A flexible transformer multiplexing type converter and a transformer parameter design method thereof
Patent Information
- Application Number
- CN202611379974.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-09-07
- Publication Date
- 2026-10-30
AI Technical Summary
[0003]本发明提供了一种柔直变压器复用型换流器及其变压器参数设计方法，以解决海上换流站占地面积大的问题
本发明提供一种柔直变压器复用型换流器的变压器参数设计方法和拓扑结构，用三绕组变压器替代传统双绕组三相变压器，三绕组变压器同时起到传统柔直变压器和桥臂电抗器的功能，省去了单独的桥臂电抗器及其布置，节约了海上换流站的占地面积，提升了海上风电轻型化柔性直流送出工程的经济性。
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Figure CN123019467A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of AC / DC power transmission and distribution technology, specifically to a flexible DC transformer multiplexing converter and its transformer parameter design method. Background Technology
[0002] As transmission capacity and distance continue to increase, the size and weight of key equipment in offshore converter stations will also increase, making their installation, transportation, and maintenance more difficult. This poses economic and technical challenges to the construction of flexible DC grid-connected systems for offshore wind power. Therefore, optimizing the design of key equipment in flexible DC converter valves to achieve economical and efficient transmission of offshore wind power is an important issue for the large-scale development of offshore wind power. Summary of the Invention
[0003] This invention provides a flexible DC-DC transformer multiplex converter and its transformer parameter design method to solve the problem of large footprint of offshore converter stations.
[0004] In a first aspect, the present invention provides a flexible DC transformer multiplex converter, including a bridge arm module and three three-winding transformers; The bridge arm module includes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, and a sixth bridge arm. The high-voltage terminals of the first, second, and third bridge arms are all connected to the positive terminal of the DC bus, while the high-voltage terminals of the fourth, fifth, and sixth bridge arms are all connected to the negative terminal of the DC bus. The three-winding transformer includes a first winding, a second winding, and a third winding. The low-voltage terminals of the first, second, and third bridge arms are respectively connected to the same-name terminals of the second windings of the corresponding three-winding transformers. The low-voltage terminals of the fourth, fifth, and sixth bridge arms are respectively connected to the same-name terminals of the third windings of the corresponding three-winding transformers. The opposite-name terminals of the second windings and the opposite-name terminals of the third windings of the corresponding three-winding transformers are connected in series. The same-name terminals of the first windings of the three three-winding transformers are respectively connected to AC bus A, B, and C. The opposite-name terminals of the first windings of the three three-winding transformers are interconnected.
[0005] In one alternative implementation, each bridge arm includes several sub-modules connected in series, and the sub-modules are half-bridge sub-modules.
[0006] In a second aspect, the present invention provides a transformer parameter design method for a flexible DC-DC transformer multiplexer converter, applicable to the converter described in the first aspect, comprising: Based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition, the constraint relationship of the converter's connection reactance is obtained. Determine the equivalent reactance of each winding of the transformer based on the constraint relationships; The short-circuit voltage percentage of each winding of the transformer is calculated based on the equivalent reactance, and the equivalent reactance and short-circuit voltage percentage are output as parameters of each winding of the transformer.
[0007] In one optional implementation, the constraint relationship of the converter's connection reactance is obtained based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition, including: The first constraint relationship of the connection reactance is obtained based on the circulating current resonance suppression condition. The first constraint relationship is used to prevent the converter from experiencing circulating current resonance. The second constraint relationship of the connecting reactance is obtained based on the AC fault current limiting condition. The second constraint relationship is used to ensure that the bridge arm current of the converter is lower than the preset current limit under AC fault. The third constraint relationship of the connecting reactance is obtained based on the DC fault current rise rate limiting condition. The third constraint relationship is used to ensure that the current rise rate of the bridge arm of the converter is lower than the preset rise rate limit under DC fault. The constraint relationship includes the first constraint relationship, the second constraint relationship and the third constraint relationship.
[0008] In one alternative implementation, the circulating resonance suppression condition includes:
[0009] in, It is the series resonant angular frequency. The inductance value of the bridge arm. The capacitance value of the submodule in the bridge arm. The number of sub-modules in the bridge arm. The system angular frequency is the rated angular frequency of the AC power system connected to the converter. is a constant and .
