CN115776225A - Redundant fault-tolerant control method for power electronic transformer - Google Patents
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Abstract
A power electronic transformer redundancy fault-tolerant control method comprises the following steps: calculating the number of fault power modules allowed by redundancy; calculating N paths of carrier phase shift pulse width modulation signals and distributing the N paths of carrier phase shift pulse width modulation signals to N power modules in input states; monitoring the working state of each power module in real time, and identifying a fault power module; judging whether a cascade power circuit of the power electronic transformer meets a redundant fault-tolerant operation condition or not; after the blocking fault power module is calculated, the low-voltage level voltage when the intermediate frequency transformer with the middle tap does not switch taps is calculated, the low-voltage level voltage is compared with the rated voltage of the low-voltage level, the voltage change proportion of the low-voltage level is calculated, the voltage change proportion of the low-voltage level is compared with the corresponding proportion of each middle tap of the intermediate frequency transformer with the middle tap, and the middle tap with the minimum difference is selected as a switching target. The invention has the advantages of low cost, simple control and high reliability.
Description
Technical Field
The invention relates to the technical field of power electronic transformers, in particular to a redundant fault-tolerant control method for a power electronic transformer.
Background
In the field of power systems, conventional transformers convert electrical energy delivered at high voltage to low voltage for consumer use. However, the industrial frequency transformer has large volume and weight, low efficiency and low power density, increases operation energy consumption and maintenance cost, and simultaneously has uncontrollable power factor and output voltage, thus easily bringing harmonic pollution and reactive pollution to the power grid. In order to solve various problems caused by industrial frequency transformers, power electronic transformers are produced. The power electronic transformer has the functions of bidirectional power flow control, electric energy quality control, self protection and self diagnosis of the device, communication, information exchange and the like besides the functions of voltage class conversion and electrical isolation of the traditional transformer, has alternating current and direct current ports with different voltage classes, and is suitable for flexible access of various distributed energy sources, energy storage and loads and interconnection of alternating current/direct current power grids. The power electronic transformer plays an irreplaceable important role in an intelligent power grid, an energy internet and a future alternating current-direct current interconnected power grid, and is a core device for realizing electric energy conversion and processing.
The power electronic transformer converts high-voltage alternating current into multi-unit direct current through an AC/DC (alternating current/direct current) converter, then realizes electrical isolation and energy transfer through a multi-unit medium/high frequency isolation type DC/DC (direct current/direct current) converter, obtains direct current at the parallel output side of the power electronic transformer, and then generates low-voltage alternating current through inversion of the DC/AC (direct current/alternating current) converter. The reliability of the power electronic transformer equipment itself is particularly important for safe and reliable operation of the distribution network. Because the power electronic transformer is a multi-unit cascade topology structure, when any one unit fails, the stable operation of the whole power electronic transformer is influenced. When the power electronic transformer is designed, the selected power electronic device can set a certain voltage and current margin, generally 50% of low-voltage elements and 100% of high-voltage elements, so that the voltage and current margin can be regarded as a redundant part of a power unit in the power electronic transformer, and the reliability of the power electronic transformer can be effectively improved by utilizing the redundant part and a corresponding fault-tolerant technology.
At present, two redundancy schemes are provided for a multi-unit structure, one scheme is that one or more units are used as standby redundancy units, the redundancy units do not participate in working when a power electronic transformer normally operates, when one unit fails, the redundancy units replace failure units to work, the failure units quit operating, and transmission power of each unit is not changed. The other scheme is that all units are put into normal operation, a fault unit is cut off when a fault occurs, and the power and the voltage of the fault unit are shared by the rest operation units, namely, the redundant voltage and current margins of the power electronic element are utilized. The redundant unit in the first scheme not only increases the hardware cost of the system, but also increases the complexity of the control circuit and causes impact on the system when the failed unit is removed and the standby unit is put into use. Compared with the first scheme, the second scheme is simpler in structure, has no additional hardware cost and does not cause impact on the system due to the input of a fault unit; however, due to the change of the number of the operating units, when the power and the voltage are redistributed, the control parameters of the DC/AC (direct current/alternating current) converter are changed to make the DC/AC converter work under the working conditions of overhigh voltage and overhigh power, and very high requirements are imposed on the voltage and current stress of power electronic devices and a control system of the DC/AC converter.
