CN107204714A - Three level indirect matrix converters and control method - Google Patents

Abstract

The present invention provides a kind of three level indirect matrix converter and control method, and the three level indirect matrix converter includes：Input voltage selector, active third-harmonic zero-sequence voltage circuit and inverter；The input voltage selector input is connected with three-phase alternating-current supply, and output end is connected with both positive and negative polarity dc bus respectively；The upper and lower ends of the active third-harmonic zero-sequence voltage circuit are connected with the both positive and negative polarity dc bus respectively, the output end connection that the 3rd end is connected with the input and the inverter of the input voltage selector respectively；The inverter upper and lower ends are connected with the both positive and negative polarity dc bus respectively, and output end is connected with load.The present invention produces third-harmonic zero-sequence voltage inductive current by active third-harmonic zero-sequence voltage circuit and is injected separately into the input voltage selector and the inverter, so that neutral point average current remains zero, and then neutral point voltage self-balancing is ensure that, improve the quality of input-output wave shape.

Description

Three-level indirect matrix converter and control method

technical field

The present invention relates to the technical field of power conversion, and more specifically, to a three-level indirect matrix converter and a control method.

Background technique

With the rapid development of power electronics technology, the power electronics industry's requirements for voltage frequency and amplitude have become more diverse. Therefore, it has become an urgent need to convert the power supply voltage into a voltage with the frequency and amplitude required by the load to meet the diverse needs of the power electronics industry for voltage.

In the prior art, a matrix converter is mainly used to directly convert the power supply voltage into the voltage required by the load. Among them, the center point clamped matrix converter (NPC MC) is widely used in many occasions because of its ability to generate multi-level output voltages, high conversion efficiency, compact structure and simple clamping circuit.

However, when NPC MC is in normal operation, it is difficult to achieve a zero-average neutral point to maintain the balance of the neutral point potential. At the same time, there are also problems that the input power control range is limited and the modulation of the rectifier and inverter stages needs to be strictly synchronized. .

Contents of the invention

The present invention provides a three-level indirect matrix converter and a control method to overcome that in the prior art, when NPC MC is in normal operation, it is difficult to achieve a zero-average neutral point to maintain the balance of the neutral point potential. The input has no limited power control range and the rectification and inverter stages need to be strictly synchronized in the modulation.

According to a first aspect of the present invention, there is provided a three-level indirect matrix converter, comprising: an input voltage selector, an active third harmonic injection circuit and an inverter; the input terminal of the input voltage selector is connected to the three-phase AC Connected to the power supply, the output ends are respectively connected to the positive and negative DC busbars, which are used to convert the three-phase AC input voltage provided by the three-phase AC power supply into a six-pulse DC voltage and provide it to the active third harmonic injection circuit and all The inverter; the upper and lower ends of the active third harmonic injection circuit are respectively connected to the positive and negative DC bus bars, and the third end is respectively connected to the input end of the input voltage selector and the inverter connected to the output end of the inverter, used to generate the third harmonic to inject the inductor current and inject it into the input voltage selector and the inverter respectively; the upper and lower ends of the inverter are respectively connected to the positive and negative DC buses, and the The terminal is connected to a load, and is used to convert the six-pulse DC voltage into a target three-phase AC output voltage and provide it to the load.

In combination with the first possible implementation of the first aspect, in the second possible implementation, the active third harmonic injection circuit includes: a high-frequency single-phase half bridge and a third harmonic injection inductor; the third harmonic injection One end of the inductor is connected to the middle point of the high-frequency single-phase half-bridge, and the other end is respectively connected to the input voltage selector and the inverter.

In combination with the second possible implementation of the first aspect, in the third possible implementation, the input voltage selector includes: a bidirectional power frequency rectifier and three front-end bidirectional switches; the bidirectional power frequency rectifier and the three front-end One end of the bidirectional switch is connected to convert the three-phase AC input voltage provided by the three-phase AC power supply into a six-pulse DC voltage and provide it to the active third harmonic injection circuit and the inverter; One ends of the three front-end bidirectional switches are respectively connected to the three-phase AC power supply, and the other ends of the three front-end bidirectional switches are all connected to the other end of the third harmonic injection inductor, which is used to connect the third harmonic A first component of the wave injection inductor current is injected into the input voltage selector.

