A control method for a low-loss photovoltaic inverter

Abstract

This invention relates to the field of photovoltaic inverter technology, and more particularly to a control method for a low-loss photovoltaic inverter, comprising: acquiring the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform of the power switching devices during the off-time period in the grid-connected operation of the photovoltaic inverter; determining the tilt angle of the photovoltaic panel based on the logarithmic decay rate; acquiring the common-mode current of the three-phase cable on the AC output side of the photovoltaic inverter within a unit fundamental frequency period and determining the redundancy vector allocation ratio in space vector pulse width modulation; acquiring the effective value of the current corresponding to the spectral component of the AC output phase current of the three-phase cable on the AC output side within a unit fundamental frequency period and calculating the loss ratio of a preset frequency band; determining the harmonic current injected into the AC output side based on the loss ratio; and controlling the photovoltaic inverter to complete the grid-connected operation process according to the redundancy vector allocation ratio and the harmonic current injected into the AC output side. This invention improves the transmission efficiency of the photovoltaic inverter.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic inverter technology, and in particular to a control method for a low-loss photovoltaic inverter. Background Technology

[0002] With the rapid development of photovoltaic (PV) power generation technology in the solar energy industry, PV inverters, as the core equipment connecting PV arrays and the power grid, directly affect the profitability and safety of the entire power generation system due to their operational efficiency and reliability. Among these, the three-level topology, with its advantages of low voltage stress on switching devices, low output harmonics, and high efficiency, has been widely used in medium- and high-power PV inverters. In large-scale PV power plants, a distributed control architecture is typically adopted, using multiple inverters to operate collaboratively to improve the overall system efficiency. This places higher demands on the long-term stability of individual inverters. However, the DC bus connection of existing PV inverters... During long-term operation, the metal contact surfaces of the inverter are susceptible to oxidation and corrosion, leading to increased contact resistance. Existing monitoring methods mostly rely on temperature sensors to directly measure the busbar temperature. However, temperature detection has significant thermal inertia, and faults are often only detected when the temperature rises abnormally. By this time, oxidation is already quite severe, posing a safety hazard. In addition, the dead time of the power switching devices in existing inverters is usually a fixed value and cannot be dynamically adjusted according to changes in busbar contact resistance. When the conduction loss of the busbar increases due to oxidation, the controller cannot reduce the commutation loss of the switching devices by optimizing the dead time to compensate for the total system loss, resulting in a decrease in the system's thermal balance capability.

[0003] Chinese Patent Publication No. CN118100292A discloses a photovoltaic inverter topology circuit, identification method, control system, and control method. The topology circuit includes: at least two DC-DC converter modules, an inverter module, and a switching switch. The input pins of each DC-DC converter module are respectively connected to both ends of a photovoltaic string. The output pins of each DC-DC converter module are connected to the input pins of the inverter module. The output pins of the inverter module are used to connect to both ends of a load. When some DC-DC converter modules are connected to the photovoltaic string and some are not connected, the DC-DC converter modules connected to the photovoltaic string and those not connected to the photovoltaic string are... A switching switch is connected in series between the modules; it can be seen that the photovoltaic inverter topology circuit, identification method, control system and control method have the following problems: due to the temperature difference between day and night and the effect of condensation, water molecules are easy to accumulate on the back of the photovoltaic panel and at the cable connection. Under the action of electrostatic adsorption, electrochemical oxidation is induced, resulting in the formation of an oxide film on the surface of the junction box terminals, which increases the contact resistance. The additional Joule heat generated by the increased contact resistance is conducted to the three-phase cable on the AC output side, which raises the temperature of the cable insulation layer, causes the dielectric constant to drift, fluctuates the parasitic capacitance to ground, reduces the leakage path impedance, increases the common mode leakage current, and thus reduces the grid-connected transmission efficiency. Summary of the Invention

[0004] To address this, the present invention provides a control method for a low-loss photovoltaic inverter, which overcomes the problems in the prior art where, due to diurnal temperature variations and condensation, water molecules easily accumulate on the back of the photovoltaic panel and at cable connections. Under electrostatic adsorption, this induces electrochemical oxidation, leading to the formation of an oxide film on the surface of the junction box terminals. This increases contact resistance, and the additional Joule heat generated by the increased contact resistance is conducted to the three-phase AC output cables, causing the cable insulation temperature to rise, dielectric constant to drift, parasitic capacitance to ground to fluctuate, leakage path impedance to decrease, and common-mode leakage current to increase, ultimately resulting in reduced grid-connected transmission efficiency.