Disclosure of Invention
The invention aims to overcome the defects and problems of high hardware cost, complex control and low system reliability in the prior art, and provides a power electronic transformer redundancy fault-tolerant control method which is low in hardware cost, simple to control and high in system reliability.
In order to achieve the above purpose, the technical solution of the invention is as follows: a redundant fault-tolerant control method of a power electronic transformer comprises N power modules, wherein the input ends of the N power modules are connected in series, the output ends of the N power modules are connected in parallel, each power module comprises a high-voltage side H-bridge converter, an isolation type DC/DC converter and a low-voltage side H-bridge converter, the input ends of the N high-voltage side H-bridge converters are connected in series, the output ends of the N low-voltage side H-bridge converters are connected in parallel, the input ends of the N low-voltage side H-bridge converters are connected in non-parallel, the isolation type DC/DC converter comprises an intermediate frequency transformer with a middle tap, the input end of the isolation type DC/DC converter is connected with the output end of the high-voltage side H-bridge converter, and the output end of the isolation type DC/DC converter is connected with the input end of the low-voltage side H-bridge converter; the control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
s2, calculating N paths of carrier phase-shift pulse width modulation signals, and distributing the N paths of carrier phase-shift pulse width modulation signals to N power modules in an input state;
s3, monitoring the working state of each power module in real time, and identifying a fault power module;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundancy fault-tolerant operation condition or not; if yes, jumping to the step S5; if the current does not meet the requirement, a tripping command is sent out, and the power electronic transformer cascade power circuit is stopped and quit operation;
s5, issuing a bypass instruction to the fault power module to lock the fault power module, calculating the low-voltage level voltage when the intermediate frequency transformer with the middle tap does not switch taps after the fault power module is locked, comparing the low-voltage level voltage with the rated voltage of the low-voltage level, calculating the voltage change proportion of the low-voltage level, comparing the voltage change proportion of the low-voltage level with the corresponding proportion of each middle tap of the intermediate frequency transformer with the middle tap, selecting the middle tap with the minimum difference as a switching target, and requiring the isolation type DC/DC converter to select the middle taps of the intermediate frequency transformer with the middle tap corresponding to the number of the fault power modules.
In step S1, the method for calculating the number of redundant allowable fault power modules includes:
calculating basic margin of each device of the power electronic transformer, selecting the minimum basic margin as a reference margin, and calculating the number of fault power modules allowed by redundancy by adopting the following formula
；
Wherein,
in order to be able to count the number of power modules,
the minimum basic margin in the basic margins of all the devices of the power electronic transformer is obtained.
In step S4, the condition that the cascade power circuit of the power electronic transformer meets the redundancy fault-tolerant operation condition means that: the number of the fault power modules is less than or equal to the number of the fault power modules allowed by redundancy.
In step S5, a bypass switch is connected to the AC terminal of the high-voltage side H-bridge converter
The low-voltage side H-bridge converter and the breaking switch
After being connected in series, the output end of the power electronic transformer is connected;
the step of issuing the bypass instruction to the fault power module is as follows: closed bypass switch
Breaking of the disconnecting switch
。
In step S5, the low-voltage level voltage change ratio
The calculation formula of (c) is:
wherein,
in order to be able to count the number of power modules,
the number of failed power modules.
The isolation type DC/DC converter comprises a high-voltage stage chopping converter and a low-voltage stage inversion converter, and the high-voltage stage chopping converter is connected with the low-voltage stage inversion converter through an intermediate frequency transformer with a middle tap.
Compared with the prior art, the invention has the beneficial effects that:
according to the redundancy fault-tolerant control method for the power electronic transformer, firstly, only the fault power module is cut off, the redundancy module and the investment thereof are not involved, and the hardware cost is saved; secondly, after the power module breaks down, the bypass of the power module is closed without changing a phase shifting angle, so that a control algorithm is simplified; finally, after the fault power module is cut off, the low-voltage direct-current bus voltage of each isolation type DC/DC converter is increased due to the sharing of the direct-current bus voltage of the fault power module, the original direct-current bus voltage can be maintained unchanged by changing the transformer transformation ratio through the middle tap, the power electronic devices of the isolation type DC/DC converter and the low-voltage side H-bridge converter are prevented from working under high voltage stress, and the system reliability is improved. Therefore, the invention has low cost, simple control and high reliability.
