164 results about "Solid state transformer" patented technology

The invention discloses a voltage-equalizing and power-equalizing control method for a multi-module cascade solid-state transformer (SST). An SST power main circuit part applied in the method comprises an input stage, an intermediate stage and an output stage, wherein a common duty ratio control policy based on single-phase d-q vector control is adopted in the input stage so as to realize the unit power factor of an input end of an electric network; a control policy based on voltage feedforward and voltage feedback is introduced for each module in the intermediate stage, the voltage feedforward is used as a reference of the voltage feedback, and an error between the voltage feedforward and the voltage feedback is used as a reference of inner current loop of the module after being output by a voltage PI adjuster. The two control policies of the front stage and the intermediate stage interact so as to realize the functions of voltage and power equalization of each SST module, electrical network side unit power factor, high electrical network current sine degree, low switching tube voltage stress and the like.

The invention provides a modular multilevel converter (MMC) based solid-state transformer and a control method thereof. The solid-state transformer comprises a high-voltage AC / DC converter, an isolated DC / DC converter and a low-voltage DC / AC converter, wherein the high-voltage AC / DC converter is connected with the isolated DC / DC converter through a high-voltage DC bus, and the isolated DC / DC converter is connected with the low-voltage DC / AC converter through a low-voltage DC bus parallel module. In the MMC based solid-state transformer, a medium- and low-voltage DC side adopts a DC bus staggered parallel topological structure, so that the unbalanced mutual influence of a high-voltage side system and a low-voltage side system of the solid-state transformer is solved from structure, and the power supply reliability is automatically improved; with the mutually-balanced solid-state transformer based on an MMC structure, the influence of a three-phase unbalance on a power grid and the influence of three-phase unbalance occurring in the power grid on an AC load can be prevented by controlling the constant voltage of a high-voltage common DC bus.

The invention relates to a high-voltage SiC solid-state transformer based high-speed rail locomotive AC-DC-AC traction system, in particular to a solid-state transformer based locomotive AC-DC-AC traction system and method. Solid-state transformer modules made of SiC materials are adopted, and input sides of the transformer modules are in series connection while output sides are in parallel connection to form the high-speed rail locomotive AC-DC-AC traction system. Compared with traction transformers made of conventional semiconductor materials, solid-state transformers made of the SiC materials have the advantages that due to good power handling capability and frequency capability of the transformers, size and weight of the transformers can be reduced, load weight of a locomotive is reduced greatly, and good economical efficiency is realized; the input sides are in series connection while the output sides are in parallel connection, so that the high-speed rail locomotive traction system is easy to expand, the locomotive can be kept in normal operation within a period of time if the secondary edge of one solid-state transformer module has loss, and accordingly reliability is improved.

The invention proposes a solid-state transformer with an alternating current-direct current fault continuous operation capability and a control method. A hybrid modular multi-level converter bridge arm consists of a half-bridge sub-module and a full-bridge sub-module; the half-bridge sub-module and the full-bridge sub-module are interconnected with input ends of isolated dual active bridge converters through a sub-module direct current capacitor; output ends of multiple isolated dual active bridge converters are connected in parallel to form a low-voltage direct current bus; and the low-voltage direct current bus is connected to a three-phase full-bridge inverter. The solid-state transformer provided by the invention can provide a medium-voltage direct current port, a medium-voltage alternating current port, a low-voltage direct current port and a low-voltage alternating current port, so that multi-voltage-grade and multi-form alternating current-direct current hybrid power distribution network interconnection can be realized; the solid-state transformer has the alternating current-direct current fault continuous operation capability, quick switching of device operation modes whena short circuit or open circuit fault occurs on a network connected to any port can be realized, and continuous operation between non-faulty ports can be ensured, thereby improving the power supply reliability of the alternating current-direct current hybrid distribution network.

