Multi-low-voltage direct-current port MMC-SST voltage ripple suppression method

Abstract

The invention relates to the technical field of power electronic electric energy conversion, and discloses a multi-low voltage direct current port MMC-SST voltage ripple suppression method, which comprises MMC control and DAB control, the MMC control comprises alternating current output control and circulating current control, the alternating current output control generates a differential mode instruction value of each phase MMC switching function, and the circulating current control generates a circulating current instruction value of each phase MMC switching function; the common-mode pulsation suppression of the capacitor voltage is realized through the circulation control; dAB control adopts phase shift modulation and comprises output control, differential mode pulsating power transmission control and input voltage-sharing control, the DAB control of each phase is performed under a fixed coordinate system and is mutually independent, direct-current power required by a low-voltage port and differential mode pulsating power of a sub-module capacitor are transmitted to a low-voltage direct-current bus through the DAB control, the voltage pulsating of the capacitor is suppressed, and the output voltage of the sub-module capacitor is improved. And voltage balance among the capacitors of the sub-modules is realized. According to the MMC-SST voltage pulsation suppression method for the multiple low-voltage direct-current ports, port power output and voltage balance of all capacitors are achieved, and capacitor voltage pulsation can be eliminated through circulation control and differential mode pulsation control.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic electric energy conversion, and in particular to a method for suppressing voltage pulsation of a multi-low-voltage DC port MMC-SST. Background Art

[0002] With the emergence of a large number of distributed power generation and energy storage systems, the demand for clean energy access to the grid continues to expand, and the traditional AC power grid will face tremendous changes. The AC / DC hybrid power grid has become an important development direction for future distribution networks. Power electronic transformers based on power electronics technology, also known as solid-state transformers (SST), as the key transmission hub of AC / DC hybrid power distribution systems, have always been a hot research topic in industry and academia. SST integrates controllable semiconductor devices based on high-frequency isolation technology, which can realize flexible allocation and conversion between various forms of power such as medium / low voltage AC / DC.

[0003] In medium and high voltage DC transmission applications, multi-level technology represented by modular multi-level converter (MMC) has gradually become a common solution for matching medium and high voltage power grids. SST needs to have electrical isolation capability for bidirectional energy conversion, and dual active bridge (DAB) is the most mature and reliable DC / DC converter that meets this requirement. Therefore, DAB is often used to assist MMC in building isolated low-voltage DC ports. The MMC type solid state transformer (MMC-SST) with MMC sub-module cascade DAB as the core structure can provide medium voltage DC, medium voltage AC and isolated low voltage DC ports at the same time. Its application and advantages in scenarios such as new energy grid connection, AC / DC hybrid distribution network, and integrated power system have been widely discussed and verified. However, the inherent sub-module capacitor voltage fluctuation of MMC-SST will lead to a capacitor with a larger capacitance and size, which greatly hinders the improvement of system power density. Therefore, designing a control strategy to actively suppress capacitor voltage pulsation and reduce the size of MMC-SST capacitors is of great significance for the practical engineering application of MMC-SST.

[0004] At present, there are mainly high-frequency injection method, variable operation mode method and pulsation transmission method for the problem of capacitor voltage fluctuation of MMC and its derivative structure submodules. The high-frequency injection method is the most widely used method in industry, but there are problems such as increased bridge arm current peak, deterioration of output harmonics and increased motor wear. Variable operation mode methods such as reducing bus voltage and simulating two-level output will increase the complexity of additional hardware and control strategies. Based on the characteristics that the odd-order components of the fluctuating power between the upper and lower bridge arms of each phase of MMC are opposite in phase, the even-order components are in phase, and the three-phase symmetry of the fluctuating power of each phase, some scholars use bidirectional converters to connect the upper and lower bridge arms, and realize the mutual cancellation of pulsating power through power transmission control. However, the added controllable devices and isolation links increase the cost and volume of the system. What is attractive is that the DAB in MMC-SST has the ability to transmit pulsation, and the pulsation transmission method can be applied without further topology modification. In addition, for the MMC-SST structure with independent low-voltage DC ports of each phase, the even-order pulsating components cannot be eliminated by three-phase symmetry, and the odd-order pulsating components can only be transmitted to the low-voltage side. However, existing pulsation transmission control strategies are unable to accurately distinguish and transmit odd-order pulsation power independently. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned technology and provide a method for suppressing voltage pulsation of MMC-SST with multiple low-voltage DC ports, so as to achieve port power output and voltage balance of all capacitors, and eliminate capacitor voltage pulsation through circulating current control and differential mode pulsation control.

