CN113726206A

CN113726206A - Five-level power electronic converter and method

Info

Publication number: CN113726206A
Application number: CN202110846712.5A
Authority: CN
Inventors: 刘进军; 邓智峰; 杜思行
Original assignee: Xian Jiaotong University
Current assignee: Xian Jiaotong University
Priority date: 2021-07-26
Filing date: 2021-07-26
Publication date: 2021-11-30
Anticipated expiration: 2041-07-26
Also published as: CN113726206B

Abstract

The invention discloses a five-level power electronic converter and a method, wherein the single-phase structure of the converter comprises a half-bridge unit, a connecting switch tube and a connecting capacitor; the direct current negative terminal of the first half-bridge unit is connected with the direct current positive terminal of the second half-bridge unit through a coupling capacitor, the alternating current terminal of the first half-bridge unit is connected with the direct current positive terminal of the third half-bridge unit through a first coupling switch tube, and the alternating current terminal of the second half-bridge unit is connected with the direct current negative terminal of the fourth half-bridge unit through a second coupling switch tube; the dc negative terminal of the third half-bridge unit is directly connected to the dc positive terminal of the fourth half-bridge unit, and the ac terminals of the third half-bridge unit and the fourth half-bridge unit are to be connected to the dc positive terminal and the dc negative terminal of the fifth half-bridge unit, respectively, and the ac terminal of the fifth half-bridge unit is an output terminal. The invention reduces the volume of the converter as a whole and is suitable for being applied to medium-high voltage high-power occasions.

Description

Five-level power electronic converter and method

Technical Field

The invention belongs to the field of converters, and relates to a five-level power electronic converter and a method.

Background

With the continuous improvement of the economic level and the consumption level of the modern society, the demand of human beings on energy sources is continuously increased. Meanwhile, environmental problems caused by frequent human social activities are also increasingly prominent. In the face of the two major crises of energy and environment, the global common advocates are 'clean green, high-efficiency and energy-saving'. From the perspective of energy consumption, it is important to improve the efficiency of energy conversion and utilization and reduce the energy loss during consumption. The variable frequency driving of the medium-voltage high-power motor is always the main part of energy consumption, and is widely applied to important industrial fields such as petrochemical industry, mining industry, sewage treatment, locomotive traction, ship driving and the like. Therefore, reduction of loss and volume cost of the medium voltage motor driving system has been the subject of extensive research.

The multilevel converter has the remarkable advantages of high equivalent switching frequency, small size of a required filter, small stress of a switching device, high output voltage waveform quality and the like, is a core component of the existing medium-voltage motor driving system, and is also the key point for optimizing the performance of the whole motor driving system. The conventional multilevel converter topology mainly includes: diode clamped converters (NPC), flying capacitor converters (FC) and cascaded H-bridge Converters (CHB). The stress of main switching devices of the NPC topology is equal, a large number of floating capacitors are not needed, the structure is simple, the NPC topology is commercialized in a large number, the number of required clamping diodes is increased sharply along with the increase of the number of electric levels, and great cost improvement and reliability reduction are brought. Meanwhile, the balance problem of the direct-current side capacitor voltage and the loss difference problem among different switching devices in the NPC topology also present great challenges to the stable operation of the system; the FC topology has no NPC direct-current side capacitance balance problem, and the control is relatively simple. However, the number of the floating capacitors required by the converter is large, and the number of the capacitors is further increased along with the increase of the number of the levels, so that a pre-charging circuit is complex, and the whole system is large in size; the CHB converter has the characteristic of modularization and is easy to realize expansion of higher levels, but in order to provide independent direct current power supply for each module, a phase-shifting transformer is generally adopted to supply power, so that the final converter is large in size and high in cost.

In summary, a more sophisticated multi-level converter topology is lacking in current industrial applications. Therefore, according to the rapidly developing needs in the field of medium-voltage motor driving, it is of great significance to provide a novel multi-level converter with high performance, high reliability and small volume.

