What is the difference between MPPT input and string input?

At present, different manufacturers have different technical routes for string inverters. There are unipolar and two-level inverters. The usual practice is to use a two-level electrical structure. The front stage is a boost booster, and the rear stage is a DC-AC inverter. Variable, the maximum power tracking MPPT is generally placed in the front stage. If the voltage input of the components is relatively high, such as single-phase exceeds about 330V, three-phase exceeds about 630V, the inverter will bypass the front-stage boost circuit and directly DC- AC inverter, at this time, the maximum power tracking MPPT will be in the latter stage.

Multiple inputs of the string inverter, each input interface is called string input, such as 30kW inverter, usually has 6 to 8 inputs, but the booster device of the front stage is not necessarily the number of string inputs In the same way, we usually call the number of booster devices the number of MPPT channels. Different manufacturers have different technical routes. There are one group connected in series with one booster device, and there are two groups connected in series and then connected to one booster device. , and there are three or four groups of strings confluent and then connected to a booster device.

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 an MPPT, the better; in terms of stability and efficiency, the number of strings behind an MPPT The more strings, the better, because the more MPPTs, the higher the system cost, the worse the stability, and the more loss. In practical applications, it is necessary to select an appropriate solution based on the actual terrain.

The advantages of MPPT with fewer strings and more strings:

1) Less functional loss: There are many MPPT algorithms, such as interference observation method, incremental conductance method, incremental conductance method, etc. No matter which algorithm is used, it judges the change of sunlight intensity by continuously changing the DC voltage. Therefore, there will be errors. For example, 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 optimal operating point. If there is more MPPT, there will be more losses.

2) Less measurement loss: When MPPT is working, the inverter needs to measure current and voltage. Generally speaking, the greater the current, the greater the anti-interference ability and the less error.

• Less circuit loss: The MPPT power circuit has an inductor and a switch tube, which will generate losses during operation. The more MPPT circuits, the greater the loss. Generally speaking, the greater the current, the smaller the inductance, and the less the loss.

The advantages of MPPT with multiple strings and fewer strings:

1) Each MPPT circuit of the inverter operates independently and does not interfere with each other. It can be different types and numbers of strings, and the strings can be in different directions and inclination angles. Therefore, the number of strings is small, and the system design has More flexible.

2) Reduce DC side fuse faults: The most common fault in photovoltaic systems is DC side faults. One MPPT is configured with 1 to 2 strings. Even if a component is short-circuited, the total current will not exceed 15%, so no configuration is required. fuse.

3) Accurate fault location: The inverter independently detects the voltage and current of each input channel, can sample the current and voltage of the string in real-time, and timely find problems such as line faults, component faults, and occlusion. Through the horizontal comparison of strings, comparison of meteorological conditions, comparison of historical data, etc., the detection accuracy is improved.

4) Matching power optimizer is more suitable: One of the current solutions to eliminate the influence of mismatch at the component end is to use a power optimizer. The photovoltaic optimizer can convert low current into high current according to the needs of the series circuit, and finally, optimize each power The output end of the inverter is connected in series and connected to the inverter, and multiple strings are connected to the optimizer. According to the principle of the same voltage in the parallel circuit, when a certain string is shaded and the power drops, the optimizer changes the voltage, and the total circuit voltage will drop, and it will also affect the voltage drop of other circuits of the same MPPT, resulting in a power drop.

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