341 results about "Dc transformers" patented technology

The invention relates to the technical field of direct-current transformers, in particular to a direct-current transformer for a power distribution network, which comprises an iron core, a coil and insulating oil, an insulating assembly is arranged between the coil and the iron core, and the insulating assembly comprises an insulating paperboard, an insulating cushion block, an insulating contour semi-ring and a semi-ring groove, the insulating paper board is of a structure with a round inner wall and an octagonal outer wall. Through the unique design of the insulating paperboards, the insulating partition strips and the insulating partition blocks, the longitudinal oil ducts and the transverse oil ducts are effectively reserved, the oil flow path of the transformer is optimized, the cooling efficiency is improved, and stable operation of the transformer is ensured; through the bisection splicing structure of the insulating cardboard and the overturning action of the insulating parting strip, the quick clamping assembly of the insulating cardboard and the iron core is realized, the time for adapting the winding on site is saved, the tightness and uniformity of the winding are improved, and the reliability and safety of the transformer are enhanced.

The invention discloses a mathematical model construction method for an external short-circuit fault of a T-type direct-current transformer. The mathematical model construction method comprises the following steps: establishing a topological structure of the T-type direct-current transformer; the transformation ratio and the output voltage of the transformer are kept constant through common point voltage control, direct current power control, alternating current power control and SPWM (Sinusoidal Pulse Width Modulation); establishing an external short-circuit fault mathematical model of the T-type direct-current transformer; simplifying the transformer into a linear time-varying circuit by adopting a submodule equivalent method; the short-circuit current is derived by using a loop current method from the angles of single-phase high-voltage bridge arm power supply independent action, single-phase multiplexing bridge arm power supply independent action, single-phase low-voltage bridge arm power supply independent action and low-voltage side power supply independent action, and the short-circuit current generated by each power supply is summed to obtain a complex frequency domain expression of the short-circuit current. The method can predict the short-circuit current impact value at the moment when the T-type direct-current transformer has the external short-circuit fault.

The invention belongs to a transformer structure, and provides a direct-current transformer, a power transmission system and a control method of the power transmission system in order to solve the technical problems that a three-level circuit topology in a power module in an existing direct-current transformer is large in number of power devices, complex in structure and large in layout and wiring difficulty. Each power module comprises a first-stage circuit, a second-stage circuit, two first direct-current bus capacitors and two second direct-current bus capacitors, the two first direct-current bus capacitors are connected in series to form a first capacitor circuit, and the two second direct-current bus capacitors are connected in series to form a second capacitor circuit. The first-stage circuit comprises two first half-bridge circuits which are connected in series, and the second-stage circuit is a dual-active bridge converter based on a series half-bridge. The number of power devices is remarkably reduced, meanwhile, the layout and wiring difficulty of the whole circuit is simplified, and popularization and application of the direct-current transformer are easier.

The invention discloses a design method and system for a frequency conversion range of a CLLC direct-current transformer based on a neural network, and the method comprises the steps: carrying out the sampling in a preset variable range, and constructing a sampling database covering all working conditions; performing simulation verification on the sampled data to generate a physical constraint data set containing forward and reverse gains; inputting the processed simulation data into a neural network model for training, and establishing a precise mapping relation between parameter variables and electrical performance; and finally, optimizing model output by integrating an optimization algorithm (such as gradient descent and genetic algorithm), and realizing global optimal solution of resonant cavity parameters under the rigid constraints that the gain matching degree error is less than 2.1% and the frequency fluctuates. According to the method, a simulation verification-model training-parameter optimization closed-loop system is innovatively constructed, through a sampling mechanism and a cross-platform data interface, the three-day optimization period needed by traditional manual trial and error is compressed, and the design is more convenient and reasonable.

