Novel AC-DC converter, DC side active filtering method and DC power transmission system

Abstract

The invention discloses a novel AC-DC converter, a DC side active filtering method and a DC power transmission system. The novel AC-DC converter comprises a voltage transformation module, wherein the first end of the voltage transformation module is connected with an AC input port; the alternating current end of the alternating current-direct current conversion module is connected with the second end of the voltage transformation module, and the first direct current end and the second direct current end of the alternating current-direct current conversion module are connected with the first direct current port and the second direct current port respectively; the switched capacitor module chain is connected with the AC-DC conversion module and is used for converting the pulsating DC signal into a DC output signal; the active filtering module chain comprises a plurality of full-bridge sub-modules which are sequentially connected in series; and the processing module is used for adjusting the switching state of each full-bridge sub-module based on the direct-current voltage measurement value and the direct-current measurement value so as to generate modulation voltage for performing filtering compensation on the direct-current output signal. According to the embodiment of the invention, active filtering can be carried out in the AC-DC conversion process, and the DC higher harmonic component is eliminated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current transmission, and in particular to a novel AC-DC converter, a direct current side active filtering method and a direct current transmission system. BACKGROUND

[0002] At present, a traditional direct current transmission system mainly uses a modular multilevel converter (MMC) as an AC-DC converter to convert AC power into DC power for direct current transmission. The MMC can realize AC / DC conversion of different voltage levels and capacity levels, and has the characteristics of high voltage resistance, large capacity and high maturity.

[0003] However, in the above-mentioned topology for realizing AC-DC conversion by using the MMC, the MMC device has the characteristics of high cost, large size and low efficiency, which restricts the traditional direct current transmission system and prevents the rapid expansion of direct current transmission.

[0004] In order to solve the problem of the above-mentioned MMC technology, a novel AC-DC converter topology is proposed in the related art, a parallel switch capacitor chain is added at the direct current side, and after the rectifier circuit converts the AC signal into a pulsating DC signal, the switch capacitor chain can modulate the pulsating DC voltage to obtain a stable DC voltage.

[0005] However, in the above-mentioned novel topology, whether in single-phase AC-DC conversion or three-phase AC-DC conversion, a ripple voltage will be generated at the direct current side. The ripple voltage will generate a corresponding harmonic current in the circuit, thereby increasing the loss of the direct current line, increasing the fluctuation of the direct current voltage, and the harmonic current will also flow into the switch capacitor chain, increasing the voltage fluctuation of each sub-module in the switch capacitor chain, and generating greater current stress on the devices in the module, thereby affecting the actual service life of the transmission system. Therefore, how to reduce the interference of the direct current side in the novel topology and improve the filtering efficiency of the direct current side has become a technical problem to be solved. SUMMARY

[0006] The embodiments of the present application provide a novel AC-DC converter, a direct current side active filtering method and a direct current transmission system, which can improve the technical problem of large harmonic signal interference at the direct current side when the novel topology in the related art performs AC-DC conversion.

[0007] In a first aspect, the embodiments of the present application provide a novel AC-DC converter for converting single-phase AC power into DC power, the novel AC-DC converter comprising: a voltage conversion module, a first end of the voltage conversion module being connected with an AC input port; The AC / DC conversion module is connected with the second end of the transformer module, and the first DC end and the second DC end of the AC / DC conversion module are connected with the first DC port and the second DC port respectively; the AC / DC conversion module is used for converting the received AC input signal into a pulsating DC signal; The switched capacitor module chain is connected with the AC / DC conversion module and is used for converting the pulsating DC signal into a DC output signal; The active filter module chain is connected between the first DC end of the AC / DC conversion module and the first DC port; the active filter module chain comprises a plurality of full-bridge sub-modules connected in series. The processing module is electrically connected with the control end of the active filter module chain; the processing module is used for adjusting the switching state of each full-bridge sub-module based on the DC voltage measurement value and the DC current measurement value, so as to generate a modulation voltage for filtering and compensating the DC output signal.

[0008] In some embodiments, the full-bridge sub-module comprises: A sub-module capacitor; A first switching device connected between the first end of the sub-module capacitor and the first node; a second switching device connected between the second end of the sub-module capacitor and the first node; a third switching device connected between the first end of the sub-module capacitor and the second node; and a fourth switching device connected between the second end of the sub-module capacitor and the second node; wherein the first node of the full-bridge sub-module is connected with the first DC end of the AC / DC conversion module or the second node of the previous full-bridge sub-module, and the second node of the full-bridge sub-module is connected with the first DC port or the first node of the next full-bridge sub-module.

[0009] In some embodiments, the processing module is used for obtaining a DC voltage measurement value between the two ends of the switched capacitor module chain and an AC voltage measurement value of the AC input port at the current time, determining a DC voltage theoretical value based on the AC voltage measurement value, and calculating a ripple component of the DC voltage based on the DC voltage measurement value and the DC voltage theoretical value; The processing module is also used for obtaining a DC current measurement value at the current time, and determining an additional component of the DC voltage corresponding to the DC current measurement value based on the closed-loop controller; The processing module is also used for determining the switching state of each full-bridge sub-module based on the ripple component and the additional component, so as to generate a modulation voltage for filtering and compensating the DC output signal.

[0010] In some embodiments, the first switching device, the second switching device, the third switching device and the fourth switching device comprise fully controlled switching devices and diodes connected in anti-parallel with the fully controlled switching devices.

[0011] In some embodiments, the switched capacitor module chain comprises a plurality of switched capacitor sub-modules connected in series, and the switched capacitor sub-module comprises a full-bridge capacitor sub-module or a half-bridge capacitor sub-module.

