A bridge arm damping resistance design method

Abstract

The application provides a bridge arm damping resistance design method, which determines the DC current when the DC side jump switch occurs after the DC short circuit fault, and the fault loop resistance and inductance of the slowest short circuit current decay fault point without considering the bridge arm damping resistance, determines the total damping resistance value of the bridge arm, determines the damping resistance value of a single damping module through the maximum value of the bridge arm current and the rated voltage of the damping module, and finally determines the number of damping modules. The bridge arm damping resistance design method provided by the application calculates the resistance value and the number of damping modules of the bridge arm damping module in the flexible DC power transmission system, avoids a large number of iterative calculations of the currently used electromagnetic transient model, and is suitable for large-scale multi-terminal systems.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology in power systems, and specifically relates to a design method for bridge arm damping resistors. Background Technology

[0002] Flexible DC transmission is widely applicable to wind power grid connection, islanded power supply, asynchronous interconnection of AC systems, distributed generation grid connection, multi-terminal DC transmission, and urban distribution network capacity expansion and upgrading. However, current flexible DC technology lacks the ability to quickly recover from DC line faults, which not only affects its application at higher DC voltage levels but also limits its applicability in multi-terminal flexible DC transmission.

[0003] To address the issue of rapid recovery from DC line faults, bridge arm damping is a feasible solution. This scheme involves inserting damping modules in series within the converter arms to limit short-circuit current. Under normal operating conditions, the damping resistors in these modules are bypassed. However, during DC faults, the damping resistors are engaged, accelerating the release of residual energy within the bridge arm reactors and reducing the short-circuit current decay time. Research indicates that the effectiveness of the bridge arm damping scheme in accelerating short-circuit current decay is primarily influenced by the value of the damping resistors. Increasing the damping resistor value accelerates the decay rate, but it also increases the risk of overvoltage in the damping modules during short circuits. This needs to be balanced by increasing the number of modules. Therefore, to ensure the effectiveness and safety of the bridge arm damping system, careful design of the damping resistor values ​​and the number of modules is essential in engineering applications.

[0004] The journal article "Rapid Fault Clearing and System Recovery Technology of Modular Multilevel Converter Based on Arm Damping Valve Group" (Electric Power Automation Equipment, Vol. 38, No. 4, April 2018) proposes the selection principle of arm damping parameters and the engineering parameter design process applicable to engineering. This method mainly relies on simulation to determine the maximum fault current. The electromagnetic transient model used requires a lot of iterative calculations and is not suitable for large-scale multi-terminal systems. Summary of the Invention

[0005] The purpose of this invention is to provide a bridge arm damping resistor design method to address the shortcomings of existing technologies.

[0006] Therefore, the above-mentioned objective of the present invention is achieved through the following technical solution:

[0007] A method for designing bridge arm damping resistors, characterized in that it is applied to the design of damping resistors for damping modules in flexible DC transmission systems employing bridge arm damping schemes, wherein the flexible DC system includes at least two modular multilevel converters, and the design method includes independently performing the design for each modular multilevel converter, comprising the following steps:

[0008] 1) Without considering the bridge arm damping resistance, determine the DC current I when the switch trips after a DC short-circuit fault occurs on the DC side. dk1 Proceed to step 2);

[0009] 2) Analyze the DC line network, determine the fault point F where the short-circuit current decays the slowest when a fault occurs on the DC line, and obtain the resistance R and inductance of the fault loop formed by the fault point F and the modular multilevel converter, then proceed to step 3).

[0010] 3) Based on the fault circuit resistance R and inductance L, and the decaying current I τ and decay time t τ The total damping resistance value R of the bridge arm is obtained. dmtot Proceed to step 4);

[0011] 4) Without considering the arm damping resistance, determine the maximum arm current I when a DC short-circuit fault occurs on the DC side of the modular multilevel converter. bk Proceed to step 5);

[0012] 5) Determine the rated voltage U of the damping module. ndm Proceed to step 6);

[0013] 6) According to I bk and U ndm Determine the damping resistance R in each damping module. dm Size, proceed to step 7);

[0014] 7) According to R dmtot and R dm Determine the number N of damping modules. dm .

