阅读说明：本技术 一种直流变压器拓扑及其控制方法 (Direct-current transformer topology and control method thereof ) 是由 陈武 姚金杰 舒良才 金浩哲 史明明 袁宇波 刘瑞煌 姜云龙 苏伟 司鑫尧 孙天 于 2020-11-18 设计创作，主要内容包括：本发明公开了一种直流变压器拓扑及其控制方法,属于发电、变电或配电的技术领域,直流变压器拓扑原边通过一个输入侧电感来连接一组N个半桥单元的阀串,从而降低开关管的电压应力；阀串通过传输电感和隔直电容连接到变压器的原边绕组,副边采用一个全桥电路,通过一个中高频变压器连接原副边,以提高整个变换器的功率密度；该拓扑结合了MMC变换器和DAB变换器的特征,具有高电压输入、故障易处理以及软开关的优点；在控制方法上采用改进的准方波调制,通过改变常开驱动信号的数量使得传输电感两端电压在较宽的输入电压范围内实现匹配,同时通过采样阀串内各个子模块电容电压进行排序后分配驱动信号,来实现子模块电容电压均衡。(The invention discloses a direct current transformer topology and a control method thereof, belonging to the technical field of power generation, power transformation or power distribution.A primary side of the direct current transformer topology is connected with a group of valve strings of N half-bridge units through an input side inductor, so that the voltage stress of a switching tube is reduced; the valve string is connected to a primary winding of the transformer through a transmission inductor and a blocking capacitor, a secondary side adopts a full-bridge circuit, and is connected with the primary side and the secondary side through a medium-high frequency transformer so as to improve the power density of the whole converter; the topology combines the characteristics of the MMC converter and the DAB converter, and has the advantages of high voltage input, easy fault handling and soft switching; the improved quasi-square wave modulation is adopted in the control method, the voltage at two ends of the transmission inductor is matched in a wider input voltage range by changing the number of normally-open driving signals, and meanwhile, the sub-module capacitor voltage balance is realized by distributing the driving signals after the sub-module capacitor voltages in the sampling valve string are sequenced.)

1. A DC transformer topology, comprising a transformer, characterized in that: the two sides of the transformer are respectively a primary side topology connected to a medium-high voltage direct current input bus and a secondary side topology connected to a low-voltage direct current output bus;

the primary side topology comprises at least two valve strings in series connection with the half-bridge units, and the valve strings are connected to a primary side winding of the transformer through a power transmission inductor and a blocking capacitor;

the secondary side topology is a full-bridge rectifier module, an input end formed by the middle points of bridge arms of the full-bridge rectifier module is connected with a secondary side winding of the transformer, and an output end of the full-bridge rectifier module is connected with an output filter capacitor in parallel.

2. The direct current transformer topology of claim 1, wherein an input side inductor is connected in series with a positive electrode of the medium and high voltage direct current input bus.

3. A control method of a direct current transformer topology is characterized in that sub-module capacitor voltage balance is realized by sequencing and distributing driving signals through sub-module capacitor voltages in a sampling valve string;

the method adopts improved quasi-square wave modulation, changes the number of normally open driving signals by comparing input voltage or output voltage to generate multi-level voltage with proper amplitude, and drives the full-bridge rectifier module in a mode of lagging a driving waveform real-time phase shift angle of a primary side half-bridge submodule until the output voltage of a direct current transformer is stable.

4. The method for controlling the topology of the direct current transformer according to claim 3, wherein the modified quasi-square wave modulation is: the number of normally open driving signals is selected according to the size of the real-time input voltage through sampling, the output voltage is subjected to feedforward control, and duty ratio control is introduced to realize soft switching.

5. The method for controlling the topology of the direct current transformer according to claim 3, wherein the method for obtaining the real-time phase shift angle comprises: the real-time output voltage value of the direct current transformer is sampled, and PI regulation and amplitude limiting regulation are carried out on the difference value of the real-time output voltage value and the voltage given value to obtain a real-time phase shift angle.

Technical Field

The invention belongs to the technical field of power generation, power transformation or power distribution, and particularly relates to a direct-current transformer topology and a control method thereof.

Background

As an important branch of power electronic integration technology, high-voltage high-power dc transformers have been the hot spot of research in recent years. The MMC (Modular Multilevel Converter) structure is widely applied to occasions such as high-voltage direct-current transmission and power electronic transformers due to the advantages of modularization, good fault handling capacity and the like. At present, researchers begin to pay attention to the application of the MMC structure in the direct-current transformer occasion, and particularly, a large number of researchers are available for the topology combining the characteristics of the MMC and the DAB;

at present, the direct current transformer based on MMC and DAB structures has more submodules, is not favorable for the promotion of power density, and simultaneously can not work in a larger input voltage range because the working principle of the direct current transformer is similar to that of a DAB converter.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a direct current transformer topology and a control method thereof, and solves the problems that the direct current transformer in the prior art is low in power density and cannot work in a wider input voltage range.

