Photovoltaic grid-connected injection inverter power adaptive soft switching control method and system

Abstract

The invention provides a power adaptive soft switching control method and system for a photovoltaic grid-connected injection inverter, and relates to the field of inverters, and the system comprises an injection-type high-power photovoltaic grid-connected inverter which at least comprises a photovoltaic side DC converter switch, an injection bridge switch unit and a main bridge switch unit, the injection bridge switch unit is used for injecting periodically changing voltage into the main bridge circuit, and the main bridge switch unit is used for converting direct current into alternating current under the action of the periodically changing voltage injected by the injection bridge switch unit; the soft switching module is used for controlling the injection bridge switch unit to inject periodically changing voltage based on photovoltaic input power and changing the switching state of a main circuit switching element under the zero voltage condition, and has the advantages of reducing the dynamic loss of a switching device, improving the efficiency of a photovoltaic inverter system under a high-power application occasion and reducing the power consumption. And meanwhile, the output harmonic quality of the alternating current side of the inverter is considered, so that the harmonic content of the alternating current side is small and a filter is not needed.

Description

Technical Field

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

[0002] As a clean and renewable energy source, photovoltaic power generation has been widely used in recent years. A photovoltaic grid-connected inverter is a core component of a photovoltaic power generation system, and its main function is to convert the direct current generated by photovoltaic modules into alternating current that meets the requirements of the power grid and achieve efficient grid connection. In a photovoltaic grid-connected system, the performance of the grid-connected inverter directly affects the efficiency and stability of the system. Therefore, how to reduce the loss of switching devices while ensuring high efficiency has become a key issue in the application of photovoltaic grid-connected inverters.

[0003] The use of high-frequency PWM (Pulse Width Modulation) in a photovoltaic grid-connected inverter can ensure grid-connected harmonics, but the dynamic loss of switching devices increases. If the frequency is reduced, a larger filter is required on the AC side to meet the grid connection standard, and the power loss of the switching devices and the filter is relatively large. The use of a modular multilevel converter (MMC) can reduce the switching frequency, but still a large dynamic loss is generated during the turn-on and turn-off of the switching devices. In order to reduce the dynamic loss generated by switching devices, the existing solution is to configure an auxiliary circuit such as a buffer circuit or a resonant circuit for the switching devices to implement soft-switching technology. Currently, to achieve soft-switching, an auxiliary circuit and resonant elements need to be introduced, and the working timing of the auxiliary switch and resonant elements must be precisely controlled. The coordinated control requirements of the main switching device and the auxiliary circuit are high, and improper design may result in poor soft-switching effects. The resonant time only accounts for a small part of the switching cycle, which may limit the full play of the soft-switching effect and affect the loss reduction effect. The resonant process may cause an increase in voltage or current peaks, increasing the requirements for the voltage withstand and current withstand of the switching devices and affecting the reliability of the devices. The introduction of the auxiliary circuit increases additional losses and affects the overall efficiency improvement effect. Especially in high-power applications, the use of resonant technology to achieve soft-switching requires a large power of the auxiliary switching device, resulting in a large increase in cost and low cost performance. Especially in high-power photovoltaic grid-connected applications, the current carried in the line is large, resulting in an extended turn-off time of the switching devices.

[0004] Therefore, it is necessary to provide a power adaptive soft-switching control method and system for a photovoltaic grid-connected injection type inverter, which is used to achieve soft-switching by using injection type technology, reduce the dynamic loss of switching devices, improve the efficiency of the photovoltaic inverter system in high-power applications, and dynamically adjust the soft-switching width of the zero-level interval of the switching devices according to the magnitude of the input power on the photovoltaic side, while taking into account the harmonic quality of the AC side output of the inverter, 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, including: an injection-type high-power photovoltaic grid-connected inverter, which at least includes a photovoltaic-side DC converter switch, an injection bridge switch unit, and a main bridge switch unit. Among them, the injection bridge switch unit is used to inject a periodically changing voltage into the main bridge circuit, and the main bridge switch unit is used to convert direct current into alternating current under the action of the periodically changing voltage injected by the injection bridge switch unit; a soft-switching module, which is used to control the injection of the periodically changing voltage by the injection bridge switch unit based on the photovoltaic input power, and change the switching state of the main circuit switching element under the zero-voltage condition.

[0006] Further, the injection bridge switch unit includes five series-connected H-bridges: the main bridge switch unit includes a Y-bridge and a Δ-bridge; the photovoltaic-side DC converter switch is connected in parallel with a capacitor bank, where the capacitor bank 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-bridge and the Δ-bridge.

