A 20MW photovoltaic power station, with a total investment of about 160 million yuan. Among them, the investment of the combiner box is less than 1 million, accounting for only 0.6% of the total investment. Therefore, in the eyes of many people, the combiner box is a trivial small device. However, according to statistics, combiner boxes are a significant cause of field failures.
The internal structure of a common combiner box is shown in the figure below.
Generally, steel spray, stainless steel, engineering plastic and other materials are used, and the protection level is above IP 54. The function is: waterproof and dustproof, which meets the requirements of long-term outdoor use of the combiner box.
The IP 54 rating system classifies electrical appliances according to their dust and moisture resistance properties. The first number "5" represents the level of the electrical appliance against foreign intrusion, and the second number "4" represents the degree of airtightness of the electrical appliance against moisture and water intrusion. The larger the number, the higher the protection level.
The DC circuit breaker is the output control device of the entire combiner box, and is mainly used for the opening and closing of the line. Its working voltage is as high as DC1000V.
Since the power generated by the solar module is direct current, it is easy to generate arcs when the circuit is disconnected, so pay attention to its temperature during inspections in the high temperature environment in summer.
The first function of the anti-reverse diode is to prevent the solar cell module or phalanx from generating heat or even damage when the solar cell module or phalanx is not generating electricity. This is because the output voltage of each branch in series cannot be absolutely equal, the voltage of each branch has a difference in height, or a branch failure or shadow occlusion will reduce the output voltage of the branch , the current of the high-voltage branch will flow to the low-voltage branch, and the anti-reverse diode is connected in series in each branch to avoid this phenomenon.
A surge, also called a surge, is an instantaneous overvoltage that exceeds normal operation. A surge protector is an electrical device that provides safety protection for the combiner box. When the electrical circuit or communication line suddenly generates peak current or transient overvoltage and lightning overvoltage due to external interference, the surge protector can turn on and shun the current in a very short time, so as to avoid the surge to other equipment in the circuit. damage.
The overload current and short-circuit current in the line will cause the wire and cable temperature to be too high, resulting in insulation damage or even breakage of the wire and cable. The fuse is arranged at the incoming or outgoing end of the wire and cable for overload protection of the wire and cable. The rated current of the fuse is about 1.25 times of the line current; the fuse must be installed at the incoming end of the wire and cable for short-circuit protection. The rated current of the fuse is about 1.45 times the tripping current.
6.PCB board/monitoring module
The main function of the PCB board is to collect and calculate the component voltage, current, temperature, average current, and communicate with the background.
The photovoltaic combiner box is a wiring device that ensures the orderly connection of photovoltaic modules and the combining function in the photovoltaic power generation system. It means that the user can connect a certain number of photovoltaic cells with the same specifications in series to form a photovoltaic series, and then connect several photovoltaic cells in series. The photovoltaic series and parallel connection are connected to the photovoltaic combiner box. After the photovoltaic combiner box is combined, the controller, DC power distribution cabinet, photovoltaic inverter, and AC power distribution cabinet are used together to form a complete photovoltaic power generation system. electricity grid.
In order to improve the reliability and practicability of the system, the photovoltaic lightning protection combiner box is equipped with photovoltaic special DC lightning protection modules, DC fuses and circuit breakers, which are convenient for users to grasp the working conditions of photovoltaic cells in a timely and accurate manner and ensure solar photovoltaic power generation. The system works to its fullest potential.
The cause of the combiner box itself
1) The layout of bus bars and fuses is unreasonable and should not overlap. In addition, the width of the bus bars is small, which is not conducive to heat dissipation. The unreasonable structural distribution causes short-circuit burnout.
2) The width of the busbar is relatively narrow, and the contact area between the terminal and the busbar is small, causing heat generation and ignition.
3) The bus bar adopts aluminum bar, and the overall temperature of the operating box is too high. It is recommended to use TMY or TMR copper bar; the quality of the protective coating of the shell is a problem.
4) The combiner box lacks effective protection devices. There is a lack of communication unit and protection unit for monitoring the current of each branch in the combiner box. Once the virtual connection of a branch is loose and ignites, the current of this circuit will fluctuate, which should alarm and drive the circuit breaker to trip; this combiner box has no circuit breaker, Even if an accident is detected, it is difficult to disconnect manually.
5) Insufficient creepage distance of high-voltage electrical clearance at the input end of the control board causes combustion;
6) The quality of the fuse: When the fuse passes the current, it bursts, or the fuse tube insurance selection is too large, which does not play a protective role. The compatibility between the melt and the base (the contact resistance is too large);
7) The IP level does not meet the requirements;
8) The insulation quality and withstand voltage of the terminal block are low.
9) The interphase partitions of the circuit breaker are not installed, or the circuit breaker is too close to the casing, and the arcing distance is not enough.