[0015] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0016] As a preferred technical solution of the present invention:

[0017] In step 1), the DC current I dk1 Determined by the following formula:

[0018]

[0019] In the formula: U sm For AC phase voltage, L ac For AC side inductance, ω is the rated angular frequency of the AC system, and L is the AC side inductance. b For bridge arm inductance.

[0020] As a preferred technical solution of the present invention:

[0021] In step 2), the fault point F is determined through the following steps:

[0022] 1) For each possible fault point X in the DC line network, calculate the DC line inductance L from fault point X to the modular multilevel converter. linex and DC line resistance R linex ;

[0023] 2) Calculate the resistance Rx and inductance Lx of the fault loop from fault point X to the modular multilevel converter:

[0024]

[0025]

[0026] In the formula: R0 is the bridge arm resistance, R linex For the resistance of a DC line, L b For the bridge arm reactance, L d For the DC smoothing reactor inductance, L linex For DC line inductance;

[0027] 3) Select the fault point with the largest Lx / Rx ratio as the fault point F.

[0028] As a preferred technical solution of the present invention:

[0029] In step 3), the total damping resistance value R of the bridge arm is... dmtot We obtain it from the following formula:

[0030]

[0031] In the formula: k d This is the margin coefficient, ranging from 1 to 10, which is adjusted by k. d The value of provides a margin for the design of the bridge arm damping system; L is the inductance; t τ I is the decay time; dk1 I is the DC current that trips when the switch is activated after a DC short-circuit fault occurs on the DC side. τ R is the decaying current; R is the fault circuit resistance.

[0032] As a preferred technical solution of the present invention:

[0033] In step 4), the maximum value of the bridge arm current I bk Determine this through the following steps:

[0034] 1) Calculate the maximum arm current I from the time the modular multilevel converter is locked out to the time it is switched off during a DC short circuit. bk1 :

[0035]

[0036] In the formula:

[0037]

[0038]

[0039] θ dc =arctan(τω) dc )

[0040]

[0041] Where N is the number of submodules, C0 is the capacitance of the submodule, and i dc0 U is the DC side current before the short-circuit fault occurs. dc0 This refers to the DC side voltage before the short-circuit fault occurred.

[0042] 2) Calculate the arm current I under steady-state conditions when the AC switch is not tripped after the modular multilevel converter is locked out during a DC short circuit. bk2 :

[0043]

[0044] 3) Maximum bridge arm current I bk Select I bk1 and I bk2 The larger value in the range.

[0045] As a preferred technical solution of the present invention:

[0046] In step 6), the damping resistor R dm satisfy:

[0047]

[0048] In the formula: U ndm This is the rated voltage of the damping module.

[0049] As a preferred technical solution of the present invention:

[0050] In step 7), the number of damping modules N dm satisfy:

[0051]

[0052] In the formula: R dmtot R is the total damping resistance value of the bridge arm. dm Damping resistors in each damping module.

[0053] This invention provides a method for designing bridge arm damping resistors. The method determines the total damping resistance of the bridge arm by determining the DC current when the switch trips after a DC short-circuit fault on the DC side, without considering the bridge arm damping resistance, and by analyzing the fault loop resistance and inductance at the fault point where the short-circuit current decays the slowest. The damping resistance of a single damping module is determined by the maximum bridge arm current and the rated voltage of the damping module. Finally, the number of damping modules is determined. This method avoids the extensive iterative calculations required by current electromagnetic transient models by calculating the resistance values ​​and the number of damping modules in a flexible DC transmission system, making it suitable for large-scale multi-terminal systems. Attached Figure Description

[0054] Figure 1 This is the equivalent circuit of the RL resistor and inductor in a flexible DC transmission system.