[0010] In one optional implementation, the DC fault current rise rate limiting condition includes: When the converter is in an AC fault, the maximum current increment of the bridge arm is calculated according to a first preset formula. The maximum current increment is lower than a preset current limit. The first preset formula is as follows:
[0011] in, For the maximum increment, The system angular frequency, The voltage modulation amplitude of the bridge arm. This is the equivalent resistance of the bridge arm. To determine the value of the connecting inductor, To control the cycle.
[0012] In one optional implementation, the DC fault current rise rate limiting condition includes: When the converter is in a DC fault, the current rise rate of the bridge arm is calculated according to the second preset formula. If the current rise rate is lower than the preset rise rate limit, the second preset formula is as follows:
[0013] in, The inductance value of the bridge arm. The DC voltage of the bridge arm. The rate of rise of current, This is the smoothing inductance value.
[0014] In one alternative implementation, the preset rate of rise limit is set according to the parameters of the power devices of the converter.
[0015] In one optional implementation, determining the equivalent reactance of each winding of the transformer based on constraint relationships includes: Based on the first, second, and third constraint relationships, the equivalent reactance constraint formulas for each winding of the transformer are determined. The equivalent reactance is then determined using these formulas. Each winding of the transformer includes the first, second, and third windings. The equivalent reactance constraint formulas are as follows:
[0016] in, The equivalent reactance of the first winding, The equivalent reactance of the second winding, The equivalent reactance of the third winding, The capacitance value of the submodule in the bridge arm. This represents the number of sub-modules in the bridge arm.
[0017] In one optional implementation, the short-circuit voltage percentage of each winding of the transformer is calculated based on the equivalent reactance, including: According to the third preset formula and the equivalent reactance calculation of the short-circuit voltage percentage of each winding of the transformer, the third preset formula is:
[0018] in, , , These represent the percentage of short-circuit voltage for the first, second, and third windings, respectively. This is the rated voltage of the transformer. This refers to the rated capacity of the transformer.
[0019] The present invention has at least the following beneficial effects: This invention provides a transformer parameter design method and topology for a flexible DC-DC converter with multiplexed transformers. It replaces the traditional two-winding three-phase transformer with a three-winding transformer. The three-winding transformer simultaneously functions as a traditional flexible DC-DC transformer and a bridge arm reactor, eliminating the need for a separate bridge arm reactor and its arrangement. This saves the floor space of the offshore converter station and improves the economic efficiency of the offshore wind power lightweight flexible DC transmission project.
[0020] The constraint relationships of the connection reactance are determined based on the circulating current resonance suppression condition, the AC fault current limiting condition, and the DC fault current rise rate limiting condition. The equivalent reactance of each winding of the transformer is determined through the constraint relationships, and the short-circuit voltage percentage of each winding is obtained based on the equivalent reactance. Multiple key factors affecting the operating characteristics of the converter are systematically solved simultaneously, so that the design results can simultaneously meet the performance requirements of circulating current resonance suppression, AC fault current limiting, and DC fault current limiting, and take into account the operating requirements of the converter under various operating conditions such as steady-state operation, AC fault transient, and DC fault transient. Attached Figure Description
[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.
[0022] Figure 1 This is a schematic diagram of the converter according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the transformer parameter design method according to an embodiment of the present invention; Figure 3 These are the system steady-state simulation results of the converter according to an embodiment of the present invention; Figure 4 These are the system transient simulation results of the converter according to an embodiment of the present invention. Detailed Implementation
[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0024] It is understood that before using the technical solutions disclosed in the various embodiments of the present invention, users should be informed of the types, scope of use, and usage scenarios of the personal information involved in the present invention and their authorization should be obtained in accordance with relevant laws and regulations through appropriate means.
[0025] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. It should be noted that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. The terms “installation,” “connection,” and “linkage” should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. The terms “parallel,” “perpendicular,” and “equal” include the described situation and situations that are similar to the described situation, within an acceptable deviation range, wherein the acceptable deviation range is determined by a person skilled in the art taking into account the measurement under discussion and the error associated with the measurement of a particular quantity (i.e., the limitations of the measurement system). For example, “parallel” includes absolute parallelism and approximate parallelism, wherein an acceptable deviation range for approximate parallelism can be, for example, within 5°; “perpendicular” includes absolute perpendicularity and approximate perpendicularity, wherein an acceptable deviation range for approximate perpendicularity can also be, for example, within 5°; “equal” includes absolute equality and approximate equality, wherein an acceptable deviation range for approximate equality can be, for example, a difference between two equal entities less than or equal to 5% of either one. For a person skilled in the art, the specific meaning of the above terms in this application can be understood on a case-by-case basis.
