Isolation transformer topology main circuit parameter selection method and system

Abstract

The application provides a selection method and system for main loop parameters of an isolated converter topology, and the method comprises the following steps: determining the number of bridge arm sub-modules in a modular multilevel converter (MMC) based on the voltage level that can be borne by a power electronic switch in a power electronic transformer; determining the type of the power electronic transformer based on the number of bridge arm sub-modules; analyzing the bridge arm circuit based on the operating state of the MMC to determine the value range of the sub-module capacitor; determining the value range of the bridge arm inductor based on the suppression of internal circulation of the MMC; the main loop parameters of the isolated converter topology comprise the type of the power electronic transformer in a DC-DC converter, and the value range of the sub-module capacitor and the value range of the bridge arm inductor in the MMC. The application can reasonably set technical parameters, thereby effectively improving the dynamic and steady-state performance of the system, reducing the initial investment and operation cost of the system, and improving the economic performance index of the system.

Description

Technical Field

[0001] This invention relates to the field of simulation technology, and specifically to a method and system for selecting the main circuit parameters of an isolated converter topology. Background Technology

[0002] With the development of electricity, DC power grids will inevitably evolve towards DC interconnection and AC / DC interconnection, forming a DC network. Therefore, connecting the emerging VSC-HVDC to LCC-HVDC is unavoidable when constructing high-voltage DC transmission networks. However, LCC-HVDC uses thyristor-based phase-controlled converters, where the voltage polarity reverses but the current direction remains unchanged during power flow reversal. In contrast, VSC-HVDC uses voltage source converters, where the voltage polarity remains unchanged but the current reverses during power flow reversal. Therefore, VSC-HVDC cannot be directly connected to LCC-HVDC. [The following is a separate, unrelated point:] ... ... Figure 1 The topology shown uses an isolated topology with two MMC converters connected on the AC side to achieve hybrid DC reversal. In this topology, each submodule on the VSC system side adopts a half-bridge topology; each submodule on the LCC system side adopts a full-bridge topology; an isolation transformer is used in the middle to achieve electrical isolation between the primary and secondary sides, and the transformer can also serve as a voltage matching function between the primary and secondary sides. Figure 1 The meanings of the symbols are shown in Table 1:

[0003] Table 1. Meaning of Labels in MMC-Based Isolated Converter Topology

[0004]

[0005]

[0006] The isolated DC-DC converter based on MMC consists of three-phase MMC converters on both the left and right sides. The left side connects to the VSC-HVDC, called the VSC side, and the right side connects to the LCC-HVDC, called the LCC side. The polarity of the VSC voltage does not change, so the VSC side module adopts a half-bridge structure. However, the polarity of the LCC voltage will reverse. From the current technology perspective, a full-bridge modular multilevel converter (FMMC) composed of cascaded full-bridge sub-modules (FBSM) has the capability to operate in four quadrants for DC voltage and DC current. The isolated DC-DC converter based on MMC consists of three-phase MMC converters on both the left and right sides. This topology has numerous components and complex parameter configurations. Generally, before practical application, it is necessary to design the model and capacity of each component in the topology. Different models and capacities of components will affect the interconnection effect. If the parameter design is unreasonable, that is, if unsuitable components are selected, not only will the interconnection of VSC-HVDC and LCC-HVDC not be achieved, affecting the stability of the power grid, but it will also lead to high initial investment costs and expensive operating costs. Summary of the Invention

[0007] To address the aforementioned shortcomings in the existing technology, this invention provides a method for selecting the main loop parameters of an isolated converter topology, comprising:

[0008] The number of bridge arm sub-modules in the modular multilevel converter (MMC) is determined based on the voltage level that the power electronic switches in the power electronic transformer can withstand.

[0009] The type of the power electronic transformer is determined based on the transformer's rated voltage and operating frequency;

[0010] The bridge arm circuit is analyzed based on the operating status of the MMC to determine the value range of the submodule capacitor.

[0011] Based on the peak value of the internal circulating current of the MMC, the range of values ​​for the bridge arm inductance is determined;

[0012] The main circuit parameters of the isolated converter topology include the number of bridge arm sub-modules in the MMC, the type of power electronic transformer, and the value range of the sub-module capacitors and the value range of the bridge arm inductors in the MMC.

