CN106058930A - Automatic detecting system and automatic detecting method for capacitance of photovoltaic grid-connected inverter bus capacitor - Google Patents

Abstract

The invention relates to a system and method for automatically detecting the capacitance value of the bus bar of a photovoltaic grid-connected inverter. At night, the inverter is in a standby state, the voltage of the DC bus bar is lowered, and the inverter is started to soft-start the circuit, and the bus bar capacitor is charged to make the inverter The bus capacitor voltage of the inverter is equal to the DC voltage. After the charging is completed, the soft start is stopped. At this time, the auxiliary test resistor circuit is controlled to work. The bus capacitor is discharged through the auxiliary test resistor, and the bus capacitor responds in zero state at this time. The capacitor value can be calculated by dividing the bus voltage integral by the bus voltage drop and the auxiliary test resistance during the discharge time period, and then judge whether the capacitor value is qualified by comparing the calculated value with the preset lower limit of the capacitor value, such as If it is unqualified, it will give a fault alarm and prompt to replace the bus capacitor to prevent further damage caused by the performance degradation or failure of the bus capacitor.

Description

Photovoltaic grid-connected inverter bus capacitance automatic detection system and method

technical field

The invention relates to the field of photovoltaic grid-connected power generation, in particular to a system and method for automatically detecting the capacitance value of a bus bar of a photovoltaic grid-connected inverter.

Background technique

As one of the renewable energy sources, solar energy has the advantages of abundant reserves, wide distribution, clean and pollution-free, green and environmental protection, and is recognized as an alternative energy source. The direct current generated by the solar panels is converted into alternating current with the same amplitude, same frequency and same phase as the grid voltage by the inverter, and then sent to the grid.

The photovoltaic grid-connected inverter is composed of key parts such as switching device unit and DC bus capacitor unit, and the DC bus capacitor is a crucial unit of the photovoltaic grid-connected inverter. In the process of exchanging with the AC side, it absorbs and provides the instantaneous current required for dynamics, maintains the voltage stability of the DC bus, and can also absorb the IGBT turn-off peak voltage, etc. Photovoltaic grid-connected inverters are required to have a service life of 25 years, but due to various reasons, the performance of the DC bus capacitor will be reduced, mainly manifested in the reduction of the capacitance value, which will lead to the increase of the DC bus voltage ripple and the output current THD increase, when the DC bus voltage ripple exceeds the tolerance value of the photovoltaic grid-connected inverter, it will also cause the IGBT to blow up, and the accident will expand. Therefore, the automatic detection of the capacitance value of the DC bus is very important.

At present, the detection method of the DC bus capacitance of the photovoltaic grid-connected inverter will inevitably affect the normal operation of the system when it is implemented. In the prior art, there is no simple and effective detection method that does not affect the system operation. In view of this situation, this paper proposes an automatic detection method for the bus capacitance of photovoltaic grid-connected inverters. This method is simple, effective, stable and reliable.

Contents of the invention

The purpose of the present invention is to provide an automatic detection system for the capacitance value of the photovoltaic grid-connected inverter busbar, and a detection method using the system to solve the problem that the detection in the prior art affects the normal operation of the system.

To achieve the above object, the solution of the present invention includes:

The photovoltaic grid-connected inverter bus capacitance automatic detection system includes DC terminals for connecting photovoltaic equipment (such as photovoltaic panels, etc.), the DC terminals are connected to the DC bus, and the DC bus is connected to the AC terminals through the converter and the soft start circuit. The AC terminal is used to connect to the AC grid; the DC bus is connected with a bus capacitor and a voltage sensor; the DC bus is connected with an auxiliary test circuit, and the auxiliary test circuit includes a series auxiliary test resistor and a control switch.

The present invention also provides a method for automatically detecting the capacitance value of the photovoltaic grid-connected inverter bus bar, the steps are as follows:

When it is detected that night is coming and the DC bus voltage is lower than the set value, disconnect the photovoltaic equipment from the DC bus, connect to the AC grid, and charge the bus capacitor;

After the charging is completed, disconnect the AC power grid, put into the auxiliary test circuit, and the bus capacitor will discharge the auxiliary test circuit;

At least detect the start-stop voltage and discharge time in the discharge process, and calculate the bus capacitance value according to the relationship between the difference of electric quantity and the integral of electric quantity.