The invention discloses a bidirectional hybrid output direct-current transformer and a fault current limiting design method thereof, and the medium-voltage side and the high-voltage side of the bidirectional hybrid output direct-current transformer are respectively composed of three thyristor valve groups and three sub-module bridge arms, and are connected through a medium-frequency transformer. The thyristor valve group is formed by reversely connecting two groups of series thyristors in parallel, and the medium-voltage side sub-module bridge arm and the high-voltage side sub-module bridge arm are respectively composed of NM half-bridge sub-modules and NH half-bridge sub-modules. According to the invention, the sub-module bridge arms with strong control capability are combined with the thyristors and the diode valve groups which are low in cost, low in loss and high in power density, bidirectional power transmission and reliable turn-off of the thyristors can be realized through coordination and cooperation of the middle-voltage side bridge arms and the high-voltage side bridge arms, smooth direct current is ensured, and a filter does not need to be installed. When a short-circuit fault occurs in the outgoing submarine cable, a fault current active suppression strategy is provided, and the scheme can effectively reduce the peak value of the fault current and eliminate the current-limiting inductance.

The invention belongs to the technical field of power distribution, and discloses a direct current transformer soft switch and a control method.The direct current transformer soft switch comprises a medium-voltage direct current bus and low-voltage direct current buses, the medium-voltage direct current bus is electrically connected with a modular multi-level multi-port direct current converter, and the modular multi-level multi-port direct current converter is electrically connected with a plurality of low-voltage direct current converters; the plurality of low-voltage direct-current converters are electrically connected with a low-voltage direct-current bus and are used for realizing bidirectional power transmission from medium-voltage direct current to low-voltage direct current; according to the method, the medium-voltage side inductance value is reduced, the medium-voltage side current ripple is enabled to pass zero, the M3DC operates in an approximate critical conduction mode, a constant valley current control strategy is adopted, the minimum value of the medium-voltage side inductance current is controlled to be constant under different working conditions, and then zero-voltage switching-on of a switching device in a wide operation range and zero-current switching-off of an anti-parallel diode are achieved.

The invention, which relates to the technical field of direct-current power transmission, discloses a co-offshore medium-high-frequency alternating-current offshore wind power full-direct-current collection system comprising: an offshore wind turbine module, which is used for obtaining offshore medium-voltage direct-current electric energy; the multi-port direct-current voltage transformation module is used for converting the offshore medium-voltage direct-current electric energy into offshore high-voltage direct-current electric energy; the direct-current submarine cable module is used for gathering the offshore high-voltage direct-current electric energy, acquiring the gathered offshore high-voltage direct-current electric energy and transmitting the gathered offshore high-voltage direct-current electric energy to the shore converter station module; the shore converter station module is used for acquiring shore high-voltage alternating-current electric energy according to the collected offshore high-voltage direct-current electric energy; the onshore alternating current module is used for receiving the onshore high-voltage alternating current electric energy and achieving full-direct-current collection of offshore wind power. By improving the medium voltage-high voltage direct current transformer topological structure and the control method, efficient and economical electric energy transmission is realized, the construction economy and the engineering application value of an open-sea wind power collection system are improved, and the average price and low price requirements of marketization development are met.

The invention discloses a trapezoidal wave operation method of a T-type direct-current transformer. The method comprises the following steps: step 1, designing waveform constraint conditions of a multi-phase T-type direct-current transformer; 2, designing an alternating current waveform of the three-phase T-type direct current transformer; and 3, designing the optimal alternating current waveform of the three-phase T-type direct current transformer. The problems of low voltage and current utilization rate and high current stress caused by the fact that the T-type direct-current transformer adopts a sine operation mode are solved, the power capacity of the T-type direct-current transformer is remarkably improved, and the T-type direct-current transformer topology can be applied to high-voltage and high-capacity occasions. On the premise that the topological structure and the number of devices of the T-type direct-current transformer are not changed, the transmission power capacity of the T-type direct-current transformer is remarkably improved, the power loss of the devices is reduced, and the T-type direct-current transformer has great significance in topological application of an existing T-type direct-current transformer.

