H-bridge submodule is connected to the input end of the i+1-th H-bridge submodule; i∈[1, N-1]; wherein N and i are both integers;

[0017] The input end of the H-bridge submodule SM1 is one end of the bridge arm A1. N The output end of is the other end of the bridge arm A1.

[0018] A startup control method for a submodule bridge arm connected in series in a DC transmission line, the method comprising three stages:

[0019] The first stage: bridge arm charging stage;

[0020] The second stage: commutation process;

[0021] The third stage: capacitor charging stage;

[0022] Phase 4: After the first, second, and third phases are completed, the startup process is completed and the system is ready to enter the operating state as required.

[0023] The bridge arm charging phase method of the bridge arm charging phase is as follows:

[0024] Step 1: Based on a submodule bridge arm suitable for being connected in series in a DC transmission line, before starting the submodule bridge arm, the H-bridge submodule SM in the submodule bridge arm N The capacitor C voltage is set to 0;

[0025] Step 2: Enter the pre-charging process. The pre-charging process is as follows:

[0026] The mechanical switch BS1 is closed, and the current I1 flows through the line 1; the two mechanical switches BSU and the mechanical switch BSD are both opened, and the bridge arm of the submodule is bypassed to prepare for isolated charging;

[0027] Step 3: Enter the isolated charging process: first, charge the capacitor of the H-bridge submodule SM1 in the bridge arm A1 through a DC voltage source.

[0028] The commutation method of the commutation process is as follows:

[0029] Close the mechanical switch BSU and the mechanical switch BSD, and control the H-bridge submodule SM1 to input the negative voltage u A1 , the remaining N-1 H-bridge submodules work in bypass state; at negative voltage u A1 Under the action, the line current I1 gradually switches from the mechanical switch BS1 to the bridge arm A1. When the current I1 flowing through the mechanical switch BS1 BS1 When the voltage drops to 0, that is, when the line current is completely switched to the bridge arm A1, the mechanical switch BS1 is disconnected to complete the commutation.

[0030] The capacitor charging method in the capacitor charging stage is as follows:

[0031] After the mechanical switch BS1 is reliably disconnected, the capacitor voltage control and current control links are enabled, the capacitor voltage control target is set to the rated value, and the current control target is set to the line current. Under the control of the capacitor voltage control and current control links, the N H-bridge submodules SM N After the capacitor is fully charged to the rated value, the startup process is completed and the system can enter the operating state as required.

[0032] Beneficial effect: The present invention proposes a DCPFC (DC-Power Flow Controller) topology and provides a startup control scheme for the DCPFC under the topology. Through the design of the topology structure and the adoption of the corresponding startup control method, the topology structure completes the bridge arm charging stage, the commutation process, and the capacitor charging stage, that is, the startup process is completed and waits to enter the operating state as required. The topology structure and the startup method do a lot of preparatory work before entering the reliable and stable operation stage, that is, the bridge arm charging stage, the commutation process, and the capacitor charging stage, so that the high-voltage direct current transmission system quickly enters the stable operation stage. The submodule bridge arm and its startup control method meet the requirements of reliable and stable operation of the high-voltage direct current transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of a submodule bridge arm suitable for being connected in series in a DC transmission line;

[0034] Figure 2 It is a schematic diagram of the isolation charging stage in the start-up control method of the bridge arm of a submodule connected in series in a DC transmission line (also known as a DCPFC isolation charging stage diagram);

[0035] Figure 3 It is a schematic diagram of the commutation phase in the starting control method of the bridge arm of a submodule connected in series in a DC transmission line (also known as a DCPFC commutation phase diagram);

[0036] Figure 4 It is a schematic diagram of the capacitor charging stage in the starting control method of the bridge arm of a submodule connected in series in a DC transmission line (also known as a DC PFC capacitor charging stage diagram);

[0037] Figure 5 It is the rated voltage and current charging control block diagram;