Drawings
Fig. 1 is a schematic circuit diagram of a single-phase power electronic transformer according to the present invention.
Fig. 2 is a flowchart of a redundant fault-tolerant control method for a power electronic transformer according to the present invention.
In the figure: power module 1, high-voltage side H-bridge converter 101, isolated DC/DC converter 102, low-voltage side H-bridge converter 103, intermediate frequency transformer with intermediate tap 104 and bypass switch
105. Breaking switch
106. A high-voltage stage chopping converter 107 and a low-voltage stage inversion converter 108.
Detailed Description
The invention is described in further detail below with reference to the figures and the detailed description of the invention.
Referring to fig. 1, the power electronic transformer includes N power modules 1, input terminals of the N power modules 1 are connected in series, output terminals of the N power modules 1 are connected in parallel, the power modules 1 include a high-voltage side H-bridge converter 101, an isolation type DC/DC converter 102 and a low-voltage side H-bridge converter 103, input terminals of the N high-voltage side H-bridge converters 101 are connected in series, output terminals of the N low-voltage side H-bridge converters 103 are connected in parallel, and input terminals of the N low-voltage side H-bridge converters 103 are connected in parallel, the isolation type DC/DC converter 102 includes an intermediate frequency transformer 104 with a tap, an input terminal of the isolation type DC/DC converter 102 is connected with an output terminal of the high-voltage side H-bridge converter 101, and an output terminal of the isolation type DC/DC converter 102 is connected with an input terminal of the low-voltage side H-bridge converter 103; a bypass switch is connected to an ac terminal of the high-voltage side H-bridge converter 101
105, the low-voltage side H-bridge converter 103 and a breaking switch
106 are connected with the output end of the power electronic transformer after being connected in series; the isolated DC/DC converter 102 comprises a high-voltage stage chopping converter 107 and a low-voltage stage inversion converter 108, wherein the high-voltage stage chopping converter 107 is connected with the low-voltage stage inversion converter 108 through an intermediate frequency transformer 104 with a middle tap.
Referring to fig. 2, a method for controlling redundancy fault tolerance of a power electronic transformer includes the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
the method for calculating the number of the fault power modules allowed by redundancy comprises the following steps:
calculating the basic margin of each device of the power electronic transformer, selecting the minimum basic margin as the reference margin, and calculating the number of fault power modules allowed by redundancy by adopting the following formula
；
Wherein,
in order to increase the number of the power modules 1,
the minimum basic margin in the basic margins of all devices of the power electronic transformer is obtained;
s2, calculating N paths of carrier phase-shift pulse width modulation signals, and distributing the N paths of carrier phase-shift pulse width modulation signals to N power modules 1 in an input state;
s3, monitoring the working state of each power module 1 in real time, and identifying a fault power module;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundancy fault-tolerant operation condition, namely whether the number of fault power modules is less than or equal to the number of fault power modules allowed by redundancy; if yes, jumping to the step S5; if the current does not meet the requirement, a tripping command is sent out, and the power electronic transformer cascade power circuit is stopped and quits the operation;
s5, issuing a bypass instruction to the fault power module, namely closing a bypass switch
105, opening the breaking switch
106, after the fault power module is locked, calculating a low-voltage level voltage when the intermediate frequency transformer 104 with the middle tap does not switch taps after the fault power module is locked, then comparing the low-voltage level voltage with a low-voltage level rated voltage, calculating a low-voltage level voltage change proportion, comparing the low-voltage level voltage change proportion with a corresponding proportion of each middle tap of the intermediate frequency transformer 104 with the middle tap, selecting the middle tap with the minimum difference as a switching target, and then requiring the isolation type DC/DC converter 102 to select the middle taps of the intermediate frequency transformer 104 with the middle tap with the number corresponding to the fault power module;
the low-voltage stage voltage change ratio
The calculation formula of (2) is as follows:
wherein,
in order to number the power modules 1,
the number of failed power modules.