In order to realize the purpose, the invention adopts the following technical scheme: a DC transformer topology, comprising a transformer, characterized in that: the two sides of the transformer are respectively a primary side topology connected to a medium-high voltage direct current input bus and a secondary side topology connected to a low-voltage direct current output bus;

furthermore, the primary side topology comprises a valve string formed by connecting at least two half-bridge units in series, and the valve string is connected to a primary side winding of the transformer through a power transmission inductor and a blocking capacitor;

furthermore, the secondary side topology is a full-bridge rectifier module, an input end formed by the middle points of the bridge arms of the full-bridge rectifier module is connected with the secondary side winding of the transformer, and an output end of the full-bridge rectifier module is connected with an output filter capacitor in parallel.

Furthermore, the positive electrode of the medium-high voltage direct current input bus is connected with an input side inductor in series.

Furthermore, the sub-module capacitor voltage balance is realized by sequencing the sub-module capacitor voltages in the sampling valve string and then distributing driving signals;

furthermore, the improved quasi-square wave modulation is adopted, the number of normally open driving signals is changed by comparing input voltage or output voltage to generate multi-level voltage with proper amplitude, and the full-bridge rectifier module is driven in a mode of lagging a driving waveform real-time phase shift angle of a primary side half-bridge submodule until the output voltage of the direct current transformer is stable.

Further, the modified quasi-square wave modulation is as follows: the number of normally open driving signals is selected according to the size of the real-time input voltage through sampling, the output voltage is subjected to feedforward control, and duty ratio control is introduced to realize soft switching.

Further, the method for acquiring the real-time phase shift angle comprises the following steps: the real-time output voltage value of the direct current transformer is sampled, and PI regulation and amplitude limiting regulation are carried out on the difference value of the real-time output voltage value and the voltage given value to obtain a real-time phase shift angle.

The invention has the beneficial effects that: the direct current transformer can work in a wider input voltage range, and the power density of the direct current transformer is increased.

Drawings

Fig. 1 is a circuit topology diagram of a wide voltage range chain dc transformer disclosed in the present application;

fig. 2 is a circuit topology diagram of a wide voltage range chain type dc transformer with N-9;

FIG. 3 is a waveform diagram of an exemplary driving signal of a primary side valve string of the wide voltage range chain-type DC transformer shown in FIG. 2;

FIG. 4 is a diagram of soft-switching waveforms of the wide voltage range chain DC transformer of FIG. 2;

FIG. 5 is a block diagram of voltage sharing control of the sub-module capacitor of the wide voltage range chain DC transformer shown in FIG. 2;

FIG. 6 is a block diagram of the output voltage control of the wide voltage range chain DC transformer shown in FIG. 2;

FIG. 7 is a simulation waveform of the main operation of the wide voltage range chain DC transformer of FIG. 2 at an input voltage of 4.5 kV;

FIG. 8 is a voltage simulation waveform of the blocking capacitor of the wide voltage range chain DC transformer shown in FIG. 2 at an input voltage of 4.5 kV;

FIG. 9 is a simulation waveform of the main operation of the wide voltage range chain DC transformer of FIG. 2 at an input voltage of 5.625 kV;

FIG. 10 is a voltage simulation waveform of the blocking capacitor of the wide voltage range chain DC transformer shown in FIG. 2 at an input voltage of 5.625 kV;

FIG. 11 is a simulated waveform of the output voltage of the wide voltage range chain DC transformer shown in FIG. 2 at an input voltage of 4.5 kV;

FIG. 12 is a block diagram of an extended output voltage control for the wide voltage range chain DC transformer of FIG. 2;

FIG. 13 is a simulation waveform of the main operation of the wide voltage range chain DC transformer shown in FIG. 2 under the control of FIG. 12 at an output voltage of 1100V;

fig. 14 is a simulation waveform of a main operation of the wide voltage range chain type dc transformer shown in fig. 2 at an output voltage of 750V when the control of fig. 12 is applied.

Detailed Description

The technical solution of the present invention will be further explained with reference to the accompanying drawings and specific embodiments.

The following are only preferred embodiments of the present invention, it being noted that: it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention.

