WO2022188063A1 - 一种光伏系统、逆变器及控制方法 - Google Patents

一种光伏系统、逆变器及控制方法 Download PDF

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Publication number
WO2022188063A1
WO2022188063A1 PCT/CN2021/079984 CN2021079984W WO2022188063A1 WO 2022188063 A1 WO2022188063 A1 WO 2022188063A1 CN 2021079984 W CN2021079984 W CN 2021079984W WO 2022188063 A1 WO2022188063 A1 WO 2022188063A1
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Prior art keywords
inverter
switch unit
current
current threshold
output
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PCT/CN2021/079984
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English (en)
French (fr)
Inventor
于心宇
王旭东
辛凯
石磊
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华为数字能源技术有限公司
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Priority to PCT/CN2021/079984 priority Critical patent/WO2022188063A1/zh
Priority to EP21929542.5A priority patent/EP4297266A4/en
Priority to CN202180065809.0A priority patent/CN116615862A/zh
Priority to AU2021432442A priority patent/AU2021432442A1/en
Publication of WO2022188063A1 publication Critical patent/WO2022188063A1/zh
Priority to US18/463,399 priority patent/US20230421074A1/en

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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/007—Plural converter units in cascade
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/487—Neutral point clamped inverters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0048—Circuits or arrangements for reducing losses
    • H02M1/0054—Transistor switching losses
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/539—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency
    • H02M7/5395—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency by pulse-width modulation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

Definitions

  • the present application relates to the technical field of power electronics, and in particular, to a photovoltaic system, an inverter and a control method.
  • inverters At present, with the increasing demand for inverters in the fields of photovoltaic power generation, electric vehicles or electric drives, for example, the inverters are required to have high conversion efficiency and a small volume. Therefore, at present, most inverters use multi-level control, which helps to improve the voltage level and power density of the inverter, and improve the quality of the AC power output by the inverter.
  • the more commonly used multi-level inverters are three-level inverters, especially the three-level inverters based on diode neutral point clamp (NPC, Neutral Point Clamped) are most commonly used.
  • NPC diode neutral point clamp
  • the present application provides a photovoltaic system, an inverter and a control method, which can improve the power conversion efficiency of the inverter and improve the harmonic quality.
  • the embodiment of the present application provides a photovoltaic system, wherein the inverter included is a three-level NPC inverter.
  • the embodiment of the present application provides a A new control method is used to shorten the high-frequency operation time of the switching unit as much as possible, thereby reducing the switching loss caused by the high-frequency operation, thereby improving the power conversion efficiency.
  • the three-level NPC inverter includes at least four switch units; when the average output current of the three-level midpoint clamp inverter is greater than the first current threshold or less than the second current threshold, the controller controls the four switch units There is only one switch unit in the high-frequency action.
  • the current average value is the average value of the output current of the three-level NPC inverter in the switching period; wherein the switching period is the switching period of the switch tube in the switching unit.
  • the output current of the inverter is positive if it flows out of the inverter, otherwise it is negative; wherein, the first current threshold is a positive number, and the second current threshold is a negative number.
  • the technical solution provided by the embodiment of the present application is that when the average value of the output current of the three-level NPC inverter is within a preset interval near zero, it means that the output current is relatively small, and it is easy to occur when the positive time is negative. Therefore, it is necessary to control the The two switches operate at high frequency to provide a current path when the output current fluctuates.
  • the average value of the output current is outside the preset range, it means that the absolute value of the output current is relatively large, and it is not near zero current, so there is no positive and negative situation. Can. In this way, the high-frequency action of the switch tube can be greatly reduced, thereby reducing the loss caused by the high-frequency action of the switch tube, and improving the power conversion efficiency of the inverter.
  • the absolute value of the first current threshold and the absolute value of the second current threshold may or may not be equal. When the absolute values of the two are not equal, the absolute value of the first current threshold may be greater than the absolute value of the second current threshold, or the absolute value of the first current threshold may be smaller than the absolute value of the second current threshold.
  • the preset interval is a symmetrical interval about 0 point.
  • the above describes the high-frequency operation time of one switch unit among the four switch units, and the following describes the high-frequency operation time of the two switch units, that is, the controller is also used to output the average value of the current greater than the second current threshold and less than the first current threshold. , control the high-frequency operation of two switching units in the four switching units, that is, control the high-frequency operation of the two switching units only when needed, so as to provide a path for the positive and negative currents.
  • the specific control method can be classified and controlled according to the positive half cycle and the negative half cycle of the output voltage, that is, the controller is specifically used to clamp the positive half cycle of the output voltage of the inverter at the three-level midpoint: control the second switch unit to turn on, The fourth switch unit is turned off, the average output current is greater than the first current threshold, the third switch unit is controlled to be turned off, the first switch unit operates at high frequency, the average output current is less than the second current threshold, and the first switch unit is controlled to be turned off , the third switch unit operates at high frequency;
  • the controller is specifically used for the negative half cycle of the output voltage: controlling the third switch unit to be turned on, the first switch unit to be turned off, the average value of the output current is greater than the first current threshold, the fourth switch unit to be turned off, and the second switch unit to be high When the average value of the output current is less than the second current threshold, the second switch unit is controlled to be turned off, and the fourth switch unit is operated at a high frequency.
  • a possible implementation method is that within a power frequency cycle, that is, the power frequency cycle corresponding to the output voltage of the inverter, according to the traditional control method, in the Each half cycle of the output voltage has two complementary high-frequency actions of the switching tubes, namely T1 and T3 high-frequency complementary actions, T2 and T4 high-frequency complementary actions, to obtain the minimum and maximum output current of the inverter in one switching cycle. value and average.
  • the minimum value of the output current in a switching cycle When the minimum value of the output current in a switching cycle is 0, it indicates that the output current in the switching cycle is a positive half cycle, and the average value of the output current in this switching cycle must be greater than 0, which is recorded as the first average current, then the first The value of the current threshold value is greater than or equal to the average value of the output current corresponding to the switching period, that is, the average value of the output current at this time is the lower limit value of the value of the first current threshold value.
  • the maximum value of the output current in a switching cycle is 0, it means that the output current in the switching cycle is a negative half cycle, and the average value of the output current in this switching cycle must be less than 0, which is recorded as the second average current, then the second The current threshold value is less than or equal to the average value of the output current corresponding to the switching period, that is, the average value of the output current at this time is the upper limit value of the second current threshold value.
  • the first current threshold and the second current threshold described above can be fixed or adjusted according to some parameters. Several adjustment methods are described below. In order to more accurately control the actions of each switch tube in the inverter, the first current threshold and/or the second current threshold can be adjusted according to the actual operating conditions, that is, only the first current threshold can be adjusted, or only the second current threshold can be adjusted. The current threshold can also be adjusted by both the first current threshold and the second current threshold.
  • the parameters that may be affected by the actual operating conditions include: total current harmonic distortion rate of the output current, modulation ratio of the three-level midpoint clamped inverter, power factor of the inverter or three-level midpoint clamped inverse output power of the inverter.
  • the controller obtains the total current harmonic distortion rate of the output current, and if the total current harmonic distortion rate increases, the first current threshold is increased, otherwise, the first current threshold is decreased.
  • the controller obtains the modulation ratio of the three-level mid-point clamped inverter, and when the modulation ratio decreases, the first current threshold is increased, otherwise the first current threshold is decreased; the modulation ratio is The ratio of the peak output phase voltage to the half DC bus voltage of the three-level neutral point clamped inverter.
  • the controller obtains the power factor of the three-level midpoint clamped inverter, and if the absolute value of the power factor decreases, the first current threshold is increased, otherwise, the first current threshold is decreased.
  • the controller obtains the output power of the three-level midpoint clamped inverter, and when the output power decreases, the first current threshold is increased, otherwise the first current threshold is decreased.
  • the controller obtains the total current harmonic distortion rate of the output current, and if the total current harmonic distortion rate increases, the second current threshold is decreased, otherwise the second current threshold is increased.
  • the controller obtains the modulation ratio of the three-level mid-point clamped inverter, and if the modulation ratio decreases, the second current threshold is decreased, otherwise the second current threshold is increased; the modulation ratio is The ratio of the peak output phase voltage to the half DC bus voltage of the three-level neutral point clamped inverter.
  • the controller obtains the power factor of the three-level midpoint clamped inverter, and if the absolute value of the power factor decreases, the second current threshold is decreased, and conversely, the second current threshold is increased.
  • the controller obtains the output power of the three-level mid-point clamped inverter, and when the output power decreases, the second current threshold is decreased, and conversely, the second current threshold is increased.
  • the three-level NPC inverter may be a centralized inverter, a string inverter, or a household single-phase inverter.
  • the three-level neutral point clamp inverter further includes: a first capacitor, a second capacitor, a first diode and a second diode; the first end of the first capacitor is connected to The DC positive busbar, the second end of the first capacitor is connected to the first end of the second capacitor, the second end of the second capacitor is connected to the DC negative busbar; the cathode of the first diode is connected to the second end of the first switch unit, and the second end of the second capacitor is connected to the DC negative busbar; The anode of a diode is connected to the second end of the first capacitor, the cathode of the second diode is connected to the anode of the first diode, and the anode of the second diode is connected to the second end of the third switch unit.
  • any scenario of the inverter can also be used for other scenarios that require DC to AC conversion, such as data centers, base stations, or electric vehicles.
  • the advantages of the solutions provided in the above embodiments are also applicable to the following inverters, which will not be repeated here.
  • Embodiments of the present application further provide a three-level neutral-point clamped inverter, which includes at least four switch units; an inverter controller, which is used for the output current of the three-level neutral-point clamped inverter whose average value is greater than The first current threshold or less than the second current threshold controls only one switch unit in the four switch units to operate at high frequency to reduce the power consumption caused by high frequency operation; the average output current is the inverse of the three-level mid-point clamp.
  • the inverter controller is specifically configured to clamp the positive half cycle of the output voltage of the inverter at the three-level midpoint: control the second switch unit to be turned on, and the fourth switch unit to be turned off , the average value of the output current is greater than the first current threshold, the third switch unit is controlled to turn off, and the first switch unit operates at high frequency; the average value of the output current is less than the second current threshold, the first switch unit is controlled to be turned off, and the third switch unit is high frequent action;
  • the inverter controller is specifically used for the negative half cycle of the output voltage: controlling the third switch unit to be turned on, the first switch unit to be turned off, the average value of the output current is greater than the first current threshold, to control the fourth switch unit to be turned off, and the first switch unit to be turned off.
  • the two switch units operate at high frequency; the average value of the output current is less than the second current threshold, the second switch unit is controlled to be turned off, and the fourth switch unit is operated at high frequency.
  • the inverter controller adjusts the first current threshold according to any one of the following parameters, and any one of the parameters includes: the total current harmonic distortion rate of the output current, the three-level midpoint clamping inverse The modulation ratio of the inverter, the power factor of the inverter or the output power of the three-level midpoint clamped inverter.
  • the inverter controller adjusts the second current threshold according to any one of the following parameters, and any one of the parameters includes: the total current harmonic distortion rate of the output current, the three-level midpoint clamping inverse The modulation ratio of the inverter, the power factor of the inverter or the output power of the three-level midpoint clamped inverter.
  • the three-level neutral point clamp inverter further includes: a first capacitor, a second capacitor, a first diode and a second diode; the first end of the first capacitor is connected to The DC positive busbar, the second end of the first capacitor is connected to the first end of the second capacitor, the second end of the second capacitor is connected to the DC negative busbar; the cathode of the first diode is connected to the second end of the first switch unit, and the second end of the second capacitor is connected to the DC negative busbar; The anode of a diode is connected to the second end of the first capacitor, the cathode of the second diode is connected to the anode of the first diode, and the anode of the second diode is connected to the second end of the third switch unit.
  • the embodiment of the present application further provides a control method for an inverter, which is applied to a three-level neutral point clamped inverter.
  • the three-level mid-point clamp inverter includes at least four switching units; the method includes: obtaining an average value of output current of the three-level mid-point clamp inverter; the average output current is the three-level mid-point clamp The average value of the output current of the inverter in the switching period; the average value of the output current of the three-level midpoint clamp inverter is greater than the first current threshold or less than the second current threshold, controlling only one of the four switching units
  • the switching unit operates at high frequency to reduce the power consumption caused by the high frequency operation; the output current of the inverter is positive if it flows out of the inverter, otherwise it is negative; the first current threshold is positive and the second current threshold is negative.
  • the average value of the output current of the three-level neutral point clamped inverter is greater than the first current threshold or less than the second current threshold, and only one switch unit among the four switch units is controlled to operate at high frequency , specifically including: clamping the positive half cycle of the output voltage of the inverter at the three-level midpoint: controlling the second switch unit to be turned on, the fourth switch unit to be turned off, the average output current is greater than the first current threshold, controlling the third When the switch unit is turned off, the first switch unit operates at high frequency; the average value of the output current is less than the second current threshold, the first switch unit is controlled to be turned off, and the third switch unit is operated at high frequency; in the negative half cycle of the output voltage: the third switch is controlled The unit is turned on, the first switch unit is turned off, the average value of the output current is greater than the first current threshold, the fourth switch unit is controlled to be turned off, and the second switch unit is operated at high frequency; the average value of the output current is less than the second current threshold
  • the embodiments of the present application have the following advantages:
  • the technical solution provided by the embodiments of the present application obtains the average output current of the inverter in one switching cycle in real time, compares the average output current with a preset interval, and controls the switching unit to act according to the comparison result.
  • the preset interval is an interval near 0, that is, a preset interval formed by a second current threshold less than 0 and a first current threshold greater than 0.
  • the technical solution provided in this embodiment can greatly reduce the high-frequency operation time of the switch unit, the power consumption caused by the high-frequency operation of the switch unit in the inverter can be reduced, thereby improving the power conversion efficiency of the inverter.
  • only one switch unit operates at high frequency most of the time, and the time period during which the two switch units complement each other at high frequency is very small. Therefore, the dead time can be reduced outside the preset interval. Therefore, the total current harmonic distortion rate of the output current of the inverter can be improved, that is, the harmonic quality can be improved.