[0005] To achieve the above objectives, the present invention provides a control method for a low-loss photovoltaic inverter, comprising: When the temperature difference at the photovoltaic panel of the photovoltaic inverter is greater than or equal to a preset temperature difference value, the DC bus voltage oscillation waveform of the power switching device of the photovoltaic inverter during the off-time period during grid-connected operation is obtained. Adjust the tilt angle of the photovoltaic panel according to the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform; Obtain the common-mode current of the three-phase cable on the AC output side of the photovoltaic inverter after the tilt angle is adjusted within a unit fundamental frequency cycle; The redundancy vector allocation ratio in space vector pulse width modulation is determined based on the effective value of the common-mode current. Obtain the spectral components of the AC output phase current of the three-phase cable on the AC output side within a unit fundamental frequency period; The loss ratio of the preset frequency band is calculated based on the effective value of the harmonic current corresponding to the spectral component within a unit fundamental period and the effective value of the total current of the AC output phase current. The harmonic current injected into the AC output side is determined based on the aforementioned loss ratio; According to the aforementioned redundancy vector allocation ratio and the injected harmonic current on the AC output side, the photovoltaic inverter is controlled to complete the grid-connected operation process, wherein... The photovoltaic inverter is a three-level photovoltaic inverter.

[0006] Further, adjusting the tilt angle of the photovoltaic panel according to the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform includes: The logarithmic decay rate is compared with the preset decay rate; If the logarithmic decay rate is greater than the preset decay rate, it is determined that the water molecules accumulated on the back of the photovoltaic panel and at the cable connection point due to condensation at the photovoltaic panel do not meet the requirements for the transmission efficiency of the photovoltaic panel and the photovoltaic inverter, and the tilt angle of the photovoltaic panel is reduced.

[0007] Furthermore, the tilt angle of the photovoltaic panel is negatively correlated with the logarithmic decay rate, wherein, The tilt angle of the photovoltaic panel is the acute angle between the plane on which the photovoltaic panel is located and the horizontal plane.

[0008] Further, determining the redundancy vector allocation ratio in space vector pulse width modulation based on the effective value of the common-mode current includes: The valid value is compared with the preset valid value; If the effective value is greater than the preset effective value, it is determined that the cumulative heat generated by the oxide layer on the back of the photovoltaic panel induced by water molecules at the cable connection does not meet the requirements for the dielectric properties of the cable insulation layer, and the redundancy vector allocation ratio in the space vector pulse width modulation is increased.

[0009] Furthermore, the redundancy vector allocation ratio is positively correlated with the effective value, wherein, The effective value is the root mean square value of the common-mode current per unit fundamental period; The redundancy vector allocation ratio is the ratio of the duration of action of the positive small vector in the space vector pulse width modulation within a single switching cycle to the duration of action of the negative small vector within the same single switching cycle.

[0010] Furthermore, the loss ratio is the ratio of the square of the effective value of the harmonic current corresponding to the spectral component within the preset frequency band to the square of the effective value of the total current of the AC output phase.

[0011] Furthermore, the preset frequency band is a frequency segment in the spectrum components whose frequency is greater than the first preset frequency and less than the second preset frequency.

[0012] Furthermore, the effective value of the harmonic current is the root mean square value of the current component with a frequency within a preset frequency band in the AC output phase current.

[0013] Further, determining the harmonic current injected into the AC output side based on the loss ratio includes: Compare the stated loss percentage with the preset loss percentage; If the loss ratio is greater than the preset loss ratio, it is determined that the reduction of the skin depth of the conductor of the AC output cable does not meet the requirements for the effect of the charge distribution in the conductor, and the harmonic current injected into the AC output side is increased.

[0014] Furthermore, the harmonic current is positively correlated with the loss ratio, wherein, The phase difference between the phase of the harmonic current and the phase of the harmonic component of the same frequency in the AC output phase current is 180°.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention, based on the temperature difference at the photovoltaic panel of the photovoltaic inverter being greater than or equal to a preset temperature difference value, indicates that the photovoltaic panel, due to the temperature difference being sufficient to cause surrounding water molecules to be adsorbed onto the surface of the photovoltaic panel, leads to water vapor condensation on the surface of the photovoltaic panel. This water vapor condensation not only increases the surface humidity of the photovoltaic panel but also, through the water bridging effect, forms capillary action between dust particles, locking the moisture in the gaps of the back panel of the photovoltaic panel, thus increasing the humidity at the back panel. This increased humidity, combined with the electrostatic adsorption at the back panel, creates a high-humidity environment on the back of the photovoltaic panel and at the cable connection points when the temperature difference is greater than or equal to the preset temperature difference value. Since a junction box is connected to the back of the photovoltaic panel, the terminal surface of the junction box undergoes electrochemical oxidation induced by water molecules at the metal contact surface, generating an oxide film. Therefore, in the DC circuit formed by the DC bus connector and power switching devices... In this context, parasitic inductance and capacitance are fixed structural parameters, while contact resistance is a variable that varies with the degree of oxidation of the oxide film on the terminal surface of the junction box. According to oscillation theory, with parasitic inductance and capacitance fixed, contact resistance determines the damping characteristics of the oscillation circuit. The oxide film reduces the microscopic contact area on the terminal surface of the junction box, increasing the contact resistance between the photovoltaic panel and the photovoltaic inverter. This increases the circuit damping coefficient between the photovoltaic panel and the photovoltaic inverter, accelerating the dissipation of oscillation energy. This manifests as a faster decay rate of the DC bus voltage oscillation waveform, i.e., a larger logarithmic decay rate. Therefore, by calculating the logarithmic decay rate of the DC bus voltage oscillation waveform, the health status of the DC bus connector can be reflected. In the early stage of increased contact resistance, before the temperature rise of the DC bus connector further increases, the oxidation trend of the contact surface can be detected, avoiding delayed fault detection and thus improving the safety of the photovoltaic inverter during grid-connected operation.