Example 1:
in the embodiment, a 10 kv power electronic transformer prototype of 500 kv-volt applied to the field of the limited company of the baohu wu steel group in china is taken as an example for explanation, a single-phase circuit of the power electronic transformer is formed by cascading 6 power modules, the single-phase circuit can independently operate, and the input phase voltage is 5774VAC; the rated voltage of a high-voltage direct-current bus of the high-voltage side H-bridge converter is 1500V, an IGBT with the rated voltage of 3300V is adopted, the rated voltage of a low-voltage direct-current bus of the isolated DC/DC converter is 400V, and the IGBT with the rated voltage of 650V is adopted in a low-voltage part of the converter and the low-voltage side H-bridge converter; it can be seen that the withstand voltage design margins of the high-side and low-side power electronic switching devices are 120% and 62.5%, respectively, and thus the reference margin can be set to 62.5%.
The transformation ratio of a normal tap of the intermediate frequency transformer with the intermediate tap is 3.75, the tap of the intermediate frequency transformer with the intermediate tap is arranged one by 1%, namely 41 taps are arranged from 0% to-40%, and the number of power modules required by the power electronic transformer to be connected in series is as follows
。
According to the above formula, when
Low voltage stage (secondary side) voltage when not switching tap
The voltage variation amplitude is 100% or more
The maximum number of fault power modules that the system can bear
。
A cascade H-bridge circuit on the high-voltage alternating current side of the power electronic transformer adopts a carrier phase-shifting type pulse width modulation method, and appropriate middle taps of corresponding intermediate frequency transformers with middle taps are respectively arranged according to 1-path, 2-path and m-path faults; once the power modules of the 1-path, the 2-path and the m-path are in fault, a bypass locking instruction is executed to bypass the fault power module, and meanwhile, a middle tap with a middle tap corresponding to the fault on the intermediate frequency transformer is input, so that the voltage of a low-voltage direct current bus of the medium/high frequency isolation type DC/DC converter is basically maintained unchanged.
In the invention, as long as the fault-tolerant operation condition is met, namely the number of the fault power modules is less than or equal to the number of the redundant fault-tolerant power modules
The fault power module can be bypassed and a middle tap corresponding to the fault on the intermediate frequency transformer with the middle tap can be input, so that the power electronic transformer can run smoothly in fault tolerance; the specific control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy
；
S2, calculating 6 paths of carrier phase-shift pulse width modulation signals, and distributing the 6 paths of carrier phase-shift pulse width modulation signals to 6 power modules in an input state;
s3, monitoring the working state of each power module in real time, and identifying that 1 power module has a fault;
s4, judging whether the cascaded power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition or not, namely the number of fault power modules
Whether or not less than or equal to the redundancy-allowed number of failed power modules
(ii) a If the redundant fault-tolerant operation condition is met, jumping to the step S5;
s5, issuing a bypass instruction to the fault power module to lock the fault power module, wherein at the moment, the system is changed from 6-path cascade operation to 5-path cascade operation, the input current ripple of the system is increased to some extent, but the basic operation is not influenced, and the total direct current bus voltage is kept unchanged (A)
) When the DC bus voltage of the high-voltage side H-bridge converter of the fault line is balanced, the bus voltage of each high-voltage side H-bridge converter is increased to the voltage of the DC bus of the remaining 5 high-voltage side H-bridge converters
The voltage-withstanding range of the IGBT is still lower than the rated 3300V, and fault-tolerant operation can be realized; and after the locking fault power module is calculated, the low-voltage level voltage of the intermediate frequency transformer with the middle tap is not switched, then the low-voltage level voltage is compared with the rated voltage of the low-voltage level, the change proportion of the low-voltage level voltage is calculated, the change proportion of the low-voltage level voltage is compared with the corresponding proportion of each middle tap of the intermediate frequency transformer with the middle tap, the middle tap with the minimum difference is selected as a switching target, and then the isolation type DC/DC converter is required to select the middle taps of the intermediate frequency transformer with the middle tap corresponding to the number of the fault power modules.
Thus, after comparing all taps from 0% to-40%, it is determined that the-17% tap is closest
Then, a-17% tap switching command with a center tap intermediate frequency transformer is sent to switch to the-17% tap.