The invention discloses a bi-direction direct-current solid-state transformer with a high-frequency alternating-current isolation link, and belongs to the technical field of electric power. The transformer is formed by connecting n identical direct-current transformer units at a low-voltage port in parallel and at a high-voltage port in series, wherein n is a positive integer larger than or equal to 2. Each direct-current transformer unit comprises two full-bridge transformers (H1 and H2), an auxiliary inductor (L), an isolation transformer (T) and two direct-current filter capacitors (C1 and C2), wherein the direct-current filter capacitors (C1 and C2) are connected with the direct-current ends of the full-bridge transformers (H1 and H2) respectively in parallel, the primary side and the secondary side of the isolation transformer are in parallel connection with the alternating current ends of the full-bridge transformers (H1 and H2) respectively, and the auxiliary inductor (L) is connected between the alternating current end of the full-bridge transformer H1 and the isolation transformer. Voltage matching of high-voltage and low-voltage direct-current links can be achieved, bi-direction flowing of power and electric isolation can be achieved, and the transformer has a certain application prospect in a direct-current power distribution network and a direct-current micro-grid.

The invention relates to a bi-directional power flow controlled multi-level mutual balance solid state transformer and a realization method thereof. The solid state transformer is a three-phase solid state transformer which is designed by using a mutual balance structure, and uses an AC-DC-AC three step type transform structure. A high voltage level, an isolation level and a low voltage level are comprised. The high voltage level and the isolation level are transformers with the same three-phase structure and are composed of sub units. The low voltage level is a single whole module which realizes an automatic mutual balance structure. Three-phase sub units of the high voltage level are connected in a star mode. The low voltage level is an alternating current output end with a three-phase four-wire wiring mode. The solid state transformer can realize power bi-directional flow. The whole three-phase system uses an independent structure, which is convenient for three-phase independent control and modular production. Through the direct current bus parallel module of the low voltage level, a three-phase electric power automatic mutual balance function is realized, so that the practical problem of the solid state transformer is solved.

The invention provides a photovoltaic solid transformer. Each of three phase circuits which are connected in a star-shaped or triangular mode of the photovoltaic solid transformer comprises a filter,a plurality of H bridge modules and a plurality of high-voltage isolation type DC / DC converters; MPPT control of the photovoltaic solid transformer is realized by the H bridge modules, so that the high-voltage isolation type DC / DC converter does not need to control MPPT of a photovoltaic plate connected to the photovoltaic solid transformer or regulate voltage in real time, therefore, the value ofa gain range of the high-voltage isolation type DC / DC converter can be small, and compared with the prior art, the high-voltage isolation type DC / DC converter has the characteristic that the efficiency is improved further.

The invention discloses a voltage-sharing and power-sharing control method of a single-phase multi-module cascading solid-state transformer. A power main circuit part of the single-phase solid-state transformer is composed of an AC / DC rectifying input stage, a DC / DC middle stage and a DC / AC output stage. The rectifying stage and the middle stage serve as a system for controlling and designing, a common duty ratio control strategy based on single-phase d-q vector control is adopted in the input stage, the output direct voltage of the middle stage serves as feedback quantity of an outer voltage loop, the stability of the output direct voltage of the middle stage is achieved, and a unity power factor of an input end of a power grid is achieved; a control strategy based on voltage feed-forward and voltage feed-backward is introduced into all modules in the middle stage at the same time, the voltage feed-forward serves the benchmark of the voltage feed-backward, and errors of the voltage feed-forward and voltage feed-backward serve as the benchmark of a module current inner loop after being output by a voltage PI regulator. The front stage control strategy is interacted with the middle stage control strategy, and therefore the purposes that voltage and the power of modules of an SST are balanced, the power grid side unity power factor is achieved, the power grid current sine degree is high, the voltage stress of a switch tube is low, and the ripple of the output direct voltage of the middle stage is low.