[0038] Figure 6 It is a schematic diagram of the simulation results of the starting process of a submodule bridge arm starting control method suitable for being connected in series in a DC transmission line;

[0039] Note: Figure 2 , 3 , 4 are all current path diagrams of the line phase circuits during the startup phase. DETAILED DESCRIPTION

[0040] Specific implementation method 1. Reference Figure 1Specifically describe this embodiment, the embodiment described is a submodule bridge arm suitable for being connected in series in a DC transmission line, the submodule bridge arm is connected in series in the DC transmission line, and the DC transmission line includes an equivalent series resistance R1, assuming that the DC transmission line is line 1, line 1 includes a node bus, a mechanical switch BS1, a bridge arm A1, a mechanical switch BSU, a mechanical switch BSD and a bridge arm series inductor L,

[0041] The output end of the equivalent series resistor R1 is simultaneously connected to one end of the mechanical switch BS1 and one end of the mechanical switch BSU; the other end of the mechanical switch BS1 is simultaneously connected to the node bus and one end of the mechanical switch BSD;

[0042] Two ends of the bridge arm A1 are connected to the other end of the mechanical switch BSU and the other end of the mechanical switch BSD respectively;

[0043] The bridge arm A1 is composed of N H bridge submodules connected in series, where N is an integer greater than or equal to 1; the N H bridge submodules are H bridge submodules SM1, SM2, SM3, SM4, SM5, SM6, SM7, SM8, SM9, SM10, SM11, SM12, SM13, SM14, SM15, SM16, SM17, SM18, SM N ;

[0044] The H-bridge submodule includes a capacitor C and two IBGT branches connected in parallel; each IGBT branch includes two IGBT units connected in series, and each IGBT unit is composed of an IBGT chip and a diode connected in reverse parallel;

[0045] The emitter of the IBGT chip (S1) and the collector of the IBGT chip (S2) in one IGBT branch are connected as the input end of the H-bridge submodule;

[0046] The emitter of another IGBT branch IBGT chip (S3) and the collector connection of the IBGT chip (S4) serve as the output end of the H-bridge submodule; the output end of the i-th H-bridge submodule is connected to the input end of the i+1-th H-bridge submodule; i∈[1, N-1]; wherein N and i are both integers;

[0047] The input end of the H-bridge submodule SM1 is one end of the bridge arm A1. N The output end of is the other end of the bridge arm A1.

[0048] Specific implementation method 2: According to the startup control method of the submodule bridge arm connected in series in the DC transmission line described in implementation method 1, in this implementation method, the startup control method of the submodule bridge arm includes three stages:

[0049] The first stage: bridge arm charging stage;

[0050] The second stage: commutation process;

[0051] The third stage: capacitor charging stage;

[0052] Phase 4: After the first, second, and third phases are completed, the startup process is completed and the system is ready to enter the operating state as required.

[0053] Specific implementation method three, this implementation method is a further description of the startup control method of the submodule bridge arm connected in series in the DC transmission line described in the implementation method two. In this implementation method, the bridge arm charging stage method of the bridge arm charging stage is as follows:

[0054] Step 1: Based on a submodule bridge arm suitable for being connected in series in a DC transmission line, before starting the submodule bridge arm, the H-bridge submodule SM in the submodule bridge arm N The capacitor C voltage is set to 0;

[0055] Step 2: Enter the pre-charging process. The pre-charging process is as follows:

[0056] The mechanical switch BS1 is closed, and the current I1 flows through the line 1; the two mechanical switches BSU and the mechanical switch BSD are both opened, and the bridge arm of the submodule is bypassed to prepare for isolated charging;

[0057] Step 3: Enter the isolated charging process: first, charge the capacitor of the H-bridge submodule SM1 in the bridge arm A1 through a DC voltage source.

