How Inverter’s MPPT Structure Affects Power Generation

At present, different inverter companies have different technical routes for string inverters. Normally, for residential, the best sellers are single-phase inverters below 6KW and 10KW which usually equip with 2 Maximum Power Point Tracking (MPPT), each MPPT take one string. As for the commercial project, it usually use 20kW to 40kW three-phase inverters which equip with 2 or 4 MPPTs, each MPPT take 2 or 4 strings. When it comes to the big PV plants, 60kW to 80kW inverters will be concerned. The number of MPPT of these kinds of inverters may range from 1 to 6 and the number of strings for each MPPT may range from 2 to 12.

Different MPPT structure will affect the solar system production. From the perspective of solving the mismatch problem, one MPPT with fewer strings is better. However, in terms of stability and efficiency, the more strings for one MPPT, the better it is, because if with more MPPT, the higher the system cost, and the more the loss. In practical applications, it is necessary to combine the actual terrain to select the appropriate solution.

Advantages of Fewer MPPT but More Strings

1. Less power loss:

There are many MPPT algorithms, such as Interference-Observation method, Incremental-Conductance method, etc. No matter which algorithm is used, the DC voltage is continuously changed to judge the intensity change of sunlight, so there will be errors, such as when the voltage is actually at the optimal operating point, the inverter will still try to change the voltage to determine whether it is the best working point. If there is more MPPT, there will be more loss.

2. Less measurement loss:

When MPPT working, the inverter need to measure the voltage and current, the MPPT will adjust the inverter according to this measurement data, the bigger the current, the greater the anti-interference ability, hence, the accurately the data. The current of two MPPT is twice of the one MPPT, therefore, the measurement data is more accurate than one MPPT and then the measurement loss is less.

3. Less circuit loss:

The MPPT power circuit has an inductor and a switching transistor that generates losses during operation. The more MPPT channels, the greater the loss. In general, the larger the current, the smaller the inductance and the less the loss.

Advantages of Fewer Strings but More MPPTs

1. More flexibility when design system: Each MPPT circuit of the inverter runs independently and does not interfere with each other. It can be a different number of strings of different models. The strings can be different directions and inclination angles. So, if there have more MPPT, users have more flexibility in designing the  system.

2. Reduce fuse failure of DC side: The most common fault of photovoltaic system is the DC side fault. If one MPPT is configured with 1 to 2 sets of strings, when a certain component is short-circuited, the total current will not exceed the Module’s Max. short circuit current. Therefore, there is no need to configure the fuse. The common failure of the fuses is overcurrent fuses, aging fuses and overheat fuses. Overcurrent fuse is a protective fuse that occurs when overload, short-circuit; Aging fuse occurs in long-term operation, it is a kind of fault fuse, because it fuses when the current is not exceed. The reason of it is self-aging, and the cut-off capability of it reduced. The current of the fuse has a great relationship with the temperature. Therefore, if the fuse is operated at a high temperature, the current intercepting capability is lowered, and a faulty use is also generated.

3. Accurate fault location: The inverter can independently detect the voltage and current of each input, and can sample the string current and voltage in real time, and find problems such as line faults, component failures, and occlusions in time. Therefore, if the inverter has more MPPT, through the horizontal comparison of meteorological conditions, comparison of historical data, etc., the detection accuracy is improved.

Conclusion

Through the above analysis, combined with the actual, scientific design, according to different terrain and component conclusion situation, select inverters with different MPPT architecture, can reduce solar plant procurement costs and maintenance costs, and improve economic efficiency. Here are some common suggestions for common situations:

1. In flat area with no cover and good lighting condition, it is recommended to select single-channel MPPT and single-stage inverter to improve system reliability and reduce system cost.

2. For hilly solar plant with complex terrain, there are inconsistencies and partial occlusions, and different hills have different occlusion characteristics. To overcome component mismatch problem, we has to choose multiple MPPT inverters, also, each MPPT with 2 strings input will be a better choice, because there no fusible consumables, but high fault location accuracy and simpler maintenance.

3. However, if the terrain of the hill stations and rooftop stations is not very complex, also if there have no component occlusion. It is recommended to select two MPPT inverters with multiple strings, which can balance string mismatch and high efficiency, in addition, the plant designing is more flexible.

- Growatt