Disclosure of Invention

The invention aims to provide a novel five-level power electronic converter with high performance and simple structure and a control method thereof aiming at a motor driving system with a voltage level of 6kV or below and considering various problems of the traditional multi-level converter. Compared with the traditional converter, less suspension capacitors are used, the capacitance is relatively reduced, the size of the converter is reduced as a whole, and the converter is suitable for being applied to medium-high voltage high-power occasions.

In order to achieve the above object, the present invention provides the following technical solutions.

A five-level power electronic converter comprises a half-bridge unit, a connecting switch tube and a connecting capacitor in a single-phase structure;

the five half-bridge units are of a three-port structure, wherein a direct-current positive terminal of the first half-bridge unit and a direct-current negative terminal of the second half-bridge unit are used for being connected with a positive electrode and a negative electrode of a direct-current bus respectively; the direct current negative terminal of the first half-bridge unit is connected with the direct current positive terminal of the second half-bridge unit through a coupling capacitor, the alternating current terminal of the first half-bridge unit is connected with the direct current positive terminal of the third half-bridge unit through a first coupling switch tube, and the alternating current terminal of the second half-bridge unit is connected with the direct current negative terminal of the fourth half-bridge unit through a second coupling switch tube; the dc negative terminal of the third half-bridge unit is directly connected to the dc positive terminal of the fourth half-bridge unit, and the ac terminals of the third half-bridge unit and the fourth half-bridge unit are to be connected to the dc positive terminal and the dc negative terminal of the fifth half-bridge unit, respectively, and the ac terminal of the fifth half-bridge unit is an output terminal.

As a further improvement of the present invention, the first half-bridge unit, the second half-bridge unit, the third half-bridge unit and the fourth half-bridge unit each include two switching tubes and a capacitor, and the two switching tubes are connected in series and then connected in parallel with the capacitor;

the fifth half-bridge unit comprises two switching tubes which are connected in series.

As a further improvement of the present invention, the first half-bridge unit and the second half-bridge unit each include two switching tubes and a capacitor, and the two switching tubes are connected in series and then connected in parallel with the capacitor;

the third half-bridge unit and the fourth half-bridge unit respectively comprise a switching tube, a clamping diode and a capacitor, and the switching tube and the clamping diode are connected and then connected in parallel with the capacitor;

As a further improvement of the invention, the rated voltage of the capacitor is one fourth of the voltage of the direct current bus.

As a further improvement of the invention, the rated voltage of the connecting capacitor is one half of the voltage of the direct current bus.

As a further improvement of the invention, the connecting switch tube and the switch tube are insulated gate bipolar transistors, integrated gate commutated thyristors or turn-off thyristors.

A modulation method of a five-level power electronic converter comprises the following steps:

control of the on and off output V of a switching tube in a converter_dc、3V_dc/4、V_dc/2、V_dc5 different levels of/4 and 0;

in the converter, the control signals of the first half-bridge unit and the second half-bridge unit are the same, and the control signals of the third half-bridge unit and the fourth half-bridge unit are the same, so that 3 paths of independent switch control signals are needed by 5 half-bridge units; in addition, the two connection switch tubes are in complementary conduction all the time, and an independent path of control signal is needed; the corresponding switch tubes are respectively controlled by four independent switch control signals, and the control signals of the other switch tubes can be obtained by taking the same or different from the four signals.

the control signals of the first half-bridge unit and the second half-bridge unit of the converter are the same, the switch tube of the third half-bridge unit and the switch tube of the fifth half-bridge unit are complementarily conducted, the switch tube of the fifth half-bridge unit and the switch tube of the fourth half-bridge unit are complementarily conducted, four independent switch signals are needed to respectively control one corresponding switch tube, and the control signals of the other switch tubes can be obtained by taking the same or different signals from the four signals.

Compared with the prior art, the invention has the following beneficial effects:

the five-level power electronic converter is provided with five half-bridge units with the same structure, has a simple integral structure, is convenient and flexible to control, uses a small number of suspension capacitors and is simple in pre-charging circuit on the basis of ensuring that the voltage stress of all switch tubes in the structure is one fourth of the voltage of a direct-current bus, and the five-level power electronic converter is provided with a common direct-current bus, so that a bulky phase-shifting transformer is omitted; as a further improved topology of the invention, the volume and the cost of the converter are further saved while the advantages of the topology are kept, and both the converter and the converter are suitable for medium-high voltage high-power application occasions.