The integrated emergency power supply module comprises an arc-shaped integrated emergency power supply module shell, and the integrated emergency power supply module shell is formed by fixedly connecting and combining at least two components; the integrated emergency power supply module shell comprises a bottom plate and a cover plate, the bottom plate is flat, an integrated emergency power supply module PCB is fixedly arranged on the bottom plate, an alternating-current and direct-current power transformation module is integrated on the integrated emergency power supply module PCB, a storage battery is detachably fixed in the integrated emergency power supply module shell, and the cover plate is arranged on the cover plate. The storage battery is electrically connected with the integrated emergency power supply module PCB; the integrated emergency power supply module also comprises a second male plug which is electrically connected with the integrated emergency power supply module PCB through a wire. And an integrated emergency power supply module clamping assembly is arranged on the outer side of the integrated emergency power supply module shell. The ceiling lamp has the beneficial effects that the ceiling lamp can be matched with the integrated emergency power supply module for use and can also be independently used, if the ceiling lamp needs to be matched with the integrated emergency power supply module, screws are not needed for fixation, and disassembly and assembly are very convenient.

The invention discloses a direct-current transformer control method, which relates to the technical field of power electronic control and comprises the following steps of: acquiring operating parameters of a direct-current transformer in real time; the operation parameters at least comprise input side voltage, output side voltage and load current; inputting the operation parameters into a model prediction controller; in the model prediction controller, on the basis of the operation parameters and a preset system model, calculating to obtain a switching device control signal which enables a control target in a future preset time domain to be optimal; a switching device control signal is applied to a switching device of the DC transformer to regulate its output voltage or power. The method has the advantages that the dynamic response speed is increased, the system robustness is enhanced, and multi-target optimization control is realized.

The invention relates to a T-connection direct current transformer control method, device and system, a computer readable storage medium and a computer program product. The T-connection direct-current transformer comprises n-phase direct-current conversion modules, and n is larger than or equal to 4. Each phase of DC conversion module comprises a bridge arm formed by connecting a plurality of energy conversion sub-module cascade structures and an electric reactor in series, and two groups of switches, and a common connection end of the two groups of switches connected in series is connected with a first end of the bridge arm; the external connection end of the DC conversion module is used for connecting the high-voltage end, the low-voltage end and the grounding end, and comprises a first end, a second end, a common connection end and a second end of the bridge arm after the two groups of switches are connected in series. The method comprises the following steps: receiving a commutation instruction; in the commutation period, the conduction state of the switch is switched, so that at least an i-phase direct current conversion module is conducted with the low-voltage end at the same moment; 2 < = i < = n-2. By adopting the method, the magnitude of the current which needs to be tolerated by each device can be reduced, so that the T-connection type direct-current transformer meets the requirement of high-capacity application.

The invention discloses an online calculation method and device for broadband impedance of a modular multi-level direct-current transformer, and relates to the field of direct-current transformers, and the method comprises the steps: obtaining the relation of all electric quantities during the steady-state operation of the modular multi-level direct-current transformer based on a circuit topological structure of the modular multi-level direct-current transformer; establishing a power circuit model based on the relation of each electrical quantity during steady-state operation; establishing a control circuit model based on the input and output of the controller group of the modularized multi-level direct current transformer; establishing a small signal impedance model based on the power circuit model and the control circuit model; and calculating the broadband impedance of the modularized multi-level direct-current transformer based on the small-signal impedance model. According to the invention, the broadband impedance of the DC transformer can be calculated on line with high precision and high efficiency in different application scenarios.

The invention discloses a DAB converter optimization method and device, equipment and a medium, and belongs to the technical field of isolated bidirectional direct-current converters, and the method comprises the following steps: firstly, analyzing the transmission power and current stress characteristics of a dual active bridge DAB converter under dual phase shift control; adding a penalty coefficient to describe the relationship between the target function and the constraint function; the target function represents current stress, and the constraint function represents transmission power; introducing a self-adaptive inertia weight and a constraint factor to improve a particle swarm algorithm; and optimizing and solving an optimization function formed by the target function and the constraint function based on an improved particle swarm optimization algorithm, and obtaining a current stress minimum value of the DAB converter, thereby obtaining an optimal internal and external phase shift angle combination corresponding to the DAB converter. The development capability and the exploration capability of the particle swarm algorithm are greatly improved, the current stress is optimized through the adaptive particle swarm algorithm, and the efficiency of the DAB converter can be effectively improved.

