A DC bias suppression method, system and storage medium

Abstract

The present invention discloses a method, system and storage medium for suppressing DC bias, which relates to the field of DC power transmission technology. A switching function model is established according to the voltage of each phase of the three-phase bridge arm in a flexible DC converter valve; according to the switching function model, the bridge arm current is expressed as the DC component and the AC fundamental frequency component of the bridge arm to obtain the bridge arm current component expression; according to the bridge arm current component expression, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained; according to the voltage input and the current input, a DC bias suppression strategy for a flexible DC system is generated. By adopting the embodiment of the present invention, the three-phase generation mechanism of DC bias can be analyzed based on the current and voltage conditions of the three-phase bridge arm in the flexible DC converter valve, the DC component in an asymmetric scenario can be identified in time, and effective suppression can be performed from the transformer valve side to optimize the control of the flexible DC system.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current power transmission, and in particular to a direct current bias suppression method, system and storage medium. Background Art

[0002] With the increasing maturity of flexible DC transmission technology, the number of flexible DC projects at home and abroad has grown rapidly, and the voltage level and transmission capacity have continued to break through. UHV flexible DC transmission projects have also become an important milestone in future DC transmission projects.

[0003] Flexible DC power transmission achieves AC / DC conversion by controlling the on and off of the converter valve submodules and isolating the electrical quantities on the AC and DC sides. This process requires the cooperation of the converter valve primary equipment and the control system. In actual projects, any bridge arm of the flexible DC converter valve is composed of hundreds of submodules. A slight error or failure in the parameters of any submodule will cause asymmetry in the bridge arm parameters, making it impossible to completely isolate the AC and DC side components of the converter valve, resulting in DC bias of the electrical quantities in the AC system.

[0004] The emergence of DC components will increase the electrical stress of the AC side equipment of the converter valve, increase equipment losses, and in extreme cases may cause equipment overheating or even damage. Existing research has not conducted relevant research on this DC bias phenomenon, and its generation mechanism is still unclear, resulting in the inability of existing control strategies to effectively suppress DC components in asymmetric scenarios. Summary of the invention

[0005] The technical problem to be solved by the present invention is how to suppress the DC bias phenomenon of a flexible DC system. A DC bias suppression method, system and storage medium are provided, which can timely identify the DC component in an asymmetric scenario and effectively suppress it from the transformer valve side, thereby optimizing the control of the flexible DC system.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a method for suppressing a DC bias, comprising:

[0007] A switching function model is established according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve;

[0008] According to the switching function model, the bridge arm current is expressed as a DC component and an AC fundamental frequency component of the bridge arm to obtain a bridge arm current component expression;

[0009] According to the bridge arm current component expression, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained;

[0010] A DC bias suppression strategy for a flexible DC system is generated according to the voltage input and the current input.

[0011] As an improvement of the above solution, the switch function model is established according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve, including:

[0012] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal, and a bridge arm switch function model of the single bridge arm is obtained;

[0013] According to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, so as to obtain the switch function model.

[0014] As an improvement of the above solution, the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal, and a bridge arm switch function model of the single bridge arm is obtained, including:

[0015] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal;

[0016] For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm;

[0017] For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm;

[0018] in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

[0019] As an improvement of the above solution, according to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, and the switch function model is obtained, including:

[0020] According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm;

[0021] pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm;

[0022] pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm;

[0023] A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

[0024] As an improvement of the above solution, the bridge arm current is expressed as a DC component and an AC fundamental frequency component of the bridge arm according to the switching function model to obtain a bridge arm current component expression, including:

[0025] According to the current characteristics of the bridge arm submodule, the component composition of the bridge arm current is obtained;

[0026] According to the switching function model and the component composition of the bridge arm current, a bridge arm current component expression is obtained.

[0027] As an improvement of the above solution, the component composition of the bridge arm current is obtained according to the current characteristics of the bridge arm submodule, including:

[0028] pass , and obtain the component composition of the bridge arm current;

[0029] in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

[0030] As an improvement of the above solution, the bridge arm current component expression is obtained according to the switching function model and the component composition of the bridge arm current, including:

[0031] From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained;

[0032] According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

[0033] As an improvement of the above solution, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained according to the bridge arm current component expression, including:

[0034] Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the bridge arm current component expression, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules;

[0035] According to the DC current of the upper bridge arm and the lower bridge arm DC current The voltage input on the valve side of the converter valve transformer in the steady state is obtained.