[0026] As transmission capacity and distance continue to increase, the size and weight of key equipment in offshore converter stations will also increase, making their installation, transportation, and maintenance more difficult. This poses economic and technical challenges to the construction of flexible DC grid-connected systems for offshore wind power. Therefore, optimizing the design of key equipment in flexible DC converter valves to achieve economical and efficient transmission of offshore wind power is an important issue for the large-scale development of offshore wind power.
[0027] The parameter design of traditional bridge arm reactors mainly considers factors such as suppressing AC side current fluctuations and limiting transient and fault currents. It lacks a systematic consideration of multiple constraints such as equivalent reactance and fault conditions under multiple operating conditions, which makes it difficult for the designed bridge arm reactor parameters to meet the performance requirements of the converter under various operating conditions.
[0028] To at least solve one of the above problems, this embodiment provides a flexible DC transformer multiplexing converter, such as... Figure 1 As shown, it includes a bridge arm module and three three-winding transformers; The bridge arm module includes a first bridge arm 101, a second bridge arm 102, a third bridge arm 103, a fourth bridge arm 104, a fifth bridge arm 105, and a sixth bridge arm 106. The high-voltage terminals of the first bridge arm 101, the second bridge arm 102, and the third bridge arm 103 are all connected to the positive terminal of the DC bus, and the high-voltage terminals of the fourth bridge arm 104, the fifth bridge arm 105, and the sixth bridge arm 106 are all connected to the negative terminal of the DC bus. The three-winding transformer includes a first winding 107, a second winding 108, and a third winding 109. The low-voltage terminals of the first bridge arm 101, the second bridge arm 102, and the third bridge arm 103 are respectively connected to the same-name terminals of the second winding 108 of the corresponding three-winding transformer. The low-voltage terminals of the fourth bridge arm 104, the fifth bridge arm 105, and the sixth bridge arm 106 are respectively connected to the same-name terminals of the third winding 109 of the corresponding three-winding transformer. The opposite-name terminals of the second winding 108 and the opposite-name terminals of the third winding 109 of the corresponding three-winding transformer are connected in series. The same-name terminals of the first windings 107 of the three three-winding transformers are respectively connected to AC bus A, B, and C. The opposite-name terminals of the first windings 107 of the three three-winding transformers are interconnected.
[0029] Specifically, the high-voltage sections of the first bridge arm 101, the second bridge arm 102, and the third bridge arm 103 are all connected to the positive terminal of the DC bus, and the low-voltage ends are respectively connected to the second winding 108 of the valve-side winding of the corresponding three-winding transformer. The high-voltage sections of the fourth bridge arm 104, the fifth bridge arm 105, and the sixth bridge arm 106 are all connected to the negative terminal of the DC bus, and the low-voltage ends are respectively connected to the third winding 109 of the valve-side winding of the corresponding three-winding transformer. The end of the second winding 108 away from the bridge arm and the end of the third winding 109 away from the bridge arm are connected to each other. The grid-side windings of the three three-winding transformers, namely the first winding 107, are respectively connected to the A, B, and C of the AC bus.
[0030] For example, in a three-winding transformer corresponding to phase A, the first end of the second winding 108 is connected to the low-voltage end of the first bridge arm 101, and the last end is connected to the last end of the third winding 109. The first end of the third winding 109 is connected to the fourth bridge arm 104, forming a series branch. The first winding 107 is connected to phase A of the AC bus.
[0031] In this embodiment, a three-winding transformer is used to replace the two-winding transformer and the independent bridge arm reactor. The equivalent leakage reactance of the second winding 108 and the third winding 109 of the three-winding transformer simultaneously undertakes the function of the converter bridge arm reactor, thereby eliminating the need for the independent bridge arm reactor and its placement space, reducing the footprint and weight.
[0032] In one embodiment, each bridge arm includes several sub-modules 110 connected in series, and the sub-modules 110 are half-bridge sub-modules 110.