The invention provides a multi-bridge-arm integrated valve tower structure, a high-voltage direct-current transformer and a power system. The multi-bridge-arm integrated valve tower structure comprises a high-voltage bridge arm branch, a multiplexing bridge arm branch, a low-voltage bridge arm branch, a high-voltage port and a low-voltage port. One end of the high-voltage bridge arm branch, one end of the low-voltage bridge arm branch and one end of the multiplexing bridge arm branch are connected; the high-voltage bridge arm branch comprises a plurality of first half-bridge sub-modules which are connected in series; the multiplexing bridge arm branch comprises a plurality of second half-bridge sub-modules which are connected in series; the low-voltage bridge arm branch comprises a plurality of full-bridge sub-modules which are connected in series; the number of the first half-bridge sub-modules, the number of the second half-bridge sub-modules and the number of the full-bridge sub-modules are in direct proportion to the voltage withstand level of the valve tower structure. According to the structure, the high-voltage bridge arm branch, the multiplexing bridge arm branch and the low-voltage bridge arm branch are connected in a T-shaped mode, the number of the first half-bridge sub-modules, the number of the second half-bridge sub-modules and the number of the full-bridge sub-modules are determined according to the voltage withstand level needed by the valve tower, and the structure can be suitable for voltages of different levels.

The invention discloses an underwater new energy power supply system. The power supply system comprises a plurality of new energy power generation modules and a first direct-current transformer which are arranged on the water surface, and a second direct-current transformer and a plurality of charging modules which are arranged underwater, the first direct-current transformer comprises a plurality of first sub-modules, and the low-voltage side of each first sub-module is connected with the corresponding new energy power generation module; the first direct-current transformer comprises a plurality of first sub-modules, the high-voltage sides of the first sub-modules are connected in series, the second direct-current transformer comprises a plurality of second sub-modules, the high-voltage sides of the second sub-modules are connected with the high-voltage sides of the first direct-current transformer after being connected in series, and the low-voltage sides of the second sub-modules are connected in parallel; the input end of each charging module is connected with the low-voltage side of the corresponding second sub-module, and the output end of each charging module is used for being connected with the corresponding underwater electric equipment. According to the scheme, stable electric energy supply can be provided for the underwater electric equipment, and the power supply cost of the underwater electric equipment is reduced.

The invention relates to a direct-current transformer circuit topology based on a series three-phase bridge circuit and a control method thereof. A high-voltage side power unit comprises N three-phase series half-bridge circuits which are sequentially connected in series; the low-voltage side power unit comprises N three-phase parallel half-bridge circuits which correspond to the three-phase series half-bridge circuits and are sequentially connected in parallel, each three-phase series half-bridge circuit is connected with the corresponding three-phase parallel half-bridge circuit through a three-phase isolation transformer to form N groups of three-phase bridge modules, and meanwhile, the N groups of three-phase bridge modules are connected with the N groups of three-phase isolation transformers. A symmetrical duty ratio and phase shift hybrid control strategy is adopted to realize bidirectional flow of energy and soft switching in a full range, and intra-module and inter-module phase shift control is adopted to realize voltage sharing of a series side, reduce current ripples of a parallel side, reduce backflow power and improve the efficiency of the converter.

The invention relates to the technical field of direct-current transformer high-voltage side fault identification and ride-through, and discloses a direct-current transformer high-voltage side fault identification and ride-through method and system, and the method comprises the steps: obtaining a first parameter of a high-voltage side of a target direct-current boosting transformer; acquiring a first fault feature according to the first parameter; the first fault feature is used for identifying a direct-current boosting transformer high-voltage side fault type; and presetting a fault ride-through strategy, and if the first fault feature of any fault type is obtained, starting the fault ride-through strategy. According to the invention, reliable identification and fault ride-through of the DC fault of the collection system can be realized by using the local measurement information of the DC transformer, the method is not influenced by photovoltaic output, the accuracy and efficiency of fault identification are improved, a new idea and method are provided for fault diagnosis of the DC boosting transformer, and the fault diagnosis efficiency of the DC boosting transformer is improved through a control and protection cooperation mode. The accuracy and efficiency of fault recognition are improved, and a new thought and method are provided for fault diagnosis of the direct-current boosting transformer.