In combination with the third possible implementation of the first aspect, in the fourth possible implementation, the inverter includes: a T-type three-level inverter and three back-end bidirectional switches; the T-type three-level The inverters are respectively connected to one end of the three back-end bidirectional switches, and are used to convert the six-pulse DC voltage into a target three-phase AC output voltage and provide it to the load; one end of the three back-end bidirectional switches is also connected to the load respectively, and the other ends of the three back-end bidirectional switches are all connected to the other end of the third harmonic injection inductor, for injecting the second component of the third harmonic injection inductor current into the inverter device.

In combination with the third possible implementation of the first aspect, in a fifth possible implementation, the bidirectional power frequency rectifier includes six rectification switches; the six rectification switches are divided into three groups, each group has two rectification switches; The two rectifier switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; the connection point of the two rectifier switches in each group is connected to the three-phase AC power supply.

With reference to the fourth possible implementation of the first aspect, in the sixth possible implementation, the T-shaped three-level inverter includes six inverter switches; the six inverter switches are divided into three groups, each group two inverter switches; the two inverter switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; the series connection point of the two inverter switches in each group is connected to the three One end of the back-end bidirectional switch is connected one-to-one.

With reference to the third possible implementation of the first aspect, in a seventh possible implementation, the three-level indirect matrix converter further includes an input filter; the input filter includes three inductors and three capacitors; The above three inductors and three capacitors are divided into three groups, each group has an inductor and a capacitor, and one end of the inductor and capacitor in each group is connected in series; the three inductors are connected to the three-phase AC power supply; the three inductors and the capacitor The series connection point is connected to one end of the three front-end bidirectional switches; the other ends of the three capacitors are connected.

According to the second aspect of the present invention, there is provided a method for controlling the above three-level indirect matrix converter, the input voltage selector and the inverter are independently controlled, the method includes: based on the three-phase AC input voltage, controlling The state of each switch of the input voltage selector; based on the input side power factor correction requirement and the neutral point potential balance of the inverter, the active third harmonic injection circuit is controlled to generate the desired third harmonic injection inductor current; based on The target three-phase AC output voltage and modulation strategy control the state of each switch of the inverter. In combination with the first possible implementation of the second aspect, in the second possible implementation, the input side power factor correction requirement and the neutral point potential balance of the inverter control the active third harmonic injection circuit Generating the desired third harmonic injection inductance current includes: based on the input side power factor correction requirement and the neutral point potential balance of the inverter, using a controller to control the active third harmonic injection circuit to generate the desired third harmonic injection inductance current.

With reference to the second possible implementation of the second aspect, in a third possible implementation, the controlling the state of each switch of the inverter based on the target AC output voltage and the modulation strategy includes: based on the target AC Output voltage, obtain the duty cycle of each switch of the T-type three-level inverter; control the state of each switch of the inverter based on the target AC output voltage, the duty cycle and the modulation strategy .

In the three-level indirect matrix converter and control method proposed by the present invention, an active third harmonic injection circuit is added between the input voltage selector and the inverter, and the active third harmonic injection circuit generates a third harmonic The wave is injected into the inductor current and injected into the input voltage selector and the inverter respectively, so that the average current of the neutral point is kept at zero, thereby ensuring the self-balancing of the neutral point voltage and improving the quality of the output waveform. In addition, the use of a three-level indirect matrix converter can not only expand the input non-power control range, but also independently control the input voltage selector and inverter, reducing the difficulty of control.

Description of drawings

1 is a schematic structural diagram of a three-level indirect matrix converter according to an embodiment of the present invention;

2 is a schematic structural diagram of a three-level indirect matrix converter according to an embodiment of the present invention;

Fig. 3 is a circuit diagram of a three-level indirect matrix converter according to an embodiment of the present invention;

4 is a flowchart of a control method for a three-level indirect matrix converter according to an embodiment of the present invention;

5 is a flowchart of a control method for a three-level indirect matrix converter according to an embodiment of the present invention;

6 is a schematic diagram of the control of the third harmonic injection inductor current according to an embodiment of the present invention;

Fig. 7 is a control schematic diagram of the third harmonic injection inductor current according to an embodiment of the present invention;

8a and 8b are schematic diagrams of input and output waveforms under different modulation indices and output frequencies according to embodiments of the present invention;

9a and 9b are input waveforms under a non-uniform input power factor according to an embodiment of the present invention;

10a and 10b are neutral point potential balance waveform diagrams under different input power factors according to an embodiment of the present invention.