[0036] As an improvement of the above solution, the DC bias suppression strategy of the flexible DC system is generated according to the voltage input and the current input, including:

[0037] Obtaining a DC bias amount according to the voltage input amount and the current input amount;

[0038] A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

[0039] The embodiment of the present invention further provides a DC bias suppression system, comprising:

[0040] A model building module, used to build a switch function model according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve;

[0041] A bridge arm current component expression generation module, used for expressing the bridge arm current as a DC component and an AC fundamental frequency component of the bridge arm according to the switching function model, to obtain a bridge arm current component expression;

[0042] A valve-side electrical quantity calculation module, used to obtain the voltage input quantity and current input quantity of the valve side of the converter valve transformer in a steady state according to the bridge arm current component expression;

[0043] The DC bias suppression strategy generation module is used to generate a DC bias suppression strategy for the flexible DC system according to the voltage input and the current input.

[0044] As an improvement of the above solution, the model building module includes:

[0045] A bridge arm switch function model establishment unit, used to set the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve to be equal, and obtain a bridge arm switch function model of the single bridge arm;

[0046] The switch function model building unit is used to obtain the switch function model by using the rated number of submodules to represent the switch state of the submodules in the bridge arm switch function model according to the engineering direct modulation strategy.

[0047] As an improvement of the above solution, the bridge arm switch function model building unit is specifically used for:

[0048] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal;

[0049] For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm;

[0050] For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm;

[0051] in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

[0052] As an improvement of the above solution, the switch function model building unit is specifically used for:

[0053] According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm;

[0054] pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm;

[0055] pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm;

[0056] A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

[0057] As an improvement of the above solution, the bridge arm current component expression generation module includes:

[0058] A component composition acquisition unit, used to obtain the component composition of the bridge arm current according to the current characteristics of the bridge arm submodule;

[0059] The component expression generating unit is used to obtain the bridge arm current component expression according to the switching function model and the component composition of the bridge arm current.

[0060] As an improvement of the above solution, the component composition acquisition unit is specifically used for:

[0061] pass , and obtain the component composition of the bridge arm current;

[0062] in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

[0063] As an improvement of the above solution, the component expression generation unit is specifically used to:

[0064] From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained;

[0065] According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

[0066] As an improvement of the above solution, the valve-side electrical quantity calculation module is specifically used for:

[0067] Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the bridge arm current component expression, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules;

[0068] According to the DC current of the upper bridge arm and the lower bridge arm DC current The voltage input on the valve side of the converter valve transformer in the steady state is obtained.

[0069] As an improvement of the above solution, the DC bias suppression strategy generation module is specifically used to:

[0070] Obtaining a DC bias amount according to the voltage input amount and the current input amount;

[0071] A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

[0072] An embodiment of the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium includes a stored computer program, wherein when the computer program is executed, the device where the computer-readable storage medium is located is controlled to execute a DC bias suppression method as described in any one of the above.

[0073] Compared with the prior art, the present invention discloses a method, system and storage medium for suppressing DC bias. The method establishes a switching function model according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve; according to the switching function model, the bridge arm current is expressed as the DC component and the AC fundamental frequency component of the bridge arm to obtain the bridge arm current component expression; according to the bridge arm current component expression, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained; according to the voltage input and the current input, a DC bias suppression strategy for the flexible DC system is generated. By adopting the embodiment of the present invention, the three-phase generation mechanism of DC bias can be analyzed based on the current and voltage conditions of the three-phase bridge arm in the flexible DC converter valve, the DC component in the asymmetric scenario can be identified in time, and effective suppression can be performed from the transformer valve side to optimize the control of the flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 It is a schematic flow chart of the steps of a method for suppressing a DC bias provided by an embodiment of the present invention;

[0075] Figure 2 It is a structural schematic diagram of a DC bias suppression system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0076] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0077] In the description of the specification and claims, it is to be understood that the terms first, second, etc. in the specification and claims are only used for the purpose of describing the same technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated, nor necessarily describing the order or time sequence. The terms are interchangeable where appropriate. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0078] Based on the above considerations, an embodiment of the present invention provides a method for suppressing DC bias. Figure 1 In this embodiment, the DC bias suppression method is specifically performed through steps S1 to S4:

[0079] S1. A switching function model is established according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve.

