A power adaptive soft switching control method and system for photovoltaic grid-connected injection inverter

Abstract

The present invention provides a power adaptive soft switching control method and system for a photovoltaic grid-connected injection inverter, relating to the field of inverters. The system comprises: an injection-type high-power photovoltaic grid-connected inverter, comprising at least a photovoltaic-side DC converter switch, an injection bridge switch unit, and a main bridge switch unit, wherein the injection bridge switch unit is used to inject a periodically varying voltage into the main bridge circuit, and the main bridge switch unit is used to convert DC power into AC power under the action of the periodically varying voltage injected by the injection bridge switch unit; a soft switching module is used to control the injection bridge switch unit to inject the periodically varying voltage based on photovoltaic input power, and to change the switching state of the main circuit switch elements under zero voltage conditions. The soft switching module has the advantages of reducing the dynamic loss of the switching devices, improving the efficiency of the photovoltaic inverter system in high-power applications, and taking into account the quality of the harmonics output on the AC side of the inverter, so that the harmonic content on the AC side is small and no filter is required.

Description

Technical Field

[0001] The present invention relates to the field of inverters, and in particular to a power adaptive soft switching control method and system for a photovoltaic grid-connected injection inverter. Background Art

[0002] Photovoltaic power generation, as a clean and renewable energy source, has gained widespread application in recent years. Grid-connected photovoltaic inverters are the core components of photovoltaic power generation systems. Their primary function is to convert the direct current (DC) generated by photovoltaic modules into AC (AC) that meets grid requirements and enables efficient grid connection. In grid-connected photovoltaic systems, the performance of the inverter directly impacts system efficiency and stability. Therefore, minimizing switching losses while maintaining high efficiency has become a key issue in the application of grid-connected photovoltaic inverters.

[0003] Grid-connected photovoltaic inverters use high-frequency PWM (Pulse Width Modulation) to minimize grid-connected harmonics, but this increases dynamic losses in switching devices. Lowering the frequency requires larger filters on the AC side to meet grid access standards, resulting in significant energy losses in the switching devices and filters. Modular multilevel converters (MMCs) can reduce switching frequency, but the switching on and off of switching devices still generates significant dynamic losses. To reduce dynamic losses in switching devices, existing solutions employ auxiliary circuits, such as snubbers or resonant circuits, to implement soft switching. Currently, soft switching requires the use of auxiliary circuits and resonant elements, requiring precise control of the operating timing of the auxiliary switches and resonant elements. Coordinated control of the main switching device and auxiliary circuits requires high standards, and improper design can lead to poor soft switching performance. The resonance time only accounts for a small fraction of the switching cycle, which can limit the full effectiveness of soft switching and compromise loss reduction. The resonance process can increase voltage or current peaks, placing higher voltage and current requirements on the switching device, impacting device reliability. The introduction of auxiliary circuits also increases losses, impacting overall efficiency gains. Especially in high-power applications, using resonant technology to achieve soft switching requires high-power auxiliary switching devices, which greatly increases costs and area, resulting in a low cost-performance ratio. In particular, in high-power photovoltaic grid-connected applications, the current carried in the line is large, which leads to a longer turn-off time of the switching device.

[0004] Therefore, it is necessary to provide a power adaptive soft switching control method and system for a photovoltaic grid-connected injection inverter, which is used to implement soft switching using injection technology, reduce the dynamic loss of switching devices, improve the efficiency of photovoltaic inverter systems in high-power applications, and dynamically adjust the soft switching width of the switching device in the zero-level interval according to the size of the photovoltaic side input power. At the same time, it takes into account the output harmonic quality of the inverter's AC side, so that the harmonic content on the AC side is small and no filter is required. Summary of the Invention

[0005] The present invention provides a power adaptive soft switching control method for a photovoltaic grid-connected injection inverter, comprising: an injection-type high-power photovoltaic grid-connected inverter, comprising at least a photovoltaic-side DC converter switch, an injection bridge switch unit, and a main bridge switch unit, wherein the injection bridge switch unit is used to inject a periodically varying voltage into the main bridge circuit, and the main bridge switch unit is used to convert DC power into AC power under the action of the periodically varying voltage injected by the injection bridge switch unit; and a soft switching module is used to control the injection bridge switch unit to inject the periodically varying voltage based on the photovoltaic input power, thereby changing the switching state of the main circuit switch element under zero voltage conditions.

[0006] Furthermore, the injection bridge switch unit includes five H-bridges connected in series: the main bridge switch unit includes a Y-type bridge and a △-type bridge; the photovoltaic side DC converter switch is connected in parallel with a capacitor group, wherein the capacitor group includes a first capacitor and a second capacitor connected in series, the input end of the injection bridge switch unit is electrically connected to the connection node of the first capacitor and the second capacitor, and the output end of the injection bridge switch unit is electrically connected to the connection node of the Y-type bridge and the △-type bridge.

[0007] Furthermore, each H-bridge cycle of the injection bridge switch unit is one sixth of the main bridge cycle of the main bridge switch unit, and the same H-bridge is turned on for different widths within five H-bridge cycles.