[0016] Furthermore, in the case where the oxide film reduces the microscopic contact area on the terminal surface of the junction box, thereby increasing the contact resistance between the photovoltaic panel and the photovoltaic inverter, the present invention reduces the tilt angle of the photovoltaic panel, making the photovoltaic panel more horizontal. This compresses the space for moisture molecules to remain between the back of the photovoltaic panel and the photovoltaic tracking bracket, as well as at the junction box, thus disrupting the conditions for increased humidity on the back of the photovoltaic panel. This reduces water vapor accumulation from a physical source and disrupts the conditions for water molecules to induce electrochemical oxidation and form an oxide film on the terminal surface of the junction box. This avoids increased conduction losses between the photovoltaic panel and the photovoltaic inverter, thereby improving the transmission efficiency of the photovoltaic panel and the photovoltaic inverter.

[0017] Furthermore, this invention obtains the common-mode current of the three-phase AC output cable of the photovoltaic inverter within a unit fundamental frequency cycle. The heat from the DC busbar connector of the photovoltaic inverter is conducted to the three-phase AC output cable. Since the three-phase AC output cable itself generates heat, the increased contact resistance between the photovoltaic panel and the photovoltaic inverter leads to increased heat in the DC busbar connector, resulting in an additional heat on the three-phase AC output cable. This temperature rise causes a change in the dielectric constant of the insulation material of the output cable, accelerating the aging of the insulation layer and causing fluctuations in the parasitic capacitance to ground. The leakage path impedance decreases, and under the same common-mode voltage excitation, the effective value of the leakage current flowing through the insulation layer to ground increases, i.e., the common-mode current increases. Simultaneously, due to the deterioration of insulation performance, high-frequency noise is more easily coupled to ground through capacitance, leading to increased electromagnetic compatibility risks. Therefore, based on the common-mode current... The effective value determines the redundancy vector allocation ratio in space vector pulse width modulation. The common-mode current refers to the leakage current flowing through the insulation layer of the three-phase cable on the AC output side to ground. The magnitude of the common-mode current reflects the change in the dielectric properties of the insulation layer. The effective value of the common-mode current represents the energy level of the common-mode current within one fundamental cycle. When the effective value is greater than the preset effective value, the redundancy vector allocation ratio in space vector pulse width modulation is increased, that is, the duration of the positive small vector in a single switching cycle is increased or the duration of the negative small vector in a single switching cycle is decreased. This makes the injected common-mode charge opposite in polarity and matched in magnitude with the error charge caused by the drift of insulation parameters, thereby actively offsetting the imbalance of the injected common-mode charge, reducing the common-mode voltage excitation from the source, reducing the leakage current flowing through the aging insulation layer, and thus reducing the risk of insulation layer aging due to the temperature rise caused by the additional heat of the three-phase cable on the AC output side, and improving the electromagnetic compatibility during grid-connected operation.

[0018] Furthermore, this invention obtains the spectral components of the AC output phase current of the three-phase AC output cable within a unit fundamental frequency period. The additional heat generated by the DC bus connector on the three-phase AC output cable, leading to a temperature rise, also affects the internal conductors, introducing additional heat. Since the internal conductors are typically made of copper or aluminum, their resistivity increases with temperature, reducing the skin depth. This means that when the high-frequency current is unevenly distributed in the conductor, the depth at which it attenuates from the conductor surface to the surface current density decreases, resulting in an increase in AC resistance inversely proportional to the skin depth. This increased AC resistance leads to increased active power losses from high-frequency harmonic currents in the conductor. Under this condition of increased active power losses from high-frequency harmonic currents, the active power losses generated by each conductor of the three-phase cable increase. The alternating magnetic fields interact, causing the charge distribution on the conductor cross-section to further concentrate on the adjacent or opposite sides. At this time, when the ratio of the square of the effective value of the harmonic current corresponding to the spectral component within the preset frequency band to the square of the effective value of the total effective value of the AC output phase current is greater than the preset loss ratio, it indicates that the AC resistance of the conductor in the preset frequency band is greater than the DC resistance. Since the charge distribution on the conductor cross-section further concentrates on the adjacent or opposite sides, it is easy to cause local hot spots on the surface of the cable, which increases the resistance of the conductor. By increasing the harmonic current injected into the AC output side with a phase difference of 180° from the harmonic component of the same frequency in the AC output phase current, the charge distribution on the conductor cross-section inside the cable further concentrates on the adjacent or opposite sides. By injecting the reverse harmonic current, the net harmonics of the conductor are reduced, thereby reducing the conductor loss inside the cable and improving the efficiency of current transmission in the conductor inside the cable. Attached Figure Description