[0080] Each phase of the three-phase bridge arm in the converter valve of the flexible DC system includes an upper and lower bridge arm. Each bridge arm is composed of hundreds of sub-modules. A slight error or fault in the parameters of any sub-module will cause asymmetry of the bridge arm parameters, making it impossible to completely isolate the AC and DC side components of the converter valve, and thus a DC bias will appear in the electrical quantities in the AC system.

[0081] It should be noted that, in the embodiment of the present invention, the bridge arm includes an upper bridge arm and a lower bridge arm, which have similar characteristics, so the electrical quantities of the upper and lower bridge arms also have similar expressions.

[0082] It should also be noted that the switching function model can represent the relationship between the switching state of the submodule and the capacitor voltage and output voltage of each submodule on the same bridge arm.

[0083] S2. According to the switching function model, the bridge arm current is expressed as a DC component and an AC fundamental frequency component of the bridge arm to obtain a bridge arm current component expression.

[0084] It should be noted that the complete expression of the bridge arm current should be composed of the DC component of the bridge arm and the AC harmonic components. Considering that the influence of high-order harmonic components on the bridge arm current will gradually weaken with the increase of the harmonic order, in the embodiment of the present invention, the high-order harmonic components are ignored and only the AC fundamental frequency components are considered.

[0085] It should also be noted that the phases of the AC components of each phase of the three-phase bridge arm are different. When calculating the expression of the bridge arm current component, it is necessary to take the phases of each phase into consideration and calculate each phase separately.

[0086] S3. According to the bridge arm current component expression, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained.

[0087] It should be noted that, in the embodiment of the present invention, the DC bias suppression strategy proposed based on the converter transformer valve side voltage or current as the input quantity can provide ideas for system optimization control from the generation mechanism of DC bias.

[0088] S4. Generate a DC bias suppression strategy for the flexible DC system according to the voltage input and the current input.

[0089] In the above scheme, starting from the current and voltage conditions of the three-phase bridge arm in the flexible DC converter valve, the three-phase generation mechanism of DC bias is analyzed, which can timely identify the DC component in the asymmetric scenario and effectively suppress it from the transformer valve side to optimize the control of the flexible DC system.

[0090] As an improvement in the above solution, step S1, establishing a switching function model according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve, includes:

[0091] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal, and a bridge arm switch function model of the single bridge arm is obtained;

[0092] According to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, so as to obtain the switch function model.

[0093] It should be noted that if the rated number of sub-modules of the bridge arm is N, and the number of sub-modules actually participating in the operation of the upper bridge arm is n1, then the number of redundant sub-modules is n1-N. Although the setting of redundant sub-modules may lead to DC bias, it is necessary to ensure the robustness of the flexible DC system.

[0094] In the embodiment of the present invention, setting the capacitor voltages of all submodules in a single bridge arm of the flexible DC converter valve to be equal means that the switching frequency is regarded as infinite.

[0095] It should also be noted that, preferably, the engineering direct modulation strategy controls the switching state of each submodule so that the output voltage is close to a sine wave.

[0096] Furthermore, the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal, and a bridge arm switch function model of the single bridge arm is obtained, including:

[0097] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal;

[0098] For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm;

[0099] For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm;

[0100] in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

[0101] It should be noted that, illustratively, in the embodiment of the present invention, the submodule is connected through two switch units. and To act, when Open, When the submodule is turned off, it is in the on state, and the capacitance will change; Shutdown, When turned on, the submodule is in the cut-off state, and the capacitor voltage remains stable. and The situation of simultaneous shutdown, but this situation generally occurs only when a fault occurs. The embodiment of the present invention considers the DC bias suppression under normal operating conditions, so this situation is not considered.