  • FIG. 1 is a schematic diagram of a photovoltaic system corresponding to a centralized inverter according to an embodiment of the present application
  • FIG. 2 is a schematic diagram of a photovoltaic system corresponding to a string inverter according to an embodiment of the present application
  • FIG. 3 is a schematic diagram of a household single-phase photovoltaic system provided by an embodiment of the present application.
  • FIG. 4 is a schematic diagram of a three-level NPC inverter provided by an embodiment of the present application.
  • 5A is a timing diagram of a pulse drive signal
  • 5B is a schematic diagram of another three-level NPC inverter provided by an embodiment of the present application.
  • FIG. 6 is a schematic diagram of a photovoltaic system provided by an embodiment of the present application.
  • FIG. 7 is a driving sequence diagram of a three-level NPC inverter in a photovoltaic system provided by an embodiment of the present application.
  • FIG. 8 is a schematic waveform diagram of an output current of an inverter provided by an embodiment of the present application.
  • FIG. 9 is a schematic diagram corresponding to the high-frequency action of the first switch tube in the positive half cycle of the output voltage provided by an embodiment of the present application.
  • FIG. 10 is a schematic diagram corresponding to the high frequency action of the third switch tube in the positive half cycle of the output voltage provided by the embodiment of the application;
  • FIG. 11 is a schematic diagram corresponding to the high-frequency action of the second switch tube in the negative half cycle of the output voltage provided by an embodiment of the present application;
  • FIG. 12 is a schematic diagram corresponding to the high-frequency action of the fourth switch tube in the negative half cycle of the output voltage provided by the embodiment of the application;
  • FIG. 13 is a schematic diagram of another three-level NPC inverter provided by an embodiment of the present application.
  • FIG. 14 is a flowchart of a control method of a three-level NPC inverter provided by an embodiment of the present application.
  • directional terms such as “upper” and “lower” may include, but are not limited to, definitions relative to the schematic placement of components in the drawings. It should be understood that these directional terms may be relative concepts, They are used for relative description and clarification, which may vary accordingly depending on the orientation in which the components are placed in the drawings.
  • connection should be understood in a broad sense.
  • connection may be a fixed connection, a detachable connection, or an integrated body; it may be directly connected, or Can be indirectly connected through an intermediary.
  • coupled may be a manner of electrical connection that enables signal transmission.
  • Coupling can be a direct electrical connection or an indirect electrical connection through an intermediate medium.
  • the embodiments of the present application relate to a photovoltaic system, and in particular, to a three-level inverter applied to the photovoltaic system. Since the output of the photovoltaic module is direct current, the inverter needs to convert the direct current into alternating current and then feed it back to the power grid. Or provide it to electrical equipment.
  • the embodiments of the present application do not limit the specific structure of the photovoltaic system, which may be a photovoltaic system including a centralized inverter, a photovoltaic system including a string inverter, or a single-phase photovoltaic system for household use. All of the above photovoltaic systems may include three-level inverters. The topological forms of the above three photovoltaic systems are described below with the accompanying drawings.
  • FIG. 1 this figure is a schematic diagram of a photovoltaic system corresponding to a centralized inverter provided in an embodiment of the present application.
  • the centralized inverter only includes the inverter circuit, that is, the power conversion only includes the DC-AC conversion circuit, and does not include the DC-DC conversion circuit.
  • the photovoltaic system includes a plurality of DC combiner boxes, such as a DC combiner box 100a to a DC combiner box 100n.
  • the output ends of the multiple DC combiner boxes are connected to the input ends of the centralized inverter 200.
  • the centralized inverter The output end of the transformer 200 is connected to the primary winding of the transformer T, and the secondary winding of the transformer T can be connected to the AC power grid.
  • each DC combiner box can be directly connected to multiple photovoltaic strings, and the number of photovoltaic strings is not specifically limited.
  • the input end of the DC combiner box can also be connected to photovoltaic strings through a photovoltaic optimizer, and each photovoltaic string corresponds to a photovoltaic optimizer. Whether the photovoltaic system has a photovoltaic optimizer is not specifically limited in the embodiments of the present application.
  • the centralized inverter 200 shown in FIG. 1 may be a three-level NPC inverter.
  • Another photovoltaic system is described below, which includes a string inverter.
  • this figure is a schematic diagram of a photovoltaic system corresponding to a string inverter provided in an embodiment of the present application.
  • the photovoltaic system shown in FIG. 2 may include a plurality of string inverters, as shown in FIG. 2 , which are respectively a string inverter 200a to a string inverter 200n. Take the input end of each string inverter connected to two DC boost converters as an example for introduction.
  • the input end of the string inverter 200a is connected to the DC boost converter 300a and the DC boost converter.
  • Streamer 300b The input end of the string inverter 200b is connected to the DC boost converter 300c and the DC boost converter 300d.
  • the DC boost converter connected to the input end of the string inverter is not limited number of devices.
  • each DC boost converter is used to connect two photovoltaic strings as an example for introduction.
  • the embodiments of the present application do not limit the number of photovoltaic strings connected to the input terminal of the DC boost converter.
  • the output ends of the string inverter 200a to the string inverter 200n are all connected to the input end of the AC combiner box 400, and the output end of the AC combiner box 400 is connected to the primary winding of the transformer T, and the secondary winding of the transformer T can be Connect to the AC grid.
  • the string inverter shown in FIG. 2 can use a three-level NPC inverter.
  • the photovoltaic system for household use is described below with reference to Figure 3. Since the photovoltaic system for household use is directly used by household electrical equipment, it is sufficient to output single-phase commercial power, such as AC 220V. If there is surplus electricity, it can also be fed back to the AC grid.
  • single-phase commercial power such as AC 220V. If there is surplus electricity, it can also be fed back to the AC grid.
  • FIG. 3 this figure is a schematic diagram of a household single-phase photovoltaic system provided by an embodiment of the present application.
  • the photovoltaic grid-connected inverter 400 in the household photovoltaic system may also use a three-level NPC inverter.
  • the input end of the photovoltaic grid-connected inverter 400 is connected to multiple DC/DC circuits, for example, the DC/DC circuit 500a to the DC/DC circuit 500n.
  • the DC/DC circuit corresponds to the DC switch one by one.
  • the input terminal of the DC/DC circuit 500a is connected to the corresponding photovoltaic string through the DC switch S1
  • the input terminal of the DC/DC circuit 500b is connected to the photovoltaic string through the DC switch Sn.
  • FIG. 3 only shows that one DC/DC circuit is connected to one photovoltaic string correspondingly.
  • one DC/DC circuit can also be connected to multiple photovoltaic strings.
  • the output end of the photovoltaic grid-connected inverter 400 is connected to the unidirectional alternating current through the alternating current switch Q.
  • the inverters in the three photovoltaic systems described above may all be three-level NPC inverters.
  • the circuit structure of the three-level NPC inverter is first introduced below with reference to the accompanying drawings.
  • FIG. 4 the figure is a schematic diagram of a three-level NPC inverter.
  • Figure 4 shows a schematic diagram of a single-phase inverter.
  • the three-level NPC inverter includes: four switch tubes, two diodes and two capacitors.
  • the four switch tubes are respectively: a first switch tube T1, a second switch tube T2, a third switch tube T3 and a fourth switch tube T4.
  • the two diodes are a first diode D1 and a second diode D2, respectively.
  • the two capacitors are: a first capacitor C1 and a second capacitor C2.
  • the first capacitor C1 and the second capacitor C2 are both bus capacitors, that is, C1 and C2 are connected in series between the DC positive bus BUS+ and the DC negative bus BUS-.
  • the first end of the first capacitor C1 is connected to the first end of the first switch tube T1, and the first end of the first capacitor C1 and the first end of the first switch tube T1 are both connected to the DC positive pole.
  • the DC positive pole It can be the DC positive bus BUS+
  • the second end of the first capacitor C1 is connected to the first end of the second capacitor C2
  • the second end of the second capacitor C2 is connected to the DC negative electrode.
  • the DC negative electrode can be the DC negative bus bar BUS- .
  • the second end of the first switch tube T1 is connected to the first end of the second switch tube T2, the second end of the second switch tube T2 is connected to the first end of the third switch tube T3, and the second end of the third switch tube T3 is connected
  • the first end of the fourth switch tube T4 and the second end of the fourth switch tube T4 are connected to the DC negative, which is connected to the DC negative bus BUS- in the photovoltaic system.
  • the cathode of the first diode D1 is connected to the second terminal of the first switch tube T1, and the anode of the first diode D1 is connected to the second terminal of the first capacitor C1, that is, the common terminal of C1 and C2.
  • the cathode of the second diode D2 is connected to the anode of the first diode D1, and the anode of the second diode D2 is connected to the second end of the third switch tube T3.
  • the implementation manner of the four switching transistors T1-T4 is not limited in the embodiments of the present application, for example, it may be a metal-oxide semiconductor field effect transistor (MOS, Metal-Oxide-Semiconductor Field-Effect Transistor) transistor, or an insulated gate bipolar transistor Type transistor (IGBT, Isolated Gate Bipolar Transistor), and the four switches T1-T4 include anti-parallel diodes, wherein the anti-parallel diodes are used to provide a current path when the switches are turned off.
  • MOS Metal-Oxide-Semiconductor Field-Effect Transistor
  • IGBT Isolated Gate Bipolar Transistor
  • the output end of the three-level NPC inverter is the second end of the second switch tube T2.
  • FIG. 5A is a timing diagram of a pulse driving signal.
  • the pulse drive signal corresponding to PWM1 is used to drive the first switch transistor T1 to act
  • the pulse drive signal corresponding to PWM2 is used to drive the second switch transistor T2 to act
  • the pulse drive signal corresponding to PWM3 is used to drive the third switch transistor T3 to act
  • the pulse drive signal corresponding to PWM4 is used to drive the fourth switch tube T4 to act.
  • the switch is turned on, and during the low-level period of the pulse drive signal, the switch is turned off.
  • the first switch transistor T1 and the third switch transistor T3 are high-frequency complementary switching, the second switch transistor T2 is turned on, and the fourth switch transistor T4 is turned off.
  • the high frequency means that the switching frequency of the switch tube is higher than the power frequency frequency, for example, the power frequency frequency is 50Hz.
  • the second switch T2 and the fourth switch T4 are high-frequency complementary switching, the first switch T1 is turned off, and the third switch T3 is turned on.
  • T1 operates at high frequency
  • T2 is turned on
  • T3 operates at high frequency
  • T4 is turned off
  • the current path is BUS+—T1—T2—Vout.
  • the current does not pass through T3 during most of the positive half cycle of the output voltage. Therefore, the high-frequency action of T3 will bring greater power consumption.
  • T2 and T4 operate at high frequency
  • T1 is turned off
  • T3 is turned on
  • the current path is BUS--T4-T3-Vout.
  • the high frequency action of T2 will bring greater power consumption. The above power consumption will cause the conversion efficiency of the three-level NPC inverter to decrease.
  • the three-level NPC inverter is simply referred to as an inverter below.
  • the embodiment of the present application provides a new control method, especially when the three-level NPC inverter is applied in a photovoltaic system, the effect is particularly prominent.
  • the output of the photovoltaic string is direct current, that is, corresponding to the direct current power, and the direct current power is transmitted to the alternating current grid, and only the active power can be generated. Therefore, in most of the time and scenarios, the direction of the output voltage of the inverter The direction of the output current is the same as the direction of the output current, that is, the same direction.
  • the direction of the output voltage of the inverter is different from the direction of the output current, that is, the opposite direction.
  • the output current is negative in a small period of time. That is, if the output current of the inverter flows out of the inverter, it is positive, otherwise, it is negative. Since the current and voltage of the AC grid are sine waves, the average value of the output current is required to be sine waves. However, when the average value of the output current is small, after the small average current superimposes a ripple current, the ripple current is generated due to the switching process. Therefore, after superimposing the ripple current, the output current will sometimes be positive. Sometimes negative, producing volatility. Therefore, only when the output current is negative, T3 and T2 are needed to temporarily provide a current path for energy release.
  • the technical solution provided by the embodiment of the present application is that when the average value of the output current of the three-level neutral point clamp inverter is within a preset interval near zero, it means that the output current is relatively small, and it is easy to be positive and negative at times. Therefore, , it is necessary to control the high-frequency action of the two switches so as to provide a current path when the output current fluctuates.
  • the average value of the output current is outside the preset range, it means that the absolute value of the output current is relatively large, and it is not near zero current, so there is no positive and negative situation. Can.
  • the high-frequency action of the switch tube can be greatly reduced, thereby reducing the loss caused by the high-frequency action of the switch tube, and improving the power conversion efficiency of the inverter.
  • only one switch operates at high frequency most of the time, and the time period during which the two switches complement each other at high frequency is very small. Therefore, the dead time can be reduced outside the preset interval. Therefore, the total harmonic current distortion (THDi, Total Harmonic Current Distortion) of the output current of the inverter can be improved.
  • TDDi Total Harmonic Current Distortion
  • the three-level NPC inverter includes at least the following four switch units: a first switch unit, a second switch unit, a third switch unit, and a fourth switch unit; the first switch unit of the first switch unit The second end of the first switch unit is connected to the first end of the second switch unit, the second end of the second switch unit is connected to the first end of the third switch unit, and the second end of the second switch unit is connected to the first end of the third switch unit.
  • each switch unit may include one switch tube, or may include multiple switch tubes, for example, each switch unit
  • the switch unit includes two switch tubes, as shown in FIG. 5B , which is a schematic diagram of another three-level NPC provided by the embodiment of the present application.
  • each switch unit includes two parallel switch tubes as an example. Since a single switch tube can withstand a limited current, when the current is large, a single switch tube may be damaged.
  • the switching tubes are connected in parallel to realize the function of a switch, and the driving signals of the multiple parallel switching tubes have the same timing, that is, the multiple parallel switching tubes act at the same time, and are turned off or turned on at the same time. Multiple switch tubes connected in parallel can increase the current that the entire switch unit can pass.
  • the first switch unit includes switch tubes T11 and T12
  • the second switch unit includes switch tubes T21 and T22
  • the third switch unit includes switch tubes T31 and T32
  • the fourth switch unit includes switch tubes T41 and T42.
