Small Equipment That Cannot Be Ignored in Solar Photovoltaic System

    A 20MW photovoltaic power station with a total investment of about 160 million yuan. The investment in combiner box is less than 1 million yuan, 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, the combiner box is an important cause of on-site failure.

                             Statistics of on-site failure rate of photovoltaic project

The internal structure of combiner box is shown in the figure above

    Generally, steel plate, stainless steel, engineering plastics and other materials are used, and the protection grade is above IP54. Its function is: waterproof and dustproof, meeting the requirements of long-term outdoor use of combiner box. The IP54 protection grade system classifies electrical appliances according to their dustproof and moisture-proof characteristics** The first digit "5" indicates the degree of protection against foreign matters, and the second digit "4" indicates the airtightness of the equipment against the invasion of moisture and water. The higher the number, the higher the protection level.

      DC circuit breaker                                                                                                                      

    DC circuit breaker is the output control device of the whole combiner box, mainly used for circuit opening and closing. Its working voltage is up to DC1000V. Since the electric energy generated by the solar module is direct current, it is easy to generate electric arc when open circuit, so the temperature should be paid attention to when checking in the high temperature environment in summer.

 Surge protection device

    Surge, also known as surge protector, is a transient overvoltage exceeding normal operation. The surge protector is an electrical device that provides safety protection for the combiner box. When the circuit or communication circuit suddenly generates peak current or transient overvoltage or lightning overvoltage due to external interference, the surge protector can conduct and shunt in a very short time, thus avoiding the damage of the surge to other equipment in the circuit.

     fuse   

         The overload current and short circuit current in the circuit will cause the temperature of wires and cables to be too high, resulting in the insulation damage of wires and cables, and even wire breakage. The fuse is arranged at the inlet and outlet of the conductor or cable for overload protection of wires and cables. The rated current of the fuse is about 1.25 times of the line current; For short circuit protection, the fuse must be installed at the incoming end of the wire or cable. The rated current of the fuse is about 1.45 times of the tripping current.

Various reasons that may cause the combiner box to burn out

1.Caused by the combiner box itself

1) The layout of bus and fuse is unreasonable and should not overlap each other. In addition, the bus width is small, which is not conducive to heat dissipation and unreasonable. Structural distribution leads to short circuit burning.
2) The width of the bus is narrow, the contact area between the terminal and the bus is small, and there is heat and fire.
3) The bus is made of aluminum, and the overall temperature of the operation box is too high. TMY or TMR copper bus is recommended; The quality of the protective coating of the shell is questionable.
4) The combiner box lacks effective protective devices. There is no communication unit and protection unit in the combiner box to monitor the current of each branch. Once the virtual connection of a branch is loose and ignited, the current of the circuit will fluctuate, and the alarm shall be given and the circuit breaker shall be driven to trip; The combiner box has no circuit breaker. Even if an accident is found, it is difficult to disconnect it manually.
5) The insufficient creepage distance of the high-voltage electrical gap at the input of the control board leads to combustion;
6) Fuse quality problem: the fuse bursts when passing the current-carrying current, or the fuse is too large to be protected. The matching of melt and matrix (contact resistance is too large);
7) IP level does not meet the requirements;
8) The insulation quality and withstand voltage of terminal blocks are low.
9) The distance between circuit breakers is not installed, or the circuit breaker is too close to the shell, and the arc striking distance is not enough.

2.Caused by non-standard construction

1) The wiring between PV string and combiner box is not firm. Due to the excessive force exerted by the construction personnel during the construction process, the fixed screw was screwed on, and the sliding wire was not replaced, or the force was too small, and the screw was not tightened, resulting in poor contact, resulting in electric arc generated during the construction process. During operation, the high temperature melted the fuse holder, causing short circuit and burning. Lower the combiner box.
2) Short circuit caused by wrong wiring. When the PV string is connected to the combiner box, the construction personnel did not correctly distinguish the positive and negative poles of the battery pack, and connected the positive pole of one battery pack with the negative pole of the other battery pack, causing a short circuit. Some construction personnel even mistakenly connected the PV module, resulting in the partial string voltage reaching 1500V or even 2500V, and then connected to the combiner box, causing the module to burn out.
3) Caused by incoming terminal and wiring. The PV bus input line enters the combiner box from the bottom of the combiner box. It is directly connected to the terminal block without fixing measures. The terminal block is fixed by a small screw. The contact area with the terminal is small and bears the gravity of the wire. When the terminal is loosened due to temperature change and current heating, it will produce sparks and gradually generate electric arc and burn, which will gradually cause other equipment and even the whole box to heat and burn completely.
4) The production process of the outgoing cable head of the combiner box is insufficient, the steel armor is not peeled enough, and it is too close to the junction nose, resulting in a short circuit to the ground; The connection plug of the element string heats up due to poor contact, causing the cable to catch fire; The copper terminal screw of the outlet switch of the combiner box is loose and hot;
5) The site protective door is not installed.

3.Reasons in operation and maintenance

1) Due to the long-term operation of the equipment, the internal failure of the power module led to arcing and burning of the combiner box.

2) The waterproof terminal at the lower part of the combiner box did not fasten the PV string or the output wiring of the combiner. Since PV modules only generate electricity in the daytime, the contact points will heat and expand during the power generation process. At night, the temperature will not decrease and the contact point will shrink. If the waterproof terminal does not fasten the cable, the downward force may cause the line to time out. The cable is loose, which may cause the arc to burn the terminal or even short circuit.
3) Mice, snakes and other small animals enter the combiner box, causing short circuit of the bus.
4) The terminal screws of the fuse board are loose, causing the fuse board to catch fire;
5) One unit fails and backflow occurs.

4.Maintenance of combiner box

The maintenance content is to master the operation status of PV module equipment, find and eliminate equipment defects in time, prevent accidents, and ensure the completion of power generation plan. The equipment maintenance work should be done carefully.
1) The combiner box shall be inspected at least once a month to find and eliminate defects in time, and recorded in detail in the operation log.
2) Check the integrity of the combiner box for damage, deformation and collapse.
3) Check that the overall combiner box is clean, free of sundries and well sealed.
4) Check whether the screws are loose or rusty.
5) Check whether the wiring terminal is burnt and whether the screw is loose.
6) Check whether the fuse is burnt out and whether the fuse box is burnt out.
7) Check whether the anti-reverse diode is burnt out.
8) Check whether the circuit voltage and current are normal.
9) Check whether the surge protector is normal.
10) Check whether the line is weathered normally.
11) Check whether the wires connected to the combiner box are tightly bound and whether the insulation is aging.
12) Check whether the communication and background of the combiner box are interrupted.
13) Check whether the connecting terminal screws of the DC circuit breaker are loose, and check the temperature of the DC circuit breaker in high temperature weather in summer.
14) Check whether the signboard of combiner box is firmly pasted.

5. Precautions for maintenance of combiner box

1) When repairing a branch of the combiner box, you must first disconnect the circuit breaker, then open the fuse of the branch to be repaired, then close the circuit breaker, and then repair the bus. Remember not to unplug the M4 plug without disconnecting the DC circuit breaker, nor directly open the fuse box without disconnecting the DC circuit breaker, so as to avoid life safety accidents.
2) When repairing the combiner box, form the habit of tightening all screws at one time. When tightening the screws, pay attention to safety, and avoid touching the positive and negative terminals with hands at the same time, or touching the positive and negative terminals. PE is connected at the same time or the negative pole is connected with PE.