Method and device for determining direct current bias current

Abstract

The invention discloses a DC bias current determination method and device. The method comprises the steps of obtaining bridge arm parameters and electrical parameters of a modular multilevel converter in a flexible DC power transmission system; establishing a correlation function of the electrical parameters according to the bridge arm parameters and the electrical parameters; establishing a first corresponding relation between the electrical parameters and the bridge arm parameters according to a correlation function; determining a second corresponding relation between the direct-current bias current and the bridge arm parameters according to the first corresponding relation; and according to the second corresponding relation, solving to obtain the direct-current bias current under the asymmetric bridge arm parameters. By considering the influence of asymmetry of MMC bridge arm parameters, the corresponding relation between the electrical parameters and the bridge arm parameters can be established, so that the one-to-one corresponding relation between the asymmetry of each bridge arm parameter and the direct current bias current is determined, the direct current bias current is determined, theoretical support is provided for research on direct current magnetic bias influence factors, and the method is suitable for popularization and application. And the reliability and the stability of the flexible direct-current power transmission system are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible DC power transmission, in particular to a DC bias current determination method and device. BACKGROUND

[0002] With the increasing maturity of flexible DC power transmission technology, the flexible DC power transmission technology has become the main choice of DC power transmission engineering due to its advantages. The flexible DC power transmission technology relies on the MMC (Modular Multilevel Converter) bridge arm. For example, the MMC bridge arm is composed of bridge arm reactance and converter valve sub-modules, and the AC / DC conversion is completed by controlling the conduction and turn-off of the converter valve, which is the core link of the flexible DC power transmission technology. Therefore, the stability of the MMC bridge arm parameters of the converter valve directly determines the efficiency and quality of the conversion process. However, in actual engineering, due to factors such as equipment manufacturing deviation and supply channel difference, there is inevitably a certain deviation in the MMC bridge arm parameters. This asymmetry of the MMC bridge arm parameters will cause the DC bias current to appear on the valve side of the converter transformer, causing the DC magnetic bias phenomenon, which will lead to the increase of transformer vibration and noise, and even damage, seriously affecting the reliability and stability of the flexible DC power transmission system. SUMMARY

[0003] The present application provides a DC bias current determination method and device, which considers the influence of the asymmetry of the MMC bridge arm parameters, establishes the corresponding relationship between the electrical parameters and the bridge arm parameters, to clearly define the one-to-one correspondence between each bridge arm parameter asymmetry and the DC bias current and determine the DC bias current, providing theoretical support for studying the DC magnetic bias influencing factors, helping to realize the engineering suppression of the DC magnetic bias from the level of the converter valve, and improving the reliability and stability of the flexible DC power transmission system.

[0004] To achieve the above purpose, the present application embodiment provides a DC bias current determination method, comprising:

[0005] Obtaining the bridge arm parameters and electrical parameters of the modular multilevel converter in the flexible DC power transmission system;

[0006] According to the bridge arm parameters and electrical parameters, a correlation function of the electrical parameters is established;

[0007] According to the correlation function, a first corresponding relationship between the electrical parameters and the bridge arm parameters is established;

[0008] According to the first corresponding relationship, a second corresponding relationship between the DC bias current and the bridge arm parameters is determined;

[0009] According to the second corresponding relationship, the DC bias current under the asymmetry of the bridge arm parameters is solved.

[0010] As an improvement of the above scheme, the establishing the correlation function of the electrical parameter according to the bridge arm parameter and the electrical parameter comprises:

[0011] According to the bridge arm parameter and the electrical parameter, the switch function of the sub-module is established.

[0012] According to the electrical parameter, the sub-module capacitor voltage function, the bridge arm current function and the bridge arm voltage function are established.

[0013] As an improvement of the above scheme, the establishing the first corresponding relationship between the electrical parameter and the bridge arm parameter according to the correlation function comprises:

[0014] The component corresponding relationship of the electrical parameter is established according to the correlation function.