[0008] Furthermore, the soft switching module controls the periodically changing voltage injected into the main bridge circuit based on the photovoltaic input power, and changes the switching state of the main circuit switching elements under zero voltage conditions, including: calculating the switch-off time of the switching device based on the photovoltaic input power and the photovoltaic output current; calculating the required width for zero voltage and other level widths based on the switch-off time of the switching device; calculating the number of levels based on the required width for zero voltage and other level widths; determining the switching state of each H-bridge based on the number of levels; and controlling the periodically changing voltage injected into the main bridge circuit according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, and changing the switching state of the main circuit switching elements under zero voltage conditions.

[0009] Furthermore, the soft switching module calculates the switch-off time of the switching device based on the photovoltaic input power and the photovoltaic output current, including: determining the current power range based on the photovoltaic input power; determining the photovoltaic output current based on the current power range; calculating the current switch-off time of the switching device based on the photovoltaic output current; and using the larger value of the current switch-off time and the standard switch-off time as the benchmark switch-off time.

[0010] Furthermore, the soft switching module determines the current switch-off time of the switch device based on the photovoltaic output current according to the following formula:

[0011] ,

[0012] in, is the current switch-off time, is the gate drive resistor, is the gate-collector capacitance, is the gate-emitter capacitance, is the gate turn-on voltage, is the gate turn-off voltage, is the critical turn-on voltage, is the equivalent inductance of the line and transformer, is the photovoltaic output current, is the collector-emitter voltage, is the tail current, is the critical current for shutoff.

[0013] Furthermore, the required width of zero voltage and other level widths are calculated based on the turn-off time of the switching device according to the following formula:

[0014] ,

[0015] ,

[0016] in, is the required width for zero voltage, is the turn-off time of the switching device, For other level widths.

[0017] Furthermore, the soft switching module calculates the number of levels based on the required width for zero voltage and other level widths according to the following rules:

[0018] When Hmod<-(1-l), Hnum=-5, where Hmod is the modulation value, Hnum is the number of levels, and l is the required width of zero voltage;

[0019] When-(1- l )≤Hmod<-(1- l -x), Hnum=-4, where x is the width of other levels;

[0020] When-(1- l -x)≤Hmod<-(1- l -2x), Hnum=-3

[0021] When-(1- l -2x)≤Hmod<-(1- l -3x), Hnum=-2;

[0022] When-(1-l -3x)≤Hmod<-(1- l -4x), Hnum=-1;

[0023] When-(1- l -4x)≤Hmod<(1- l -4x), Hnum=0;

[0024] When (1- l -4x)≤Hmod<(1- l -3x), Hnum=1;

[0025] When (1- l -3x)≤Hmod<(1- l -2x), Hnum=2;

[0026] When (1- l -2x)≤Hmod<(1- l -x), Hnum=3;

[0027] When (1- l -x)≤Hmod<(1- l ), Hnum=4;

[0028] When Hmod>(1- l ), Hnum=5;

[0029] Among them, Hmod is obtained based on the following transformation:

[0030] when hour, ;

[0031] when hour, ;

[0032] when hour, ;

[0033] when hour, .

[0034] Furthermore, according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled, and the switching state of the main circuit switching element is changed under zero voltage conditions, including: determining the d-axis and q-axis voltage signals at the output end of the inverter through a dual-loop control structure, wherein the dual-loop control structure includes a voltage outer loop and a current inner loop, the voltage outer loop is used to determine the reference of the system active current based on the photovoltaic array output voltage and the DC side voltage according to maximum power point tracking, and the current inner loop is used to determine the d-axis voltage signal and the q-axis voltage signal at the output end of the inverter according to the reference of the system active current; according to the d-axis voltage signal and the q-axis voltage signal at the output end of the inverter, the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled, and the switching state of the main circuit switching element is changed under zero voltage conditions.

[0035] The present invention provides a power adaptive soft switching control system for a photovoltaic grid-connected injection inverter, which is applied to the above-mentioned power adaptive soft switching control system for a photovoltaic grid-connected injection inverter, comprising: calculating the turn-off time of a switching device based on the photovoltaic input power and the photovoltaic output current; calculating the required width for zero voltage and other level widths based on the turn-off time of the switching device; calculating the number of levels based on the required width for zero voltage and other level widths; determining the switching state of each H-bridge based on the number of levels; and controlling the periodically changing voltage injected into the main bridge circuit according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, thereby changing the switching state of the main circuit switch elements under zero voltage conditions.

[0036] Compared with the prior art, the power adaptive soft switching control method and system for a photovoltaic grid-connected injection inverter provided by the present invention have at least the following beneficial effects:

[0037] (1) Significantly reduce switching losses and improve system efficiency

[0038] At low switching frequencies, the zero voltage range is dynamically adjusted to optimize the turn-on and turn-off conditions of the switching devices, significantly reducing switching losses. Simultaneously, due to circuit simplification, component losses are further reduced, thereby improving the overall efficiency of the photovoltaic inverter, especially in high-power applications.

