阅读说明：本技术 基于三端口高频变压器的电力电子变压器及其控制方法 (Power electronic transformer based on three-port high-frequency transformer and control method thereof ) 是由 欧阳少迪 刘进军 陈晖� 杨跃 陈星星 宋曙光 于 2019-09-27 设计创作，主要内容包括：本发明公开了一种基于三端口高频变压器的电力电子变压器及其控制方法,每个H桥变换器与三有源全桥变换器的一次侧直流端口连接,三有源全桥变换器的二次侧的第一个直流端口的正极作为该变换器的输出正极,第二个直流端口的负极作为该变换器的输出负极,二次侧的第一个直流端口的负极与第二个直流端口的正极连接作为该变换器的输出中性点；所有的三有源全桥变换器的输出正极连接成为电力电子变压器低压直流端口的正极；所有输出负极连接成为电力电子变压器低压直流端口的负极；所有输出中性点连接成为电力电子变压器低压直流端口的中性点。该拓扑结构可以使电力电子变压器实现双极性低压直流输出与三相四线制低压交流输出。(The invention discloses a power electronic transformer based on a three-port high-frequency transformer and a control method thereof, wherein each H-bridge converter is connected with primary side direct current ports of three active full-bridge converters, the positive pole of the first direct current port on the secondary side of the three active full-bridge converters is used as the output positive pole of the converter, the negative pole of the second direct current port is used as the output negative pole of the converter, and the negative pole of the first direct current port on the secondary side is connected with the positive pole of the second direct current port to be used as the output neutral point of the converter; the output anodes of all the three active full-bridge converters are connected to form the anode of the low-voltage direct-current port of the power electronic transformer; all output cathodes are connected to form the cathode of the low-voltage direct current port of the power electronic transformer; all output neutral points are connected to become neutral points of the low-voltage direct current ports of the power electronic transformer. The topological structure can enable the power electronic transformer to realize bipolar low-voltage direct current output and three-phase four-wire system low-voltage alternating current output.)

1. A power electronic transformer based on a three-port high-frequency transformer is characterized by comprising an input stage, an isolation stage and an output stage which are connected in sequence;

the input stage is a three-phase series H bridge, each phase comprises N first single-phase H bridge converters which are connected in series, and N is a positive integer;

the isolation stage comprises three-phase three-active full-bridge converters, each phase comprises N three-active full-bridge converters, each three-active full-bridge converter comprises a three-port high-frequency transformer, a high-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a second single-phase H-bridge converter, a first low-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a third single-phase H-bridge converter, and a second low-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a fourth single-phase H-bridge converter; the direct current port of the third single-phase H-bridge converter is a first direct current output end of the three-active full-bridge converter, and the direct current port of the fourth single-phase H-bridge converter is a second direct current output end of the three-active full-bridge converter;

the positive poles of the first direct current output ends of all the three active full-bridge converters are connected together and used as the positive pole of the low-voltage direct current port of the power transformer; the negative electrodes of the second direct current output ends of all the three active full bridges are connected to be used as the negative electrode of the low-voltage direct current port; the negative electrodes of all the first direct current output ends are connected with the positive electrodes of all the second direct current output ends to serve as neutral points of the low-voltage direct current ports; a port formed by the positive electrode of the low-voltage direct current port and the neutral point is a high-side low-voltage direct current port, and a port formed by the negative electrode of the low-voltage direct current port and the neutral point is a low-side low-voltage direct current port;

the output stage is a three-phase two-level converter, a direct current port of the three-phase two-level converter is connected with the anode and the cathode of the low-voltage direct current port of the power electronic transformer, and an alternating current port of the three-phase two-level converter is used for connecting a load.

2. The power electronic transformer based on the three-port high-frequency transformer as claimed in claim 1, wherein an LC filter is connected to an ac port of the three-phase two-level converter, and a midpoint of the LC filter is connected to a neutral point of a low-voltage dc port of the power electronic transformer.