detailed description

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

Referring to Fig. 1, according to a first aspect of the present invention, a kind of three-level indirect matrix converter is provided, comprising: an input voltage selector, an active third harmonic injection circuit and an inverter; the input voltage selector input terminal It is connected to a three-phase AC power supply, and the output terminals are respectively connected to the positive and negative DC busbars, which are used to convert the three-phase AC input voltage provided by the three-phase AC power supply into a six-pulse DC voltage and provide it to the active third harmonic Injection circuit and the inverter; the upper and lower ends of the active third harmonic injection circuit are respectively connected to the positive and negative DC bus bars, and the third end is respectively connected to the input end of the input voltage selector and the The output terminal connected to the inverter is used to generate the third harmonic to inject the inductor current and inject it into the input voltage selector and the inverter respectively; the upper and lower ends of the inverter are respectively connected to the positive and negative DC The bus bar is connected, and the output end is connected to the load, which is used to convert the six-pulse DC voltage into a target three-phase AC output voltage and provide it to the load.

In the three-level indirect matrix converter proposed by the present invention, by adding an active third harmonic injection circuit to the three-level indirect matrix converter, the active third harmonic injection circuit generates a third harmonic injection inductor current and injects the three harmonics respectively into the The input voltage selector and the inverter keep the average current of the neutral point at zero, thereby ensuring the self-balancing of the neutral point voltage and improving the quality of the output waveform. In addition, the use of a three-level indirect matrix converter can not only expand the input non-power control range, but also independently control the input voltage selector and inverter, reducing the difficulty of control.

As an optional embodiment, the active third harmonic injection circuit includes: a high-frequency single-phase half-bridge and a third harmonic injection inductor; one end of the third harmonic injection inductor is connected to the high-frequency single-phase half-bridge point connection, and the other end is respectively connected to the input voltage selector and the inverter.

As an optional embodiment, the input voltage selector includes: a bidirectional power frequency rectifier and three front-end bidirectional switches; the bidirectional power frequency rectifier is respectively connected to one end of the three front-end bidirectional The three-phase AC input voltage provided by the three-phase AC power supply is converted into a six-pulse DC voltage and provided to the active third harmonic injection circuit and the inverter; one end of the three front-end bidirectional switches is also connected to the The three-phase AC power supply is connected, and the other ends of the three front-end bidirectional switches are all connected to the other end of the third harmonic injection inductor for injecting the first component of the third harmonic injection inductor current into the input voltage selector.

As an optional embodiment, the inverter includes: a T-type three-level inverter and three back-end bidirectional switches; the T-type three-level inverter is connected with the three back-end bidirectional switches One end is connected to convert the six-pulse DC voltage into the target three-phase AC output voltage and provide it to the load; one end of the three rear-end bidirectional switches is also connected to the load respectively, and the three rear-end bidirectional The other ends of the switches are connected to the other end of the third harmonic injection inductance for injecting the second component of the third harmonic injection inductor current into the inverter.

As an optional embodiment, the bidirectional power frequency rectifier includes six rectification switches; the six rectification switches are divided into three groups, each group has two rectification switches; two rectification switches in each group are connected in series to form three The three series circuits are connected in parallel; the series connection points of the two rectifier switches in each group are connected to the three-phase AC power supply.

As an optional embodiment, the T-type three-level inverter includes six inverter switches; the six inverter switches are divided into three groups, each group has two inverter switches; two inverter switches in each group The three inverter switches are connected in series to form three series circuits, and the three series circuits are connected in parallel; the series connection points of the two inverter switches in each group are connected one-to-one with one end of the three back-end bidirectional switches.

As an optional embodiment, the three-level indirect matrix converter also includes an input filter; the input filter includes three inductors and three capacitors; the three inductors and three capacitors are divided into three groups , one inductor and one capacitor in each group, one end of the inductor and capacitor in each group is connected in series; three inductors are connected to the three-phase AC power supply; three inductors and capacitor series connection points are connected to one end of three front-end bidirectional switches ; The other ends of the three capacitors are connected.