As shown in fig. 1, the present application discloses a dc transformer topology, including: a transformer, a primary side topology and a secondary side topology; the primary side topology is composed of N half-bridge units connected in series, the ith half-bridge unit comprises an upper switch tube S_i1And a lower switch tube S_i2Bridge arm formed by connecting in series and ith capacitor C connected in parallel at two ends of bridge arm_iGo up switch tube S_i1And the ith capacitor C_iThe connecting point of (a) is the positive pole of the ith half-bridge unit, and the switching tube S_i2And the ith capacitor C_iThe connecting point of the first half-bridge unit is the negative pole of the ith half-bridge unit, the middle point of the bridge arm of the 1 st half-bridge unit is connected to one end of the primary winding of the transformer through a power transmission inductor and a DC blocking capacitor, the middle point of the bridge arm of the 2 nd half-bridge unit is connected with the negative pole of the 1 st half-bridge unit, the middle point of the bridge arm of the ith half-bridge unit is connected with the negative pole of the (i-1) th half-bridge unit, the middle point of the bridge arm of the Nth half-bridge unit is connected with the negative pole of the (N-1) th half-bridge unit, the negative pole of the Nth half;

the secondary side topology is a full-bridge rectification module, and specifically includes: first switch tube Q₁And a second switch tube Q₂A first bridge arm and a third switching tube Q which are connected in series₃And a fourth switching tube Q₄A second bridge arm formed by connecting in series, wherein the midpoint of the first bridge arm and one end connected with the secondary winding of the transformer T are connected with the power transmission inductor L_dDC blocking capacitor C_dConnected to the primary windingThe connected ends are the same-name ends, and the middle point of the second bridge arm is connected with the other end of the secondary winding; an input side inductor L is connected in series on the positive polarity input direct current bus_inThe output end of the full-bridge rectifier module is connected with an output filter capacitor C in parallel_oThe transformer topology is connected to a medium-high voltage direct current power supply V through a positive and negative polarity input direct current bus_in。

In the control method, an improved quasi-square wave control strategy is adopted to realize that the topological output voltage of the transformer follows the set value of the topological output voltage, firstly, the real-time input voltage is sampled, the number of normally open driving signals is selected according to the size of the input voltage, the feedforward control is carried out on the output voltage, and the duty ratio control is introduced in the switching process to realize soft switching; meanwhile, sampling real-time voltages of all sub-module capacitors in the valve string, sequencing the voltages, and distributing driving signals to realize sub-module capacitor voltage balance, and on the basis, generating a multi-level voltage of a primary side; and then, acquiring a topological real-time output voltage value, making a difference between the real-time output voltage value and a voltage given value, obtaining a real-time phase shift angle by the difference value through a PI regulator and an amplitude limiter, and driving a full-bridge rectifier module in a mode of lagging a primary side driving waveform real-time phase shift angle to stabilize the output voltage of the whole direct current transformer.

The transformer valve string driving signals are as follows:

the driving signals of the upper switching tubes of the first K half-bridge units are always at a high level, the driving signals of the lower switching tubes are always at a low level, the duty ratios of the driving waveforms of the upper switching tubes of the remaining N-K half-bridge units are equal and are all 50% (no dead zone is considered), and the phase difference between every two half-bridge units is one theta; the driving signals of the upper and lower switching tubes of the half-bridge unit are complementary; the actual driving signals are redistributed to realize voltage sharing after being sequenced according to the capacitor voltages of the sub-modules.

The driving signals of the switching tube on the secondary side of the transformer are as follows:

the driving signals of the first switching tube and the second switching tube are complementary, the duty ratio is 50%, the driving signals of the third switching tube and the fourth switching tube are complementary, the duty ratio is 50%, the driving signals of the first switching tube and the fourth switching tube are the same, and the driving signals of the second switching tube and the third switching tube are the same(ii) a Primary side driving waveform and first switch tube Q₁A phase shift angle phi exists between the driving waveforms, the size of the output voltage of the whole direct current transformer is controlled by adjusting the size of the phase shift angle phi, and the phase shift angle phi is obtained by carrying out PI (proportional-integral) adjustment and amplitude limiting processing on the difference value of the output voltage.

The working principle of the technical solution of the present invention is described below by taking a wide voltage range chain type dc transformer system (as shown in fig. 2) with N-9 as an example and combining the simulation result. The simulation parameters are as follows:

simulation main parameters

Fig. 2 is a schematic diagram of a main circuit of the wide voltage range chain type dc transformer system with N being 9, and referring to the control mode of fig. 3, the driving signal of the converter is given according to the control method proposed above. As can be seen from fig. 3, as the input voltage increases, the voltage at one end of the transmission inductor can be reduced by increasing the number of normally-on driving signals, so as to ensure the voltage matching between the two ends of the transmission inductor.