[0039] (2) Reduce circuit complexity

[0040] By designing a dynamic zero-voltage time adjustment scheme, soft switching is achieved directly through the injection branch, eliminating the need for additional resonant networks or complex auxiliary circuits. Compared to traditional resonant soft switching, this significantly simplifies circuit design, reduces component count, and lowers system size and implementation cost. This invention achieves soft switching solely through the injection branch and real-time control, eliminating the need for additional resonant networks or redundant auxiliary switches. This reduces implementation costs, eliminates potential points of failure, and significantly improves system reliability.

[0041] (3) Improving dynamic adaptability

[0042] Existing soft-switching technologies are mostly fixed designs, making them difficult to adapt to dynamic changes in load current. To address this issue, this method and system introduces real-time current detection and dynamic adjustment mechanisms. This accurately calculates the zero voltage time based on changes in load current, ensuring efficient soft-switching under varying load conditions. This is particularly suitable for the frequently changing load conditions in photovoltaic grid-connected systems.

[0043] (4) Improve power quality and reduce harmonic distortion

[0044] By utilizing specific subharmonic elimination patterns, a periodically varying DC voltage is injected into the DC terminals of two series-connected three-phase full-bridges. This optimizes the output voltage waveform, bringing it closer to an ideal sine wave, reducing low-order harmonics and improving power quality. In high-power grid-connected photovoltaic applications, this reduces the system's harmonic distortion, helping to minimize or eliminate the need for additional filtering equipment, lowering overall system design and operation costs. It also enhances the system's adaptability to grid connections, meeting the stringent harmonic requirements of photovoltaic grid-connected systems.

[0045] (5) Improve system reliability and extend the life of switching devices

[0046] By dynamically adjusting the zero voltage time, the voltage and current stress of the switching device during turn-on and turn-off can be effectively reduced, reducing the heat loss and overheating risk of the switching device under high-power conditions, extending its service life, and thus improving the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] This specification will be further described in the form of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting, and in these embodiments, like numbers represent like structures, wherein:

[0048] Figure 1 This is a module diagram of a power adaptive soft switching control system for a photovoltaic grid-connected injection inverter according to some embodiments of this specification;

[0049] Figure 2is a circuit diagram of an injection-type high-power photovoltaic grid-connected inverter according to some embodiments of this specification;

[0050] Figure 3 is a circuit diagram of an injection bridge switch unit according to some embodiments of this specification;

[0051] Figure 4 is a circuit diagram of a main bridge switch unit according to some embodiments of this specification;

[0052] Figure 5 is a timing diagram of injection branch cycle triggering according to some embodiments of this specification;

[0053] Figure 6 is a flow chart of calculating the turn-off time of a switching device according to some embodiments of this specification;

[0054] Figure 7 is a schematic diagram of the phase change relationship between the main bridge and the injection branch according to some embodiments of this specification;

[0055] Figure 8 is a schematic diagram of level numbers according to some embodiments of this specification;

[0056] Figure 9 is a schematic diagram of an output step waveform according to some embodiments of this specification;

[0057] Figure 10 is a control strategy block diagram according to some embodiments of this specification;

[0058] Figure 11 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the YY bridge below;

[0059] Figure 12 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the Y-△ bridge below;

[0060] Figure 13 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the output voltage waveform of phase A on the lower AC side;

[0061] Figure 14 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the harmonic distortion rate of the A-phase output voltage on the lower AC side;

[0062] Figure 15The illumination intensity is 800W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the YY bridge below;

[0063] Figure 16 The illumination intensity is 800W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the Y-△ bridge below;

[0064] Figure 17 The illumination intensity is 800W / m according to some embodiments of this specification. 2 Schematic diagram of the output voltage waveform of phase A on the lower AC side;

[0065] Figure 18 The illumination intensity is 800W / m according to some embodiments of this specification. 2 Schematic diagram of the harmonic distortion rate of the A-phase output voltage on the lower AC side;

[0066] Figure 19 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the YY bridge below;

[0067] Figure 20 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the voltage waveform injected into the Y-△ bridge below;

[0068] Figure 21 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the output voltage waveform of phase A on the lower AC side;

[0069] Figure 22 The light intensity is 400W / m according to some embodiments of this specification. 2 Schematic diagram of the harmonic distortion rate of the A-phase output voltage on the lower AC side;

[0070] Figure 23 This is a flow chart of a power adaptive soft switching control method for a photovoltaic grid-connected injection inverter according to some embodiments of this specification. DETAILED DESCRIPTION

[0071] To more clearly illustrate the technical solutions of the embodiments of this specification, the following briefly describes the drawings required for describing the embodiments. Obviously, the drawings described below are merely examples or embodiments of this specification. Those skilled in the art can apply this specification to other similar scenarios based on these drawings without inventive effort. Unless otherwise apparent from the context or otherwise noted, the same reference numerals in the figures represent the same structure or operation.

[0072] Figure 1 This is a module diagram of a photovoltaic grid-connected injection inverter power adaptive soft switching control system according to some embodiments of this specification, such as Figure 1 As shown, a photovoltaic grid-connected injection inverter power adaptive soft switching control system may include an injection-type high-power photovoltaic grid-connected inverter and a soft switching module.