[0006] To achieve the above-mentioned purpose, the multi-low-voltage DC port MMC-SST voltage pulsation suppression method designed by the present invention includes MMC control and DAB control, the MMC control includes AC output control and circulating current control, the AC output control adopts feedforward cross-decoupling control in a traditional rotating coordinate system to generate differential mode command values ​​of each phase MMC switch function, and the circulating current control realizes the suppression of common mode pulsation of capacitor voltage;

[0007] DAB control adopts phase-shift modulation, including output control, differential-mode pulsating power transmission control and input voltage equalization control. Each phase of DAB control is carried out in a fixed coordinate system and is independent of each other. Through DAB control, the DC power required by the low-voltage port and the differential-mode pulsating power of the sub-module capacitor are transmitted to the low-voltage DC bus, suppressing the capacitor voltage pulsation while achieving voltage balance between the sub-module capacitors.

[0008] Preferably, the circulation control comprises the following steps:

[0009] A) The MMC upper arm current i xp With the lower bridge arm current i xn Add and divide by 2 to get the common-mode bridge arm current i of this phase cmx , the MMC upper bridge arm current ixp With the lower bridge arm current i xn Subtract and divide by 2 to get the phase difference mode bridge arm current i dmx , the MMC common mode switch function s cmx Multiply by the differential mode bridge arm current i dmx and divided by the MMC differential mode switching function s dmx The feedforward compensation term s of circulating current control is obtained cmx i dmx / s dmx , where the right subscript x represents the xth phase, the right subscript p represents the upper bridge arm, the right subscript n represents the lower bridge arm, the right subscript k represents the kth submodule, and the MMC differential mode switching function s dmx Generated by AC output control, the initial value is 0, MMC common mode switching function s cmx The common-mode bridge arm current i cmx Control is generated, the initial value is 0.5, and the control method of each phase is symmetrical;

[0010] B) Add the capacitor voltages of all submodules in the upper bridge arm and divide by N to obtain the average capacitor voltage u of the upper bridge arm submodule. cxp_avg , add the capacitor voltages of all submodules in the lower bridge arm and divide by N to get the average capacitor voltage u of the lower bridge arm submodule cxn_avg , N represents the number of sub-modules in each bridge arm, and the average voltage u of the bridge arm sub-module capacitor on the MMC is cxp_avg The average voltage u of the capacitor of the lower bridge arm submodule cxn_avg Add and divide by 2 to get the differential mode voltage component u of the phase submodule capacitor ccmx , the square value of the submodule capacitor common mode voltage command value (u ccmx_ref ) 2 Subtract the square value of the actual differential mode voltage component of the phase submodule capacitor (u ccmx ) 2 The difference is outputted by the proportional-integral regulator, and the feedforward compensation term of the circulating current control calculated in step A) is further subtracted to obtain the MMC common-mode bridge arm current command value i cmx_ref , the right subscript _ref indicates the instruction value;

[0011] C) Set the MMC common mode bridge arm current command value i cmx_ref Subtract the actual common-mode bridge arm current i generated in this phase cmx The difference is outputted by the proportional resonant regulator and then added to 0.5 to obtain the MMC common-mode switching function s for this phase. cmx ;

[0012] D)MMC upper bridge arm switching function s xp The common mode switching function s cmx and differential mode switching function s dmxAdd together to get the MMC lower bridge arm switching function s xn The common mode switching function s cmx and differential mode switching function s dmx Subtract and get;

[0013] E) Set the MMC upper bridge arm switch function s xp and MMC lower bridge arm switching function s xn Compare with the carrier value to generate PWM pulse signals for the upper and lower arms of the MMC.