[0012] In some embodiments, the AC-DC conversion module comprises a first bridge arm group and a second bridge arm group; The upper bridge arm of the first bridge arm group is connected between the first DC end and the third node of the AC-DC conversion module, and the lower bridge arm of the first bridge arm group is connected between the second DC end and the third node of the AC-DC conversion module; The upper bridge arm of the second bridge arm group is connected between the first DC end and the fourth node of the AC-DC conversion module, and the lower bridge arm of the second bridge arm group is connected between the second DC end and the fourth node of the AC-DC conversion module; The upper bridge arm and the lower bridge arm of the first bridge arm group and the upper bridge arm and the lower bridge arm of the second bridge arm group comprise a plurality of power switching devices connected in series, and at least part of the power switching devices in a single upper bridge arm and a single lower bridge arm are all-controllable power switching devices.

[0013] In a second aspect, the embodiments of the present application provide a novel AC-DC converter for converting three-phase alternating current into direct current, which comprises: Three voltage transformation modules, the first end of each voltage transformation module being connected with a corresponding alternating current input port; Three AC-DC conversion modules, the alternating current end of each AC-DC conversion module being connected with the second end of a corresponding voltage transformation module, the first DC end of each AC-DC conversion module being connected with a first DC port or the second DC end of a previous AC-DC conversion module, and the second DC end of each AC-DC conversion module being connected with a second DC port or the first DC end of a subsequent AC-DC conversion module; the AC-DC conversion module is used for converting the received alternating current input signal into a pulsating direct current signal; Three switched capacitor module chains, each switched capacitor module chain being connected in parallel with a corresponding AC-DC conversion module and being used for converting the pulsating direct current signal into a direct current output signal; An active filter module chain is connected between the first DC end of the first AC-DC conversion module and the first DC port; the active filter module chain comprises a plurality of full-bridge sub-modules connected in series; A processing module is electrically connected with the control end of the active filter module chain; the processing module is used for adjusting the switching state of each full-bridge sub-module based on the direct current voltage measurement value and the direct current current measurement value, so as to generate a modulation voltage for filtering and compensating the direct current output signal.

[0014] In a third aspect, the embodiments of the present application provide a direct current side active filtering method, which is characterized in that it is applied to the novel AC-DC converter of the first aspect or the second aspect, and comprises: Obtaining a direct current voltage measurement value between the two ends of the switched capacitor module chain and an alternating current voltage measurement value of the alternating current input port at the current time, and determining a ripple component of the direct current voltage based on the direct current voltage measurement value and the alternating current voltage measurement value; The direct current measurement value at the current moment is obtained, and the direct current measurement value is input to the closed-loop controller to obtain an additional component of the direct current voltage; An ideal modulation voltage value is determined based on the ripple component and the additional component, and a switching state of each full-bridge sub-module is determined according to the ideal modulation voltage value; The switching control signal of each full-bridge sub-module is generated based on the switching state of each full-bridge sub-module.

[0015] In some embodiments, the ripple component of the direct current voltage is determined based on the direct current voltage measurement value and the alternating current voltage measurement value, including: The corresponding direct current voltage theoretical value is calculated based on the alternating current voltage measurement value; The ripple component of the direct current voltage is calculated based on the direct current voltage measurement value and the direct current voltage theoretical value.

[0016] In some embodiments, the switching state of each full-bridge sub-module is determined according to the ideal modulation voltage value, including: The number of full-bridge sub-modules in different switching states is determined according to the ideal modulation voltage value and the rated voltage of the full-bridge sub-module, the switching states including a positive input state, a negative input state and a bypass state; The full-bridge sub-module in the positive input state is selected according to a first preset rule and the full-bridge sub-module in the negative input state is selected according to a second preset rule based on the size order of the actual voltage of each full-bridge sub-module.

[0017] In a fourth aspect, the embodiments of the present application provide a direct current transmission system, including the novel AC-DC converter of the first aspect or the second aspect.

[0018] Compared with the related art, the novel AC-DC converter, the direct current side active filtering method and the direct current transmission system provided by the embodiments of the present application can determine the size of the modulation voltage to be provided by the processing module according to the direct current voltage measurement value and the direct current current measurement value of the direct current side in the running process through the setting of the source filtering module chain between the AC-DC conversion module and the direct current port. Based on the calculated modulation voltage, the switching state adjustment of each full-bridge sub-module in the active filtering module chain can be controlled. Based on the number of full-bridge sub-modules in the positive input and the negative input of each full-bridge sub-module, the negative voltage and the positive voltage can be provided respectively. Based on the respective switching state of all full-bridge sub-modules, the voltage formed by the whole ultimately can be used as the modulation voltage for filtering and compensating the direct current output signal, so that the active filtering of the direct current output signal is realized. By using the above active filtering method, the multiple harmonic components of the direct current can be effectively reduced under typical working conditions, the quality of the direct current waveform is significantly improved, the power loss on the direct current line and the current stress on the module device are reduced, and the stability and reliability of the direct current transmission system are improved. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0020] Figure 1 is a circuit structure schematic diagram of a novel AC-DC converter provided by an embodiment of the present application; Figure 2 is a circuit structure schematic diagram of an active filter module chain provided by an embodiment of the present application; Figure 3 is a circuit structure schematic diagram of a switched capacitor module chain provided by an embodiment of the present application; Figure 4 is a circuit structure schematic diagram of an AC-DC conversion module provided by an embodiment of the present application; Figure 5 is a circuit structure schematic diagram of a novel AC-DC converter provided by another embodiment of the present application; Figure 6 is a circuit structure schematic diagram of a novel AC-DC converter provided by still another embodiment of the present application; Figure 7 is a flowchart of a direct current side active filtering method provided by an embodiment of the present application; Figure 8 is a generation schematic diagram of a modulated voltage provided by an embodiment of the present application.

[0021] In the drawings: 10, transformer module; 20, AC-DC conversion module; 30, switched capacitor module chain; 40, active filter module chain; DC+, first direct current port; DC-, second direct current port; C1, sub-module capacitor; Q1, first switching device; Q2, second switching device; Q3, third switching device; Q4, fourth switching device; 21, first bridge arm group; 22, second bridge arm group; 31, full-bridge capacitor sub-module; 32, half-bridge capacitor sub-module; DETAILED DESCRIPTION

[0022] The features and exemplary embodiments of the various aspects of the present application will be described in detail below with reference to the drawings. For the purpose of clarity, the description is divided into the following sections: technical field, brief description of the drawings, detailed description of the embodiments, and conclusion. It should be understood that the specific embodiments described herein are intended to be illustrative only and are not intended to limit the scope of the present application. The present application can be practiced without some or all of these specific details. The following description of the embodiments is provided as an example of the present application.