[0007] Further, the period of each H-bridge of the injection bridge switch unit is one-sixth of the main bridge period of the main bridge switch unit, and the same H-bridge conducts different widths within five H-bridge periods.

[0008] Further, 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 element under the zero-voltage condition, including: calculating the turn-off time of the switching device based on the photovoltaic input power and the photovoltaic output current; calculating the required width of zero voltage and the widths of other levels based on the turn-off time of the switching device; calculating the number of levels based on the required width of zero voltage and the widths of other levels; determining the switching state of each H-bridge based on the number of levels; 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 period, and changing the switching state of the main circuit switching element under the zero-voltage condition.

[0009] Further, the soft-switching module calculates the turn-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 according to the current power range; calculating the current turn-off time of the switching device according to the photovoltaic output current; taking the larger value of the current turn-off time of the switching device and the standard turn-off time of the switching device as the reference turn-off time of the switching device.

[0010] Further, the soft-switching module determines the current turn-off time of the switching device based on the photovoltaic output current according to the following formula: , wherein, is the turn-off time of the current switching device, is the gate drive resistance, is the gate-collector capacitance, is the gate-emitter capacitance, is the gate turn-on voltage, is the gate turn-off voltage, is the conduction critical voltage, is the equivalent inductance of the line and the transformer, is the photovoltaic output current, is the collector-emitter voltage, is the tail current, is the turn-off critical current.

[0011] Furthermore, based on the turn-off time of the switching device, the required width of zero voltage and other level widths are calculated according to the following formula: , , wherein, is the required width of zero voltage, is the turn-off time of the switching device, is the other level width.

[0012] Furthermore, the soft-switching module calculates the number of levels based on the required width of zero voltage and the other level width 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 other level width; 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; Wherein, Hmod is obtained based on the following transformation: When , ; When , ; When , ; When , .

[0013] Furthermore, according to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, control the periodically varying voltage injected into the main bridge circuit, and change the switching state of the main circuit switching elements under zero voltage conditions, including: determining the d-axis and q-axis voltage signals at the output end of the inverter through a double-loop control structure, wherein the double-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 active current of the system based on the maximum power point tracking of the output voltage of the photovoltaic array and the DC-side voltage, 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 active current of the system; control the periodically varying voltage injected into the main bridge circuit according to the d-axis voltage signal, q-axis voltage signal, switching state of each H-bridge and the conduction width in the current H-bridge cycle, and change the switching state of the main circuit switching elements under zero voltage conditions.

[0014] The present invention provides a power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter. Applied to the above-mentioned power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter, it includes: calculating the turn-off time of the switching device based on the photovoltaic input power and the photovoltaic output current; calculating the required width of zero voltage and the widths of other levels based on the turn-off time of the switching device; calculating the number of levels based on the required width of zero voltage and the widths of other levels; determining the switching states of each H-bridge based on the number of levels; and controlling the periodically varying voltage injected into the main bridge circuit according to the switching states of each H-bridge and the conduction width in the current H-bridge cycle, and changing the switching states of the main circuit switching elements under zero voltage conditions.

[0015] 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: (1) Significantly reduce switching losses and improve system efficiency At a low switching frequency, by dynamically adjusting the zero voltage interval and optimizing the turn-on and turn-off conditions of the switching device, the switching losses are significantly reduced. At the same time, due to the simplification of the circuit, the component losses are further reduced, thereby improving the overall efficiency of the photovoltaic inverter, especially more obvious in high-power applications.

[0016] (2) Reduce circuit complexity By designing a dynamic zero voltage time adjustment scheme, there is no need to introduce an additional resonant network or a complex auxiliary circuit. The soft-switching function is directly realized by using the injection branch. Compared with the traditional resonant soft-switching, the circuit design is significantly simplified, the number of components is reduced, and the volume and implementation cost of the system are reduced. The present invention does not require an additional resonant network or redundant auxiliary switches, and the soft-switching function can be realized only through the injection branch and real-time control, which not only reduces the implementation cost but also reduces potential failure points, significantly improving the reliability of the system.

[0017] (3) Improve dynamic adaptability Most of the existing soft-switching technologies are fixed designs and are difficult to adapt to the dynamic changes of the load current. To address this problem, this method and system introduce a real-time current detection and dynamic adjustment mechanism, which can accurately calculate the zero voltage time according to the change of the load current to ensure that the soft-switching function can be efficiently realized under different load conditions, especially suitable for the frequently changing load conditions in the photovoltaic grid-connected system.

