阅读说明：本技术 三电平逆变器控制方法及pcs系统 (Three-level inverter control method and PCS system ) 是由 林伟民 陈海森 黄凯伦 焦保帅 于 2021-08-23 设计创作，主要内容包括：本发明适用于电力电子技术领域,提供了一种三电平逆变器控制方法及PCS系统,上述方法包括：获取逆变器的第一相的逆变电流的直流分量及逆变器的第二相的逆变电流的直流分量；根据第一相的逆变电流的直流分量和第二相的逆变电流的直流分量,确定第三相的逆变电流的直流分量；分别根据第一相的逆变电流的直流分量、第二相的逆变电流的直流分量及第三相的逆变电流的直流分量,对逆变器进行控制。本发明根据两相的逆变电流的直流分量确定第三相的逆变电流的直流分量,基于三相逆变电流的直流分量的关联性,整体对逆变器进行调节,提高了调节精度,母线不平衡问题得到了有效的改善。(The invention is suitable for the technical field of power electronics, and provides a three-level inverter control method and a PCS system, wherein the method comprises the following steps: acquiring a direct-current component of an inverter current of a first phase of an inverter and a direct-current component of an inverter current of a second phase of the inverter; determining a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase; and controlling the inverter according to the direct current component of the inverter current of the first phase, the direct current component of the inverter current of the second phase and the direct current component of the inverter current of the third phase. The method determines the direct-current component of the inverter current of the third phase according to the direct-current components of the inverter currents of the two phases, integrally adjusts the inverter based on the relevance of the direct-current components of the inverter currents of the three phases, improves the adjustment precision, and effectively improves the problem of bus imbalance.)

1. The control method of the three-level inverter is characterized in that the output end of the inverter is connected with a transformer; wherein the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the method comprises the following steps:

acquiring a direct-current component of an inverter current of a first phase of the inverter and a direct-current component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

determining a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and controlling the inverter according to the direct current component of the inverter current of the first phase, the direct current component of the inverter current of the second phase and the direct current component of the inverter current of the third phase.

2. The three-level inverter control method according to claim 1, wherein the determining the dc component of the inverter current of the third phase from the dc component of the inverter current of the first phase and the dc component of the inverter current of the second phase comprises:

and subtracting the direct-current component of the inverter current of the second phase from the direct-current component of the inverter current of the first phase to obtain the direct-current component of the inverter current of the third phase.

3. The method of controlling a three-level inverter according to claim 1, wherein the controlling the inverter based on the dc component of the inverter current of the first phase, the dc component of the inverter current of the second phase, and the dc component of the inverter current of the third phase, respectively, comprises:

acquiring the inversion voltage of each phase of the inverter;

and aiming at each phase of the inverter, controlling the phase of the inverter according to the inversion voltage of the phase and the direct current component of the inversion current of the phase.

4. The method of claim 3, wherein said controlling the phase of the inverter according to the inverted voltage of the phase and the dc component of the inverted current of the phase comprises:

inputting the direct current component of the inverter current of the phase into the corresponding first PI controller to obtain a voltage reference value of the phase;

subtracting the inversion voltage of the phase from the voltage reference value of the phase to obtain a voltage difference value of the phase;

and inputting the voltage difference value of the phase into the corresponding second PI controller to obtain the control quantity of the phase, and controlling the phase of the inverter according to the control quantity of the phase.

5. The three-level inverter control method according to any one of claims 1 to 4, characterized in that the sampling frequency f of the DC component of the three-phase inverter current of the inverter is such that_sComprises the following steps:

f_s＝10*f

wherein f is the frequency of the inverter voltage of the inverter.

6. The three-level inverter control method according to any one of claims 1 to 4, wherein the inverter includes: a three-phase full-bridge inverter circuit and a three-phase LC filter circuit;

the input end of the three-phase full-bridge inverter circuit is used for being connected with a direct current power supply, and the output end of the three-phase full-bridge inverter circuit is connected with the input end of the three-phase LC filter circuit;

and the output end of the three-phase LC filter circuit is connected with the output end of the inverter.

7. The control device of the three-level inverter is characterized in that the output end of the inverter is connected with a transformer; wherein the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the device comprises:

the direct current component acquisition module is used for acquiring a direct current component of an inverter current of a first phase of the inverter and a direct current component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

the direct-current component determining module is used for determining the direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and the control module is used for controlling the inverter according to the direct-current component of the inverter current of the first phase, the direct-current component of the inverter current of the second phase and the direct-current component of the inverter current of the third phase.