The invention provides a novel modular multi-level solid-state transformer and an internal model control method thereof. The solid-state transformer comprises a modular multi-level converter, a DC-DC isolator and a DC-AC inverter, wherein the alternating current side of the modular multi-level converter is connected to a high-voltage AC network and the direct current side of the modular multi-level converter is connected to the input end of the DC-DC isolator; the direct current side of the DC-AC inverter is connected to the output end of the DC-DC isolator and the alternating current side of the DC-AC inverter is connected to a low-voltage AC network or load; through changing the number of power sub-modules connected to each bridge arm of the modular multi-level solid-state transformer in series, the voltage application range of the solid-state transformer is flexibly adjusted, and a problem of application of the solid-state transformer in middle-high voltage occasions is effectively solved. According to the characteristics of internal model control, an internal model current inner loop is combined with a PI voltage outer loop, and a new double closed loop control structure is constructed, so that the modular multi-level converter and the DC-AC inverter are controlled; and through the control method, the current has a quicker response speed and stronger anti-disturbance capability.

The present invention relates to a domestic energy local area network energy router. The router comprises a controller, a solid state transformer and a plurality of interfaces. The interfaces are respectively connected with a major network and a distributed power supply, an energy storage device and a load in a domestic energy local area network, the controller regulates the voltage of the solid state transformer which is a dual-way structure and comprise an input-level high-voltage three-phase rectifier, an isolated DC / DC convertor and an output-level low-voltage three-phase inverter which are connected in order, the DC / DC convertor is a dual-way symmetrical structure and comprises a single-phase full-bridge inversion circuit, a high-frequency transformer and a one-way bridge total control rectifying circuit, the single-phase full-bridge inversion circuit converts the high-voltage direct current of the major network to high frequency square wave to realize secondary side voltage reduction through the high-frequency transformer and output to the low-voltage direct current bus through the single-phase full-bridge inversion circuit, and the low-voltage direct current bus is connected with the domestic energy local area network through the low-voltage three-phase inverter. Compared to the prior art, the domestic energy local area network energy router and the control strategy thereof optimize the energy configuration, enrich the electric energy flow and has good application prospect.

This invention relates to a method of controlling a Solid State Transformer (SST). The method comprises dividing a dual active bridge (DAB) power reference on a Low Voltage (LV) side Direct Current (DC) link voltage to obtain a total DAB reference current; dividing the total DAB current reference by N number of DAB modules to obtain a reference current for each DAB module; in response to SST being in a normal power regulation, determining a deviation of a capacitor voltage at the HV side of each of a k DAB modules with reference from an average voltage from the HV DC bus; determining a current adjustment signal for each of the k DAB modules based on the total DAB reference current and a corresponding deviation of the capacitor voltage of each of the k DAB modules; adding a saturated current adjustment signal together with the reference current for each of the k DAB modules to obtain a total current reference for each of the k DAB modules; and subtracting a sum of the total current reference of each of the k DAB modules from the total DAB reference current to determine a total current reference for an Nth DAB module.

The invention discloses a voltage-equalizing and power-equalizing control method for a three-phase multi-module cascade solid-state transformer. A three-phase SST system power main circuit part applied in the method comprises an input stage, an intermediate stage and an output stage, wherein a control policy based on three-phase d-q-0 vector control is adopted in the input stage, a common duty ratio control policy is adopted in modules in the same phase so as to achieve the unit power factor of an input end of an electric network; a control policy based on voltage feedforward and voltage feedback is introduced for each module in the intermediate stage, the voltage feedforward is used as a reference of the voltage feedback, and an error between the voltage feedforward and the voltage feedback is used as a reference of an inner current loop of each module after being output by a voltage PI adjuster. The two control policies of the front stage and the intermediate stage interact so as to achieve the functions of voltage and power equalization of each SST module, electrical network side unit power factor, high electrical network current sine degree, low switching tube voltage stress and the like.

