Converter low-voltage side duty ratio selection method, system, equipment and medium

Abstract

The invention discloses a converter low-voltage side duty ratio selection method, system and device and a medium. The method comprises the following steps: constructing a topology model of an MMDC converter; analyzing a basic working principle of the MMDC converter according to the topological model and by combining a plurality of typical working modes of the MMDC converter, obtaining a transmission power calculation formula of the MMDC converter, and realizing a selection requirement of a duty ratio during capacitor energy balance of sub-modules of the MMDC converter; calculating the transmission power of the converter by adopting a transmission power calculation formula of the MMDC; and obtaining the operation range of the corresponding duty ratio under the working condition of the transmission power through a table look-up method, and selecting the maximum operable duty ratio in the operation range according to the selection requirement. According to the method provided by the invention, the secondary voltage duty ratio not only can realize the energy balance of the MMDC sub-module, but also can have the maximum voltage utilization rate. Therefore, the problem that how to select the duty ratio value cannot be determined by an existing method is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics technology, and in particular to a method, system, device and medium for selecting a duty cycle on a low-voltage side of a converter. Background Art

[0002] The vigorous development of renewable energy has brought new challenges and opportunities to power technology. On the one hand, renewable energy such as wind and solar energy is highly intermittent and random. The large-scale access of such energy affects the safety of grid operation and power quality, and it is urgent to develop a highly flexible grid architecture and power operation control method. On the other hand, renewable energy bases such as offshore wind farms are often characterized by wide distribution and long transmission distances, and it is urgent to develop a lightweight, efficient and reliable large-scale power collection and transmission network system.

[0003] With the development of semiconductor device and power equipment manufacturing technology, the advantages of DC transmission and DC grid technology based on power electronic conversion in power density, economy, control flexibility and other aspects are becoming increasingly apparent. It is increasingly considered to be an effective solution to the above-mentioned renewable energy collection and transmission problems, and has received more and more attention and favor.

[0004] Similar to the transformer in the AC power system, the high-voltage and large-capacity DC-DC converter is the core equipment for voltage matching and interconnection in the DC power grid. The modular multi-level DC-DC converter (MMDC) has the advantages of high modularity and strong voltage resistance, and is an ideal solution for grid-level DC-DC conversion. Unlike traditional DC-DC converters, MMDC contains a large number of energy buffer units and complex power interaction processes because of the use of module combination to achieve voltage control. Controlling the energy stability and balance of each buffer unit is the prerequisite for the normal operation of MMDC. Up to now, the research on MMDC at home and abroad is still in its infancy, and there is a lack of targeted and systematic research on its energy control problem. The existing method achieves its energy balance by adjusting the duty cycle of the secondary square wave voltage, but the existing scheme cannot determine how to select the duty cycle value. Therefore, it is urgent to design a duty cycle selection method for modular multi-level DC-DC converters. Summary of the invention

[0005] The present invention provides a method, system, device and medium for selecting a duty cycle on a low-voltage side of a converter, which are used to solve the problem that the existing method cannot determine how to select the value of the duty cycle.

[0006] In view of this, a first aspect of the present invention provides a method for selecting a duty cycle of a low-voltage side of a converter, the method comprising:

[0007] Constructing a topological model of the MMDC converter, the topological model comprising: an MMDC full-bridge submodule structure, an intermediate frequency transformer and a full-bridge structure connected in sequence;

[0008] According to the topological model and in combination with several typical working modes of the MMDC converter, the basic working principle of the MMDC converter is analyzed to obtain the transmission power calculation formula of the MMDC and the selection requirements of the duty cycle when realizing the energy balance of the sub-module capacitors of the MMDC;

[0009] According to the circuit parameters of the MMDC converter in the working mode, the transmission power of the converter is calculated using the transmission power calculation formula of the MMDC;

[0010] The operating range of the duty cycle corresponding to the transmission power condition is obtained by table lookup, and the maximum operable duty cycle is selected within the operating range according to the selected requirement.