[0058] Specific implementation mode 4: This implementation mode is a further description of the startup control method of the submodule bridge arm connected in series in the DC transmission line described in the implementation mode 2. In this implementation mode, the commutation method of the commutation process is as follows:

[0059] Close the mechanical switch BSU and the mechanical switch BSD, and control the H-bridge submodule SM1 to input the negative voltage u A1 , the remaining N-1 H-bridge submodules work in bypass state; at negative voltage u A1 Under the action, the line current I1 gradually switches from the mechanical switch BS1 to the bridge arm A1. When the current I1 flowing through the mechanical switch BS1 BS1 When the voltage drops to 0, that is, when the line current is completely switched to the bridge arm A1, the mechanical switch BS1 is disconnected to complete the commutation.

[0060] Specific implementation mode 5: This implementation mode is a further description of the startup control method of the submodule bridge arm connected in series in the DC transmission line described in the implementation mode 2. In this implementation mode, the capacitor charging method in the capacitor charging stage is as follows:

[0061] After the mechanical switch BS1 is reliably disconnected, the capacitor voltage control and current control links are enabled, the capacitor voltage control target is set to the rated value, and the current control target is set to the line current. Under the control of the capacitor voltage control and current control links, the N H-bridge submodules SM N After the capacitor is charged to the rated value, the startup process is completed and the DC power flow control operation state can be entered as required.

[0062] The present invention proposes a DCPFC (DC-Power Flow Controller) topology and provides a start-up control scheme of the DCPFC under the topology.

[0063] A submodule bridge arm suitable for series connection in a DC transmission line, also known as a DC PFC topology, has a structure such as Figure 1 As shown: R1 is the equivalent series resistance of line 1, BS1 is the mechanical switch in line 1, BSU and BSD are two mechanical switches connected in series with bridge arm A1, L is the bridge arm series inductance of bridge arm A1, and bridge arm A1 is composed of N = 10 H bridge sub-modules connected in series. The positive direction of the relevant electrical quantities is as follows Figure 1 As indicated in the markings.

[0064] The startup control method of the submodule bridge arm includes:

[0065] (1) Bridge arm charging stage

[0066] Before starting the proposed DCPFC, the voltage of each submodule capacitor in the bridge arm is 0. In order to put the DCPFC into operation, the submodule capacitor needs to be charged first. The pre-charging process is designed as follows:

[0067] 1) Pre-startup state: Take the start-up process of bridge arm A1 as an example for analysis. The pre-startup state of the proposed DCPFC is as follows: Figure 1 As shown, the mechanical switch BS1 is connected in series in line 1, and two other mechanical switches BS are connected in series with bridge arm A1. Before starting, the mechanical switch BS1 is closed, and current I1 flows through line 1; the two mechanical switches BS are disconnected, and the DCPFC is bypassed to prepare for isolated charging;

[0068] 2) Isolation charging process: First, a submodule capacitor in the bridge arm A1 is charged by a DC voltage source (taking the first submodule A1_1 as an example), such as Figure 2 Because the bridge arm A1 is isolated from the DC grid by the mechanical switch BS, the charging power supply does not need to bear the insulation requirements of the DC grid, and a general DC voltage source can be used.

[0069] (2) Commutation process

[0070] Close the two mechanical switches BS and control the submodule A1_1 to input negative voltage uA1 , the other submodules work in bypass state, such as Figure 3 As shown. At negative voltage u A1 Under the action, the line current I1 gradually switches from the mechanical switch BS1 to the bridge arm A1. When the current I1 flowing through the mechanical switch BS1 BS1 When the line current drops to 0, that is, when the line current is completely switched to the bridge arm A1, the mechanical switch BS1 is disconnected (zero current shutdown) to complete the commutation.

[0071] (3) Capacitor charging stage

[0072] After the mechanical switch BS1 is reliably disconnected, the capacitor voltage control and current control links are enabled, the capacitor voltage control target is set to the rated value, and the current control target is set to the line current, such as Figure 4 Under the control, after each submodule capacitor is charged to the rated value, the startup process is completed and the DC power flow control operation state can be entered as required.