The invention relates to a controller and a control method for voltage stabilization of a wind power DC (direct current) bus based on a hybrid system. At a low voltage DC side of a novel permanent magnetic wind power integration system based on a solid state transformer, a super capacitor is added by a bidirectional DC-DC conversion unit, a hybrid system model is built based on a Boost circuit, a quasi-sliding mode control policy is introduced. By utilizing the quick charge and discharge characteristics of the super capacitor, the stability of DC voltage is improved. The invention further discloses a control implementation method of the controller. The DC-side voltage fluctuation of the integration system, charge and discharge voltages as well as charge and discharge currents of the super capacitor are diminished, thereby implementing fast and accurate inhibition on the voltage fluctuation of the DC bus of the wind power system, and improving power quality and stability of the integration system.

The invention relates to a hybrid energy storage wind / PV hybrid generation system based on a solid state transformer. The system comprises a wind driven generator, a photovoltaic array, the solid state transformer, a hybrid energy storage module, a power grid, and a control module. The solid state transformer is respectively connected with the wind driven generator, the photovoltaic array, the hybrid energy storage module, the power grid, and the control module. The control module is connected with the hybrid energy storage module. Compared with the prior art, the system obviously reduces system size and cost and improves a device utilization rate. Power fluctuation the system can be preferably restrained, power distribution reliability and electric energy quality of a distributed power supply can be improved, and the system can provide short-time energy support under conditions of low illumination intensity, low wind speed, or the like.

The invention discloses an electric vehicle charging station power supply system based on a solid-state transformer. The system comprises an alternating-current / direct-current solid-state transformer, a direct-current power distribution cabinet, a direct-current charging cabinet, a direct-current charging pile, an inverter, an alternating-current charging cabinet, an alternating-current charging pile and other equipment. In the system, a power supply line, where a 400V direct current as a main portion, is set and is connected to a power grid through the alternating-current / direct-current solid-state transformer. During an electric vehicle charging process, an influence of a generated harmonic wave on power-grid electric energy quality is effectively restrained and a power factor is effectively increased so that energy saving and consumption reduction are realized and structures of a power supply system and charging equipment are simplified. Based on that, because of an application of an energy storage unit, a power grid load can be effectively regulated and controlled, which is good for power supply stabilization; and because that the alternating-current / direct-current solid-state transformer and the power supply line where the 400V direct current as the main portion are arranged, access of renewable energy sources of solar energy, wind energy and the like can be convenient.

Embodiments of the present invention provide a power source equipment (PSE) network device operable to provide a network signal that may include both power and data. This PSE network device includes a network connector and an integrated circuit. The network connector physically couples the PSE network device to the network. The integrated circuit further includes a power feed circuit. This power feed circuit is operable to combine and pass the received data signals and power signal as a single network signal. A PSE controller electrically couples to the integrated circuit but is not necessarily part of the integrated circuit. The PSE controller is operable to govern the production and distribution of the power portion of the network signal.

The invention provides a control method for suppressing a capacitor voltage fluctuation in a modular solid-state transformer. The method comprises a step of transmitting a fluctuation amount in half-bridge unit input current to a low-voltage DC port and a medium-voltage DC port according to a preset wave power transmission strategy, and a step of controlling the fluctuation power of the low-voltage DC port and the medium-voltage DC port to maintain self-balancing by modulating a phase shift angle of a dual-active bridge unit. Therefore, the capacitor voltage fluctuation of half-bridge units ina multi-level converter is transmitted to a low-voltage DC side through a dual-active bridge of a back stage, therefore, the low-frequency fluctuation in half-bridge unit capacitor voltage in the multi-level converter is suppressed, the cost and volume of a capacitor are reduced, and the compactness of a device is achieved. In addition, the fluctuating power transmitted to the low-voltage DC sideof each half-bridge unit achieves self balance, and the normal operation of the low-voltage DC side is not affected.