[0019] Figure 1 This is an overall flowchart of the control method for a low-loss photovoltaic inverter according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the adjustment of the tilt angle of the photovoltaic panel in the control method of the low-loss photovoltaic inverter according to an embodiment of the present invention. Figure 3 This is a flowchart illustrating the determination of the redundancy vector allocation ratio in space vector pulse width modulation in the control method of a low-loss photovoltaic inverter according to an embodiment of the present invention. Figure 4 This is a flowchart illustrating the determination of the harmonic current injected into the AC output side in the control method of the low-loss photovoltaic inverter according to an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0021] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0022] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0023] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0024] Please see Figure 1 The diagram shown is an overall flowchart of the control method for a low-loss photovoltaic inverter according to an embodiment of the present invention. The control method for a low-loss photovoltaic inverter according to an embodiment of the present invention includes: Step S1: When the temperature difference at the photovoltaic panel of the photovoltaic inverter is greater than or equal to a preset temperature difference value, obtain the DC bus voltage oscillation waveform of the power switching device of the photovoltaic inverter during the off-time period during grid-connected operation. Step S2: Adjust the tilt angle of the photovoltaic panel according to the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform; Step S3: Obtain the common-mode current of the three-phase cable on the AC output side of the photovoltaic inverter after the tilt angle is adjusted within a unit fundamental frequency cycle; Step S4: Determine the redundancy vector allocation ratio in space vector pulse width modulation based on the effective value of the common-mode current; Step S5: Obtain the spectral components of the AC output phase current of the three-phase cable on the AC output side within a unit fundamental frequency period; Step S6: Calculate the loss ratio of the preset frequency band based on the effective value of the harmonic current corresponding to the spectral component within a unit fundamental period and the effective value of the total current of the AC output phase current. Step S7: Determine the harmonic current injected into the AC output side based on the loss ratio; Step S8: According to the redundancy vector allocation ratio and the harmonic current injected into the AC output side, control the photovoltaic inverter to complete the grid-connected operation process, wherein... The photovoltaic inverter is a three-level photovoltaic inverter.

[0025] Specifically, the grid-connected operation of a photovoltaic inverter involves converting the DC power generated by the photovoltaic panels into AC power through the photovoltaic inverter, and then injecting the AC power into the AC grid after synchronizing the frequency and phase of the AC power with the AC grid.

[0026] As will be understood by those skilled in the art, the grid-connected operation of photovoltaic inverters is a conventional technical process well-known to them, and therefore the grid-connected operation of photovoltaic inverters will not be described in detail here.

[0027] Optionally, under the conditions that the photovoltaic inverter is installed in an outdoor environment; the photovoltaic panel uses crystalline silicon modules; the backsheet of the photovoltaic panel is made of polymer; and the ambient air temperature around the photovoltaic inverter ranges from -20℃ to +50℃ and the relative humidity ranges from 30% to 95%, the preset temperature difference value can be selected within the range of [5℃, 15℃].

[0028] Preferably, the preset temperature difference value in the preferred embodiment is 10°C.

[0029] Specifically, the temperature difference at the photovoltaic panel of the photovoltaic inverter is obtained by a first temperature sensor installed on the back surface of the photovoltaic panel and a second temperature sensor in the installation environment of the photovoltaic inverter. The shortest straight-line distance between the first temperature sensor and the second temperature sensor is greater than 1.5m.

[0030] Specifically, the temperature difference is the difference between the surface temperature of the back panel of the photovoltaic panel in a photovoltaic inverter and the ambient air temperature.

[0031] Specifically, the topology of the three-level photovoltaic inverter is a three-level T-type.

[0032] Specifically, the DC bus voltage oscillation waveform during the off-time period of the power switching devices of the photovoltaic inverter during grid-connected operation is obtained by using Hall voltage sensors installed on both sides of the DC bus capacitor.

[0033] Specifically, the common-mode current of the three-phase cable on the AC output side of the photovoltaic inverter is obtained by a Hall current sensor located near the output port of the photovoltaic inverter.

[0034] Specifically, the Hall voltage sensor has a voltage sampling frequency of 2MHz.

[0035] Specifically, the shutdown period lasts for 20 μs.

[0036] Specifically, the logarithmic decay rate of the DC bus voltage oscillation waveform is calculated using a DSP algorithm.

[0037] As will be understood by those skilled in the art, the calculation process of logarithmic decay rate is a conventional technique well-known to them, and therefore the calculation process of logarithmic decay rate will not be elaborated here.