At this time, because the voltage of the high-voltage bus rises to 1800V, the voltage of the low-voltage direct-current bus of the isolated DC/DC converter is equal to that before the-17% tap is switched
And the rated working voltage of the H-bridge converter exceeds 400V of the low-voltage side by 20%, so that the voltage stress of corresponding devices is increased, and the controller of the H-bridge converter is greatly influenced. After the-17% tap is switched, the voltage of the low-voltage direct current bus of the isolated DC/DC converter is
Comparison of 400V onlyThe voltage of the power electronic devices of the isolated DC/DC converter and the low-voltage side H-bridge converter is kept unchanged basically, and the voltage of the power electronic devices of the isolated DC/DC converter and the low-voltage side H-bridge converter is reduced by 1.6V, namely 0.4%.
Example 2:
if the 2-path power module which is serious simultaneously fails, the specific control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy
；
S2, 6 paths of carrier phase-shifting pulse width modulation signals are calculated, and the 6 paths of carrier phase-shifting pulse width modulation signals are distributed to 6 power modules in an input state;
s3, monitoring the working state of each power module in real time, and identifying that 2 power modules have faults;
s4, judging whether the cascaded power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition or not, namely the number of fault power modules
Whether or not less than or equal to the redundancy-allowed number of failed power modules
(ii) a If the redundancy fault-tolerant operation condition is met, jumping to the step S5;
s5, issuing a bypass instruction to the fault power module to lock the fault power module, wherein at the moment, the system is changed from 6-path cascade operation to 4-path cascade operation, the input current ripple of the system is increased, but the basic operation is not influenced, and the total direct current bus voltage is kept unchanged (A)
) And the voltage drop of the bus of each high-voltage side H-bridge converter is increased to the voltage drop of the bus of the remaining 4 high-voltage side H-bridge converters due to the balance of the direct-current bus voltage of the high-voltage side H-bridge converter of the fault circuit
Within the range of the IGBT voltage withstanding still lower than the rated 3300VFault-tolerant operation; and then calculating the low-voltage level voltage when the intermediate frequency transformer with the intermediate taps does not switch taps after the fault power module is locked, then comparing the low-voltage level voltage with the rated low-voltage level voltage, calculating the voltage change proportion of the low-voltage level, comparing the voltage change proportion of the low-voltage level with the corresponding proportion of each intermediate tap of the intermediate frequency transformer with the intermediate taps, selecting the intermediate tap with the minimum difference as a switching target, and then requiring the isolated DC/DC converter to select the intermediate taps of the intermediate frequency transformer with the intermediate taps corresponding to the number of the fault power modules.
Thus, after comparing all taps from 0% to-40%, it is determined that the-33% tap is closest
And sending a tap switching command of-33% of the intermediate frequency transformer with the intermediate tap, and switching to the-33% tap.
At the moment, because the voltage of the high-voltage bus rises to 2250V, the voltage of the low-voltage direct-current bus of the isolated DC/DC converter is equal to that before the tap is switched to-33 percent
The rated working voltage of the H-bridge converter exceeding the low-voltage side reaches 400V to 50%, the voltage is close to the 650V rated working voltage limit of the IGBT, the voltage stress of corresponding devices is increased, and the controller of the H-bridge converter is greatly influenced. After the-33% tap is switched, the voltage of a low-voltage direct-current bus of the isolated DC/DC converter is
Compared with the situation that the voltage of 400V exceeds 2V, namely 0.5%, the method can effectively ensure the smoothness of fault switching, and can basically maintain the voltage stress of the power electronic devices of the isolated DC/DC converter and the low-voltage side H-bridge converter unchanged.
The above method can be generalized to be applied to a three-phase power electronic transformer, and the taps may not be limited to 0% to-40%, and one tap is provided for every 1%.
After the system has a power module fault, the fault power module can be bypassed as long as the fault-tolerant operation condition is met, namely the number of the fault power modules is less than or equal to the voltage and current margins of each device, an appropriate middle tap of the middle-tap intermediate frequency transformer of the medium/high frequency isolation type DC/DC converter is selected according to the specific fault condition, the transformation ratio of the intermediate frequency transformer is changed, the voltage change of a direct current bus of the low-voltage side H-bridge converter is greatly reduced, the power electronic transformer is switched in a smooth and redundant mode, and the risk of voltage stress rise of the power electronic devices of the low-voltage side H-bridge converter is reduced.