[0011] Optionally, the transmission power calculation formula is expressed as:

[0012] ;

[0013] In the formula, is the transmission power, is the transformer medium voltage side voltage, is the current flowing through the primary side of the transformer, For half a cycle, is the duty cycle value, , , is the moment in the working mode, is the phase shift between the primary and secondary sides of the transformer, is the maximum phase shift rate within half a period, is the primary voltage of MMDC, is the secondary voltage of MMDC, is the equivalent inductance of the medium voltage side of the transformer.

[0014] Optionally, the selected requirements include:

[0015] The duty cycle is selected so that the energy absorbed by the lower arm SMk of the MMDC converter per cycle is Q k It exhibits monotonicity;

[0016] The duty cycle is selected so that the energy Q absorbed by the lower arm SMk per cycle is guaranteed k While presenting monotonicity, the voltage utilization is also taken into consideration.

[0017] Optionally, the calculating the transmission power of the converter according to the circuit parameters of the MMDC converter in the working mode and using the transmission power calculation formula of the MMDC includes:

[0018] By obtaining the primary voltage of the MMDC, the secondary voltage of the MMDC, the bridge arm inductance of the MMDC, and the phase shift between the primary and secondary sides of the transformer in the working mode, the transmission power calculation formula of the MMDC is used to calculate the transmission power of the converter.

[0019] A second aspect of the present invention provides a system for selecting a duty cycle of a low-voltage side of a converter, the system comprising:

[0020] A construction unit, used to construct a topological model of the MMDC converter, wherein the topological model includes: an MMDC full-bridge submodule structure, an intermediate frequency transformer and a full-bridge structure connected in sequence;

[0021] An analysis unit, configured to analyze the basic working principle of the MMDC converter according to the topological model and in combination with several typical working modes of the MMDC converter, obtain a transmission power calculation formula of the MMDC, and a duty cycle selection requirement for achieving sub-module capacitance energy balance of the MMDC;

[0022] A calculation unit, configured to calculate the transmission power of the converter according to the circuit parameters of the MMDC converter in the working mode and using the transmission power calculation formula of the MMDC;

[0023] A selection unit is used to obtain the operating range of the duty cycle corresponding to the transmission power working condition by a table lookup method, and select the maximum operable duty cycle within the operating range according to the selected requirement.

[0024] Optionally, the transmission power calculation formula is expressed as:

[0025] ;

[0026] In the formula, is the transmission power, is the transformer medium voltage side voltage, is the current flowing through the primary side of the transformer, For half a cycle, is the duty cycle value, , , is the moment in the working mode, is the phase shift between the primary and secondary sides of the transformer, is the maximum phase shift rate within half a period, is the primary voltage of MMDC, is the secondary voltage of MMDC, is the equivalent inductance of the medium voltage side of the transformer.

[0027] Optionally, the selected requirements include:

[0028] The duty cycle is selected so that the energy absorbed by the lower arm SMk of the MMDC converter per cycle is Q k It exhibits monotonicity;

[0029] The duty cycle is selected so that the energy Q absorbed by the lower arm SMk per cycle is guaranteed k While presenting monotonicity, the voltage utilization is also taken into consideration.

[0030] Optionally, the computing unit is specifically configured to:

[0031] By obtaining the primary voltage of the MMDC, the secondary voltage of the MMDC, the bridge arm inductance of the MMDC, and the phase shift between the primary and secondary sides of the transformer in the working mode, the transmission power calculation formula of the MMDC is used to calculate the transmission power of the converter.

[0032] A third aspect of the present invention provides a device for selecting a duty cycle of a low-voltage side of a converter, the device comprising a processor and a memory:

[0033] The memory is used to store program code and transmit the program code to the processor;

[0034] The processor is used to execute the steps of the method for selecting the duty cycle of the low-voltage side of the converter as described in the first aspect according to the instructions in the program code.

[0035] A fourth aspect of the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium is used to store program code, and the program code is used to execute the method for selecting the duty cycle of the low-voltage side of the converter described in the first aspect.