[0073] Verification of the startup process of the startup control method for a submodule bridge arm connected in series in a DC transmission line:

[0074] To verify the rationality of the above startup process design, Figure 6 As shown in the figure, before 0.18 seconds, the capacitor C in the H-bridge submodule SM1 has been completed under the condition that the BS is disconnected from the isolation. A1_1 Charging; at 0.2 seconds, the mechanical switch BS is turned on to charge the submodule capacitor C A1_1 Reverse input, the other sub-modules are in bypass state. During the commutation phase, the bridge arm current I BS Gradually increases, the current I flowing through the mechanical switch BS1 BS1 Gradually decreases, the line current remains stable, and the current I BS1 After it drops to 0, the switch BS1 is disconnected to complete the commutation. During this process, the capacitor voltage u CA1_1 It gradually decreases, but is still not greater than 0 when the commutation is completed; after confirming that the switch BS1 is disconnected or accepting the control command, it switches to the charging stage at 0.22s. Under the control of the capacitor voltage, the voltage of each submodule of the bridge arm A1 gradually increases and reaches the rated value at 0.26s. At this time, the startup process is completed and the DC power flow control operation state can be entered as required.

[0075] Capacitor C A1_1 is the mark of the capacitor in the H-bridge submodule SM1, capacitor C A1_2 is the label of the capacitor in the H-bridge submodule SM2, and so on.

[0076] Theoretical analysis:

[0077] 1. Submodule capacitor pre-charge voltage design

[0078] During the commutation process, as the bridge arm current I A1 Gradually increases, the bridge arm submodule A1_1 (ie, H bridge submodule SM1) capacitor voltage u CA1_1 Gradually decreases. To achieve reliable startup, it is necessary to ensure that the submodule capacitor voltage u is CA1_1 does not drop to 0. Therefore, how to set the output capacitor voltage u CA1_1 It is the key to ensure reliable commutation. According to the circuit relationship of the commutation process, the relationship between the submodule voltage and current can be obtained:

[0079]

[0080]

[0081] Among them, L is the bridge arm inductance, C is the submodule capacitance, is the initial value of the capacitor voltage of the submodule, t0 is the bridge arm current i A1 The moment when the line current i1 is equal.

[0082] By solving the second-order differential equation and substituting the initial value of the DC grid line current, the setting requirements of the initial value of the submodule capacitor voltage can be solved. This paper takes the Zhangbei DC grid parameters as an example, substituting the maximum line current of 3000A, and using the bridge arm inductance and capacitance parameters designed in this paper to obtain the required capacitor voltage initial value of about 2200V. At the same time, considering the redundant design (about 10%), the final capacitor rated voltage is selected as 2400V.

[0083] 2. Specific control strategy for capacitor voltage controllable charging

[0084] The controllable charging control strategy includes two control loops: rated voltage loop and rated current loop, such as Figure 5 As shown, the submodule capacitor voltage and bridge arm current are controlled by the PI controller to reach the rated value. Since the submodule A1_1 is not fully discharged at the end of the commutation phase, its capacitor voltage is higher than the capacitor voltages of the other bypassed submodules. The present invention introduces the difference between each submodule capacitor voltage and the average bridge arm capacitor voltage as the negative feedback amount of the submodule modulation signal to ensure the consistency of the charging progress of all submodule capacitors.

[0085] The present invention is described by several specific embodiments, and it should be understood by those skilled in the art that various changes and equivalent substitutions may be made to the present invention without departing from the scope of the present invention. In addition, various modifications may be made to the present invention for specific situations or specific circumstances without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed, but should include all implementation methods falling within the scope of the claims of the present invention.