[0018] (4) Improve power quality and reduce harmonic distortion By following the rule of specific sub - harmonic elimination and injecting a periodically varying DC voltage across the two ends of the DC sides of two series - connected three - phase full - bridges, the output voltage waveform can be optimized to be closer to an ideal sine wave, reducing the content of low - order harmonics and improving the power quality. In high - power photovoltaic grid - connected applications, the harmonic distortion rate of the system is reduced, which helps to reduce or avoid the use of additional filtering equipment, lowering the overall cost of system design and operation. At the same time, it enhances the grid - connection adaptability of the system and meets the strict requirements of the photovoltaic grid - connected system for harmonics.

[0019] (5) Improve the reliability of the system and extend the lifespan of switching devices By dynamically adjusting the zero - voltage time, the voltage and current stresses of the switching devices during turn - on and turn - off can be effectively reduced, reducing the thermal losses and overheating risks of the switching devices under high - power conditions, extending their service life, and thus improving the reliability of the system. Description of the Drawings

[0020] This specification will further illustrate by way of exemplary embodiments, which will be described in detail through the drawings. These embodiments are not restrictive. In these embodiments, the same numbers represent the same structures, where: Figure 1 is a schematic diagram of the modules of a power - adaptive soft - switching control system for a photovoltaic grid - connected injection inverter according to some embodiments of this specification; Figure 2 is a schematic circuit diagram of an injection - type high - power photovoltaic grid - connected inverter according to some embodiments of this specification; Figure 3 is a schematic circuit diagram of an injection - bridge switch unit according to some embodiments of this specification; Figure 4 is a schematic circuit diagram of a main - bridge switch unit according to some embodiments of this specification; Figure 5 is a timing diagram of the cyclic triggering of the injection branch according to some embodiments of this specification; Figure 6 is a flowchart for calculating the turn - off time of the switching device according to some embodiments of this specification; 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; Figure 8 is a schematic diagram of the number of levels according to some embodiments of this specification; Figure 9 is a schematic diagram of the output stepped waveform according to some embodiments of this specification; Figure 10 is a block diagram of the control strategy according to some embodiments of this specification; Figure 11 is a schematic diagram of the Y-Y bridge injection voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 12 is a schematic diagram of the Y-Δ bridge injection voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 13 is a schematic diagram of the AC side A-phase output voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 14 is a schematic diagram of the AC side A-phase output voltage harmonic distortion rate under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 15 is a schematic diagram of the Y-Y bridge injection voltage waveform under a light intensity of 800 W / m 2 shown in some embodiments of this specification; Figure 16 is a schematic diagram of the Y-Δ bridge injection voltage waveform under a light intensity of 800 W / m 2 shown in some embodiments of this specification; Figure 17 is a schematic diagram of the AC side A-phase output voltage waveform under a light intensity of 800 W / m 2 shown in some embodiments of this specification; Figure 18 is a schematic diagram of the AC side A-phase output voltage harmonic distortion rate under a light intensity of 800 W / m 2 shown in some embodiments of this specification; Figure 19 is a schematic diagram of the Y-Y bridge injection voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 20 is a schematic diagram of the Y-Δ bridge injection voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 21 is a schematic diagram of the AC side A-phase output voltage waveform under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 22 is a schematic diagram of the AC side A-phase output voltage harmonic distortion rate under a light intensity of 400 W / m 2 shown in some embodiments of this specification; Figure 23It is a schematic flowchart of a power adaptive soft-switching control method for a grid-connected injection type photovoltaic inverter according to some embodiments of this specification. Detailed implementation manners

[0021] To more clearly illustrate the technical solutions of the embodiments of this specification, the accompanying drawings required for the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some examples or embodiments of this specification. For those of ordinary skill in the art, without creative efforts, this specification can also be applied to other similar scenarios based on these drawings. Unless obvious from the language context or otherwise stated, the same reference numerals in the figures represent the same structures or operations.

[0022] Figure 1 It is a schematic diagram of the modules of a power adaptive soft-switching control system for a grid-connected injection type photovoltaic inverter according to some embodiments of this specification. As Figure 1 shown, a power adaptive soft-switching control system for a grid-connected injection type photovoltaic inverter may include an injection type high-power grid-connected photovoltaic inverter and a soft-switching module.