Drawings

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes, the proportional sizes, and the like of the respective members in the drawings are merely schematic for facilitating the understanding of the present invention, and do not specifically limit the shapes, the proportional sizes, and the like of the respective members of the present invention. In the drawings:

fig. 1 a single phase block diagram of a five level converter 1 of the present invention;

FIG. 2 is a block diagram of a half bridge unit;

fig. 3 is a three-phase structural diagram of the five-level converter 1 of the present invention;

fig. 4 is a single phase block diagram of the five level converter 2 of the present invention;

fig. 5 is a three-phase structural view of the five-level converter 2 of the present invention;

the current flow path of converter 1 in the switching state B1 of fig. 6;

FIG. 7 illustrates the principle of five-level carrier phase-shift modulation;

FIG. 8 illustrates a carrier phase shift modulation waveform with a reference voltage between-1 and-0.5;

FIG. 9 a three-phase reference voltage waveform;

fig. 10 is a waveform diagram of a simulation of the five-level converter 1.

Detailed Description

In order to make those skilled in the art better understand the technical solution of the present invention, the technical solution in the embodiment of the present invention will be clearly and completely described below with reference to the drawings in the embodiment of the present invention, and it is obvious that the described embodiment is only a part of the embodiment of the present invention, and not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, shall fall within the scope of protection of the present invention.

It will be understood that when an element is referred to as being "disposed on" another element, it can be directly on the other element or intervening elements may also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. The terms "vertical," "horizontal," "left," "right," and the like as used herein are for illustrative purposes only and do not denote a single embodiment.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

The converter structure, modulation method and converter capacitor voltage of the present invention will be analyzed in detail in four subsections.

First, the five-level converter topology of the invention

The single-phase structure of the five-level converter 1 provided by the invention is shown in fig. 1 and comprises 5 half-bridge units and 2 connected switching tubes (S)_x6、S_x6') and 1 coupling capacitor (C)_d2). Wherein, the half-bridge unit is a three-port structure, as shown in FIG. 2, and comprises two switching tubes S₁～S₁' and a capacitor C, a switching tube S₁And S₁' connected in series and then connected in parallel with a capacitor C.

Fig. 1 is a single-phase structure diagram of a five-level converter 1 according to the present invention; fig. 2 is a block diagram of a half-bridge cell.

With reference to fig. 1, the connection mode of each device in the single-phase structure of the five-level converter 1 of the present invention is described in detail as follows: in the structure, the positive electrode and the negative electrode of the direct current bus are respectively and directly connected with the direct current positive electrode terminal of the first half-bridge unit I and the direct current negative electrode terminal of the second half-bridge unit II; the DC negative terminal of the first half-bridge unit (I) and the DC positive terminal of the second half-bridge unit (II) are connected via a coupling capacitor (C)_d2The alternating current terminals of the first half bridge unit are connected through a connecting switch tube S_x6The second half-bridge unit II is connected with a direct current positive terminal of the third half-bridge unit III, and an alternating current terminal of the second half-bridge unit II is connected with a switch tube S_x6' connected to the dc negative terminal of the fourth half-bridge unit (iv); the DC negative terminal of the third half-bridge unit (c) is directly connected with the DC positive terminal of the fourth half-bridge unit (c), the AC terminal of the third half-bridge unit (c) and the AC terminal of the fourth half-bridge unit (c) are respectively connected with the DC positive terminal and the DC negative terminal of the fifth half-bridge unit (c), and the AC terminal of the fifth half-bridge unit (c) is used as the output end of the single-phase circuit and is connected with the load. It should be additionally noted that unlike the other 4 half-bridge units, the capacitor of the fifth half-bridge unit (c) is omitted in the present structure.

