1. First, understand why photovoltaic system is malfunctioning
Multiple risks of long-term exposure of Photovoltaic modules to the outdoors:
Hot spot effect: When a cell is obstructed, damaged or aging, it no longer generates electricity but becomes a resistor, consuming electricity generated by other cells. Temperatures in the area can suddenly climb into hundreds of degrees Celsius, enough to burn back panels and even ignite components.
Wire internal short circuit: cable insulation aging, seam water ingress, connector virtual connection, etc., can cause short circuit, immediately produce many tens of times the normal working current fault current.
Reverse current: When a string stops due to a fault, the current of other normal strings returns through the fault string, causing secondary damage.
power semiconductor devices such as IGBT in inverters are extremely sophisticated. Once the DC side current is out of control, it may be permanently damaged in milliseconds and the entire inverter may be abandoned.
The whole mission of a photovoltaic fuse is to completely cut off the circuit in the first milliseconds after a disaster.
2. Protection mechanism for photovoltaic components
Protection 1: Isolate hot spots from burning.
When a particular battery experiences a hot spot, the fault current increases rapidly along the string circuit. When the current exceeds a set threshold, the fuse quickly melts, completely isolating the entire faulty line from the system. Other normal strings are unaffected and continue to generate electricity. In the absence of a fuse, the persistently high current generated by the heat source will travel along the cable, not only burning defective components but also potentially affecting adjacent components.
Protection 2: Cut short-circuit current to prevent cable from catching fire.
When a series short circuit occurs, the short-circuit circuit current can reach more than 10 times the normal working current. If the fuse is not cut in time, the cable will overheat, the insulation will melt and even ignite the surrounding material in a short amount of time. The millisecond response of the fuse can prevent a fire at the source by cutting off the circuit before the cable temperature rises to a dangerous value.
Protection 3: Prevent reflux.
When a string is disconnected from the network by a fuse, the current of other normal strings will not flow back to the branch in question. This appears to be a "side effect"of fuses, but is in fact a extremely important form of protection-it ensures that faults are kept to a minimum and do not proliferate or expand.
3. Protective mechanism of inverters
Protection 1: Serve as a the 'last line line of defense "for inverters.
Although the inverter itself has the function of overflow protection, its response speed and breaking ability are limited. When a severe short circuit occurs on the DC side, the fault current rises rapidly and may have burned out the IGBT before the inverter's electronic protection circuit is activated. The pure physical melting mechanism inverter does not depend on any electronic signal, has fast response speed and strong fracture ability, and is an irreplaceable hardware-level protection for the inverter DC input terminal.
Protection 2: Limit failure power and reduce damage.
Even in the event of a fault, the fast fuse of the fuse can greatly shorten the duration of the fault current. The shorter the current duration, the less energy is released to the inverter, and the less impact there is on the power device. In many cases, it is because the fuse cuts the circuit in milliseconds that the inverter is able to "survive" by simply replacing the fuse to get back to work, rather than replacing the entire device.
Protection 3: Achieve precise isolation of fault branches to ensure overall system operation
In a system where multiple strings are parallel to the same inverter, if one string fails without a fuse, the DC input of the entire inverter is affected, which can trigger a protective shutdown of the inverter and cause all strings to stop generating electricity. With cascading fuses, only the faulty branches are cut off and the remaining normal series continues to supply the the inverter, minimizing the loss of system power generation.
4, Complete logical chain of fuse protection action
In the event of a malfunction, the entire protection process is as follows:
The first is the current anomaly caused by short circuit, hot spot backflow, overload, etc..
The second step is to increase the current rapidly, and when the current reaches the rated current of the fuse, the inside of the melt begins to heat and melt.
The third step is to melt the melt completely, cutting off the circuit in milliseconds to tens of milliseconds.
Step four, reset the fault current to zero and remove all components, cables, and inverters from danger.
Step five, operators and maintenance personnel locate and replace the fuse to restore normal operation of the system.
The process requires no external power supply, is independent of any control signals, is not influenced by any software logic, and is the purest and most reliable method of protection.
5, Why can't fuses be completely replaced with electronic protection?
Many may ask: Doesn't the inverter already have electronic overcurrent protection? Why do we need fuses?
The reason is that electronic protection is a kind of ``soft protection"which relies on sensor detection, control chip judgment and IGBT shutdown. If any of the links on this link run into problems, protection may fail. A fuse is a "hard protection" that recognizes only the size of the current, and when it arrives, it doesn't melt, has no intermediate links, doesn't malfunction, doesn't misjudge, and doesn't require maintenance or calibration.
"Soft/ hard combination" is the safest solution in a high-voltage, high-current, high-fault energy environment on the PV DC side: electronic protection is responsible for daily overload and minor anomalies, while fuses are responsible for the most extreme short-circuit and high-current failures. The two are complementary and indispensable.