2 and 3, an embodiment of the present invention provides a three-level indirect matrix converter, the three-level indirect matrix converter includes: an input filter, an input voltage selector, an active third harmonic injection circuit and Inverter; the input end of the input filter is connected to the three-phase AC power supply, and the output end is connected to the input end of the input voltage selector, which is used to filter out the high-order harmonics generated by the three-level indirect matrix converter Wave components, and the three-phase AC input voltage provided by the three-phase AC power supply is sent to the input voltage selector; the output terminals of the input voltage selector are respectively connected to the positive and negative DC bus bars for connecting the three Phase AC input voltage is converted into six-pulse DC voltage and provided to the active third harmonic injection circuit and the inverter; the upper and lower ends of the active third harmonic injection circuit are respectively connected to the positive and negative DC The bus bar is connected, and the third terminal is respectively connected to the input terminal of the input voltage selector and the output terminal connected to the inverter, and is used to generate the third harmonic and inject the inductor current into the input voltage selector and the input voltage selector respectively. Inverter; the upper and lower ends of the inverter are respectively connected to the positive and negative DC buses, and the output end is connected to the load, which is used to convert the six-pulse DC voltage into a target three-phase AC output voltage and provide it to the the above load.

In this embodiment, the input filter includes three inductors LF and three capacitors _CF , and the input voltage selector includes a bidirectional power frequency rectifier and three front-end bidirectional switches ( _{S ay} , S _by , S _cy ), the active third harmonic injection circuit includes a high-frequency single-phase half-bridge and third harmonic injection inductance L _y , and the inverter includes a T-type three-level inverter and three back-end bidirectional switches (S _ro , S _by , S _cy ).

The three inductors and three capacitors of the input filter are divided into three groups, each group has an inductor and a capacitor, and one end of the inductor and capacitor in each group is connected in series; the three inductors are connected to the three-phase AC power supply; the three The series connection point of the inductor and the capacitor is connected one-to-one with one end of the three front-end bidirectional switches of the input voltage selector at points a, b, and c; the other ends of the three capacitors are connected to each other at m.

In the specific working process, the input filter is used to filter out the high-order harmonic components generated by the three-level indirect matrix converter, and avoid the high-order harmonic components generated by the three-level indirect matrix converter from being transmitted to the external network interference, and deliver the three-phase AC input voltage ( _usa , u _sb , u _sc ) and the three-phase sinusoidal input current ( _{ia, i b , ic} ) to the input voltage selector.

The bidirectional power frequency rectifier of the input voltage selector includes six rectification switches (S _a+ , S _b+ , S _c+ , S _a- , S _b- , S _c- ); the six rectification switches are divided into three groups, Each group has two rectifier switches; the two rectifier switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; the series connection point of the two rectifier switches in each group is connected to the input voltage selector One end of the three front-end bidirectional switches is connected one-to-one to points a, b, c; the parallel connection point of the three series circuits is connected to the two ends of the high-frequency single-phase half-bridge of the active third harmonic injection circuit One-to-one connection to p,n.

The other ends of the three front-end bidirectional switches are all connected to the other end of the third harmonic injection inductor at o.

In a specific working process, the bidirectional power frequency rectifier is used to convert the three-phase AC input voltage into a six-pulse DC voltage and provide it to the third harmonic injection circuit and the inverter. The three front-end bidirectional switches are used to inject the first component of the third harmonic injection inductor current into the input voltage selector.

Specifically, each rectifying switch of the bidirectional power frequency rectifier is composed of an IGBT transistor and a diode connected in antiparallel, that is, the collector of the IGBT transistor is connected to the cathode of the diode, and the emitter of the IGBT transistor is connected to the anode of the diode. In each working cycle, among the six rectifying switches of the bidirectional power frequency rectifier, only the rectifying switches with the maximum and minimum instantaneous input voltage values are turned on to provide six-pulse direct current for the third harmonic injection circuit and the inverter , the other four rectifier switches are disconnected; the front-end bidirectional switch corresponding to the intermediate value of the instantaneous input phase voltage is closed, and the third harmonic is injected into the inductor current for the input voltage selection circuit to achieve neutral point voltage balance . Also, in a diode-conducting rectifier switch, the IGBT transistor in antiparallel to the conducting diode is turned off. Each switch of the input voltage selector performs phase commutation at a power frequency.

The high-frequency single-phase half-bridge includes two high-frequency bidirectional switches (S _y+ , S _y- ) connected in series, and each high-frequency bidirectional switch is composed of an IGBT transistor and a diode connected in antiparallel, that is, the collector of the IGBT transistor and the diode The cathode of the IGBT transistor is connected to the anode of the diode.

One end of the third harmonic injection inductor is connected to the series connection point of two high-frequency bidirectional switches, and the other end is also connected to one end of the three back-end bidirectional switches of the inverter.