Based on the single-phase topology described above, a three-phase topology of the five-level converter 1 of the present invention can be obtained, as shown in fig. 3. The three-phase topology is composed of three single-phase topologies (marked as a phase, b phase and C phase) with the same structure, the direct current positive terminals of the first half bridge units (I) of each phase are connected together in parallel, the direct current negative terminals of the first half bridge units (I) of each phase are connected together in parallel, then the capacitors of the 3 first half bridge units (I) are connected together in parallel, and then the 3 capacitors are simplified into 1 common capacitor (C)_d1(ii) a Similarly, the dc positive terminal and the dc negative terminal of the second half-bridge unit (ii) of each phase are also connected in parallel, and the capacitances of the 3 second half-bridge units (ii) are also simplified to 1 common capacitance C_d3For the coupling capacitance C similarly_d2Also simplified to 1 common capacitance. Finally, the three phases of the converter 1 share the same DC bus and are connected to a common DC-side capacitor C_d1～C_d3Thus constituting a three-phase topology of the five-level converter 1. Note C_d1And C_d2Is N₁，C_d2And C_d3Is N₂。

description of the drawings: in order to make the converter output five-level voltage, a DC side capacitor C_d1The rated voltage is set to be DC bus voltage (V)_dc) One fourth of (2), a DC side capacitance C_d2The rated voltage is set to be one half of the DC bus voltage, and the DC side capacitor C_d3The rated voltage is set to be one fourth of the DC bus voltage, and each phase of the suspension capacitor C_f1xAnd C_f2xThe set rated voltage is one fourth of the voltage of the direct current bus.

It should be noted that the switching tube of the present invention may be all fully-controlled power semiconductor switching devices such as an Insulated Gate Bipolar Transistor (IGBT), an Integrated Gate Commutated Thyristor (IGCT), or a turn-off thyristor (GTO), and the present invention is not limited thereto.

Second, the improved five-level converter topology of the invention

As a further improvement of the invention, the figures are respectivelySwitch tube S of third half-bridge unit c in 1_x3' and fourth half-bridge unit_x4Replacement by a clamping diode D_x1And D_x2And keeping the connection mode of other switching tubes and capacitors unchanged to obtain the single-phase topology of the five-level converter 2, as shown in fig. 4. Further, the abc three phases are connected in parallel to a common dc bus in the same manner as the converter 1, and a three-phase topology of the five-level converter 2 as shown in fig. 5 is obtained.

Fig. 4 is a single-phase structure diagram of the five-level converter 2 of the present invention; fig. 5 is a three-phase configuration diagram of the five-level converter 2 of the present invention.

Modulation method of converter

Having described the topology of the converter in detail, this section will first introduce the proposed switching states of the five-level converter, explain how to control the switching on and off of the switching tubes to output five levels, and then further explain how to control the switching states to output a five-level waveform that varies sinusoidally based on carrier phase shift modulation.

The five-level converter of the invention needs to control the on and off of the switch tube in the converter according to the instruction value of the output reference voltage in the operation process to output V_dc、3V_dc/4、V_dc/2、V_dcThere are 5 different levels of/4 and 0. For the converter 1, the control signals of the two switching tubes in each half-bridge unit are complementary all the time, that is, one switching tube is on, the other switching tube is off, and vice versa, so that only one control signal is needed for the two switching tubes of each half-bridge unit; meanwhile, in the converter 1, the control signals of the first half-bridge unit I and the second half-bridge unit II are the same, and the control signals of the third half-bridge unit III and the fourth half-bridge unit IV are the same, so that only 3 independent switch control signals are needed for 5 half-bridge units; in addition to the connecting switch tube S_x6And S_x6' complementary conduction all the time, which requires a separate control signal. In summary, the converter 1 needs four independent switch control signals to respectively control S_x1、S_x6、S_x3、S_x5Control of the other switching tubesThe control signal can be obtained by taking the same or different from the four paths of signals.