The invention discloses a direct-current solid-state transformer with the fault ride-through capability and a control method thereof. The direct-current solid-state transformer is composed of a filterinductor, a full-bridge sub module, N-1 half-bridge sub modules, and N DC-DC isolated converters. The bridge sub modules are in cascaded connection and then the cascaded connection unit is connectedin series with the filter inductor to connect an intermediate-voltage direct-current grid. The output terminal of each bridge sub module is connected with a direct-current input terminal of each DC-DCisolated converter; and direct-current output terminals of all DC-DC isolated converters are connected in parallel and then are connected with a low-voltage direct-current grid. When a short-circuitfault occurs at the intermediate-voltage direct-current grid, if the voltage of the intermediate-voltage direct-current grid with the fault is higher than a preset threshold, the duty ratio of all thebridge sub modules of the direct-current solid-state transformer are adjusted to realize a fault ride-through effect; and if the direct-current voltage is lower than or equal to the preset threshold,the half-bridge sub modules and the connected DC-DC isolated converters are turned off and only the full-bridge sub module and the connected DC-DC isolated converters work to realize the fault ride-through effect.

The invention discloses a direct current-direct current solid-state transformer, which belongs to the technical field of transformers and solves the technical problems that a power tapping device in an existing high-voltage direct current transmission system is large and heavy and cannot convert high, medium and low voltage levels. The direct current-direct current solid-state transformer comprises a first MMC (Modular Multilevel Converter) valve, a transformer unit and a second MMC valve, wherein one end of the first MMC valve is a direct current input end, and the first MMC valve is used for converting a direct current voltage input by the direct current input end into a first alternating current voltage and then outputting the first alternating current voltage; the input of the transformer unit is connected with the output of the first MMC valve, and the transformer unit is used for transforming the amplitude of the first alternating current voltage output by the first MMC valve to a second alternating current voltage and outputting the second alternating current voltage; and one end of the second MMC valve is connected with the transformer unit, the other end of the second MMC valve is a direct current output end, and the second MMC valve is used for rectifying the second alternating current voltage into a direct current voltage and then outputting the direct current voltage from the direct current output end.

The invention discloses a direct current power supply system for a data center. The direct current power supply system comprises a first alternating-current power grid (1), a solid-state transformer (3), a set of UPS system (9), a plurality of IT loads (7), a plurality of fan loads (8) and a direct current power supply system; normally, the power supply for the IT loads (7) and fan loads (8) is performed by the first alternating-current power grid (1) through high-tension direct current bus (4), and charged by a energy storage battery (12); in the event of power failure due to the faults of the first alternating-current power grid (1) or the high-tension direct current bus (4), uninterruptible power supply for the IT loads (7) and fan loads (8) is realized by the energy storage battery (12). Compared with the prior art, the direct current power supply system for the data center greatly reduces the volume of a transformer, the volume of a distribution wire and cost, is convenient for flexible access of renewable energy sources, and further improves the utilization rate of electrical energy; the system configuration is flexible, so it is beneficial to great reduction of the battery charge and discharge loss, and power supply system efficiency of the data center is improved.

The invention discloses a method for starting a solid-state transformer. The solid-state transformer allows grid-connecting to be conducted after high-voltage side capacitance rushes to a rated voltage, so that stable and impact-free starting of the solid-state transformer can be realized. The starting process includes dual active bridge operation and high-voltage side grid-connected operation; impact-free operation is realized in the two links; no harmful impact will be generated on equipment and power grids; the whole process will not generate any harmful impact on the system itself and the power grids; and the usage life of the equipment is prolonged.

The invention provides a solid-state transformer with medium-voltage direct-current and true bipolar low-voltage direct-current ports. The solid-state transformer comprises a modular multilevel converter and a plurality of isolated bidirectional DC-DC converters, the three-phase output of the modular multilevel converter and the positive and negative direct current buses form the stable medium-voltage alternating current and medium-voltage direct current ports respectively, and the input sides of the plurality of isolated bidirectional DC-DC converters are interconnected with the DC side of the modular multilevel converter submodule, wherein a plurality of isolated bidirectional DC-DC converters interconnected with bridge arm sub-modules on the modular multilevel converter are connected inparallel at the output side to form a positive low-voltage DC port, a plurality of isolated bidirectional DC-DC converters interconnected with a lower bridge arm sub-module of the modular multilevelconverter are connected in parallel at the output side to form a negative low-voltage DC port, a bridge arm of the modular multilevel converter adopts a fundamental frequency positive-sequence circulation injection control loop, and the low-voltage direct-current port is the true bipolar port.

