Frequency conversion control method of photovoltaic grid-connected inverter

Abstract

The invention discloses a frequency conversion control method of a photovoltaic grid-connected inverter. The photovoltaic grid-connected inverter is internally provided with a power switch tube, the frequency conversion control method is used for controlling the power switch tube, and the frequency conversion control method comprises the following steps: obtaining a switching period of the power switch tube, setting the switching period and a semi-resonance period of the photovoltaic grid-connected inverter to form a positive incidence relation, and comparing the switching period with a pre-stored period lower limit; when the switching period is greater than or equal to the period lower limit, updating the switching period to the pulse width modulation signal to control the on-off of the power switch tube; and when the switching period is less than the period lower limit, adding one resonance period to the switching period, and comparing the switching period with the added resonance period with the period lower limit again until the switching period is greater than or equal to the period lower limit. The technical scheme of the invention can be turned on at the resonance valley bottom stage, the turn-on loss is reduced, the switching of working modes is not involved, and the whole control process is simplified.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and particularly to a variable-frequency control method for a photovoltaic grid-connected inverter. Background Art

[0002] An inverter is a converter that converts direct current into alternating current with a fixed frequency and voltage or a variable frequency and voltage. The inverter can be applied in a photovoltaic power generation system and used as the core grid-connected device of the photovoltaic power generation system. Among them, the energy conversion efficiency is an important technical index for evaluating the inverter.

[0003] Among them, when the flyback micro-inverter operates in the discontinuous current mode (DCM), the control method is simple, and it has a high energy conversion efficiency in the low-power output state. However, the drain-source voltage value of the switching transistor is random when it is turned on, and the conduction loss cannot be guaranteed to be always the lowest; when it operates in the continuous conduction mode (CCM), the drain-source voltage and the equivalent current flowing through the switching transistor are not zero when the switching transistor is turned on and off. The switching loss in the hard-switching state is large, and the energy conversion efficiency is low; when it operates in the boundary conduction mode (BCM), the equivalent current flowing through the switching transistor is zero when it is turned on, and the conversion efficiency is high when the output power is large. However, when the output power is small, the switching frequency is extremely high, the switching loss is very large, and the energy conversion efficiency is extremely low.

[0004] Currently, in order to enable the inverter to have a high energy conversion efficiency at the full output power, the existing method is to mix the DCM and BCM control modes. When the output power is small, the inverter is switched to the DCM mode; when the output power is large, the inverter is switched to the BCM mode; the two modes of DCM mode and BCM mode are switched according to the grid-connected power size. However, even so, there is still a problem of large turn-on loss in the DCM mode. Summary of the Invention

[0005] Aiming at the defects in the prior art, the present invention provides a variable-frequency control method for a photovoltaic grid-connected inverter, which can effectively reduce the turn-on loss and improve the energy conversion efficiency.

[0006] A variable-frequency control method for a photovoltaic grid-connected inverter provided by the present application, the photovoltaic grid-connected inverter includes a primary circuit, a transformer, and a secondary circuit. One end of the transformer is connected to the primary circuit, and the other end of the transformer is connected to the secondary circuit. A power switching transistor is arranged in the primary circuit. The variable-frequency control method is used to control the power switching transistor, and the variable-frequency control method includes:

[0007] Obtain the switching period of the power switch tube, and set a positive correlation relationship between the switching period and the half-resonant period of the photovoltaic grid-connected inverter;

[0008] Compare the switching period with a pre-stored lower period limit;

[0009] When the switching period is greater than or equal to the lower period limit, update the switching period to a pulse width modulation signal to control the on and off of the power switch tube;

[0010] When the switching period is less than the lower period limit, increase the switching period by one resonant period, and compare the switching period increased by one resonant period with the lower period limit again until the switching period is greater than or equal to the lower period limit.

[0011] In one aspect, before the step of comparing the switching period with a pre-stored lower period limit, it includes:

[0012] Set the upper operating frequency of the photovoltaic grid-connected inverter, calculate the lower period limit based on the upper operating frequency, and save the lower period limit.

[0013] In one aspect, define the upper operating frequency as f max , and the lower period limit as T min , then it satisfies: T min = 1 / f max .

[0014] In one aspect, the step of setting a positive correlation relationship between the switching period and the half-resonant period of the photovoltaic grid-connected inverter includes:

[0015] The switching period is the sum of the time required for the primary-side current to rise to the peak value, the time required for the secondary-side current to drop to zero, and n times the half-resonant period, where n is an odd number greater than or equal to 1.