[0023] Figure 2 It is a schematic circuit diagram of an injection type high-power grid-connected photovoltaic inverter according to some embodiments of this specification. As Figure 2 shown, an injection type high-power grid-connected photovoltaic inverter at least includes a photovoltaic side DC converter switch, an injection bridge switch unit, and a main bridge switch unit. Among them, the injection bridge switch unit is used to inject a periodically changing voltage into the main bridge circuit, and the main bridge switch unit is used to convert direct current into alternating current under the action of the periodically changing voltage injected by the injection bridge switch unit. In the figure, S Y1 - S Y6 , S D1 - S D6 are the main bridge switches, and H 11 -H 54 are the injection bridge switches. 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 are the secondary side voltages of the transformer, U SA , USB , U SC is the three-phase voltage on the AC side, I Ydc , I △dc are the currents of the Y-bridge and Δ-bridge, U yy , U △y are the voltages of the Y-bridge and Δ-bridge, U j is the multi-level injection voltage.

[0024] It can be understood that the DC side of the 12-pulse inverter circuit is a stable and constant DC power, and the AC side contains 12k±1 harmonics. A power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter converts the stable and constant DC power into a periodically changing DC power through the injection bridge switching unit, so that the harmonic content on the AC side is greatly reduced. Even without installing a filter, it can output AC electric energy that meets the grid connection standard, and the injection branch voltage is exactly zero when the switching device of the main bridge switching unit is turned on or off, thus realizing the soft-switching technology. In a traditional DC / AC inverter, the current and voltage values applied to each bridge are constant, while in an injection-type high-power photovoltaic grid-connected inverter, a periodically changing voltage is superimposed on the main bridge switching unit, and the changing injection waveform is beneficial to eliminating harmonics and improving the performance of the inverter.

[0025] Figure 3 is a circuit schematic diagram of the injection bridge switching unit shown in some embodiments of this specification. As Figure 3 shown, in order to add a periodically changing voltage, the injection bridge switching unit includes five series-connected H-bridges. Each H-bridge is composed of fully controlled devices. The injection bridge switching unit does not conduct DC current, and their current ratings are only related to the AC components, so the current ratings are relatively low. In high-voltage and high-power applications, devices with lower power ratings can be selected for the injection bridge.

[0026] Figure 4 is a circuit schematic diagram of the main bridge switching unit shown in some embodiments of this specification. As Figure 4 shown, the main bridge switching unit includes a series-connected Y-bridge and Δ-bridge. Its AC side is formed by connecting the grid-side windings of the commutation transformer in series. The secondary sides of the transformer are respectively connected in star and delta, and are respectively connected to two three-phase full bridges. Due to different winding connections, the turn ratio of the secondary windings of the Y-connected and Δ-connected transformers is a fixed constant, and the main bridge switching unit is composed of 12 switching devices.

[0027] As Figure 2As shown, a capacitor bank is connected in parallel with the switches of the DC inverter converter on the photovoltaic side. Among them, the capacitor bank 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-bridge and the Δ-bridge. It can be seen that by changing U j the direction and magnitude of, the voltage on the bridge can be changed, and the switching devices on the main bridge can be turned on and off under zero voltage. That is, the voltage determined by the conduction state of the switching devices of the injection bridge switch unit clamps the switching devices of the main bridge switch unit, and the switching state of the switching devices of the main bridge switch unit is changed under zero voltage conditions. The switching devices of the main bridge switch unit commutate at the moment of U j = 0, and at this time, the conditions of soft switching technology are satisfied.

[0028] In an injection-type high-power photovoltaic grid-connected inverter, the bridge inverter can be regarded as a whole, designed according to its voltage and current levels, and there is no danger of direct conduction of the components on the two series bridge arms. Therefore, there is no need to leave a dead zone for safety due to the turn-on of one bridge arm and the turn-off of another bridge arm, which improves the operation reliability.

[0029] Figure 5 is the injection branch cycle trigger timing diagram shown in some embodiments of this specification. As Figure 5 shown, each H-bridge cycle of the injection bridge switch unit is one-sixth of the main bridge cycle of the main bridge switch unit. The same H-bridge conducts different widths within five H-bridge cycles, and any two H-bridges conduct different widths within the same H-bridge cycle. By making each H-bridge conduct different widths within five H-bridge cycles and rotating the conduction of different H-bridges, the charge and discharge balance of the cascaded H-bridges is achieved. Let J1, J2, J3, J4, and J5 be different conduction widths, and the conduction widths increase in sequence. Within five H-bridge cycles, let the conduction widths of the first H-bridge be J1, J2, J3, J4, J5 in sequence; then the conduction widths of the second H-bridge are J2, J3, J4, J5, J1; the conduction widths of the third H-bridge are J3, J4, J5, J1, J2; the conduction widths of the fourth H-bridge are J4, J5, J1, J2, J3; the conduction widths of the fifth H-bridge are J5, J1, J2, J3, J4; and so on in a cycle to achieve the charge and discharge balance of the cascaded H-bridges.