[0015] The first corresponding relationship between the electrical parameter and the bridge arm parameter is established according to the component corresponding relationship and the bridge arm KVL equation.

[0016] As an improvement of the above scheme, the establishing the component corresponding relationship of the electrical parameter according to the correlation function comprises:

[0017] According to the switch function, the sub-module capacitor voltage function and the bridge arm voltage function, the voltage component corresponding relationship of the sub-module capacitor voltage and the direct current component corresponding relationship between the bridge arm voltage in the electrical parameter and the sub-module capacitor voltage are established.

[0018] According to the switch function, the sub-module capacitor voltage function and the bridge arm current function, the electrical component corresponding relationship between the bridge arm current in the electrical parameter and the sub-module capacitor voltage and the current component corresponding relationship of the bridge arm current are established.

[0019] As an improvement of the above scheme, the establishing the first corresponding relationship between the electrical parameter and the bridge arm parameter according to the component corresponding relationship and the bridge arm KVL equation comprises:

[0020] According to the bridge arm KVL equation, the electrical corresponding relationship between the direct current voltage in the electrical parameter and the bridge arm voltage and the bridge arm resistance and the bridge arm reactance in the bridge arm parameter is established.

[0021] The first corresponding relationship between the electrical parameter and the bridge arm parameter is established according to the electrical corresponding relationship and the component corresponding relationship.

[0022] As an improvement of the above scheme, the establishing the voltage component corresponding relationship of the sub-module capacitor voltage and the direct current component corresponding relationship between the bridge arm voltage in the electrical parameter and the sub-module capacitor voltage according to the switch function, the sub-module capacitor voltage function and the bridge arm voltage function comprises:

[0023] establishing a function correspondence relationship among the switching function, the submodule capacitor voltage function and the bridge arm voltage function;

[0024] According to the function correspondence relationship, a voltage component correspondence relationship of the submodule capacitor voltage is established.

[0025] According to the function correspondence relationship and the voltage component correspondence relationship, a direct current component correspondence relationship between the bridge arm voltage of the electrical parameter and the submodule capacitor voltage is established.

[0026] As an improvement of the above-mentioned scheme, the electrical component correspondence relationship between the bridge arm current of the electrical parameter and the submodule capacitor voltage is established according to the switching function, the submodule capacitor voltage function and the bridge arm current function; and the current component correspondence relationship of the bridge arm current comprises:

[0027] According to the switching function, the submodule capacitor voltage function and the bridge arm current function, the electrical component correspondence relationship between the bridge arm current of the electrical parameter and the submodule capacitor voltage is established.

[0028] According to the electrical component correspondence relationship, the current component correspondence relationship of the bridge arm current is established.

[0029] As an improvement of the above-mentioned scheme, the first correspondence relationship is a correspondence relationship between a direct current component of the bridge arm current of the electrical parameter and the bridge arm parameter.

[0030] As an improvement of the above-mentioned scheme, the bridge arm parameter comprises a bridge arm resistance, a bridge arm reactance, a submodule capacitor and a submodule number.

[0031] In order to achieve the above-mentioned purpose, the embodiment of the present application provides a direct current bias current determination device, comprising:

[0032] A related parameter acquisition module is configured to acquire a bridge arm parameter and an electrical parameter of a modular multilevel converter in a flexible direct current power transmission system.

[0033] A related function establishing module is configured to establish a related function of the electrical parameter according to the bridge arm parameter and the electrical parameter.

[0034] A first relationship establishing module is configured to establish a first correspondence relationship between the electrical parameter and the bridge arm parameter according to the related function.

[0035] A second relationship establishing module is configured to determine a second correspondence relationship between a direct current bias current and the bridge arm parameter according to the first correspondence relationship.

[0036] The bias current determination module is configured to solve the DC bias current under the asymmetry of the bridge arm parameter according to the second correspondence.