[0016] In one aspect, define the switching period of the power switch tube as T SW , the time required for the primary-side current to rise to the peak value as t on , the time required for the secondary-side current to drop to zero as t off , the half-resonant period as T R , then it satisfies: T SW = t on + t off + n×T R .

[0017] In one aspect, when the switching period is less than the lower period limit, increase the switching period by one resonant period, then the switching period satisfies: T SW = ton +t off +(n + 2)×T R 。

[0018] In one aspect, the calculation formula for the semi-resonant period is: where L M is the inductance of the primary circuit, and C oss is the parasitic capacitance of the primary circuit.

[0019] In one aspect, the primary circuit further includes an electrolytic capacitor and a photovoltaic module. The electrolytic capacitor is connected in parallel with the photovoltaic module. The positive electrode of the electrolytic capacitor is connected to the positive electrode of the photovoltaic module. The positive electrode of the electrolytic capacitor is connected to one end of the primary winding of the transformer. The drain of the power switch is connected to the other end of the primary winding of the transformer. The source of the power switch is connected to the negative electrode of the electrolytic capacitor and the negative electrode of the photovoltaic module;

[0020] The secondary circuit includes a rectifier diode, a decoupling capacitor, a first control switch, a second control switch, a third control switch, and a fourth control switch. The non-corresponding terminal of the transformer is connected to the positive electrode of the rectifier diode. The decoupling capacitor is connected in parallel to the secondary side of the transformer. One end of the secondary side of the decoupling capacitor is connected to the negative electrode of the rectifier diode. The other end of the secondary side of the decoupling capacitor is connected to the corresponding terminal of the secondary side of the transformer. The drains of the first control switch and the fourth control switch are connected. The source of the first control switch is connected to the drain of the second control switch. The sources of the second control switch and the third control switch are connected. The source of the fourth control switch is connected to the drain of the third control switch. The sources of the second control switch and the third control switch are connected to the corresponding terminal of the secondary side of the transformer;

[0021] The photovoltaic grid-connected inverter further includes a filter circuit. The filter circuit includes a filter capacitor and a filter inductor. The filter capacitor is connected in parallel between the source of the first control switch and the drain of the third control switch. One end of the filter capacitor is connected to the drain of the third control switch. The other end of the filter capacitor is connected to one end of the power grid. One end of the filter inductor is connected to one end of the filter capacitor. The other end of the filter inductor is connected to the other end of the power grid.

[0022] The beneficial effects of the present invention are embodied in that the on or off of the power switch tube is controlled by the switching period. Among them, a positive correlation is set between the switching period and the half-resonant period of the photovoltaic grid-connected inverter. The switching period changes with the change of the half-resonant period. As the time length of the half-resonant period decays, the time length of the switching period also changes, thereby realizing the variable-frequency control of the power switch tube. Then, the switching period is compared with the pre-stored period lower limit. When the switching period is greater than or equal to the period lower limit, the switching period is updated to the pulse width modulation signal, and the on or off of the power switch tube is directly controlled through the pulse width modulation signal, so that the power switch tube is turned on at the resonant valley stage where the energy is basically consumed, reducing the turn-on loss. It can be seen that the technical solution of this application can turn on at the resonant valley stage, reduce the turn-on loss, and at the same time does not involve the switching of working modes, thereby simplifying the entire control process. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts do not necessarily draw to actual scale.

[0024] Figure 1 It is a schematic flow chart of the variable-frequency control method of the photovoltaic grid-connected inverter of this application;

[0025] Figure 2 It is a schematic flow chart of the formation of a positive relationship between the switching period and the half-resonant period in the variable-frequency control method of the photovoltaic grid-connected inverter of this application;

[0026] Figure 3 It is a corresponding schematic diagram of the switching period and the half-resonant period in the coordinate system in the variable-frequency control method of the photovoltaic grid-connected inverter of this application;

[0027] Figure 4 It is a schematic circuit diagram of the photovoltaic grid-connected inverter of this application.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS: 10, primary circuit; 20, secondary circuit; T, transformer;

[0029] Cin, electrolytic capacitor; PV, photovoltaic module; Q M , power switch tube; D M , rectifier diode; C O , decoupling capacitor; Q1, first control switch; Q2, second control switch; Q3, third control switch; Q4, fourth control switch; Cf, filter capacitor; Lf, filter inductor; Vg, power grid. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The embodiments of the technical solution of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, so they are only examples and cannot be used to limit the protection scope of the present invention.