  • the first ends of the switch tubes T11 and T12 are both connected to the DC positive bus BUS+, the second ends of the switch tubes T11 and T12 are connected to the first ends of the switch tubes T21 and T22, and the second ends of the switch tubes T21 and T22 are connected to the switch tube T31. and the first end of T32, the second ends of switch tubes T31 and T32 are connected to the first ends of switch tubes T41 and T42, and the second ends of switch tubes T41 and T42 are connected to the DC negative bus BUS-;
  • the two terminals are the output terminals of the three-level midpoint clamped inverter.
  • FIG. 5B only two switch tubes are connected in parallel to realize the function of one switch, and more switch tubes may be connected in parallel, for example, 3 or 4, or more, which will not be repeated here.
  • this figure is a schematic diagram of a photovoltaic system provided by an embodiment of the present application.
  • the photovoltaic system provided in this embodiment includes: a three-level neutral point clamp inverter and a controller 401 .
  • the three-level neutral point clamped inverter includes at least the following four switches: a first switch T1, a second switch T2, a third switch T3 and a fourth switch T4; the first switch T1 The terminal is connected to the DC positive bus, the second end of the first switch tube T1 is connected to the first end of the second switch tube T2, the second end of the second switch tube T2 is connected to the first end of the third switch tube T3, and the third switch tube The second end of T3 is connected to the first end of the fourth switch tube T4, and the second end of the fourth switch tube T4 is connected to the DC negative bus; the second end of the second switch tube T2 is a three-level midpoint clamp inverter The output terminal of the output terminal, the output voltage of the output terminal is Vout. In general, an inductor is connected to the output of the inverter.
  • the controller 401 is configured to control only one switch of the four switches to operate at high frequency when the average value of the output current of the three-level midpoint clamped inverter is greater than the first current threshold or less than the second current threshold, In order to reduce the power consumption caused by high frequency operation.
  • the controller 401 may be a controller independently provided in the photovoltaic system, or may be a controller integrated with the inverter, which is not specifically limited in the embodiment of the present application.
  • the output current of the inverter is positive if it flows out of the inverter, otherwise it is negative; the absolute value of the first current threshold and the absolute value of the second current threshold may be equal or unequal. When the absolute values of the two are not equal, the absolute value of the first current threshold may be greater than the absolute value of the second current threshold, or the absolute value of the first current threshold may be smaller than the absolute value of the second current threshold.
  • the preset interval is a symmetrical interval about 0 point.
  • the average value of the output current is the average value of the output current of the three-level midpoint clamped inverter in the switching period; the first current threshold is a positive number, and the second current threshold is a negative number. That is, the first current threshold value I1 is a number greater than 0, and the second current threshold value is a number less than 0.
  • the technical solutions provided by the embodiments of the present application control a switch tube to operate at a high frequency when the average value of the output current of the inverter is outside the preset interval near 0. Only in the preset interval near 0, the high-frequency action of the two switches is controlled. That is, the controller 401 is further configured to output the average value of the current greater than the second current threshold and less than the first current threshold, and to control the high-frequency operation of two switches among the four switches.
  • FIG. 7 is a driving timing diagram of the three-level NPC inverter in the photovoltaic system provided by the embodiment of the present application.
  • the four driving signals in FIG. 7 provided in the embodiment of the present application are respectively represented by PWMOUT1, PWMOUT1, PWMOUT1 and PWMOUT1, and the above four pulse driving signals are the four driving signals of the inverter respectively.
  • the high frequency operation time of the second switch tube T2 and the third switch tube T3 can be realized by detecting the output current of the inverter.
  • the output current of the inverter is the output current of the bridge arm of the inverter, that is, the output current of the second end of the second switch tube T2.
  • the output current is an AC parameter similar to the output voltage, which can be positive or negative.
  • the output current superimposes the ripple current on the basis of the sine wave.
  • FIG. 8 is a schematic diagram of a waveform of an output current of an inverter provided by an embodiment of the present application.
  • the sine wave Ug with a larger amplitude in FIG. 8 represents the output voltage
  • the sine wave ig with a smaller amplitude represents the output current
  • the sawtooth wave represents the ripple current
  • the ripple current is small.
  • the superimposed output current will not be positive and negative.
  • the relatively small output current superimposes the ripple current, and the superimposed output current may be positive and negative.
  • the output current after superimposing the ripple current is easy to be positive and negative.
  • the output current provides the current path.
  • the amplitude of the sine wave is large at this time, so the output current after superimposing the ripple current will not be positive and negative, so only one switch can be controlled to operate at high frequency. No more current paths have to be provided.
  • the output current after superimposing the ripple current is positive and negative, so it is necessary to control the two switches to high. It operates at high frequency and provides a current path for the positive and negative output current.
  • the amplitude of the sine wave is large at this time, so the output current after superimposing the ripple current will not be positive and negative, so only one switch can be controlled to operate at high frequency. No more current paths have to be provided.
  • the output current corresponding to the switch tube of the inverter in one switching cycle can be averaged
  • the values are compared to the first current threshold and the second current threshold, respectively.
  • a possible implementation is that in a power frequency cycle, that is, the power frequency cycle corresponding to the output voltage of the inverter, according to the traditional control method, there are two complementary high-frequency action of two switches in each half cycle of the output voltage. , namely T1 and T3 high-frequency complementary action, T2 and T4 high-frequency complementary action, to obtain the minimum value, maximum value and average value of the output current of the inverter in one switching cycle.
  • the minimum value of the output current in a switching cycle When the minimum value of the output current in a switching cycle is 0, it indicates that the output current in the switching cycle is a positive half cycle, and the average value of the output current in this switching cycle must be greater than 0, which is recorded as the first average current, then the first The value of the current threshold value is greater than or equal to the average value of the output current corresponding to the switching period, that is, the average value of the output current at this time is the lower limit value of the value of the first current threshold value.
  • the maximum value of the output current in a switching cycle is 0, it means that the output current in the switching cycle is a negative half cycle, and the average value of the output current in this switching cycle must be less than 0, which is recorded as the second average current, then the second The current threshold value is less than or equal to the average value of the output current corresponding to the switching period, that is, the average value of the output current at this time is the upper limit value of the second current threshold value.
  • the controller is specifically used to clamp the positive half cycle of the output voltage of the inverter at the three-level midpoint: control the second switch to be turned on, the fourth switch to be turned off, and the average output current is greater than the first current Threshold, control the third switch to turn off, the first switch to operate at high frequency, the average output current is less than the second current threshold, control the first switch to turn off, and the third switch to operate at high frequency;
  • the controller is specifically used for the negative half cycle of the output voltage: control the third switch to turn on, the first switch to turn off, the average output current is greater than the first current threshold, control the fourth switch to turn off, and the second switch to high frequency action, the average value of the output current is less than the second current threshold value, the second switch tube is controlled to be turned off, and the fourth switch tube is operated at high frequency.
  • the control method of the three-level inverter provided by the embodiment of the present application reduces the number of switching transistors operating at high frequency, and only two switching transistors operate at high frequency for part of the time. Therefore, the switching loss of the switching transistors is reduced. Thereby, the power consumption caused by the high-frequency action is reduced, thereby improving the power conversion efficiency of the three-level NPC inverter.
  • just to provide a current path there are two switches operating at high frequency during a small part of the positive and negative half cycles of the output voltage.
  • the magnitude of the ripple current is generally related to the size of the inductor.
  • the ripple current is smaller; The smaller the inductance, the larger the ripple current.
  • the working conditions of the inverter are fixed, for example, the bus voltage is fixed, the power is fixed, and the inductance is fixed, when the fundamental component of the output current reaches a certain level, that is, the fundamental component of the output current is greater than the preset current threshold, the ripple If the change of the current is less than the preset current threshold, the output current will be greater than zero, or the output current will be less than zero, but it will not fluctuate negatively from time to time. to improve the current path.
  • the photovoltaic system also includes a current sampling circuit, which is used to collect the output current of the inverter.
  • the controller can use the current sampling circuit to collect multiple samples at multiple sampling moments in one switching cycle. current to obtain the average value of the output current for one switching cycle. There is no need to sample the positive and negative half cycles of the output voltage.
  • a reference voltage is given to control the output voltage. Therefore, the controller can determine the positive and negative half cycles of the known reference voltage respectively. Positive and negative half cycles of the inverter's output voltage.
  • FIG. 9 this figure is a schematic diagram corresponding to the high frequency action of the first switch tube in the positive half cycle of the output voltage provided by the embodiment of the present application.
  • FIG. 10 is a schematic diagram corresponding to the high frequency action of the third switch tube in the positive half cycle of the output voltage provided by the embodiment of the present application.
  • the second switch tube T2 is controlled to be turned on all the time
  • the fourth switch tube T4 is controlled to be turned off all the time.
  • the first switch tube T1 when the average value of the output current of the inverter is greater than the first current threshold, the first switch tube T1 is controlled to operate at a high frequency, and the third switch tube T3 is controlled to be turned off, that is, the frequency of T1 is higher than the power frequency Switching action, T3 has been turned off. At this time, the current path is switched between the first switch tube T1 and the first diode D1.
  • the current path is from BUS+ to the output end through T1 and T2 in sequence.
  • the current path is from the midpoint O of the DC bus, that is, the common terminal of C1 and C2, to the output terminal through the first diode D1 and the second switch tube T2.
  • the first switch tube T1 is controlled to turn off, and the third switch tube T3 is controlled to operate at high frequency, that is, T1 is always turned off, and T3 is switched at a frequency higher than the power frequency. action.
  • the current path at this time is switched between the third switch tube T3 and the second diode D2.
  • the output current reaches the midpoint O of the DC bus through the third switch tube T3 and the second diode D6.
  • the current path is from the anti-parallel diode of the second switch tube T2 and the anti-parallel diode of the first switch tube T1 to the DC positive bus BUS+.
  • this figure is a schematic diagram corresponding to the high frequency action of the second switch tube in the negative half cycle of the output voltage provided by the embodiment of the present application.
  • this figure is a schematic diagram corresponding to the high frequency action of the fourth switch tube in the negative half cycle of the output voltage provided by the embodiment of the present application.
  • the third switch tube T3 is controlled to be turned on all the time, and the first switch tube T1 is controlled to be turned off all the time.
  • the fourth switch T4 is controlled to be turned off all the time, and the second switch T2 is controlled to operate at a high frequency, that is, T2 operates at a higher frequency than the power frequency. Frequency switching action. At this time, the current path is switched between the second switch tube T2 and the first diode D1.
  • the current path is to flow from the midpoint O of the DC bus through the first diode D1 and the second switch transistor T2 to reach the output end.
  • the current path is from the DC negative bus BUS- through the anti-parallel diode of the fourth switch T4 and the anti-parallel diode of the third switch T3 to the output end.
  • the second switch tube T2 is controlled to be turned off, and the fourth switch tube T4 is controlled to operate at high frequency, that is, T2 is always turned off, and T4 switches at a frequency higher than the power frequency.
  • the current path at this time is switched between the fourth switch transistor T4 and the second diode D6.
  • the current path is from the third switch tube T4 and the fourth switch tube T4 to the DC negative bus BUS-.
  • the fourth switch T4 is turned off, since the third switch T3 is always on, the current path is from the third switch T3 and the second diode D2 to the midpoint O of the DC bus.
  • the first current threshold and the second current threshold may be fixed values, that is, after setting, the first current threshold and the second current threshold will not be adjusted.
  • the first current threshold and/or the second current threshold may be adjusted according to the actual operating conditions, that is, only the first current may be adjusted Threshold, only the second current threshold may be adjusted, or both the first current threshold and the second current threshold may be adjusted.
  • the parameters that may be affected by the actual operating conditions include: total current harmonic distortion rate of the output current, modulation ratio of the three-level midpoint clamped inverter, power factor of the inverter or three-level midpoint clamped inverse output power of the inverter.
  • one of the following can be selected to adjust the first current threshold value.
  • the controller is further configured to obtain the THDi of the output current. When the THDi increases, the first current threshold is increased, otherwise, the first current threshold is decreased.
  • the first current threshold when the first current threshold is set too small, it may be inappropriate, and the harmonics of the output current will become larger, resulting in a larger THDi. Therefore, in order to reduce the harmonics, the first current threshold can be set larger.
  • the controller is also used to obtain the modulation ratio of the three-level mid-point clamp inverter.
  • the modulation ratio decreases, the first current threshold is increased, and vice versa, the first current threshold is decreased; the modulation ratio is the three-level midpoint The ratio of the output phase voltage peak value of the clamped inverter to the half DC bus voltage, that is, the ratio of the output phase voltage peak voltage to half the DC bus voltage.
  • the modulation ratio represents the voltage difference between the peak value of the output phase voltage and the half-DC bus voltage.
  • the voltage difference between the two is smaller, that is, the closer the two are, the greater the modulation ratio; The smaller the modulation ratio.
  • the first current threshold value needs to be reduced, that is, the switch tube is more precisely controlled to perform high-frequency action, so as to improve the ripple current.
  • the controller is further configured to obtain the power factor of the three-level midpoint clamped inverter. If the absolute value of the power factor decreases, the first current threshold is increased, otherwise, the first current threshold is decreased.
  • the power factor of the inverter becomes smaller, it means that the inverter is sending reactive power to the AC grid, and the ripple current will be relatively large at this time. Therefore, the first current threshold needs to be increased.
  • the controller is further configured to obtain the output power of the three-level mid-point clamped inverter, if the output power decreases, the first current threshold is increased, otherwise, the first current threshold is decreased.
  • the output power is positively correlated with the average value of the output current. Therefore, when the output power decreases, the effective value of the output current is smaller, and the first current threshold needs to be increased. Conversely, when the output power increases, the average value of the output current increases, and the first current threshold value can be decreased.
  • the second current threshold is a positive number greater than 0, and the second current threshold is a negative number less than 0, when adjusted based on the same parameters, the adjustment trends for the first current threshold and the second current threshold are opposite.
  • the controller is further configured to obtain the total current harmonic distortion rate of the output current. If the total current harmonic distortion rate increases, the second current threshold is decreased, and vice versa, the second current threshold is increased.