[0102] As a preferred implementation, according to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, and the switch function model is obtained, including:

[0103] According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm;

[0104] pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm;

[0105] pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm;

[0106] A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

[0107] It should be noted that the output voltage of the bridge arm is composed of the output voltages of each submodule, that is, the number of submodules needs to be considered in the engineering direct modulation strategy. and These are modulation parameters.

[0108] As a preferred implementation, step S2, according to the switching function model, expressing the bridge arm current as a DC component and an AC fundamental frequency component of the bridge arm, and obtaining a bridge arm current component expression, includes:

[0109] According to the current characteristics of the bridge arm submodule, the component composition of the bridge arm current is obtained;

[0110] According to the switching function model and the component composition of the bridge arm current, a bridge arm current component expression is obtained.

[0111] It should be noted that, according to the current characteristics of the bridge arm submodule, the bridge arm current is composed of a DC component and AC harmonic components, and when the bridge arm is symmetrical, the harmonic components only have even-order harmonic components.

[0112] Further, preferably, obtaining the component composition of the bridge arm current according to the current characteristics of the bridge arm submodule includes:

[0113] pass , and obtain the component composition of the bridge arm current;

[0114] in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

[0115] Furthermore, in some preferred embodiments, obtaining the bridge arm current component expression according to the switching function model and the component composition of the bridge arm current includes:

[0116] From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained;

[0117] According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

[0118] It should be noted that in the embodiment of the present invention, for the AC component of the bridge arm current, only the AC fundamental frequency component is considered, and the first component is obtained by combining the DC current. The bridge arm current component expression obtained based on this can be used to analyze whether the upper and lower bridge arms operate symmetrically.

[0119] By combining the switching function model and the component composition of the bridge arm current, it can be obtained ,in, is the current of the upper bridge arm, The final operation point of the flexible DC system will satisfy the DC component in the bridge arm current component expression to be 0. When only the DC current and the AC fundamental frequency component are considered, the bridge arm current component expression described in the embodiment of the present invention can be obtained.

[0120] Furthermore, by jointly analyzing the a, b, and c phases of the three-phase bridge arm, we can obtain: ;

[0121] in, is the DC component of the upper bridge arm of phase a, is the DC component of the lower bridge arm of phase a, is the AC fundamental frequency component of the upper bridge arm of phase a, is the AC fundamental frequency component of the lower bridge arm of phase a; is the DC component of the upper bridge arm of phase b, is the DC component of the lower bridge arm of phase b, is the AC fundamental frequency component of the upper bridge arm of phase b, is the AC fundamental frequency component of the lower bridge arm of phase b; is the DC component of the upper bridge arm of phase c, is the DC component of the lower bridge arm of phase c, is the AC fundamental frequency component of the upper bridge arm of phase c, is the AC fundamental frequency component of the lower bridge arm of phase c; is the total DC current of the upper bridge arm of each phase; is the total DC current of the lower bridge arm of each phase.

[0122] Since the current on the DC side is determined by the control links of the peer station and the local station, it is preferred to set .

[0123] Further, as a preferred implementation, step S3, obtaining the voltage input and current input on the valve side of the converter valve transformer in a steady state according to the bridge arm current component expression, includes:

[0124] Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the bridge arm current component expression, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules;

[0125] According to the DC current of the upper bridge arm and the lower bridge arm DC current The voltage input on the valve side of the converter valve transformer in the steady state is obtained.

[0126] For example, As the total DC current condition of the upper and lower bridge arms, combined with the switching function model and the component composition of the bridge arm current, the upper bridge arm is analyzed to obtain .

[0127] It can be understood that in steady state, the current relationship satisfies:

[0128] .

[0129] When the AC component only considers the AC fundamental frequency component, the steady-state condition is:

[0130] .

[0131] Solving the steady-state condition yields ;in, , .

[0132] It can be understood that the above is obtained by analyzing phase a in the three-phase bridge arm. Similarly, similar expressions can be obtained for each phase in the three-phase bridge arm, except that their phases and specific parameters may be different.