Thus, a switching state table corresponding to the inverter 1 outputting 5 different levels is obtained, as shown in table 1, where "1" indicates that the switching tube is on, and "0" indicates that the switching tube is off. At the same time, the table also shows the influence of each switch state on the current and voltage of each capacitor in the converter, i_cf1x、i_cf2x、i_N1x、i_N2x、V_cf1x、V_cf2x、V_d1、V_d2And V_d3Respectively representing each phase flowing through the floating capacitor C_f1xCurrent of (2) flowing through the floating capacitor C_f2xCurrent of each phase, N₁Current at point, N per phase₂Current at point, suspension capacitance per phase C_f1xVoltage, suspension capacitance C_f2xVoltage, DC side capacitance C_d1Voltage, DC side capacitance C_d2Voltage and dc side capacitance C_d3The voltage, "↓" indicates that the corresponding capacitor is charged, the voltage of the capacitor rises, "↓" indicates that the corresponding capacitor is discharged, the voltage of the capacitor decreases, and "-" indicates that the voltage of the capacitor is not influenced. It is particularly emphasized that the effect of the switching states shown in Table 1 is given by the assumed load current i_xIf the load current is less than 0, the effect will be opposite to the result in table 1.

Table 1 switching state table of five-level converter 1

The converter 1 outputs 3V_dcTable 1 illustrates the case of a/4 level and a switch state of B1: when the switching state of the converter 1 is B1, the switch tube S_x1、S_x3、S_x5Conducting, switching tube S_x6Turning off; corresponding switch tube S_x2、S_x4、S_x6' conducting, switching tube S_x1'、S_x2'、S_x3'、S_x4'、S_x5' turn off; FIG. 6 shows the current flow path, and the solid red line indicates the current flow when the current is greater than 0And a red dotted line indicates that the current flows into the inverter when the current is less than 0. N is a radical of₂Point potential is V_dcV at two voltages_dcAfter the/4 suspension capacitor, the 3V is finally output at the output end of the converter_dcThe/4 level.

Assuming that the load current is greater than 0 at this time, the effect of switch state B1 on the converter capacitor voltage is: flows through the floating capacitor C_f1xAnd C_f2xWill be-i_xThen the two floating capacitors will be discharged and the voltage will decrease; outflow of N₂The current at the point will be i_xA capacitor C on the DC side_d1And C_d2Will be charged and the voltage will rise, the capacitor C_d3Will be discharged and the voltage will decrease. The analysis of the current less than 0 and other switch states is the same and will not be described again.

The control mode of each switch tube of the converter 2 is similar to that of the converter 1, and the difference lies in that two switch tubes in the third half-bridge unit (c), (d) and (c) of the converter 2 are not complementarily conducted, but are S of the third half-bridge unit (c)_x3And S of the fifth half-bridge unit-_x3Complementary conducting, S of fifth half-bridge unit_x4And S of the fourth half-bridge unit_x4' complementary conduction, also only four independent switching signals are needed to obtain the switching state table thereof as shown in table 2, and the analysis method is the same as that of the converter 1 and will not be described.

Table 2 switching state table of five level converter 2

Since the two converters involved in the present invention are similar in control, the five-level converter 1 is only taken as an example here, and carrier phase shift modulation is used to illustrate how the converter can output a sinusoidally varying five-level waveform.

FIG. 7 shows the basic principle of five-level carrier phase-shift modulation, m_xThe amplitude of the sine reference voltage waveform can be set between 0 and 1, the amplitudes of the reference voltage waveforms of the abc three phases are consistent, and the reference voltage waveforms are mutually different in phase by 120 degrees; c_r1～C_r4Is fourThe phase difference of the triangular carrier wave with the amplitude of 1 is 90 degrees. By comparing the reference voltage value with the four paths of triangular carrier values, four paths of independent switching signals can be generated respectively: if the reference voltage value is larger than the triangular carrier wave value, a high level is generated, and the corresponding switch tube is conducted; if the reference voltage value is smaller than the triangular carrier wave value, a low level is generated, and the corresponding switch tube is turned off.

Combined with a reference voltage m_xCan be varied within a range of m_xDivided into four regions, namely-1 to-0.5, -0.5 to 0, 0 to 0.5 and 0.5 to 1. When m is_xBetween-1 and-0.5, 0 and V will be output by modulation_dcA/4 level; when m is_xWhen the voltage is between-0.5 and 0, V is output by modulation_dc/4 and V_dcA/2 level; when m is_xWhen the voltage is between 0 and 0.5, V is output by modulation _dc2 and 3V_dcA/4 level; when m is_xWhen the voltage is between 0.5 and 1, 3V is output by modulation _dc/4 and V_dcA level.

