CN113204261A - Photovoltaic cell output power control method under load cycle fluctuation condition - Google Patents

Photovoltaic cell output power control method under load cycle fluctuation condition Download PDF

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CN113204261A
CN113204261A CN202110392249.1A CN202110392249A CN113204261A CN 113204261 A CN113204261 A CN 113204261A CN 202110392249 A CN202110392249 A CN 202110392249A CN 113204261 A CN113204261 A CN 113204261A
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load
photovoltaic cell
power
output power
duty ratio
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CN113204261B (en
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宋新阳
赵文博
许芮豪
应展烽
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Nanjing University of Science and Technology
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    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/66—Regulating electric power
    • G05F1/67—Regulating electric power to the maximum power available from a generator, e.g. from solar cell
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    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

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Abstract

The invention discloses a photovoltaic cell output power control method under the condition of load cyclic fluctuation, which comprises the following steps of firstly, collecting photovoltaic cell output voltage and current, load voltage and current and junction temperature of a power device through a sampling circuit; then, judging the load fluctuation condition by using the acquired data, and finally, enabling the photovoltaic controller to selectively adopt a traditional disturbance observation method or an MPPT method for inhibiting junction temperature fluctuation to control the output power of the photovoltaic cell in real time according to different load conditions through the judgment; the method for controlling the output power of the photovoltaic cell under the condition of the cyclic fluctuation of the load can track the maximum power point when the load fluctuates, simultaneously restrain the junction temperature fluctuation of a power device and improve the reliability and the thermal stability of the photovoltaic cell.