[0055] Figure 2 This is the equivalent circuit of a modular multilevel converter.

[0056] Figure 3 The flowchart illustrates the bridge arm damping resistor design method provided by this invention.

[0057] Figure 4 Flowchart of a method for determining the fault point F where the short-circuit current decays slowest when a fault occurs on a DC line in a modular multilevel converter;

[0058] Figure 5 To determine the maximum arm current I when a DC short-circuit fault occurs on the DC side of the modular multilevel converter. bk The method flowchart. Detailed Implementation

[0059] The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

[0060] Figure 1 The RL resistor-inductance equivalent circuit is simplified from the flexible DC transmission system. Whether a bipolar short circuit or a unipolar short circuit occurs on the DC side, the short circuit current circuit can be simplified to this circuit after the converter valve is locked and the AC incoming switch is tripped.

[0061] I dk1 Let I(t) be the DC current when the switch trips after a DC short-circuit fault occurs on the DC side, i(t) be the instantaneous value of the loop current at different times, L be the equivalent inductance of the loop, and R be the equivalent resistance of the loop. According to circuit principles, the initial current of the equivalent loop is known to be I. dk1 The instantaneous values ​​of the short-circuit current i(t) at each moment after the short circuit can be calculated:

[0062]

[0063] In the formula: τ is the decay time constant of the short-circuit current, and the decay rate of the short-circuit current can be determined by formula (1).

[0064] Figure 2 This is the equivalent circuit of a modular multilevel converter. Based on the principle of flexible DC transmission, I in equation (1) dk1 The maximum value can be expressed as:

[0065]

[0066] Considering the damping resistance value R dmtot In this case, the decay time constant τ can be expressed as:

[0067]

[0068] In the formula: Rx and Lx can be expressed as:

[0069]

[0070] In equations (2)-(4): R0 is the bridge arm resistance, L ac For AC side inductance, U sm For AC phase voltage, L b For the bridge arm inductance, L d R is the inductance of the DC-side smoothing reactor. linex For the resistance of a DC line, L line x represents the inductance of a DC line.

[0071] When the fault location is different, R linex and L linex The attenuation time constant τ = Lx / Rx will vary depending on the length of the DC line and the wiring method of the main circuit. By comparing the attenuation time constant τ when a fault occurs at different locations, the fault point F with the slowest short-circuit current attenuation can be determined. This can be analyzed through the following steps:

[0072] 1) For each possible fault point X in the DC line network, calculate the DC line inductance L from fault point X to the modular multilevel converter. linex and DC line resistance R linex ;

[0073] 2) Calculate the resistance Rx and inductance Lx of the fault loop from fault point X to the modular multilevel converter using equation (4);

[0074] 3) Select the fault point with the largest Lx / Rx ratio as the fault point F.

[0075] After determining the fault point F, the attenuating current I is used. τ and decay time t τAs a design objective for the bridge arm damping resistor, that is, assuming at time t τ The short-circuit current decays to I at any given time. τ , t = t τ i(t) = I τ Substituting the fault loop resistance R and fault loop inductance L corresponding to fault point F into equation (1), the total damping resistance R of the bridge arm can be calculated. dmtot ,Right now:

[0076]

[0077] In the formula: k d This is the margin coefficient, ranging from 1 to 10, which is adjusted by k. d The value of provides a margin for the design of the bridge arm damping system; L is the inductance; t τ I is the decay time; dk1 I is the DC current that trips when the switch is activated after a DC short-circuit fault occurs on the DC side. τ R is the decaying current; R is the fault circuit resistance.