8. A terminal device comprising a memory, a processor and a computer program stored in said memory and executable on said processor, characterized in that said processor implements the steps of the three-level inverter control method according to any one of claims 1 to 6 when executing said computer program.

9. A computer-readable storage medium, in which a computer program is stored which, when being executed by a processor, carries out the steps of the method for controlling a three-level inverter according to any one of claims 1 to 6.

10. A PCS system comprising: a three-level inverter, a battery and the terminal device of claim 8;

the input end of the three-level inverter is connected with the battery, the output end of the three-level inverter is used for being connected with external equipment, and the control end of the three-level inverter is connected with the terminal equipment.

Technical Field

The invention belongs to the technical field of power electronics, and particularly relates to a three-level inverter control method and a PCS system.

Background

When the output end of a Power Conversion System (PCS) is connected to a transformer, when a small dc component exists on the output side, the small dc component on the output side of the PCS acts on the transformer to cause a large dc component of the output current due to the small dc impedance of the transformer, thereby causing an imbalance of the midpoint voltage of the bus.

In the prior art, the direct current components of three-phase inverter currents are generally sampled respectively, and the direct current components of each phase of PCS inverter currents are adjusted according to the sampling values. However, because the current sampling frequency is limited, the accuracy of the control method is not enough, and the improvement effect of the bus imbalance problem is not good.

Disclosure of Invention

In view of this, embodiments of the present invention provide a three-level inverter control method and a PCS system, so as to solve the problem in the prior art that the problem of bus imbalance is not well improved when loop control is adopted to respectively control the dc components of each phase.

A first aspect of an embodiment of the present invention provides a method for controlling a three-level inverter, where an output terminal of the inverter is connected to a transformer; wherein the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the method comprises the following steps:

acquiring a direct-current component of an inverter current of a first phase of an inverter and a direct-current component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

determining a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and controlling the inverter according to the direct current component of the inverter current of the first phase, the direct current component of the inverter current of the second phase and the direct current component of the inverter current of the third phase.

A second aspect of an embodiment of the present invention provides a three-level inverter control apparatus, wherein an output terminal of an inverter is connected to a transformer; the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the above-mentioned device includes:

the direct current component acquisition module is used for acquiring a direct current component of an inverter current of a first phase of the inverter and a direct current component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

the direct-current component determining module is used for determining the direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and the control module is used for controlling the inverter according to the direct-current component of the inverter current of the first phase, the direct-current component of the inverter current of the second phase and the direct-current component of the inverter current of the third phase.

A third aspect of the embodiments of the present invention provides a terminal device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, where the processor implements the steps of the three-level inverter control method provided in the first aspect of the embodiments of the present invention when executing the computer program.

A fourth aspect of the embodiments of the present invention provides a computer-readable storage medium, which stores a computer program, and when the computer program is executed by a processor, the steps of the three-level inverter control method according to the first aspect of the embodiments of the present invention are implemented.

A fifth aspect of an embodiment of the present invention provides a PCS system, including: a three-level inverter, a battery, and a terminal device as provided in the third aspect of the embodiments of the present invention;

and the input end of the three-level inverter is connected with the battery, the output end of the three-level inverter is used for being connected with external equipment, and the control end of the three-level inverter is connected with terminal equipment.

The embodiment of the invention provides a three-level inverter control method and a PCS system, wherein the output end of an inverter is connected with a transformer; the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the method comprises the following steps: acquiring a direct-current component of an inverter current of a first phase of an inverter and a direct-current component of an inverter current of a second phase of the inverter; determining a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase; and controlling the inverter according to the direct current component of the inverter current of the first phase, the direct current component of the inverter current of the second phase and the direct current component of the inverter current of the third phase. When the sampling frequency of the direct current component of the three-phase inverter current of the inverter is small, the embodiment of the invention can determine the direct current component of the inverter current of the third phase according to the direct current components of the inverter currents of the two phases, and the inverter is integrally adjusted based on the relevance of the direct current components of the three-phase inverter current, so that the adjustment precision is improved, and the problem of unbalanced bus is effectively improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.

Fig. 1 is a schematic circuit diagram of an I-type three-level inverter according to an embodiment of the present invention;

fig. 2 is a schematic circuit diagram of a phase a in the I-type three-level inverter according to the embodiment of the present invention;

fig. 3 is a schematic flow chart of an implementation of a three-level inverter control method according to an embodiment of the present invention;

fig. 4 is a schematic diagram of a three-level inverter control apparatus provided by an embodiment of the present invention;

fig. 5 is a schematic diagram of a terminal device provided in an embodiment of the present invention;

fig. 6 is a schematic structural diagram of a PCS system according to an embodiment of the present invention.

Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular system structures, techniques, etc. in order to provide a thorough understanding of the embodiments of the invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

In order to explain the technical means of the present invention, the following description will be given by way of specific examples.

Fig. 1 shows a topology of a type I three-level inverter, comprising: the three-phase full-bridge inverter circuit 11 and the three-phase LC filter circuit 12 are not described in detail, and refer to fig. 1. Fig. 2 shows a circuit diagram of a phase a in a type I three-level inverter, and referring to fig. 2,

i_a0＝s_2a*i_aL1-s_3a*i_aL1

wherein s is_2aIs a switch tube S_a2Corresponding switching function, s_3aIs a switch tube S_a3Corresponding switching function, when the corresponding switching tube is on, s_2aAnd s_3aIs 1, when the switch tube is turned off, s_2aAnd s_3aIs 0; i.e. i_aL1Is an inverter current.

When positive and negative half cycle switch tube S_a2And a switching tube S_a3When the conduction times are unequal, i_a0When the voltage is not 0 in one fundamental wave period, the midpoint potential of the bus bar is shifted. Wherein the direction of the offset depends on i in one fundamental period_a0In the direction of (a). If i_a0>0, the potential of the O point is reduced; if i_a0<0, the O point potential rises, causing an imbalance at the bus midpoint.

For a device with a three-level inverter, when an output end is connected with a transformer, because the transformer is a resistance-inductance load, pure resistance internal resistance is generally small, direct current impedance is small, slight direct current components exist in inverted output voltage of the three-level inverter and act on the transformer, and the inverted output current generates large direct current components (for example, 1A), so that the midpoint voltage of a bus is seriously unbalanced.

For example, for a PCS adopting an I-type three-level inversion topology, a dc bus is directly connected to an energy storage battery. The PCS has a grid-connected mode and an off-grid mode. When the PCS works in an off-grid mode, the three-phase three-wire output voltage is 800Vac, and when the output end is connected with a transformer, the output voltage is reduced to 400 Vac. Based on the analysis, the problem of unbalanced bus midpoint voltage exists in the soft start process and the steady state process of the inversion voltage of the PCS. The highest bus voltage of the PCS capable of working is as high as 1500V, and the single-tube IGBT voltage resistance of the selected I-type three-level module is only 1000V. Under the condition of bus midpoint balance, the average reverse peak platform voltage of each IGBT tube is 750V, the withstand voltage margin is only 250V, the IGBT tubes are damaged, and the PCS reliability is seriously influenced.

Referring to fig. 2, the inverter current i_aL1Can be decomposed into alternating current parts i_{aL1_ac}And a DC part i_{aL1_dc}

i_aL1＝i_{aL1_dc}+i_{aL1_ac}

i_{aL1_ac}Containing 50Hz fundamental and higher harmonics, i can be considered_{aL1_ac}The sum in one fundamental period is 0. Thus, the above equation can be simplified to:

i_a0＝s_2a*i_{aL1_dc}-s_3a*i_{aL1_dc}

therefore, to ensure bus voltage balance, i should be ensured within one fundamental period_a0Is 0.

In the prior art, the direct current components of three-phase inverter currents are generally sampled respectively, and the direct current components of each phase of PCS inverter currents are adjusted according to the sampling values. However, because the current sampling frequency is limited, the accuracy of the control method is not enough, and the improvement effect of the bus imbalance problem is not good.

Based on the above, referring to fig. 3, an embodiment of the present invention provides a three-level inverter control method, where an output end of an inverter is connected to a transformer; the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the method comprises the following steps:

s101: acquiring a direct-current component of an inverter current of a first phase of an inverter and a direct-current component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

s102: determining a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

s103: and controlling the inverter according to the direct current component of the inverter current of the first phase, the direct current component of the inverter current of the second phase and the direct current component of the inverter current of the third phase.