[0033] Specifically, the half-bridge submodule 110 includes two switching devices, two anti-parallel diodes, and a DC capacitor. The two switching devices are connected in series, forming a series branch consisting of an upper switching device and a lower switching device. Each switching device is connected in anti-parallel to a diode, i.e., the upper switching device is connected in anti-parallel to the upper diode, and the lower switching device is connected in anti-parallel to the lower diode. The DC capacitor is connected in parallel across the two ends of the series branch, i.e., the positive terminal of the capacitor is connected to the collector of the upper switching device, and the negative terminal of the capacitor is connected to the emitter of the lower switching device. The half-bridge submodule 110 has two external connection terminals, namely a positive terminal and a negative terminal. In the same bridge arm, the negative terminal of the previous submodule 110 is connected to the positive terminal of the next submodule 110, realizing the sequential series connection of multiple submodules 110.
[0034] In this embodiment, by setting the submodule 110 as a half-bridge submodule 110, the cooperation of the two switching devices in the half-bridge submodule 110 and the DC capacitor can realize two working states: capacitor in and capacitor bypass, thereby superimposing a multi-level AC voltage waveform at the output end of each bridge arm.
[0035] This embodiment provides a transformer parameter design method for a flexible DC-DC transformer-multiplexed converter, applicable to the converter described in the above embodiment, such as... Figure 2 As shown, it includes: Step S201: Obtain the constraint relationship of the converter's connection reactance based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition. Specifically, the linkage reactance refers to the reactance value in the converter arm used to suppress circulating current and limit fault current. In this invention, the linkage reactance is not provided by an independent arm reactor, but is jointly constituted by the equivalent reactance of the second winding 108 and the third winding 109 of the three-winding transformer. Determining the constraint relationship of the linkage reactance essentially imposes constraint requirements on the equivalent reactance of the transformer valve-side winding. The circulating current resonance suppression condition corresponds to the steady-state operating condition of the converter and is used to avoid resonance between the capacitor of submodule 110 and the arm inductor; the AC fault current limiting condition corresponds to the transient operating condition of AC system faults and is used to limit the increase of arm current; the DC fault current rise rate limiting condition corresponds to the transient operating condition of DC system faults and is used to limit the rate of change of arm current.
[0036] Step S202: Determine the equivalent reactance of each winding of the transformer according to the constraint relationship; Specifically, after obtaining the constraint relationship of the converter's connection reactance through the circulating current resonance suppression condition, the AC fault current limiting condition, and the DC fault current rise rate limiting condition, the constraint relationship is used to solve the problem simultaneously to determine the specific values or ranges of the equivalent reactance of the first winding 107, the second winding 108, and the third winding 109 of the three-winding transformer.
[0037] Step S203: Calculate the short-circuit voltage percentage of each winding of the transformer based on the equivalent reactance, and output the equivalent reactance and short-circuit voltage percentage as parameters of each winding of the transformer.
[0038] Specifically, the short-circuit voltage percentage refers to the percentage of the impedance voltage drop across the transformer winding when the rated current flows through it, relative to the rated voltage. Equivalent reactance and short-circuit voltage percentage can be converted to each other. Outputting the equivalent reactance and short-circuit voltage percentage as parameters allows for operational characteristic analysis and transformer design and manufacturing.
[0039] This embodiment obtains the constraint relationship of the converter's connection reactance based on the circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition. Then, it determines the equivalent reactance of each winding of the transformer based on the constraint relationship and finally calculates the short-circuit voltage percentage. This incorporates multiple key factors affecting the converter's operating characteristics into a unified parameter design framework, enabling the design results to simultaneously meet the performance requirements under various operating conditions.
[0040] In one embodiment, step S201 involves obtaining the constraint relationship of the converter's connection reactance based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition, including: Step S2011: Obtain the first constraint relationship of the connection reactance according to the circulating current resonance suppression condition. The first constraint relationship is used to prevent the converter from experiencing circulating current resonance. Specifically, circulating current resonance refers to the amplification of specific frequency harmonics within the converter caused by improper matching of the parameters of the 110 capacitor in the submodule and the inductor in the bridge arm. When circulating current resonance occurs, a large-amplitude circulating current component appears inside the converter, leading to bridge arm current distortion, increased losses, and increased device stress. To avoid circulating current resonance, the value of the connecting reactance should be selected so that the bridge arm series resonant angular frequency is as far away as possible from the circulating current resonant angular frequency.