[0031] It should be noted that unless otherwise specified, the technical terms or scientific terms used in this application should have the ordinary meaning understood by those skilled in the art to which the present invention belongs.

[0032] This application provides a frequency conversion control method for a photovoltaic grid-connected inverter. The inverter in this embodiment can be a flyback micro-inverter. Micro-inverters have the advantages of high efficiency, high safety, strong adaptability, long life, etc. Among them, the flyback topology is widely used because of its simple structure, easy control, and high energy conversion efficiency.

[0033] The photovoltaic grid-connected inverter in this application includes a primary circuit, a transformer, and a secondary circuit. One end of the transformer is connected to the primary circuit, and the other end of the transformer is connected to the secondary circuit. A power switch tube is provided in the primary circuit, and the frequency conversion control method is used to control the power switch tube.

[0034] As Figure 1 shown, the frequency conversion control method includes:

[0035] Step S10, obtaining the switching period of the power switch tube, and setting a positive correlation relationship between the switching period and the half-resonant period of the photovoltaic grid-connected inverter; from the positive correlation relationship between the switching period and the half-resonant period of the photovoltaic grid-connected inverter, it can be seen that the switching period increases as the half-resonant period of the photovoltaic grid-connected inverter increases, and decreases as the half-resonant period decreases. Among them, the resonant period is formed by the inductor and capacitor in the circuit, and the resonant energy gradually decays, so the resonant period also gradually changes, and the half-resonant period is half of the resonant period.

[0036] Step S20, comparing the switching period with a pre-stored period lower limit; the period lower limit can be understood as an inherent property of the inverter. Usually, the period lower limit is a fixed value, and different inverters have different period lower limits. The period lower limit can be calculated from the upper operating frequency of the inverter, and the reciprocal of the upper operating frequency is the period lower limit.

[0037] Step S30, when the switching period is greater than or equal to the period lower limit, updating the switching period to a pulse width modulation signal to control the on and off of the power switch tube; the power switch tube can be a MOS tube. The MOS tube has a gate, a source, and a drain. The pulse width modulation signal acts on the gate of the power switch tube to control the conduction of the source and drain.

[0038] Step S40: When the switching period is less than the period lower limit, increase the switching period by one resonant period, and then compare the switching period increased by one resonant period with the period lower limit again until the switching period is greater than or equal to the period lower limit. That is, when the switching period is less than the period lower limit, after increasing one resonant period, return to step S20 again to repeat the comparison between the switching period and the period lower limit.

[0039] In this embodiment, the conduction or cut-off of the power switch tube is controlled by the switching period. Among them, a positive correlation is set between the switching period and the half-resonant period of the photovoltaic grid-connected inverter. The switching period changes with the change of the half-resonant period. As the time length of the half-resonant period decays, the time length of the switching period also changes, so as to realize the variable-frequency control of the power switch tube. Then, the switching period is compared with the pre-stored period lower limit. When the switching period is greater than or equal to the period lower limit, the switching period is updated to the pulse width modulation signal, and the conduction or disconnection of the power switch tube is directly controlled through the pulse width modulation signal, so that the power switch tube is made to conduct at the resonant valley stage when the energy is basically consumed, reducing the on-energy consumption. As can be seen from the above, the technical solution of this application can turn on at the resonant valley stage, reduce the turn-on loss, and at the same time does not involve the switching of the working mode, thus simplifying the entire control process.

[0040] As Figure 2 shown, in an embodiment of this application, before the step of comparing the switching period with the pre-stored period lower limit, it includes:

[0041] Step S01: Set the upper limit operating frequency of the photovoltaic grid-connected inverter, calculate the period lower limit based on the upper limit operating frequency, and save the period lower limit. When the period lower limit is needed, the saved period lower limit is retrieved. Step S01 can be performed after setting a new inverter or replacing the inverter to re-set the period lower limit. Usually, the period lower limit only needs to be set once before the inverter works normally.

[0042] Specifically, define the upper limit operating frequency as f max and the period lower limit as T min , then it satisfies: T min = 1 / f max .

[0043] As Figure 2 shown, in an embodiment of this application, the step of setting a positive correlation between the switching period and the half-resonant period of the photovoltaic grid-connected inverter includes:

[0044] Step S11, the switching period is the sum of the time required for the primary current to rise to the peak value, the time required for the secondary current to drop to zero, and n times the half-resonant period, where n is an odd number greater than or equal to 1. After correlating the half-resonant period with the switching period, the secondary current has dropped to zero, and it can be understood that there is basically no current on the secondary side at this time. Turn on the power switch tube, and the power consumed at this time is relatively low.