[0038] Please see Figure 2 The diagram shows a flowchart of adjusting the tilt angle of a photovoltaic panel in a control method for a low-loss photovoltaic inverter according to an embodiment of the present invention. The step of adjusting the tilt angle of the photovoltaic panel based on the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform includes: The logarithmic decay rate is compared with the preset decay rate; If the logarithmic decay rate is greater than the preset decay rate, it is determined that the water molecules accumulated on the back of the photovoltaic panel and at the cable connection point due to condensation at the photovoltaic panel do not meet the requirements for the transmission efficiency of the photovoltaic panel and the photovoltaic inverter, and the tilt angle of the photovoltaic panel is reduced.

[0039] In implementation, this invention uses a temperature difference at the photovoltaic panel of the photovoltaic inverter that is greater than or equal to a preset temperature difference value to indicate that the photovoltaic panel has reached a temperature difference sufficient to cause surrounding water molecules to be adsorbed onto its surface, resulting in water vapor condensation. This water vapor condensation not only increases the surface humidity of the photovoltaic panel but also creates capillary action between dust particles through a water bridge effect, locking the moisture in the gaps of the photovoltaic panel's back panel, thus increasing the humidity at the back panel. This increased humidity, combined with electrostatic adsorption at the back panel, creates a high-humidity environment on the back of the photovoltaic panel and at the cable connection points when the temperature difference is greater than or equal to the preset temperature difference value. Since a junction box is connected to the back of the photovoltaic panel, the terminal surfaces of the junction box undergo electrochemical oxidation induced by water molecules at the metal contact surface, forming an oxide film. Therefore, in the DC circuit composed of the DC bus connector and power switching devices, due to parasitic electricity... Parasitic inductance and capacitance are fixed structural parameters, while contact resistance is a variable that varies with the degree of oxidation of the oxide film on the terminal surface of the junction box. According to oscillation theory, with parasitic inductance and capacitance fixed, contact resistance determines the damping characteristics of the oscillation circuit. The oxide film reduces the microscopic contact area on the terminal surface of the junction box, increasing the contact resistance between the photovoltaic panel and the photovoltaic inverter. This increases the circuit damping coefficient between the photovoltaic panel and the photovoltaic inverter, accelerating the dissipation of oscillation energy. This manifests as a faster decay rate of the DC bus voltage oscillation waveform, i.e., a larger logarithmic decay rate. Therefore, by calculating the logarithmic decay rate of the DC bus voltage oscillation waveform, the health status of the DC bus connector can be reflected. In the early stage of increased contact resistance, before the temperature rise of the DC bus connector increases further, the oxidation trend of the contact surface can be detected, avoiding delayed fault detection and thus improving the safety of the photovoltaic inverter during grid-connected operation.

[0040] Specifically, the tilt angle of the photovoltaic panel is adjusted by a photovoltaic tracking bracket connected to the photovoltaic panel.

[0041] As will be understood by those skilled in the art, the operating principle and process of photovoltaic tracking brackets are conventional technical means well known to them, and therefore the operating principle and process of photovoltaic tracking brackets will not be described in detail here.

[0042] Optionally, under the conditions that the photovoltaic inverter uses IGBT as the power switching device; the switching frequency range of the power switching device is 8kHz to 16kHz; and the DC bus voltage range is 800V to 1500V, the preset attenuation rate can be selected within the range of [0.2, 0.8].

[0043] Preferably, the preset attenuation rate is 0.5 in this embodiment.

[0044] Specifically, the tilt angle of the photovoltaic panel is negatively correlated with the logarithmic decay rate, wherein, The tilt angle of the photovoltaic panel is the acute angle between the plane on which the photovoltaic panel is located and the horizontal plane.

[0045] In implementation, when the logarithmic decay rate is greater than the preset decay rate by less than 0.02, the tilt angle of the photovoltaic panel is adjusted to 95% of the current tilt angle. When the logarithmic decay rate exceeds the preset decay rate by more than 0.02, the tilt angle of the photovoltaic panel is reduced by 1° for every 0.01 exceeding the preset decay rate. In a specific embodiment, the current logarithmic decay rate is 0.54, the current tilt angle of the photovoltaic panel is 10°, and the reduced tilt angle of the photovoltaic panel is 10°×95%-(0.02 / 0.01)×1°=7.5°.

[0046] In practice, when the oxide film reduces the microscopic contact area on the terminal surface of the junction box, increasing the contact resistance between the photovoltaic panel and the photovoltaic inverter, this invention reduces the tilt angle of the photovoltaic panel, making it more horizontal. This compresses the space for moisture molecules to remain between the back of the photovoltaic panel and the photovoltaic tracking bracket, as well as at the junction box, thus disrupting the conditions for increased humidity on the back of the photovoltaic panel. This reduces water vapor accumulation from a physical source and disrupts the conditions for water molecules to induce electrochemical oxidation and form an oxide film on the terminal surface of the junction box. This avoids increased conduction losses between the photovoltaic panel and the photovoltaic inverter, thereby improving the transmission efficiency of the photovoltaic panel and the photovoltaic inverter.