[0030] A 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 elements under zero voltage conditions.

[0031] Specifically, it includes: Calculating the turn-off time of the switching device based on the photovoltaic input power and the photovoltaic output current; Calculate the width required for zero voltage and the widths of other levels based on the turn-off time of the switching device; Calculate the number of levels based on the width required for zero voltage and the widths of other levels; Determine the switching state of each H-bridge based on the number of levels; According to the switching state of each H-bridge and the conduction width in the current H-bridge cycle, control the periodically varying voltage injected into the main bridge circuit to change the switching state of the main circuit switching element under zero voltage conditions.

[0032] Figure 6 is a flowchart for calculating the turn-off time of the switching device shown in some embodiments of this specification, as Figure 6 shown. Preferably, the soft-switching module calculates the turn-off time of the switching 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 according to the current power range; Calculate the current turn-off time of the switching device according to the photovoltaic output current; Take the larger value between the current turn-off time of the switching device and the standard turn-off time of the switching device as the reference turn-off time of the switching device. Among them, the standard turn-off time of the switching device can be obtained from the instruction manual of the switching device. Specifically, multiple power ranges can be divided, and each power range corresponds to a power correction coefficient. The photovoltaic output current can be obtained by dividing the product of the maximum power and the power correction coefficient by the DC voltage.

[0033] For example, divide six power ranges according to [0.9, 1], [0.8, 0.9], [0.6, 0.8], [0.4, 0.6], [0.2, 0.4], [0, 0.2], and the corresponding power correction coefficients are 0.9, 0.8, 0.6, 0.4, 0.2, 0 respectively.

[0034] It can be understood that in large-power photovoltaic grid connection, the DC voltage remains constant. Therefore, the change in current can reflect the change in power. By detecting the magnitude of the current flowing through the circuit and calculating the turn-off time of the switching device according to this current, making the zero voltage time provided by the injection branch greater than or equal to the turn-off time of the switching device, the dynamic adjustment of the zero voltage interval is realized, ensuring that the soft-switching technology can be achieved at any power, and significantly reducing the dynamic loss of the switching device.

[0035] Preferably, the soft-switching module determines the current turn-off time of the switching device based on the photovoltaic output current according to the following formula: , where, is the current turn-off time of the switching device, is the gate drive resistance, is the gate-collector capacitance, is the gate-emitter capacitance, is the gate turn-on voltage, is the gate turn-off voltage, is the conduction critical 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 turn-off critical current.

[0036] Preferably, based on the turn-off time of the switching device, the width required for zero voltage and other level widths are calculated according to the following formula: , , wherein, is the width required for zero voltage, is the turn-off time of the switching device, is the other level width.

[0037] Preferably, the soft-switching module calculates the number of levels based on the width required for zero voltage and the other level width according to the following rules: Figure 8 is a schematic diagram of the number of levels shown in some embodiments of this specification, such as Figure 8 shown, when Hmod < -(1 - l), Hnum = -5, wherein, Hmod is the modulation value, Hnum is the number of levels, and l is the width required for zero voltage; when -(1 - l ) ≤ Hmod < -(1 - l -x), Hnum = -4, wherein, x is the other level width; 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-(1 - 4x) ≤ Hmod < (1 - l -(1 - 3x)), Hnum = 1; When (1 - l -(1 - 3x)) ≤ Hmod < (1 - l -(1 - 2x)), Hnum = 2; When (1 - l -(1 - 2x)) ≤ Hmod < (1 - l -(1 - x)), Hnum = 3; When (1 - l -(1 - x)) ≤ Hmod < (1 - l ), Hnum = 4; When Hmod > (1 - l ), Hnum = 5.

[0038] For example, when Hnum = 5, it means that 5 H - bridges output high level, that is, H n1 and H n3 are turned on simultaneously (n = 1, 2, 3, 4, 5). When Hnum = - 5, it means that in 5 H - bridges, H n2 and H n4 are turned on simultaneously. When Hnum = 4, it means that in 4 H - bridges, H n1 and H n3 are turned on simultaneously, and in 1 H - bridge, H n2 and H n4 are turned on simultaneously. And so on for other values of Hnum, thus realizing the adjustment of the zero - voltage interval width.