[0013] Preferably, determining the number of bridge arm sub-modules in the modular multilevel converter (MMC) based on the voltage levels that the power electronic switches in the power electronic transformer can withstand includes:

[0014] Obtain the DC voltage on each bridge arm;

[0015] The number of cascaded submodules on each bridge arm is determined based on the average value of the DC voltage on each bridge arm and the capacitor voltage of all submodules on that bridge arm.

[0016] Preferably, determining the type of the power electronic transformer includes:

[0017] Select a transformer with a rated power of 50nHZ, where n is a positive integer.

[0018] Preferably, the value range of the submodule capacitor in the MMC is determined by the following formula:

[0019]

[0020] In the formula: P ac ω is the average power at the AC terminal of the MMC; m is the fundamental modulation frequency of the AC link; N is the modulation ratio. sub U represents the number of submodules in the MMC bridge arm; ε represents the pre-selected fluctuation range of the submodule capacitor voltage; U sub The average voltage of the submodule; This represents the phase difference between voltage and current.

[0021] Preferably, the range of values ​​for the bridge arm inductance is determined by the following formula:

[0022]

[0023] In the formula: L vsc For the bridge arm inductance on the VSC side; L lcc ω is the bridge arm inductance on the LCC side; ω is the fundamental modulation frequency of the AC link; C sublcc For the submodule capacitor on the LCC side; U sublcc For the submodule voltage on the LCC side; U lcc The voltage on the LCC side; I lcc I is the current on the LCC side. 2flcc The peak value of the second harmonic circulating current on the LCC side.

[0024] Based on the same inventive concept, the present invention also provides a selection system for the main loop parameters of an isolated converter topology, comprising:

[0025] The module for determining the number of submodules is used to determine the number of bridge arm submodules in the modular multilevel converter (MMC) based on the voltage levels that the power electronic switches in the power electronic transformer can withstand.

[0026] A type determination module is used to determine the type of the power electronic transformer based on the transformer's rated voltage and operating frequency;

[0027] The capacitor module is identified to analyze the bridge arm circuit based on the operating status of the MMC and determine the value range of the submodule capacitor.

[0028] The inductor module is used to determine the range of values ​​for the bridge arm inductance based on the peak value of the internal circulating current of the MMC.

[0029] The main circuit parameters of the isolated converter topology include the number of bridge arm sub-modules in the MMC, the type of power electronic transformer, and the value range of the sub-module capacitors and the value range of the bridge arm inductors in the MMC.

[0030] Preferably, the module for determining the number of sub-modules is specifically used for:

[0031] Obtain the DC voltage on each bridge arm;

[0032] The number of cascaded submodules on each bridge arm is determined based on the average value of the DC voltage on each bridge arm and the capacitor voltage of all submodules on that bridge arm.

[0033] Preferably, the type determination module is specifically used for:

[0034] Select a transformer with a rated power of 50nHZ, where n is a positive integer.

[0035] Preferably, the value range of the submodule capacitor in the MMC is determined by the following formula:

[0036]

[0037] In the formula: P ac ω is the average power at the AC terminal of the MMC; m is the fundamental modulation frequency of the AC link; N is the modulation ratio. sub U represents the number of submodules in the MMC bridge arm; ε represents the pre-selected fluctuation range of the submodule capacitor voltage; U sub The average voltage of the submodule; This represents the phase difference between voltage and current.

[0038] Preferably, the range of values ​​for the bridge arm inductance is determined by the following formula:

[0039]

[0040] In the formula: L vsc For the bridge arm inductance on the VSC side; L lcc ω is the bridge arm inductance on the LCC side; ω is the fundamental modulation frequency of the AC link; C sublcc For the submodule capacitor on the LCC side; U sublcc For the submodule voltage on the LCC side; U lcc The voltage on the LCC side; I lcc I is the current on the LCC side.2flcc The peak value of the second harmonic circulating current on the LCC side.

[0041] Compared with the closest existing technology, the technical solution provided by the present invention has the following beneficial effects:

[0042] The technical solution provided by this invention determines the number of bridge arm sub-modules in a modular multilevel converter (MMC) based on the voltage level that the power electronic switches in the power electronic transformer can withstand; determines the type of the power electronic transformer based on the rated voltage and operating frequency of the transformer; analyzes the bridge arm circuit based on the operating state of the MMC to determine the value range of the sub-module capacitor; and determines the value range of the bridge arm inductance based on the peak value of the internal circulating current of the MMC. It also provides constraints and selection methods for the main circuit technical parameters of a DC-DC interface converter. By reasonably setting the technical parameters, the dynamic and steady-state performance of the system can be effectively improved, the initial investment and operating costs of the system can be reduced, and the economic performance indicators of the system can be improved. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the topology of an isolated DC-DC converter based on MMC in the prior art;

[0044] Figure 2 This is a flowchart illustrating a method for selecting main loop parameters of an isolated converter topology according to an embodiment of the present invention.