[0036] It can be seen from the above technical solutions that the present invention has the following advantages:

[0037] The embodiment of the present invention provides a method for selecting the duty cycle of the low-voltage side of the converter, 1) a model is built for the MMDC converter, and the basic working principle of the MMDC converter is analyzed. In the method of realizing the energy balance of the sub-modules of the MMDC by modifying the low-voltage side voltage duty cycle D, each mode after the MMDC duty cycle is transformed is analyzed, and then a mathematical model and formula derivation are established to prepare for the specific selection of the duty cycle D value. 2) After the MMDC duty cycle range changes, the duty cycle operating conditions for the energy balance of each modal sub-module are determined for each mode, and they are combined to determine a duty cycle D selected value suitable for the full working condition of the MMDC. The method of the present invention enables the secondary side voltage duty cycle to achieve both the energy balance of the sub-modules of the MMDC and the maximum voltage utilization. This solves the problem that the existing method cannot determine how to select the size of the duty cycle value. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0039] Figure 1 A schematic flow chart of a method for selecting a duty cycle of a low-voltage side of a converter provided by an embodiment of the present invention;

[0040] Figure 2 MMDC topology diagram of the energy balancing control method of the modular multi-level DC-DC converter provided by the embodiment of the present invention;

[0041] Figure 3 A submodule half-bridge structure diagram of the MMDC of the energy balancing control method of the modular multi-level DC-DC converter provided by an embodiment of the present invention;

[0042] Figure 4 A structural diagram of a single submodule of the energy balancing control method of a modular multi-level DC-DC converter provided by an embodiment of the present invention;

[0043] Figure 5 A structural diagram of MMDC series submodules of an energy balancing control method for a modular multi-level DC-DC converter provided by an embodiment of the present invention;

[0044] Figure 6 An equivalent voltage waveform diagram at both ends of a modular multi-level DC-DC converter provided by an embodiment of the present invention;

[0045] Figure 7The operating condition of the modular multi-level DC-DC converter after the duty cycle changes provided by the embodiment of the present invention;

[0046] Figure 8 The operating range of the duty cycle of the modular multi-level DC-DC converter provided by the embodiment of the present invention;

[0047] Fig. 9 A simulation of the duty cycle selection of a modular multi-level DC-DC converter provided by an embodiment of the present invention;

[0048] Fig.10 A schematic diagram of the structure of a system for selecting a duty cycle on the low-voltage side of a converter provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below 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.

[0050] See also Figure 1 , a method for selecting a duty cycle of a low-voltage side of a converter provided in an embodiment of the present invention comprises:

[0051] Step 101: construct a topology model of the MMDC converter, wherein the topology model includes: an MMDC full-bridge submodule structure, a medium-frequency transformer and a full-bridge structure connected in sequence.

[0052] It should be noted that if Figure 2 As shown, the MMDC topology structure includes an MMDC full-bridge submodule structure, an intermediate frequency transformer and a full-bridge structure connected in sequence, an MMDC full-bridge submodule structure composed of submodule strings, an intermediate frequency transformer for electrical isolation, and a half-bridge structure composed of IGBT switches. The MMDC full-bridge submodule structure includes at least two phase units, each phase unit is composed of an upper bridge arm and a lower bridge arm, the positive pole of the DC side is interconnected with the positive pole of the upper bridge arm of each phase unit, and the negative pole of the DC side is interconnected with the negative pole of the lower bridge arm. In each bridge arm, n submodules (Submodule, represented by SM1~SMn respectively) with the same circuit parameters are connected in series, as well as a bridge arm inductor L connected in series in the bridge arm. p The negative electrode of the upper bridge arm and the positive electrode of the lower bridge arm are each connected in series with the bridge arm inductor L p Then they are connected to each other and led out as the AC output side of each phase unit.

[0053] Step 102: Analyze the basic working principle of the MMDC converter based on the topological model and in combination with several typical working modes of the MMDC converter to obtain the transmission power calculation formula of the MMDC and the duty cycle selection requirements for achieving sub-module capacitor energy balance of the MMDC.