[0041] Step S2012: Obtain the second constraint relationship of the connecting reactance according to the AC fault current limiting condition. The second constraint relationship is used to ensure that the bridge arm current of the converter is lower than the preset current limit under AC fault. Specifically, when a fault occurs in the AC system connected to the converter, the power exchange between the converter and the AC system changes abruptly, and the arm current increases rapidly. If the connecting reactance is too small, the ability to suppress the AC fault current will be insufficient, and the arm current may exceed the withstand current of the power devices, leading to device damage. Therefore, the core of the second constraint is to ensure that the value of the connecting reactance is large enough to limit the arm current under AC faults to within a preset current limit.
[0042] Step S2013: Obtain the third constraint relationship of the connecting reactance according to the DC fault current rise rate limiting condition. The third constraint relationship is used to ensure that the current rise rate of the bridge arm of the converter is lower than the preset rise rate limit under DC fault. The constraint relationship includes the first constraint relationship, the second constraint relationship and the third constraint relationship.
[0043] Specifically, when a fault occurs on the DC side of the converter, the DC voltage is applied to the arm inductance and the transformer leakage inductance, causing the arm current to rise rapidly at a certain rate. If the connecting reactance is too small, the total inductance of the circuit will be insufficient, resulting in an excessively fast current rise rate, causing the power devices to exceed their safe operating range before the protection system operates. Therefore, the core of the third constraint is to ensure that the connecting reactance is large enough to limit the current rise rate under DC fault conditions to within a preset rise rate limit.
[0044] This embodiment obtains the first, second, and third constraint relationships based on the circulating current resonance suppression condition, the AC fault current limiting condition, and the DC fault current rise rate limiting condition, respectively. These three constraint relationships are used together as the constraint conditions for the connecting reactance. Constraint boundaries are proposed for the connecting reactance from three operating conditions: steady-state operation of the converter, AC fault transient, and DC fault transient, to ensure that the final design result can simultaneously take into account the safety and stability of the converter under different operating conditions.
[0045] In one embodiment, step S2011, the circulating current resonance suppression condition is:
[0046] in, It is the series resonant angular frequency. The inductance value of the bridge arm. For the capacitor value of submodule 110 of the bridge arm, The number of sub-modules in the bridge arm is 110. is a constant and .
[0047] Specifically, the range of the circulating resonant angular frequency can be obtained from the modulation ratio variation range of 0~1: ,in, It is the resonant angular frequency of the circulating current. This represents the system angular frequency. Considering that a large series resonant angular frequency in the bridge arms can cause circulating current resonance at fourth harmonics or higher, the bridge arm reactance value should be selected so that the series resonant angular frequency of the bridge arms is less than 1.55. ω.
[0048] This embodiment limits the series resonant angular frequency of the bridge arm to 1.55 to 2 times the system angular frequency and provides a quantitative relationship between the series resonant angular frequency and the system angular frequency. It can directly calculate the constraint boundary that the bridge arm reactance value needs to meet based on known parameters such as the system angular frequency, the number of sub-modules 110, and the capacitance value of sub-modules 110. This provides a clear and quantifiable design criterion for suppressing circulating current resonance and effectively avoids circulating current resonance caused by improper parameter matching in the converter.
[0049] In one embodiment, step S2012, the DC fault current rise rate limiting condition, includes: When the converter is in an AC fault, the maximum current increment of the bridge arm is calculated according to a first preset formula. The maximum current increment is lower than a preset current limit. The first preset formula is as follows:
[0050] in, For the maximum increment, The system angular frequency, The voltage modulation amplitude of the bridge arm. This is the equivalent resistance of the bridge arm. To determine the value of the connecting inductor, To control the cycle.
[0051] Specifically, when a fault occurs in the AC system connected to the converter, the converter's control strategy adjusts the arm voltage modulation waveform within one control cycle. Due to the existence of the control cycle, the adjustment is not instantaneous; the arm current will experience an increment shortly after the fault occurs. The magnitude of this increment is related to the system angular frequency, the amplitude of the arm voltage modulation waveform, the equivalent resistance of the arm, the connection reactance, and the control cycle.