[0045] Figure 3 This is a schematic diagram of the equivalent phasor model of phase A on the AC side of the converter in an embodiment of the present invention;

[0046] Figure 4 This is a schematic diagram of the circulating current analysis of the three-phase bridge arm of the MMC in an embodiment of the present invention. Detailed Implementation

[0047] To better understand this invention, the following description, in conjunction with the accompanying drawings and examples, will further illustrate the invention.

[0048] Example 1: As Figure 2 As shown, the present invention provides a method for selecting the main loop parameters of an isolated converter topology, including:

[0049] The number of bridge arm sub-modules in the modular multilevel converter (MMC) is determined based on the voltage level that the power electronic switches in the power electronic transformer can withstand.

[0050] The type of the power electronic transformer is determined based on the transformer's rated voltage and operating frequency;

[0051] The bridge arm circuit is analyzed based on the operating status of the MMC to determine the value range of the submodule capacitor.

[0052] Based on the peak value of the internal circulating current of the MMC, the range of values ​​for the bridge arm inductance is determined;

[0053] The main circuit parameters of the isolated converter topology include the number of bridge arm submodules in the MMC, the type of power electronic transformer, and the value ranges of the submodule capacitors and bridge arm inductors in the MMC. The core component in a DC-DC converter is the power electronic transformer, which contains power electronic switches.

[0054] like Figure 2 The method for selecting the main loop parameters of an isolated converter topology, as shown, includes the following steps:

[0055] I. Number of sub-modules and selection of connecting transformers;

[0056] II. Determine the submodule capacitance through bridge arm circuit analysis;

[0057] III. Determine the bridge arm inductance through bridge arm circulating current analysis.

[0058] Furthermore, in step I, the number of sub-modules and the selection of connecting transformers are carried out.

[0059] Figure 3 For the equivalent phasor model of phase A of the AC section of the interface converter, the voltage level that the power electronic switches can withstand is the decisive factor in determining the number of bridge arm submodules in the MMC. Each bridge arm should be able to withstand the entire DC voltage U allocated to the MMC. dc And leave sufficient margin. If the average capacitor voltage of each submodule is denoted as U... C Let N be the total number of cascaded submodules in a bridge arm. Then, it needs to satisfy...

[0060] U C N≥U dc ………………………………………(1)

[0061] When the number of levels in an MMC is small, the number of levels n level It is directly related to the number of cascaded submodules N, and generally satisfies

[0062] n level =N+1…………………………………………(2)

[0063] To facilitate the formation of a zero-level signal, N is generally an even number. In high-voltage applications, the number of MMCs may reach hundreds. In this case, the number of levels depends not only on the number of sub-modules, but also on the controller's control frequency and output voltage modulation ratio.

[0064] Based on the phase-shifting working principle of the interface converter, the phase voltage e of phase a of the MMC converter on the VSC side can be taken. vscfor:

[0065]

[0066] In equation (3), m is the modulation ratio, U vsc ωt represents the DC voltage on the VSC side, and ωt represents the phase of the AC voltage. This represents the amplitude of phase A voltage on the VSC side.

[0067] Take the phase voltage e of phase a of the MMC converter on the LCC side. lcc for:

[0068]

[0069] In equation (4), U lcc This is the DC voltage on the LCC side. δ is the amplitude of phase A voltage on the LCC side, and δ is the phase difference between the AC voltage on the VSC side and the LCC side.

[0070] When using phase-shift control mode, the AC voltage amplitude of the MMC is only related to the DC voltage, i.e., a constant modulation ratio is used; taking m=1, the AC voltage amplitudes of the primary and secondary sides of the transformer are:

[0071]

[0072]

[0073] Effective value of rated line voltage U of primary and secondary sides of high frequency transformer P U S They are respectively:

[0074]

[0075]

[0076] In order to reduce the size of the transformer by increasing the AC voltage frequency in the isolated DC-DC converter, the rated frequency of the isolation transformer can be taken as 50nHz (n=1,2,3…) in combination with the actual engineering situation. The larger n is, the smaller the transformer size, but the higher the manufacturing difficulty.