[0045] It should be noted that during the resonant period stage, the energy of the inductor and the parasitic capacitance has begun to decay. In order to further reduce the energy consumption, the power switch tube can be turned on at the valley stage of the energy waveform during the resonant period, that is, the power switch tube is turned on at the moment when n is an odd number, ensuring that the power switch tube is turned on at the valley stage of the energy waveform during the half-resonant period.

[0046] In an embodiment of the present application, the switching period of the power switch tube is defined as T SW , the time required for the primary current to rise to the peak value is t on , the time required for the secondary current to drop to zero is t off , the half-resonant period is T R , then it satisfies: T SW = t on + t off + n × T R . It can be seen from this that the switching period of the power switch tube in the present application has passed through the time required for the primary current to rise to the peak value and the time required for the secondary current to drop to zero, and on this basis, the vibration time of the half-resonant period is superimposed, and the variable frequency of the power switch tube is realized through the half-resonant period.

[0047] In an embodiment of the present application, when the switching period is less than the lower limit of the period, add a resonant period to the switching period, then the switching period satisfies: T SW = t on + t off + (n + 2) × T R .

[0048] As Figure 3 shown, for example, when n starts from 1 to calculate the switching period, when the switching period is less than the lower limit of the period, n is increased by 2, then the above formula is T SW = t on + t off + 3 × T R , so as to ensure that the switching period is always related to an odd multiple of the half-resonant period, and the power switch tube can be turned on at the valley of the vibration waveform of the resonant energy. The voltage across the power switch tube is V DS , V DS is the sum of V in and NV O . The voltage across the photovoltaic module is V PV .

[0049] In an embodiment of the present application, the calculation formula for the half-resonant period is: where L M is the inductance of the primary circuit, and C oss is the parasitic capacitance of the primary circuit. The half-resonant period is related to the magnitudes of the inductance and the parasitic capacitance. Generally, after the circuit design is fixed, the magnitude of the half-resonant period is also fixed, which is an inherent property of the inverter. Then, in the actual variable-frequency control process, the key is to calculate the change of n. The whole process is simple to control, without the need to distinguish between the DCM mode and the BCM mode, and there is no switching between the DCM mode and the BCM mode. The control strategy is more concise, and at the same time, the power consumption can be reduced.

[0050] As Figure 4 shown, in an embodiment of the present application, the primary circuit 10 further includes an electrolytic capacitor Cin and a photovoltaic module PV. The electrolytic capacitor Cin is connected in parallel with the photovoltaic module PV. The positive electrode of the electrolytic capacitor Cin is connected to the positive electrode of the photovoltaic module PV, and the positive electrode of the electrolytic capacitor Cin is connected to one end of the primary winding of the transformer T. The drain of the power switch Q M is connected to the other end of the primary winding of the transformer T. The source of the power switch Q M is connected to the negative electrode of the electrolytic capacitor Cin and the negative electrode of the photovoltaic module PV; the secondary circuit 20 includes a rectifying diode D M , a decoupling capacitor C O , a first control switch Q1, a second control switch Q2, a third control switch Q3, and a fourth control switch Q4. The opposite-named end of the transformer T is connected to the positive electrode of the rectifying diode D M . The decoupling capacitor C O is connected in parallel to the secondary side of the transformer T. One end of the secondary side of the decoupling capacitor C O is connected to the negative electrode of the rectifying diode D M . The other end of the secondary side of the decoupling capacitor C OThe other end of the secondary side is connected to the same-named end of the secondary side of the transformer T. The drains of the first control switch Q1 and the fourth control switch Q4 are connected. The source of the first control switch Q1 is connected to the drain of the second control switch Q2. The sources of the second control switch Q2 and the third control switch Q3 are connected. The source of the fourth control switch Q4 is connected to the drain of the third control switch Q3. The sources of the second control switch Q2 and the third control switch Q3 are connected to the same-named end of the secondary side of the transformer T. The photovoltaic grid-connected inverter further includes a filter circuit. The filter circuit includes a filter capacitor Cf and a filter inductor Lf. The filter capacitor Cf is connected in parallel between the source of the first control switch Q1 and the drain of the third control switch Q3. One end of the filter capacitor Cf is connected to the drain of the third control switch Q3, and the other end of the filter capacitor Cf is connected to one end of the power grid Vg. One end of the filter inductor Lf is connected to one end of the filter capacitor Cf, and the other end of the filter inductor Lf is connected to the other end of the power grid Vg. Through the cooperation of the above primary circuit 10, the transformer T and the secondary circuit 20, under the frequency conversion control of the power switch tube Q M the photovoltaic module PV is connected to the power grid Vg, reducing the power consumption of the inverter during the connection process.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered by the scope of the claims and the specification of the present invention.