The invention discloses a direct-current power distribution system of a fully-flexible structure. A voltage converter of the direct-current power distribution system is a solid-state transformer and comprises an alternating current / direct current solid-state transformer, a direct current / direct current solid-state transformer and a direct current / direct current / alternating-direct current solid-state transformer, an alternating current side of the alternating current / direct current solid-state transformer is connected with different alternating current power distribution lines or an alternating-current distributed-type power source while a direct current side of the same is connected with a direct current power distribution line, a low voltage side of the direct current / direct current solid-state transformer is connected with a direct-current distributed-type power source while a high voltage side of the same is connected with the direct current power distribution line, a high voltage direct current side of the direct current / direct current / alternating current solid-state transformer is connected with the direct current power distribution line while an alternating current side of the same is connected with an alternating current power consumption line to provide alternating current power, and a low voltage direct current side of the direct current / direct current / alternating current solid-state transformer is connected with a direct current power consumption line to provide direct current power. By the direct-current power distribution system, flexible conversion of alternating current and direct current for constructing the direct-current power distribution system is realized, convenience can be brought to connection-in of the distributed-type power source and stored energy in a direct-current manner, high quality alternating and direct current power is provided, and direct-current power distribution networks with various structures can be formed flexibly.

The invention discloses a DC solid state transformer and a control method thereof; the DC solid state transformer comprises a filter inductor, a plurality of half-bridge submodules, and a plurality of DC-DC isolated form converters; the plurality of half-bridge submodules are mutually cascaded and connected in series with the filter inductor in a middle voltage DC power grid; the output end of each the half-bridge submodule is connected with the DC input end of each DC-DC isolated form converter; all DC output ends of the DC-DC isolated form converters are connected in parallel, and connected with a low voltage DC power grid or a load. The DC solid state transformer can real time calculate the operation numbers of the half-bridge submodules and the DC-DC isolated form converters according to a middle voltage DC power grid voltage and the DC solid state transformer real operation power, and the resting half-bridge submodules and the DC-DC isolated form converters can remain in a hot standby state, thus realizing the whole machine high efficiency operations.

The invention provides a modular solid-state transformer based on a half-bridge cascade type three-level, wherein a sub-module unit and a DAB unit of the MMC are interconnected, Both the MMC submoduleand the DAB input use a half-bridge cascaded three-level structure. to reduce the number of modules required and the number of high frequency transformers. The AC side of DAB is connected in series with capacitors to isolate the DC component of the three-level output voltage, and the DAB output converter adopts a full-bridge structure, which forms a low-voltage DC bus bar through a plurality of DAB outputs in parallel, and a three-phase full-bridge inverter is connected to the low-voltage DC bus bar and forms a low-voltage AC port. The application of the topology of the invention can realizethe interconnection of the distribution network with multi-voltage levels and various ACDC forms, thereby laying the equipment foundation for the construction of the intelligent distribution network in the future.

The invention provides a solid-state transformer topology family applied to a direct-current and alternating-current hybrid power distribution network. The direct-current end of a sub-module of a modular multilevel converter and the input end of each isolated dual-active-bridge converter are interconnected to form a modular structure, the output ends of the multiple isolated dual-active-bridge converters are connected in parallel to form a low-voltage direct-current bus, and a three-phase full-bridge inverter is connected to the low-voltage direct-current bus to provide medium-voltage direct-current, medium-voltage alternating-current, low-voltage direct-current and low-voltage alternating-current ports, so the four kinds of ports are suitable for interconnection of multi-voltage-level polymorphic alternating-current and direct-current hybrid power distribution networks. The invention further provides a design method of the solid-state transformer topology family, so that the number ofsub-modules and the number of high-frequency transformers in the solid-state transformer device are reduced on the premise of obtaining the solid-state transformer device meeting specific scene requirements, and the size and the cost are reduced.