[0077] In step II of the embodiment, the capacitance of the submodule capacitor is determined by performing bridge arm circuit analysis.

[0078] To avoid loss of generality, this embodiment uses a three-phase circuit as an example for analysis. In MMC, energy storage is mainly achieved by submodule capacitors. To determine the capacity of the capacitor, it is necessary to first analyze the changes in the energy stored in the capacitor.

[0079] According to the working principle of MMC, the voltage u of each phase upper and lower bridge arm ju u jlThey are respectively:

[0080]

[0081] In equation (9), φ is the phase of the modulating wave. The current i in each phase upper and lower bridge arm... ju i jl for:

[0082]

[0083] In the formula: I dc This is the DC side current of the MMC; i j It is the alternating current of each phase of MMC.

[0084] From the above formula, we can obtain the transient value i of the upper half-bridge arm current of phase a. au It can be expressed by equation (11):

[0085]

[0086] In the formula: It is the phase difference between voltage and current. The amplitude of phase current in phase a.

[0087] Based on the above relationships, the average power P at the AC terminal of a single three-phase MMC unit is... ac for:

[0088]

[0089] In the formula: U a It is the effective value of phase a voltage; I a It is the effective value of the phase current.

[0090] Neglecting converter losses, the AC side power P ac and DC side power P dc satisfy:

[0091] P ac =P dc =U dc ·I dc (13)

[0092] According to equations (12) and (13), we can obtain:

[0093]

[0094] The instantaneous power P of the upper bridge arm au It can be expressed as follows:

[0095]

[0096] Substituting equation (14) into equation (15), we can obtain the function P.au The two zeros of (ωt) can be expressed by the following formula:

[0097]

[0098] Therefore, the charging or discharging energy ΔW of the bridge arm within one AC cycle arm It can be expressed as follows:

[0099]

[0100] Analysis of the above formula shows that the charging or discharging energy of the bridge arms is related to the operating state of the converter. The higher the apparent power of the converter, the higher the charging or discharging energy of the bridge arms; the lower the power factor of the converter, the higher the charging or discharging energy of the bridge arms; the lower the modulation ratio of the converter, the higher the charging or discharging energy of the bridge arms.

[0101] Let the average voltage of the submodule capacitor be U. sub If the upper and lower limits of voltage fluctuation are ε, then the maximum fluctuation of bridge arm energy is ΔW. arm It can be expressed as follows:

[0102]

[0103] Where: N sub ε represents the number of submodules in the bridge arm; ε is the upper and lower limits of voltage fluctuation, i.e., the pre-selected fluctuation range of the submodule capacitor voltage; C sub It is a submodule capacitor.

[0104] Combining equations (17) and (18), we obtain the following equation:

[0105]

[0106] The range of values ​​for the submodule capacitor is determined according to equation (19), which is the average power P through the AC terminal of a single three-phase MMC unit. ac Modulation ratio m, phase difference φ between voltage and current, and number of submodules N in the bridge arm. sub U sub The average voltage, ε, and fundamental modulation frequency ω of the AC link of the submodule determine the range of values ​​for the submodule capacitor.

[0107] In step III of the embodiment, the bridge arm inductance is determined by performing bridge arm circulating current analysis.

[0108] As we know from the working principle of the MMC (Multi-phase Controller), the DC terminals of the three-phase bridge arms of the converter are connected in parallel, and their structure and operation are identical. If a voltage difference occurs between the bridge arms, circulating current will inevitably be generated between them. This circulating current component is superimposed on the bridge arm current, occupying the current capacity of the switching devices and increasing the system cost. To reduce the harmful effects of circulating current, it is necessary to suppress the circulating current in the MMC bridge arms. Based on the working principle of the MMC, using the midpoint of the DC bus voltage of a single MMC unit as the reference voltage, the circuit diagram of a three-phase MMC structure can be drawn as follows: Figure 4 As shown.

[0109] Taking phase a as an example, the circulation analysis is performed, such as... Figure 4 As shown, circulating current is defined as the portion of the current that flows from the upper arm to the lower arm without flowing into the AC side. Applying the KCL equation at point A:

[0110]

[0111] In equation (20), i au For the upper bridge arm current, i al This represents the current in the lower bridge arm. adiff This refers to the circulating current from the DC terminal to the phase or between phases, i.e., the circulating current of the MMC.