[0023] It should be noted that the relational terms herein such as first and second and the like are used solely to distinguish one entity or action from another, without necessarily requiring or implying any such actual relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element preceded by "comprises... " does not, without more constraints, foreclose the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0024] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The embodiments will be described in detail below with reference to the accompanying drawings.

[0025] At present, the traditional direct current transmission system mainly uses a modular multilevel converter (MMC) as an alternating current-direct current converter to convert alternating current of a large voltage into direct current for direct current transmission. The MMC can realize AC / DC conversion of different voltage levels and capacity levels, and has the characteristics of high voltage resistance, large capacity, and high maturity.

[0026] However, in the above-mentioned topology for realizing AC / DC conversion by using MMC, due to the characteristics of high cost, large size, and low efficiency of the MMC device, the traditional direct current transmission system is subject to certain restrictions, and the rapid expansion of direct current transmission cannot be realized.

[0027] In order to solve the problem of the above-mentioned MMC technology, a new AC / DC converter topology is proposed in the related art, which increases a parallel switch capacitor chain at the direct current side. After the alternating current signal is converted into a pulsating direct current signal in the rectifier circuit, the switch capacitor chain can modulate the pulsating direct current voltage to obtain a stable direct current voltage.

[0028] However, in the above new topology, whether in single-phase AC-DC conversion or three-phase AC-DC conversion, a ripple voltage will be generated at the DC side, which will generate a corresponding harmonic current in the circuit, thereby increasing the loss of the DC line, increasing the fluctuation of the DC voltage, and the harmonic current will also flow into the switched capacitor chain, increasing the voltage fluctuation of each sub-module in the switched capacitor chain, and generating greater current stress on the devices in the module, thereby affecting the actual service life of the power transmission system. Therefore, how to reduce the interference of the DC side in the new topology and improve the filtering efficiency of the DC side has become a technical problem to be solved.

[0029] To solve the above technical problems, the embodiments of the present application provide a new AC-DC converter, a DC side active filtering method and a DC power transmission system. First, the new AC-DC converter provided by the embodiments of the present application will be introduced.

[0030] Please refer to Figure 1 , the new AC-DC converter can convert single-phase AC into DC, and the new AC-DC converter includes a transformer module 10, an AC-DC conversion module 20, a switched capacitor module chain 30, an active filter module chain 40 and a processing module (not shown).

[0031] The first end of the transformer module 10 is connected with the AC input port, and the transformer module 10 can set the corresponding winding ratio according to the AC voltage of the AC input signal provided by the AC input port and the size of the DC voltage actually required to be converted, so as to perform voltage conversion on the AC input signal provided by the AC input port.

[0032] The AC end of the AC-DC conversion module 20 can be connected with the second end of the transformer module 10, and the first DC end and the second DC end of the AC-DC conversion module 20 are connected with the first DC port DC+ and the second DC port DC- respectively. After the transformer module 10 performs voltage conversion on the AC input signal provided by the AC input port, the AC-DC conversion module 20 can perform AC-DC conversion on the received AC input signal to generate a pulsating DC signal.

[0033] The switched capacitor module chain 30 can be connected in parallel with the AC-DC conversion module 20, and the pulsating DC voltage is modulated through the charging and discharging of the capacitor to generate a stable DC output signal, which is output through the first DC port DC+ and the second DC port DC-.

[0034] The active filter module chain 40 can be connected between the first DC end of the AC-DC conversion module 20 and the first DC port DC+, and the active filter module chain 40 can include a plurality of full-bridge sub-modules connected in series. Each full-bridge sub-module can be connected to the circuit in different switching states to provide positive voltage, negative voltage or no voltage respectively.

[0035] The processing module can be electrically connected with the control end of the active filter module chain 40. During operation of the novel AC-DC converter, the processing module can detect the output voltage of the DC side, i.e. the voltage across the switching capacitor module chain 30, to obtain a DC voltage measurement value. The processing module can also detect the loop current of the DC side to obtain a DC current measurement value. Based on the DC voltage measurement value and the DC current measurement value, the processing module can determine the deviation of the actual voltage of the DC side from the theoretical voltage, and adjust the switching state of each full-bridge submodule according to the deviation value, and use the overall voltage generated by each full-bridge submodule in different switching states as the modulation voltage for filtering and compensating the DC output signal.

[0036] In this embodiment, by arranging the active filter module chain 40 between the AC-DC conversion module 20 and the DC port, the processing module can determine the size of the modulation voltage to be provided according to the DC voltage measurement value and the DC current measurement value of the DC side during operation. Based on the calculated modulation voltage, the switching state of each full-bridge submodule in the active filter module chain 40 can be adjusted. Based on the number of full-bridge submodules that are positively or negatively switched on, negative voltage or positive voltage can be provided, respectively. The voltage formed by all full-bridge submodules based on their respective switching states can be used as the modulation voltage for filtering and compensating the DC output signal, thereby achieving active filtering of the DC output signal. By using the above active filtering method, the multiple harmonic components of the DC current can be effectively reduced under typical operating conditions, the quality of the DC current waveform is significantly improved, the power loss on the DC line and the current stress on the module devices are reduced, and the stability and reliability of the DC power transmission system are improved.

[0037] Please refer to Figure 2 In some embodiments, the active filter module chain 40 described above can include n full-bridge submodules, namely full-bridge submodule 1, full-bridge submodule 2, and full-bridge submodule n.