  • the controller is also used to obtain the modulation ratio of the three-level mid-point clamp inverter. If the modulation ratio decreases, the second current threshold is reduced, and vice versa, the second current threshold is increased; the modulation ratio is the three-level midpoint The ratio of the output phase voltage peak value of the clamped inverter to the half DC bus voltage, that is, the ratio of the output phase voltage peak value to half the DC bus voltage.
  • the controller is further configured to obtain the power factor of the three-level midpoint clamped inverter. If the absolute value of the power factor decreases, the second current threshold is decreased, and vice versa, the second current threshold is increased.
  • the controller is further configured to obtain the output power of the three-level mid-point clamped inverter. If the output power decreases, the second current threshold is decreased, and vice versa, the second current threshold is increased.
  • the photovoltaic system provided by the embodiments of the present application does not limit the specific topology architecture, for example, it may be any one of the architectures shown in FIG. 1 to FIG. 3 .
  • the three-level NPC inverter introduced above may be a centralized inverter
  • the inverter, the string inverter, or the household single-phase inverter are not limited in the embodiments of the present application. Since most of the photovoltaic system transmits active power to the grid, the output voltage of the inverter and the output current have the same direction. Therefore, the time period in which the directions of the output voltage and the output current are different is very short. The solution provided by the embodiment of the present application is applied.
  • the three-level NPC inverter in the photovoltaic system introduced in the above embodiments may be a centralized inverter, a string inverter, or a household single-phase inverter.
  • the three-level NPC inverter in the embodiment of the present application is not only applicable to photovoltaic systems, but also applicable to other power supply systems, such as the power supply of a data center, or the power supply system of a base station, etc. In addition, it can also be applied to fields such as electric vehicles , therefore, the embodiments of the present application do not limit the application scenario of the three-level NPC inverter.
  • FIG. 13 is a schematic diagram of a three-level NPC inverter provided by an embodiment of the present application.
  • the three-level NPC inverter includes at least the following four switch tubes: a first switch tube T1, a second switch tube T2, a third switch tube T3 and a fourth switch tube T4; a first switch tube
  • the first end of T1 is connected to the DC positive bus BUS+
  • the second end of the first switch tube T1 is connected to the first end of the second switch tube T2
  • the second end of the second switch tube T2 is connected to the first end of the third switch tube T3
  • the second end of the third switch tube T3 is connected to the first end of the fourth switch tube T4
  • the second end of the fourth switch tube T4 is connected to the DC negative bus BUS-
  • the second end of the second switch tube T2 is three-level The midpoint clamps the output of the inverter, Vout.
  • the inverter further includes: an inverter controller 402, configured to control the output current average value of the three-level neutral point clamp inverter to be greater than the first current threshold value or less than the second current threshold value, and to control the output current of the four switch tubes. Only one switch tube operates at high frequency to reduce the power consumption caused by high frequency operation; the average output current is the average value of the output current of the three-level mid-point clamped inverter in the switching cycle; the output of the inverter If the current flows out of the inverter, it is positive, otherwise, it is negative; the first current threshold is a positive number, and the second current threshold is a negative number.
  • the inverter controller 402 is specifically used to clamp the positive half cycle of the output voltage of the inverter at the three-level midpoint: control the second switch to be turned on, the fourth switch to be turned off, and the average output current is greater than the first
  • the current threshold controls the third switch to turn off, the first switch operates at high frequency, and when the average output current is less than the second current threshold, the first switch is controlled to turn off, and the third switch operates at high frequency.
  • the inverter controller is specifically used for the negative half cycle of the output voltage: controlling the third switch to be turned on, the first switch to be turned off, the average value of the output current is greater than the first current threshold, to control the fourth switch to be turned off, and the second switch to be turned off.
  • the switch tube operates at high frequency and the average output current is less than the second current threshold, the second switch tube is controlled to be turned off, and the fourth switch tube is operated at high frequency.
  • the inverter controller 402 is further configured to adjust the first current threshold according to any one of the following parameters, where any one of the parameters includes: the total current harmonic distortion rate of the output current, the modulation of the three-level midpoint clamped inverter ratio, the power factor of the inverter, or the output power of the three-level midpoint clamped inverter.
  • the inverter controller 402 is further configured to adjust the second current threshold according to any one of the following parameters, where any one of the parameters includes: the total current harmonic distortion rate of the output current, the modulation of the three-level midpoint clamped inverter ratio, the power factor of the inverter, or the output power of the three-level midpoint clamped inverter.
  • the inverter controller 402 adjusts the first current threshold and the second current threshold according to any of the above parameters, reference may be made to the description of the photovoltaic system embodiments above.
  • the three-level midpoint clamp inverter provided by the embodiment of the present application further includes: a first capacitor C1, a second capacitor C2, a first diode D1 and a second diode D2;
  • the first end of the first capacitor C1 is connected to the DC positive bus BUS+, the second end of the first capacitor C1 is connected to the first end of the second capacitor C2, and the second end of the second capacitor C2 is connected to the DC negative bus BUS-;
  • the cathode of the first diode D1 is connected to the second end of the first switch tube T1
  • the anode of the first diode D1 is connected to the second end of the first capacitor C1
  • the cathode of the second diode D2 is connected to the first diode
  • the anode of the tube D1 and the anode of the second diode D2 are connected to the second end of the third switch tube T3.
  • the three-level NPC inverter provided by the embodiment of the present application does not control two of the four switches to perform high-frequency complementary actions every half cycle, but according to the average output current and the preset threshold in real time Make a comparison, and control the switch tube to act according to the comparison result.
  • control the two switches to perform high-frequency action when the average value of the output current is outside the preset interval, only control the A switch tube can perform high frequency action.
  • the preset interval is an interval near 0, that is, a preset interval formed by a second current threshold less than 0 and a first current threshold greater than 0. Since the technical solution provided in this embodiment can greatly reduce the high-frequency action time of the switch tube, the power consumption caused by the high-frequency action of the switch tube in the inverter can be reduced, thereby improving the power conversion efficiency of the inverter. .
  • the embodiment of the present application further provides a control method of the inverter, which will be described in detail below with reference to the accompanying drawings.
  • FIG. 14 is a flowchart of an inverter control method provided by an embodiment of the present application.
  • the inverter control method provided by the embodiment of the present application is applied to the three-level midpoint clamp inverter introduced in the above embodiment.
  • the three-level midpoint clamp inverter shown in FIG. 13 It includes at least the following four switch tubes: a first switch tube T1, a second switch tube T2, a third switch tube T3 and a fourth switch tube T4; the first end of the first switch tube T1 is connected to the DC positive bus BUS+, the first switch tube T1
  • the second end of the switch tube T1 is connected to the first end of the second switch tube T2, the second end of the second switch tube T2 is connected to the first end of the third switch tube T3, and the second end of the third switch tube T3 is connected to the fourth
  • the first end of the switch tube T4 and the second end of the fourth switch tube T4 are connected to the DC negative bus BUS-; the second end of the second switch tube T2 is the output end of the three-level midpoint clamp inverter, namely Vout .
  • the method includes:
  • S1401 Obtain an average output current of the three-level midpoint clamped inverter; wherein, the average output current may be an average value of the output current of the three-level midpoint clamped inverter in a switching period.
  • the average value of the output current in one cycle of the switch tube can be collected.
  • the switching cycle collects the output currents of several sampling points, and then obtains the average value of the output currents at the sampling points.
  • the average value of the output current of the three-level mid-point clamp inverter is greater than the first current threshold or less than the second current threshold, and only one of the four switches is controlled to operate at high frequency to reduce the high-frequency operation band
  • the output current of the inverter is positive if it flows out of the inverter, otherwise it is negative; the first current threshold is a positive number, and the second current threshold is a negative number.
  • the average value of the output current of the inverter is obtained in real time, the average value of the output current is compared with a preset threshold, and the switch tube is controlled to act according to the comparison result.
  • the preset interval is an interval near 0, that is, a preset interval formed by a second current threshold less than 0 and a first current threshold greater than 0.
  • the average output current of the three-level mid-point clamp inverter is greater than the first current threshold or less than the second current threshold, and controls only one of the four switches to operate at high frequency, which specifically includes:
  • the controller is specifically used for the negative half cycle of the output voltage: control the third switch to turn on, the first switch to turn off, the average output current is greater than the first current threshold, control the fourth switch to turn off, and the second switch to high
  • control the third switch to turn on the first switch to turn off
  • the average output current is greater than the first current threshold
  • control the fourth switch to turn off
  • the second switch to high
  • the second switch tube is controlled to be turned off, and the fourth switch tube is operated at a high frequency.
  • the absolute value of the first current threshold and the absolute value of the second current threshold may be equal or may not be equal, which are not specifically limited in the embodiments of the present application.
  • the first current threshold can be adjusted according to any one of the following parameters.
  • Any one of the parameters includes: the total current harmonic distortion rate of the output current, the modulation ratio of the three-level midpoint clamped inverter , the power factor of the inverter or the output power of the three-level midpoint clamped inverter.
  • Adjust the second current threshold according to any one of the following parameters, any one of which includes: the total current harmonic distortion rate of the output current, the modulation ratio of the three-level midpoint clamp inverter, the power factor of the inverter or the three The midpoint of the level clamps the output power of the inverter.
  • At least one (item) refers to one or more, and "a plurality” refers to two or more.
  • “And/or” is used to describe the relationship between related objects, indicating that there can be three kinds of relationships, for example, “A and/or B” can mean: only A, only B, and both A and B exist , where A and B can be singular or plural.
  • the character “/” generally indicates that the associated objects are an “or” relationship.
  • At least one item(s) below” or similar expressions thereof refer to any combination of these items, including any combination of single item(s) or plural items(s).
  • At least one (a) of a, b or c can mean: a, b, c, "a and b", “a and c", “b and c", or "a and b and c" ", where a, b, c can be single or multiple.