By controlling the two high-frequency bidirectional switches in the active third harmonic injection circuit, desired third harmonic injection inductor current can be generated. The third harmonic injects the first component of the inductor current, the injection current, and the second component, the neutral point current, respectively into the input voltage selector through the closed front-end bidirectional switch, and into the inverter through the closed rear-end bidirectional switch device. The desired third harmonic injection inductor current generated by the active third harmonic injection circuit is injected into the input voltage selector and the inverter respectively, so as to realize input power factor correction and neutral point voltage balance.

The T-type three-level inverter of the inverter includes six inverter switches (S _r+ , S _s+ , S _t+ , S _r- , S _s- , S _t- ); the six inverter switches Divided into three groups, each group has two inverter switches; two inverter switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; two inverter switches in each group are connected in series The points are connected one-to-one to one end of the three back-end bidirectional switches, and are also connected to the load at points r, s, and t to provide the load with a target three-phase AC output voltage with variable frequency and amplitude; the three The parallel connection points of the two series circuits are respectively connected one-to-one with the two ends of the high-frequency single-phase half-bridge of the active third harmonic injection circuit.

One end of the three back-end bidirectional switches of the inverter is connected to points r, s, and t in series with the two inverter switches in each group, and the other ends are connected to the third harmonic injection inductance the other end of the connection.

The T-type three-level inverter is used to convert the six-pulse DC voltage into a target three-phase AC output voltage and provide it to the load; the three back-end bidirectional switches are used to inject the third harmonic A second component of inductor current is injected into the inverter.

Referring to Fig. 4, according to the second aspect of the present invention, a method for controlling the above-mentioned three-level indirect matrix converter is provided, the input voltage selector and the inverter are independently controlled, the method includes: based on the three-phase AC Input voltage, control the state of each switch of the input voltage selector; based on the power factor correction requirement on the input side and the neutral point potential balance of the inverter, control the active third harmonic injection circuit to generate the desired third harmonic injection Inductor current: based on the target three-phase AC output voltage and modulation strategy, controlling the state of each switch of the inverter.

In the three-level indirect matrix converter control method proposed by the present invention, by adding an active third harmonic injection circuit to the three-level indirect matrix converter, the active third harmonic injection circuit generates third harmonic injection inductor current and injects them respectively The input voltage selector and the inverter keep the neutral point average current at zero, thereby ensuring the self-balancing of the neutral point voltage and improving the quality of the output waveform. In addition, the use of a three-level indirect matrix converter can not only expand the input non-power control range, but also independently control the input voltage selector and inverter, reducing the difficulty of control.

As an optional embodiment, the power factor correction requirement on the input side is balanced with the neutral point potential of the inverter, and controlling the active third harmonic injection circuit to generate the desired third harmonic injection inductor current includes: based on the input The side power factor correction requirement and the neutral point potential balance of the inverter are balanced, and a controller is used to control the active third harmonic injection circuit to generate a desired third harmonic injection inductor current.

As an optional embodiment, the controlling the state of each switch of the inverter based on the target AC output voltage and the modulation strategy includes: obtaining the T-type three-level inverter based on the target AC output voltage. The duty cycle of each switch of the inverter; based on the target AC output voltage, the duty cycle and the modulation strategy, the state of each switch of the inverter is controlled.

Referring to Fig. 5, based on the above-mentioned embodiment corresponding to Fig. 4, the present invention provides a control method of a three-level indirect matrix converter, the method is used to control the indirect matrix conversion as described above, the method includes: based on The three-phase AC input voltage controls the state of each switch of the input voltage selector; based on the input side power factor correction requirement and the neutral point potential balance of the inverter, a controller is used to control the active third harmonic The wave injection circuit generates the expected third harmonic injection inductor current; based on the target AC output voltage, the duty ratio of each switch of the T-type three-level inverter is obtained; based on the target AC output voltage, the duty cycle ratio and the modulation strategy, and control the state of each switch of the inverter.

In this embodiment, the state of each switch of the input voltage selector is controlled according to Table 1. Table 1 is a state table of six rectifying switches and three front-end bidirectional switches in the bidirectional power frequency rectifier of the input voltage selector.