[0052] This embodiment combines the maximum increment of the arm current under AC fault with the system angular frequency, voltage modulation amplitude, equivalent resistance of the arm, connection inductance, and control cycle through a first preset formula. It can deduce the constraint conditions that the connection reactance needs to meet based on the preset current limit, providing a quantifiable design basis for AC fault current limiting and effectively ensuring that the arm current under AC fault does not exceed the withstand capability of the power devices.
[0053] In one embodiment, step S2013, the DC fault current rise rate limiting condition, includes: When the converter is in a DC fault, the current rise rate of the bridge arm is calculated according to the second preset formula. If the current rise rate is lower than the preset rise rate limit, the second preset formula is as follows:
[0054] in, The inductance value of the bridge arm. The DC voltage of the bridge arm. The rate of rise of current, This is the smoothing inductance value.
[0055] Specifically, under DC fault conditions, the rate of rise of the bridge arm current is determined by both the DC voltage and the total inductance of the circuit. To limit the rate of rise of the current to within a preset limit, the bridge arm inductance must be greater than [a certain value]. The determined lower limit value. The smoothing inductance value is the existing inductance value in the DC line.
[0056] This embodiment combines the quantitative relationship between the bridge arm current rise rate under DC faults, DC voltage, bridge arm reactance, and smoothing reactance through a second preset formula. It can directly calculate the minimum value that the bridge arm reactance needs to meet based on the preset rise rate limit, providing a clear and quantifiable design criterion for limiting the rise rate of DC fault current, and effectively avoiding the problem of power device damage caused by excessively rapid current rise under DC faults.
[0057] In one embodiment, the preset rate of rise limit is set according to the parameters of the power devices of the converter.
[0058] Specifically, power devices are the components that determine the fault tolerance capability of the converter. The parameters of the component can be obtained from the component's datasheet, the range of the rate of rise limit can be calculated based on the component's parameters, and a safety margin can be left in the calculation results to obtain the preset rate of rise limit.
[0059] This embodiment ensures that the design results utilize the fault tolerance capability of the power device while remaining within its safe operating range by matching the preset rise rate limit with the characteristic parameters of the actual selected power device.
[0060] In one embodiment, step S202, determining the equivalent reactance of each winding of the transformer according to the constraint relationship, includes: Based on the first, second, and third constraint relationships, the equivalent reactance constraint formulas for each winding of the transformer are determined. The equivalent reactance is then determined using these formulas. The transformer windings include the first winding 107, the second winding 108, and the third winding 109. The equivalent reactance constraint formulas are as follows:
[0061] in, The equivalent reactance of the first winding 107 is... The equivalent reactance of the second winding 108, The equivalent reactance of the third winding 109 is... For the capacitor value of submodule 110 of the bridge arm, The number of sub-modules 110 for the bridge arm.
[0062] Specifically, in this embodiment, the series resonant angular frequency is less than the system angular frequency of 1.55. Taking the square root of the relationship between the series resonant angular frequency and the system angular frequency yields:
[0063] Deformation and solution Lower limit:
[0064] Multiply both sides by the fundamental angular frequency Then the bridge arm inductor This corresponds to the reactance of the second winding 108 of a three-winding transformer. : Thus, the first constraint formula of the equivalent reactance constraint formula is obtained:
[0065] Given connection reactance Substituting it into the first preset formula, we get:
[0066] Determined by deformation on both sides :
[0067] Square both sides and rearrange terms. Combining with the definition of connection reactance: the equivalent connection reactance on the AC side of the MMC is half the sum of the reactances of winding 1 plus the reactances of windings 2 and 3 of a three-winding transformer, i.e. To meet the requirements for safe operation, the second constraint formula of the equivalent reactance constraint formula can be obtained:
[0068] Simplifying the second preset formula, to ensure the system's dynamic response and the minimum constraint on the bridge arm inductance / reactance, the equivalent reactance of the second winding 108 is... The inductance value must be greater than this critical inductance limit, resulting in the third constraint formula for the reactance constraint formula:
[0069] The fourth constraint formula is derived based on the symmetrical structure of the three-term, three-winding transformer: .
[0070] This embodiment transforms the first, second, and third constraint relationships into four simultaneous constraint formulas for the equivalent reactance of each winding, establishing a direct quantitative relationship between circulating current resonance suppression, AC fault current limiting, DC fault current limiting, and the equivalent reactance of the three transformer windings. By solving the simultaneous inequality set, the value or range of the equivalent reactance of each winding can be directly determined, providing a clear numerical basis for the engineering design of transformer windings.