Abstract

本申请公开了一种光伏系统、逆变器及控制方法，三电平中点箝位逆变器包括四个开关单元，三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；输出电流平均值为三电平中点箝位逆变器的输出电流在开关周期内的平均值；第一电流阈值为正数，第二电流阈值为负数。可以降低逆变器中由于开关单元的高频动作带来的功耗，从而可以提高逆变器的电能转换效率。两个开关单元在高频互补动作的时间段很小，因此，在一个开关单元高频动作期间减少死区时间的设置，或者不设置死区时间，因此可以改善逆变器的输出电流的电流谐波总畸变率，即改善谐波质量。

Description

一种光伏系统、逆变器及控制方法 技术领域
本申请涉及电力电子技术领域，尤其涉及一种光伏系统、逆变器及控制方法。
背景技术
目前，随着光伏发电、电动汽车或电气传动等领域对逆变器的需求不断增加，例如需要逆变器的转换效率较高而且体积较小。因此，目前大部分逆变器采用多电平控制，这样有助于提升逆变器的电压等级和功率密度，提高逆变器输出的交流电的质量。
目前，比较常用的多电平逆变器为三电平逆变器，尤其是基于二极管中点箝位(NPC，Neutral Point Clamped)的三电平逆变器最为常用。
但是，目前对于三电平NPC逆变器的控制方式导致电能转换的效率较低，而且谐波较差。
发明内容
本申请提供了一种光伏系统、逆变器及控制方法，能够提高逆变器的电能转换效率，而且改善谐波质量。
本申请实施例提供一种光伏系统，其中包括的逆变器为三电平NPC逆变器，为了减低逆变器中开关单元的高频动作带来的开关损耗，本申请实施例提供了一种新的控制方式来尽量缩短开关单元高频动作的时间，进而降低高频动作带来的开关损耗，从而提高电能转换效率。三电平NPC逆变器至少包括四个开关单元；在三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值时，控制器控制四个开关单元中的仅一个开关单元高频动作，由于相对于传统的动作方式少了一个开关管的高频动作，因此可以降低高频动作带来的功耗；为了更加准确地控制开关单元动作，其中输出电流平均值为三电平NPC逆变器的输出电流在开关周期内的平均值；其中开关周期为开关单元中开关管的开关周期。逆变器的输出电流以流出逆变器为正，反之为负；其中，第一电流阈值为正数，第二电流阈值为负数。
本申请实施例提供的技术方案是三电平NPC逆变器的输出电流平均值在零附近的预设区间内时，说明输出电流比较小，容易出现时正时负的情况，因此，需要控制两个开关管高频动作，以便于当输出电流波动时提供电流路径。但是当输出电流平均值在预设区间以外时，说明输出电流的绝对值比较大，不在零电流附近，因此不会出现时正时负的情况，因此，仅控制其中一个开关管高频动作即可。这样可以大大减少开关管的高频动作，从而降低开关管高频动作带来的损耗，提高逆变器的电能转换效率。另外，由于本申请实施例提供的技术方案，大部分时间仅有一个开关管在高频动作，两个开关管在高频互补动作的时间段很小，因此，在预设区间以外可以减少死区时间的设置，或者不设置死区时间，因此可以改善逆变器的输出电流的电流谐波总畸变率。
第一电流阈值的绝对值和第二电流阈值的绝对值可以相等，也可以不相等。当两者的绝对值不相等时，第一电流阈值的绝对值可以大于第二电流阈值的绝对值，也可以第一电流阈值的绝对值小于第二电流阈值的绝对值。
当第一电流阈值的绝对值和第二电流阈值的绝对值相等时，即第二电流阈值和第一电 流阈值关于0点对称，即预设区间是关于0点的一个对称区间。
以上介绍的是四个开关单元中一个开关单元高频动作的时间，下面介绍两个开关单元高频动作的时间，即控制器还用于输出电流平均值大于第二电流阈值小于第一电流阈值，控制四个开关单元中的两个开关单元高频动作，即仅在需要时才控制两个开关单元高频动作，为了正负变化的电流提供路径。
具体的控制方式可以根据输出电压的正半周和负半周来分类控制，即控制器具体用于在三电平中点箝位逆变器的输出电压的正半周：控制第二开关单元导通，第四开关单元关断，输出电流平均值大于第一电流阈值，控制第三开关单元关断，第一开关单元高频动作，输出电流平均值小于第二电流阈值，控制第一开关单元关断，第三开关单元高频动作；
控制器具体用于在输出电压的负半周：控制第三开关单元导通，第一开关单元关断，输出电流平均值大于第一电流阈值，控制第四开关单元关断，第二开关单元高频动作，输出电流平均值小于第二电流阈值，控制第二开关单元关断，第四开关单元高频动作。
下面介绍第一电流阈值和第二电流阈值的取值原则，一种可能的实现方式，在一个工频周期内，即逆变器的输出电压对应的工频周期，按照传统的控制方式，在输出电压的每半个周期有两个开关管互补高频动作，即T1和T3高频互补动作，T2和T4高频互补动作，获得逆变器的输出电流在一个开关周期的最小值、最大值和平均值。当一个开关周期内输出电流的最小值为0时，则表明该开关周期内输出电流为正半周，此时该开关周期的输出电流平均值肯定大于0，记为第一平均电流，则第一电流阈值取值为大于等于该开关周期对应的输出电流平均值，即此时输出电流平均值为第一电流阈值的取值下限值。当一个开关周期内输出电流的最大值为0时，则表明该开关周期内输出电流为负半周，此时该开关周期的输出电流平均值肯定小于0，记为第二平均电流，则第二电流阈值小于等于该开关周期对应的输出电流平均值，即此时的输出电流平均值为第二电流阈值的取值上限值。
以上介绍的第一电流阈值和第二电流阈值可以固定不变，也可以根据一些参数来调整，下面介绍几种调整方式。为了更加准确地控制逆变器中各个开关管的动作，可以根据实际运行工况来调整第一电流阈值和/或第二电流阈值，即可以仅调整第一电流阈值，也可以仅调整第二电流阈值，也可以第一电流阈值和第二电流阈值均调节。其中实际运行工况可能影响的参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
在一种可能的实现方式中，控制器获得输出电流的电流谐波总畸变率，电流谐波总畸变率增大，则增大第一电流阈值，反之减小第一电流阈值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的调制比，调制比减小，则增大第一电流阈值，反之减小第一电流阈值；调制比为三电平中点箝位逆变器的输出相电压峰值与半直流母线电压的比值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的功率因数，功率因数的绝对值减小，则增大第一电流阈值，反之减小第一电流阈值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的输出功率，输出功率减小，则增大第一电流阈值，反之减小第一电流阈值。
在一种可能的实现方式中，控制器获得输出电流的电流谐波总畸变率，电流谐波总畸变率增大，则减小第二电流阈值，反之增大第二电流阈值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的调制比，调制比减小，则减小第二电流阈值，反之增大第二电流阈值；调制比为三电平中点箝位逆变器的输出相电压峰值与半直流母线电压的比值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的功率因数，功率因数的绝对值减小，则减小第二电流阈值，反之增大第二电流阈值。
在一种可能的实现方式中，控制器获得三电平中点箝位逆变器的输出功率，输出功率减小，则减小第二电流阈值，反之增大第二电流阈值。
本申请实施例不限定光伏系统的具体类型，即该三电平NPC逆变器可以为集中式逆变器、组串式逆变器或户用单相逆变器。
在一种可能的实现方式中，三电平中点箝位逆变器还包括：第一电容、第二电容、第一二极管和第二二极管；第一电容的第一端连接直流正母线，第一电容的第二端连接第二电容的第一端，第二电容的第二端连接直流负母线；第一二极管的阴极连接第一开关单元的第二端，第一二极管的阳极连接第一电容的第二端，第二二极管的阴极连接第一二极管的阳极，第二二极管的阳极连接第三开关单元的第二端。
以上实施例介绍的是一种光伏系统，本申请实施例还提供一种三电平NPC逆变器，下面进行介绍，本申请实施例不限定三电平NPC的具体应用场景，可以为需要逆变器的任何场景，除了光伏发电场景以外，还可以为其他需要直流转换为交流的场景，例如数据中心、基站或者电动汽车等。以上实施例提供的各个方案的优点同样适用于以下的逆变器，在此不再赘述。
本申请实施例还提供一种三电平中点箝位逆变器，至少包括四个开关单元；逆变器控制器，用于三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；输出电流平均值为三电平中点箝位逆变器的输出电流在开关周期内的平均值；逆变器的输出电流以流出逆变器为正，反之为负；第一电流阈值为正数，第二电流阈值为负数。
在一种可能的实现方式中，逆变器控制器，具体用于在三电平中点箝位逆变器的输出电压的正半周：控制第二开关单元导通，第四开关单元关断，输出电流平均值大于第一电流阈值，控制第三开关单元关断，第一开关单元高频动作；输出电流平均值小于第二电流阈值，控制第一开关单元关断，第三开关单元高频动作；
逆变器控制器，具体用于在输出电压的负半周：控制第三开关单元导通，第一开关单元关断，输出电流平均值大于第一电流阈值，控制第四开关单元关断，第二开关单元高频动作；输出电流平均值小于第二电流阈值，控制第二开关单元关断，第四开关单元高频动作。
在一种可能的实现方式中，逆变器控制器根据以下任意一种参数调节第一电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
在一种可能的实现方式中，逆变器控制器根据以下任意一种参数调节第二电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
在一种可能的实现方式中，三电平中点箝位逆变器还包括：第一电容、第二电容、第一二极管和第二二极管；第一电容的第一端连接直流正母线，第一电容的第二端连接第二电容的第一端，第二电容的第二端连接直流负母线；第一二极管的阴极连接第一开关单元的第二端，第一二极管的阳极连接第一电容的第二端，第二二极管的阴极连接第一二极管的阳极，第二二极管的阳极连接第三开关单元的第二端。
基于以上实施例提供的一种三电平NPC逆变器，本申请实施例还提供一种逆变器的控制方法，应用于三电平中点箝位逆变器，具体的实现方式的优点也适用于以下的方法，在此不再赘述。
三电平中点箝位逆变器至少包括四个开关单元；该方法包括：获得三电平中点箝位逆变器的输出电流平均值；输出电流平均值为三电平中点箝位逆变器的输出电流在开关周期内的平均值；三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；逆变器的输出电流以流出逆变器为正，反之为负；第一电流阈值为正数，第二电流阈值为负数。
在一种可能的实现方式中，三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关单元中的仅一个开关单元高频动作，具体包括：在三电平中点箝位逆变器的输出电压的正半周：控制第二开关单元导通，第四开关单元关断，输出电流平均值大于第一电流阈值，控制第三开关单元关断，第一开关单元高频动作；输出电流平均值小于第二电流阈值，控制第一开关单元关断，第三开关单元高频动作；在输出电压的负半周：控制第三开关单元导通，第一开关单元关断，输出电流平均值大于第一电流阈值，控制第四开关单元关断，第二开关单元高频动作；输出电流平均值小于第二电流阈值，控制第二开关单元关断，第四开关单元高频动作。
从以上技术方案可以看出，本申请实施例具有以下优点：
本申请实施例提供的技术方案，实时获得逆变器的在一个开关周期内的输出电流平均值，根据输出电流平均值与预设区间进行比较，根据比较结果来控制开关单元进行动作。当输出电流平均值位于预设区间内时，控制四个开关单元中的两个开关单元进行高频动作；当输出电流平均值位于预设区间之外时，仅控制四个开关单元中的一个开关单元进行高频动作即可。其中预设区间是0附近的一个区间，即由小于0的第二电流阈值和大于0的第一电流阈值形成的预设区间。该方案区别于传统的对于四个开关单元，每半个周期控制其中两个开关单元进行高频互补动作。由于本实施例提供的技术方案可以大大降低开关单元高频动作的时间，因此，可以降低逆变器中由于开关单元的高频动作带来的功耗，从而可以提高逆变器的电能转换效率。另外，由于本申请实施例提供的技术方案，大部分时间仅有一个开关单元在高频动作，两个开关单元在高频互补动作的时间段很小，因此，在预设区间以外可以减少死区时间的设置，或者不设置死区时间，因此可以改善逆变器的输出电 流的电流谐波总畸变率，即改善谐波质量。
附图说明
图1为本申请实施例提供的一种集中式逆变器对应的光伏系统示意图；
图2为本申请实施例提供的一种组串式逆变器对应的光伏系统的示意图；
图3为本申请实施例提供的一种户用单相光伏系统的示意图；
图4为本申请实施例提供的一种三电平NPC逆变器的示意图；
图5A为一种脉冲驱动信号的时序图；
图5B为本申请实施例提供的另一种三电平NPC逆变器的示意图；
图6为本申请实施例提供的一种光伏系统的示意图；
图7为本申请实施例提供的光伏系统中的三电平NPC逆变器的驱动时序图；
图8为本申请实施例提供的逆变器的输出电流的波形示意图；
图9为本申请实施例提供的输出电压正半周第一开关管高频动作对应的示意图；
图10为本申请实施例提供的输出电压正半周第三开关管高频动作对应的示意图；
图11为本申请实施例提供的输出电压负半周第二开关管高频动作对应的示意图；
图12为本申请实施例提供的输出电压负半周第四开关管高频动作对应的示意图；
图13为本申请实施例提供的另一种三电平NPC逆变器的示意图；
图14为本申请实施例提供的一种三电平NPC逆变器的控制方法流程图。
具体实施方式
以下说明中的“第一”、“第二”等用词仅用于描述目的，而不能理解为指示或暗示相对重要性或者隐含指明所指示的技术特征的数量。由此，限定有“第一”、“第二”等的特征可以明示或者隐含地包括一个或者更多该特征。在本申请的描述中，除非另有说明，“多个”的含义是两个或两个以上。
此外，本申请中，“上”、“下”等方位术语可以包括但不限于相对附图中的部件示意置放的方位来定义的，应当理解到，这些方向性术语可以是相对的概念，它们用于相对于的描述和澄清，其可以根据附图中部件附图所放置的方位的变化而相应地发生变化。
在本申请中，除非另有明确的规定和限定，术语“连接”应做广义理解，例如，“连接”可以是固定连接，也可以是可拆卸连接，或成一体；可以是直接相连，也可以通过中间媒介间接相连。此外，术语“耦接”可以是实现信号传输的电性连接的方式。“耦接”可以是直接的电性连接，也可以通过中间媒介间接电性连接。