Description

Photovoltaic cell output power control method under load cycle fluctuation condition
Technical Field
The invention belongs to the photovoltaic power generation technology, and particularly relates to a photovoltaic cell output power control method under the condition of load cyclic fluctuation.
Background
Due to the increasing shortage of conventional resources such as petroleum, the photovoltaic cell is a novel green renewable energy source without pollution and exhaustion risks, and has attracted wide attention of countries in the world. How to improve the output power of the photovoltaic cell becomes a research hotspot at home and abroad recently. The power output by the photovoltaic cell can be effectively improved by tracking the maximum power of the photovoltaic cell.
Common maximum power tracking algorithms include a disturbance observation method, a conductance gain method, a fuzzy control method, a neural network control method and the like. However, these maximum power control methods do not take into account the influence of the load cycle fluctuation on the output power, and patent 201410178381.2 discloses "a photovoltaic cell power control system and method with adaptive load demand variation" in which the relationship between the output power of the photovoltaic cell and the load demand is obtained by the photovoltaic output current and the load current, which does not take into account the influence of the change in the output power on the junction temperature of the power device, although the relationship between the load cycle fluctuation and the output power is taken into account. Excessive junction temperature fluctuations can reduce the thermal safety of the photovoltaic cell and even cause damage to the power devices.
Disclosure of Invention
The invention aims to provide a photovoltaic cell output power control method under the condition of load cycle fluctuation, which adjusts the photovoltaic cell output power by controlling a duty ratio when the load cycle fluctuates, and inhibits the fluctuation of junction temperature caused by the change of the output power.
The technical solution for realizing the purpose of the invention is as follows: a method for controlling the output power of a photovoltaic cell under the condition of load cycle fluctuation is particularly suitable for a photovoltaic system in which the junction temperature of a power device generates larger fluctuation along with the change of the output power of the photovoltaic cell, and comprises the following steps:
step 1, setting sampling frequency f and collecting photovoltaic cell output voltage U in real timepvOutput current IpvLoad current IoutAnd voltage UoutAnd a power device junction temperature t (n); and calculating to obtain the output power P of the photovoltaic cellpvLoad power P.
Step 2, providing an initial duty ratio D for a photovoltaic controller in charge of controlling the power device to be switched on and off in the photovoltaic system0Let D0＝0。
Step 3, monitoring the fluctuation condition of the junction temperature T of the power device, and calculating the junction temperature variation delta T in a specified sampling interval by adopting a Cauer structure thermal network model; and the junction temperature variation delta T is used as the input quantity of the PI controller, and a duty ratio increment delta D is generated after the temperature PI is adjusted.
Step 4, judging the load power P obtained by sampling at the moment n, and if the load power P is in a high-power period, accurately tracking a maximum power point according to a traditional disturbance observation method so as to meet the high-load requirement, thereby obtaining the current corresponding duty ratio D (n); if the load power P is in the low power period, in order to meet the low load requirement, the duty ratio D (n-1) at the previous time should be superimposed with the duty ratio increment Δ D controlled by the temperature PI, so as to obtain the current corresponding duty ratio D (n).
And 5: and (3) providing the current corresponding duty ratio D (n) for a photovoltaic controller to realize the control of the output power of the photovoltaic cell, and returning to the step 1 for real-time control.
Compared with the prior art, the invention has the remarkable advantages that:
the invention considers the influence of the output power reduction on the junction temperature, adopts a mode of increasing the duty ratio to enable the output power of the photovoltaic cell to be far away from the maximum power point, effectively restrains the junction temperature fluctuation while reducing the output power, prolongs the service life of a power device, and has important theoretical significance and engineering practical value.
Drawings
Fig. 1 is a schematic diagram of a photovoltaic system according to the present invention.
Fig. 2 is a flowchart of a method for controlling the output power of a photovoltaic cell under the condition of cyclic fluctuation of the load according to the present invention.
FIG. 3 is a model diagram of a thermal network of a Cauer structure of a power device.
FIG. 4 is a graph of the cyclic fluctuation of the load (Phigh for high load and Plow for low load).
Detailed Description
The present invention is described in further detail below with reference to the attached drawing figures.
With reference to fig. 1 and fig. 2, the method for controlling the output power of the photovoltaic cell under the condition of the cyclic fluctuation of the load according to the present invention is based on the conventional disturbance observation method, and combines the active thermal control of the power device, and different control strategies are respectively adopted for the high power load output period and the low power load output period, so as to ensure the load carrying capacity of the photovoltaic cell to the maximum extent and suppress the fluctuation of the junction temperature at the same time, and is particularly suitable for a photovoltaic system in which the junction temperature of the power device fluctuates greatly with the change of the output power of the photovoltaic cell, and has important theoretical significance and engineering practical value, and the specific steps are as follows:
step 1, setting sampling frequency and collecting photovoltaic cell output voltage U in real timepvOutput current IpvLoad current IoutVoltage UoutAnd a power device junction temperature t (n); and calculating to obtain the output power P of the photovoltaic cellpvLoad power P.
Attention should be paid to the setting of the sampling point position to avoid the load voltage UoutAnd the output voltage U of the photovoltaic cellpvObfuscating; after sampling, the collected data are transmitted to the photovoltaic controller, and the sampling frequency can be set to be 5 kHz.
Step 2, providing an initial duty ratio D for a photovoltaic controller in charge of controlling the power device to be switched on and off in the photovoltaic system0And order D0＝0。
Step 3, monitoring the fluctuation condition of the junction temperature T of the power device, and calculating the junction temperature variation delta T in unit time by adopting a Cauer structure thermal network model; and the junction temperature variation delta T is used as the input quantity of the PI controller, and a duty ratio increment delta D is generated after the temperature PI is adjusted.
In the Cauer structural thermal network model, T is shown in FIG. 3jAnd TcJunction temperature and case temperature, T, of the power devicehAnd TaRespectively radiator temperature and ambient temperature, RchIs the conduction thermal resistance of heat conduction material between the power device shell and the radiator, RhaIs convective thermal resistance between the heat sink and the environment, ChIs the heat capacity of a heat sink, C1～CmAnd R1～RmRespectively is the heat capacity and the heat resistance of each node of the heat network, and m is the order of the heat network and depends on the specific structure in the power device; the power device junction-ambient thermal network model can be expressed in terms of the following system of differential equations according to fig. 3:
Figure BDA0003017175410000031
in the formula, the variables A, B and C are:
Figure BDA0003017175410000032
Figure BDA0003017175410000041
Figure BDA0003017175410000042
knowing the loss P of the power device and the ambient temperature TaAnd after the parameter values of the thermal resistance and the thermal capacity in the thermal network model are obtained, the junction temperature of the power device can be estimated by solving an equation set.
Step 4, judging the sampling condition as shown in fig. 4, and if the load power P is required to be in a high-power period at the moment n, accurately tracking the maximum power point according to a traditional disturbance observation method in order to meet the high-load requirement, so as to obtain the current corresponding duty ratio D (n); if the required load power P is in the low-power period, in order to meet the low-load requirement, the duty ratio D (n-1) at the previous time should be superimposed with the duty ratio increment Δ D controlled by the temperature PI, so as to obtain the current corresponding duty ratio D (n).
Compared with the conventional mode of reducing the output power of the photovoltaic cell, the step increases the output power P of the photovoltaic cell by increasing the duty ratiopvFar away from the maximum power point, the method can reduce the output power, inhibit the severe fluctuation of the junction temperature of the power device in the photovoltaic system and prolong the service life of the power device.
And 5: and (4) providing the current corresponding duty ratio D (n) obtained in the step (4) for a photovoltaic controller to realize the control of the output power of the photovoltaic cell, and returning to the step (1) for real-time control.
When the photovoltaic system is connected with a circuit, a Boost type (Boost) circuit is adopted between the photovoltaic cell and the load; a Boost (Boost) circuit may provide a relatively high load voltage with the same voltage polarity as the photovoltaic cell output voltage. As shown in the Boost (Boost) topology model of fig. 1, the input terminal capacitor C1The function of the voltage stabilizer is voltage stabilization, and the larger the capacitance value is, the better the voltage stabilization effect is; l is an energy storage inductor; filter capacitor C of output end2May be used to limit the load voltage ripple.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims (3)