Fig. 1 shows a topology of a type I three-level inverter.

i_aL1+i_bL1+i_cL1＝0

i_aL2＝i_aL1+i_aC

i_bL2＝i_bL1+i_bC

i_cL2＝i_cL1+i_cC

From the above, i_aL1+i_aC+i_bL1+i_bC+i_cL1+i_cC＝0

If only the DC component is considered, then i_{aL1_dc}+i_{aC_dc}+i_{bL1_dc}+i_{bC_dc}+i_{cL1_dc}+i_{cC_dc}＝0

Considering that the filter capacitor has the function of isolating direct current from direct current_{aC_dc}+i_{bC_dc}+i_{cC_dc}When is equal to 0, then i_{aL1_dc}+i_{bL1_dc}+i_{cL1_dc}＝0。

Based on the analysis, i to reduce the problem of poor control effect of the neutral point imbalance of the bus caused by inaccurate sampling of the DC component of the inverter current_{aL1_dc}+i_{bL1_dc}+i_{cL1_dc}0, the embodiment of the invention is based on the off between three-phase direct current componentsThe direct-current component of the third-phase inverter current is determined through the direct-current components of the two-phase inverter current, the inverter is integrally adjusted, the influence caused by inaccurate sampling is reduced, the adjusting precision is improved, and the problem of bus imbalance is effectively improved.

In some embodiments, S102 may include:

s1021: and subtracting the direct-current component of the inversion current of the second phase from the direct-current component of the inversion current of the negative first phase to obtain the direct-current component of the inversion current of the third phase.

In some embodiments, S103 may include:

s1031: acquiring the inversion voltage of each phase of the inverter;

s1032: and aiming at each phase of the inverter, controlling the phase of the inverter according to the inversion voltage of the phase and the direct current component of the inversion current of the phase.

In some embodiments, S1032 may comprise:

1. inputting the direct current component of the inverter current of the phase into the corresponding first PI controller to obtain a voltage reference value of the phase;

2. subtracting the inversion voltage of the phase from the voltage reference value of the phase to obtain a voltage difference value of the phase;

3. and inputting the voltage difference value of the phase into the corresponding second PI controller to obtain the control quantity of the phase, and controlling the phase of the inverter according to the control quantity of the phase.

In the embodiment of the invention, based on fig. 1, to ensure the neutral point voltage balance of the bus, the inverter can be adjusted to enable i_{aL1_dc}、i_{bL1_dc}And i_{cL1_dc}Are all 0. In the embodiment of the invention, the current reference value is set to be 0, the sampled direct current component of the inverter current of the phase is directly input into the first PI controller to obtain the voltage reference value of the phase, and then the second PI controller outputs the control quantity of the phase of the inverter to control the phase, so that the direct current component of the inverter current of the phase is controlled to be 0.

In some embodiments, the method may further include:

s104: and respectively sampling the first phase inversion current and the second phase inversion current to obtain a direct current component of the first phase inversion current and a direct current component of the second phase inversion current.

In some embodiments, the sampling frequency f of the direct current component of the three-phase inverter current of the inverter is set to be lower than the sampling frequency f of the direct current component of the three-phase inverter current of the inverter_sComprises the following steps:

f_s＝10*f

wherein f is the frequency of the inverter voltage of the inverter.

By adopting the control method provided by the embodiment of the invention, the adjustment precision is improved, the requirement on the sampling period is reduced, and the requirement can be met by sampling for 10 times in one power frequency period.

When the inversion current is sampled, because the input port of the control chip is limited, a multi-selection one-gating chip can be usually adopted for sampling, for example, an 8-selection 1-gating chip (the model can be CD74HC4051M96, 2007) 00501 of TI corporation, but when the sampling rate of the gating chip is set too fast, the sampling signal cannot be completely stabilized, the sampling accuracy is greatly affected, the problem of bus imbalance is caused, the use of the gating chip is limited, and the requirement for the control chip is improved.

The three-level inverter control method provided by the embodiment of the invention reduces the requirement on sampling frequency, can sample the inverter current by adopting the gating chip on the premise of not influencing the balance of the bus, and reuses one port for three phases, thereby saving the ports of the control chip and reducing the requirement on the control chip. For example, the sampling rate of the direct current component of each phase of the inverted current is set to be 2ms, the period of the inverted voltage of the inverter is set to be 20ms, and the direct current component of the inverted current can be collected for 10 times in each period, so that the requirement of practical application can be met.

In some embodiments, referring to fig. 1, the inverter may include: a three-phase full-bridge inverter circuit 11 and a three-phase LC filter circuit 12;

the input end of the three-phase full-bridge inverter circuit 11 is used for being connected with a direct-current power supply, the output end of the three-phase full-bridge inverter circuit 11 is connected with the input end of the three-phase LC filter circuit 12, and the output end of the three-phase LC filter circuit 12 is connected with the output end of the inverter.