3. The power electronic transformer based on the three-port high-frequency transformer as claimed in claim 1, wherein the DC terminal of each first single-phase H-bridge converter is connected with a converter capacitor C1.

4. The power electronic transformer based on the three-port high-frequency transformer as claimed in claim 1, wherein the high-voltage coil of the three-port high-frequency transformer is connected with the ac port of the second single-phase H-bridge converter through a reactor L1, the first low-voltage coil of the three-port high-frequency transformer is connected with the ac port of the third single-phase H-bridge converter through a reactor L2, and the second low-voltage coil of the three-port high-frequency transformer is connected with the ac port of the fourth single-phase H-bridge converter through a reactor L3.

5. The power electronic transformer based on three-port high-frequency transformer as claimed in claim 1, wherein a capacitor C is connected between the positive pole of the low voltage DC port of the power electronic transformer and the neutral point of the low voltage DC port₂₁A capacitor C is connected between the negative electrode and the neutral point of the low-voltage DC port of the power electronic transformer₂₂。

6. A control method of a power electronic transformer based on a three-port high-frequency transformer as claimed in claim 1, characterized by comprising the steps of:

step 1, sampling the voltage of capacitors C1 of all single-phase H-bridge converters corresponding to the input stage and the high-side low-voltage direct-current port

step 2, averaging the voltage average values V of all the capacitors C1_{dc_MV}And the set input stage DC voltage instruction

Step 3, determining a reactive current instruction according to the reactive demand

Step 4, detecting the alternating current side current i of the input stage_A、i_B、i_CAnd calculating the current i_A、i_B、i_CActive component i of_dAnd a reactive component i_q；

Step 5, the active component i of each phase is calculated_dAnd active current command

Step 6, commanding the active voltage of each phaseAnd reactive voltage command

Step 7, sampling the voltage V of the high-side low-voltage direct-current port_{dc_LV_up}；

Step 8, the voltage V of the high-side low-voltage direct current port obtained in the step 7 is used_{dc_LV_up}And the set high-side low-voltage direct-current command voltage

Step 9, the capacitance voltage of each input stage single-phase H-bridge converter detected in the step 1 is used for measuring the capacitance voltage of each input stage single-phase H-bridge converter

Step 10: converting the high-side single-phase H-bridge converter obtained in the step 8Average phase shift instruction of

Step 11: sampling voltage V of low-side low-voltage DC port_{dc_LV_down}；

Step 12: the voltage V of the low-side low-voltage direct current port obtained in the step 11 is measured_{dc_LV_down}And the set low-side low-voltage direct-current command voltage

Step 13: sampling output current of secondary side low-side single-phase H-bridge converter of each three-active full-bridge converter of isolation levelAnd averaging the values

Step 14: the output current of the secondary side low-side single-phase H-bridge converter of the three-active full-bridge converter detected in the step 13 is detected

Step 15: averaging the phase shift commands of the low-side single-phase H-bridge converter of each three-active full-bridge converter of the isolation level obtained in the step 12

Technical Field

The invention belongs to the technical field of power electronic transformers, and particularly relates to a power electronic transformer based on a three-port high-frequency transformer and a control method thereof.

Background

The traditional power transformer has the advantages of simple structure, high efficiency and high reliability, and is widely applied to power systems. However, the traditional transformer is large and heavy due to the low working frequency, and potential hazards of fire and environmental pollution exist when mineral oil, epoxy resin, flame retardant oil and the like are used as insulating or cooling media. In addition, it can only realize the relatively single functions of electrical isolation, voltage class conversion, power bidirectional transmission and the like, but does not have the functions of network side power quality regulation, harmonic transmission isolation, overload and fault protection, load voltage regulation and the like. These weaknesses of the conventional transformer make it unable to meet the functional requirements of some new applications such as smart grids. In the past decades, power electronic technology has been developed rapidly and comprehensively, and more power electronic devices are applied to power systems. In this context, researchers and engineers have proposed Power Electronic transformers (Power Electronic transformers) or Solid-State transformers (Solid-State transformers) to address the above-mentioned weaknesses of conventional transformers.