[0014] Preferably, the submodule capacitor common mode voltage command value u ccmx_ref Equal to one Nth of the medium voltage DC voltage, that is, u MVDC / N.

[0015] Preferably, the switching functions of the MMC submodules in the same bridge arm are completely the same, and the modulation method adopts carrier phase-shift pulse width modulation.

[0016] Preferably, the DAB control comprises the following steps:

[0017] 1) The square of the voltage command value of the low voltage DC bus (u LVDCx_ref ) 2 Subtract the square of the actual voltage feedback value (u LVDCx ) 2 , the common mode transmission power command P of the x-phase DAB is obtained through the proportional integral regulator DABcmx ;

[0018] 2) Set the MMC upper bridge arm switch function s xp Multiply the upper arm current i xp Get the input current i of the upper bridge arm submodule SMxp , the upper bridge arm submodule input current i SMxp Multiply the average voltage u of the upper bridge arm submodule capacitor cxp_avg Get the average input power P of the upper bridge arm submodule SMxp ;

[0019] Set the MMC lower bridge arm switch function s xn Multiply the following bridge arm current i xn Get the input current i of the lower bridge arm submodule SMxn , the lower bridge arm submodule input current i SMxn Multiply the average voltage u of the bridge arm submodule capacitor below cxn_avg Get the average input power P of the lower bridge arm submodule SMxn , the upper bridge arm submodule input power average value P SMxp Subtract the average input power P of the lower bridge arm submodule SMxn And divided by 2 to get the differential mode component P of the input power of the phase submodule SMdmx ;

[0020] 3) The average voltage u of the upper bridge arm submodule capacitor cxp_avg Subtract the average voltage u of the lower bridge arm submodule capacitor cxn_avg And divided by 2 to get the phase submodule capacitor differential mode voltage u cdmx , the square of the submodule capacitor differential mode voltage command value (u cdmx_ref ) 2 Subtract the square value of the actual submodule capacitance differential mode voltage (u cdmx ) 2 The difference is output through the proportional integral regulator, and the differential mode component P of the input power of the phase submodule calculated in step 2) is converted into SMdmx Subtract the output generated by the proportional integral regulator to obtain the common differential mode power command P of the phase DAB DABdmx When the capacitor differential mode voltage command value is set to 0, all differential mode pulsating power is transmitted; when it is set to a non-zero value, part of the pulsating power is transmitted;

[0021] 4) Add the capacitor voltage values ​​of the xth phase and divide by 2N to obtain the average capacitor voltage u of the sub-module of this phase cx_avg , N represents the number of submodules in each bridge arm, and the square of the actual value of the capacitor voltage of the kth submodule on the bridge arm of the xth phase (u cxpk ) 2 The square of the average voltage of the phase submodule capacitor (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the upper bridge arm through the proportional integral regulator. DABxpk , calculate the square of the actual value of the capacitor voltage of the kth submodule of the lower bridge arm of the xth phase (u cxnk ) 2 The square of the average value of the phase capacitor voltage (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the lower bridge arm through the proportional integral regulator. DABxnk ;

[0022] 5) Transmission power command value P of the kth DAB on the upper bridge arm of the xth phase DABxpk The common mode transmission power command P DABcmx , differential mode power command P DABdmx With △P DABxpk Add together to get the transmission power command value P of the kth DAB in the lower bridge arm DABxnk The common mode transmission power command P DABcmx Subtract the differential mode power command P SMdmx Plus the corresponding △P DABxnk Obtained; The shift ratio of DAB is calculated based on the power transfer characteristics.

[0023] Preferably, the kth DAB shift phase of the upper bridge arm of the xth phase is d xpk Compared with the kth DAB shift of the lower bridge arm, d xnk , according to the power transfer characteristics, it is calculated as follows:

[0024]

[0025] Where, f represents the switching frequency of DAB, and L represents the leakage inductance of DAB transformer.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. It can significantly improve the power density of MMC-SST. The circulating current control of MMC can suppress the common-mode voltage pulsation of the sub-module capacitor. DAB can transmit the differential-mode pulsating power input by the MMC sub-module, further suppressing the differential-mode voltage pulsation of the capacitor, and minimizing the system's demand for large-value and large-volume capacitors.

