The DC trunk line is the transmission line used for the photovoltaic module system to be connected to the inverter after the converging box. If the inverter is the heart of the whole phalanx system, then the DC trunk system is one aorta. Because the DC trunk system is not grounded, the grounding fault of the cable will bring considerably greater harm to the system and even the equipment than that of the AC. Therefore, photovoltaic system engineers should be more cautious about the DC cable than electrical engineers in different industries. Based on the analysis of various cable accidents, we conclude that the grounding fault of the cable accounts for 90-95% of the total cable faults.
There are three main causes of ground faults.
First, cable manufacturing defects, for unqualified products;
second, bad operating environment, natural aging, and external damage;
third, the installation is not standard, the wiring is rough. Pick up
The root cause of the ground failure is only one - the cable insulation material.
The operating environment of the DC trunk line of the photovoltaic power station is harsh. China's large ground power stations are typically located in the west, where desert, saline-alkali land and the temperature difference between days are large, and the environment is also extremely humid. iec 62930 solar cable is buried and laid, and the filling and digging requirements of cable trench are relatively steep.
Distributed power station cable operating environment is not better than the above ground, the cable will bear extremely high temperature, technical personnel measurement and control, the roof temperature can even reach 100-110℃ high temperature, the cable fire retardant requirements, and high temperature has a great impact on the insulation breakdown voltage of the cable. Therefore, the selection and design of DC trunk cable of photovoltaic power station should consider the following points:
Insulation performance of cable
Resistance to moisture, cold and weather
Heat resistance and flame resistance of cable
The way the cable is laid
Conductor material of cable (copper core, aluminum core, aluminum core)
Section specification of cable
At present, most of the DC mains cables of Chinese photovoltaic power stations are replaced by general low-voltage AC cables. commonly used models are ZR-YJV22 0.6/1kv and ZRYJY23 0.6/1kv. Most of the cables are copper core cables, and some power stations gradually begin to use aluminum alloy conductor cables. But the insulation material of the cable is essentially produced according to the standard of 1kv low-voltage cable. In other words, our PV system engineers are aware of the great relationship of DC cable, but they do not pay enough attention to the technical solution of cable.