Since the power grid needs to be accessed with distributed energy in a large scale in the future, the low-voltage power grid can be widely in an alternating-current and direct-current mixed mode. According to the latest standards of dc power distribution published in recent years, a low-voltage dc power grid tends to adopt a bipolar structure, so that a low-voltage stage of a future power electronic transformer needs to provide a bipolar dc port and a three-phase four-wire ac port at the same time. Power electronic transformer topologies that can accomplish this function are not mentioned or discussed in the literature at present.

Disclosure of Invention

In order to solve the above problems, the present invention provides a power electronic transformer based on a three-port high-frequency transformer and a control method thereof, so as to realize a low-voltage ac/dc hybrid function, and realize a three-phase four-wire system output without using a fourth bridge arm.

In order to achieve the purpose, the power electronic transformer based on the three-port high-frequency transformer comprises an input stage, an isolation stage and an output stage which are connected in sequence;

the input stage is a three-phase series H bridge, each phase comprises N first single-phase H bridge converters which are connected in series, and N is a positive integer;

the isolation stage comprises three-phase three-active full-bridge converters, each phase comprises N three-active full-bridge converters, each three-active full-bridge converter comprises a three-port high-frequency transformer, a high-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a second single-phase H-bridge converter, a first low-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a third single-phase H-bridge converter, and a second low-voltage coil of each three-port high-frequency transformer is connected with an alternating current port of a fourth single-phase H-bridge converter; the direct current port of the third single-phase H-bridge converter is a first direct current output end of the three-active full-bridge converter, and the direct current port of the fourth single-phase H-bridge converter is a second direct current output end of the three-active full-bridge converter;

the positive poles of the first direct current output ends of all the three active full-bridge converters are connected together and used as the positive pole of the low-voltage direct current port of the power transformer; the negative electrodes of the second direct current output ends of all the three active full bridges are connected to be used as the negative electrode of the low-voltage direct current port; the negative electrodes of all the first direct current output ends are connected with the positive electrodes of all the second direct current output ends to serve as neutral points of the low-voltage direct current ports; a port formed by the positive electrode of the low-voltage direct current port and the neutral point is a high-side low-voltage direct current port, and a port formed by the negative electrode of the low-voltage direct current port and the neutral point is a low-side low-voltage direct current port;

the output stage is a three-phase two-level converter, a direct current port of the three-phase two-level converter is connected with the anode and the cathode of the low-voltage direct current port of the power electronic transformer, and an alternating current port of the three-phase two-level converter is used for connecting a load.

Furthermore, an alternating current port of the three-phase two-level converter is connected with an LC filter, and the middle point of the LC filter is connected with the neutral point of the low-voltage direct current port of the power electronic transformer.

Further, a converter capacitor C1 is connected to the dc terminal of each first single-phase H-bridge converter.

Further, the high-voltage coil of the three-port high-frequency transformer is connected with the ac port of the second single-phase H-bridge converter through a reactor L1, the first low-voltage coil of the three-port high-frequency transformer is connected with the ac port of the third single-phase H-bridge converter through a reactor L2, and the second low-voltage coil of the three-port high-frequency transformer is connected with the ac port of the fourth single-phase H-bridge converter through a reactor L3.

Furthermore, a capacitor C is connected between the positive electrode of the low-voltage direct-current port of the power electronic transformer and the neutral point of the low-voltage direct-current port₂₁A capacitor C is connected between the negative electrode and the neutral point of the low-voltage DC port of the power electronic transformer₂₂。

The control method of the power electronic transformer based on the three-port high-frequency transformer is characterized by comprising the following steps of:

step 1, sampling the voltage of capacitors C1 of all single-phase H-bridge converters corresponding to the input stage and the high-side low-voltage direct-current portSampling the voltage of the capacitor C1 of all single-phase H-bridge converters with the input stages corresponding to the low-side low-voltage direct-current ports