[0133] As a preferred implementation, step S4, generating a DC bias suppression strategy for a flexible DC system according to the voltage input and the current input, includes:

[0134] Obtaining a DC bias amount according to the voltage input amount and the current input amount;

[0135] A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

[0136] According to the above analysis, when the number of sub-modules or the capacitance of the upper and lower bridge arms of any phase are asymmetric, it will cause asymmetry in the DC components of the upper and lower bridge arm currents or the bridge arm capacitor voltages; when there is no asymmetry in the number of sub-modules or the capacitance of the upper and lower bridge arms, the DC components of the upper and lower bridge arm currents or the bridge arm capacitor voltages are in a corresponding relationship, that is, when the DC currents of the upper and lower bridge arms are asymmetric, the bridge arm capacitor voltage is also asymmetric.

[0137] For a three-phase six-bridge MMC, when there is an asymmetric DC component in the upper and lower bridge arms of any phase, the DC bias current generated by it forms a phase-to-phase path through the converter transformer, causing a DC bias voltage and current to appear in the symmetrical phase. It can be seen that the asymmetry of any bridge arm will cause DC bias to appear in all three phases of the system.

[0138] It should be noted that when designing the controller through the dq axis components, the dq decomposition process filters out most of the harmonics in the voltage and current. However, some DC components still leak into the controller and cause a response to the input modulation wave.

[0139] In the embodiment of the present invention, by analyzing the voltage input and the current input, the DC bias phenomenon of the three-phase bridge arm can be further analyzed, and DC bias suppression from the transformer valve side will achieve a better DC bias suppression effect.

[0140] A method for suppressing DC bias provided in an embodiment of the present invention can analyze the three-phase generation mechanism of DC bias based on the current and voltage conditions of the three-phase bridge arms in the flexible DC converter valve, timely identify the DC component in an asymmetric scenario, and effectively suppress it from the transformer valve side, thereby optimizing the control of the flexible DC system.

[0141] The embodiment of the present invention provides a DC bias suppression system. Figure 2 The DC bias suppression system includes a model building module 11, a bridge arm current component expression generation module 12, a valve side electrical quantity calculation module 13 and a DC bias suppression strategy generation module 14, wherein:

[0142] The model building module 11 is used to build a switch function model according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve;

[0143] A bridge arm current component expression generation module 12 is used to express the bridge arm current as a DC component and an AC fundamental frequency component of the bridge arm according to the switching function model, so as to obtain a bridge arm current component expression;

[0144] The valve-side electrical quantity calculation module 13 is used to obtain the voltage input quantity and current input quantity of the valve side of the converter valve transformer in a steady state according to the bridge arm current component expression;

[0145] The DC bias suppression strategy generation module 14 is used to generate a DC bias suppression strategy for the flexible DC system according to the voltage input and the current input.

[0146] As a preferred implementation, the model building module 11 includes:

[0147] A bridge arm switch function model establishment unit, used to set the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve to be equal, and obtain a bridge arm switch function model of the single bridge arm;

[0148] The switch function model building unit is used to obtain the switch function model by using the rated number of submodules to represent the switch state of the submodules in the bridge arm switch function model according to the engineering direct modulation strategy.

[0149] Further, preferably, the bridge arm switch function model building unit is specifically used to:

[0150] The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal;

[0151] For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm;

[0152] For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm;

[0153] in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

[0154] Preferably, the switch function model building unit is specifically used to:

[0155] According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm;

[0156] pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm;

[0157] pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm;

[0158] A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

[0159] As a preferred implementation, the bridge arm current component expression generation module 12 includes:

[0160] A component composition acquisition unit, used to obtain the component composition of the bridge arm current according to the current characteristics of the bridge arm submodule;

[0161] The component expression generating unit is used to obtain the bridge arm current component expression according to the switching function model and the component composition of the bridge arm current.

[0162] Further, preferably, the component composition acquisition unit is specifically used for:

[0163] pass , and obtain the component composition of the bridge arm current;

[0164] in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

[0165] Furthermore, preferably, the component expression generating unit is specifically used for:

[0166] From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained;

[0167] According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

[0168] As a preferred implementation, the valve-side electrical quantity calculation module 13 is specifically used for:

[0169] Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the bridge arm current component expression, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules;

[0170] According to the DC current of the upper bridge arm and the lower bridge arm DC current The voltage input on the valve side of the converter valve transformer in the steady state is obtained.