[0028] 2. Both the MMC circulating current control method and the DAB control method are implemented in a fixed coordinate system, without the need to sense the AC frequency. They are simple to implement and each phase is independent, which is very suitable for variable frequency systems and is very conducive to the free expansion of the system.

[0029] 3. Adopting a feedforward plus closed-loop control structure, the feedforward term can accelerate the convergence of the control target, so that a simple PI can achieve higher control accuracy for the AC quantity. At the same time, closed-loop feedback control can also overcome the problem that the open-loop strategy is easily affected by non-ideal circuits. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a topological diagram of a multi-low-voltage DC port MMC-SST circuit in the present invention;

[0031] Figure 2 This is the single-phase equivalent circuit diagram of MMC-SST;

[0032] Figure 3 Schematic diagram of the internal power flow of MMC-SST;

[0033] Figure 4 This is the control block diagram of MMC;

[0034] Figure 5 This is the control block diagram of DAB;

[0035] Figure 6 Schematic diagram of the power flow of the submodule capacitor. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] The present invention relates to a method for suppressing voltage pulsation of a multi-low-voltage DC port MMC-SST. In this embodiment, the MMC-SST topology is first described. The multi-low-voltage DC port MMC-SST consists of two main parts: Figure 1 As shown, they are:

[0038] 1) The MMC structure is used to match the medium-voltage DC bus, and a medium-voltage AC output / input port is provided at the midpoint of each phase. MMC has the advantages of large capacity, fast redundancy, and modular structure;

[0039] 2) The DAB converter is used to construct an isolated low-voltage DC bus. The input end of the DAB is connected in parallel to the MMC sub-module capacitor, and the output end is connected in parallel to the phase-independent low-voltage DC bus. The sub-module capacitor is reused as the DAB input capacitor, reducing the number of capacitors required.

[0040] The standard power module composed of MMC sub-modules and cascaded DAB converters is the smallest unit of the MMC-SST system. Each phase consists of 2N standard power modules and bridge arm inductors in series.

[0041] Figure 2 is the single-phase equivalent circuit of MMC-SST. In the figure, the right subscript x of each variable represents the xth phase, the right subscript p represents the upper bridge arm, the right subscript n represents the lower bridge arm, the right subscript k represents the kth module, the right subscript “_ref” represents the command value, L and R are the bridge arm inductance and bridge arm equivalent resistance respectively, uMVDC is the medium voltage DC bus voltage, u x and i x Respectively represent the AC output voltage and current of the x phase; i xp 、i xn are the upper bridge arm current and the lower bridge arm current, u xp 、u xn are the sum of the voltages of the upper and lower bridge arms, respectively, LVDCx is the low voltage DC port voltage, i zx is the circulating current of the x-phase bridge arm.

[0042] In order to suppress the voltage pulsation of the submodule capacitor, the MMC part adds a circulating current control method to suppress the even-order (common-mode) pulsation component, and the DAB part adds a differential-mode pulsation power transmission method to suppress the odd-order (differential-mode) pulsation component. Under the control method designed by the present invention, the power flow of the MMC-SST system is as follows: Figure 3 As shown, the active power of the medium voltage DC port is transmitted to the medium voltage AC port through the bridge arm, and the remaining active power of the bridge arm is transmitted to the low voltage DC port through DAB. The odd-order or differential mode pulsating power circulates between the upper and lower bridge arms through DAB, and the even-order or common mode pulsating power circulates between the medium voltage DC port and the medium voltage AC port through the MMC bridge arm. Due to the three-phase symmetry of the pulsating power, the pulsating power will not affect the voltage and output current of the medium voltage DC bus; due to the anti-phase characteristics of the upper and lower bridge arms of the differential mode pulsating power, the differential mode pulsating power will not affect the output of the low voltage DC port.