[0054] In one embodiment, the transmission power calculation formula of the MMDC in step 102 is expressed as:

[0055] ;

[0056] In the formula, is the transmission power, is the transformer medium voltage side voltage, is the current flowing through the primary side of the transformer, For half a cycle, is the duty cycle value, , , is the moment in the working mode, is the phase shift between the primary and secondary sides of the transformer, is the maximum phase shift rate within half a period, is the primary voltage of MMDC, is the secondary voltage of MMDC, is the equivalent inductance of the medium voltage side of the transformer.

[0057] Selected requirements include:

[0058] The duty cycle is selected so that the energy absorbed by the lower arm SMk of the MMDC converter per cycle is Q k The duty cycle is selected so that the energy Q absorbed by the lower arm SMk per cycle is guaranteed. k While presenting monotonicity, the voltage utilization is also taken into consideration.

[0059] The basic working principle analysis process of the MMDC converter in step 102 is described as follows:

[0060] It should be noted that the MMDC full-bridge submodule structure includes a full-bridge, a half-bridge and other submodules, and the submodules are composed of IGBTs and capacitors, such as Figure 3 The working principle of the half-bridge submodule is described by taking the half-bridge structure shown in FIG. 1 as an example. In this embodiment, the working principle of the half-bridge submodule includes controlling Figure 4 The different on or off states of the switch device T1 and the switch device T2 shown can switch the half-bridge submodule among three working states.

[0061] In the embodiment of the present invention, the working principle of the MMDC full-bridge submodule structure includes:

[0062] When the switch device T1 or the freewheeling diode D1 is turned on, the submodule is put into operation and the bridge arm current i arm Flowing through the submodule capacitor C sm , submodule output voltage v sm =v c ;

[0063] When the switch device T2 or the freewheeling diode D2 is turned on, the submodule is cut off and the output voltage v sm =0.

[0064] It should be noted that the output voltage v of the entire bridge arm arm is the sum of the output voltages of each submodule, and the average value of the capacitor voltage of each submodule is V c , the sum of the average voltages of all capacitors in the bridge arm V arm =nV c (n is the number of submodules in the bridge arm). Ignoring the submodule capacitor voltage ripple, S sm is the submodule switch function, S sm =1 corresponds to the submodule being put into operation; S sm =0 corresponds to the cut-off working state of the submodule. Ts is the switching period, Ds is S sm The bridge arm switching function S sm is the sum of the switching functions of each submodule. The submodule chain input voltage v arm It is the sum of the input voltages of each submodule. Figure 6 It can be seen from the figure that in the quasi-two-level modulation mode, except for the step wave voltage edge, the control process and voltage and current waveforms of the bridge arm of the MMDC full-bridge submodule structure are basically the same as those of a single submodule. Ignoring the transient conversion process of the bridge arm voltage between zero level and high level, the quasi-two-level voltage is regarded as a two-level voltage. The bridge arm of the MMDC full-bridge submodule structure can be regarded as a single large submodule with a capacitor voltage of NVc, such as Figure 5 shown.

[0065] In the embodiment of the present invention, the submodule switch function S sm The expression is:

[0066] ;

[0067] Where t0 is the starting point of the switching cycle, t is the independent variable time, and Ds is the switch on time in one switching cycle.

[0068] The output voltage of the submodule is v sm It can be written as the switching function S sm and the submodule capacitor voltage v c The product of In the quasi-two-level modulation mode, ignoring the transient conversion process of capacitor voltage ripple and bridge arm voltage, the entire submodule chain can be equivalent to a single submodule with capacitor voltage enlarged N times. At this time, the sum of the bridge arm capacitor voltage V arm for:

[0069] ;

[0070] In the formula, v c,i is the capacitor voltage of the ith submodule in the submodule chain. The isolated MMDC quasi-two-level modulation is realized by controlling the pulse phase shift. The equivalent voltage waveform at both ends of the modular DC transformer in the quasi-two-level modulation mode is as follows: Figure 6 shown.

