A 20 MW photovoltaic power station has a total investment of approximately 160 million yuan. Of this, the investment in combiner boxes is less than 1 million yuan, accounting for only 0.6% of the total investment. As a result, many people view combiner boxes as insignificant, minor equipment. However, according to statistical data, combiner boxes are a significant cause of on-site failures.

PV combiner box 3

1.Causes attributed to the combiner box itself

  • An unreasonable layout of busbars and fuses, which should not overlap, coupled with narrow busbar widths, hinders heat dissipation and causes short-circuit damage due to improper structural distribution.
  • The narrow width of busbars and the small contact area between terminals and busbars lead to overheating and arcing.
  • The use of aluminum busbars results in excessive overall operating temperatures for the combiner box. It is recommended to adopt TMY or TMR copper busbars. The protective coating of the outer shell may have quality issues.
  • The combiner box lacks effective protective devices. It is missing communication and protection units for monitoring branch currents. Loose connections and arcing should trigger alarms and drive circuit breakers to trip. This combiner box has no circuit breakers, making it difficult to manually disconnect even when accidents are detected.
  • Insufficient creepage distance on the high-voltage electrical input side of the control board results in combustion.
  • Fuse quality issues: fuses may explode when carrying current or may not provide adequate protection if oversized. Poor compatibility between the fusible element and the base (excessive contact resistance).
  • The IP rating does not meet requirements.
  • Terminal block insulation quality and withstand voltage values are low.
  • The circuit breaker interphase barrier is not installed, or the circuit breaker is too close to the outer shell, providing insufficient arc distance.

PV combiner box1

2.Causes due to non-standard construction practices

  • Insecure connections between photovoltaic module strings and the combiner box. Construction workers may over-tighten or under-tighten fixing screws, leading to poor contact and current arcing. High temperatures can melt fuse holders and cause short circuits, burning the combiner box.
  • Incorrect wiring leading to short circuits. Workers may fail to correctly identify the polarity of incoming module strings, leading to incorrect connections and short circuits. Some workers may even improperly connect photovoltaic modules, resulting in voltages exceeding 1500V or even 2500V, causing component burnout when connected to the combiner box.
  • Issues with incoming terminal and wiring connections. Photovoltaic combiner input lines enter the combiner box from the bottom without proper fixation, and the contact area between the terminal block and the wire head is minimal. Over time, temperature fluctuations and current-induced heating may cause these connections to loosen and arc, ultimately leading to widespread combustion within the box.
  • Inadequate cable head manufacturing techniques for the combiner box output line, poor steel armor stripping, and insufficient distance from the terminal block may result in ground short circuits. Poor contact in module string connectors may cause cable overheating and ignition. Combiner box output switch copper terminal screws may loosen and overheat.
  • Protective doors are not installed on site.

PV combiner box

3.Causes due to operations and maintenance:

  • Long-term equipment operation may lead to internal faults in power supply modules, causing arcing and the subsequent destruction of the combiner box.
  • Waterproof terminals at the bottom of the combiner box may not securely fasten photovoltaic module strings or combiner output lines. Over time, downward force may cause cable disconnection, leading to arcing, terminal burnout, and even short-circuiting.
  • Rodents and snakes may enter the combiner box and cause short circuits in the busbars.
  • Loose screws on fuse block terminals may lead to fuse block ignition.
  • Reverse current may occur in the event of a malfunction in a specific unit, causing potential damage to the combiner box and other components. Proper maintenance, inspection, and timely intervention can help prevent such issues and ensure the smooth operation of the photovoltaic system.

Precautions for Maintaining Combiner Boxes

  1. When maintaining a specific branch of the combiner box, always disconnect the circuit breaker first, then open the fuse box of the branch to be maintained, close the circuit breaker, and proceed with the maintenance of the wiring. It is crucial to remember not to unplug the M4 connector without first disconnecting the DC circuit breaker or directly opening the fuse box without disconnecting the DC circuit breaker to avoid personal safety accidents.
  2. When maintaining combiner boxes, develop the habit of tightening all screws. Ensure safety during the process by avoiding simultaneous contact with the positive and negative terminal connections or simultaneously touching the positive and PE wires or negative and PE wires.

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