Further, the difference of the electric quantity is the product of the voltage difference and the bus capacitance, and the relationship between the electric quantity difference and the integral of the electric quantity is equal to The formula for calculating the capacitance of the busbar is:

Among them, U _c (t1) and U _c (t2) are the starting and ending voltages respectively, t1 and t2 are the starting and ending times, R ₄₁ is the auxiliary test resistor, and C is the bus capacitance.

Further, when the difference between the calculated bus capacitance value and the set bus capacitance value exceeds a limit value, an alarm is given to prompt replacement of the bus capacitor.

The beneficial effect of the present invention is that the capacitance detection method can be automatically detected every night, and the performance index of the bus capacitance can be grasped in time without affecting the normal power generation and grid connection during the day, so the method does not affect the system operation, and the detection frequency High, simple, effective, and easy to implement and popularize in practical engineering.

In addition, the accurate bus voltage value can be detected through the voltage hall of the inverter itself, without adding additional detection devices, and then the capacitance value of the capacitor can be obtained through voltage integral calculation. Comparing with calculation by current integral, not only the detection variables are reduced, but also the detection error caused by inaccurate detection due to small current can be avoided, thereby reducing the calculation error.

Description of drawings

Fig. 1 is an automatic detection system for the bus capacitance of a photovoltaic grid-connected inverter according to an embodiment of the present invention;

Fig. 2 is a method for automatically detecting the capacitance value of the bus bar of a photovoltaic grid-connected inverter according to an embodiment of the present invention.

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings.

As shown in Figure 1, the photovoltaic grid-connected inverter bus capacitance automatic detection system includes DC terminals for connecting photovoltaic equipment (such as photovoltaic panels, etc.). The starting circuit is connected to the AC terminal. The AC terminal is used to connect to the AC grid; the DC bus is connected with a bus capacitor. A voltage sensor for detecting the bus voltage is also arranged on the DC bus.

The soft-start circuit includes the main contactor, soft-start contactor, and soft-start resistor, which are used for photovoltaic grid connection and reduce the impact of starting current. The converter adopts a bridge converter, that is, the power bridge in the figure.

The above-mentioned circuit parts all belong to the photovoltaic grid-connected part of the prior art. The structural improvement of the present invention lies in that an auxiliary test circuit is connected to the DC bus, and the auxiliary test circuit includes series auxiliary test resistors and corresponding control switches. In this embodiment, the voltage sensor is a Hall voltage sensor. As other implementation manners, other types of voltage sensors may also be used.

The present invention carries out the principle of busbar capacitance capacitance value test is:

As the sun's rays gradually weaken every night, the voltage at the battery panel port gradually decreases, and the inverter stops working. After the DC side voltage drops to 0V, the inverter starts the AC soft-start circuit (soft-start contactor and soft-start Start-up resistance) to charge the DC bus capacitor, stop the soft-start circuit after the charging is completed, and then start the auxiliary test circuit to discharge, and use the DC discharge process to detect the capacity of the bus capacitor. Capacitance detection mainly uses the AC power grid, soft start circuit, power bridge, auxiliary test circuit, bus capacitor and Hall voltage sensor. The AC power grid provides power, and the soft start circuit and power bridge form a rectifier circuit to charge the bus capacitor. The auxiliary test circuit is to discharge the bus capacitor, and the voltage Hall sensor detects the bus voltage. The inverter then uses its own controller to perform capacitance calculations.

According to the nature of capacitance, there are the following expressions:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 41</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; C</span>}\frac{{\mathrm{<span\; class="notranslate">\; du</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}}{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

R ₄₁ is an auxiliary test resistor.

Integrating both sides of (1) can get:

${<span\; class="notranslate">\; \&Integral;</span>}_{{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 41</span>}}}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; C</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; du</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

which is Thus, the capacitance value can be obtained, that is, the calculation formula of capacity:

$\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; t</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; t</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 41</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

It can be seen from formula (3) that the capacitance value can be calculated according to formula (3) by knowing the voltage change △U across the bus capacitor C at time t1~t2 and the voltage u _c (t) across the bus capacitor.

The specific test process is shown in Figure 2:

When it is detected that the DC voltage is less than 20V after the inverter stops at night (that is, a set value close to 0V, those skilled in the art can choose according to the actual situation), the soft start circuit is started to charge the capacitor.