[0073] Figure 2 is a circuit diagram of an injection-type high-power photovoltaic grid-connected inverter according to some embodiments of this specification, such as Figure 2 As shown, the injection-type high-power photovoltaic grid-connected inverter includes at least a photovoltaic-side DC converter switch, an injection bridge switch unit, and a main bridge switch unit. The injection bridge switch unit is used to inject a periodically varying voltage into the main bridge circuit, and the main bridge switch unit is used to convert DC power into AC power under the action of the periodically varying voltage injected by the injection bridge switch unit. S Y1 - S Y6 、 S D1 - S D6 For the main bridge switch, H 11 -H 54 Injection bridge switch. S 1 is the photovoltaic side DC converter switch. U pv is the photovoltaic voltage, U dc is the DC side voltage, U j is the injection voltage, U △a 、 U ya is the transformer secondary voltage, U SA 、 U SB 、 U SC is the three-phase voltage on the AC side, I Ydc 、 I △dcis the current of Y-bridge and △-bridge, U yy 、 U △y is the voltage between the Y-bridge and the △-bridge, U j is the multi-level injection voltage.

[0074] As you can understand, the DC side of a 12-pulse inverter circuit is a stable DC power source, while the AC side contains 12k±1 harmonics. A power-adaptive soft-switching control system for a photovoltaic grid-connected injection-type inverter converts this stable DC power into a periodically varying DC power source through the injection bridge switching unit, significantly reducing the harmonic content on the AC side. Even without a filter, AC power that meets grid-connected standards can be output. Furthermore, the injection branch voltage is precisely zero when the switching devices of the main bridge switching unit are turned on or off, thus implementing soft switching technology. In traditional DC / AC inverters, the current and voltage applied to each bridge are constant. However, in injection-type high-power photovoltaic grid-connected inverters, a periodically varying voltage is superimposed on the main bridge switching unit. The variable injection waveform helps eliminate harmonics and improve inverter performance.

[0075] Figure 3 is a circuit diagram of an injection bridge switch unit according to some embodiments of this specification, such as Figure 3 As shown in the figure, to introduce a periodically varying voltage, the injection bridge switching unit comprises five H-bridges connected in series. Each H-bridge is composed of fully controlled devices. The injection bridge switching units do not carry DC current; their current levels are dependent only on the AC component. Therefore, the current levels are relatively low. In high-voltage, high-power applications, the injection bridge components can use lower power levels.

[0076] Figure 4 is a circuit diagram of a main bridge switch unit according to some embodiments of this specification, such as Figure 4 As shown, the main bridge switch unit consists of a Y-type bridge and a Δ-type bridge connected in series. The AC side is formed by connecting the grid-side windings of the converter transformers in series. The transformer secondary sides are connected in star and delta configurations, respectively, to two three-phase full-bridges. Due to the different winding connections, the turns ratio of the secondary windings of the Y- and Δ-type transformers is a fixed constant. The main bridge switch unit consists of 12 switching devices.

[0077] like Figure 2 As shown, the photovoltaic side DC inverter converter switch is connected in parallel with a capacitor group, wherein the capacitor group includes a first capacitor and a second capacitor connected in series, the input end of the injection bridge switch unit is electrically connected to the connection node of the first capacitor and the second capacitor, and the output end of the injection bridge switch unit is electrically connected to the connection node of the Y-type bridge and the △-type bridge. U jThe direction and magnitude of the voltage on the bridge can change, and the switch devices on the main bridge can be turned on and off at zero voltage. That is, the voltage determined by the conduction state of the switch devices of the injection bridge switch unit is de-embedded to the switch devices of the main bridge switch unit, and the switch state of the switch devices of the main bridge switch unit is changed under zero voltage conditions. U j = 0, the phase is changed, and the conditions of soft switching technology are met.

[0078] In the injection-type high-power photovoltaic grid-connected inverter, the bridge inverter can be regarded as a whole and designed according to its voltage and current levels. There is no danger of direct conduction of the components on the two series bridge arms, so there is no need to leave a dead zone for safety when opening one bridge arm and closing the other bridge arm, which improves operational reliability.

[0079] Figure 5 This is a timing diagram of injection branch cycle triggering according to some embodiments of this specification, such as Figure 5 As shown, each H-bridge cycle of the injection bridge switching unit is one-sixth of the main bridge cycle of the main bridge switching unit. The same H-bridge switches on for different widths within five H-bridge cycles, and any two H-bridges switch on for different widths within the same H-bridge cycle. By switching on different H-bridges for different widths within five H-bridge cycles, charge and discharge balance is achieved for the cascaded H-bridges. Assume that J1, J2, J3, J4, and J5 have different conduction widths, and the conduction widths increase in sequence. In five H-bridge cycles, let the conduction width of the first H-bridge be J1, J2, J3, J4, and J5 in sequence; the conduction width of the second H-bridge is J2, J3, J4, J5, and J1; the conduction width of the third H-bridge is J3, J4, J5, J1, and J2; the conduction width of the fourth H-bridge is J4, J5, J1, J2, and J3; and the conduction width of the fifth H-bridge is J5, J1, J2, J3, and J4. Repeat this cycle to achieve charge and discharge balance of the cascaded H-bridge.

[0080] The soft switching module is used to control the injection bridge switch unit to inject a periodically changing voltage based on the photovoltaic input power, and change the switching state of the main circuit switching element under zero voltage conditions.