[0038] Taking the full-bridge submodule 1 in Figure 2 for example, the full-bridge submodule 1 can include a submodule capacitor C1 and four switching devices. Among the four switching devices, a first switching device Q1 is connected between a first end of the submodule capacitor C1 and a first node N1; a second switching device Q2 is connected between a second end of the submodule capacitor C1 and the first node N1; a third switching device Q3 is connected between the first end of the submodule capacitor C1 and a second node N2; and a fourth switching device Q4 is connected between the second end of the submodule capacitor C1 and the second node N2. The submodule capacitor C1 and the four switching devices can constitute a full-bridge circuit.

[0039] Since the plurality of full-bridge sub-modules are connected in series, the first node N1 of the full-bridge sub-module is connected with the first DC end of the AC-DC conversion module 20 or the second node N2 of the previous full-bridge sub-module, and the second node N2 of the full-bridge sub-module is connected with the first DC port DC+ or the first node N1 of the next full-bridge sub-module. That is, for the first full-bridge sub-module, the first node N1 can be directly connected with the first DC end of the AC-DC conversion module 20; and for the last full-bridge sub-module, the second node N2 can be directly connected with the first DC port DC+.

[0040] In the above embodiment, when the first switch device Q1 and the fourth switch device Q4 are turned on, the full-bridge sub-module is in a positive input state, and at this time, the DC output signal generated by the switched-capacitor module chain 30 can charge the sub-module capacitor C1; when the second switch device Q2 and the third switch device Q3 are turned on, the full-bridge sub-module is in a negative input state, and at this time, the DC output signal generated by the switched-capacitor module chain 30 generates a negative current relative to the sub-module capacitor C1, and under the action of the negative current, the sub-module capacitor C1 can be discharged.

[0041] When the first switch device Q1 and the third switch device Q3 are turned on, the full-bridge sub-module is in a bypass state, and the sub-module capacitor C1 is short-circuited by the loop formed by the first switch device Q1 and the third switch device Q3, and does not perform charging and discharging; similarly, when the second switch device Q2 and the fourth switch device Q4 are turned on, the full-bridge sub-module is also in a bypass state.

[0042] It should be noted that since the plurality of full-bridge sub-modules in the active filter module chain 40 are connected in series, during normal operation, each full-bridge sub-module can only be switched between the positive input state, the negative input state and the bypass state, and cannot be switched to the latching state.

[0043] As an optional embodiment, the specific number of full-bridge sub-modules in the active filter module chain 40 can be obtained by dividing the ripple amplitude of the DC voltage output by the switched-capacitor module chain by the voltage rating of a single full-bridge sub-module, and then adding a certain margin. The ripple amplitude of the DC voltage output by the switched-capacitor module chain can be calculated according to the peak value of a single-phase sinusoidal half-wave or the superposition value of a three-phase sinusoidal half-wave.

[0044] It can be understood that for a single full-bridge sub-module, a preset dead time or transition control logic can be used when switching between the states to reduce the voltage stress and switching loss of the switch device.

[0045] In some embodiments, the above processing module can obtain the DC voltage measurement value between the two ends of the switched-capacitor module chain 30 and the AC voltage measurement value of the AC input port at the current time.

[0046] The processing module can calculate the corresponding DC voltage theoretical value based on the calculation formula of AC-DC conversion, in combination with the winding ratio of the two-end coil of the transformer module 10 and other relevant parameters in the novel AC-DC converter, based on the AC voltage measurement value of the AC input port.

[0047] The processing module can calculate the ripple component of the DC voltage actually generated at the current time according to the difference between the DC voltage measurement value and the DC voltage theoretical value.

[0048] The processing module can also input the sampled DC current measurement value at the current time into the pre-set closed-loop controller, and the closed-loop controller can perform closed-loop control on the multiple harmonic components of the DC current to make them approach zero. The output of the closed-loop controller is the additional component of the DC voltage.

[0049] After obtaining the ripple component and the additional component of the DC current, the switching state of each full-bridge sub-module can be determined to generate the modulation voltage for filtering and compensating the DC output signal.

[0050] It should be noted that, after determining the ripple component of the DC current, the voltage value of the inverse of the ripple component is taken as part of the modulation voltage, and the additional component of the DC voltage is taken as another part of the modulation voltage. The theoretical modulation voltage value can be obtained by adding the two voltage values. After obtaining the theoretical modulation voltage value, the switching state of each full-bridge sub-module can be determined to make the voltage value formed by all the full-bridge sub-modules approach the modulation voltage value as much as possible.

[0051] In some embodiments, the first switching device Q1, the second switching device Q2, the third switching device Q3 and the fourth switching device Q4 include fully-controlled switching devices and diodes anti-parallel to the fully-controlled switching devices.

[0052] As an exemplary embodiment, the fully-controlled switching devices can include IGBT (Insulated Gate Bipolar Transistor), IGCT (Intergrated Gate Commutated Thyristors) or other types of fully-controlled power electronic devices, without limitation.

[0053] In the above embodiments, the diodes anti-parallel to the switching devices can act as freewheeling diodes to prevent sudden changes in current and voltage in the circuit and protect the elements in the circuit from being damaged.

[0054] In some embodiments, the above-mentioned switching capacitor module chain 30 can include a plurality of series-connected switching capacitor sub-modules, and the switching capacitor sub-modules include full-bridge capacitor sub-modules 31 or half-bridge capacitor sub-modules 32.

[0055] In the plurality of series-connected switched-capacitor sub-modules, the full-bridge switched-capacitor sub-module 31 can be included, and the half-bridge switched-capacitor sub-module 32 can also be included. For example Figure 3 As shown, the plurality of switched-capacitor sub-modules in the switched-capacitor module chain 30 can all be the half-bridge switched-capacitor sub-module 32, can all be the full-bridge switched-capacitor sub-module 31, or can be a mixture of the full-bridge switched-capacitor sub-module 31 and the half-bridge switched-capacitor sub-module 32, which is not limited herein.

[0056] Please refer to Figure 4 In some embodiments, the AC-DC conversion module 20 described above can include a first bridge arm group 21 and a second bridge arm group 22.