[0043] In the present invention, a method for suppressing voltage pulsation of a multi-low-voltage DC port MMC-SST includes MMC control and DAB control. The MMC control includes AC output control and circulating current control. The AC output control adopts feedforward cross-decoupling control in a traditional rotating coordinate system to generate differential mode command values ​​of MMC switch functions of each phase. The circulating current control realizes the suppression of common mode pulsation of capacitor voltage.

[0044] DAB control uses phase shift modulation, including output control, differential mode pulsating power transmission control and input voltage control. Each phase of DAB control is carried out in a fixed coordinate system and is independent of each other. The block diagram of the x-phase DAB control is shown in Figure 5 As shown, through DAB control, the power flow of the submodule is as follows Figure 6 As shown, the DC power required by the low-voltage port and the differential-mode pulsating power of the sub-module capacitor are transmitted to the low-voltage DC bus, which suppresses the capacitor voltage pulsation and realizes voltage balance between the sub-module capacitors.

[0045] Among them, combined Figure 4 As shown, circulation control includes the following steps:

[0046] A) The MMC upper arm current i xp With the lower bridge arm current i xn Add and divide by 2 to get the common-mode bridge arm current i of this phase cmx , the MMC upper bridge arm current i xp With the lower bridge arm current i xn Subtract and divide by 2 to get the phase difference mode bridge arm current i dmx , the MMC common mode switch function s cmx Multiply by the differential mode bridge arm current i dmx and divided by the MMC differential mode switching function s dmx The feedforward compensation term s of circulating current control is obtained cmx idmx / s dmx , where the right subscript x represents the xth phase, the right subscript p represents the upper bridge arm, the right subscript n represents the lower bridge arm, the right subscript k represents the kth submodule, and the MMC differential mode switching function s dmx Generated by AC output control, the initial value is 0, MMC common mode switching function s cmx The common-mode bridge arm current i cmx Control is generated, the initial value is 0.5, and the control method of each phase is symmetrical;

[0047] B) Add the capacitor voltages of all submodules in the upper bridge arm and divide by N to obtain the average capacitor voltage u of the upper bridge arm submodule. cxp_avg , add the capacitor voltages of all submodules in the lower bridge arm and divide by N to get the average capacitor voltage u of the lower bridge arm submodule cxn_avg , N represents the number of sub-modules in each bridge arm, and the average voltage u of the bridge arm sub-module capacitor on the MMC is cxp_avg The average voltage u of the capacitor of the lower bridge arm submodule cxn_avg Add and divide by 2 to get the differential mode voltage component u of the phase submodule capacitor ccmx , the square value of the submodule capacitor common mode voltage command value (u ccmx_ref ) 2 Subtract the square value of the actual differential mode voltage component of the phase submodule capacitor (u ccmx ) 2 The difference is outputted by the proportional-integral regulator, and the feedforward compensation term of the circulating current control calculated in step A) is further subtracted to obtain the MMC common-mode bridge arm current command value i cmx_ref , the right subscript _ref indicates the instruction value;

[0048] C) Set the MMC common mode bridge arm current command value i cmx_ref Subtract the actual common-mode bridge arm current i generated in this phase cmx The difference is outputted by the proportional resonant regulator and then added to 0.5 to obtain the MMC common-mode switching function s for this phase. cmx ;

[0049] D)MMC upper bridge arm switching function s xp The common mode switching function s cmx and differential mode switching function s dmx Add together to get the MMC lower bridge arm switching function s xn The common mode switching function s cmx and differential mode switching function s dmx Subtract and get;

[0050] E) Set the MMC upper bridge arm switch function s xp and MMC lower bridge arm switching function s xnCompare with the carrier value to generate PWM pulse signals for the upper and lower arms of the MMC.

[0051] Among them, the submodule capacitor common mode voltage command value u ccmx_ref Equal to one Nth of the medium voltage DC voltage, that is, u MVDC / N, while achieving capacitor common mode voltage ripple suppression, maintaining energy balance between the phase unit and the medium voltage DC bus, between each phase unit, and within the phase unit. The switching functions of each MMC sub-module in the same bridge arm are exactly the same, and the modulation method adopts carrier phase-shift pulse width modulation.