Find all

And

average value of (V)_{dc_MV}(ii) a Wherein k represents phase, k belongs to A, B, C and i represent ith single-phase H-bridge converter，i∈1,2......N

Step 2, averaging the voltage average values V of all the capacitors C1_{dc_MV}And the set input stage DC voltage instruction

Comparing, and outputting active current command via PI regulator

Step 3, determining a reactive current instruction according to the reactive demand

Step 4, detecting the alternating current side current i of the input stage_A、i_B、i_CAnd calculating the current i_A、i_B、i_CActive component i of_dAnd a reactive component i_q；

Step 5, the active component i of each phase is calculated_dAnd active current commandComparing the reactive components i of the phases_qAnd reactive current command

Comparing, and outputting an active voltage instruction of the input stage under the dq coordinate system through the PI regulator

And reactive voltage command

Step 6, commanding the active voltage of each phase

And reactive voltage command

Obtaining total command voltage of the input stage in a static coordinate system through coordinate transformation respectively

Step 7, sampling the voltage V of the high-side low-voltage direct-current port_{dc_LV_up}；

Step 8, the voltage V of the high-side low-voltage direct current port obtained in the step 7 is used_{dc_LV_up}And the set high-side low-voltage direct-current command voltage

Comparing; outputting average phase shift instruction of high-side single-phase H-bridge converter of each three-active full-bridge converter of isolation stage through PI regulator

Step 9, the capacitance voltage of each input stage single-phase H-bridge converter detected in the step 1 is used for measuring the capacitance voltage of each input stage single-phase H-bridge converterAnd the average capacitance voltage V of the input stage converter_{dc_MV}Comparing, outputting phase shift regulating quantity commands of secondary side high-side single-phase H-bridge converters of three active full-bridge converters corresponding to each input stage single-phase H-bridge converter through PI regulators

Step 10: the average phase shift instruction of the high-side single-phase H-bridge converter obtained in the step 8 is used

The phase shift adjustment amount command of the secondary side high-side single-phase H-bridge converter of the three-active full-bridge converter corresponding to each input stage single-phase H-bridge converter obtained in step 9

Summations, as isolation stages, of three eachPhase shift command of high-side single-phase H-bridge converter of active full-bridge converter

Step 11: sampling voltage V of low-side low-voltage DC port_{dc_LV_down}；

Step 12: the voltage V of the low-side low-voltage direct current port obtained in the step 11 is measured_{dc_LV_down}And the set low-side low-voltage direct-current command voltage

Comparing; outputting average phase shift instruction of low-side single-phase H-bridge converter of each three-active full-bridge converter of isolation stage through PI regulator

Step 13: sampling output current of secondary side low-side single-phase H-bridge converter of each three-active full-bridge converter of isolation level

And averaging the values

Step 14: the output current of the secondary side low-side single-phase H-bridge converter of the three-active full-bridge converter detected in the step 13 is detected

Andcomparing, outputting phase shift regulating quantity instruction of secondary side low side single phase H-bridge converter of each three active full-bridge converter through PI regulator

Step 15: all three active full-bridge converters of the isolation level obtained in the step 12Average phase shift instruction of low-side single-phase H-bridge converter

And step 14, obtaining phase shift regulating quantity commands of the secondary side low-side single-phase H-bridge converter of each three-active full-bridge converter

Summing, as phase shift commands for the low-side single-phase H-bridge converter of each three-active full-bridge converter of the isolation stage

Compared with the prior art, the power electronic transformer based on the three-port high-frequency transformer has the following beneficial effects:

1) the isolation transformer of the isolation level can realize the basic functions of transformation and isolation of the traditional power transformer;

2) three-phase four-wire system alternating current output can be realized through a three-phase two-level converter of an output stage and a neutral point of a low-voltage direct current port of the power electronic transformer, and the routine of a low-voltage distribution network in China is met.

3) The direct-current ports of the third single-phase H-bridge converters of all the isolation level DC/DC converters are connected to form a high-side low-voltage direct-current port; the direct current ports of the fourth single-phase H-bridge converters of all the isolation level DC/DC converters are connected to form a low-side low-voltage direct current port, so that bipolar direct current output is realized, and the standard structure of the latest direct current power distribution network is met.