Table 1

Among them, "1" means closed, and "0" means open. θ _sa is the phase of the input phase voltage u _sa . Sectors are defined as follows:

The interval of u _sa >u _sb >u _sc is set as sector Ⅰ;

The interval of u _sb >u _sa >u _sc is set as sector II;

The interval of u _sb >u _sc >u _sa is set as sector III;

The interval of u _sc >u _sb >u _sa is set as sector IV;

The interval of u _sc >u _sa >u _sb is set as sector Ⅴ;

The interval of u _sa >u _sc >u _sb is set as sector VI;

Taking sector Ⅰ as an example, at this time, u _sa > _usb >us _sc , the rectifier bidirectional switch in the input voltage selector

S _a+ , S _c- and the front-end bidirectional switch S _by are closed and turned on. At this point, node a is connected to p, node b is connected to o, and node c is connected to n. The input line voltages u _ab =u _sa -u _sb and u _bc = _usb -u _sc respectively provide the upper end DC power supply voltage u _po and the lower end DC power supply voltage u _on of the split DC bus voltage. DC link voltage u _pn =u _sa -u _sb -u _sc . After sectors I-VI in turn, the DC bus voltages are: u _sa -u _sb -u _sc , u _sb -u _sa -u _sc , u _sb -u _sc -u _sa , u _sc -u _sb -u _sa , u _sc -u _sa -u _sb , u _sa -u _sc -u _sb , that is, a 6-pulse split intermediate DC bus voltage is generated.

In this embodiment, since the correction of the power factor at the input side of the three-level indirect matrix converter and the balance of the neutral point voltage need to be realized by synthesizing the correct third harmonic and injecting the inductor current. The third harmonic injection inductor current is the current flowing through the inductor L _y . The mathematical model of the third harmonic injection circuit can be described as follows:

Among them, u _Ly is the voltage applied to the third harmonic injection inductor _{Ly, and i y is} the inductor current. Therefore, the control of the third harmonic injection circuit can realize the control of the inductor current through the control of the inductor voltage u _Ly . Specifically, by performing Laplace transformation on the above formula, it can be found that the changed inductor current can be obtained from the transformed inductor voltage and the transfer function G _p (s)=1/L _y s.

Referring to FIG. 6, FIG. 6 is a control diagram of the third harmonic injection circuit. Specifically, based on the magnitude and phase angle of the median input phase voltage, the active power of the three-level indirect matrix converter, and the required input shift angle, the expected injection current of the input voltage selector is obtained Obtain the expected neutral point current based on the average value of the neutral point current of the T-type three-level inverter Based on desired injection current and the desired neutral current Obtaining the Reference Third Harmonic Injected Inductor Current Through the combination of the PI controller and the feedforward term, the actual third harmonic injection inductor current is adjusted to the reference third harmonic injection inductor current, so that the average current value at the neutral point o is 0, and then the balance of the neutral point voltage is realized .

In this embodiment, the inverter is modulated based on the carrier.

First, assuming a three-phase modulated signal, the desired output phase voltage with respect to the neutral point of the star-connected load is:

Where u _rN , u _sN and u _tN are the expected output phase voltages relative to the neutral point of the star-connected load; U _om , ω _o and represent the amplitude, angular frequency and initial phase of the modulating signal, respectively.

To maximize the utilization of the DC link voltage, the desired output phase voltage relative to the neutral point of the star-connected load is modified by adding a zero-sequence voltage to obtain the output phase voltage relative to the virtual midpoint of the DC link:

Where u _rO , u _sO and u _tO represent the output phase voltages relative to the virtual midpoint of the DC link, u _NO is the zero-sequence voltage, and max() and min() are operators for maximum and minimum values.

In order to maximize the utilization of the DC bus voltage, an additional zero-sequence voltage is introduced for compensation, and the expected output phase voltage with the neutral point of the split DC power supply as the reference is obtained:

Among them, u _ro , u _so and u _to are the expected output phase voltages relative to the neutral point of the split DC power supply, and u _Oo is the additional zero-sequence voltage.

For the convenience of numerical implementation, the desired output phase voltages referenced to the neutral point of the split DC power supply are normalized according to the upper and lower DC voltages of the split DC power supply, respectively:

in, with is the normalized expected output phase voltage with respect to the neutral point of the split DC power supply.

Obtain the duty cycle based on the normalized desired output phase voltages referenced to the neutral point of the split DC supply:

Among them, d _rp , d _sp , d _tp represent the duty cycle of the switch on the output r, s and t phase respectively; d _rn , d _sn , d _tn represent the duty cycle of the switch on the output r, s and t phase respectively.