Claims (6)
1. A redundant fault-tolerant control method of a power electronic transformer is characterized by comprising the following steps:
the power electronic transformer comprises N power modules (1), wherein the input ends of the N power modules (1) are connected in series, the output ends of the N power modules are connected in parallel, each power module (1) comprises a high-voltage side H-bridge converter (101), an isolation type DC/DC converter (102) and a low-voltage side H-bridge converter (103), the input ends of the N high-voltage side H-bridge converters (101) are connected in series, the output ends of the N low-voltage side H-bridge converters (103) are not connected with each other, the output ends of the N low-voltage side H-bridge converters (103) are connected in parallel, the input ends of the N low-voltage side H-bridge converters (103) are not connected with each other, each isolation type DC/DC converter (102) comprises an intermediate frequency transformer (104) with a middle tap, the input end of the isolation type DC/DC converter (102) is connected with the output end of the high-voltage side H-bridge converter (101), and the output end of the isolation type DC/DC converter (103) is connected with the input end of the low-voltage side H-bridge converter (103);
the control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
s2, calculating N paths of carrier phase-shift pulse width modulation signals, and distributing the N paths of carrier phase-shift pulse width modulation signals to N power modules (1) in an input state;
s3, monitoring the working state of each power module (1) in real time, and identifying a fault power module;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundancy fault-tolerant operation condition or not; if yes, jumping to the step S5; if the current does not meet the requirement, a tripping command is sent out, and the power electronic transformer cascade power circuit is stopped and quits the operation;
s5, issuing a bypass instruction to the fault power module to lock the fault power module, calculating a low-voltage level voltage when the intermediate frequency transformer (104) with the middle tap does not switch taps after the fault power module is locked, comparing the low-voltage level voltage with a low-voltage level rated voltage, calculating a low-voltage level voltage change proportion, comparing the low-voltage level voltage change proportion with a corresponding proportion of each middle tap of the intermediate frequency transformer (104) with the middle tap, selecting the middle tap with the minimum difference as a switching target, and requiring the isolation type DC/DC converter (102) to select the middle taps of the intermediate frequency transformer (104) with the middle tap corresponding to the quantity of the fault power module.
2. The power electronic transformer redundancy fault-tolerant control method according to claim 1, characterized in that: in step S1, the method for calculating the number of the redundancy-allowed fault power modules includes:
calculating basic margin of each device of the power electronic transformer, selecting the minimum basic margin as a reference margin, and calculating the number of fault power modules allowed by redundancy by adopting the following formula
；
Wherein,
is the number of the power modules (1),
the minimum basic margin in the basic margins of all the devices of the power electronic transformer is obtained.
3. The power electronic transformer redundancy fault-tolerant control method according to claim 1, characterized in that: in step S4, the condition that the cascade power circuit of the power electronic transformer meets the redundancy fault-tolerant operation condition means that: the number of the fault power modules is less than or equal to the number of the fault power modules allowed by redundancy.
4. The power electronic transformer redundancy fault-tolerant control method according to claim 1, characterized in that:
in step S5, a bypass switch is connected to the AC terminal of the high-voltage side H-bridge converter (101)
(105) The low-voltage side H-bridge converter (103) and the breaking switch
(106) After being connected in series, the power electronic transformer is connected with the output end of the power electronic transformer;
the step of issuing the bypass instruction to the fault power module is as follows: closed bypass switch
(105) Breaking of the disconnecting switch
(106)。
5. The power electronic transformer redundancy fault-tolerant control method according to claim 1, characterized in that: in step S5, the low-voltage level voltage change ratio
The calculation formula of (2) is as follows:
wherein,
is the number of the power modules (1),
the number of failed power modules.
6. The power electronic transformer redundancy fault-tolerant control method according to claim 1, characterized in that: the isolation type DC/DC converter (102) comprises a high-voltage stage chopping converter (107) and a low-voltage stage inversion converter (108), wherein the high-voltage stage chopping converter (107) is connected with the low-voltage stage inversion converter (108) through a middle-frequency transformer (104) with a middle tap.
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