[0047] Please see Figure 3 The diagram shows a flowchart illustrating the determination of the redundancy vector allocation ratio in space vector pulse width modulation (SVM) using a control method for a low-loss photovoltaic inverter according to an embodiment of the present invention. The determination of the redundancy vector allocation ratio in SVM based on the effective value of the common-mode current includes: The valid value is compared with the preset valid value; If the effective value is greater than the preset effective value, it is determined that the cumulative heat generated by the oxide layer on the back of the photovoltaic panel induced by water molecules at the cable connection does not meet the requirements for the dielectric properties of the cable insulation layer, and the redundancy vector allocation ratio in the space vector pulse width modulation is increased.

[0048] Optionally, under the conditions that the rated power of the photovoltaic inverter is 100kW to 150kW; the voltage on the AC output side is 400V; the length of the three-phase cable on the AC output side is 50m to 200m; and the three-phase cable on the AC output side uses XLPE as the insulation layer, the preset effective value can be selected in the range of [30mA, 300mA].

[0049] Preferably, the preferred embodiment of the preset effective value is 100mA.

[0050] Specifically, the redundancy vector allocation ratio in space vector pulse width modulation is adjusted by adjusting the comparator register of the DSP chip located inside the photovoltaic inverter.

[0051] As will be understood by those skilled in the art, the operating principle and process of adjusting the redundancy vector allocation ratio in space vector pulse width modulation by adjusting the comparator register of the DSP chip is a conventional technique well known to those skilled in the art. Therefore, the operating principle and process of adjusting the redundancy vector allocation ratio in space vector pulse width modulation by adjusting the comparator register of the DSP chip will not be described in detail here.

[0052] In implementation, when the effective value is within 5mA of the preset effective value, the redundancy vector allocation ratio in the spatial vector pulse width modulation is adjusted to 1.1 times the current redundancy vector allocation ratio. When the effective value exceeds the preset effective value by more than 5mA, the redundancy vector allocation ratio in the spatial vector pulse width modulation is increased by 0.01 times for every 1mA exceeding the preset effective value. In a specific embodiment, the current effective value is 108mA, the current redundancy vector allocation ratio in the spatial vector pulse width modulation is 1.0, and the increased redundancy vector allocation ratio in the spatial vector pulse width modulation is 1.0 × 1.1 + (3mA / 1mA) × 0.01 times = 1.13.

[0053] In implementation, this invention obtains the common-mode current of the three-phase AC output cable of the photovoltaic inverter within a unit fundamental frequency cycle. Heat is conducted from the DC bus connector of the photovoltaic inverter to the three-phase AC output cable. Since the three-phase AC output cable itself generates heat, the increased contact resistance between the photovoltaic panel and the photovoltaic inverter leads to increased heat in the DC bus connector, resulting in an additional heat on the three-phase AC output cable. This temperature rise causes a change in the dielectric constant of the output cable's insulation material, accelerating the aging of the insulation layer and causing fluctuations in the parasitic capacitance to ground. The leakage path impedance decreases, and under the same common-mode voltage excitation, the effective value of the leakage current flowing through the insulation layer to ground increases, i.e., the common-mode current increases. Simultaneously, due to the deterioration of insulation performance, high-frequency noise is more easily coupled to ground through capacitance, increasing electromagnetic compatibility risks. Therefore, based on the common-mode current... The effective value determines the redundancy vector allocation ratio in space vector pulse width modulation. The common-mode current refers to the leakage current flowing through the insulation layer of the three-phase cable on the AC output side to ground. The magnitude of the common-mode current reflects the change in the dielectric properties of the insulation layer. The effective value of the common-mode current represents the energy level of the common-mode current within one fundamental cycle. When the effective value is greater than the preset effective value, the redundancy vector allocation ratio in space vector pulse width modulation is increased, that is, the duration of the positive small vector in a single switching cycle is increased or the duration of the negative small vector in a single switching cycle is decreased. This makes the injected common-mode charge opposite in polarity and matched in magnitude with the error charge caused by the drift of insulation parameters, thereby actively offsetting the imbalance of the injected common-mode charge, reducing the common-mode voltage excitation from the source, reducing the leakage current flowing through the aging insulation layer, and thus reducing the risk of insulation layer aging due to the temperature rise caused by the additional heat of the three-phase cable on the AC output side, and improving the electromagnetic compatibility during grid-connected operation.

[0054] Specifically, the redundancy vector allocation ratio is positively correlated with the effective value, wherein, The effective value is the root mean square value of the common-mode current per unit fundamental period; The redundancy vector allocation ratio is the ratio of the duration of action of the positive small vector in the space vector pulse width modulation within a single switching cycle to the duration of action of the negative small vector within the same single switching cycle.

[0055] Specifically, the fundamental period is 20ms.

[0056] Specifically, the loss ratio is the ratio of the square of the effective value of the harmonic current corresponding to the spectral component within the preset frequency band to the square of the effective value of the total current of the AC output phase.

[0057] Specifically, by performing a fast Fourier transform on the time-domain waveform of the AC output phase current within a unit fundamental period and decomposing it into several spectral components, each spectral component corresponds to a harmonic current at a frequency. The effective value of the harmonic current is the effective value of the harmonic current at the coordinate point of each frequency on the time-domain waveform of the AC output phase current within a unit fundamental period.