The invention provides a self-coupling direct current transformer and a control method thereof. Power transmission and energy transfer are achieved through the energy storage branches, multiplexing of the single energy storage branch is achieved through series connection of the first capacitor and the second capacitor, a primary side and a secondary side are formed, and the secondary side is formed through on-off of the first control switch and the second control switch and control over the voltage of the energy storage branches. And the current flow direction and the power transmission condition of each working mode are realized. In some embodiments, an energy storage bridge arm based on a half-bridge sub-module string is adopted as an energy transfer medium for self-coupling, so that the number of sub-modules is reduced, and thyristors which are low in cost and easy to connect in series are adopted as switching devices, so that the device cost is further reduced. Besides, the active characteristic of the energy storage bridge arm not only can actively construct a current waveform through a control signal to realize soft switching, but also can realize reliable turn-off of the thyristor, and is low in element loss and higher in control efficiency.

A renewable energy source off-grid hydrogen production system based on a high-capacity hydrogen production power source is characterized in that the hydrogen production system adopts a direct current collection-direct current transmission-direct current hydrogen production topological structure, and a wind power part and a photovoltaic part are arranged in a wind and light equipment area; the wind power part converts the output of a wind turbine generator into direct current through alternating current, then is connected with a direct current transformer in the direct current transformer area for boosting, and then is connected with an input end direct current bus in the direct current line area; an outgoing line of the combiner box is connected with a direct-current transformer in the direct-current transformer area for boosting and then connected with an input end direct-current bus in the direct-current line area. A novel DC off-grid hydrogen production topological structure is provided, the capacity of a hydrogen production power supply is improved on the basis of an off-grid condition, and a new scheme with advantages in the aspects of electric energy quality, stability, reliability, transmission efficiency and the like is provided; in the topology, a control protection and black-start strategy is provided. And a relatively complete system scheme is formed.

The invention provides a valve assembly equivalent test circuit and a valve assembly full-condition equivalent test method, and the circuit comprises a DC power supply unit, a first phase unit, a second phase unit and a common bridge arm which are connected in parallel. Each of the first phase unit and the second phase unit comprises two bridge arms which are connected in series; a common bridge arm is electrically connected between a connection point of the two serially connected bridge arms in the first phase unit and a connection point of the two serially connected bridge arms in the second phase unit; the common bridge arm is connected in series with the modular multi-level valve assembly to be tested; the direct-current power supply unit is used for charging the two bridge arms in the first phase unit, the two bridge arms in the second phase unit and the common bridge arm respectively; the two bridge arms in the first phase unit, the two bridge arms in the second phase unit and the common bridge arm are all used for generating test current used for testing the modularized multi-level valve assembly. As the bridge arms in the first phase unit and the second phase unit and the common bridge arm are independently adjusted, the test requirements of the direct-current transformer under all working conditions are met.

The invention provides a high-voltage high-capacity self-coupling direct-current transformer. The high-voltage high-capacity self-coupling direct-current transformer comprises a support capacitor, an energy buffer bridge arm and a converter valve assembly, the support capacitor is used for forming a part of power transmission loop; the energy buffer bridge arm is used for realizing main power transmission and forming the other part of power transmission loop; converting of the transformer is achieved by controlling the input number of the bridge arm sub-modules of the energy buffer bridge arm and trigger signals of the converter valve assembly. Meanwhile, the invention provides a commutation working method and a control method of the transformer. Energy transfer from a low-voltage side to a high-voltage side is realized, the efficiency of the transformer is improved, and energy balance of the energy buffer bridge arm is realized; reliable turn-off and approximate zero-voltage conduction of the converter valve are realized by controlling the input number of the energy buffer bridge arm sub-modules, and two closed-loop modes of direct-current voltage and power control are provided at the same time. High-voltage and high-capacity direct-current transformation is realized, and meanwhile, the semi-control device converter valve is used, so that the device cost is reduced.