[0112] The reason for the circulating current is that when alternating current flows through a DC capacitor, the capacitor inevitably undergoes a charging and discharging process, causing the capacitor voltage to change periodically. Since the voltage change pattern is different on each bridge arm, a circulating current will inevitably form between phases.

[0113] The circulating current originates from the real-time changes in capacitor voltage, thus the relationship between capacitor voltage and current is shown in the following equation:

[0114]

[0115] Where: n u (t), n l (t) represents the number of modules that are instantly turned on in the upper and lower bridge arms, respectively; C sub It is a submodule capacitor.

[0116] The voltage drop generated by the KVL circulation on the bridge arm can be expressed by the following formula:

[0117]

[0118] Therefore, considering the circulating current, the voltages of the upper and lower bridge arms of phase a are:

[0119]

[0120] Combining equations (22) and (23), the instantaneous power P of the upper and lower bridge arms au and P alThis can be expressed as the differential of energy:

[0121]

[0122] The sum of energy stored in the upper and lower bridge arm capacitors (W) au W al The difference in energy stored in the upper and lower bridge arm capacitors can be expressed by the following formula:

[0123]

[0124] Differentiating the two equations above and substituting equation (23) into them, we get:

[0125]

[0126] The above formula shows that the circulation i adiff It plays a crucial role in the energy storage of the module capacitor. On the other hand, i adiff The fundamental frequency component and u a The frequencies are the same, and this also affects the energy distribution of the capacitors in the upper and lower bridge arms.

[0127] Assume i adiff In the ideal case, i.e., with only a pure DC component, the instantaneous state of a single-phase circuit can be expressed by the following formula:

[0128]

[0129] In the formula: i diff0 It represents the DC component; '^' indicates the peak value.

[0130] Substituting equation (27) into equation (25), the energy of the upper bridge arm is now:

[0131]

[0132] In the formula: W au0 This indicates the DC component in the energy of the upper bridge arm.

[0133] Therefore, the voltage of the upper bridge arm can be expressed as:

[0134]

[0135] From the above formula, we can see that u a1 (t) contains multiple frequency components. Considering the DC component, first harmonic component, and second harmonic component of the upper and lower bridge arm voltages, the upper and lower bridge arm voltages can be set as:

[0136]

[0137] In the formula: V 2f The second amplitude value of the bridge arm voltage.

[0138] The alternating current of the upper and lower bridge arms can be expressed as:

[0139]

[0140] The instantaneous power flowing through the upper and lower arms of phase a is:

[0141] P a =u au i au +u al i al (32)

[0142] Substituting equations (30) and (31) into (32), the integral yields the expression for energy, where the AC component can be represented by the following equation:

[0143]

[0144] If the third term in the formula is relatively small, it can be ignored, and the above formula can be transformed into:

[0145]

[0146] Additionally, when the submodule voltages are balanced, the second-harmonic voltage fluctuation V 2f It will be evenly distributed among the sub-modules. If the average voltage of the sub-modules is U... sub The instantaneous voltage value of each submodule is then calculated using the following formula:

[0147]

[0148] Then the energy in phase a capacitor is also equal to:

[0149]

[0150] Comparing equations (34) and (36), we can obtain the following from the fact that the second harmonic amplitudes are equal:

[0151]

[0152] The amplitude of the second harmonic voltage is obtained by solving:

[0153]

[0154] Therefore, under the premise of MMC three-phase symmetry, the peak value of the second harmonic circulating current can be expressed as:

[0155]

[0156] Solving for the bridge arm inductance:

[0157]

[0158] The average voltage of the submodule is U. sub Submodule capacitor C sub The fundamental modulation frequency ω and U in the AC link dc MMC DC side current I dc The second amplitude value I of the bridge arm voltage 2f Determine the bridge arm inductance.

[0159] As can be seen from the above formula, with other parameters constant, the larger L is, the greater I is. 2f The smaller the value, the greater the need for L to suppress the second harmonic circulating current.

[0160] The circulating current within the MMC is caused by the inconsistency in the sum of the voltages of the upper and lower arms of each phase. This circulating current is of second harmonic negative sequence nature and flows between the three-phase arms of the MMC, having no effect on the external AC system. Therefore, the circulating current between each phase is actually superimposed on the DC current of that phase, flowing together through the upper and lower arms of the same phase.