[0057] The first bridge arm group 21 includes an upper bridge arm and a lower bridge arm, and the upper bridge arm of the first bridge arm group 21 is connected between the first DC end and the third node N3 of the AC-DC conversion module 20, and the lower bridge arm of the first bridge arm group 21 is connected between the second DC end and the third node N3 of the AC-DC conversion module 20.

[0058] The second bridge arm group 22 includes an upper bridge arm and a lower bridge arm, and the upper bridge arm of the second bridge arm group 22 is connected between the first DC end and the fourth node N4 of the AC-DC conversion module 20, and the lower bridge arm of the second bridge arm group 22 is connected between the second DC end and the fourth node N4 of the AC-DC conversion module 20.

[0059] The upper bridge arm and the lower bridge arm of the first bridge arm group 21 and the upper bridge arm and the lower bridge arm of the second bridge arm group 22 each include a plurality of power switching devices connected in series. Among the plurality of power switching devices included in a single upper bridge arm or a single lower bridge arm, at least part of the power switching devices are fully controlled power switching devices.

[0060] That is, a single upper bridge arm or a single lower bridge arm can be composed of all fully controlled power switching devices, or can be composed of fully controlled power switching devices and half-controlled power switching devices.

[0061] Please refer to Figure 5 The embodiments of the present application also provide a novel AC-DC converter for converting three-phase alternating current into direct current, which includes three voltage transformation modules 10, three AC-DC conversion modules 20, three switched-capacitor module chains 30, an active filter module chain 40, and a processing module.

[0062] The first end of each of the three voltage transformation modules 10 is respectively connected with a corresponding AC input port, i.e., the three voltage transformation modules 10 are respectively connected with an A-phase AC port, a B-phase AC port, and a C-phase AC port.

[0063] The AC end of each of the three AC-DC conversion modules 20 is connected with the second end of a corresponding voltage transformation module 10.

[0064] The first DC end of the AC-DC conversion module 20 is connected with the first DC port DC+ or the second DC end of the previous AC-DC conversion module 20, and the second DC end of the AC-DC conversion module 20 is connected with the second DC port DC- or the first DC end of the next AC-DC conversion module 20. That is, the first DC end of the first AC-DC conversion module 20 is connected with the first DC port DC+, the second DC end of the first AC-DC conversion module 20 is connected with the first DC end of the second AC-DC conversion module 20, the second DC end of the second AC-DC conversion module 20 is connected with the first DC end of the third AC-DC conversion module 20, and the second DC end of the third AC-DC conversion module 20 is connected with the second DC port DC-. Each AC-DC conversion module 20 can convert the AC input signal output by the corresponding voltage conversion module 10 into a pulsating DC signal.

[0065] In each of the three switched capacitor module chains 30, the switched capacitor module chain 30 is connected in parallel with the corresponding AC-DC conversion module 20, and can convert the pulsating DC signal generated by the corresponding AC-DC conversion module 20 into a DC output signal. At this time, the DC voltage between the first DC port DC+ and the second DC port DC- is the superposition of the modulated DC voltages of the three switched capacitor module chains 30.

[0066] The active filter module chain 40 can be connected between the first DC end of the first AC-DC conversion module 20 and the first DC port DC+. The active filter module chain 40 includes a plurality of full-bridge sub-modules connected in series.

[0067] The processing module can be electrically connected with the control end of the active filter module chain 40. During the operation of the new AC-DC converter, the processing module can detect the output voltage of the DC side, i.e., the voltage across the switched capacitor module chain 30, to obtain a DC voltage measurement value. The processing module can also detect the loop current of the DC side to obtain a DC current measurement value. Based on the DC voltage measurement value and the DC current measurement value, the processing module can determine the deviation of the actual voltage of the current DC side from the theoretical voltage, and adjust the switching state of each full-bridge sub-module according to the deviation value, and use the overall voltage generated by each full-bridge sub-module in different switching states as the modulation voltage for filtering and compensating the DC output signal.

[0068] It can be understood that in the new AC-DC converter applied to three-phase AC power, the principle of determining the switching state of each full-bridge sub-module of the active filter module chain 40 by the processing module is similar to the principle of the new AC-DC converter applied to single-phase AC power. The DC voltage measurement value sampled by the processing module is the superposition of the modulated DC voltages of the three switched capacitor module chains 30, and based on the same calculation method as in the above embodiment, the switching state of each full-bridge sub-module can be determined according to the calculated theoretical modulation voltage.

[0069] As an optional implementation, please refer to Figure 5 and Figure 6 As shown in Figure 5 , a small-sized passive filter can also be arranged in the new AC-DC converter to further filter out high-frequency harmonic current. The passive filter can be two DC smoothing reactors, one of which is arranged between the active filter module chain 40 and the first DC port DC+, and the other is electrically connected with the second DC port DC-. Figure 6 In addition, as shown in , an LCL filter can also be arranged as a passive filter in the new AC-DC converter to further weaken the residual harmonic components.

[0070] In the above implementation, the main target of the passive filter added on the basis of the active filter is the high-frequency harmonic component. Therefore, compared with the passive filter only, not only the filtering performance can be improved, but also a smaller capacitor or inductor can be selected as the reactor of the passive filter, thereby reducing the volume and cost of the passive filter device and the overall device.

[0071] It can be understood that the combination of active filtering and passive filtering can realize the attenuation of high-frequency switching noise on the DC side, thereby improving the electromagnetic compatibility (EMC) performance.

[0072] As an optional implementation, as shown in Figure 6 , the line voltage of each AC port is 35 kV, the DC port voltage is ±15 kV, and the device capacity is 50 MVA; the number of switch capacitor submodules of the switch capacitor module chain is 10, and the rated voltage of the module capacitor of the switch capacitor submodule is set to 2.3 kV.

[0073] The active filter module chain includes 5 full-bridge submodules connected in series, which is used to realize the active filtering function on the DC side; the rated voltage of the module capacitor of the full-bridge submodule is also set to 2.3 kV. The active filter module chain can effectively cover and filter out the main 6th harmonic component of the DC current. In the above implementation, the 6th harmonic component of the DC current can be filtered to below 2% of the rated value of the DC current under typical operating conditions, thereby significantly improving the waveform quality of the DC current, reducing the DC line loss and module current stress, and improving the stability and reliability of the system operation.