The invention discloses a land wind power transmission system, which is used for solving the problems of alternating current collection and transmission such as harmonic resonance and reactive power transmission in the prior art. The land wind power transmission system comprises a wind power sending end unit and a wind power receiving end unit. The wind power sending end unit comprises a direct current transformer and more than two wind turbine generators; a generator-side converter is arranged in each wind generator set and corresponds to one low-voltage circuit breaker; two poles of the direct-current transformer are respectively provided with a direct-current circuit breaker; for each wind power generation set, electric energy generated by the wind power generation set is rectified into first direct current through a machine-side converter, and the first direct current is connected into a direct-current transformer in parallel through a low-voltage circuit breaker to be boosted into second direct current which is sent into a wind power receiving end unit through a bipolar direct-current circuit breaker of the direct-current transformer; the wind power receiving end unit comprises a receiving end converter station, a connection transformer and a starting loop; and the second direct current is subjected to secondary boosting through the connection transformer after being subjected to current conversion through the receiving-end converter station, and is accessed to the alternating-current power grid through the starting loop.

The invention relates to the technical field of flexible direct-current power transmission, and provides an offshore wind power direct-current sending-out system and a starting control method thereof.The system is characterized in that part of wind turbine generators are gathered through an offshore alternating-current gathering bus, then connected to an offshore booster station, connected to a DRU converter valve of a diode uncontrolled rectifying unit and then connected to an offshore direct-current bus through a first switch; an onshore converter valve of the onshore converter station is connected through a direct-current line, and is finally connected to an onshore alternating-current power grid; part of the wind turbine generators are gathered through the offshore direct current gathering bus, then connected to the direct current transformer, then connected to the offshore direct current bus, connected to the land converter valve of the land converter station through a direct current line and finally connected to the land alternating current power grid. According to the offshore wind power direct-current sending-out system, the DRU converter valve and the direct-current transformer are adopted, the DRU does not need to adopt a series / parallel MMC structure, the problem of black start of the DRU converter valve is solved through the power bidirectional flow characteristic of the direct-current transformer, and the direct-current transformer can conduct power transmission.

The invention discloses an overvoltage suppression method of a resonant direct-current transformer and the resonant direct-current transformer, and relates to the technical field of power electronics. The overvoltage suppression method of the resonant direct-current transformer comprises the following steps: detecting a direct-current output voltage provided by a direct-current output port, and comparing the direct-current output voltage with an output voltage target value of the resonant direct-current transformer; and under the condition that the direct current output voltage is greater than the output voltage target value, dynamically adjusting the internal phase shift delay between the control signals of the first half bridge and the second half bridge so as to adjust the direct current output voltage to be less than or equal to the output voltage target value. According to the embodiment of the invention, the suppression cost of the light-load overvoltage of the resonant direct-current transformer can be reduced, and the reliability of the resonant direct-current transformer is improved.

The invention discloses a thermal power energy storage segmented boosting and segmented conversion method and system, and relates to the technical field of segmented boosting and segmented conversion, and the method comprises the steps: dividing a thermal power energy storage system into two segments, connecting the two segments to a DC bus, and achieving the boosting through a DC transformer; converging the boosted energy storage section to a direct current convergence bus of a boosted voltage level; the energy storage power is adjusted through the high-voltage segmented inversion device, and the frequency of the thermal power generating unit is adjusted; an improved droop control scheme is implemented to control power, and on-off control of the inversion device is realized through electronic components. According to the invention, an improved droop control scheme is adopted in a control system of the high-voltage segmented inversion device, so that the power can be accurately adjusted, and meanwhile, transient fluctuation caused by the voltage of the direct-current bus is avoided. According to the high-voltage segmented inversion device, the loop on-off function of the high-voltage segmented inversion device can be achieved only by controlling on / off of power electronic components in the high-voltage segmented inversion device, a traditional high-voltage large-current circuit breaker is replaced, and the equipment cost is not increased.

A modular multi-level direct-current transformer topology applicable to a multi-voltage-class direct-current power grid, and a control method therefor. The topology comprises a bridge arm inductor, an upper bridge arm, a lower bridge arm and a filter inductor, wherein one end of the bridge arm inductor is connected to a positive electrode of a high-voltage side, and the other end thereof is connected to one end of the upper bridge arm; the other end of the upper bridge arm is connected to one end of the lower bridge arm and one end of the filter inductor; the other end of the lower bridge arm is separately connected to a negative electrode of the high-voltage side and a negative electrode of a low-voltage side; the other end of the filter inductor is connected to a positive electrode of the low-voltage side; and the upper bridge arm and the lower bridge arm each comprise a sub-module string, the sub-module string comprising several sub-modules, which are sequentially connected in series, and the upper bridge arm and the lower bridge arm regulate respective bridge arm voltages by means of controlling the number of inserted sub-modules, so as to realize direct-current voltage conversion between different voltage classes. In the present application, the use of upper and lower sets of chained sub-modules reduces the use of complex assemblies, thereby reducing the cost and the control difficulty, and also improving the safety, reliability and efficiency of a system.