[0071] In the embodiment of the present invention, based on the principle that the area enclosed by the time axis is equal, the primary voltage v of the intermediate frequency transformer in the modular DC transformer is po The rising and falling stages of the step wave can be equivalent to a slope of 2V dc1 / d N The first-order curve, d N is the maximum phase shift within half a cycle of the submodule, V dc1 is the input voltage of the modular DC transformer. Figure 2 As shown, the working mode decomposition of the steady-state operation process of the isolated MMDC is carried out in combination with the equivalent voltage conversion process of the primary and secondary sides. Since the voltage and current waveforms of the AC side in the front and rear half working cycles are symmetrical, the working mode analysis of the isolated MMDC in the first half cycle is used as a case illustration. Before time t0, all the sub-modules of the lower bridge arm of the A phase of the primary side of the modular DC transformer are removed, and all the upper bridge arm sub-modules are put into operation; all the lower bridge arm sub-modules of the B phase of the primary side of the modular DC transformer are put into operation, and all the upper bridge arm sub-modules are removed; the upper bridge arm switch tube of the A phase of the secondary side of the modular DC transformer is disconnected, and the lower bridge arm switch tube is closed; the upper bridge arm switch tube of the B phase of the secondary side of the modular DC transformer is closed, and the lower bridge arm switch tube is disconnected. At this time, the voltage and current of the intermediate frequency transformer can be expressed by the first formula. The first formula is:

[0072] ;

[0073] Where V dc2 is the output voltage of the modular DC transformer, V po (t) is the primary AC voltage of the medium frequency transformer, nv so (t) is the equivalent voltage of the primary AC side of the medium frequency transformer, i p (t) is the primary AC side current of the medium frequency transformer.

[0074] The working principle of MMDCT indicates that the adjustment of duty cycle will affect the phase shift ratio to maintain the transmission power of the system. Therefore, the following four typical operating modes are considered in the analysis of the proposed scheme, as Figure 7 shown, where the cases with extremely low duty cycles are ignored. Due to the similarity, only Mode 1 is analyzed in detail.

[0075] When t < t0, due to the topology and operating symmetry, the lower-arm SM of Phase A is bypassed, the upper-arm SM is switched in, the lower-arm SM of Phase B is switched in, and the upper-arm SM is bypassed. At this time, the voltage V po on the medium-voltage side of the transformer is -V dc1 , and the voltage nv so on the low-voltage side is 0.

[0076] When t0 ≤ t < t1, the lower-arm SM of Phase A will be switched in in the delay sequence of d1 - dk - dN, and the upper-arm SM of this phase unit will also be switched in in this sequence. In the Phase B unit, the lower-arm SM will be bypassed in the sequence of d1 - d k -d N , and the upper-arm SM will participate in normal operation in the sequence of d1 - d k -d N . In this state, the voltage V po on the medium-voltage side of the MMDC rises linearly. The voltage on the low-voltage side of the MMDC remains 0.

[0077] When t1 ≤ t < t2, all the lower-arm SMs of Phase A are inserted and the lower-arm units are bypassed. The SMs of Phase B work in the opposite way. The low-voltage switches remain in the working state. At this time, the medium-voltage and low-voltage side voltages of the transformer are Vdc1 and 0 respectively.

[0078] When t2 ≤ t < t3, all the lower-arm SMs of Phase A are inserted and the upper-arm SMs are bypassed. Similarly, the lower (or upper) arm SMs of Phase B are bypassed (or inserted). At this time, the low-voltage side voltage of the MMDC is nV dc2 .

[0079] When t3 ≤ t < t4, the medium-voltage side of the MMDCT maintains the same operation as in the state of t2 ≤ t < t3. And the low-voltage side voltage of the MMDC is regulated to 0.

[0080] Therefore, the medium-voltage and low-voltage side voltages and currents of the transformer can be expressed as:

[0081] ;

[0082] ;

[0083] ;

[0084] where, v po , nv soare the transformer medium voltage side and equivalent low voltage side voltage respectively; L ac is the equivalent inductance of the medium voltage side. N is the maximum phase shift rate within half a period; T h Combining the above two equations, the medium voltage side current i in one cycle can be calculated p .

[0085] ;

[0086] Therefore, the transmission power of MMDC can be derived as:

[0087] ;

[0088] The energy absorbed by the lower arm SMk per cycle can be obtained. The variable 𝑡 represents the insertion time of the kth submodule.