[0161] Based on the above analysis, the circulating current should consist of a normally operating DC current component and a second harmonic negative-sequence AC component. Because the DC side current I... dc The current is evenly distributed among the three phases, therefore the DC current on each phase in the circulation should be I. dc / 3, while communication corresponds to the internal circulation of MMC.

[0162] Since other high-frequency components in the circulating current are very small, the main measure to suppress the circulating current is to suppress the second harmonic AC current. If PWM modulation is mainly used or the step level is very high, a second harmonic component can be added to the reference voltage. As we already know, the inductance on the bridge arm has a suppressive effect on the circulating current, so the bridge arm must be selected to be large enough.

[0163] In this invention, to suppress the peak value of the second harmonic circulating current to below 10% of the fundamental current, the lower limit values ​​of the bridge arm inductances on the LCC side and VSC side are obtained by substituting data and calculation:

[0164]

[0165] Through L vsc For the bridge arm inductance on the VSC side; L lcc ω is the bridge arm inductance on the LCC side; ω is the fundamental modulation frequency of the AC link; C sublcc For the submodule capacitor on the LCC side; U sublcc For the submodule voltage on the LCC side; U lcc The voltage on the LCC side; I lcc I is the current on the LCC side. 2flcc The peak value of the second harmonic circulating current on the LCC side.

[0166] This invention proposes constraints and selection methods for the main circuit technical parameters of the basic topology of an MMC-based DC-DC interface converter. It focuses on the selection of the number of sub-modules and connecting transformers, analysis of bridge arm circuits and determination of sub-module capacitors, analysis of bridge arm circulating current and determination of bridge arm inductance. This allows for the reasonable setting of technical parameters, thereby effectively improving the dynamic and steady-state performance of the system, reducing the initial investment and operating costs of the system, and improving the economic performance indicators of the system.

[0167] Example 2: Based on the same inventive concept, this embodiment of the invention also provides a selection system for the main loop parameters of an isolated converter topology, including:

[0168] The module for determining the number of submodules is used to determine the number of bridge arm submodules in the modular multilevel converter (MMC) based on the voltage levels that the power electronic switches in the power electronic transformer can withstand.

[0169] A type determination module is used to determine the type of the power electronic transformer based on the transformer's rated voltage and operating frequency;

[0170] The capacitor module is identified to analyze the bridge arm circuit based on the operating status of the MMC and determine the value range of the submodule capacitor.

[0171] The inductor module is used to determine the range of values ​​for the bridge arm inductance based on the peak value of the internal circulating current of the MMC.

[0172] The main circuit parameters of the isolated converter topology include the number of bridge arm sub-modules in the MMC, the type of power electronic transformer, and the value range of the sub-module capacitors and the value range of the bridge arm inductors in the MMC.

[0173] In this embodiment, the module for determining the number of sub-modules is specifically used for:

[0174] Obtain the DC voltage on each bridge arm;

[0175] The number of cascaded submodules on each bridge arm is determined based on the average value of the DC voltage on each bridge arm and the capacitor voltage of all submodules on that bridge arm.

[0176] In this embodiment, the type determination module is specifically used for:

[0177] Select a transformer with a rated power of 50nHZ, where n is a positive integer.

[0178] In this embodiment, the value range of the submodule capacitor in the MMC is determined by the following formula:

[0179]

[0180] In the formula: Pac ω is the average power at the AC terminal of the MMC; m is the fundamental modulation frequency of the AC link; N is the modulation ratio. sub U represents the number of submodules in the MMC bridge arm; ε represents the pre-selected fluctuation range of the submodule capacitor voltage; U sub The average voltage of the submodule; This represents the phase difference between voltage and current.

[0181] In this embodiment, the range of values ​​for the bridge arm inductance is determined by the following formula:

[0182]

[0183] In the formula: L vsc For the bridge arm inductance on the VSC side; L lcc The inductance of the bridge arm on the LCC side; ω is; C sublcc For the submodule capacitor on the LCC side; U sublcc For the submodule voltage on the LCC side; U lcc The voltage on the LCC side; I lcc I is the current on the LCC side. 2flcc The peak value of the second harmonic circulating current on the LCC side.