[0039] In some embodiments, the state of the H - bridge can be determined based on Hnum according to a preset conduction sequence. For example, as Figure 5 shown, when Hnum = 4 for the first time, in H 1 , H 2 H 3 , H 4 in H n1 and H n3 are turned on simultaneously, and in H 5 in H n2 and H n4 are turned on simultaneously. When Hnum = 4 for the second time, in H 2 , H 3 , H 4 , H 5 in H n1 and H n3 are turned on simultaneously, and in H 1 in H n2 and H n4 are turned on simultaneously. When Hnum = 4 for the second time, in H 3 , H 4 , H 5 , H1 H in the middle n1 and H n3 are turned on simultaneously, H 2 H in the middle n2 and H n4 are turned on simultaneously.

[0040] In some embodiments, the state of the H-bridge can also be determined based on Hnum according to the voltage. For example, each H-bridge has a capacitor. When 1 and 3 are connected, the capacitor is charged, and when 2 and 4 are connected, the capacitor is discharged. Therefore, the H-bridge with a lower voltage is turned on first, and the H-bridge with a higher voltage is turned off first. For example, when Hnum = 4, H n1 and H n3 in the 4 H-bridges with lower voltages can be turned on simultaneously, and H n2 and H n4 in the H-bridge with the highest voltage can be turned on simultaneously.

[0041] Figure 9 is a schematic diagram of constructing Hmod according to some embodiments of this specification. As Figure 9 shown, in order to achieve injecting the waveform with six times the fundamental frequency and realizing the automatic adjustment of the zero-voltage interval, the following transformation is performed on ThetaH, and Hmod is obtained based on the following transformation: When , ; When , ; When , ; When , .

[0042] Specifically, the grid phase ThetaY is obtained by the phase-locked loop, and the phase of the inverter output is obtained by the closed-loop control system. That is, the angle of the triggering moment of the first switching device of the main bridge switching unit is alphaY = arctan(U Tq / U Td ) . When ThetaY > alphaY, the first switching device of the main bridge switching unit starts to conduct, and other switching devices conduct 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: , where is the reference phase of the inverter output voltage.

[0043] The injection - type high - power photovoltaic grid - connected inverter is mainly applied to high - power photovoltaic grid connection. Therefore, it is necessary to control the amplitude and phase of the alternating current output by the inverter to meet the grid - connection conditions. The phase of the alternating current output by the injection - type high - power photovoltaic grid - connected inverter is consistent with the conduction angle of the first switching device, that is, to control the first switching device of the main - bridge switching unit to turn on at the moment corresponding to the angle output by the closed - loop control, and through the determined switching sequence, the triggering of all the remaining switching devices can be determined. Coordinate the control of the switching devices of the main - bridge switching unit and the injection - bridge switching unit, so that the zero - level time of the injection - bridge switching unit is synchronized with the commutation operation of the main - bridge switching unit. Through this control strategy, the main - bridge switching unit always maintains a zero - voltage state during commutation, realizing zero - voltage switching (ZVS) and achieving the purpose of soft switching. Therefore, as long as the H - bridge combination in the injection - bridge switching unit outputs the lowest level when the switching device of the main - bridge switching unit turns on or off, it can ensure that the switching device of the main - bridge switching unit commutates at zero voltage. Then, according to the constructed Hnum, trigger the injection - bridge switching unit, so that the time for the injection - bridge switching unit to output a low level is automatically adjusted according to the current magnitude, and thus the time for injecting the zero level can be adjusted. Figure 7 is a schematic diagram of the phase change relationship between the main bridge and the injection branch shown in some embodiments of this specification, as Figure 7 shown. Since the frequency of the injection - bridge switching unit is 6 times that of the main - bridge frequency, the change of the phase ThetaH of the H - bridge with time can be obtained from ThetaY.

[0044] Figure 10 is a control - strategy block diagram shown in some embodiments of this specification, as Figure 10 shown. Preferably, the soft - switching module controls the periodically varying 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: through a double - loop control structure, determining the d - axis and q - axis voltage signals at the output end of the inverter. Among them, the double - 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 active current of the system based on the output voltage of the photovoltaic array and the DC - side voltage according to the maximum power point tracking (MPPT). The current inner loop is used to determine the d - axis voltage signal and q - axis voltage signal at the output end of the inverter according to the reference of the active current of the system; Control the periodically varying voltage injected into the main - bridge circuit according to the d - axis voltage signal, q - axis voltage signal, switching state of each H - bridge and the conduction width in the current H - bridge cycle, and change the switching state of the main - circuit switching element under zero - voltage conditions.