[0081] Specifically include:

[0082] Calculate the switch device turn-off time based on the photovoltaic input power and photovoltaic output current;

[0083] Calculate the required width of zero voltage and other level widths based on the turn-off time of the switching device;

[0084] Calculate the number of levels based on the required width of zero voltage and the widths of other levels;

[0085] Based on the number of levels, determine the switching state of each H-bridge;

[0086] According to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions.

[0087] Figure 6 is a flow chart for calculating the turn-off time of a switching device according to some embodiments of this specification, such as Figure 6 As shown, preferably, the soft switching module calculates the switch-off time of the switch device based on the photovoltaic input power and the photovoltaic output current, including:

[0088] Determine the current power range based on the photovoltaic input power;

[0089] Determine the photovoltaic output current based on the current power range;

[0090] Calculate the current off time of the switch device according to the photovoltaic output current;

[0091] The larger of the current switch device off time and the standard switch device off time is used as the reference switch device off time. The standard switch device off time can be obtained from the switch device manual. Specifically, multiple power ranges can be divided, each with a corresponding power correction factor. The PV output current can be calculated by multiplying the maximum power and the power correction factor by the DC voltage.

[0092] For example, six power intervals are divided into [0.9, 1], [0.8, 0.9], [0.6, 0.8], [0.4, 0.6], [0.2, 0.4], and [0, 0.2], and the corresponding power correction coefficients are 0.9, 0.8, 0.6, 0.4, 0.2, and 0 respectively.

[0093] Understandably, the DC voltage remains constant in high-power photovoltaic grid-connected systems, so current changes reflect power changes. By detecting the current flowing through the circuit and calculating the switch-off time based on this current, the zero-voltage time provided by the injection branch is ensured to be greater than or equal to the switch-off time. This allows for dynamic adjustment of the zero-voltage range, ensuring soft switching at all power levels and significantly reducing dynamic losses in the switch.

[0094] Preferably, the soft switching module determines the current switch-off time of the switching device based on the photovoltaic output current according to the following formula:

[0095] ,

[0096] in, is the current switch-off time, is the gate drive resistor, is the gate-collector capacitance, is the gate-emitter capacitance, is the gate turn-on voltage, is the gate turn-off voltage, is the critical turn-on voltage, is the equivalent inductance of the line and transformer, is the photovoltaic output current, is the collector-emitter voltage, is the tail current, is the critical current for shutoff.

[0097] Preferably, the required width for zero voltage and other level widths are calculated based on the turn-off time of the switching device according to the following formula:

[0098] ,

[0099] ,

[0100] in, is the required width for zero voltage, is the turn-off time of the switching device, For other level widths.

[0101] Preferably, the soft switching module calculates the number of levels based on the required width of zero voltage and other level widths according to the following rules:

[0102] Figure 8 is a schematic diagram of the level numbers shown in some embodiments of this specification, such as Figure 8 As shown, when Hmod<-(1-l), Hnum=-5, where Hmod is the modulation value, Hnum is the number of levels, and l is the required width of zero voltage;

[0103] When-(1- l )≤Hmod<-(1- l -x), Hnum=-4, where x is the width of other levels;

[0104] When-(1- l -x)≤Hmod<-(1- l -2x), Hnum=-3

[0105] When-(1- l -2x)≤Hmod<-(1- l -3x), Hnum=-2;

[0106] When-(1- l -3x)≤Hmod<-(1- l-4x), Hnum=-1;

[0107] When-(1- l -4x)≤Hmod<(1- l -4x), Hnum=0;

[0108] When (1- l -4x)≤Hmod<(1- l -3x), Hnum=1;

[0109] When (1- l -3x)≤Hmod<(1- l -2x), Hnum=2;

[0110] When (1- l -2x)≤Hmod<(1- l -x), Hnum=3;

[0111] When (1- l -x)≤Hmod<(1- l ), Hnum=4;

[0112] When Hmod>(1- l ), Hnum=5.

[0113] For example, when Hnum=5, it means that 5 H bridges output high level, that is, H n1 and H n3 When Hnum=-5, it means that the H in the 5 H bridges is open at the same time (n=1,2,3,4,5). n2 and H n4 When Hnum=4, it means that the H in the 4 H bridges n1 and H n3 At the same time, one H bridge H n2 and H n4 They are turned on at the same time, and the other Hnums are turned on by analogy, thereby achieving the adjustment of the zero voltage interval width.

[0114] In some embodiments, the state of the H-bridge can be determined based on Hnum according to a preset conduction sequence. Figure 5 As shown, when Hnum=4 for the first time, H1, H2 H3, H4 n1 and H n3 At the same time, H5 in H n2 and H n4 At the same time, in the second time Hnum=4, H2, H3, H4, H5 n1 and H n3 Opened at the same time, H1 in H n2 and Hn4 Open at the same time, the second time Hnum=4, H3, H4, H5, H1 n1 and H n3 Opened at the same time, H2 in H n2 and H n4 Opened at the same time.