[0078] To prevent overvoltage in the damping modules during short circuits, given the known values ​​of the bridge arm damping resistances and the bridge arm current analysis, the number of damping modules required to meet the overvoltage requirements can be determined. First, the maximum value I of the bridge arm current needs to be determined. bk According to the principle of modular multilevel converter, the maximum value of the bridge arm current I bk The arm current I is the maximum blocking moment between the latching of the modular multilevel converter and the AC tripping switch during a DC short circuit. bk1 The arm current I under steady-state condition after the modular multilevel converter is locked out during a DC short circuit and the AC switch is not tripped. bk2 The larger value in the range can be calculated using the following steps:

[0079] 1) Calculate the maximum arm current I from the time the modular multilevel converter is locked out to the time it is switched off during a DC short circuit. bk1 :

[0080]

[0081] In the formula:

[0082]

[0083]

[0084] θ dc =arctan(τω) dc )

[0085]

[0086] Where N is the number of submodules, C0 is the capacitance of the submodule, and i dc0 U is the DC side current before the short-circuit fault occurs. dc0 This refers to the DC side voltage before the short-circuit fault occurred.

[0087] 2) Calculate the arm current I under steady-state conditions when the AC switch is not tripped after the modular multilevel converter is locked out during a DC short circuit. bk2 :

[0088]

[0089] 3) Maximum bridge arm current I bk Select I bk1 and I bk2 The larger value in the range.

[0090] Assume the rated voltage U on the DC side dn The number of damping modules is N d Then the rated voltage U of each damping module ndm for:

[0091]

[0092] Determining the maximum value of the bridge arm current I bk Then, according to Ohm's law, to prevent overvoltage in the damping modules, the resistance value of each damping module must satisfy the following:

[0093]

[0094] Then, the damping resistance value R of each module is used. dm Calculate the total damping resistance value R. dmtot Number of damping modules N dm ,Right now

[0095]

[0096] Figure 3 The flowchart of the bridge arm damping resistor design method provided by the present invention is used to guide the design of bridge arm damping systems in flexible DC transmission systems, and includes the following steps:

[0097] (1) Without considering the bridge arm damping resistance, determine the DC current I when the switch trips after a DC short-circuit fault occurs on the DC side. dk1 According to the bridge arm inductance L b AC phase voltage U sm AC side inductor L ac The rated angular frequency ω of the AC system is determined by equation (2).

[0098] (2) Analyze the DC line network to determine the fault point F where the short-circuit current of the modular multilevel converter decays the slowest when a fault occurs on the DC line, and obtain the resistance R and inductance L of the fault loop formed by the fault point F and the modular multilevel converter. See the detailed steps below. Figure 4 .

[0099] (3) Based on the fault circuit resistance R and inductance L, and the decaying current I τ and decay time t τ The total damping resistance value R of the bridge arm is obtained by equation (5). dmtot k can be adjusted appropriately. d The value of provides a margin for the design of the bridge arm damping system.

[0100] (4) Without considering the arm damping resistance, determine the maximum arm current I when a DC short-circuit fault occurs on the DC side of the modular multilevel converter. bk For detailed steps, please see Figure 5 .

[0101] (5) Determine the rated voltage U of the damping module. ndm Rated voltage U ndm Calculated using equation (8).

[0102] (6) According to I bk and U ndm Determine the damping resistance R in each damping module dm The size of each damping module and the resistance value of each damping module must meet the requirements of equation (9).

[0103] (7) According to R dmtot and R dm Determine the number N of damping modules dm The number of damping modules must meet the requirements of equation (10).

[0104] Figure 4 A flowchart illustrating a method for determining the fault point F where the short-circuit current decays most slowly in a modular multilevel converter when a fault occurs on a DC line includes the following steps:

[0105] (1) For each possible fault point X in the DC line network, calculate the DC line inductance L from fault point X to the modular multilevel converter. linex and DC line resistance R linex .

[0106] (2) Calculate the resistance Rx and inductance Lx of the fault loop from the fault point X to the modular multilevel converter using equation (4).

[0107] (3) Select the fault point with the largest Lx / Rx as the fault point F.