1. A method for controlling the output power of a photovoltaic cell under the condition of load cycle fluctuation is particularly suitable for a photovoltaic system in which the junction temperature of a power device generates larger fluctuation along with the change of the output power of the photovoltaic cell, and is characterized by comprising the following steps of:
step 1, setting sampling frequency f and collecting photovoltaic cell output voltage U in real timepvOutput current IpvLoad current IoutAnd voltage UoutAnd a power device junction temperature t (n); and calculating to obtain the output power P of the photovoltaic cellpvAnd load power P, turning to the step 2;
step 2, providing an initial duty ratio D for a photovoltaic controller in charge of controlling the power device to be switched on and off in the photovoltaic system0Let D0If the value is 0, the step 3 is carried out;
step 3, monitoring the fluctuation condition of the junction temperature T of the power device, and calculating the junction temperature variation delta T in a specified sampling interval by adopting a Cauer structure thermal network model; taking the junction temperature variation delta T as the input quantity of a PI controller, generating a duty ratio increment delta D after the temperature PI is adjusted, and turning to the step 4;
step 4, judging the load power P obtained by sampling at the moment n, and if the load power P is in a high-power period, accurately tracking a maximum power point according to a traditional disturbance observation method so as to meet the high-load requirement, thereby obtaining the current corresponding duty ratio D (n); if the load power P is in a low-power period, in order to meet the low-load requirement, overlapping the duty ratio D (n-1) at the last moment and the duty ratio increment delta D obtained by controlling the temperature PI, thereby obtaining the current corresponding duty ratio D (n), and turning to the step 5;
and 5: and (3) providing the current corresponding duty ratio D (n) for a photovoltaic controller to realize the control of the output power of the photovoltaic cell, and returning to the step 1 for real-time control.
2. The method of claim 1 for controlling photovoltaic cell output power under load cycle fluctuating conditions, characterized by: and connecting the photovoltaic cell with a load through a Boost type Boost circuit.
3. The method of claim 1 for controlling photovoltaic cell output power under load cycle fluctuating conditions, characterized by: in step 4, if the load required by the load is in a low-power period, the output power is reduced by increasing the duty ratio to make the output power of the photovoltaic cell far away from the maximum power point, specifically according to the following formula:
when in use
Figure FDA0003017175400000011
D (n) ═ D (n-1) + Δ D;
wherein
Figure FDA0003017175400000012
Representing the derivative of the photovoltaic cell output power P to the duty cycle D.
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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103956745A (en) * 2014-04-29 2014-07-30 武汉大学 Photovoltaic cell power control system and method capable of automatically adapting to load demand changes
CN110266187A (en) * 2019-07-30 2019-09-20 阳光电源股份有限公司 A kind of DC/DC translation circuit control method, device and photovoltaic DC-to-AC converter
US20190384340A1 (en) * 2015-06-30 2019-12-19 Gd Midea Airconditioning Equipment Co., Ltd. Method for tracking control of maximum power point of solar cell and tracking device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103956745A (en) * 2014-04-29 2014-07-30 武汉大学 Photovoltaic cell power control system and method capable of automatically adapting to load demand changes
US20190384340A1 (en) * 2015-06-30 2019-12-19 Gd Midea Airconditioning Equipment Co., Ltd. Method for tracking control of maximum power point of solar cell and tracking device
CN110266187A (en) * 2019-07-30 2019-09-20 阳光电源股份有限公司 A kind of DC/DC translation circuit control method, device and photovoltaic DC-to-AC converter

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