[0115] In some embodiments, the state of the H-bridge can also be determined based on the voltage and Hnum. For example, each H-bridge has a capacitor. 1 and 3 are connected to charge the capacitor, and 2 and 4 are connected to discharge the capacitor. Therefore, the H-bridge with low voltage is turned on first, and the H-bridge with high voltage is turned off first. For example, if Hnum=4, the H-bridges with low voltage can be turned off first. n1 and H n3 At the same time, the H bridge with the highest voltage is opened. n2 and H n4 Opened at the same time.

[0116] Figure 9 is a schematic diagram of the structure Hmod according to some embodiments of this specification, such as Figure 9 As shown, in order to realize the injection of the injected waveform at six times the main frequency and realize automatic adjustment of the zero voltage interval, ThetaH is transformed as follows, and Hmod is obtained based on the following transformation:

[0117] when hour, ;

[0118] when hour, ;

[0119] when hour, ;

[0120] when hour, .

[0121] Specifically, the grid phase ThetaY is obtained by the phase-locked loop, and the inverter output phase is obtained by the closed-loop control system, that is, the angle at which the first switch device of the main bridge switch unit is triggered is alphaY=arctan(U Tq / U Td ) When ThetaY>alphaY, the first switch device of the main bridge switch unit starts to conduct, and the other switch devices are turned on in sequence according to the preset time interval. The phase of the injection branch is constructed from the grid phase, and the expression is as follows:

[0122] ,

[0123] in, is the reference phase of the inverter output voltage.

[0124] Injection-type high-power photovoltaic grid-connected inverters are primarily used for high-power photovoltaic grid-connected applications. Therefore, the AC output amplitude and phase of the inverter must be controlled to meet grid-connection requirements. The phase of the AC output of an injection-type high-power photovoltaic grid-connected inverter is aligned with the conduction angle of the first switch. Specifically, the first switch of the main bridge switch unit is controlled to turn on at the moment corresponding to the angle output by the closed-loop control. This predetermined switching sequence determines the triggering of all remaining switches. The switching devices of the main and injection bridge switches are controlled in a coordinated manner, synchronizing the zero-level timing of the injection bridge switch unit with the commutation of the main bridge switch unit. This control strategy ensures that the main bridge switch unit maintains a zero voltage state during commutation, achieving zero voltage switching (ZVS) and achieving soft switching. Therefore, as long as the H-bridge combination in the injection bridge switch unit produces the lowest level when the switching device of the main bridge switch unit is turned on or off, it can ensure that the switching device of the main bridge switch unit is switched at zero level, and then the injection bridge switch unit is triggered according to the constructed Hnum, so that the time for the injection bridge switch unit to output a low level is automatically adjusted according to the current size, and the time for injecting zero level can be adjusted. Figure 7 is a schematic diagram of the phase change relationship between the main bridge and the injection branch according to some embodiments of this specification, such as Figure 7 As shown, since the frequency of the injection bridge switching unit is 6 times the main bridge frequency, the time variation of the H-bridge phase ThetaH can be obtained from ThetaY.

[0125] Figure 10 is a control strategy block diagram according to some embodiments of this specification, such as Figure 10 As shown, as an example, the soft switching module controls the periodically changing voltage injected into the main bridge circuit according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, and changes the switching state of the main circuit switching element under zero voltage conditions, including: determining the d-axis and q-axis voltage signals at the output end of the inverter through a dual-loop control structure, wherein the dual-loop control structure includes a voltage outer loop and a current inner loop, the voltage outer loop is used to determine the reference of the system active current based on the photovoltaic array output voltage and the DC side voltage according to maximum power point tracking (MPPT), and the current inner loop is used to determine the d-axis voltage signal and the q-axis voltage signal at the output end of the inverter according to the reference of the system active current;

[0126] According to the d-axis voltage signal and q-axis voltage signal at the inverter output, the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions.

[0127] Specifically, Figure 10 middle,U pv 、 I pv is the voltage and current of photovoltaic output, U dc is the DC side voltage, U Sd 、 I Sd is the grid d-axis voltage and current, U Sq 、 I Sq are the grid q-axis voltage and current, U Td 、 I Tq The inverter outputs d and q axis voltages, t off is the final determined turn-off time of the switching device. According to the instantaneous power theory, the expressions of the instantaneous active power and reactive power of the system in the dq coordinate system can be obtained:

[0128] ,

[0129] ,

[0130] It can be seen that the active power of photovoltaic grid-connected is only related to the d-axis current, and the reactive power is only related to the q-axis current. In order to improve the grid-connected efficiency, the q-axis current can be controlled to zero to achieve unity power factor grid-connected. According to the output characteristics of photovoltaic cells, when under a certain light intensity, the output current of the photovoltaic array is I pv Maintains a constant voltage, while the voltage amplitude on the grid side U sd is also a constant, so I sd can be U dc It is determined by the output voltage of the photovoltaic array. Therefore, the DC side voltage U dc As the feedback signal, the maximum power point voltage is used as the given command value to establish a voltage control loop to achieve tracking control of the photovoltaic array output voltage. At the same time, the output of the voltage control loop is used as the system active current I sd The given command I dref , then the active power connected to the grid can be controlled. In this way, a dual-loop control structure with voltage as the outer loop and current as the inner loop is established, realizing the unified control of MPPT and grid-connected power generation operation.