[0171] As a preferred implementation, the DC bias suppression strategy generation module 14 is specifically used for:

[0172] Obtaining a DC bias amount according to the voltage input amount and the current input amount;

[0173] A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

[0174] A DC bias suppression system provided by an embodiment of the present invention can analyze the three-phase generation mechanism of DC bias based on the current and voltage conditions of the three-phase bridge arms in the flexible DC converter valve, can timely identify the DC component in an asymmetric scenario, and effectively suppress it from the transformer valve side, thereby optimizing the control of the flexible DC system.

[0175] An embodiment of the present invention also provides a DC bias suppression device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, a DC bias suppression method as described above is implemented. The working principles and beneficial effects of the two correspond one to one, and thus are not described in detail.

[0176] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0177] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for suppressing a DC bias, characterized in that: include: A switching function model is established according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve; According to the switching function model, the bridge arm current is expressed as a DC component and an AC fundamental frequency component of the bridge arm to obtain a bridge arm current component expression; According to the bridge arm current component expression, the voltage input and current input on the valve side of the converter valve transformer in a steady state are obtained; Generating a DC bias suppression strategy for a flexible DC system according to the voltage input and the current input; The method of obtaining the voltage input and current input on the valve side of the converter valve transformer in a steady state according to the bridge arm current component expression includes: Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the bridge arm current component expression, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules; According to the DC current of the upper bridge arm and the lower arm DC current The current input on the valve side of the converter valve transformer in the steady state is obtained.

2. A DC bias suppression method as claimed in claim 1, characterized in that: The method of establishing a switch function model according to the voltage of each phase of the three-phase bridge arm in the flexible direct current converter valve includes: The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal, and a bridge arm switch function model of the single bridge arm is obtained; According to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, so as to obtain the switch function model.

3. A DC bias suppression method as claimed in claim 2, characterized in that: The setting of the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve to be equal, and obtaining a bridge arm switch function model of the single bridge arm, includes: The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal; For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm; For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm; in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

4. A DC bias suppression method as claimed in claim 3, characterized in that: According to the engineering direct modulation strategy, the rated number of submodules is used in the bridge arm switch function model to represent the switch state of the submodule, and the switch function model is obtained, including: According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm; pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm; pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm; A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

5. A DC bias suppression method as claimed in claim 4, characterized in that: The bridge arm current is expressed as a DC component and an AC fundamental frequency component of the bridge arm according to the switching function model to obtain a bridge arm current component expression, including: According to the current characteristics of the bridge arm submodule, the component composition of the bridge arm current is obtained; According to the switching function model and the component composition of the bridge arm current, a bridge arm current component expression is obtained.

6. A DC bias suppression method as claimed in claim 5, characterized in that: The component composition of the bridge arm current is obtained according to the current characteristics of the bridge arm submodule, including: pass , and obtain the component composition of the bridge arm current; in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

7. A DC bias suppression method as claimed in claim 6, characterized in that: The obtaining the bridge arm current component expression according to the switching function model and the bridge arm current component comprises: From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained; According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

8. A DC bias suppression method as claimed in claim 1, characterized in that: The step of generating a DC bias suppression strategy for a flexible DC system according to the voltage input and the current input comprises: Obtaining a DC bias amount according to the voltage input amount and the current input amount; A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

9. A DC bias suppression system, characterized in that: include: A model building module, used to build a switch function model according to the voltage of each phase of the three-phase bridge arm in the flexible DC converter valve; A bridge arm current component expression generation module, used for expressing the bridge arm current as a DC component and an AC fundamental frequency component of the bridge arm according to the switching function model, to obtain a bridge arm current component expression; A valve-side electrical quantity calculation module, used to obtain the voltage input quantity and current input quantity of the valve side of the converter valve transformer in a steady state according to the bridge arm current component expression; A DC bias suppression strategy generation module, used to generate a DC bias suppression strategy for a flexible DC system according to the voltage input and the current input; The valve-side electrical quantity calculation module is specifically used for: Substituting the corresponding relationship between the AC fundamental frequency components of the three-phase bridge arm under steady-state conditions into the expression of the bridge arm current component, we can obtain ;in, is the voltage input on the valve side of the converter valve transformer in steady state; is the total resistance of the submodules of this phase; is the number of submodules; According to the DC current of the upper bridge arm and the lower bridge arm DC current The current input on the valve side of the converter valve transformer in the steady state is obtained.