[0052] In addition, DAB control includes the following steps:

[0053] 1) The square of the voltage command value of the low voltage DC bus (u LVDCx_ref ) 2 Subtract the square of the actual voltage feedback value (u LVDCx ) 2 , the common mode transmission power command P of the x-phase DAB is obtained through the proportional integral regulator DABcmx ;

[0054] 2) Set the MMC upper bridge arm switch function s xp Multiply the upper arm current i xp Get the input current i of the upper bridge arm submodule SMxp , the upper bridge arm submodule input current i SMxp Multiply the average voltage u of the upper bridge arm submodule capacitor cxp_avg Get the average input power P of the upper bridge arm submodule SMxp ;

[0055] Set the MMC lower bridge arm switch function s xn Multiply the following bridge arm current i xn Get the input current i of the lower bridge arm submodule SMxn , the lower bridge arm submodule input current i SMxn Multiply the average voltage u of the bridge arm submodule capacitor below cxn_avg Get the average input power P of the lower bridge arm submodule SMxn , the upper bridge arm submodule input power average value P SMxp Subtract the average input power P of the lower bridge arm submodule SMxn And divided by 2 to get the differential mode component P of the input power of the phase submodule SMdmx ;

[0056] 3) The average voltage u of the upper bridge arm submodule capacitor cxp_avg Subtract the average voltage u of the lower bridge arm submodule capacitor cxn_avg And divided by 2 to get the phase submodule capacitor differential mode voltage u cdmx , the square of the submodule capacitor differential mode voltage command value (ucdmx_ref ) 2 Subtract the square value of the actual submodule capacitance differential mode voltage (u cdmx ) 2 The difference is output through the proportional integral regulator, and the differential mode component P of the input power of the phase submodule calculated in step 2) is converted into SMdmx Subtract the output generated by the proportional integral regulator to obtain the common differential mode power command P of the phase DAB DABdmx When the capacitor differential mode voltage command value is set to 0, all differential mode pulsating power is transmitted; when it is set to a non-zero value, part of the pulsating power is transmitted;

[0057] 4) Add the capacitor voltage values ​​of the xth phase and divide by 2N to obtain the average capacitor voltage u of the sub-module of this phase cx_avg , N represents the number of submodules in each bridge arm, and the square of the actual value of the capacitor voltage of the kth submodule on the bridge arm of the xth phase (u cxpk ) 2 The square of the average voltage of the phase submodule capacitor (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the upper bridge arm through the proportional integral regulator. DABxpk , calculate the square of the actual value of the capacitor voltage of the kth submodule of the lower bridge arm of the xth phase (u cxnk ) 2 The square of the average value of the phase capacitor voltage (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the lower bridge arm through the proportional integral regulator. DABxnk ;

[0058] 5) Transmission power command value P of the kth DAB on the upper bridge arm of the xth phase DABxpk The common mode transmission power command P DABcmx , differential mode power command P DABdmx With △P DABxpk Add together to get the transmission power command value P of the kth DAB in the lower bridge arm DABxnk The common mode transmission power command P DABcmx Subtract the differential mode power command P SMdmx Plus the corresponding △P DABxnk Obtained; The shift ratio of DAB is calculated based on the power transfer characteristics.

[0059] The kth DAB shift phase of the upper bridge arm of the xth phase is d xpk Compared with the kth DAB shift of the lower bridge arm, d xnk , according to the power transfer characteristics, it is calculated as follows:

[0060]

[0061] Where, f represents the switching frequency of DAB, and L represents the leakage inductance of DAB transformer.

[0062] Finally, it should be pointed out that the above content is a further detailed description of the invention in combination with specific implementation methods. It cannot be considered that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, simple replacements made without departing from the concept of the present invention should be regarded as belonging to the protection scope of the present invention. The above embodiments are only more representative examples of the present invention. Obviously, the present invention is not limited to the above embodiments, and there can be many variations. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention should be considered to belong to the protection scope of the present invention.