本申请实施例涉及一种光伏系统，尤其涉及应用于光伏系统中的一种三电平逆变器，由于光伏组件输出的为直流电，因此，需要逆变器将直流电变换为交流电后反馈给电网或提供给用电设备。本申请实施例不限定光伏系统的具体结构，可以为包括集中式逆变器的光伏系统，也可以为包括组串式逆变器的光伏系统，还可以为家庭户用的单相光伏系统。以上几种光伏系统均可以包括三电平逆变器。下面集合附图介绍以上三种光伏系统的拓扑形式。
参见图1，该图为本申请实施例提供的一种集中式逆变器对应的光伏系统示意图。
当光伏系统中包括集中式逆变器时，集中式逆变器中仅包括逆变电路，即功率变换仅 包括直流-交流转换电路，不包括直流-直流转换电路。
如图1所示，光伏系统包括多个直流汇流箱，例如包括直流汇流箱100a至直流汇流箱100n，多个直流汇流箱的输出端均连接集中式逆变器200的输入端，集中式逆变器200的输出端连接变压器T的原边绕组，变压器T的副边绕组可以连接交流电网。
每个直流汇流箱的输入端可以直接连接多个光伏组串，光伏组串的数量不做具体限定。另外，直流汇流箱的输入端也可以通过光伏优化器连接光伏组串，每个光伏组串对应一个光伏优化器。该光伏系统是否具有光伏优化器，本申请实施例中不做具体限定。
图1所示的集中式逆变器200可以为三电平NPC逆变器。
下面介绍另一种光伏系统，该光伏系统包括组串式逆变器。
参见图2，该图为本申请实施例提供的一种组串式逆变器对应的光伏系统的示意图。
图2所示的光伏系统可以包括多个组串式逆变器，如图2所示，分别为组串式逆变器200a至组串式逆变器200n。以每个组串式逆变器的输入端连接两个直流升压变流器为例进行介绍，例如组串式逆变器200a的输入端连接直流升压变流器300a和直流升压变流器300b。组串式逆变器200b的输入端连接直流升压变流器300c和直流升压变流器300d，本申请实施例中不限定组串式逆变器的输入端连接的直流升压变流器的数量。
其中，以每个直流升压变流器的输入端连接两个光伏组串为例进行介绍，本申请实施例不限定直流升压变流器的输入端连接的光伏组串的数量。
组串式逆变器200a至组串式逆变器200n的输出端均连接交流汇流箱400的输入端，交流汇流箱400的输出端连接变压器T的原边绕组，变压器T的副边绕组可以连接交流电网。
图2所述的组串式逆变器可以采用三电平NPC逆变器。
下面结合图3介绍家庭户用的光伏系统，由于家庭用的光伏系统直接供家庭用电设备使用，因此输出单相市电即可，例如交流220V。如果电量有富余，还可以反馈给交流电网，
参见图3，该图为本申请实施例提供的一种户用单相光伏系统的示意图。
户用光伏系统中的光伏并网逆变器400也可以采用三电平NPC逆变器。其中光伏并网逆变器400的输入端连接多路DC/DC电路，例如DC/DC电路500a至DC/DC电路500n。DC/DC电路与直流开关一一对应，例如DC/DC电路500a的输入端通过直流开关S1连接对应的光伏组串，DC/DC电路500b的输入端通过直流开关Sn连接光伏组串。图3中仅是示意一路DC/DC电路对应连接一路光伏组串，另外，也可以一路DC/DC电路连接多路光伏组串。
光伏并网逆变器400的输出端通过交流开关Q连接单向交流电。
以上介绍的三种光伏系统中的逆变器均可以为三电平NPC逆变器。为了使本领域技术人员更好地理解本申请实施例提供的技术方案，下面先结合附图介绍三电平NPC逆变器的电路结构。
参见图4，该图为一种三电平NPC逆变器的示意图。
图4所示的为单相逆变器的示意图。
该三电平NPC逆变器包括：四个开关管、两个二极管和两个电容。其中四个开关管分 别为：第一开关管T1、第二开关管T2、第三开关管T3和第四开关管T4。两个二极管分别为第一二极管D1和第二二极管D2。两个电容分别为：第一电容C1和第二电容C2。在光伏系统中，第一电容C1和第二电容C2均为母线电容，即C1和C2串联后连接在直流正母线BUS+和直流负母线BUS-之间。
其中第一电容C1的第一端连接第一开关管T1的第一端，并且第一电容C1的第一端和第一开关管T1的第一端均连接直流正极，在光伏系统中直流正极可以为直流正母线BUS+，第一电容C1的第二端连接第二电容C2的第一端，第二电容C2的第二端连接直流负极，在光伏系统中直流负极可以为直流负母线BUS-。
第一开关管T1的第二端连接第二开关管T2的第一端，第二开关管T2的第二端连接第三开关管T3的第一端，第三开关管T3的第二端连接第四开关管T4的第一端，第四开关管T4的第二端连接直流负极，在光伏系统中连接直流负母线BUS-。
第一二极管D1的阴极连接第一开关管T1的第二端，第一二极管D1的阳极连接第一电容C1的第二端，即C1和C2的公共端。第二二极管D2的阴极连接第一二极管D1的阳极，第二二极管D2的阳极连接第三开关管T3的第二端。
本申请实施例中不限定四个开关管T1-T4的实现方式，例如可以为金属-氧化物半导体场效应晶体管(MOS，Metal-Oxide-Semiconductor Field-Effect Transistor)管，或者为绝缘栅双极型晶体管(IGBT，Isolated Gate Bipolar Transistor)，并且四个开关管T1-T4均包括反并联二极管，其中反并联二极管是为了在开关管关断时提供电流路径。
该三电平NPC逆变器的输出端为第二开关管T2的第二端。
三电平NPC逆变器的四个开关管对应的脉冲驱动信号参见图5A，该图为一种脉冲驱动信号的时序图。
图5A中PWM1对应的脉冲驱动信号用于驱动第一开关管T1动作，PWM2对应的脉冲驱动信号用于驱动第二开关管T2动作，PWM3对应的脉冲驱动信号用于驱动第三开关管T3动作，PWM4对应的脉冲驱动信号用于驱动第四开关管T4动作。一般情况下，在脉冲驱动信号的高电平时间段内，开关管导通，在脉冲驱动信号的低电平时间段内，开关管关断。
在三电平NPC逆变器的输出电压的正半周内，第一开关管T1和第三开关管T3高频互补开关，第二开关管T2导通，第四开关管T4关断。其中高频是指开关管的开关频率高于工频频率，例如工频频率为50Hz。相反，在三电平NPC逆变器的输出电压的负半周内，第二开关管T2和第四开关管T4高频互补开关，第一开关管T1关断，第三开关管T3导通。
结合图4分析图5A的脉冲驱动信号的弊端，在输出电压Vout的正半周，T1高频动作，T2导通，T3高频动作，T4关断，电流路径为BUS+—T1—T2—Vout。但是实际工作过程中，在输出电压的正半周的大部分时间段，电流并没有通过T3，因此，T3高频动作将带来较大的功耗。同理，在输出电压Vout的负半周，T2和T4高频动作，T1关断，T3导通，电流路径为BUS-—T4—T3—Vout，实际工作时，在输出电压的负半轴的大部分时间段内，电流并没有通过T2，因此，T2高频动作将带来较大的功耗。以上带来的功耗会均造成三电平NPC逆变器的转换效率降低。
为了描述方便，以下将三电平NPC逆变器简称为逆变器。
因此，为了解决传统对于三电平NPC逆变器控制的弊端，本申请实施例提供一种新的控制方式，尤其当光伏系统中应用该三电平NPC逆变器时效果尤为突出，因此在光伏系统中，光伏组串输出的为直流电，即对应直流功率，将该直流功率传输到交流电网，只发有功功率即可，因此，大部分时间和场景下，逆变器的输出电压的方向和输出电流的方向相同，即同向，仅有一小部分时间段或场景下，逆变器的输出电压的方向和输出电流的方向不同，即反向。如果定义从第二开关管T2的第二端流出的方向为正，则在一小部分时间段，输出电流为负。即逆变器的输出电流以流出所述逆变器为正，反之为负。由于交流电网的电流和电压为正弦波，则需要输出电流的平均值为正弦波。但是，当输出电流的平均值较小时，较小的平均电流叠加一个纹波电流后，该纹波电流是由于开关管动作过程产生的，因此，叠加上纹波电流以后，输出电流会有时正有时负，产生波动。因此，只有当输出电流为负时，才需要T3和T2来暂时提供电流路径，供能量释放。
本申请实施例提供的技术方案是三电平中点箝位逆变器的输出电流平均值在零附近的预设区间内时，说明输出电流比较小，容易出现时正时负的情况，因此，需要控制两个开关管高频动作，以便于当输出电流波动时提供电流路径。但是当输出电流平均值在预设区间以外时，说明输出电流的绝对值比较大，不在零电流附近，因此不会出现时正时负的情况，因此，仅控制其中一个开关管高频动作即可。这样可以大大减少开关管的高频动作，从而降低开关管高频动作带来的损耗，提高逆变器的电能转换效率。另外，由于本申请实施例提供的技术方案，大部分时间仅有一个开关管在高频动作，两个开关管在高频互补动作的时间段很小，因此，在预设区间以外可以减少死区时间的设置，或者不设置死区时间，因此可以改善逆变器的输出电流的电流谐波总畸变率(THDi，Total Harmonic Current Distortion)。
本申请实施例提供的三电平NPC逆变器至少包括以下四个开关单元：第一开关单元、第二开关单元、第三开关单元和第四开关单元；所述第一开关单元的第一端连接直流正母线，所述第一开关单元的第二端连接第二开关单元的第一端，所述第二开关单元的第二端连接所述第三开关单元的第一端，所述第三开关单元的第二端连接所述第四开关单元的第一端，所述第四开关单元的第二端连接直流负母线；所述第二开关单元的第二端为所述三电平中点箝位逆变器的输出端；本申请实施例不限定四个开关单元的具体实现方式，例如每个开关单元中可以包括一个开关管，也可以包括多个开关管，例如每个开关单元包括两个开关管，如图5B所示，该图为本申请实施例提供的另一种三电平NPC的示意图。
本实施例中以每个开关单元包括两个并联的开关管为例进行介绍，由于单个开关管可以承受的电流有限，当电流较大时，单个开关管可能会被损坏，因此，可以利用多个开关管并联来实现一个开关的功能，并联的多个开关管的驱动信号的时序相同，即多个并联的开关管同时动作，同时关断或同时导通。多个开关管并联在一起可以增大整个开关单元可以通过的电流。例如第一开关单元包括开关管T11和T12，第二开关单元包括开关管T21和T22，第三开关单元包括开关管T31和T32，第四开关单元包括开关管T41和T42。
开关管T11和T12的第一端均连接直流正母线BUS+，开关管T11和T12的的第二端 连接开关管T21和T22的第一端，开关管T21和T22的第二端连接开关管T31和T32的第一端，开关管T31和T32的第二端连接开关管T41和T42的第一端，开关管T41和T42的第二端连接直流负母线BUS-；开关管T21和T22的第二端为三电平中点箝位逆变器的输出端。
图5B中仅是以两个开关管并联来实现一个开关的功能，也可以更多的开关管并联，例如3个或4，或者更多，在此不再一一赘述。
下面结合附图详细介绍本申请实施例提供的光伏系统的工作原理。
参见图6，该图为本申请实施例提供的一种光伏系统的示意图。
本实施例提供的光伏系统包括：三电平中点箝位逆变器和控制器401。
三电平中点箝位逆变器至少包括以下四个开关管：第一开关管T1、第二开关管T2、第三开关管T3和第四开关管T4；第一开关管T1的第一端连接直流正母线，第一开关管T1的第二端连接第二开关管T2的第一端，第二开关管T2的第二端连接第三开关管T3的第一端，第三开关管T3的第二端连接第四开关管T4的第一端，第四开关管T4的第二端连接直流负母线；第二开关管T2的第二端为三电平中点箝位逆变器的输出端，输出端的输出电压为Vout。一般情况下逆变器的输出端会连接电感。
控制器401，用于在三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值时，控制四个开关管中的仅一个开关管高频动作，以降低高频动作带来的功耗。
控制器401可以为光伏系统中独立设置的控制器，也可以为与逆变器集成在一起的控制器，本申请实施例中不做具体限定。
其中，逆变器的输出电流以流出逆变器为正，反之为负；第一电流阈值的绝对值和第二电流阈值的绝对值可以相等，也可以不相等。当两者的绝对值不相等时，第一电流阈值的绝对值可以大于第二电流阈值的绝对值，也可以第一电流阈值的绝对值小于第二电流阈值的绝对值。
当第一电流阈值的绝对值和第二电流阈值的绝对值相等时，即第二电流阈值和第一电流阈值关于0点对称，即预设区间是关于0点的一个对称区间。
其中，输出电流平均值为三电平中点箝位逆变器的输出电流在开关周期内的平均值；第一电流阈值为正数，第二电流阈值为负数。即第一电流阈值I1为大于0的数，第二电流阈值为小于0的数。本申请实施例提供的技术方案在逆变器的输出电流平均值在0附近的预设区间以外时，控制一个开关管高频动作。仅在0附近的预设区间内时，才控制两个开关管高频动作。即控制器401，还用于输出电流平均值大于第二电流阈值小于第一电流阈值，控制四个开关管中的两个开关管高频动作。
为了使本领域技术人员更好地理解本申请实施例提供的技术方案的优点，下面结合图6和图7来分析本申请实施例提供的技术方案的工作原理。
参见图7，该图为本申请实施例提供的光伏系统中的三电平NPC逆变器的驱动时序图。
为了与图5中的脉冲驱动信号进行区别，本申请实施例提供的图7中的四个驱动信号分别用PWMOUT1、PWMOUT1、PWMOUT1和PWMOUT1表示，以上四个脉冲驱动信 号分别为逆变器的四个开关管T1-T4的脉冲驱动信号。
从图7中可以看出，在输出电压的正半周的大部分时间，只有一个开关管在高频动作，即只有第一开关管T1在高频动作，仅有一小部分时间，第三开关管T3在高频动作。其余大部分时间，第三开关管T3关断。
同理，在输出电压的负半周的大部分时间，只有一个开关管在高频动作，即只有第四开关管T4在高频动作，仅有一小部分时间，第二开关管T2在高频动作。
其中，第二开关管T2和第三开关管T3高频动作的时间可以通过检测逆变器的输出电流来实现。应该理解，逆变器的输出电流为逆变器的桥臂输出电流，即第二开关管T2的第二端的输出电流。该输出电流与输出电压类似为交流参数，可正可负。并且由于纹波电流的存在，该输出电流在正弦波的基础上叠加了纹波电流。
具体可以参见图8，该图为本申请实施例提供的逆变器的输出电流的波形示意图。
图8中的幅值较大的正弦波Ug表示输出电压，幅值较小的正弦波ig表示输出电流，锯齿波表示纹波电流。
一般情况下，纹波电流较小，当正弦波的电流幅值较大时，即使叠加了纹波电流，叠加后的输出电流也不会时正时负。但是当正弦波的输出电流幅值较小时，由于纹波电流在一个开关周期内，有时上升有时下降，因此，比较小的输出电流叠加纹波电流，叠加后的输出电流就可能时正时负。
例如图8中，对于输出电压的正半周的T1时间段，叠加纹波电流后的输出电流就容易时正时负，此时需要控制两个开关管高频动作，进而为时正时负的输出电流提供电流路径。在输出电压的正半周的T2时间段内，此时正弦波的幅值较大，因此叠加纹波电流后的输出电流不会时正时负，因此仅控制一个开关管高频动作即可，不必提供更多的电流路径。
同理，图8中的输出电压的负半周的T3时间段，由于正弦波的输出电流的幅值比较小，叠加纹波电流后的输出电流时正时负，因此需要控制两个开关管高频动作，进而为时正时负的输出电流提供电流路径。在输出电压的负半周的T4时间段内，此时正弦波的幅值较大，因此叠加纹波电流后的输出电流不会时正时负，因此仅控制一个开关管高频动作即可，不必提供更多的电流路径。