[0089] ;

[0090] To achieve energy balance of sub-module capacitors of MMDC, the duty cycle D needs to meet the following conditions:

[0091] 1) The selection of D needs to make Q k It is monotonic, either monotonically increasing or monotonically decreasing.

[0092] 2) Ensure Q k In order to ensure monotonicity and voltage utilization, the duty cycle D should be large enough.

[0093] Through Q k The duty cycle D needs to satisfy the following working range: Figure 8 As shown in FIG. 1 , the optimal duty cycle under each parameter is obtained by table lookup method, that is, it can achieve energy balance of MMDC submodules and maximize voltage utilization.

[0094] Step 103: According to the circuit parameters of the MMDC converter in the working mode, the transmission power of the converter is calculated using the transmission power calculation formula of the MMDC.

[0095] In one embodiment, step 103 includes:

[0096] By obtaining the primary voltage of the MMDC, the secondary voltage of the MMDC, the bridge arm inductance of the MMDC, and the phase shift between the primary and secondary sides of the transformer in the working mode, the transmission power calculation formula of the MMDC is used to calculate the transmission power of the converter.

[0097] It should be noted that in the specific implementation, the MMDC primary voltage Vdc1 and the secondary voltage Vdc2 are obtained through the voltage and current sensors in the circuit, and the current i flowing through the primary side of the transformer is p ; By obtaining the MMDC primary voltage Vdc1, secondary voltage Vdc2, bridge arm inductance L in the working mode p , the phase shift d of the primary and secondary sides of the transformer, and the transmission power calculation formula of MMDC is used to calculate the transmission power P of the converter at this time

[0098] Step 104: Obtain the operating range of the duty cycle corresponding to the transmission power condition by table lookup, and select the maximum operable duty cycle within the operating range according to the selected requirements.

[0099] It should be noted that, through the table lookup method, Figure 8 In the figure, find the operating range of the corresponding duty cycle under this condition and find the maximum operable duty cycle D within the operating range. At this time, the duty cycle D can not only meet the capacitor voltage balance of the MMDC submodules, but also maximize the voltage utilization of the MMDC.

[0100] The following is a further explanation of the application of the method in the above implementation case in conjunction with a specific simulation example.

[0101] In combination with the above implementation case, MATLAB / Simulink software is used to simulate and verify the system. The simulation system parameters are shown in Table 1.

[0102] Table 1 shows the simulation parameters.

[0103]

[0104] Simulation Example:

[0105] Fig. 9 The present invention provides a simulation of the secondary voltage duty cycle design of the modular DC transformer described in an embodiment of the present invention.

[0106] according to Figure 2 The modular DC transformer shown in the figure is constructed using MATLAB / Simulink software. Figure 2 The modular DC transformer structure shown in the figure is simulated and verified for this topology, and the simulation parameters are shown in Table 1. Fig. 9 As shown in the figure, based on the following three working states, the effectiveness of the proposed MMDC secondary voltage duty cycle design scheme is verified. The specific working states are described as follows:

[0107] State 1: During 0-0.5s, the primary voltage Vdc1 of the MMDC is 20kV, the transmission power is 0.4pu, the secondary voltage Vdc2 is 750V, the duty cycle D of the secondary voltage is 50%, and the submodule capacitor voltage is balanced.

[0108] State 2: During 0.5-1.5s, the primary voltage Vdc1 changes from 20kV to 18kV, the duty cycle D of the secondary voltage is 50%, and the submodule capacitor voltage is unbalanced.

[0109] State 3: During the period of 1.5-3.5 s, the duty cycle D is adjusted to 40% by looking up the table according to the proposed method.

[0110] According to the primary voltage Vdc1, the secondary voltage Vdc2, and the inductance L p , calculate the transmission power P:

[0111] ;

[0112] According to the present invention, the table obtained by traversal is calculated, such as Figure 7 As shown, at this time, the range of the duty cycle D is 25%-40%. In order to ensure the maximum voltage, the duty cycle D is 40%.