If the number of commonly-used submodules K is switched to K +1 when the input voltage is suddenly changed, if the duty ratio of the commonly-used submodules K is directly converted to 100% from the original 50%, the inductive current has a large peak, the switch tube can be damaged, the reliability of the circuit is reduced, and meanwhile, the output voltage can drop. In order to reduce the peak of the inductor current during switching and prevent the output voltage from dropping, as shown in fig. 4, when the K +1 th sub-module changes from 50% of the duty ratio to 100%, a duty ratio control variable D is introduced_K+1. When the input voltage is detected to have sudden change, D is enabled_K+1Linearly increasing from 1 to 2, thereby achieving a soft handover.

The sub-module capacitor voltage balance control strategy is shown in fig. 5, and the sub-modules capacitor voltages are sampled to be sequenced, and the drive signals of the previous items are sent to the sub-modules with small sub-module capacitor voltages.

Fig. 6 shows a whole output voltage control block diagram, which includes that firstly, the number K of commonly-used sub-modules is determined through hysteresis loops according to the size of a sampled input voltage and an output voltage reference value to realize rough control of the output voltage, and then an outward phase shift angle is adjusted through an output voltage loop to realize accurate control of the output voltage.

FIG. 7 shows a basic working simulation waveform when the input voltage is 4.5kV, and a 10-level quasi square wave with the highest voltage of 9kV and the lowest voltage of 0V is applied to one end of the transmission inductor; primary voltage v of transformer_aThe voltage of the transmission inductor is square wave with the highest voltage of 4.5kV and the lowest voltage of-4.5 kV, and the voltages of the two ends of the transmission inductor are matched.

Fig. 8 shows a voltage simulation diagram of the blocking capacitor of the system when the input voltage is 4.5kV, the voltage of which is stabilized at-4.5 kV, and by superimposing the blocking capacitor voltage and the voltage at the AB point, a square wave waveform with both positive and negative peaks of 4.5kV can be obtained to be applied to one side of the power transmission inductor.

Fig. 9 shows a basic work simulation waveform when the input voltage is 5.625kV, and a 9-level quasi square wave with a highest voltage of 10.125kV and a lowest voltage of 1.125kV is provided at one end of the transmission inductor; primary voltage v of transformer_aThe voltage of the transmission inductor is square wave with the highest voltage of 4.5kV and the lowest voltage of-4.5 kV, and the voltages of the two ends of the transmission inductor are matched.

Fig. 10 shows a voltage simulation diagram of the blocking capacitor of the system when the input voltage is 5.625kV, the voltage of the blocking capacitor is stabilized at-5.625 kV, and a square wave waveform with both positive and negative peaks of 4.5kV can be obtained by superposing the blocking capacitor voltage and the voltage at the AB point to be applied to one side of the power transmission inductor.

Fig. 11 shows a simulation of the output voltage of the system, and it can be seen that the output voltage of the system can be stabilized at a given 750V after a short adjustment.

Fig. 12 is a block diagram of an extended output voltage control of the wide voltage range chain dc transformer shown in fig. 2, which first samples the real-time output voltage, determines the number K of sub-modules that are frequently used according to the magnitude of the output voltage, and then adjusts the phase angle of the phase shift through the output power loop to achieve the precise control of the output power.

FIG. 13 shows the basic operation simulation waveform at an output voltage of 1100VOne end of the transmission inductor is a 9-level quasi square wave with the highest voltage of 13.5kV and the lowest voltage of 0 kV; primary voltage v of transformer_aThe voltage of the transmission inductor is square wave with the highest voltage of 6.6kV and the lowest voltage of-6.6 kV, and the voltages of the two ends of the transmission inductor are matched.

FIG. 14 shows a basic operation simulation waveform with an output voltage of 750V, and a 7-level quasi square wave with a highest voltage of 11.045kV and a lowest voltage of 24.54kV at one end of a transmission inductor; primary voltage v of transformer_aThe voltage of the transmission inductor is square wave with the highest voltage of 4.5kV and the lowest voltage of-4.5 kV, and the voltages of the two ends of the transmission inductor are matched.

The working principle is as follows:

the primary side of the direct current transformer is connected with a valve string of a group of N half-bridge units through an input side inductor, so that the voltage stress of a switching tube is reduced; the valve string is connected to a primary winding of the transformer through a transmission inductor and a blocking capacitor; the secondary side adopts a full-bridge circuit and is connected with the primary side through a medium-high frequency transformer so as to improve the power density of the whole converter; the topology combines the characteristics of the MMC converter and the DAB converter, and has the advantages of high voltage input, easy fault handling and soft switching; in the control method, improved quasi-square wave modulation is adopted, the voltage at two ends of the transmission inductor is matched in a wider input voltage range by changing the number of normally-open driving signals, and meanwhile, the sub-module capacitor voltage balance is realized by sampling each sub-module capacitor voltage in the valve string and distributing the driving signals after sequencing.