[0045] Specifically, Figure 10 in U pv 、 I pvare the voltage and current of the PV output, U dc is the DC - side voltage, U Sd 、 I Sd are the grid d - axis voltage and current, U Sq 、 I Sq are the grid q - axis voltage and current, U Td 、 I Tq are the inverter output d - axis and q - axis voltages, t off is the finally determined turn - off time of the switching device. According to the instantaneous power theory, the expressions of the system instantaneous active power and reactive power in the dq coordinate system can be obtained: , , It can be seen from this that the active power of PV grid - connection 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 - connection efficiency, controlling the q - axis current to zero can achieve unity power factor grid - connection. According to the output characteristics of the PV cell, when at a certain light intensity, the output current of the PV array, i.e., I pv basically remains constant, and the amplitude of the grid - side voltage U sd is also a constant value. Therefore, I sd can be determined by U dc , that is, determined by the output voltage of the PV array. Therefore, taking the DC - side voltage U dc as the feedback signal and the maximum power point voltage as the given command value, a voltage control loop is established to achieve the tracking control of the output voltage of the PV array. At the same time, taking the output of the voltage control loop as the given command I sd of the system active current I dref , then the control of the in - grid active power can be achieved. In this way, a double - loop control structure of voltage outer - loop and current inner - loop is established to realize the unified control of MPPT and grid - connected power generation operation.

[0046] Next, combined with experiments, the beneficial effects of a power adaptive soft - switching control system for a PV grid - connected injection - type inverter will be described.

[0047] Combined with the requirements of photovoltaic power sources and grid-connected inverters, system debugging and waveform optimization are carried out through PSCAD / EMTDC simulation software to ensure the performance of the injected high-power photovoltaic grid-connected inverter in a high-power environment, providing an innovative and highly stable soft-switching solution for the optimization of high-power photovoltaic systems. Considering the actual operation of the photovoltaic system, that is, the randomness and volatility of photovoltaic power generation, in order to make the system operate stably and improve the response speed, according to different light intensities, that is, when the light is greater than 1000 W / m 2 less than 1200 W / m 2 and greater than 700 W / m 2 less than 1000 W / m 2 as well as less than 700 W / m 2 , three turn-off times are calculated according to the maximum power in the interval, and the system adjusts the zero-voltage injection time according to the power generated under different light intensities.

[0048] A 50-kW photovoltaic array is obtained through the series-parallel combination of photovoltaic cells, and then 20 identical photovoltaic arrays are used to construct a 1-MW photovoltaic grid-connected system. The following lists several specific situations of this system at different powers.

[0049] (1) Temperature T = 25 °C, light intensity 400 W / m 2 By adding the waveforms shown in Figure 11 and Figure 12 to the DC sides of two three-phase full bridges, a high-quality sine wave as shown in Figure 13 is obtained after passing through the transformer. As shown in Figure 14 , the total harmonic distortion rate is 2.01%, and only the 95th and 97th harmonics have relatively large proportions. It meets the grid connection requirements, and the zero-level time is tested to be 167 us in the simulation.

[0050] (2) Temperature T = 25 °C, light intensity 800 W / m 2 After increasing the light intensity, the simulation waveform is as shown in Figures 15 - 18 . As can be seen from Figure 17 , when the zero-level time is extended, the harmonics will increase slightly. Figure 18 is the distortion rate waveform of the AC side phase A output voltage, and the total harmonic distortion rate is 2.39%. After the light intensity increases, the output power of the photovoltaic grid-connected system increases, and the zero-level time of the injected waveform is extended, indicating that the adjustment of the zero-voltage time is realized, and the soft-switching technology is better implemented, verifying the correctness and rationality of the present invention. The zero-voltage time measured in this simulation is 243 us.

[0051] (3) Temperature T = 25 °C, light intensity 1000 W / m 2 After continuing to increase the light intensity, the simulation waveform is as followsFigures 19 - 22 As shown Figure 19 and Figure 20 it shows that after the light intensity increases, the output power of the photovoltaic grid-connected system increases, and the zero-level time of the injected waveform is extended. From Figure 21 it can be seen that after the zero-level time is extended, the harmonics will increase slightly Figure 22 is the waveform of the distortion rate of the output voltage of phase A on the AC side, and the harmonic distortion rate is 3.02%. The measured zero-voltage time by simulation is 362 us.