[0108] Figure 5 To determine the maximum arm current I when a DC short-circuit fault occurs on the DC side of the modular multilevel converter. bk The method flowchart includes the following steps:

[0109] (1) Calculate the maximum bridge arm current I from the time the modular multilevel converter is locked out to the time the AC trip switch is reached during a DC short circuit using equation (6). bk1 .

[0110] (2) Calculate the arm current I under steady-state conditions when the AC switch is not tripped after the modular multilevel converter is locked out during a DC short circuit using equation (7). bk2 .

[0111] (3) Maximum bridge arm current I bk Select I bk1 and I bk2 The larger value in the range.

[0112] The above specific embodiments are used to explain and illustrate the present invention, and are only preferred embodiments of the present invention, not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for designing bridge arm damping resistors, characterized in that: The damping resistor design is applied to the damping module of a flexible DC transmission system employing a bridge arm damping scheme. The flexible DC transmission system includes at least two modular multilevel converters. The design method involves independently performing the design for each modular multilevel converter, including the following steps: 1) Without considering the bridge arm damping resistance, determine the DC current when the switch trips after a DC short-circuit fault occurs on the DC side. Proceed to step 2). 2) Analyze the DC line network to determine the fault point F where the short-circuit current decays slowest when a fault occurs on the DC line, and obtain the resistance R and inductance of the fault loop formed by the fault point F and the modular multilevel converter, then proceed to step 3). 3) Based on the fault circuit resistance R and inductance L, and the decaying current... and decay time The total damping resistance value of the bridge arm is obtained. Proceed to step 4). 4) Without considering the arm damping resistance, determine the maximum arm current when a DC short-circuit fault occurs on the DC side of the modular multilevel converter. Proceed to step 5). 5) Determine the rated voltage of the damping module. Proceed to step 6). 6) According to and Determine the damping resistance in each damping module. Size, proceed to step 7); 7) According to and Determine the number of damping modules .

2. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 1), the direct current Determined by the following formula: In the formula: For AC phase voltage, For AC side inductance, The rated angular frequency of the AC system, For bridge arm inductance.

3. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 2), the fault point F is determined through the following steps: 1) For each possible fault point X in the DC line network, calculate the DC line inductance from fault point X to the modular multilevel converter. and DC line resistance ; 2) Calculate the resistance Rx and inductance Lx of the fault loop from fault point X to the modular multilevel converter: In the formula: For the bridge arm resistance, For DC line resistance, For the bridge arm reactance, For DC smoothing reactor inductance, For DC line inductance; 3) Select the fault point with the largest Lx / Rx ratio as the fault point F.

4. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 3), the total damping resistance value of the bridge arm We obtain it from the following formula: ] In the formula: This is the margin coefficient, ranging from 1 to 10, which is adjusted... The value of provides a margin for the design of the bridge arm damping system; L is the inductance; This refers to the decay time; This refers to the DC current that trips when the switch is activated after a DC short-circuit fault occurs on the DC side. R is the decaying current; R is the fault circuit resistance.

5. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 4), the maximum value of the bridge arm current. Determine this through the following steps: 1) Calculate the maximum arm current from the time the modular multilevel converter is locked out to the time it reaches the AC trip switch during a DC short circuit. : In the formula: in, Number of submodules For submodule capacitors, This refers to the DC-side current before the short-circuit fault occurs. This refers to the DC side voltage before the short-circuit fault occurred. 2) Calculate the arm current under steady-state conditions when the AC switch is not tripped after the modular multilevel converter is locked out during a DC short circuit. : 3) Maximum bridge arm current Select and The larger value in the range.

6. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 6), the damping resistor satisfy: In the formula: This is the rated voltage of the damping module.

7. The bridge arm damping resistor design method according to claim 1, characterized in that: In step 7), the number of damping modules satisfy: In the formula: This represents the total damping resistance value of the bridge arm. Damping resistors in each damping module.

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