For the modulation strategy of the carrier of the three-level NPC inverter (including the T-type three-level inverter), there are two carrier waves, which makes the realization of the modulation very flexible. Modulation strategies include but are not limited to: in-phase stacked carrier (PD) scheme, reversed phase stacked carrier scheme and interleaved stacked carrier scheme. Because in this embodiment, the PD scheme has the advantage of lower output voltage harmonic distortion. Therefore, this embodiment selects the PD scheme. Figure 7 shows the Schematic diagram of the PD modulation scheme and switching sequence for the case of , where Ts and fs are the switching period and switching frequency, respectively. Through this carrier modulation, a three-phase three-level output voltage can be obtained.

In order to verify that the present invention realizes the effectiveness of the neutral point voltage balance by introducing third harmonics into the inductor current, the present invention has carried out the following simulation experiments:

Fig. 8a shows the waveform generated by the three-level indirect matrix converter when the modulation index is mi=0.5, the output frequency is fo=50Hz, and the expected input displacement angle is 0. Fig. 8b shows the waveform generated by the three-level indirect matrix converter when the modulation index is mi=0.9, the output frequency is fo=60Hz, and the expected input displacement angle is 0.

The waveforms shown in Figure 8a include input phase voltage u _sa , input current ia , output line voltage u _rs and output current _{i r} . Except for a slight phase lead caused by the capacitive current absorbed by the filter capacitor, the input current is almost sinusoidal, in phase with the input phase voltage, and the resulting three-level output line voltage and sinusoidal output current. This is because from the point of view of the SVM, it only uses the small, medium and zero vectors in the output voltage vector to synthesize the expected voltage at low modulation index. Unlike the three-level output line voltage in Figure 8a, it produces a distinct five-level output line voltage in Figure 8b. This is because at high modulation index it utilizes 5 different levels of vectors to synthesize the output voltage, but also achieves the desired characteristics of sinusoidal input and output currents and unity power factor on the input side.

Figure 9a and Figure 9b show the wide-range input reactive power control results of the three-level indirect matrix converter. Among them, Figure 9a shows the current generated on the input side when the amplitude of the input reactive current reference is 4A, and the expected input displacement angle is π/2; Figure 9b shows that the amplitude of the input reactive current reference is 4A, and the expected input The current generated on the input side when the displacement angle is -π/2. From Figure 9a and Figure 9b, it can be found that a pure reactive current with the required magnitude is generated on the input side.

In order to verify the effectiveness of the developed algorithm for balancing the neutral point voltage, the ability of the three-level indirect matrix converter to balance the neutral point potential under different input power factors was tested. The test results are shown in Figure 10a and Figure 10b. Fig. 10a shows the waveform generated by the three-level indirect matrix converter when the modulation coefficient is mi=0.9, the output frequency is fo=50Hz, and the input displacement angle is π/12. Fig. 10b shows the waveform generated by the three-level indirect matrix converter when the modulation coefficient is mi=0.9, the output frequency is fo=50Hz, and the input displacement angle is -π/12.

First, do not start the control of the active third harmonic injection circuit, start after 0.1s. It can be seen from Figure 10a and Figure 10b that when the control of the active third harmonic injection circuit is not activated, the upper and lower DC power supply voltages are uneven, and the input current is seriously distorted. This is due to the fact that the neutral point current generated by the inverter directly flows into the input side of the three-level indirect matrix converter, which causes the input current of the three-level indirect matrix converter and the capacitor voltage in the input filter to be in the form of low-order harmonics Distortion, voltage division DC power supply unbalanced. However, the sinusoidal output is not distorted because the modulation signal of the inverter is compensated according to the real-time DC link voltage. When the balancing algorithm is activated, the neutral point current is fully compensated by the third harmonic injection circuit, achieving a sinusoidal input current as well as balanced upper and lower DC supply voltages. In addition, the amplitude of the average injected current i _j is reduced, which is beneficial to reduce power loss.