Claims

1. A startup control method for a submodule bridge arm connected in series in a DC transmission line, wherein the submodule bridge arm is connected in series in the DC transmission line, and the DC transmission line contains an equivalent series resistance R 1. Assume that the DC transmission line is line 1, which includes the node bus, mechanical switch BS1, bridge arm A1, mechanical switch BSU, mechanical switch BSD and bridge arm series inductor L , Equivalent series resistance R The output end of 1 is simultaneously connected to one end of the mechanical switch BS1 and one end of the mechanical switch BSU; the other end of the mechanical switch BS1 is simultaneously connected to the node bus and one end of the mechanical switch BSD; Two ends of the bridge arm A1 are connected to the other end of the mechanical switch BSU and the other end of the mechanical switch BSD respectively; It is characterized in that The bridge arm A1 is composed of N H-bridge sub-modules are connected in series. N is an integer greater than or equal to 1; N The H-bridge submodules are H-bridge submodules SM1, SM2, SM3, SM4, SM5, SM6, SM7, SM8, SM9, SM10, SM11, SM12, SM13, SM14, SM15, SM16, SM17, SM18, SM19, SM20, SM21, SM22, SM23, SM24, SM N ; The H-bridge submodule includes a capacitor C and two IGBT branches connected in parallel; each IGBT branch includes two IGBT units connected in series, and each IGBT unit is composed of an IGBT chip and a diode connected in reverse parallel; The connection point between the emitter of IGBT chip S1 and the collector of IGBT chip S2 in one IGBT branch serves as the input end of the H-bridge submodule; The emitter of the IGBT chip S3 of another IGBT branch and the collector connection of the IGBT chip S4 serve as the output end of the H-bridge submodule; i The output of the first H-bridge submodule is connected to the i+ 1 input end of the H-bridge submodule; i∈[1, N -1]; among them, N and i All are integers; The input end of the H-bridge submodule SM1 is one end of the bridge arm A1. N The output end is the other end of the bridge arm A1; The method consists of three stages: The first stage: bridge arm charging stage; The second stage: commutation process; The third stage: capacitor charging stage; Phase 4: After the first, second and third phases are completed, the startup process is completed and the system is ready to enter the running state as required; The method of the bridge arm charging stage is as follows: Step 1: Based on a submodule bridge arm suitable for being connected in series in a DC transmission line, before starting the submodule bridge arm, the H bridge submodule SM in the submodule bridge arm N The capacitor C voltage is set to 0; Step 2: Enter the pre-charging process. The pre-charging process is as follows: Mechanical switch BS1 is closed and current flows through line 1 I 1; Both mechanical switches BSU and BSD are disconnected, and the bridge arm of the submodule is bypassed to prepare for isolated charging; Step 3: Enter the isolated charging process: First, the capacitor of the H-bridge submodule SM1 in the bridge arm A1 is charged by a DC voltage source.

2. A startup control method for a submodule bridge arm connected in series in a DC transmission line according to claim 1, characterized in that: The commutation method of the commutation process is as follows: Close the mechanical switch BSU and the mechanical switch BSD, and control the H-bridge submodule SM1 to input negative voltage at the same time u A1 ,the remaining N -1 H-bridge submodule works in bypass state; at negative voltage u A1 Under the action, the line current I 1 gradually switches from mechanical switch BS1 to bridge arm A1. When the current flowing through mechanical switch BS1 I BS1 When the voltage drops to 0, that is, when the line current is completely switched to the bridge arm A1, the mechanical switch BS1 is disconnected to complete the commutation.

3. A startup control method for a submodule bridge arm connected in series in a DC transmission line according to claim 2, characterized in that: The capacitor charging method in the capacitor charging stage is as follows: After the mechanical switch BS1 is reliably disconnected, the capacitor voltage control and current control links are enabled, the capacitor voltage control target is set to the rated value, and the current control target is set to the line current. Under the control of the capacitor voltage control and current control links, N H-bridge submodule SM N After the capacitor is fully charged to the rated value, the startup process is completed and the system enters the operating state as required.

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