Claims

1. A frequency conversion control method for a photovoltaic grid-connected inverter, characterized in that: The photovoltaic grid-connected inverter comprises a primary circuit, a transformer and a secondary circuit, one end of the transformer is connected to the primary circuit, the other end of the transformer is connected to the secondary circuit, a power switch tube is arranged in the primary circuit, the frequency conversion control method is used to control the power switch tube, and the frequency conversion control method comprises: Acquire a switching cycle of a power switch tube, and set the switching cycle to form a positive correlation with a semi-resonant cycle of the photovoltaic grid-connected inverter; Comparing the switching cycle with a pre-stored cycle lower limit; When the switching period is greater than or equal to the period lower limit, updating the switching period to a pulse width modulation signal to control the on and off of the power switch tube; When the switching period is less than the period lower limit, the switching period is increased by one resonance period, and the switching period increased by one resonance period is compared with the period lower limit again until the switching period is greater than or equal to the period lower limit.

2. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 1, characterized in that: The switch Expect Before the step of comparing with the pre-stored cycle lower limit, include: An upper limit operating frequency of the photovoltaic grid-connected inverter is set, a cycle lower limit is calculated according to the upper limit operating frequency, and the cycle lower limit is saved.

3. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 2, characterized in that: Define the upper limit operating frequency as f max , the lower limit of the period is T min , then: T min =1 / f max .

4. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 1, characterized in that: The step of setting the switching period to form a positive correlation with the semi-resonant period of the photovoltaic grid-connected inverter comprises: The switching period is the sum of the time required for the primary current to rise to a peak value, the time required for the secondary current to drop to zero, and n times the half-resonance period, where n is an odd number greater than or equal to 1.

5. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 4, characterized in that: The switching period of the power switch tube is defined as T SW The time required for the primary current to rise to the peak value is t on , the time required for the secondary current to drop to zero is t off , the half-resonance period is T R , then: T SW =t on +t off +n×T R .

6. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 5, characterized in that: When the switching period is less than the period lower limit, the switching period is increased by one resonant period, and the switching period satisfies: T SW =t on +t off +(n+2)×T R .

7. The frequency conversion control method of photovoltaic grid-connected inverter according to claim 1, characterized in that: The calculation formula of the half-resonance period is: Among them, L M is the inductance of the primary circuit, C oss is the parasitic capacitance of the primary circuit.

8. The frequency conversion control method for a photovoltaic grid-connected inverter according to any one of claims 1 to 7, characterized in that: The primary circuit also includes an electrolytic capacitor and a photovoltaic module, the electrolytic capacitor is connected in parallel with the photovoltaic module, the positive electrode of the electrolytic capacitor is connected to the positive electrode of the photovoltaic module, the positive electrode of the electrolytic capacitor is connected to one end of the primary winding of the transformer, the drain of the power switch tube is connected to the other end of the primary winding of the transformer, and the source of the power switch tube is connected to the negative electrode of the electrolytic capacitor and the negative electrode of the photovoltaic module; The secondary circuit includes a rectifier diode, a decoupling capacitor, a first control switch, a second control switch, a third control switch and a fourth control switch. The opposite-name end of the transformer is connected to the positive electrode of the rectifier diode, the decoupling capacitor is connected in parallel to the secondary side of the transformer, one end of the secondary side of the decoupling capacitor is connected to the negative electrode of the rectifier diode, and the other end of the secondary side of the decoupling capacitor is connected to the same-name end of the secondary side of the transformer. The drain of the first control switch is connected to the drain of the second control switch, the source of the second control switch is connected to the source of the third control switch, the source of the fourth control switch is connected to the drain of the third control switch, and the source of the second control switch and the source of the third control switch are connected to the same-name end of the secondary side of the transformer. The photovoltaic grid-connected inverter also includes a filter circuit, which includes a filter capacitor and a filter inductor. The filter capacitor is connected in parallel between the source of the first control switch and the drain of the third control switch, one end of the filter capacitor is connected to the drain of the third control switch, the other end of the filter capacitor is connected to one end of the power grid, one end of the filter inductor is connected to one end of the filter capacitor, and the other end of the filter inductor is connected to the other end of the power grid.

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