[0113] Fig. 9 It shows that the submodule capacitor voltage is unbalanced in state 2, and the secondary voltage is 50% under this condition. With the proposed scheme, the duty cycle D at this time can restore the submodule capacitor voltage balance in the MMDC. The simulation proves that the selection of the secondary voltage duty cycle of the proposed modular multilevel DC-DC converter is effective.

[0114] The embodiment of the present invention provides a method for selecting the duty cycle of the low-voltage side of the converter, 1) a model is built for the MMDC converter, and the basic working principle of the MMDC converter is analyzed. In the method of realizing the energy balance of the sub-modules of the MMDC by modifying the low-voltage side voltage duty cycle D, each mode after the MMDC duty cycle is transformed is analyzed, and then a mathematical model and formula derivation are established to prepare for the specific selection of the duty cycle D value. 2) After the MMDC duty cycle range changes, the duty cycle operating conditions for the energy balance of each modal sub-module are determined for each mode, and they are combined to determine a duty cycle D selected value suitable for the full working condition of the MMDC. The method of the present invention enables the secondary side voltage duty cycle to achieve both the energy balance of the sub-modules of the MMDC and the maximum voltage utilization. This solves the problem that the existing method cannot determine how to select the size of the duty cycle value.

[0115] The above is a method for selecting a duty cycle on the low-voltage side of a converter provided in an embodiment of the present invention. The following is a system for selecting a duty cycle on the low-voltage side of a converter provided in an embodiment of the present invention.

[0116] See also Fig.10 , a converter low-voltage side duty cycle selection system provided in an embodiment of the present invention includes:

[0117] The construction unit 201 is used to construct a topological model of the MMDC converter, and the topological model includes: an MMDC full-bridge sub-module structure, a medium-frequency transformer and a full-bridge structure connected in sequence.

[0118] The analysis unit 202 is used to analyze the basic working principle of the MMDC converter according to the topological model and in combination with several typical working modes of the MMDC converter, to obtain the transmission power calculation formula of the MMDC, and the duty cycle selection requirements when realizing the sub-module capacitor energy balance of the MMDC.

[0119] The calculation unit 203 is used to calculate the transmission power of the converter according to the circuit parameters of the MMDC converter in the working mode using the transmission power calculation formula of the MMDC.

[0120] The selection unit 204 is used to obtain the operating range of the duty cycle corresponding to the transmission power condition by a table lookup method, and select the maximum operable duty cycle within the operating range according to the selected requirements.

[0121] Furthermore, an embodiment of the present invention also provides a device for selecting a duty cycle of a low-voltage side of a converter, the device comprising a processor and a memory:

[0122] The memory is used to store program code and transmit the program code to the processor;

[0123] The processor is used to execute the steps of the method for selecting the duty cycle of the low-voltage side of the converter as described in the above method embodiment according to the instructions in the program code.

[0124] Furthermore, a computer-readable storage medium is provided in an embodiment of the present invention, and the computer-readable storage medium is used to store program codes, and the program codes are used to execute the method for selecting the duty cycle of the low-voltage side of the converter described in the above method embodiment.

[0125] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0126] In the several embodiments provided by the present invention, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic, for example, the division of units is only a logical function division, and there may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0127] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0128] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0129] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0130] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for selecting a duty cycle of a converter low voltage side, characterized in that: include: Constructing a topological model of the MMDC converter, the topological model comprising: an MMDC full-bridge submodule structure, an intermediate frequency transformer and a full-bridge structure connected in sequence; According to the topological model and in combination with several typical working modes of the MMDC converter, the basic working principle of the MMDC converter is analyzed to obtain the transmission power calculation formula of the MMDC and the selection requirements of the duty cycle when realizing the energy balance of the sub-module capacitors of the MMDC; According to the circuit parameters of the MMDC converter in the working mode, the transmission power of the converter is calculated using the transmission power calculation formula of the MMDC; The operating range of the duty cycle corresponding to the transmission power condition is obtained by table lookup, and the maximum operable duty cycle is selected within the operating range according to the selected requirement.