The invention relates to a charging system and a control method thereof. The charging system comprises a DC source; the direct-current bus is used for transmitting direct current obtained by converting electric energy of the alternating-current power grid or transmitting direct current output by the direct-current source; the charging host is used for outputting charging current to external equipment; the direct-current transformer is used for transforming the direct current transmitted by the direct-current bus and / or the direct current output by the direct-current source; the switching device has a first switching state and a second switching state; in the first switching state, the direct current source is electrically connected with the direct current bus through the switching device so as to realize electric energy transmission between the direct current bus and the direct current source; and in the second switching state, the direct current source is electrically connected with the charging host through the switching device so as to realize electric energy transmission between the direct current source and the charging host. According to the charging system and the control method thereof, electric energy transmission between the direct-current bus and the direct-current source or between the direct-current source and the charging host can be realized through the switching device, so that the energy conversion efficiency is improved.

The invention provides a modularized multi-level direct-current transformer topology suitable for a multi-voltage-level direct-current power grid and a control method. The topology comprises a bridge arm inductor, an upper bridge arm, a lower bridge arm and a filter inductor. One end of the bridge arm inductor is connected with the high-voltage side anode, and the other end is connected with one end of the upper bridge arm; the other end of the upper bridge arm is connected with one end of the lower bridge arm and one end of the filter inductor; the other end of the lower bridge arm is respectively connected with the high-voltage side cathode and the low-voltage side cathode; the other end of the filter inductor is connected with the low-voltage side anode; each of the upper bridge arm and the lower bridge arm comprises a sub-module string, each sub-module string comprises a plurality of sub-modules which are sequentially connected in series, and the voltage of each of the upper bridge arm and the lower bridge arm is adjusted by controlling the input number of the sub-modules, so that direct-current voltage conversion among different voltage levels is realized. Through the application of the upper and lower chain type sub-modules, the use of complex components is reduced, the cost and the control difficulty are reduced, and meanwhile, the safety, reliability and efficiency of the system are improved.

The invention relates to a fault ride-through method of a direct current transformer and a fault ride-through system of the direct current transformer. Working modes of low-voltage bridge arm direct-current voltage component adjustment, low-voltage port direct-current voltage control and high-voltage bridge arm alternating-current voltage control are switched, and direct current of a low-voltage port is adjusted through a first fault ride-through mode, a constant direct-current power mode and a bridge arm energy balance control mode; during the fault ride-through period, the low-voltage port is controlled to output negative voltage, current opposite to fault current is generated, the direct-current transformer quickly blocks the fault current, voltage balance of the bridge arm sub-module is maintained during the fault period, and stable fault ride-through is achieved.

The invention discloses an onshore wind power transmission system which is used for solving the problems of alternating current collection and transmission such as harmonic resonance and reactive power transmission in the prior art. The land wind power transmission system adopts a bipolar topological structure, and each pole comprises a wind power sending end unit and a wind power receiving end unit; the wind power sending end unit comprises a direct current transformer and more than two wind turbine generators; a generator-side converter is arranged in each wind generator set and corresponds to one low-voltage circuit breaker; the direct-current transformer is provided with a sending-end direct-current circuit breaker; for each wind power set, electric energy generated by the wind power set is rectified into first direct current through a machine-side converter, and the first direct current is connected into a direct-current transformer in parallel through a low-voltage circuit breaker to be boosted into second direct current which is sent into a wind power receiving end unit through a sending end direct-current circuit breaker of the direct-current transformer; the wind power receiving end unit comprises a receiving end converter station, a connection transformer and a starting loop; the second direct current is converted by the receiving end converter station and then is connected to the alternating current power grid through the connection transformer and the starting loop.