[0058] As will be understood by those skilled in the art, the calculation process for obtaining the effective value of the harmonic current corresponding to the spectral components through Fast Fourier Transform is a conventional technique well-known to those skilled in the art. Therefore, the calculation process for obtaining the effective value of the harmonic current corresponding to the spectral components through Fast Fourier Transform will not be described in detail here.

[0059] Specifically, the preset frequency band is a frequency segment in the spectrum components whose frequency is greater than the first preset frequency and less than the second preset frequency.

[0060] Optionally, the switching frequency range of the power switching devices in the photovoltaic inverter is 8kHz to 16kHz; the conductor material of the three-phase AC output cable is copper; and the cross-sectional area of ​​the three-phase AC output cable is 50... Up to 240 With the operating temperature range of the conductor of the three-phase cable on the AC output side being 40℃ to 90℃, the selectable range of the first preset frequency is [5kHz, 15kHz]; the selectable range of the second preset frequency is [20kHz, 30kHz].

[0061] Preferably, the first preset frequency is 10kHz in a preferred embodiment; the second preset frequency is 20kHz in a preferred embodiment.

[0062] Specifically, the effective value of the harmonic current is the root mean square value of the current component with a frequency in the preset frequency band in the AC output phase current. The effective value of the total current is the root mean square value of the AC output phase current per unit fundamental period.

[0063] Please see Figure 4 The diagram shows a flowchart of the control method for a low-loss photovoltaic inverter according to an embodiment of the present invention, illustrating the determination of the harmonic current injected into the AC output side. The determination of the harmonic current injected into the AC output side based on the loss ratio includes: Compare the stated loss percentage with the preset loss percentage; If the loss ratio is greater than the preset loss ratio, it is determined that the reduction of the skin depth of the conductor of the AC output cable does not meet the requirements for the effect of the charge distribution in the conductor, and the harmonic current injected into the AC output side is increased.

[0064] Specifically, the harmonic current injected into the AC output side is adjusted by the harmonic injection controller and PWM generation module of the DSP chip set inside the photovoltaic inverter.

[0065] As will be understood by those skilled in the art, the operating principle and process of adjusting the harmonic current injected into the AC output side through the harmonic injection controller and PWM generation module of the DSP chip are conventional techniques well known to those skilled in the art. Therefore, the operating principle and process of adjusting the harmonic current injected into the AC output side through the harmonic injection controller and PWM generation module of the DSP chip will not be described in detail here.

[0066] Optionally, the photovoltaic inverter has a rated power of 100kW to 150kW, an initial carrier frequency setting range of 8kHz to 16kHz, and an AC output cable cross-sectional area of ​​50... Up to 240 Under these conditions, the selectable range for the preset loss percentage is [0.3, 0.6].

[0067] Preferably, the preset loss ratio in the preferred embodiment is 0.4.

[0068] Specifically, the harmonic current is positively correlated with the loss ratio, wherein, The phase difference between the phase of the harmonic current and the phase of the harmonic component of the same frequency in the AC output phase current is 180°.

[0069] In implementation, when the loss ratio is greater than the preset loss ratio by less than 0.02, the harmonic current injected into the AC output side is adjusted to 1.1 times the current harmonic current. When the loss ratio exceeds the preset loss ratio by more than 0.02, the harmonic current is increased by 0.1A for every 0.01 increase. In a specific embodiment, the current loss ratio is 0.45, the current harmonic current injected into the AC output side is 0.75A, and the increased harmonic current is 0.75A×1.1+(0.03 / 0.01)×0.1A=1.125A.

[0070] In implementation, this invention obtains the spectral components of the AC output phase current of the three-phase AC output cable within a unit fundamental frequency period. The additional heat generated by the DC bus connector on the three-phase AC output cable, leading to a temperature rise, also affects the internal conductors, introducing additional heat. Since the internal conductors are typically made of copper or aluminum, their resistivity increases with temperature, reducing the skin depth. This means that when the high-frequency current is unevenly distributed in the conductor, the depth at which it attenuates from the conductor surface to the surface current density decreases, resulting in an increase in AC resistance inversely proportional to the skin depth. This increased AC resistance leads to increased active power losses from high-frequency harmonic currents in the conductor. Under these conditions of increased active power losses from high-frequency harmonic currents, the active power losses generated by each conductor of the three-phase cable increase. The alternating magnetic fields interact, causing the charge distribution on the conductor cross-section to further concentrate on the adjacent or opposite sides. At this time, when the ratio of the square of the effective value of the harmonic current corresponding to the spectral component within the preset frequency band to the square of the effective value of the total effective value of the AC output phase current is greater than the preset loss ratio, it indicates that the AC resistance of the conductor in the preset frequency band is greater than the DC resistance. Since the charge distribution on the conductor cross-section further concentrates on the adjacent or opposite sides, it is easy to cause local hot spots on the surface of the cable, which increases the resistance of the conductor. By increasing the harmonic current injected into the AC output side with a phase difference of 180° from the harmonic component of the same frequency in the AC output phase current, the charge distribution on the conductor cross-section inside the cable further concentrates on the adjacent or opposite sides. By injecting the reverse harmonic current, the net harmonics of the conductor are reduced, thereby reducing the conductor loss inside the cable and improving the efficiency of current transmission in the conductor inside the cable.