[0184] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0185] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0186] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0187] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0188] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A method for selection of main loop parameters of an isolated converter topology, characterized in that, The method comprises the following steps: determining the number of bridge arm sub-modules in a modular multilevel converter (MMC) based on the voltage level that a power electronic switch in a power electronic transformer can withstand; determining the type of the power electronic transformer based on the rated voltage and working frequency of the transformer; analyzing the bridge arm circuit based on the operating state of the MMC to determine the value range of the sub-module capacitor; determining the value range of the bridge arm inductor based on the internal circulating current peak value of the MMC; the main loop parameters of the isolation type converter topology include the number of bridge arm sub-modules in the MMC, the type of the power electronic transformer, and the value range of the sub-module capacitor and the value range of the bridge arm inductor in the MMC.

2. The method of claim 1, wherein, The method for determining the number of bridge arm sub-modules in the MMC based on the voltage level that the power electronic switch in the power electronic transformer can withstand comprises the following steps: obtaining the DC voltage on each bridge arm; determining the number of cascaded sub-modules on each bridge arm based on the average value of the DC voltage on each bridge arm and the voltage of all sub-module capacitors on the bridge arm.

3. The method of claim 1, wherein, The method for determining the type of the power electronic transformer comprises the following steps: selecting a transformer with a rated power of 50nHZ, wherein n is a positive integer.

4. The method of claim 1, wherein, The value range of the sub-module capacitor in the MMC is determined according to the following formula: In the formula: P ac is the average power of the MMC AC end; ω is the fundamental modulation frequency of the AC link; m is the modulation ratio; N sub is the number of sub-modules in the MMC bridge arm; ε is the pre-selected fluctuation range of the sub-module capacitor voltage; U sub is the average voltage of the sub-module; is the phase difference between voltage and current.

5. The method of claim 1, wherein, The value range of the bridge arm inductor is determined according to the following formula: where: L vsc Lb is the bridge leg inductance on the VSC side; L lcc Lc is the bridge leg inductance on the LCC side; ω is the fundamental frequency of the AC link modulation; C sublcc Cc is the sub-module capacitance on the LCC side; U sublcc Uc is the sub-module voltage on the LCC side; U lcc U is the voltage on the LCC side; I lcc I is the current on the LCC side; I 2flcc Ic is the peak value of the double frequency circulating current on the LCC side.

6. A system for selection of main loop parameters of an isolated converter topology, characterized by The method comprises the following steps: a sub-module number determination module is configured to determine the number of bridge arm sub-modules in a modular multilevel converter (MMC) based on the voltage level that a power electronic switch in a power electronic transformer can withstand; a type determination module is configured to determine the type of the power electronic transformer based on the rated voltage and working frequency of the transformer; a capacitor determination module is configured to analyze the bridge arm circuit based on the operating state of the MMC to determine the value range of the sub-module capacitor; an inductor determination module is configured to determine the value range of the bridge arm inductor based on the internal circulating current peak value of the MMC; the main loop parameters of the isolation type converter topology include the number of bridge arm sub-modules in the MMC, the type of the power electronic transformer, and the value range of the sub-module capacitor and the value range of the bridge arm inductor in the MMC.

7. The system of claim 6, wherein, The sub-module number determination module is specifically configured to: obtain the DC voltage on each bridge arm; determine the number of cascaded sub-modules on each bridge arm based on the average value of the DC voltage on each bridge arm and the voltage of all sub-module capacitors on the bridge arm.

8. The system of claim 6, wherein, The type determination module is specifically configured to: select a transformer with a rated power of 50nHZ, wherein n is a positive integer.

9. The system of claim 6, wherein, The value range of the sub-module capacitor in the MMC is determined according to the following formula: In the formula: P ac is the average power of the MMC AC end; ω is the fundamental modulation frequency of the AC link; m is the modulation ratio; N sub is the number of submodules in the MMC bridge arm; ε is the pre-selected fluctuation range of the submodule capacitor voltage; U sub is the average voltage of the submodule; is the phase difference between voltage and current.

10. The system of claim 6, wherein, The value range of the bridge arm inductor is determined according to the following formula: where: L vsc Lb is the bridge leg inductance on the VSC side; L lcc Lc is the bridge leg inductance on the LCC side; ω is the fundamental frequency of the AC link modulation; C sublcc Cc is the sub-module capacitance on the LCC side; U sublcc Uc is the sub-module voltage on the LCC side; U lcc U is the voltage on the LCC side; I lcc I is the current on the LCC side; I 2flcc Ic is the peak value of the double frequency circulating current on the LCC side.

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