[0131] The following is an experiment to illustrate the beneficial effects of a power adaptive soft switching control system for a photovoltaic grid-connected injection inverter.

[0132] In combination with the requirements of photovoltaic power supply and grid-connected inverter, system debugging and waveform optimization are carried out through PSCAD / EMTDC simulation software to ensure the performance of injection-type high-power photovoltaic grid-connected inverter in high-power environment, providing an innovative and stable soft switching solution for the optimization of high-power photovoltaic system. Considering the actual operation of photovoltaic system, that is, the randomness and volatility of photovoltaic power generation, in order to make the system run stably and improve the response speed, according to the different light intensity, that is, when the light intensity is greater than 1000W / m 2 Less than 1200W / m 2 , greater than 700W / m 2 Less than 1000W / m 2 and less than 700W / m 2 , three turn-off times are calculated according to the maximum power of the interval, and the system adjusts the zero voltage injection time according to the power emitted under different light conditions.

[0133] By combining PV cells in series and parallel, a 50kW PV array is created. Twenty identical PV arrays are then used to build a 1MW PV grid-connected system. The following lists several specific scenarios for this system at different power levels.

[0134] (1) Temperature T = 25°C, light intensity 400W / m 2

[0135] By adding two three-phase full-bridge DC Figure 11 and Figure 12 The waveform shown is obtained after passing through the transformer Figure 13 As shown in the figure, the high-quality sine waveform is Figure 14 As shown, the harmonic distortion rate is 2.01%, with only the 95th and 97th harmonics accounting for a significant proportion. This meets grid connection requirements, with a zero-level time of 167µs tested in the simulation.

[0136] (2) Temperature T = 25°C, light intensity 800W / m 2

[0137] After increasing the light intensity, the simulation waveform is as follows Figure 15-18 As shown by Figure 17 It can be seen that when the zero-level time is extended, the harmonics will increase slightly. Figure 18The waveform of the AC-side A-phase output voltage distortion rate is 2.39%. As the light intensity increases, the output power of the photovoltaic grid-connected system increases, and the zero-level time of the injected waveform is prolonged. This indicates that the zero-voltage time has been adjusted, which better implements the soft-switching technology and verifies the correctness and rationality of this invention. The zero-voltage time measured in this simulation is 243 μs.

[0138] (3) Temperature T = 25°C, light intensity 1000W / m 2

[0139] After continuing to increase the light intensity, the simulation waveform is as follows Figure 19-22 As shown, Figure 19 and Figure 20 This shows that when the light intensity increases, the output power of the photovoltaic grid-connected system increases, and the zero-level time of the injected waveform is prolonged. Figure 21 It can be seen that after extending the zero-level time, the harmonics will increase slightly. Figure 22 Figure 1 shows the AC side phase A output voltage distortion waveform. The harmonic distortion is 3.02%. The simulated zero voltage time is 362 μs.

[0140] In summary, a photovoltaic grid-connected injection inverter power adaptive soft switching control system can automatically adjust the zero voltage interval width according to the change in power size, and the soft switching effect is significant, which verifies the correctness and rationality of the present invention. However, as the zero voltage time increases, the harmonic content of the AC side output waveform will increase. Therefore, a photovoltaic grid-connected injection inverter power adaptive soft switching control system injects waveforms of equal width and height under light load to achieve optimal waveform output and the lowest harmonic distortion rate; when heavily loaded, in order to reduce the dynamic loss of the switching device, the voltage is always zero during the opening or closing process of the switching device, so that the injection zero voltage interval is widened. A photovoltaic grid-connected injection inverter power adaptive soft switching control system provides an efficient, reliable and adaptable solution for high-power, large-scale photovoltaic grid connection.

[0141] Figure 23 This is a flow chart of a power adaptive soft switching control method for a photovoltaic grid-connected injection inverter according to some embodiments of this specification, such as Figure 23 As shown, a power adaptive soft switching control method for a photovoltaic grid-connected injection inverter may include the following process:

[0142] Calculate the switch device turn-off time based on the photovoltaic input power and photovoltaic output current;

[0143] Calculate the required width of zero voltage and other level widths based on the turn-off time of the switching device;

[0144] Calculate the number of levels based on the required width of zero voltage and the widths of other levels;

[0145] Based on the number of levels, determine the switching state of each H-bridge;

[0146] According to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions.

[0147] A power adaptive soft switching control method for a photovoltaic grid-connected injection inverter can be applied to a power adaptive soft switching control system for a photovoltaic grid-connected injection inverter, which will not be described in detail here.

[0148] Finally, it should be understood that the embodiments described in this specification are intended only to illustrate the principles of the embodiments of this specification. Other variations may also fall within the scope of this specification. Therefore, by way of example and not limitation, alternative configurations of the embodiments of this specification may be considered consistent with the teachings of this specification. Accordingly, the embodiments of this specification are not limited to the embodiments explicitly described and illustrated in this specification.