[0052] In summary, a power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter can automatically adjust the width of the zero-voltage interval according to the change of the power magnitude, and the soft-switching effect is remarkable, verifying the correctness and rationality of the present invention. However, as the zero-voltage time extends, the harmonic content of the output waveform on the AC side will increase. Therefore, a power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter injects waveforms with equal width and equal height under light load to achieve the optimal waveform output with the lowest harmonic distortion rate; when under heavy load, in order to reduce the dynamic loss of the switching devices and make the voltage zero during the on or off process of the switching devices, the width of the injected zero-voltage interval is made wider. A power adaptive soft-switching control system for a photovoltaic grid-connected injection inverter provides an efficient, reliable and highly adaptable solution for large-power and large-scale photovoltaic grid connection.

[0053] Figure 23 is a schematic flow chart of a power adaptive soft-switching control method for a photovoltaic grid-connected injection inverter according to some embodiments of this specification. As Figure 23 shown, a power adaptive soft-switching control method for a photovoltaic grid-connected injection inverter may include the following processes: Based on the photovoltaic input power and the photovoltaic output current, calculate the turn-off time of the switching device; Based on the turn-off time of the switching device, calculate the required width of the zero voltage and the widths of other levels; Based on the required width of the zero voltage and the widths of other levels, calculate the number of levels; Based on the number of levels, determine the switching states of each H-bridge; According to the switching states of each H-bridge and the conduction width in the current H-bridge cycle, control the periodically varying voltage injected by the main bridge circuit, and change the switching states of the main circuit switching elements under zero-voltage conditions.

[0054] 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 elaborated here.

[0055] Finally, it should be understood that the embodiments described in this specification are only used 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 regarded as consistent with the teachings of this specification. Accordingly, the embodiments of this specification are not limited to the embodiments explicitly presented and described in this specification.

Claims

1. A photovoltaic grid-connected injection inverter power adaptive soft switch control system, characterized in that: include: An injection-type high-power photovoltaic grid-connected inverter comprises 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 changing voltage into the main bridge circuit, and the main bridge switch unit is used to convert direct current into alternating current under the action of the periodically changing voltage injected by the injection bridge switch unit; 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 switch element under zero voltage conditions.

2. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 1, characterized in that: The injection bridge switch unit includes five H bridges connected in series: 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 switch 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 in five H-bridge cycles.

4. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 3, characterized in that: The soft switch 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 switch element under the zero voltage condition, including: Calculate the switch-off time of the switch device based on the PV input power and PV output current; Based on the turn-off time of the switch device, the required width of zero voltage and other level widths are calculated; Calculate the number of levels based on the required width of zero voltage and other level widths; Based on the number of levels, determine the switch 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 switch element under zero voltage conditions.

5. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 4, characterized in that: The soft switch module calculates the switch-off time of the switch device based on the photovoltaic input power and the photovoltaic output current, including: Based on the photovoltaic input power, determine the current power range; Determine the photovoltaic output current according to the current power range; calculate the current switch-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.

6. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 5, characterized in that: The soft switch module determines the current switch-off time of the switch device based on the photovoltaic output current according to the following formula: ， in, is the current switch-off time of the switch device, 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 turn-on critical 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.

7. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to any one of claims 4 to 6, 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 switch off time, For other level widths.

8. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 7, characterized in that: 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: 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, .

9. A photovoltaic grid-connected injection inverter power adaptive soft switch control system according to claim 8, 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 switch element under zero voltage conditions, including: Determine 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 the 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, q-axis voltage signal, switching state of each H-bridge and the conduction width in the current H-bridge cycle at the output end of the inverter, the periodically changing voltage injected into the main bridge circuit is controlled to change the switching state of the main circuit switching element under zero voltage conditions.

10. A power adaptive soft switch control method for a photovoltaic grid-connected injection inverter, characterized in that: A photovoltaic grid-connected injection inverter power adaptive soft switch control system as described in any one of claims 1 to 9, comprising: Calculate the switch-off time of the switch device based on the PV input power and PV output current; Based on the turn-off time of the switch device, the required width of zero voltage and other level widths are calculated; Calculate the number of levels based on the required width of zero voltage and other level widths; Based on the number of levels, determine the switch 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 switch element under zero voltage conditions.

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