[0071] In one embodiment, step S203, calculating the short-circuit voltage percentage of each winding of the transformer based on the equivalent reactance, includes: According to the third preset formula and the equivalent reactance calculation of the short-circuit voltage percentage of each winding of the transformer, the third preset formula is:
[0072] in, , , These represent the percentage of short-circuit voltage for the first winding 107, the second winding 108, and the third winding 109, respectively. This is the rated voltage of the transformer. This refers to the rated capacity of the transformer.
[0073] Specifically, the three formulas correspond to the three windings of the transformer, respectively. In this embodiment, in addition to using equivalent reactance to calculate the short-circuit voltage percentage of each winding of the transformer, the short-circuit voltage percentage of each winding can also be calculated based on the short-circuit voltage percentage between any two windings:
[0074]
[0075]
[0076] in, , , These are the percentages of short-circuit voltages between the first winding 107 and the second winding 108, the second winding 108 and the third winding 109, and the first winding 107 and the third winding 109 of a three-winding transformer. They can be obtained by sequentially opening one winding, passing the rated current between the remaining two windings, and measuring the percentage of voltage drop across the transformer impedance.
[0077] This embodiment establishes a conversion relationship between the equivalent reactance of each winding and the percentage of short-circuit voltage, which can convert the calculated equivalent reactance into the percentage of short-circuit voltage. This transforms the reactance parameter requirements at the system design level into the short-circuit voltage percentage parameter commonly used in transformer design and manufacturing, thereby improving the engineering applicability of the design method.
[0078] The converter of this invention will be described below in conjunction with specific scenarios.
[0079] A simulation was conducted using a 1000MW offshore wind power system transmitted via a dual-end flexible DC transmission line. The offshore converter station utilizes the flexible DC transformer multiplexing converter of this invention. The rectifier-side converter employs islanded control. The inverter-side converter employs constant DC voltage and constant reactive power control. The receiving-end grid uses an equivalent voltage source. The offshore converter station has a three-winding transformer with a turns ratio of 66 kV / 505 kV / 505 kV, a transformer short-circuit voltage ratio of 10%, and a rated DC voltage of ±320 kV.
[0080] The control timing is as follows: wind turbine starts at 0.5s, wind farm stabilizes and transmits power at 1s. The system's offshore converter grid-side AC bus voltage, converter valve outlet AC current, DC voltage, DC current, and wind farm active and reactive power output are respectively as follows: Figure 3 As shown.
[0081] The system reached steady state at 2 seconds. At 3.0 seconds, a DC bipolar short-circuit fault occurred at the output of the smoothing reactor, with a fault resistance of 0.01Ω. The converter was locked out 5ms after the fault occurred. The instantaneous maximum voltage, instantaneous current, AC current, and average voltage of the 110 capacitor in submodule of the system's offshore converter arm are as follows: Figure 4 As shown.
[0082] Depend on Figure 4It can be seen that the voltage on the valve side of the upper arm transformer rapidly rises to 550 kV after the fault, and then drops rapidly after reaching the peak value. During the lockout, the voltage on the transformer valve side is approximately 50 kV. During the lockout, the voltage of capacitor 110 in submodule is approximately 61.3% of its rated value. This shows that the lightweight arm reactor scheme does not consume most of the energy after a capacitor failure, which helps the system quickly enter a stable state after restarting. During the system transient process, the maximum arm current is approximately 13.21 kA, and the maximum AC current is approximately 14.66 kA, which is 1.2 pu during normal operation. This demonstrates that the three-winding transformer can replace the arm reactor in traditional modular multilevel converters to effectively limit the fault current.
[0083] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.
Claims
1. A flexible DC transformer multiplex converter, characterized in that, Includes bridge arm modules and three three-winding transformers; The bridge arm module includes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, and a sixth bridge arm. The high-voltage terminals of the first bridge arm, the second bridge arm, and the third bridge arm are all connected to the positive terminal of the DC bus, and the high-voltage terminals of the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm are all connected to the negative terminal of the DC bus. The three-winding transformer includes a first winding, a second winding, and a third winding. The low-voltage terminals of the first arm, the second arm, and the third arm are respectively connected to the same-name terminals of the second winding of the corresponding three-winding transformer. The low-voltage terminals of the fourth arm, the fifth arm, and the sixth arm are respectively connected to the same-name terminals of the third winding of the corresponding three-winding transformer. The opposite-name terminals of the second winding and the opposite-name terminals of the third winding of the corresponding three-winding transformer are connected in series. The same-name terminals of the first windings of the three three-winding transformers are respectively connected to AC bus A, B, and C. The opposite-name terminals of the first windings of the three three-winding transformers are interconnected.
