Is it good to use fuse for inverter?

In a photovoltaic system, the DC cable is exposed outdoors and may suffer from short circuit and ground fault, which requires protection. Fuses are an overcurrent protection device connected in series in the circuit. They can cut off the fault circuit in time when a short circuit occurs in the system to ensure system safety. Inverters and combiner boxes generally use photovoltaic fuses, but fuses are also unreliable. If they are not designed well, misjudgment is likely to occur.

Common failure modes of fuses are overcurrent fusing, aging fusing, and overtemperature fusing. Overcurrent fusing is a protective fusing that occurs when the fuse is overloaded, short-circuited, or in excess of the rated value. Aging fusing refers to a fault fusing that occurs when the fuse ages over a long period of time and the current-cutting capability decreases without overcurrent. The current of a fuse is closely related to its temperature. If the fuse works at a high temperature, its current-cutting capability decreases, and the possibility of fault fusing is relatively high.

The commonly used photovoltaic fuse is 15A. The photovoltaic module is a non-linear power supply with internal resistance, which is generally around 5 ohms. Even if the module is short-circuited, the current will not be too large. Taking the specification of Trina's monocrystalline module as an example, 300W, the open circuit voltage is 39.9V, and the short-circuit current is 9.55A. The calculated internal resistance of the module is about 4.2 ohms.

If an MPPT is configured with 1 to 2 strings, even if a component in one string is short-circuited, the total current will not exceed 15%, so there is no need to configure a fuse. If an MPPT is configured with N strings (N≥3), if a component in one string is short-circuited, this string will have a short-circuit current of (N-1)*, and a fuse needs to be configured at this time. After theoretical analysis and years of practice, it has been proven that this method is correct. The principle is as follows:

As shown in the figure above, an MPPT is connected to two components, namely S1 and S2. When a short circuit occurs to the ground somewhere in S2, it can be seen from the figure that the negative current of S2 flows to the grounding point without passing through the fuse, and the negative current of S1 flows to the grounding point through the common collection point and the fuse of S2. The total current of the fuse does not exceed 15% of the rated current, which does not meet the conditions for melting and there will be no fire hazard, so no fuse is required.

When an MPPT is configured with N strings (N≥3), the short-circuit circuit will increase. As shown in the figure above, an MPPT is connected to three components, namely S1, S2 and S3. When a short circuit to ground occurs somewhere in S3, it can be seen from the figure that the negative current of S3 flows to the grounding point without passing through the fuse, and the negative current of S1 and S2 flows to the grounding point through the common collection point and the fuse of S3. The total current of the fuse is twice the short-circuit current, reaching the melting condition, which will cause a fire hazard. Therefore, multi-strings need to be configured with fuses for protection.