从图8可以看出，仅在输出电压的正半周和负半周交替对应的一小部分时间段内，控制两个开关管高频动作，其余时间段仅是一个开关管高频动作，因此，相对于图5所示的控制方式，可以降低开关管的高频损耗。
由于实际检测的输出电流是正弦波叠加锯齿波的纹波电流，因此，为了更准确地控制逆变器的开关管动作，可以将逆变器的开关管的一个开关周期内对应的输出电流平均值分别与第一电流阈值和第二电流阈值进行比较。
下面介绍第一电流阈值和第二电流阈值的取值原则。
一种可能的实现方式，在一个工频周期内，即逆变器的输出电压对应的工频周期，按照传统的控制方式，在输出电压的每半个周期有两个开关管互补高频动作，即T1和T3高频互补动作，T2和T4高频互补动作，获得逆变器的输出电流在一个开关周期的最小值、最大值和平均值。当一个开关周期内输出电流的最小值为0时，则表明该开关周期内输出 电流为正半周，此时该开关周期的输出电流平均值肯定大于0，记为第一平均电流，则第一电流阈值取值为大于等于该开关周期对应的输出电流平均值，即此时输出电流平均值为第一电流阈值的取值下限值。当一个开关周期内输出电流的最大值为0时，则表明该开关周期内输出电流为负半周，此时该开关周期的输出电流平均值肯定小于0，记为第二平均电流，则第二电流阈值小于等于该开关周期对应的输出电流平均值，即此时的输出电流平均值为第二电流阈值的取值上限值。
具体实现时，控制器具体用于在三电平中点箝位逆变器的输出电压的正半周：控制第二开关管导通，第四开关管关断，输出电流平均值大于第一电流阈值，控制第三开关管关断，第一开关管高频动作，输出电流平均值小于第二电流阈值，控制第一开关管关断，第三开关管高频动作；
控制器具体用于在输出电压的负半周：控制第三开关管导通，第一开关管关断，输出电流平均值大于第一电流阈值，控制第四开关管关断，第二开关管高频动作，输出电流平均值小于第二电流阈值，控制第二开关管关断，第四开关管高频动作。
相对比图5，在输出电压的正半周和负半周均有两个开关管在开关动作。因此，本申请实施例提供的三电平逆变器的控制方式减少了高频动作的开关管的数量，仅有部分时间两个开关管高频动作，因此，降低了开关管的开关损耗，从而降低由于高频动作带来的功耗，从而提高三电平NPC逆变器的电能转换效率。另一方面，仅是为了提供电流路径，在输出电压的正半周和负半周的一小部分时间内，有两个开关管在高频动作。
下面介绍本申请实施例提供的技术方案中将逆变器的输出电流与预设电流阈值进行比较，根据比较结果来控制开关管动作的原因。
由于逆变器的输出端一般连接电感(图中未示出)，因此，纹波电流的大小一般与电感的大小有关系，当电感的感量越大时，纹波电流越小；当电感的感量越小时，纹波电流越大。在逆变器的工况固定的情况下，例如母线电压固定，功率固定，电感固定，当输出电流的基波分量达到一定程度以后，即输出电流的基波分量大于预设电流阈值，纹波电流的变化量小于预设电流阈值，则输出电流会大于零，或者输出电流会小于零，但是不会时正时负波动了，因此，不必两个开关管高频动作，利用其中一个开关管来提高电流路径。
该光伏系统除了包括逆变器和控制器以外，还包括电流采样电路，用于采集逆变器的输出电流，控制器可以利用电流采样电路采集的一个开关周期内多个采样时刻的多个采样电流，获得一个开关周期的输出电流平均值。对于输出电压的正半周和负半周可以不必采样，一般逆变器控制时，会给定参考电压来控制输出电压，因此，控制器可以根据已知的参考电压的正半周和负半周分别来确定逆变器的输出电压的正半周和负半周。
下面结合图9和图10分析逆变器的输出电压的正半周对应的开关管的控制方式。参见图9，该图为本申请实施例提供的输出电压正半周第一开关管高频动作对应的示意图。
参见图10，该图为本申请实施例提供的输出电压正半周第三开关管高频动作对应的示意图。
如图9和图10所示，在逆变器的输出电压的正半周内，控制第二开关管T2一直导通，控制第四开关管T4一直关断。
如图9所示，当逆变器的输出电流平均值大于第一电流阈值时，控制第一开关管T1高频动作，控制第三开关管T3关断，即T1以高于工频的频率开关动作，T3一直关断。此时电流路径在第一开关管T1和第一二极管D1之间切换。
例如，当第一开关管T1导通时，由于第二开关管T2一直导通，因此，电流路径为从BUS+依次经过T1和T2到达输出端。当第一开关管T1关断时，电流路径为从直流母线中点O，即C1和C2的公共端经过第一二极管D1和第二开关管T2达到输出端。
从以上结合图9的分析可知，在第一开关管T1高频动作期间，第三开关管T3关断不进行高频动作，并不影响电流路径。
如图10所示，当逆变器的输出电流平均值小于第二电流阈值时，由于第二电流阈值为负数，因此，逆变器的输出电流的方向为从外流进第二开关管T2的第二端，如图10中的箭头所示，此时控制第一开关管T1关断，控制第三开关管T3高频动作，即T1一直关断，T3以高于工频的频率开关动作。此时的电流路径在第三开关管T3和第二二极管D2之间切换。
例如，当第三开关管T3导通时，此时输出电流经过第三开关管T3和第二二极管D6到达直流母线的中点O。当第三开关管T3关断时，电流路径为从第二开关管T2的反并联二极管和第一开关管T1的反并联二极管到达直流正母线BUS+。
从以上结合图10的分析可知，在第三开关管T3高频动作期间，第一开关管T1关断不进行高频动作，并不影响电流路径。
以上结合图9和图10分析的为逆变器的输出电压正半周的开关动作情况，下面结合图11和图12分析逆变器的输出电压的正半周对应的开关管的控制方式。
参见图11，该图为本申请实施例提供的输出电压负半周第二开关管高频动作对应的示意图。
参见图12，该图为本申请实施例提供的输出电压负半周第四开关管高频动作对应的示意图。
如图11和图12，在逆变器的输出电压的负半周内，控制第三开关管T3一直导通，控制第一开关管T1一直关断。
如图11所示，当逆变器的输出电流平均值大于第一电流阈值时，控制第四开关管T4一直关断，控制第二开关管T2高频动作，即T2以高于工频的频率开关动作。此时，电流路径在第二开关管T2和第一二极管D1之间切换。
例如，当第二开关管T2导通时，电流路径为从直流母线中点O流经第一二极管D1和第二开关管T2达到输出端。当第二开关管T2关断时，电流路径为从直流负母线BUS-流经第四开关管T4的反并联二极管和第三开关管T3的反并联二极管达到输出端。
从以上结合图11的分析可知，在第二开关管T2高频动作期间，第四开关管T4关断不进行高频动作，并不影响电流路径。
如图12所示，当逆变器的输出电流平均值小于第二电流阈值时，由于第二电流阈值为负数，因此，逆变器的输出电流的方向为从外流进第二开关管T2的第二端，如图12中的箭头所示。此时控制第二开关管T2关断，控制第四开关管T4高频动作，即T2一直关断， T4以高于工频的频率开关动作。此时的电流路径在第四开关管T4和第二二极管D6之间切换。
例如，当第四开关管T4导通时，由于第三开关管T3一直导通，因此，电流路径为从第三开关管T4和第四开关管T4到达直流负母线BUS-。当第四开关管T4关断时，由于第三开关管T3一直导通，因此，电流路径为从第三开关管T3和第二二极管D2到达直流母线中点O。
从以上结合图12的分析可知，在第四开关管T4高频动作期间，第二开关管T2关断不进行高频动作，并不影响电流路径。
在一种实现方式中，第一电流阈值和第二电流阈值可以为固定值，即设置了以后不再调整第一电流阈值和第二电流阈值的大小。
在另外一种实现方式中，为了更加准确地控制逆变器中各个开关管的动作，可以根据实际运行工况来调整第一电流阈值和/或第二电流阈值，即可以仅调整第一电流阈值，也可以仅调整第二电流阈值，也可以第一电流阈值和第二电流阈值均调节。其中实际运行工况可能影响的参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
下面首先介绍第一电流阈值的四种调节原则，具体实现时可以任选以下一种对第一电流阈值进行调节。
第一种：
控制器，还用于获得输出电流的THDi，THDi增大，则增大第一电流阈值，反之减小第一电流阈值。
例如，当第一电流阈值设置的太小时，可能不合适，会输出电流的谐波变大，造成THDi变大，因此，为了减小谐波，可以将第一电流阈值设置的更大一些。
第二种：
控制器，还用于获得三电平中点箝位逆变器的调制比，调制比减小，则增大第一电流阈值，反之减小第一电流阈值；调制比为三电平中点箝位逆变器的输出相电压电压峰值与半直流母线电压的比值，即输出相电压电压峰值与一半的直流母线电压的比值。
调制比表征的是输出相电压的峰值与半直流母线电压的压差，当两者的压差越小时，即两者越接近时，调制比越大；当两者的压差越大时，调制比越小。为了电感上的纹波电流较小，则希望两者的压差越小越好。因此，当调制比增大时，需要调小第一电流阈值，即更精确地控制开关管进行高频动作，来改善纹波电流。
第三种：
控制器，还用于获得三电平中点箝位逆变器的功率因数，功率因数的绝对值减小，则增大第一电流阈值，反之减小第一电流阈值。
当逆变器的功率因数变小时，说明逆变器在向交流电网发送无功功率，此时纹波电流将比较大，因此，需要调大第一电流阈值。
第四种：
控制器，还用于获得三电平中点箝位逆变器的输出功率，输出功率减小，则增大第一 电流阈值，反之减小第一电流阈值。
由于逆变器的输出电流的瞬时值为输出电流平均值叠加纹波电流，输出功率与输出电流平均值正相关。因此，当输出功率减小时，输出电流的有效值越小，需要增大第一电流阈值。反之，当输出功率增大时，输出电流平均值增大，可以减小第一电流阈值。
下面介绍第二电流阈值的四种调节原则，具体实现时可以任选以下一种对第二电流阈值进行调节。由于第一电流阈值为大于0的正数，第二电流阈值为小于0的负数，因此基于相同的参数调节时，对于第一电流阈值和第二电流阈值的调整趋势相反。
第一种：
控制器，还用于获得输出电流的电流谐波总畸变率，电流谐波总畸变率增大，则减小第二电流阈值，反之增大第二电流阈值。
第二种：
控制器，还用于获得三电平中点箝位逆变器的调制比，调制比减小，则减小第二电流阈值，反之增大第二电流阈值；调制比为三电平中点箝位逆变器的输出相电压峰值与半直流母线电压的比值，即输出相电压峰值与一半的直流母线电压的比值。
第三种：
控制器，还用于获得三电平中点箝位逆变器的功率因数，功率因数的绝对值减小，则减小第二电流阈值，反之增大第二电流阈值。
第四种：
控制器，还用于获得三电平中点箝位逆变器的输出功率，输出功率减小，则减小第二电流阈值，反之增大第二电流阈值。
本申请实施例提供的光伏系统不限定具体的拓扑架构，例如可以为图1-图3所示的任意一种架构，对应地，以上介绍的三电平NPC逆变器，可以为集中式逆变器、组串式逆变器或户用单相逆变器，本申请实施例中均不作限定。由于光伏系统中大部分为向电网传输有功功率，逆变器的输出电压和输出电流的方向相同，因此，输出电压和输出电流的方向不同的时间段很短，应用本申请实施例提供的方案可以很大程度降低开关管的开关损耗，即高频动作带来的开关损耗，从而增加光伏系统的发电效率。由于大部分时间仅有一个开关管高频动作，因此，在仅一个开关管高频动作期间可以减少死区时间的设置，或者不设置死区时间，从而改善逆变器的输出电流的THDi。
以上实施例介绍的光伏系统中的三电平NPC逆变器可以为集中式逆变器，也可以为组串式逆变器，也可以为户用单相逆变器。本申请实施例对于三电平NPC逆变器，不但适用于光伏系统，也可以适用于其他电源系统，例如数据中心的电源，或者基站的供电系统等，另外，还可以应用于电动汽车等领域，因此，本申请实施例不限定三电平NPC逆变器的应用场景。
下面介绍本申请实施例提供的三电平NPC逆变器，具体的实现方式可以参见以上光伏系统实施例的描述，对应的各种有益效果也适用于以下的逆变器实施例，下面仅简要介绍三电平NPC逆变器的实现方式。
参见图13，该图为本申请实施例提供的一种三电平NPC逆变器的示意图。
本申请实施例提供的三电平NPC逆变器，至少包括以下四个开关管：第一开关管T1、第二开关管T2、第三开关管T3和第四开关管T4；第一开关管T1的第一端连接直流正母线BUS+，第一开关管T1的第二端连接第二开关管T2的第一端，第二开关管T2的第二端连接第三开关管T3的第一端，第三开关管T3的第二端连接第四开关管T4的第一端，第四开关管T4的第二端连接直流负母线BUS-；第二开关管T2的第二端为三电平中点箝位逆变器的输出端，即Vout。
该逆变器还包括：逆变器控制器402，用于三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关管中的仅一个开关管高频动作，以降低高频动作带来的功耗；输出电流平均值为三电平中点箝位逆变器的输出电流在开关周期内的平均值；逆变器的输出电流以流出逆变器为正，反之为负；第一电流阈值为正数，第二电流阈值为负数。
逆变器控制器402，具体用于在三电平中点箝位逆变器的输出电压的正半周：控制第二开关管导通，第四开关管关断，输出电流平均值大于第一电流阈值，控制第三开关管关断，第一开关管高频动作，且在输出电流平均值小于第二电流阈值时，控制第一开关管关断，第三开关管高频动作。
逆变器控制器具体用于在输出电压的负半周：控制第三开关管导通，第一开关管关断，输出电流平均值大于第一电流阈值，控制第四开关管关断，第二开关管高频动作，且输出电流平均值小于第二电流阈值时，控制第二开关管关断，第四开关管高频动作。
逆变器控制器402，还用于根据以下任意一种参数调节第一电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
逆变器控制器402，还用于根据以下任意一种参数调节第二电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
逆变器控制器402根据以上任意一种参数具体调节第一电流阈值和第二电流阈值的方式可以参见以上光伏系统实施例的介绍。
继续参见图13，本申请实施例提供的三电平中点箝位逆变器还包括：第一电容C1、第二电容C2、第一二极管D1和第二二极管D2；
第一电容C1的第一端连接直流正母线BUS+，第一电容C1的第二端连接第二电容C2的第一端，第二电容C2的第二端连接直流负母线BUS-；
第一二极管D1的阴极连接第一开关管T1的第二端，第一二极管D1的阳极连接第一电容C1的第二端，第二二极管D2的阴极连接第一二极管D1的阳极，第二二极管D2的阳极连接第三开关管T3的第二端。
本申请实施例提供的三电平NPC逆变器，并不是对于四个开关管，每半个周期控制其中两个开关管进行高频互补动作，而是实时根据输出电流平均值与预设阈值进行比较，根据比较结果来控制开关管进行动作，当输出电流平均值位于预设区间内时，控制两个开关管进行高频动作；当输出电流平均值位于预设区间之外时，仅控制一个开关管进行高频动 作即可。其中预设区间是0附近的一个区间，即由小于0的第二电流阈值和大于0的第一电流阈值形成的预设区间。由于本实施例提供的技术方案可以大大降低开关管高频动作的时间，因此，可以降低逆变器中由于开关管的高频动作带来的功耗，从而可以提高逆变器的电能转换效率。
基于以上实施例提供的一种三电平NPC逆变器，本申请实施例还提供一种逆变器的控制方法，下面结合附图进行详细介绍。