A DC solid-state transformer belongs to the field of DC power transmission and distribution, and the problems of relatively high loss and relatively high cost of an existing DC solid-state transformercaused by relatively large voltage stress of a switch device at a high-voltage side are solved. In the DC solid-state transformer, each power module comprises a front-stage DC conversion unit and anintermediate-frequency isolation conversion unit which are cascaded, wherein the intermediate-frequency isolation conversion unit comprises an inversion sub-unit which is implemented by a middle-pointclamping half-bridge three-level inverter, N front-stage DC conversion units are connected in series to form a front-stage DC conversion series unit, a positive DC input end of the series unit is connected with a positive electrode of an intermediate-voltage DC power grid by a filtering inductor L1, a negative DC input end of the series unit is connected with a negative electrode of the intermediate-voltage DC power grid by a filtering inductor L2, and a low-voltage DC power grid of a load is connected in series between a public end of a positive DC output end of the intermediate-frequency isolation conversion unit and a public end of a negative DC output end of the N intermediate-frequency isolation conversion unit.

The invention discloses a control method for a double PWM solid-state transformer. The control method is implemented based on a double PWM solid-state transformer structure. For rectifier control, a double closed-loop control mode (direct-current voltage at outer loop and grid-side current at inner loop) is adopted under a dq synchronous rotating coordinate system to realize sinusoidal control on the grid-side current and make the unit power factor be 1; for direct-current transformation control, a double closed-loop control mode (output voltage at outer loop and inductive current at inner loop) is adopted to realize voltage level transformation, power transmission and electrical isolation; and for inverter control, a double closed-loop control mode (output voltage at outer loop and inductive current at inner loop) is adopted under the dq synchronous rotating coordinate system. Reliable high-quality power can be provided for users, and the bad influence of nonlinear load on the power grid can be improved effectively. The control method has higher load adaptability and dynamic response.

The invention discloses a module power equalization control method for a cascade photovoltaic solid-state transformer and is to realize equalization control of active power between modules when a system operates under non-unit power factor and the number of modules contained in the three phases is not equal. The method comprises the following main steps: controlling average DC side capacitor voltage of all H-bridge converters and obtaining an active current instruction value; controlling grid current and compensating three-phase modulation voltage with the calculated zero-sequence voltage to realize module power equalization control; controlling the output voltage of a two-level full-bridge LLC converter; and controlling input bus voltage of a Boost converter to realize maximum power pointtracking control of a pre-stage photovoltaic array. Compared with existing technologies, the method can ensure that all modules transmit the same active power when the number of three-phase modules is not equal regardless of whether the system operates under unit power factor, thereby facilitating modular design.

The invention discloses a high-thermal-conductivity strong-insulation epoxy composite material for a solid-state transformer under a low doping amount and a preparation method thereof. The preparationmethod comprises the following steps: 1) preparing core-shell structure BN@SiO2 micron particles, and carrying out coupling agent surface modification and ball-milling treatment on nano BN; 2) preparing 100 parts of epoxy resin, 80 parts of a curing agent, 1 part of an accelerant and 20-80 parts of a micro-nano filler system, wherein nano BN accounts for 1-5wt% of the specific gravity of the filler system; (3) mixing the components in the step (2), and carrying out dispersion and defoaming treatment by virtue of an autoroatation and revolution stirrer; and 4) pouring the mixture obtained in the step 3) into a mold, and heating and curing the mixture in a drying box. Through a micro-nano co-doping technology, micron particles adopt a core-shell structure, the surfaces of the nano particlesare modified, and the epoxy-based composite material with excellent electrical and thermal properties can be obtained under the selected co-doping ratio; in addition, the preparation method is simplein step, low in filler doping amount, low in cost and suitable for industrial production.