[0074] The embodiment of the application also provides a DC side active filtering method, which is applied to the new AC-DC converter in the above embodiment, Figure 7 shows a flowchart of the DC side active filtering method provided by one embodiment of the application. The method comprises the following steps: S110, obtain a direct current voltage measurement value of both ends of the switched capacitor module chain and an alternating current voltage measurement value of an alternating current input port at the current moment, and determine a ripple component of the direct current voltage based on the direct current voltage measurement value and the alternating current voltage measurement value; S120, obtain a direct current measurement value at the current moment, input the direct current measurement value to a closed loop controller, and obtain an additional component of the direct current voltage; S130, determine an ideal modulation voltage value based on the ripple component and the additional component, and determine a switching state of each full-bridge sub-module according to the ideal modulation voltage value; S140, generate a switching control signal of each full-bridge sub-module based on the switching state of each full-bridge sub-module.

[0075] In the embodiment, by obtaining the direct current voltage measurement value and the alternating current voltage measurement value at the current moment, the ripple component of the direct current voltage at the current moment can be determined according to the calculation formula corresponding to the circuit topology. Based on the direct current measurement value measured at the current moment, the additional component that makes the multiple harmonic components of the direct current tend to zero can be obtained through the closed loop controller. Based on the ripple component and the additional component, the ideal modulation voltage value required at the moment can be determined, and the switching state of each full-bridge sub-module is determined accordingly, and the active filtering of the interference signal at the current moment can be realized by controlling the state adjustment of each full-bridge sub-module. Compared with the passive filtering method at the direct current side in the related art, the active filtering method in the above embodiment effectively improves the filtering performance of the direct current side and reduces the overall volume and cost of the filtering module in the system.

[0076] The specific implementation of each step is introduced below.

[0077] In S110, the processing module can obtain a direct current voltage measurement value of both ends of the switched capacitor module chain and an alternating current voltage measurement value of an alternating current input port at the current moment.

[0078] It can be understood that, when applied to a single-phase alternating current to direct current topology, the direct current voltage measurement value is the voltage of both ends of a single switched capacitor module chain; and when applied to a three-phase alternating current to direct current topology, the direct current voltage measurement value is the superposition of the voltages of both ends of all switched capacitor module chains.

[0079] The processing module can calculate the ripple component of the direct current voltage at the current moment according to the collected direct current voltage measurement value and alternating current voltage measurement value in combination with other related parameters in the new AC-DC converter.

[0080] In some embodiments, the determination of the ripple component of the direct current voltage based on the direct current voltage measurement value and the alternating current voltage measurement value includes: S210, calculate a corresponding direct current voltage theoretical value based on the alternating current voltage measurement value; S220, the ripple component of the DC voltage is calculated based on the DC voltage measurement value and the DC voltage theoretical value.

[0081] In this embodiment, based on the AC voltage measurement value, the theoretical value of the DC voltage can be determined in combination with the winding ratio of the transformer module and related parameters. According to the DC voltage measurement value sampled at the current time and the calculated DC voltage theoretical value, the difference value can be taken as the ripple component of the DC voltage at the current time.

[0082] In S210, the processing module can calculate the corresponding DC voltage theoretical value based on the AC-DC conversion calculation formula based on the AC voltage measurement value of the AC input port, in combination with the winding ratio of the two coils at both ends of the transformer module and other related parameters in the new AC-DC converter.

[0083] In S220, after determining the DC voltage theoretical value, the processing module can calculate the difference between the DC voltage measurement value and the DC voltage theoretical value, which is the ripple component actually generated by the DC voltage at the current time.

[0084] As an optional implementation, as shown in Figure 8 , the DC voltage measurement value can be U dc , the DC voltage theoretical value can be U dc_ref , and the ripple component Δ U dc can be represented as: Δ U dc = U dc- U dc_ref ; In S120, the processing module can also input the sampled DC current measurement value at the current time to the pre-set closed-loop controller. The closed-loop controller can perform closed-loop control on the multiple harmonic components of the DC current to make it tend to zero. The output of the closed-loop controller is the additional component of the DC voltage.

[0085] In S130, after obtaining the ripple component and the additional component of the DC current, the switching state of each full-bridge sub-module can be determined accordingly.

[0086] As shown in Figure 8 , after determining the ripple component of the DC current, the voltage value of the inverted ripple component Δ U dc is taken as the modulation voltage rating U m0 . The DC current measurement value at the current time I dcThe input is to a pre-set closed-loop controller, and an additional component of the DC voltage, i.e., a modulation voltage additional value Δ U m .

[0087] The processing module calculates a modulation voltage rated value U m0 The sum of the modulation voltage additional value Δ U m , i.e., a theoretical modulation voltage value U m : U m = U m0 + Δ U m ; As an optional embodiment, the above-mentioned closed-loop controller can be a PR (Proportional Resonant) controller. In a new AC-DC converter for single-phase AC-to-DC conversion, the PR controller aims to control the 2nd harmonic component of the DC current to 0, generates an additional component of the DC voltage, and realizes closed-loop control of the harmonic current on the DC side. In a new AC-DC converter for three-phase AC-to-DC conversion, the PR controller aims to control the 6th harmonic component of the DC current to 0, generates an additional component of the DC voltage, and realizes closed-loop control of the harmonic current on the DC side.

[0088] In some embodiments, the determination of the switching state of each full-bridge sub-module according to the ideal modulation voltage value comprises: S310, determining the number of full-bridge sub-modules in different switching states in the plurality of full-bridge sub-modules according to the ideal modulation voltage value and the rated voltage of the full-bridge sub-modules; the switching states include a positive input state, a negative input state, and a bypass state; S320, selecting the full-bridge sub-module in the positive input state according to a first preset rule and selecting the full-bridge sub-module in the negative input state according to a second preset rule based on the size order of the actual voltage of each full-bridge sub-module.