10. A DC bias suppression system as claimed in claim 9, characterized in that: The model building module comprises: A bridge arm switch function model establishment unit, used to set the capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve to be equal, and obtain a bridge arm switch function model of the single bridge arm; The switch function model building unit is used to obtain the switch function model by using the rated number of submodules to represent the switch state of the submodules in the bridge arm switch function model according to the engineering direct modulation strategy.

11. A DC bias suppression system as claimed in claim 10, characterized in that: The bridge arm switch function model building unit is specifically used for: The capacitor voltages of all submodules in a single bridge arm of the flexible direct current converter valve are set to be equal; For the upper bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the upper bridge arm; For the lower bridge arm of each phase of the three-phase bridge arm in the flexible DC converter valve, , Establishing a single submodule switch function model of the i-th submodule in the lower bridge arm; in, is the switch state of the i-th submodule in the upper bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the upper bridge arm; is the capacitor voltage of a submodule in the upper bridge arm; is the output voltage of the upper bridge arm; is the number of submodules in the upper bridge arm; is the switch state of the jth submodule in the lower bridge arm, hour, Open, Shutdown, hour, Shutdown, Open; is the current flowing through the lower bridge arm; is the capacitor voltage of a submodule in the lower bridge arm; is the output voltage of the lower bridge arm; is the number of submodules in the lower bridge arm.

12. A DC bias suppression system as claimed in claim 11, characterized in that: The switch function model building unit is specifically used for: According to the engineering direct modulation strategy, Get the relationship between the number of submodules put into use in the upper bridge arm and the switch status of the submodule; The relationship between the number of submodules put into operation in the upper bridge arm and the switch state of the submodule is obtained; wherein, is the rated number of submodules of the bridge arm; pass Obtain the upper bridge arm switching function model of each phase of the three-phase bridge arm; pass Obtain the lower bridge arm switch function model of each phase of the three-phase bridge arm; A switching function model is obtained according to the upper bridge arm switching function model and the lower bridge arm switching function model.

13. A DC bias suppression system as claimed in claim 12, characterized in that: The bridge arm current component expression generation module comprises: A component composition acquisition unit, used to obtain the component composition of the bridge arm current according to the current characteristics of the bridge arm submodule; The component expression generating unit is used to obtain the bridge arm current component expression according to the switching function model and the component composition of the bridge arm current.

14. A DC bias suppression system as claimed in claim 13, characterized in that: The component composition acquisition unit is specifically used for: pass , and obtain the component composition of the bridge arm current; in, is the DC component of the upper arm current; is the amplitude of the mth AC component of the upper arm current; is the phase of the mth AC component of the upper arm current; is the DC component of the lower arm current; is the amplitude of the mth AC component of the lower arm current; is the phase of the mth AC component of the lower arm current.

15. A DC bias suppression system as claimed in claim 14, characterized in that: The component expression generation unit is specifically used for: From the component composition of the bridge arm current, a first component composition of a direct current and an alternating current fundamental frequency component is obtained; According to the switching function model and the first component of the bridge arm current, the bridge arm current component expression is obtained: .

16. A DC bias suppression system as claimed in claim 9, characterized in that: The DC bias suppression strategy generation module is specifically used for: Obtaining a DC bias amount according to the voltage input amount and the current input amount; A DC bias suppression strategy is provided based on the DC bias amount and generated from the valve side of the converter transformer of the flexible DC system.

17. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored computer program, wherein when the computer program is executed, the device where the computer-readable storage medium is located is controlled to execute a method for suppressing a DC bias as claimed in any one of claims 1 to 8.

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