After the charging is completed (it can be stopped after charging to a set voltage value), stop the soft start circuit, and start the auxiliary test resistor at the same time (start the auxiliary test circuit by closing the control switch connected in series with the auxiliary test resistor), when the bus voltage changes from U _c During the process from (t1) to U _c (t2), the DC bus voltage is integrated, and when it is detected that the bus voltage is less than U _c (t3), the auxiliary test circuit is cut off (by turning off the control switch), where U _c ( t3)<U _c (t2)<U _c (t1), then use the formula (3) to calculate the capacitance value of the busbar, compare the calculated result with the set capacitance value, when the calculated busbar capacitance value Compared with the set capacitance value (by dividing or subtracting), if the gap is too large, it will prompt a message that the bus capacitance has a serious problem and needs to be replaced.

In the above embodiments, the time period for detecting the bus capacitance is after night, so that the normal operation of the photovoltaic will not be affected. Therefore, it is necessary to judge the coming of night; the judgment of night can be realized by timing, such as the automatic timing chip of the controller or the timing information of the main control system; accurately judging the coming of night can avoid the dark clouds during the day and cause shutdowns, and the bus capacitance detection. As other implementation manners, it can also be combined with the shutdown information to determine that night is coming.

Depending on the frequency required, you can choose to test every night, or once every few days.

In the above embodiments, the bus voltage is detected, and the capacitance value is detected by using the relationship that the electric quantity difference (that is, the product of the voltage difference and the capacitance) is equal to the voltage integral. As other implementations, the discharge current can also be detected by connecting a current sensor (such as a current transformer) in series in the auxiliary test circuit, and the capacitance value can be detected by the relationship between the integral of the discharge current and the power difference. The specific calculation formula is the following formula (3) It is sufficient to change the voltage integral to the current integral (remove the resistance on the denominator), so it will not be repeated here. However, since the Hall voltage sensor is an inherent device on the DC bus, the use of voltage integration for calculation does not require the introduction of additional equipment. Compared with the current integration scheme, the cost is lower.

In the above embodiment, the DC terminal in Figure 1 is the output terminal of the photovoltaic device, so when the auxiliary test circuit is used for charging and discharging, it is necessary to disconnect the connection between the photovoltaic device and the DC terminal in Figure 1 to prevent the photovoltaic device from participating in the charging of the bus capacitor and the discharge of the auxiliary test resistor.

The specific embodiments related to the present invention are given above, but the present invention is not limited to the described embodiments. Under the idea given by the present invention, the technical means in the above-mentioned embodiments are transformed, replaced, and modified in ways that are easy for those skilled in the art, and the functions played are basically the same as those of the corresponding technical means in the present invention. 1. The purpose of the invention realized is also basically the same, and the technical solution formed in this way is formed by fine-tuning the above-mentioned embodiments, and this technical solution still falls within the protection scope of the present invention.

Claims (4)

1. Photovoltaic grid-connected inverter bus capacitance automatic detection system, including DC terminals for connecting photovoltaic equipment, DC terminals are connected to DC bus, DC bus is connected to AC terminals through converters and soft start circuits, and AC terminals are used for connected to the AC power grid; the DC bus is connected with a bus capacitor and a voltage sensor; it is characterized in that the DC bus is connected with an auxiliary test circuit, and the auxiliary test circuit includes a series auxiliary test resistor and a control switch. 2. the detection method of detection system as claimed in claim 1, is characterized in that, step is as follows: When it is detected that night is coming and the DC bus voltage is lower than the set value, disconnect the photovoltaic equipment from the DC bus, connect to the AC grid, and charge the bus capacitor; After the charging is completed, disconnect the AC power grid, put into the auxiliary test circuit, and the bus capacitor will discharge the auxiliary test circuit; At least detect the start-stop voltage and discharge time in the discharge process, and calculate the bus capacitance value according to the relationship between the difference of electric quantity and the integral of electric quantity. 3. detection method according to claim 2, is characterized in that, the relation that the difference of electric quantity is equal to integral of electric quantity is The formula for calculating the capacitance of the busbar is: Among them, U _c (t1) and U _c (t2) are the starting and ending voltages respectively, t1 and t2 are the starting and ending times, R ₄₁ is the auxiliary test resistor, and C is the bus capacitance. 4. The detection method according to claim 2, characterized in that when the difference between the calculated bus capacitance and the set bus capacitance exceeds a limit value, an alarm is given to prompt replacement of the bus capacitance.