4) Compared with the existing topological structure for realizing bipolar direct current output through the voltage balancer, the topological structure does not need the voltage balancer, saves the capacity of a semiconductor device, simultaneously omits the control of the voltage balancer and simplifies the control.

The control strategy provided by the invention averagely distributes the unbalanced load power of the positive electrode and the negative electrode to each input-stage converter, so that the power of each input-stage converter is balanced, and the electric energy quality of the input-stage side is not influenced. The working condition that the positive and negative power of the direct current port is unbalanced can be well dealt with.

Drawings

FIG. 1 is a diagram of a single-phase H-bridge converter;

FIG. 2 is a diagram of a three-active full-bridge converter;

FIG. 3 is a block diagram of a three-phase two-level converter;

FIG. 4 is a graph of output stage voltage waveforms;

FIG. 5 is a graph of output stage current waveforms;

FIG. 6 is a low voltage DC port voltage waveform diagram;

FIG. 7 is a waveform diagram of the output current at the low voltage DC port;

FIG. 8 is a waveform of the output power of each converter of the input stage;

FIG. 9 is a waveform of DC voltage of each converter of the input stage;

FIG. 10 is a waveform diagram of an input current for an input stage;

FIG. 11 is a power electronic transformer topology of the present invention;

FIG. 12 is a high frequency square wave voltage waveform;

FIG. 13 is a schematic representation of steps 1 through 6 of the method of the present invention;

FIG. 14 is a schematic diagram of step 8 of the method of the present invention;

FIG. 15 is a schematic diagram of step 9 of the method of the present invention;

FIG. 16 is a schematic diagram of step 10 of the method of the present invention;

FIG. 17 is a schematic representation of step 12 of the method of the present invention;

FIG. 18 is a schematic representation of step 14 of the method of the present invention;

fig. 19 is a schematic diagram of step 15 of the method of the present invention.

Detailed Description

In order to make the objects and technical solutions of the present invention clearer and easier to understand. The present invention will be described in further detail with reference to the following drawings and examples, wherein the specific examples are provided for illustrative purposes only and are not intended to limit the present invention.

In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified. In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

The invention provides a topological structure of an input-stage three-phase medium-voltage alternating current input power electronic transformer, which can simultaneously output bipolar direct current and three-phase four-wire system alternating current at a low-voltage stage so as to realize the function of low-voltage alternating current and direct current mixing.

A power electronic transformer based on a three-port high-frequency transformer. The method comprises three steps: an input stage, an isolation stage and an output stage. The input stage adopts a three-phase series H-bridge converter; the isolation stage adopts a three-active full-bridge converter; the output stage adopts a three-phase two-level converter.

The direct current port of each H-bridge converter of the input stage is connected with the primary side direct current port of one three-active full-bridge converter; the three active full-bridge converters of the isolation level are divided into two groups, secondary side direct current ports of the two groups of three active full-bridge converters are respectively connected in parallel, wherein a positive electrode of the parallel-connected secondary side direct current ports of the first group of three active full-bridge converters is used as a positive electrode of a low-voltage direct current port of the power electronic transformer, a negative electrode of the parallel-connected secondary side direct current ports of the second group of three active full-bridge converters is used as a negative electrode of the low-voltage direct current port of the power electronic transformer, and the negative electrode of the first group and the positive electrode of the second group are connected to be used as a neutral point of the low-voltage direct current. The DC port of the output stage converter is connected with the positive electrode and the negative electrode of the low-voltage DC port of the power electronic transformer, and the AC port is connected with the LC filter and supplies power to a three-phase load; the middle point of the LC filter is connected with the neutral point of the low-voltage direct-current port of the power electronic transformer. The topological structure can enable the power electronic transformer to realize bipolar low-voltage direct current output and three-phase four-wire system low-voltage alternating current output.