For example, when m_xIn the range of-1 to 0.5, as shown in fig. 8, assuming that the carrier frequency is much larger than the frequency of the reference voltage, the value of the reference voltage can be considered to remain unchanged within one carrier period. As can be seen, m_xAnd C_r1Comparison yields S_x1M of the switching signal_xAnd C_r2Comparison yields S_x6M of the switching signal_xAnd C_r3Comparison yields S_x3M of the switching signal_xAnd C_r4Comparison yields S_x5The final combination of the switching signals (D) corresponds to 5 switching states D1-D4 and E, and is expressed in the output voltage V_oxAbove is in line with V_dcTwo levels,/4 and 0. When m is_xIn other intervals, the same analysis can be done, whereby the converter is based on m_xA five-level waveform varying sinusoidally is output.

In addition, it can be seen from fig. 8 that the switch states D1-D4 obtained by the modulation method are always equal in duration in one carrier cycle, and all of them are V_dcOne fourth of the level duration/4. It is also theorized that the durations of the switch states B1-B4 in a carrier cycle will also be related to the phaseEtc. are 3V_dcOne fourth of the/4 level duration, the switch states C1-C4 are of equal duration and V in one carrier cycle_dcOne quarter of the level duration. Based on this conclusion, the next section will analyze the self-balancing characteristics of the capacitor voltages in the present converter.

Capacitance voltage equalization analysis

The section takes a five-level converter 1 as an example, and proves that the converter can realize self-balance of voltage of each capacitor under a carrier phase shift modulation method.

Assume that the reference command values of the three-phase voltages are

m_x＝m·sin(ωt-α) (1)

Wherein, subscript x represents a, b and c three phases; m is the amplitude of the modulation voltage; alpha represents the phase difference between the three phases, the phase a takes 0, the phase b takes 2 pi/3, and the phase c takes 4 pi/3.

The three-phase load current is set as:

i_x＝I·sin(ωt-α-θ) (2)

wherein I is the load current amplitude; and theta is the phase difference between the voltage and the current, namely the power factor angle.

As shown in fig. 9, according to the characteristics of the three-phase symmetrical waveform, the three-phase voltage and current has the same waveform every 120 degrees and the abc three-phase waveforms are alternately replaced every 120 degrees. Therefore, the state of each capacitor voltage of the converter only needs to be analyzed within the range of 120 degrees.

Consider the 120 ℃ range of- π/6 to π/2 for analysis using π/6 as a small fragment. Recording output level V_dc、3V_dc/4、V_dc/2、V_dcThe/4 and 0 are respectively 4, 3, 2, 1 and 0 levels, and the duty ratio corresponding to each level in one carrier wave period is D_4x、D_3x、D_2x、D_1xAnd D_0x. Then, the three-phase output level duty ratio shown in Table 3 can be obtained, for example, in the time period of- π/6 ~ 0, phase a only outputs 1 level and 2 level, and the duty ratio corresponding to 1 level is-2 m_aDuty ratio of 2 level is 1+ m_aAnd the duty cycle of the other levels is 0.

TABLE 3 Duty ratio of three-phase output level in-pi/6-pi/2 range

As mentioned above, each level may correspond to a different switch state during the modulation process, and the different switch states will have different effects on the voltages of the capacitors. Therefore, the change of each capacitance voltage in each pi/6 time period can be calculated by combining the table 1 and the table 3, and is characterized by the accumulated charge amount of the current on each capacitance, wherein the accumulated charge amount is larger than 0 to indicate that the capacitance voltage rises, the accumulated charge amount is smaller than 0 to reduce the capacitance voltage, and the accumulated charge amount is equal to 0 to keep the capacitance voltage unchanged. Note Q_N1a、Q_N2a、Q_N1b、Q_N2b、Q_N1c、Q_N2cRespectively represent three relative N of abc₁Point sum N₂Accumulated charge amount of dots, Q_cf1a、Q_cf2a、Q_cf1b、Q_cf2b、Q_cf1c、Q_cf2cRespectively representing three opposite suspension capacitors C of abc_f1xAnd C_f2xThe accumulated charge amount of (a). (for ease of reading, Table 1 is reproduced below)