2. The method for selecting the duty cycle of the low-voltage side of the converter according to claim 1, characterized in that: The transmission power calculation formula is expressed as: ； In the formula, is the transmission power, is the transformer medium voltage side voltage, is the current flowing through the primary side of the transformer, For half a cycle, is the duty cycle value, , , is the moment in the working mode, is the phase shift between the primary and secondary sides of the transformer, is the maximum phase shift rate within half a period, is the primary voltage of MMDC, is the secondary voltage of MMDC, is the equivalent inductance of the medium voltage side of the transformer.

3. The method for selecting the duty cycle of the low voltage side of the converter according to claim 1, characterized in that: The selected requirements include: The duty cycle is selected so that the energy absorbed by the lower arm SMk of the MMDC converter per cycle is Q k It exhibits monotonicity; The duty cycle is selected so that the energy Q absorbed by the lower arm SMk per cycle is guaranteed k While presenting monotonicity, the voltage utilization is also taken into consideration.

4. The method for selecting the duty cycle of the low voltage side of the converter according to claim 1 or 2, characterized in that: The method of calculating the transmission power of the converter according to the circuit parameters of the MMDC converter in the working mode and using the transmission power calculation formula of the MMDC includes: By obtaining the primary voltage of the MMDC, the secondary voltage of the MMDC, the bridge arm inductance of the MMDC, and the phase shift between the primary and secondary sides of the transformer in the working mode, the transmission power calculation formula of the MMDC is used to calculate the transmission power of the converter.

5. A converter low voltage side duty cycle selection system, characterized in that: include: A construction unit, used to construct a topological model of the MMDC converter, wherein the topological model includes: an MMDC full-bridge submodule structure, an intermediate frequency transformer and a full-bridge structure connected in sequence; An analysis unit, configured to analyze the basic working principle of the MMDC converter according to the topological model and in combination with several typical working modes of the MMDC converter, obtain a transmission power calculation formula of the MMDC, and a duty cycle selection requirement for achieving sub-module capacitance energy balance of the MMDC; A calculation unit, configured to calculate the transmission power of the converter according to the circuit parameters of the MMDC converter in the working mode and using the transmission power calculation formula of the MMDC; A selection unit is used to obtain the operating range of the duty cycle corresponding to the transmission power working condition by a table lookup method, and select the maximum operable duty cycle within the operating range according to the selected requirement.

6. The converter low voltage side duty cycle selection system according to claim 5, characterized in that: The transmission power calculation formula is expressed as: ； In the formula, is the transmission power, is the transformer medium voltage side voltage, is the current flowing through the primary side of the transformer, For half a cycle, is the duty cycle value, , , is the moment in the working mode, is the phase shift between the primary and secondary sides of the transformer, is the maximum phase shift rate within half a period, is the primary voltage of MMDC, is the secondary voltage of MMDC, is the equivalent inductance of the medium voltage side of the transformer.

7. The converter low voltage side duty cycle selection system according to claim 5, characterized in that: The selected requirements include: The duty cycle is selected so that the energy absorbed by the lower arm SMk of the MMDC converter per cycle is Q k It exhibits monotonicity; The duty cycle is selected so that the energy Q absorbed by the lower arm SMk per cycle is guaranteed k While presenting monotonicity, the voltage utilization is also taken into consideration.

8. The converter low voltage side duty cycle selection system according to claim 5 or 6, characterized in that: The computing unit is specifically used for: By obtaining the primary voltage of the MMDC, the secondary voltage of the MMDC, the bridge arm inductance of the MMDC, and the phase shift between the primary and secondary sides of the transformer in the working mode, the transmission power calculation formula of the MMDC is used to calculate the transmission power of the converter.

9. A device for selecting duty cycle of a converter low voltage side, characterized in that: The device comprises a processor and a memory: The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the method for selecting the duty cycle of the low-voltage side of the converter according to any one of claims 1 to 4 according to the instructions in the program code.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium is used to store program codes, and the program codes are used to execute the method for selecting the duty cycle of the low-voltage side of the converter according to any one of claims 1 to 4.

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