Fig. 1 can be referred to for a specific circuit schematic diagram, which is not described herein again.

It should be understood that, the sequence numbers of the steps in the foregoing embodiments do not imply an execution sequence, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present invention.

Corresponding to the above embodiment, referring to fig. 4, an embodiment of the present invention further provides a three-level inverter control device, where an output terminal of an inverter is connected to a transformer; the sampling frequency of the direct current component of the three-phase inverter current of the inverter is less than the switching frequency of the inverter; the above-mentioned device includes:

a dc component obtaining module 21, configured to obtain a dc component of an inverter current of a first phase of the inverter and a dc component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

the direct-current component determining module 22 is configured to determine a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and the control module 23 is configured to control the inverter according to the dc component of the inverter current of the first phase, the dc component of the inverter current of the second phase, and the dc component of the inverter current of the third phase.

In some embodiments, the dc component determining module 22 may include:

the third-phase direct-current component determining unit 221 is configured to subtract the direct-current component of the inverted current of the second phase from the direct-current component of the inverted current of the negative first phase to obtain the direct-current component of the inverted current of the third phase.

In some embodiments, the control module 23 may include:

a voltage obtaining unit 231 for obtaining an inversion voltage of each phase of the inverter;

the phase splitting control unit 232 is configured to control, for each phase of the inverter, the phase of the inverter according to the inversion voltage of the phase and the dc component of the inversion current of the phase.

In some embodiments, the phase separation control unit 232 is specifically configured to:

1. inputting the direct current component of the inverter current of the phase into the corresponding first PI controller to obtain a voltage reference value of the phase;

2. subtracting the inversion voltage of the phase from the voltage reference value of the phase to obtain a voltage difference value of the phase;

3. and inputting the voltage difference value of the phase into the corresponding second PI controller to obtain the control quantity of the phase, and controlling the phase of the inverter according to the control quantity of the phase.

In some embodiments, the apparatus may further include:

the sampling module 24 is configured to sample the first phase inverted current and the second phase inverted current respectively to obtain a direct current component of the first phase inverted current and a direct current component of the second phase inverted current.

In some embodiments, the sampling frequency f of the direct current component of the three-phase inverter current of the inverter_sComprises the following steps:

f_s＝10*f

where f is the frequency of the inverter voltage of the inverter.

In some embodiments, the inverter may include: a three-phase full-bridge inverter circuit 11 and a three-phase LC filter circuit 12;

the input end of the three-phase full-bridge inverter circuit 11 is used for being connected with a direct-current power supply, the output end of the three-phase full-bridge inverter circuit 11 is connected with the input end of the three-phase LC filter circuit 12, and the output end of the three-phase LC filter circuit 12 is connected with the output end of the inverter.

It is obvious to those skilled in the art that, for convenience and simplicity of description, the foregoing division of the functional units and modules is merely used as an example, and in practical applications, the above function distribution may be performed by different functional units and modules as needed, that is, the internal structure of the terminal device is divided into different functional units or modules to perform all or part of the above described functions. Each functional unit and module in the embodiments may be integrated in one processing unit, or each unit may exist alone physically, or two or more units are integrated in one unit, and the integrated unit may be implemented in a form of hardware, or in a form of software functional unit. In addition, specific names of the functional units and modules are only for convenience of distinguishing from each other, and are not used for limiting the protection scope of the present application. The specific working processes of the units and modules in the above-mentioned apparatus may refer to the corresponding processes in the foregoing method embodiments, and are not described herein again.

Fig. 5 is a schematic block diagram of a terminal device according to an embodiment of the present invention. As shown in fig. 5, the terminal device 4 of this embodiment includes: one or more processors 40, a memory 41, and a computer program 42 stored in the memory 41 and executable on the processors 40. The processor 40, when executing the computer program 42, implements the steps in the various three-level inverter control method embodiments described above, such as the steps S101 to S103 shown in fig. 3. Alternatively, the processor 40, when executing the computer program 42, implements the functions of the respective modules/units in the above-described three-level inverter control apparatus embodiment, such as the functions of the modules 21 to 23 shown in fig. 4.

Illustratively, the computer program 42 may be divided into one or more modules/units, which are stored in the memory 41 and executed by the processor 40 to accomplish the present application. One or more of the modules/units may be a series of computer program instruction segments capable of performing specific functions, which are used to describe the execution of the computer program 42 in the terminal device 4. For example, the computer program 42 may be divided into the direct-current component acquisition module 21, the direct-current component determination module 22, and the control module 23.

A dc component obtaining module 21, configured to obtain a dc component of an inverter current of a first phase of the inverter and a dc component of an inverter current of a second phase of the inverter; the first phase of the inverter is any one of the three phases of the inverter, and the second phase of the inverter is any one of the three phases of the inverter except the first phase;

the direct-current component determining module 22 is configured to determine a direct-current component of the inverter current of the third phase according to the direct-current component of the inverter current of the first phase and the direct-current component of the inverter current of the second phase;

and the control module 23 is configured to control the inverter according to the dc component of the inverter current of the first phase, the dc component of the inverter current of the second phase, and the dc component of the inverter current of the third phase.

Other modules or units are not described in detail herein.

Terminal device 4 includes, but is not limited to, processor 40, memory 41. Those skilled in the art will appreciate that fig. 5 is only one example of a terminal device and does not constitute a limitation of terminal device 4 and may include more or fewer components than shown, or combine certain components, or different components, e.g., terminal device 4 may also include input devices, output devices, network access devices, buses, etc.

The Processor 40 may be a Central Processing Unit (CPU), other general purpose Processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other Programmable logic device, discrete Gate or transistor logic device, discrete hardware component, etc. A general purpose processor may be a microprocessor or the processor may be any conventional processor or the like.

The storage 41 may be an internal storage unit of the terminal device, such as a hard disk or a memory of the terminal device. The memory 41 may also be an external storage device of the terminal device, such as a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) Card, a Flash memory Card (Flash Card), and the like provided on the terminal device. Further, the memory 41 may also include both an internal storage unit of the terminal device and an external storage device. The memory 41 is used for storing the computer program 42 and other programs and data required by the terminal device. The memory 41 may also be used to temporarily store data that has been output or is to be output.

In the above embodiments, the descriptions of the respective embodiments have respective emphasis, and reference may be made to the related descriptions of other embodiments for parts that are not described or illustrated in a certain embodiment.

Those of ordinary skill in the art will appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware or combinations of computer software and electronic hardware. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the implementation. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present application.

In the embodiments provided in the present application, it should be understood that the disclosed terminal device and method may be implemented in other ways. For example, the above-described terminal device embodiments are merely illustrative, and for example, a module or a unit may be divided into only one logical function, and may be implemented in other ways, for example, multiple units or components may be combined or integrated into another system, or some features may be omitted or not executed. In addition, the shown or discussed mutual coupling or direct coupling or communication connection may be an indirect coupling or communication connection through some interfaces, devices or units, and may be in an electrical, mechanical or other form.

Units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment.

In addition, functional units in the embodiments of the present application may be integrated into one processing unit, or each unit may exist alone physically, or two or more units are integrated into one unit. The integrated unit can be realized in a form of hardware, and can also be realized in a form of a software functional unit.

The integrated modules/units, if implemented in the form of software functional units and sold or used as separate products, may be stored in a computer readable storage medium. Based on such understanding, all or part of the flow in the method according to the embodiments described above may be implemented by a computer program, which is stored in a computer readable storage medium and used by a processor to implement the steps of the embodiments of the methods described above. Wherein the computer program comprises computer program code, which may be in the form of source code, object code, an executable file or some intermediate form, etc. The computer readable medium may include: any entity or device capable of carrying computer program code, recording medium, usb disk, removable hard disk, magnetic disk, optical disk, computer Memory, Read-Only Memory (ROM), Random Access Memory (RAM), electrical carrier wave signals, telecommunications signals, software distribution medium, and the like. It should be noted that the computer readable medium may include any suitable increase or decrease as required by legislation and patent practice in the jurisdiction, for example, in some jurisdictions, computer readable media may not include electrical carrier signals and telecommunications signals in accordance with legislation and patent practice.

Corresponding to the above embodiment, referring to fig. 6, an embodiment of the present invention further provides a PCS system, including: a three-level inverter 1, a battery 2, and a terminal device 4 as provided by the embodiment of the present invention;

and the input end of the three-level inverter 1 is connected with the battery 2, the output end of the three-level inverter is used for being connected with the external equipment 3, and the control end of the three-level inverter is connected with the terminal equipment 4.

The external device 3 may be a transformer. The terminal device 4 controls the three-level inverter 1 so that the midpoint potential of the three-level inverter 1 is balanced.

The above embodiments are only used to illustrate the technical solutions of the present application, and not to limit the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not substantially depart from the spirit and scope of the embodiments of the present application and are intended to be included within the scope of the present application.