How many strings can be connected to an MPPT without a fuse?

At present, different manufacturers have different technical routes for string inverters. Single-phase inverters below 6kW and three-phase inverters below 10kW generally use two MPPTs, and each MPPT is equipped with 1 string. Small industrial and commercial projects generally use 20kW to 40kW inverters, with 2 to 4 MPPTs, and each MPPT is equipped with 2 to 4 strings. Large power stations generally choose 60kW to 80kW high-power string inverters, with 1 to 6 MPPTs, and each MPPT is equipped with 2 to 12 strings.

Choosing different MPPT routes will have a certain impact on the power generation of the system. From the perspective of solving the mismatch problem, the fewer strings behind one MPPT, the better, because each MPPT loop operates independently, and the system design is more flexible. From the perspective of stability and efficiency, the more strings behind one MPPT, the better, because the more MPPTs there are, the higher the system cost, the worse the stability, and the more losses.

The most common fault in photovoltaic systems is the DC side fault. Common failure modes of fuses are overcurrent fusing, aging fusing, and overtemperature fusing. Overcurrent fusing is a protective fusing that occurs when the load, short circuit, or other conditions exceed the rated value. Aging fusing refers to a fault fusing that occurs when the fuse has aged and its current-cutting ability has decreased in the absence of overcurrent. The current of a fuse is closely related to its temperature. If a fuse works at a high temperature, its current-cutting ability will decrease, and the possibility of a fault fusing is relatively high.

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 occurs to the ground 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-channel strings need to be equipped with fuses for protection.

Combined with actual conditions and scientific design, inverters with different MPPT architectures are selected according to different terrains and component shielding conditions to reduce power station procurement and maintenance costs and improve economic benefits.

(1) In areas with good lighting conditions and no obstructions on flat ground, it is recommended to choose a single-channel MPPT, single-stage inverter, which can improve system reliability and reduce system costs, such as Growatt 50K-60KTL3-HE series inverters;

(2) For power stations on hills with complex terrain, such as large power stations such as the Leader Base, there are problems of inconsistent orientation and partial shielding. In addition, different hills have different shielding characteristics, which leads to component mismatch problems. Multiple MPPTs have to be selected. In this case, an inverter with 2 string inputs per MPPT is a better choice. It has no fuses, high fault location accuracy, and simpler maintenance; such as Growatt's MAX 60K-100K TL3 series inverters.

(3) For hilly power stations and rooftop power stations with not very complex terrain and no obstruction of components, it is recommended to choose an inverter with one MPPT and multiple strings, which can take into account both string mismatch and high efficiency and has more flexible design, such as the Growatt 30-50KTL3-S series inverter.