参见图14，该图为本申请实施例提供的一种逆变器的控制方法的流程图。
本申请实施例提供的逆变器的控制方法，应用于以上实施例介绍的三电平中点箝位逆变器，具体可以参见图13所示的三电平中点箝位逆变器，其中至少包括以下四个开关管：第一开关管T1、第二开关管T2、第三开关管T3和第四开关管T4；第一开关管T1的第一端连接直流正母线BUS+，第一开关管T1的第二端连接第二开关管T2的第一端，第二开关管T2的第二端连接第三开关管T3的第一端，第三开关管T3的第二端连接第四开关管T4的第一端，第四开关管T4的第二端连接直流负母线BUS-；第二开关管T2的第二端为三电平中点箝位逆变器的输出端，即Vout。
该方法包括：
S1401：获得三电平中点箝位逆变器的输出电流平均值；其中，输出电流平均值可以为三电平中点箝位逆变器的输出电流在开关周期内的平均值。
由于逆变器的输出电流实际会叠加一个纹波电流，因此，为了更加精确地根据输出电流来控制开关管的动作，可以采集一个开关管周期内的输出电流平均值，具体地，可以在一个开关周期采集几个采样点的输出电流，然后获得采样点的输出电流的平均值。
S1402：三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关管中的仅一个开关管高频动作，以降低高频动作带来的功耗；逆变器的输出电流以流出逆变器为正，反之为负；第一电流阈值为正数，第二电流阈值为负数。
本申请实施例提供的三电平NPC逆变器的控制方法，实时获得逆变器的输出电流平均值，根据输出电流平均值与预设阈值进行比较，根据比较结果来控制开关管进行动作。当输出电流平均值位于预设区间内时，控制两个开关管进行高频动作；当输出电流平均值位于预设区间之外时，仅控制一个开关管进行高频动作即可。其中预设区间是0附近的一个区间，即由小于0的第二电流阈值和大于0的第一电流阈值形成的预设区间。该控制方法区别于传统的对于四个开关管，每半个周期控制其中两个开关管进行高频互补动作。由于本实施例提供的技术方案可以大大降低开关管高频动作的时间，因此，可以降低逆变器中由于开关管的高频动作带来的功耗，从而可以提高逆变器的电能转换效率。
三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制四个开关管中的仅一个开关管高频动作，具体包括：
在三电平中点箝位逆变器的输出电压的正半周：控制第二开关管导通，第四开关管关断，输出电流平均值大于第一电流阈值，控制第三开关管关断，第一开关管高频动作，且在输出电流平均值小于第二电流阈值时，控制第一开关管关断，第三开关管高频动作；
控制器具体用于在输出电压的负半周：控制第三开关管导通，第一开关管关断，输出 电流平均值大于第一电流阈值，控制第四开关管关断，第二开关管高频动作，且输出电流平均值小于第二电流阈值时，控制第二开关管关断，第四开关管高频动作。
其中，第一电流阈值的绝对值和第二电流阈值的绝对值可以相等，也可以不相等，本申请实施例中不做具体限定。
另外为了提高输出电流的质量，可以根据以下任意一种参数调节第一电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
根据以下任意一种参数调节第二电流阈值，任意一种参数包括：输出电流的电流谐波总畸变率、三电平中点箝位逆变器的调制比、逆变器的功率因数或三电平中点箝位逆变器的输出功率。
根据以上任意一种参数具体调节第一电流阈值和第二电流阈值的方式可以参见以上光伏系统实施例的介绍。
应当理解，在本申请中，“至少一个(项)”是指一个或者多个，“多个”是指两个或两个以上。“和/或”，用于描述关联对象的关联关系，表示可以存在三种关系，例如，“A和/或B”可以表示：只存在A，只存在B以及同时存在A和B三种情况，其中A，B可以是单数或者复数。字符“/”一般表示前后关联对象是一种“或”的关系。“以下至少一项(个)”或其类似表达，是指这些项中的任意组合，包括单项(个)或复数项(个)的任意组合。例如，a，b或c中的至少一项(个)，可以表示：a，b，c，“a和b”，“a和c”，“b和c”，或“a和b和c”，其中a，b，c可以是单个，也可以是多个。
以上所述，以上实施例仅用以说明本申请的技术方案，而非对其限制；尽管参照前述实施例对本申请进行了详细的说明，本领域的普通技术人员应当理解：其依然可以对前述各实施例所记载的技术方案进行修改，或者对其中部分技术特征进行等同替换；而这些修改或者替换，并不使相应技术方案的本质脱离本申请各实施例技术方案的精神和范围。

Claims (21)

  1. 一种光伏系统，其特征在于，包括：三电平中点箝位逆变器和控制器；
    所述三电平中点箝位逆变器至少包括以下四个开关单元：第一开关单元、第二开关单元、第三开关单元和第四开关单元；所述第一开关单元的第一端连接直流正母线，所述第一开关单元的第二端连接第二开关单元的第一端，所述第二开关单元的第二端连接所述第三开关单元的第一端，所述第三开关单元的第二端连接所述第四开关单元的第一端，所述第四开关单元的第二端连接直流负母线；所述第二开关单元的第二端为所述三电平中点箝位逆变器的输出端；
    所述控制器，用于所述三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制所述四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；所述输出电流平均值为所述三电平中点箝位逆变器的输出电流在开关周期内的平均值；所述逆变器的输出电流以流出所述逆变器为正，反之为负；所述第一电流阈值为正数，所述第二电流阈值为负数。
  2. 根据权利要求1所述的光伏系统，其特征在于，所述控制器，还用于所述输出电流平均值大于第二电流阈值小于第一电流阈值，控制所述四个开关单元中的两个开关单元高频动作。
  3. 根据权利要求1或2所述的光伏系统，其特征在于，所述控制器具体用于在所述三电平中点箝位逆变器的输出电压的正半周：控制所述第二开关单元导通，所述第四开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第三开关单元关断，所述第一开关单元高频动作，所述输出电流平均值小于第二电流阈值，控制所述第一开关单元关断，所述第三开关单元高频动作；
    所述控制器具体用于在所述输出电压的负半周：控制所述第三开关单元导通，所述第一开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第四开关单元关断，所述第二开关单元高频动作，所述输出电流平均值小于所述第二电流阈值，控制所述第二开关单元关断，所述第四开关单元高频动作。
  4. 根据权利要求1-3任一项所述的光伏系统，其特征在于，一个所述开关周期内所述输出电流的最小值为0，则所述第一电流阈值大于等于该开关周期对应的所述输出电流平均值；一个所述开关周期内所述输出电流的最大值为0，则所述第二电流阈值小于等于该开关周期对应的所述输出电流平均值。
  5. 根据权利要求1-4任一项所述的光伏系统，其特征在于，所述控制器，还用于获得所述输出电流的电流谐波总畸变率，所述电流谐波总畸变率增大，则增大所述第一电流阈值，反之减小所述第一电流阈值。
  6. 根据权利要求1-4任一项所述的光伏系统，其特征在于，所述控制器，还用于获得所述三电平中点箝位逆变器的调制比，所述调制比减小，则增大所述第一电流阈值，反之减小所述第一电流阈值；所述调制比为所述三电平中点箝位逆变器的输出相电压峰值与半直流母线电压的比值。
  7. 根据权利要求1-4任一项所述的光伏系统，其特征在于，所述控制器，还用于获得所 述三电平中点箝位逆变器的功率因数，所述功率因数的绝对值减小，则增大所述第一电流阈值，反之减小所述第一电流阈值。
  8. 根据权利要求1-4任一项所述的光伏系统，其特征在于，所述控制器，还用于获得所述三电平中点箝位逆变器的输出功率，所述输出功率减小，则增大所述第一电流阈值，反之减小所述第一电流阈值。
  9. 根据权利要求1-5任一项所述的光伏系统，其特征在于，所述控制器，还用于获得所述输出电流的电流谐波总畸变率，所述电流谐波总畸变率增大，则减小所述第二电流阈值，反之增大所述第二电流阈值。
  10. 根据权利要求1-4任一项或6所述的光伏系统，其特征在于，所述控制器，还用于获得所述三电平中点箝位逆变器的调制比，所述调制比减小，则减小所述第二电流阈值，反之增大所述第二电流阈值；所述调制比为所述三电平中点箝位逆变器的输出相电压峰值与半直流母线电压的比值。
  11. 根据权利要求1-4任一项或7所述的光伏系统，其特征在于，所述控制器，还用于获得所述三电平中点箝位逆变器的功率因数，所述功率因数的绝对值减小，则减小所述第二电流阈值，反之增大所述第二电流阈值。
  12. 根据权利要求1-4任一项或8所述的光伏系统，其特征在于，所述控制器，还用于获得所述三电平中点箝位逆变器的输出功率，所述输出功率减小，则减小所述第二电流阈值，反之增大所述第二电流阈值。
  13. 根据权利要求1-12任一项所述的光伏系统，其特征在于，所述三电平中点箝位逆变器为集中式逆变器、组串式逆变器或户用单相逆变器。
  14. 根据权利要求1-13任一项所述的光伏系统，其特征在于，所述三电平中点箝位逆变器还包括：第一电容、第二电容、第一二极管和第二二极管；
    所述第一电容的第一端连接所述直流正母线，所述第一电容的第二端连接所述第二电容的第一端，所述第二电容的第二端连接所述直流负母线；
    所述第一二极管的阴极连接所述第一开关单元的第二端，所述第一二极管的阳极连接所述第一电容的第二端，所述第二二极管的阴极连接所述第一二极管的阳极，所述第二二极管的阳极连接所述第三开关单元的第二端。
  15. 一种三电平中点箝位逆变器，其特征在于，至少包括以下四个开关单元：第一开关单元、第二开关单元、第三开关单元和第四开关单元；所述第一开关单元的第一端连接直流正母线，所述第一开关单元的第二端连接第二开关单元的第一端，所述第二开关单元的第二端连接所述第三开关单元的第一端，所述第三开关单元的第二端连接所述第四开关单元的第一端，所述第四开关单元的第二端连接直流负母线；所述第二开关单元的第二端为所述三电平中点箝位逆变器的输出端；还包括：
    逆变器控制器，用于所述三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制所述四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；所述输出电流平均值为所述三电平中点箝位逆变器的输出电流在开关周期内的平均值；所述逆变器的输出电流以流出所述逆变器为正，反之为负；所述第一电流 阈值为正数，所述第二电流阈值为负数。
  16. 根据权利要求15所述的逆变器，其特征在于，所述逆变器控制器，具体用于在所述三电平中点箝位逆变器的输出电压的正半周：控制所述第二开关单元导通，所述第四开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第三开关单元关断，所述第一开关单元高频动作；所述输出电流平均值小于所述第二电流阈值，控制所述第一开关单元关断，所述第三开关单元高频动作；
    所述逆变器控制器，具体用于在所述输出电压的负半周：控制所述第三开关单元导通，所述第一开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第四开关单元关断，所述第二开关单元高频动作；所述输出电流平均值小于所述第二电流阈值，控制所述第二开关单元关断，所述第四开关单元高频动作。
  17. 根据权利要求15或16所述的逆变器，其特征在于，所述逆变器控制器，还用于根据以下任意一种参数调节所述第一电流阈值，所述任意一种参数包括：所述输出电流的电流谐波总畸变率、所述三电平中点箝位逆变器的调制比、所述逆变器的功率因数或所述三电平中点箝位逆变器的输出功率。
  18. 根据权利要求15-17任一项所述的逆变器，其特征在于，所述逆变器控制器，还用于根据以下任意一种参数调节所述第二电流阈值，所述任意一种参数包括：所述输出电流的电流谐波总畸变率、所述三电平中点箝位逆变器的调制比、所述逆变器的功率因数或所述三电平中点箝位逆变器的输出功率。
  19. 根据权利要求15-18任一项所述的逆变器，其特征在于，所述三电平中点箝位逆变器还包括：第一电容、第二电容、第一二极管和第二二极管；
    所述第一电容的第一端连接所述直流正母线，所述第一电容的第二端连接所述第二电容的第一端，所述第二电容的第二端连接所述直流负母线；
    所述第一二极管的阴极连接所述第一开关单元的第二端，所述第一二极管的阳极连接所述第一电容的第二端，所述第二二极管的阴极连接所述第一二极管的阳极，所述第二二极管的阳极连接所述第三开关单元的第二端。
  20. 一种逆变器的控制方法，其特征在于，应用于三电平中点箝位逆变器，所述三电平中点箝位逆变器至少包括以下四个开关单元：第一开关单元、第二开关单元、第三开关单元和第四开关单元；所述第一开关单元的第一端连接直流正母线，所述第一开关单元的第二端连接第二开关单元的第一端，所述第二开关单元的第二端连接所述第三开关单元的第一端，所述第三开关单元的第二端连接所述第四开关单元的第一端，所述第四开关单元的第二端连接直流负母线；所述第二开关单元的第二端为所述三电平中点箝位逆变器的输出端；
    该方法包括：
    获得所述三电平中点箝位逆变器的输出电流平均值；所述输出电流平均值为所述三电平中点箝位逆变器的输出电流在开关周期内的平均值；
    所述三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制所述四个开关单元中的仅一个开关单元高频动作，以降低高频动作带来的功耗；所述 逆变器的输出电流以流出所述逆变器为正，反之为负；所述第一电流阈值为正数，所述第二电流阈值为负数。
  21. 根据权利要求20所述的方法，其特征在于，所述三电平中点箝位逆变器的输出电流平均值大于第一电流阈值或小于第二电流阈值，控制所述四个开关单元中的仅一个开关单元高频动作，具体包括：
    在所述三电平中点箝位逆变器的输出电压的正半周：控制所述第二开关单元导通，所述第四开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第三开关单元关断，所述第一开关单元高频动作；所述输出电流平均值小于第二电流阈值，控制所述第一开关单元关断，所述第三开关单元高频动作；
    在所述输出电压的负半周：控制所述第三开关单元导通，所述第一开关单元关断，所述输出电流平均值大于所述第一电流阈值，控制所述第四开关单元关断，所述第二开关单元高频动作；所述输出电流平均值小于所述第二电流阈值，控制所述第二开关单元关断，所述第四开关单元高频动作。
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