For example, in the period of-pi/6-0, the a-phase outputs 1 level and 2 level, which correspond to the switching states D1-D2 and C1-C2, respectively, and it is known that the switching states D1-D4 each account for a quarter of the duration of 1 level, and the switching states C1-C2 each account for a quarter of the duration of 2 level, it can be obtained that the accumulated charge amount of the a-phase for the N1 point in the period of-pi/6-0 is:

wherein S is_iAnd indicating the influence mark of the ith switch state corresponding to the j level. According to Table 1, if the ith is onThe off state corresponds to an influence current of i_xThen S is_i1 is ═ 1; if the impact current corresponding to the ith switch state is-i_xThen S is_i-1; if the influence current corresponding to the ith switch state is 0, S_iSubstitution is simplified to 0:

in the same way, phase a is in phase C_f1aThe accumulated charge amount on is:

finally, the amount of charge accumulated on each capacitor at each time period for the three phases can be obtained as shown in table 4.

TABLE 4 accumulated charge amount on each capacitor at each time period in- π/6- π/2 for three-phase abc

In this 120 ° range, the amounts of charges accumulated at the point N1 in the three abc phases are summed:

the result shows that the total charge quantity accumulated by the three-phase unit at the point N1 is 0 in each 120-degree period, namely the potential at the point N1 is kept unchanged and is independent of the power factor angle; it can also be seen from Table 4 that Q is measured every pi/6 time period_N1xAnd Q_N2xAlways equal, the potential at the final point N2 will remain unchanged, i.e. the dc side capacitance C_d1～C_d3The voltage of (2) can realize self-balancing.

Table 4 also shows that Q is present every π/6 time period_cf1xAnd Q_N1xThe expressions are also always equal, Q_cf1xAnd Q_cf2xExpression assemblyOn the contrary, according to the characteristic that three-phase voltage and current are alternately replaced every 120 degrees, the total charge quantity accumulated in the range of 360 degrees by considering the suspension capacitor is as follows:

equation (7) shows that the floating capacitance C of each phase_f1xAnd C_f2xWill also remain constant during a power frequency cycle. The detailed mathematical analysis proves that the voltages of all capacitors of the five-level converter can realize self balance under the carrier phase shift modulation method.

In order to make the technical solution of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. The specific embodiments described herein are merely illustrative of the invention and are not intended to be limiting.

Examples

The single-phase structure of the five-level converter 1 according to the present invention is shown in fig. 1, and includes 5 half-bridge units, each of which is a three-port structure as shown in fig. 2. In the single-phase structure of the converter 1, the dc negative terminal of the first half-bridge unit (i) and the dc positive terminal of the second half-bridge unit (ii) are connected via a coupling capacitor (C)_d2The alternating current terminals of the first half bridge unit are connected through a connecting switch tube S_x6The second half-bridge unit II is connected with a DC positive terminal of the third half-bridge unit III, and an AC terminal of the second half-bridge unit II is connected with a switch tube S_x6The positive dc terminal and the negative dc terminal of the fifth half-bridge unit are connected to the ac terminals of the third half-bridge unit, the fifth half-bridge unit, and the fourth half-bridge unit. Unlike the other 4 half-bridge units, the capacitance of the fifth half-bridge unit (c) is omitted in the present structure.

Based on the single-phase topology, three-phase topologies with the same structure are connected in parallelThe connection may result in a three-phase topology of the five-level converter 1 of the invention, as shown in fig. 3. At this time, the dc positive terminal and the dc negative terminal of the first half-bridge unit (i) of each phase are connected in parallel, and the dc positive terminal and the dc negative terminal of the second half-bridge unit (ii) of each phase are also connected in parallel, so that the first half-bridge unit (i), the second half-bridge unit (ii) and the coupling capacitor (C) are connected in parallel_d2Finally, the capacitors are simplified into a common direct current side capacitor C_d1～C_d3And three phases share the same direct current bus. Meanwhile, the output ends of the three phases are connected with the same resistance inductance load, and the loads are connected by adopting a Y-shaped connection method.