[0037] Compared with the prior art, the DC bias current determination method and device provided by the embodiment of the present application can obtain the bridge arm parameter and the electrical parameter of the modular multilevel converter in the flexible DC power transmission system, establish a correlation function of the electrical parameter according to the bridge arm parameter and the electrical parameter, establish a first correspondence between the electrical parameter and the bridge arm parameter according to the correlation function, determine a second correspondence between the DC bias current and the bridge arm parameter according to the first correspondence, and solve the DC bias current under the asymmetry of the bridge arm parameter according to the second correspondence. The influence of the asymmetry of the MMC bridge arm parameter can be considered to establish the correspondence between the electrical parameter and the bridge arm parameter, so as to clearly determine the one-to-one correspondence between each bridge arm parameter asymmetry and the DC bias current and determine the DC bias current, provide theoretical support for researching the DC bias magnetic influencing factor, help to realize the engineering suppression of the DC bias magnetic from the converter valve level, reduce the occurrence of the DC bias magnetic phenomenon, reduce the vibration and damage of the transformer, and improve the reliability and stability of the flexible DC power transmission system. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a flowchart of a DC bias current determination method provided by the embodiment of the present application;

[0039] Figure 2 is a DC bias electrical correspondence diagram of an MMC provided by the embodiment of the present application;

[0040] Figure 3 is a structure diagram of a DC bias current determination device provided by the embodiment of the present application. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0042] It should be noted that the terms "include" and "specifically" and any variants thereof in the present application are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to the clearly listed steps or units, but can include other steps or units that are not clearly listed or inherent to the process, method, product or device.

[0043] Referring to Figure 1 , Figure 1 is a flowchart of a direct current bias current determination method provided by an embodiment of the present application. The direct current bias current determination method comprises the following steps:

[0044] S1, obtaining bridge arm parameters and electrical parameters of a modular multilevel converter in a flexible direct current power transmission system;

[0045] S2, establishing a correlation function of the electrical parameters according to the bridge arm parameters and the electrical parameters;

[0046] S3, establishing a first correspondence between the electrical parameters and the bridge arm parameters according to the correlation function;

[0047] S4, determining a second correspondence between a direct current bias current and the bridge arm parameters according to the first correspondence;

[0048] S5, solving the direct current bias current under asymmetry of the bridge arm parameters according to the second correspondence.

[0049] Exemplarily, the direct current bias current determination method is implemented by a direct current bias current determination server. The direct current bias current determination server can be loaded in a direct current power transmission system management server or exist independently of the direct current power transmission system management server. The direct current bias current determination server can interact with a target user and interact with the direct current power transmission system management server. The direct current bias current determination server obtains bridge arm parameters (such as sub-module capacitance, sub-module number, bridge arm resistance and bridge arm reactance) and electrical parameters (such as sub-module capacitance voltage, bridge arm current, bridge arm voltage and direct current voltage) of a modular multilevel converter in a flexible direct current power transmission system. It can be understood that the bridge arm parameters are design parameters of the MMC bridge arm, which are determined by manufacturing and selection and directly affect the electrical characteristics of the bridge arm. Asymmetry of the bridge arm parameters (such as capacitance deviation and reactance deviation) can cause imbalance of the electrical parameters (such as asymmetric bridge arm voltage and asymmetric bridge arm current), and ultimately generate the direct current bias current at the valve side of the transformer. According to the bridge arm parameters and the electrical parameters, a correlation function (such as a sub-module switching function, a sub-module capacitance voltage function, a bridge arm current function and a bridge arm voltage function) of the electrical parameters is established. According to the correlation function, a corresponding relationship between a bridge arm current direct current component, a sub-module capacitance voltage fundamental frequency component and the sub-module capacitance and the sub-module number is established. According to a bridge arm KVL equation, a corresponding relationship between a direct current voltage and a bridge arm voltage fundamental frequency component and the bridge arm resistance and the bridge arm reactance is established, to obtain a first corresponding relationship (a corresponding relationship between the bridge arm current direct current component and the bridge arm parameters in the electrical parameters). According to the first corresponding relationship, a second corresponding relationship between the direct current bias current and the bridge arm parameters is determined. According to the second corresponding relationship, the direct current bias current under asymmetry of the bridge arm parameters is solved. By considering the influence of the MMC bridge arm parameter asymmetry, the corresponding relationship between the electrical parameters and the bridge arm parameters is established, to clearly define the one-to-one corresponding relationship between each bridge arm parameter asymmetry and the direct current bias current and determine the direct current bias current, which provides theoretical support for studying the direct current magnetic bias influencing factors, helps to realize engineering suppression of the direct current magnetic bias from the level of the converter valve, reduces the occurrence of the direct current magnetic bias phenomenon, reduces vibration and damage of the transformer and improves the reliability and stability of the flexible direct current power transmission system.

[0050] Specifically, the step S2 comprises:

[0051] S21, a sub-module switching function is established according to the bridge arm parameters and the electrical parameters.

[0052] S22, a sub-module capacitance voltage function, a bridge arm current function and a bridge arm voltage function are established according to the electrical parameters.

[0053] For example, in the three-phase six-arm topology of MMC, each arm mainly consists of arm reactors and converter valve submodules, and AC / DC conversion is achieved by controlling the opening and closing of the converter valves. Taking the deviation of the equipment parameters of the upper arm of phase A as an example, such as... Figure 2 As shown, Figure 2 This is a schematic diagram of the DC bias electrical correspondence of an MMC provided in an embodiment of the present invention. Figure 2 Middle,U s For AC system voltage, u va_dc u vb_dc u vc_dc i represents the DC component of the valve-side voltage of the converter transformer. ga i gb i gc For the three-phase current on the grid side of the converter transformer, i va_dc i vb_dc i vc_dc For the DC component of the valve side current of the converter transformer, u p_dc u n_dc i represents the DC component of the voltage on the upper and lower bridge arms of the converter valve. p_dc i n_dc U represents the DC components of the current in the upper and lower bridge arms. dc The voltage is DC. To simplify the derivation, based on the bridge arm parameters and electrical parameters, the switching function of the submodule containing DC component, fundamental component and second harmonic component is established, as shown in equation (1).

[0054]

[0055] In the formula, S pi S ni M is the switching function of the i-th submodule in the upper and lower bridge arms; M is the fundamental modulation ratio; M2 is the second harmonic modulation ratio; is the initial phase of the second harmonic switching function; N is the number of individual bridge arm submodules (without redundancy). Since the controller measurement points are usually located on the converter transformer grid side in engineering, the isolation effect of the transformer makes the controller insensitive to small DC bias currents. Therefore, it is assumed that when the bridge arm parameters are asymmetrical, the modulated wave voltage output by the controller will not change compared to the symmetrical condition.

[0056] To simplify the derivation, based on the electrical parameters, a submodule capacitor voltage function containing DC component, fundamental component, and second harmonic component is established as shown in equation (2).

[0057]

[0058] In the formula, u cp_dc u cn_dc These are the DC components of the capacitor voltages of the upper and lower bridge arm submodules, respectively. cp(1) cn (1) u is the amplitude of the fundamental component of the upper and lower bridge arm sub-module capacitor voltage, u cp (2) cn (2) u is the second harmonic component of the upper and lower bridge arm sub-module capacitor voltage, θ (1) (2) θ and θ are the phases of the fundamental and second harmonic, respectively.

[0059] It is worth noting that the asymmetry of the bridge arm parameters is mainly based on the device manufacturing tolerance and the difference in supply channels, which is relatively small compared to the design value. For example, the device allowed manufacturing tolerance of bridge arm reactance is 0-±5% of the design value. In order to simplify the derivation, the phase change of voltage and current caused by parameter deviation is ignored in the following, and only the influence of bridge arm parameter asymmetry on amplitude is considered. Considering that the phases of the fundamental component and the second harmonic component of the upper and lower bridge arms are the same under symmetrical working conditions, the upper and lower bridge arms are still considered to have the same phase in the following.

[0060] Assuming that the circulating current suppression effect is ideal, the bridge arm current and the bridge arm voltage do not contain the second harmonic component, i.e. according to the electrical parameters, the bridge arm current function and the bridge arm voltage function containing the direct current component and the fundamental component are established; let the upper and lower bridge arm voltages be u p n , the upper and lower bridge arm currents be i p n , and the bridge arm current function and the bridge arm voltage function be as shown in formula (3).

[0061]

[0062] In the formula, u p_dc n_dc are the direct current components of the upper and lower bridge arm voltages, u p (1) n (1) are the fundamental amplitudes of the upper and lower bridge arm voltages, and γ is the phase of the fundamental voltage; i p_dc n_dc are the direct current components of the upper and lower bridge arm currents, i p (1) n (1) are the fundamental amplitudes of the upper and lower bridge arm currents, and β is the phase of the fundamental current.

[0063] Specifically, the step S3 comprises:

[0064] S31, establishing a component correspondence relationship of the electrical parameters according to the correlation function;

[0065] ​​​​​​​​​S32, according to the component correspondence relationship and the bridge arm KVL equation, establishing a first correspondence relationship between the electrical parameters and the bridge arm parameters.

[0066] More specifically, the step S31 comprises:

[0067] S311, according to the switching function, the submodule capacitor voltage function and the bridge arm voltage function, establishing a voltage component correspondence relationship of the submodule capacitor voltage, and a direct current component correspondence relationship between the bridge arm voltage in the electrical parameters and the submodule capacitor voltage;

[0068] S312, according to the switching function, the submodule capacitor voltage function and the bridge arm current function, establishing a bridge arm current in the electrical parameters and an electrical component correspondence relationship of the submodule capacitor voltage; and a current component correspondence relationship of the bridge arm current.

[0069] More specifically, the step S32 comprises:

[0070] S321, according to the bridge arm KVL equation, establishing an electrical correspondence relationship between the direct current voltage in the electrical parameters and the bridge arm voltage, and the bridge arm resistance and the bridge arm reactance in the bridge arm parameters;

[0071] S322, according to the electrical correspondence relationship and the component correspondence relationship, establishing a first correspondence relationship between the electrical parameters and the bridge arm parameters.

[0072] More specifically, the step S311 comprises:

[0073] establishing a function correspondence relationship of the switching function, the submodule capacitor voltage function and the bridge arm voltage function;

[0074] According to the function correspondence relationship, establishing a voltage component correspondence relationship of the submodule capacitor voltage;

[0075] According to the function correspondence relationship and the voltage component correspondence relationship, establishing a direct current component correspondence relationship between the bridge arm voltage in the electrical parameters and the submodule capacitor voltage.

[0076] Exemplarily, the function correspondence relationship of the switching function, the submodule capacitor voltage function and the bridge arm voltage function is as shown in formula (4).

[0077]

[0078] Formula (4) can be divided into three parts of direct current component and fundamental component, and double frequency component. By substituting formula (1) and formula (2) into formula (4), solving the double frequency component of the converter valve output voltage can obtain formula (5).

[0079]

[0080] According to formula (5), the fundamental component, the second harmonic component amplitude and the DC component of the submodule capacitor voltage present one-to-one correspondence (such as, the voltage component correspondence of the submodule capacitor voltage), when the DC component of the submodule capacitor voltage changes, the fundamental component and the second harmonic component will also change correspondingly.

[0081] Substitute formula (5) into formula (4), the DC component U p_dc , n_dc and the fundamental component amplitude u p (1) , n (1) As shown in formula (6).

[0082]

[0083] Wherein,

[0084]

[0085] According to formula (6), the DC component of the upper and lower bridge arm voltage and the DC component of the submodule capacitor voltage present one-to-one correspondence, when the DC component of the submodule capacitor voltage changes, the DC component of the upper and lower bridge arm voltage will also change correspondingly (such as, the DC component of the bridge arm voltage in the electrical parameter and the submodule capacitor voltage corresponding relationship), when the DC component of the submodule capacitor voltage changes, the fundamental component of the upper and lower bridge arm voltage will also change correspondingly.

[0086] More specifically, the step S312 comprises:

[0087] According to the switching function, the submodule capacitor voltage function and the bridge arm current function, the electrical component correspondence between the bridge arm current in the electrical parameter and the submodule capacitor voltage is established;

[0088] According to the electrical component correspondence, the current component correspondence of the bridge arm current is established.

[0089] For example, according to the switching function, the submodule capacitor voltage function and the bridge arm current function, the corresponding relationship between the bridge arm current DC component, the submodule capacitor voltage fundamental frequency component and the submodule capacitor, the number of submodules (such as, the electrical component correspondence between the bridge arm current in the electrical parameter and the submodule capacitor voltage) is established as shown in formula (8).

[0090]

[0091] In the formula, n pn n is the number of sub-modules under any period; C p , C n are the upper and lower bridge arm single sub-module capacitances respectively. Substituting formula (1) to formula (2) into formula (8) can obtain expression (9).

[0092]

[0093] It can be seen from formula (9) that the direct current component and the fundamental component of the bridge arm current present a corresponding relationship (such as the current component corresponding relationship of the bridge arm current), when the direct current component or the fundamental component changes, the other component will also change.

[0094] Substituting formula (9) into the expansion formula of formula (8), the fundamental component expression of the upper and lower bridge arm single sub-module capacitance voltage can be obtained, as shown in formula (10).

[0095]

[0096] wherein,

[0097] Specifically, the first corresponding relationship is a corresponding relationship between the direct current component of the bridge arm current in the electrical parameter and the bridge arm parameter.

[0098] Specifically, the bridge arm parameter includes bridge arm resistance, bridge arm reactance, sub-module capacitance and sub-module number.

[0099] Based on the bridge arm KVL (Kirchhoff's voltage law) equation, the direct current voltage U dcm and the corresponding relationship between the bridge arm voltage fundamental component and the bridge arm resistance, bridge arm reactance is shown in formula (12).

[0100]

[0101] In the formula, R arm_p , R arm_n are the bridge arm resistances of the upper and lower bridge arms, L arm_p , L arm_n are the bridge arm reactances of the upper and lower bridge arms.

[0102] Substituting formula (6), formula (7), formula (9) and formula (10) into formula (13) can obtain the first corresponding relationship between the electrical parameter and the bridge arm parameter as shown in formula (13).

[0103]

[0104] wherein,

[0105] According to the first correspondence relationship, a second correspondence relationship between the direct current bias current and the bridge arm parameter is determined as shown in formula (15).

[0106] i v_dc =i n_dc -i p_dc , (15)

[0107] Substituting formula (14) into formula (15), the transformer valve side line current direct current component (direct current bias current) under the asymmetry of the bridge arm parameter can be solved.

[0108] It can be understood that the inconsistency of the sub-module capacitor between the upper and lower bridge arms, the number of redundant sub-modules, the bridge arm reactance, the bridge arm resistance and other parameters will all cause the direct current bias current at the valve side of the converter transformer, and the influence degree of the above-mentioned parameter asymmetry on the direct current bias will be analyzed one by one.

[0109] 1) Bridge arm reactance

[0110] Taking a designed value of 50mH of the bridge arm reactance as an example, based on the reactance manufacturing tolerance of 0% to ±5%, the influence of the bridge arm reactance deviation on the direct current. With the increase of the bridge arm reactance deviation, the transformer valve side direct current bias current gradually increases, and when the upper bridge arm reactance deviation is 5%, the transformer valve side direct current bias current is 7.2A.

[0111] 2) Bridge arm resistance

[0112] Taking a designed value of 0.3Ω of the bridge arm resistance as an example, the resistance deviation range is 0-0.3Ω, and the influence of the bridge arm resistance deviation on the direct current. With the increase of the bridge arm resistance deviation, the transformer valve side direct current gradually increases, and when the bridge arm asymmetry deviation is 0.3Ω, the transformer valve side direct current can reach 3.6A.

[0113] 3) Sub-module number and capacitor

[0114] In addition to the bridge arm resistance and reactance, the asymmetry of the sub-module capacitor and the number of redundant sub-modules between the upper and lower bridge arms will also affect the size of the direct current bias current. Taking a bridge arm rated sub-module number of 226 and a capacitor of 18mF as an example, when the deviation range of the number of redundant sub-modules is 0-16 and the allowable manufacturing tolerance of the sub-module capacitor is 0-5%, the direct current bias current increases with the gradual increase of the deviation. With the increase of the sub-module capacitor and the sub-module number deviation, the transformer valve side direct current bias current gradually increases. When the sub-module capacitor deviation is 5%, the transformer valve side direct current bias current can reach 2.3A. When the number of redundant sub-modules is exhausted, the transformer valve side direct current bias current can reach 4A.

[0115] The DC bias current determination method disclosed by the embodiment of the present application can establish the corresponding relationship between the electrical parameters and the bridge arm parameters by considering the influence of the MMC bridge arm parameter asymmetry, so as to determine the one-to-one corresponding relationship between each bridge arm parameter asymmetry and the DC bias current and the DC bias current, provide theoretical support for the research on the DC magnetic bias influencing factors, help to realize the engineering suppression of the DC magnetic bias from the converter valve level, reduce the occurrence of the DC magnetic bias phenomenon, reduce the vibration and damage of the transformer, and improve the reliability and stability of the flexible DC power transmission system.

[0116] Referring to Figure 3 , Figure 3 is a structural schematic diagram of a DC bias current determination device 10 provided by the embodiment of the present application. The DC bias current determination device 10 comprises:

[0117] The related parameter acquisition module 11 is configured to acquire the bridge arm parameters and the electrical parameters of the modular multilevel converter in the flexible DC power transmission system.

[0118] The correlation function establishment module 12 is configured to establish the correlation function of the electrical parameters according to the bridge arm parameters and the electrical parameters.

[0119] The first relationship establishment module 13 is configured to establish the first corresponding relationship between the electrical parameters and the bridge arm parameters according to the correlation function.

[0120] The second relationship establishment module 14 is configured to determine the second corresponding relationship between the DC bias current and the bridge arm parameters according to the first corresponding relationship.

[0121] The bias current determination module 15 is configured to solve the DC bias current under the asymmetry of the bridge arm parameters according to the second corresponding relationship.

[0122] The DC bias current determination device 10 provided by the embodiment of the present application can realize all the processes of the DC bias current determination method of the above-mentioned embodiment. The functions of each module in the device and the technical effects achieved are the same as the functions of the DC bias current determination method of the above-mentioned embodiment and the technical effects achieved, which will not be repeated here.

[0123] The above is the preferred embodiment of the present application, it should be pointed out that, for those skilled in the art, without departing from the principles of the present application, can also make a number of improvements and refinements, these improvements and refinements are also considered to be within the scope of the present application.

Claims

1. A direct current bias current determination method characterized by, The method comprises the following steps: obtaining bridge arm parameters and electrical parameters of a modular multilevel converter in a flexible direct current power transmission system; establishing a correlation function of the electrical parameters according to the bridge arm parameters and the electrical parameters; establishing a first correspondence between the electrical parameters and the bridge arm parameters according to the correlation function; determining a second correspondence between a direct current bias current and the bridge arm parameters according to the first correspondence; solving the direct current bias current under asymmetry of the bridge arm parameters according to the second correspondence.

2. The DC bias current determination method of claim 1, wherein, The step of establishing the correlation function of the electrical parameters according to the bridge arm parameters and the electrical parameters comprises the following steps: establishing a switching function of a sub-module according to the bridge arm parameters and the electrical parameters; establishing a sub-module capacitor voltage function, a bridge arm current function and a bridge arm voltage function according to the electrical parameters.

3. The DC bias current determination method of claim 2, wherein, The step of establishing the first correspondence between the electrical parameters and the bridge arm parameters according to the correlation function comprises the following steps: establishing a component correspondence of the electrical parameters according to the correlation function; establishing the first correspondence between the electrical parameters and the bridge arm parameters according to the component correspondence and a bridge arm KVL equation.

4. The DC bias current determination method of claim 3, wherein, The step of establishing the component correspondence of the electrical parameters according to the correlation function comprises the following steps: establishing a voltage component correspondence of a sub-module capacitor voltage and a direct current component correspondence between a bridge arm voltage in the electrical parameters and the sub-module capacitor voltage according to the switching function, the sub-module capacitor voltage function and the bridge arm voltage function; establishing an electrical component correspondence between a bridge arm current in the electrical parameters and the sub-module capacitor voltage and a current component correspondence of the bridge arm current according to the switching function, the sub-module capacitor voltage function and the bridge arm current function.

5. The DC bias current determination method of claim 3, wherein, The step of establishing the first correspondence between the electrical parameters and the bridge arm parameters according to the component correspondence and the bridge arm KVL equation comprises the following steps: establishing an electrical correspondence between a direct current voltage in the electrical parameters and a bridge arm voltage and a bridge arm resistance and a bridge arm reactance in the bridge arm parameters according to the bridge arm KVL equation; establishing the first correspondence between the electrical parameters and the bridge arm parameters according to the electrical correspondence and the component correspondence.

6. The DC offset current determination method of claim 4, wherein, The step of establishing the voltage component correspondence of the sub-module capacitor voltage and the direct current component correspondence between the bridge arm voltage in the electrical parameters and the sub-module capacitor voltage according to the switching function, the sub-module capacitor voltage function and the bridge arm voltage function comprises the following steps: establishing a function correspondence of the switching function, the sub-module capacitor voltage function and the bridge arm voltage function; establishing the voltage component correspondence of the sub-module capacitor voltage according to the function correspondence; establishing the direct current component correspondence between the bridge arm voltage in the electrical parameters and the sub-module capacitor voltage according to the function correspondence and the voltage component correspondence.

7. The DC offset current determination method of claim 4, wherein, The step of establishing the electrical component correspondence between the bridge arm current in the electrical parameters and the sub-module capacitor voltage according to the switching function, the sub-module capacitor voltage function and the bridge arm current function comprises the following steps: and a current component corresponding relationship of the bridge arm current, comprising: According to the switching function, the submodule capacitor voltage function and the bridge arm current function, an electrical component corresponding relationship between the bridge arm current and the submodule capacitor voltage in the electrical parameters is established; According to the electrical component corresponding relationship, a current component corresponding relationship of the bridge arm current is established.

8. The DC bias current determination method of claim 1, wherein, The first corresponding relationship is a corresponding relationship between a direct current component of the bridge arm current and the bridge arm parameter in the electrical parameters.

9. The DC bias current determination method of claim 1, wherein, The bridge arm parameter comprises a bridge arm resistance, a bridge arm reactance, a submodule capacitor and a submodule number.

10. A direct current bias current determination apparatus characterized by comprising: Comprise: A related parameter acquisition module is used to acquire a bridge arm parameter and an electrical parameter of a modular multilevel converter in a flexible direct current power transmission system; A related function establishing module is used to establish a related function of the electrical parameter according to the bridge arm parameter and the electrical parameter; A first relationship establishing module is used to establish a first corresponding relationship between the electrical parameter and the bridge arm parameter according to the related function; A second relationship establishing module is used to determine a second corresponding relationship between a direct current bias current and the bridge arm parameter according to the first corresponding relationship; A bias current determining module is used to solve the direct current bias current under asymmetry of the bridge arm parameter according to the second corresponding relationship.

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