[0071] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A control method of a low-loss photovoltaic inverter, characterized by, include: When the temperature difference at the photovoltaic panel of the photovoltaic inverter is greater than or equal to a preset temperature difference value, the DC bus voltage oscillation waveform of the power switching device of the photovoltaic inverter during the off-time period during grid-connected operation is obtained. Adjust the tilt angle of the photovoltaic panel according to the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform; Obtain the common-mode current of the three-phase cable on the AC output side of the photovoltaic inverter after the tilt angle is adjusted within a unit fundamental frequency cycle; The redundancy vector allocation ratio in space vector pulse width modulation is determined based on the effective value of the common-mode current. Obtain the spectral components of the AC output phase current of the three-phase cable on the AC output side within a unit fundamental frequency period; The loss ratio of the preset frequency band is calculated based on the effective value of the harmonic current corresponding to the spectral component within a unit fundamental period and the effective value of the total current of the AC output phase current. The harmonic current injected into the AC output side is determined based on the aforementioned loss ratio; According to the aforementioned redundancy vector allocation ratio and the injected harmonic current on the AC output side, the photovoltaic inverter is controlled to complete the grid-connected operation process, wherein... The photovoltaic inverter is a three-level photovoltaic inverter.

2. The control method for a low-loss photovoltaic inverter according to claim 1, characterized in that, The step of adjusting the tilt angle of the photovoltaic panel according to the logarithmic decay rate corresponding to the DC bus voltage oscillation waveform includes: The logarithmic decay rate is compared with the preset decay rate; If the logarithmic decay rate is greater than the preset decay rate, it is determined that the water molecules accumulated on the back of the photovoltaic panel and at the cable connection point due to condensation at the photovoltaic panel do not meet the requirements for the transmission efficiency of the photovoltaic panel and the photovoltaic inverter, and the tilt angle of the photovoltaic panel is reduced.

3. The control method for a low-loss photovoltaic inverter according to claim 2, characterized in that, The tilt angle of the photovoltaic panel is negatively correlated with the logarithmic decay rate, wherein, The tilt angle of the photovoltaic panel is the acute angle between the plane on which the photovoltaic panel is located and the horizontal plane.

4. The control method for a low-loss photovoltaic inverter according to claim 3, characterized in that, Determining the redundancy vector allocation ratio in space vector pulse width modulation based on the effective value of the common-mode current includes: The valid value is compared with the preset valid value; If the effective value is greater than the preset effective value, it is determined that the cumulative heat generated by the oxide layer on the back of the photovoltaic panel induced by water molecules at the cable connection does not meet the requirements for the dielectric properties of the cable insulation layer, and the redundancy vector allocation ratio in the space vector pulse width modulation is increased.

5. The control method for a low-loss photovoltaic inverter according to claim 4, characterized in that, The redundancy vector allocation ratio is positively correlated with the effective value, wherein, The effective value is the root mean square value of the common-mode current per unit fundamental period; The redundancy vector allocation ratio is the ratio of the duration of action of the positive small vector in the space vector pulse width modulation within a single switching cycle to the duration of action of the negative small vector within the same single switching cycle.

6. The control method for a low-loss photovoltaic inverter according to claim 5, characterized in that, The loss ratio is the ratio of the square of the effective value of the harmonic current corresponding to the spectral component within the preset frequency band to the square of the effective value of the total current of the AC output phase.

7. The control method for a low-loss photovoltaic inverter according to claim 6, characterized in that, The preset frequency band is a frequency segment in the spectrum components whose frequency is greater than the first preset frequency and less than the second preset frequency.

8. The control method for a low-loss photovoltaic inverter according to claim 7, characterized in that, The effective value of the harmonic current is the root mean square value of the current component with frequency in the preset frequency band in the AC output phase current. The effective value of the total current is the root mean square value of the AC output phase current per unit fundamental period.

9. The control method for a low-loss photovoltaic inverter according to claim 8, characterized in that, The step of determining the harmonic current injected into the AC output side based on the loss ratio includes: Compare the stated loss percentage with the preset loss percentage; If the loss ratio is greater than the preset loss ratio, it is determined that the reduction of the skin depth of the conductor of the AC output cable does not meet the requirements for the effect of the charge distribution in the conductor, and the harmonic current injected into the AC output side is increased.

10. The control method for a low-loss photovoltaic inverter according to claim 9, characterized in that, The harmonic current is positively correlated with the loss ratio, wherein... The phase difference between the phase of the harmonic current and the phase of the harmonic component of the same frequency in the AC output phase current is 180°.

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