[0089] In this embodiment, after the ideal modulation voltage value is determined, the actual voltage of each full-bridge sub-module can be sorted based on the size, and the full-bridge sub-module in the positive input state and the full-bridge sub-module in the negative input state can be respectively determined from the plurality of full-bridge sub-modules sorted according to different rules.

[0090] In S310, after determining the ideal modulation voltage value, the switching state of the main full-bridge sub-module can be determined according to the positive and negative of the ideal modulation voltage value. For example, if the ideal modulation voltage value is positive, the switching state of the main full-bridge sub-module is positive input state to compensate for a positive voltage value; on the contrary, if the ideal modulation voltage value is negative, the switching state of the main full-bridge sub-module is negative input state to compensate for a negative voltage value.

[0091] It can be understood that after determining the number of full-bridge sub-modules in positive input state and the number of full-bridge sub-modules in negative input state according to the modulation strategy of recent level approximation modulation, carrier phase shift modulation, etc., the remaining full-bridge sub-modules can be in bypass state and do not participate in the voltage modulation process.

[0092] In S320, after determining the number of full-bridge sub-modules in positive input state and the number of full-bridge sub-modules in negative input state, the actual voltage across the sub-module capacitor in each full-bridge sub-module can be collected and sorted in order according to the size of the actual voltage.

[0093] After obtaining the sorted full-bridge sub-modules, the full-bridge sub-modules in positive input state can be selected based on a first preset rule, and the full-bridge sub-modules in negative input state can be selected based on a second preset rule.

[0094] The above-mentioned first preset rule is: in the process of converting alternating current into direct current by the new AC-DC converter, when the direct current is in positive direction (the current flows from the alternating current side to the direct current side, the rectification working condition), the full-bridge sub-modules in positive input state are selected in order of actual voltage from small to large; in the process of converting direct current into alternating current by the new AC-DC converter, when the direct current is in negative direction (the current flows from the direct current side to the alternating current side, the inverter working condition), the full-bridge sub-modules in positive input state are selected in order of actual voltage from large to small.

[0095] Correspondingly, the above-mentioned second preset rule is: in the process of converting alternating current into direct current by the new AC-DC converter, when the direct current is in positive direction (the current flows from the alternating current side to the direct current side, the rectification working condition), the full-bridge sub-modules in negative input state are selected in order of actual voltage from large to small; in the process of converting direct current into alternating current by the new AC-DC converter, when the direct current is in negative direction (the current flows from the direct current side to the alternating current side, the inverter working condition), the full-bridge sub-modules in negative input state are selected in order of actual voltage from small to large.

[0096] As an optional implementation, taking the rectification mode of the new AC-DC converter as an example, if the number of full-bridge sub-modules that need to be switched to the positive input state is n, the first n full-bridge sub-modules can be selected in the order of actual voltage from small to large to be switched to the positive input state, so as to drive the sub-module capacitors with higher voltage to discharge; if the number of full-bridge sub-modules that need to be switched to the negative input state is m, the first m full-bridge sub-modules can be selected in the order of actual voltage from large to small to be switched to the negative input state, so as to drive the sub-module capacitors with lower voltage to charge.

[0097] In S140, after determining the switching state of each full-bridge sub-module, the corresponding switching control signal can be generated according to the switching state of each full-bridge sub-module, and the switching control signal is output to realize the switching state control of all full-bridge sub-modules.

[0098] The application also provides a direct current power transmission system, which comprises the new AC-DC converter in the above embodiments.

[0099] It should be noted that the present application is not limited to the specific configurations and processes described above and shown in the drawings. For the sake of brevity, detailed descriptions of well-known methods are omitted herein. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present application is not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications and additions, or change the order of the steps, after understanding the spirit of the present application.

[0100] The functional blocks shown in the above structural block diagram can be implemented as hardware, software, firmware or a combination thereof. When implemented in hardware, it can be, for example, an electronic circuit, an application specific integrated circuit (ASIC), appropriate firmware, a plug-in, a functional card, etc. When implemented in software, the elements of the present application are program or code segments used to perform the required tasks. The program or code segments can be stored in a machine-readable medium or transmitted through a data signal carried in a carrier wave over a transmission medium or communication link. The "machine-readable medium" can include any medium capable of storing or transmitting information. Examples of machine-readable media include electronic circuits, semiconductor memory devices, ROM, flash memory, erasable ROM (EROM), floppy disks, CD-ROMs, optical disks, hard disks, optical fiber media, radio frequency (RF) links, etc. The code segments can be downloaded via a computer network such as the Internet, an intranet, etc.

[0101] It should also be noted that the example embodiments mentioned in the present application describe some methods or systems based on a series of steps or devices. However, the present application is not limited to the order of the above steps, that is, the steps can be performed in the order mentioned in the embodiments, can be different from the order in the embodiments, or several steps can be performed simultaneously.

[0102] The computer program instructions can also be loaded onto a computer, other programmable data processing apparatus, or other processing device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other processing device to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0103] The above is merely specific implementation of the present application, and those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process of the above-described system, module and unit can refer to the corresponding process in the foregoing method embodiments, which will not be described herein. It should be understood that the protection scope of the present application is not limited to this, and any person skilled in the art can easily think of various equivalent modifications or replacements within the technical range disclosed in the present application, and these modifications or replacements shall be covered within the protection scope of the present application.

Claims

1. A novel AC-DC converter, characterized in that, The novel AC-DC converter, used to convert single-phase alternating current to direct current, includes: A transformer module, wherein the first end of the transformer module is connected to an AC input port; An AC-DC conversion module is provided, wherein the AC terminal of the AC-DC conversion module is connected to the second terminal of the transformer module, and the first DC terminal and the second DC terminal of the AC-DC conversion module are respectively connected to the first DC port and the second DC port; the AC-DC conversion module is used to convert the received AC input signal into a pulsating DC signal. A switched capacitor module chain, connected to the AC-DC conversion module, is used to convert the pulsating DC signal into a DC output signal; An active filter module chain is connected between the first DC terminal and the first DC port of the AC-DC conversion module; the active filter module chain includes multiple full-bridge sub-modules connected in series. The processing module is electrically connected to the control terminal of the active filter module chain; the processing module is used to adjust the switching state of each full-bridge sub-module based on the DC voltage measurement value and the DC current measurement value, so as to generate a modulation voltage for filtering and compensating the DC output signal.