[0063] At the same time, it should be noted that the description of the above technical solutions is exemplary, and this specification can be embodied in different forms and should not be construed as being limited to the technical solutions set forth herein. On the contrary, providing these descriptions will make the disclosure of the present invention thorough and complete, and will fully convey the scope disclosed in this specification to those skilled in the art. In addition, the technical solutions of the present invention are limited only by the scope of the claims. The features of the various embodiments of the present invention may be partially or completely combined or spliced ​​with each other, and may be performed in various different configurations as those skilled in the art may fully understand. The embodiments of the present invention may be performed independently of each other, or may be performed together in a mutually dependent relationship.

[0064] For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, and the above structures should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for suppressing voltage pulsation of MMC-SST with multiple low-voltage DC ports. Features: It includes MMC control and DAB control. MMC control consists of AC output control and circulating current control. AC output control adopts feedforward cross-decoupling control in the traditional rotating coordinate system to generate differential mode command values ​​of MMC switching functions of each phase. Circulating current control realizes the suppression of common mode pulsation of capacitor voltage. DAB control adopts phase-shift modulation, including output control, differential-mode pulsating power transmission control and input voltage equalization control. Each phase of DAB control is carried out in a fixed coordinate system and is independent of each other. Through DAB control, the DC power required by the low-voltage port and the differential-mode pulsating power of the sub-module capacitor are transmitted to the low-voltage DC bus, suppressing the capacitor voltage pulsation while achieving voltage balance between the sub-module capacitors.

2. According to the multi-low voltage DC port MMC-SST voltage pulsation suppression method of claim 1, Features: Circulation control includes the following steps: A) The MMC upper arm current i xp With the lower bridge arm current i xn Add and divide by 2 to get the common-mode bridge arm current i of this phase cmx , the MMC upper bridge arm current i xp With the lower bridge arm current i xn Subtract and divide by 2 to get the phase difference mode bridge arm current i dmx , the MMC common mode switch function s cmx Multiply by the differential mode bridge arm current i dmx and divided by the MMC differential mode switching function s dmx The feedforward compensation term s of circulating current control is obtained cmx i dmx / s dmx , where the right subscript x represents the xth phase, the right subscript p represents the upper bridge arm, the right subscript n represents the lower bridge arm, the right subscript k represents the kth submodule, and the MMC differential mode switching function s dmx Generated by AC output control, the initial value is 0, MMC common mode switching function s cmx The common-mode bridge arm current i cmx Control is generated, the initial value is 0.5, and the control method of each phase is symmetrical; B) Add the capacitor voltages of all submodules in the upper bridge arm and divide by N to obtain the average capacitor voltage u of the upper bridge arm submodule. cxp_avg , add the capacitor voltages of all submodules in the lower bridge arm and divide by N to get the average capacitor voltage u of the lower bridge arm submodule cxn_avg , N represents the number of sub-modules in each bridge arm, and the average voltage u of the bridge arm sub-module capacitor on the MMC is cxp_avg The average voltage u of the capacitor of the lower bridge arm submodule cxn_avg Add and divide by 2 to get the differential mode voltage component u of the phase submodule capacitor ccmx , the square value of the submodule capacitor common mode voltage command value (u ccmx_ref ) 2 Subtract the square value of the actual differential mode voltage component of the phase submodule capacitor (u ccmx ) 2 The difference is outputted by the proportional-integral regulator, and the feedforward compensation term of the circulating current control calculated in step A) is further subtracted to obtain the MMC common-mode bridge arm current command value i cmx_ref , the right subscript _ref indicates the instruction value; C) Set the MMC common mode bridge arm current command value i cmx_ref Subtract the actual common-mode bridge arm current i generated in this phase cmx The difference is output through the proportional resonant regulator and then added to 0.5 to obtain the MMC common-mode switching function s of this phase. cmx ; D)MMC upper bridge arm switching function s xp The common mode switching function s cmx and differential mode switching function s dmx Add together and you get the MMC lower bridge arm switching function s xn The common mode switching function s cmx and differential mode switching function s dmx Subtract and get; E) Set the MMC upper bridge arm switch function s xp and MMC lower bridge arm switching function s xn Compare with the carrier value to generate PWM pulse signals for the upper and lower arms of the MMC.