Claims

1. A photovoltaic grid-connected injection inverter power adaptive soft switching control system, characterized in that: include: An injection-type high-power photovoltaic grid-connected inverter, comprising at least a photovoltaic-side DC converter switch, an injection bridge switch unit, and a main bridge switch unit, wherein the injection bridge switch unit is used to inject a periodically varying voltage into the main bridge circuit, and the main bridge switch unit is used to convert DC power into AC power under the action of the periodically varying voltage injected by the injection bridge switch unit, and the injection bridge switch unit comprises five H-bridges connected in series; The soft switching module is used to control the injection of periodically changing voltage into the injection bridge switch unit based on the photovoltaic input power, and change the switching state of the main circuit switching elements under zero voltage conditions. Specifically, it includes: Calculate the turn-off time of the switching device of the main bridge switch unit based on the photovoltaic input power and photovoltaic output current; Calculate the required width of zero voltage and other level widths based on the turn-off time of the switching device; Calculate the number of levels based on the required width of zero voltage and the widths of other levels; Based on the number of levels, determine the switching state of each H-bridge; According to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions; The soft switching module calculates the switch-off time of the switch device based on the photovoltaic input power and the photovoltaic output current, including: Determine the current power range based on the photovoltaic input power; Determine the photovoltaic output current based on the current power range; Calculate the current off time of the switch device according to the photovoltaic output current; The larger value between the current switch device turn-off time and the standard switch device turn-off time is used as the reference switch device turn-off time.

2. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to claim 1, characterized in that: The main bridge switch unit includes a Y-shaped bridge and a △-shaped bridge; The photovoltaic side DC converter switch is connected in parallel with a capacitor group, wherein the capacitor group includes a first capacitor and a second capacitor connected in series, the input end of the injection bridge switch unit is electrically connected to the connection node of the first capacitor and the second capacitor, and the output end of the injection bridge switch unit is electrically connected to the connection node of the Y-type bridge and the △-type bridge.

3. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to claim 2, characterized in that: Each H-bridge cycle of the injection bridge switch unit is one sixth of the main bridge cycle of the main bridge switch unit, and the same H-bridge is turned on for different widths within five H-bridge cycles.

4. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to claim 1, characterized in that: The soft switching module determines the current switch-off time based on the photovoltaic output current according to the following formula: ， in, is the current switch-off time, is the gate drive resistor, is the gate-collector capacitance, is the gate-emitter capacitance, is the gate turn-on voltage, is the gate turn-off voltage, is the critical turn-on voltage, is the equivalent inductance of the line and transformer, is the photovoltaic output current, is the collector-emitter voltage, is the tail current, is the critical current for shutoff.

5. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to any one of claims 1 to 4, characterized in that: The soft switching module calculates the required width of zero voltage and other level widths based on the switch device turn-off time according to the following formula: ， in, is the required width for zero voltage, is the turn-off time of the switching device, For other level widths.

6. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to claim 5, characterized in that: The soft switching module calculates the number of levels based on the required width for zero voltage and other level widths according to the following rules: When Hmod<-(1- l ), Hnum=-5, where Hmod is the modulation value, Hnum is the number of levels, and l is the required width of zero voltage; When-(1- l )≤Hmod<-(1- l -x), Hnum=-4, where x is the width of other levels; When-(1- l -x)≤Hmod<-(1- l -2x), Hnum=-3 When-(1- l -2x)≤Hmod<-(1- l -3x), Hnum=-2; When-(1- l -3x)≤Hmod<-(1- l -4x), Hnum=-1; When-(1- l -4x)≤Hmod<(1- l -4x), Hnum=0; When (1- l -4x)≤Hmod<(1- l -3x), Hnum=1; When (1- l -3x)≤Hmod<(1- l -2x), Hnum=2; When (1- l -2x)≤Hmod<(1- l -x), Hnum=3; When (1- l -x)≤Hmod<(1- l ), Hnum=4; When Hmod>(1- l ), Hnum=5; Among them, Hmod is obtained based on the following transformation: when hour, ; when hour, ; when hour, ; when hour, .

7. A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to claim 6, characterized in that: The soft switching module controls the periodically changing voltage injected into the main bridge circuit according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, and changes the switching state of the main circuit switching elements under zero voltage conditions, including: Determining d-axis and q-axis voltage signals at the inverter output terminal through a dual-loop control structure, wherein the dual-loop control structure includes a voltage outer loop and a current inner loop, the voltage outer loop being used to determine a reference for the system active current based on the photovoltaic array output voltage and the DC side voltage according to maximum power point tracking, and the current inner loop being used to determine the d-axis voltage signal and the q-axis voltage signal at the inverter output terminal based on the system active current reference; According to the d-axis voltage signal and q-axis voltage signal at the inverter output, the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions.

8. A power adaptive soft switching control method for a photovoltaic grid-connected injection inverter, characterized in that: A photovoltaic grid-connected injection inverter power adaptive soft switching control system according to any one of claims 1 to 7, comprising: Calculate the switch device turn-off time based on the photovoltaic input power and photovoltaic output current; Calculate the required width of zero voltage and other level widths based on the turn-off time of the switching device; Calculate the number of levels based on the required width of zero voltage and the widths of other levels; Based on the number of levels, determine the switching state of each H-bridge; According to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching elements under zero voltage conditions.

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