2. The novel AC-DC converter according to claim 1, characterized in that, The full-bridge submodule includes: Submodule capacitors; A first switching device is connected between the first terminal of the submodule capacitor and the first node; a second switching device is connected between the second terminal of the submodule capacitor and the first node; a third switching device is connected between the first terminal of the submodule capacitor and the second node; a fourth switching device is connected between the second terminal of the submodule capacitor and the second node; wherein, the first node of the full-bridge submodule is connected to the first DC terminal of the AC-DC conversion module or the second node of the preceding full-bridge submodule, and the second node of the full-bridge submodule is connected to the first DC port or the first node of the following full-bridge submodule.

3. The novel AC-DC converter according to claim 2, characterized in that, The processing module is used to obtain the measured DC voltage values ​​at both ends of the switched capacitor module chain and the measured AC voltage value at the AC input port at the current moment, determine the theoretical DC voltage value based on the measured AC voltage value, and calculate the ripple component of the DC voltage based on the measured DC voltage value and the theoretical DC voltage value. The processing module is also used to obtain the DC current measurement value at the current moment, and determine the additional component of the DC voltage corresponding to the DC current measurement value based on the closed-loop controller; The processing module is also used to determine the switching state of each full-bridge submodule based on the ripple component and the additional component, so as to generate a modulation voltage for filtering and compensating the DC output signal.

4. The novel AC-DC converter according to claim 2, characterized in that, The first switching device, the second switching device, the third switching device, and the fourth switching device include fully controlled switching devices and diodes connected in antiparallel with the fully controlled switching devices.

5. The novel AC-DC converter according to claim 1, characterized in that, The switched capacitor module chain includes multiple switched capacitor sub-modules connected in series, and the switched capacitor sub-modules include full-bridge capacitor sub-modules or half-bridge capacitor sub-modules.

6. The novel AC-DC converter according to claim 1, characterized in that, The AC-DC conversion module includes a first bridge arm group and a second bridge arm group; The upper arm of the first bridge arm group is connected between the first DC terminal and the third node of the AC-DC conversion module, and the lower arm of the first bridge arm group is connected between the second DC terminal and the third node of the AC-DC conversion module. The upper arm of the second bridge arm group is connected between the first DC terminal and the fourth node of the AC-DC conversion module, and the lower arm of the second bridge arm group is connected between the second DC terminal and the fourth node of the AC-DC conversion module. The upper and lower bridge arms of the first bridge arm group and the upper and lower bridge arms of the second bridge arm group include multiple power switching devices connected in series. Among the multiple power switching devices included in a single upper bridge arm and a single lower bridge arm, at least some of the power switching devices are fully controlled power switching devices.

7. A novel AC-DC converter, characterized in that, The novel AC-DC converter, used to convert three-phase alternating current to direct current, includes: Three transformer modules, the first end of which is connected to the corresponding AC input port; Three AC-DC conversion modules are provided. The AC terminal of each AC-DC conversion module is connected to the second terminal of the corresponding transformer module. The first DC terminal of each AC-DC conversion module is connected to the first DC port or the second DC terminal of the preceding AC-DC conversion module. The second DC terminal of each AC-DC conversion module is connected to the second DC port or the first DC terminal of the following AC-DC conversion module. The AC-DC conversion module is used to convert the received AC input signal into a pulsating DC signal. Three switched capacitor module chains are connected in parallel with corresponding AC-DC conversion modules to convert the pulsating DC signal into a DC output signal. An active filter module chain is connected between the first DC terminal and the first DC port of the first AC-DC converter module; the active filter module chain includes multiple full-bridge sub-modules connected in series. The processing module is electrically connected to the control terminal of the active filter module chain; the processing module is used to adjust the switching state of each full-bridge sub-module based on the DC voltage measurement value and the DC current measurement value, so as to generate a modulation voltage for filtering and compensating the DC output signal.

8. A DC-side active filtering method, characterized in that, The DC-side active filtering method, applied to the novel AC-DC converter as described in any one of claims 1-7, comprises: Obtain the DC voltage measurement values ​​at both ends of the switched capacitor module chain and the AC voltage measurement value at the AC input port at the current moment, and determine the ripple component of the DC voltage based on the DC voltage measurement values ​​and the AC voltage measurement values; Obtain the DC current measurement value at the current moment, input the DC current measurement value to the closed-loop controller, and obtain the additional component of DC voltage; The ideal modulation voltage value is determined based on the ripple component and the additional component, and the switching state of each full-bridge submodule is determined according to the ideal modulation voltage value. The switching control signals for each full-bridge submodule are generated based on the switching status of each full-bridge submodule.

9. The DC-side active filtering method according to claim 8, characterized in that, The determination of the DC voltage ripple component based on the DC voltage measurement and the AC voltage measurement includes: Calculate the corresponding theoretical value of DC voltage based on the measured AC voltage value; The ripple component of the DC voltage is calculated based on the measured DC voltage value and the theoretical DC voltage value.

10. The DC-side active filtering method according to claim 8, characterized in that, The step of determining the switching state of each full-bridge submodule based on the ideal modulation voltage value includes: Based on the ideal modulation voltage value and the rated voltage of the full-bridge submodule, determine the number of full-bridge submodules in different switching states among the multiple full-bridge submodules; the switching states include positive switching state, negative switching state, and bypass state; Based on the order of the actual voltage magnitudes of each full-bridge submodule, the full-bridge submodules in the positive input state are selected according to the first preset rule, and the full-bridge submodules in the negative input state are selected according to the second preset rule.

11. A DC transmission system, characterized in that, The novel AC-DC converter includes any one of claims 1-7.

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