2. The converter according to claim 1, characterized in that, Each bridge arm includes several sub-modules connected in series, and the sub-modules are half-bridge sub-modules.
3. A method for designing transformer parameters for a flexible DC-DC converter, applied to the converter as described in any one of claims 1 to 2, characterized in that, include: Based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition, the constraint relationship of the converter's connection reactance is obtained. Determine the equivalent reactance of each winding of the transformer based on the aforementioned constraint relationship; The short-circuit voltage percentage of each winding of the transformer is calculated based on the equivalent reactance, and the equivalent reactance and the short-circuit voltage percentage are output as parameters of each winding of the transformer.
4. The method according to claim 3, characterized in that, The step of obtaining the constraint relationship of the converter's connection reactance based on the converter's circulating current resonance suppression condition, AC fault current limiting condition, and DC fault current rise rate limiting condition includes: A first constraint relationship for the connection reactance is obtained based on the circulating current resonance suppression condition. The first constraint relationship is used to prevent the converter from experiencing circulating current resonance. A second constraint relationship for the connection reactance is obtained based on the AC fault current limiting condition. The second constraint relationship is used to ensure that the arm current of the converter is lower than the preset current limit under AC fault. The third constraint relationship of the connecting reactance is obtained according to the DC fault current rise rate limiting condition. The third constraint relationship is used to ensure that the current rise rate of the bridge arm of the converter is lower than the preset rise rate limit under DC fault. The constraint relationship includes the first constraint relationship, the second constraint relationship and the third constraint relationship.
5. The method according to claim 3, characterized in that, The circulating current resonance suppression condition is: in, The series resonant angular frequency is... The inductance value of the bridge arm. The capacitance value of the submodule of the bridge arm. The number of sub-modules in the bridge arm. The system angular frequency is the rated angular frequency of the AC power system to which the converter is connected. is a constant and .
6. The method according to claim 3, characterized in that, The DC fault current rise rate limiting condition includes: When the converter is in an AC fault, the maximum current increment of the bridge arm is calculated according to a first preset formula. The maximum current increment is lower than the preset current limit. The first preset formula is: in, For the maximum increment, Let be the angular frequency of the system. The voltage modulation amplitude value of the bridge arm. The equivalent resistance of the bridge arm is given. To determine the value of the connecting inductor, To control the cycle.
7. The method according to claim 3, characterized in that, The DC fault current rise rate limiting condition includes: When the converter is in a DC fault, the current rise rate of the bridge arm is calculated according to a second preset formula. The current rise rate is lower than the preset rise rate limit. The second preset formula is: in, The inductance value of the bridge arm. The DC voltage of the bridge arm. The current rise rate, This is the smoothing inductance value.
8. The method according to claim 3, characterized in that, The preset rate of rise limit is set according to the parameters of the power devices of the converter.
9. The method according to any one of claims 5 to 7, characterized in that, Determining the equivalent reactance of each winding of the transformer based on the constraint relationship includes: Based on the first constraint relationship, the second constraint relationship, and the third constraint relationship, the equivalent reactance constraint formula for each winding of the transformer is determined, and the equivalent reactance is determined based on the equivalent reactance constraint formula. The transformer windings include a first winding, a second winding, and a third winding. The equivalent reactance constraint formula is as follows: in, The equivalent reactance of the first winding, The equivalent reactance of the second winding, The equivalent reactance of the third winding, The capacitance value of the submodule of the bridge arm. The number of sub-modules in the bridge arm.
10. The method according to claim 9, characterized in that, The calculation of the short-circuit voltage percentage of each winding of the transformer based on the equivalent reactance includes: The short-circuit voltage percentage of each winding of the transformer is calculated according to the third preset formula and the equivalent reactance, wherein the third preset formula is: in, , , These are the percentages of the short-circuit voltage for the first winding, the second winding, and the third winding, respectively. The rated voltage of the transformer is... This refers to the rated capacity of the transformer.