Finally, the method of the present invention is only a preferred embodiment, and is not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A three-level indirect matrix converter is characterized in that, comprising: an input voltage selector, an active third harmonic injection circuit and an inverter; The input terminal of the input voltage selector is connected to the three-phase AC power supply, and the output terminal is respectively connected to the positive and negative DC bus bars, and is used to convert the three-phase AC input voltage provided by the three-phase AC power supply into a six-pulse DC voltage and provide to said active third harmonic injection circuit and said inverter; The upper and lower ends of the active third harmonic injection circuit are respectively connected to the positive and negative DC bus bars, and the third end is respectively connected to the input end of the input voltage selector and the output end connected to the inverter, for generating a third harmonic to inject inductor current and injecting it into the input voltage selector and the inverter respectively; The upper and lower ends of the inverter are respectively connected to the positive and negative DC buses, and the output end is connected to a load for converting the six-pulse DC voltage into a target three-phase AC output voltage and providing it to the load. 2. The three-level indirect matrix converter according to claim 1, wherein the active third harmonic injection circuit comprises a high-frequency single-phase half-bridge and a third harmonic injection inductor; One end of the third harmonic injection inductor is connected to the middle point of the high-frequency single-phase half-bridge, and the other end is respectively connected to the input voltage selector and the inverter. 3. The three-level indirect matrix converter according to claim 2, wherein the input voltage selector comprises a bidirectional power frequency rectifier and three front-end bidirectional switches; The bidirectional power frequency rectifiers are respectively connected to one end of the three front-end bidirectional switches, and are used to convert the three-phase AC input voltage provided by the three-phase AC power supply into a six-pulse DC voltage and provide it to the active third harmonic wave injection circuit and said inverter; One end of the three front-end bidirectional switches is also respectively connected to the three-phase AC power supply, and the other ends of the three front-end bidirectional switches are all connected to the other end of the third harmonic injection inductor, which is used to connect the three-phase A first component of the harmonic injection inductor current is injected into the input voltage selector. 4. The three-level indirect matrix converter according to claim 3, wherein the inverter comprises a T-type three-level inverter and three back-end bidirectional switches; The T-type three-level inverter is respectively connected to one end of three back-end bidirectional switches for converting the six-pulse DC voltage into a target three-phase AC output voltage and providing it to the load; One end of the three back-end bidirectional switches is also respectively connected to the load, and the other ends of the three back-end bidirectional switches are connected to the other end of the third harmonic injection inductor for injecting the third harmonic A second component of inductor current is injected into the inverter. 5. The three-level indirect matrix converter according to claim 3, wherein the bidirectional power frequency rectifier comprises six rectification switches; The six rectification switches are divided into three groups, and each group has two rectification switches; The two rectifier switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; The series connection point of the two rectifier switches in each group is connected to the three-phase AC power supply. 6. The three-level indirect matrix converter according to claim 4, wherein the T-type three-level inverter comprises six inverter switches; The six inverter switches are divided into three groups, each group has two inverter switches; Two inverter switches in each group are connected in series to form three series circuits, and the three series circuits are connected in parallel; The series connection points of the two inverter switches in each group are connected one-to-one with one end of the three back-end bidirectional switches. 7. three-level indirect matrix converter according to claim 3, is characterized in that, described three-level indirect matrix converter also comprises input filter; The input filter includes three inductors and three capacitors; The three inductors and the three capacitors are divided into three groups, each group has an inductor and a capacitor, and one end of the inductor and capacitor in each group is connected in series; Three inductors are connected to the three-phase AC power supply; Three inductors and capacitors are connected in series with one end of three front-end bidirectional switches; The other ends of the three capacitors are connected. 8. A method for controlling the three-level indirect matrix converter described in any one of claims 1-7, wherein the input voltage selector and the inverter are independently controlled; the method comprises: controlling the state of each switch of the input voltage selector based on the three-phase AC input voltage; Based on the input side power factor correction requirement and the neutral point potential balance of the inverter, controlling the active third harmonic injection circuit to generate a desired third harmonic injection inductor current; Based on the target three-phase AC output voltage and modulation strategy, the state of each switch of the inverter is controlled. 9. The method according to claim 8, characterized in that, the input side power factor correction requirement and the neutral point potential balance of the inverter control the active third harmonic injection circuit to generate the desired third harmonic The injected inductor current consists of: Based on the power factor correction requirement on the input side and the neutral point potential balance of the inverter, a controller is used to control the active third harmonic injection circuit to generate a desired third harmonic injection inductor current. 10. The method according to claim 8, wherein the controlling the state of each switch of the inverter based on the target AC output voltage and the modulation strategy comprises: Obtaining the duty cycle of each switch of the T-type three-level inverter based on the target AC output voltage; Based on the target AC output voltage, the duty cycle and the modulation strategy, the state of each switch of the inverter is controlled.