3. According to the multi-low voltage DC port MMC-SST voltage pulsation suppression method of claim 2, Features: Submodule capacitor common mode voltage command value u ccmx_ref Equal to one Nth of the medium voltage DC voltage, that is, u MVDC / N.

4. According to the multi-low voltage DC port MMC-SST voltage pulsation suppression method of claim 2, Features: The switching functions of each MMC submodule in the same bridge arm are exactly the same, and the modulation method adopts carrier phase-shift pulse width modulation.

5. According to the multi-low voltage DC port MMC-SST voltage pulsation suppression method of claim 1, Features: DAB control includes the following steps: 1) The square of the voltage command value of the low voltage DC bus (u LVDCx_ref ) 2 Subtract the square of the actual voltage feedback value (u LVDCx ) 2 , the common mode transmission power command P of the x-phase DAB is obtained through the proportional integral regulator DABcmx ; 2) Set the MMC upper bridge arm switch function s xp Multiply the upper arm current i xp Get the input current i of the upper bridge arm submodule SMxp , the upper bridge arm submodule input current i SMxp Multiply the average voltage u of the upper bridge arm submodule capacitor cxp_avg Get the average input power P of the upper bridge arm submodule SMxp ; The MMC lower bridge arm switch function s xn Multiply the following bridge arm current i xn Get the input current i of the lower bridge arm submodule SMxn , the lower bridge arm submodule input current i SMxn Multiply the average voltage u of the bridge arm submodule capacitor below cxn_avg Get the average input power P of the lower bridge arm submodule SMxn , the input power average value P of the upper bridge arm submodule SMxp Subtract the average input power P of the lower bridge arm submodule SMxn And divided by 2 to get the differential mode component P of the input power of the phase submodule SMdmx ; 3) The average voltage u of the upper bridge arm submodule capacitor cxp_avg Subtract the average voltage u of the lower bridge arm submodule capacitor cxn_avg And divided by 2 to get the phase submodule capacitor differential mode voltage u cdmx , the square of the submodule capacitor differential mode voltage command value (u cdmx_ref ) 2 Subtract the square value of the actual submodule capacitance differential mode voltage (u cdmx ) 2 The difference is output through the proportional integral regulator, and the differential mode component P of the input power of the phase submodule calculated in step 2) is converted into SMdmx Subtract the output generated by the proportional integral regulator to obtain the common differential mode power command P of the phase DAB DABdmx When the capacitor differential mode voltage command value is set to 0, all differential mode pulsating power is transmitted; when it is set to a non-zero value, part of the pulsating power is transmitted; 4) Add the capacitor voltage values ​​of the xth phase and divide by 2N to obtain the average capacitor voltage u of the sub-module of this phase cx_avg , N represents the number of submodules in each bridge arm, and the square of the actual value of the capacitor voltage of the kth submodule in the bridge arm of the xth phase (u cxpk ) 2 The square of the average voltage of the phase submodule capacitor (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the upper bridge arm through the proportional integral regulator. DABxpk , calculate the square of the actual value of the capacitor voltage of the kth submodule of the lower bridge arm of the xth phase (u cxnk ) 2 The square of the average value of the phase capacitor voltage (u cx_avg ) 2 The deviation is then used to generate the additional power command △P of the kth DAB of the lower bridge arm through the proportional integral regulator. DABxnk ; 5) Transmission power command value P of the kth DAB on the upper bridge arm of the xth phase DABxpk The common mode transmission power command P DABcmx , differential mode power command P DABdmx With △P DABxpk Add together to get the transmission power command value P of the kth DAB in the lower bridge arm DABxnk The common mode transmission power command P DABcmx Subtract the differential mode power command P SMdmx Plus the corresponding △P DABxnk Obtained; The shift ratio of DAB is calculated based on the power transfer characteristics.

6. The method for suppressing voltage pulsation of MMC-SST with multiple low-voltage DC ports according to claim 5, Features: The kth DAB shift phase of the upper bridge arm of the xth phase is d xpk Compared with the kth DAB shift of the lower bridge arm, d xnk , according to the power transfer characteristics, it is calculated as follows: Where f represents the switching frequency of DAB, and L represents the leakage inductance of DAB transformer.

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