Specific parameters of the converter are given in table 5 in conjunction with specific embodiments, when the dc bus voltage (V) is_dc) 11.2kV, the direct current side capacitance C_d1Rated voltage of V_dc/4＝2800V，C_d2Rated voltage of V_dc/2＝5600V，C_d3Rated voltage of V_dc2800V for each phase_f1xAnd C_f2xRated voltage of V_dcAnd/4 is 2800V. Rated withstand voltage of each switching tube is V_dcA 4500V 800A IGBT from the company infi may be used: FZ800R45KL3_ B5.

TABLE 5 converter specific parameters

As a further improvement of the invention, the switching tubes S of the third half-bridge unit (c) in FIG. 1 are respectively_x3' and fourth half-bridge unit_x4Replacement by a clamping diode D_x1And D_x2And keeping the connection mode of other switching tubes and capacitors unchanged to obtain the single-phase topology of the five-level converter 2, as shown in fig. 4. Further, the abc three phases are connected in parallel to a common dc bus in the same manner as the converter 1, and a three-phase topology of the five-level converter 2 as shown in fig. 5 is obtained. In practice, the converter parameters shown in Table 5 are still used, the capacitor rated voltage and the switch tube type are consistent with the converter 1, and the clamp is clampedRated withstand voltage of the diode is also V_dc2800V, an 4500V 400A diode from the company infi may be used: DD400S45KL3_ B5.

Since the five-level converter 2 and the five-level converter 1 are similar in structure and control, the converter 1 will be taken as an example only, and simulation is performed on the converter through MATLAB/Simulink, and the related simulation waveforms are obtained as shown in fig. 10.

In FIG. 10, three-phase output voltage V is shown from top to bottom_oa～V_ocThree-phase load current i_a～i_cDC side capacitor voltage V_d1～V_d3And abc three-phase suspension capacitor voltage V_cf1xAnd V_cf2x. As can be seen from the figure, the converter outputs a sinusoidal five-level PWM waveform and three-phase symmetrical sinusoidal current; DC side capacitor C_d1～C_d3All stabilized around their rated voltage, where V_d1And V_d3Undulate around 2800V, V_d2Fluctuated around 5600V; the three-phase suspension capacitor voltage is also stabilized at 2800V. As previously described, each capacitor voltage achieves self-balancing.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

It should be noted that, in the description of the present invention, the terms "first", "second", and the like are used for descriptive purposes only and for distinguishing similar objects, and no precedence between the two is considered as indicating or implying relative importance. In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many embodiments and many applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the present teachings should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are hereby incorporated by reference for all purposes. The omission in the foregoing claims of any aspect of subject matter that is disclosed herein is not intended to forego such subject matter, nor should the applicant consider that such subject matter is not considered part of the disclosed subject matter.

Claims

1. A five-level power electronic converter is characterized in that a single-phase structure of the converter comprises a half-bridge unit, a connection switch tube and a connection capacitor;

2. Five-level power electronic converter according to claim 1,

the first half-bridge unit, the second half-bridge unit, the third half-bridge unit and the fourth half-bridge unit respectively comprise two switching tubes and a capacitor, and the two switching tubes are connected in series and then connected in parallel with the capacitor;

3. Five-level power electronic converter according to claim 1,

the first half-bridge unit and the second half-bridge unit respectively comprise two switching tubes and a capacitor, and the two switching tubes are connected in series and then connected in parallel with the capacitor;

4. Five-level power electronic converter according to claim 2 or 3,

the rated voltage of the capacitor is one fourth of the voltage of the direct-current bus.

5. Five-level power electronic converter according to claim 2 or 3,

and the rated voltage of the connecting capacitor is one half of the voltage of the direct current bus.

6. Five-level power electronic converter according to claim 2 or 3,

the connecting switch tube and the switch tube are insulated gate bipolar transistors, integrated gate commutated thyristors or turn-off thyristors.

7. A method of modulating a five-level power electronic converter according to claim 2, characterized in that it comprises the steps of:

8. A method of modulating a five-level power electronic converter according to claim 3, characterized in that it comprises the following steps: