Currently string inverter, different manufacturers of different technical routes.Generally home to single-phase 6kW inverters and three-phase inverters below 10kW most of the following, using two MPPT, each with a road MPPT string; Small business projects, the general use of 20kW to 40kW inverter, MPPT number 2 to 4, each MPPT with 2 to 4 strings; large power plants, generally will be selected 60kW to 80kW high-power string inverters , MPPT number 1 to 6, each MPPT with 2 to 12 strings.
The choice of different MPPT routes has some effect on the power generation of the system.From the point of view of solving the problem of mismatch, the less one string after MPPT is the better, and the stability and efficiency of one MPPT The more strings the better, because the more the MPPT number the higher the system cost, the worse the stability, the more loss .In practical applications, with the actual terrain, select the appropriate program.
First, MPPT less string more advantages:
1) less functional loss: many MPPT algorithms, interference observation, incremental conductivity method, conductance incremental method, etc. No matter what kind of algorithm, are constantly changing the DC voltage to determine the intensity of the sun changes, So there will be errors, for example, when the voltage is actually at the best working point, the inverter will still try to change the voltage to determine is not the best working point, more MPPT all the way, it will all the way to loss.
2) Low measurement loss: When the MPPT is working, the inverter needs to measure the current and voltage. Generally speaking, the bigger the current is, the more the anti-interference ability is and the less the error is.
3) Low circuit loss: The MPPT power circuit has an inductor and a switch tube, which will cause loss during operation. The more MPPT circuits, the more loss. In general, the larger the current, the smaller the inductance can be. , Less wear and tear.
Second, MPPT multi-string less advantages:
1) Each MPPT loop of the inverter runs independently and does not interfere with each other. Strings can be different types, different numbers, different directions and tilt angles. Therefore, the number of strings is small and the system design flexibility is more Big.
2) Reduce the fault of the DC link fuse: The most common fault of the PV system is the DC link fault. One MPPT is configured with 1 to 2 strings. Even if a short circuit occurs on a certain component, the total current does not exceed 15%. Therefore, it is not necessary to configure Fuse, fuse common failure mode is divided into over-current fuse, aging fuse, over-temperature fuse. Over-current fuse is in the overload, short circuit and other conditions beyond the protection of the occurrence of the fuse; aging fuse refers to the long-term work , With its own aging, the interception of capacity decline in the case of no over-current fault occurred fusing; fuse current and temperature have a great relationship, if the fuse work at high temperatures, the interception of decline, the occurrence of fault fuse The possibility is relatively large.
3) Precise Fault Location: The inverter independently detects the input voltage and current of each channel and can sample the current and voltage of the string in real time to find out the line faults, component faults and occlusions in time, etc. By comparing the strings horizontally and comparing the meteorological conditions , Historical data, etc., to improve the detection accuracy.
4) Matching power optimizer is more suitable: One of the solutions to eliminate the effect of mismatch on the component side currently is to use a power optimizer that can convert low current to high current according to the requirements of the series circuit, and finally optimize each power The output of the device is connected in series and connected to the inverter. A plurality of strings are connected to the optimizer. According to the principle that the voltages of the parallel circuits are consistent, when the power of one string is shaded and the power is decreased, the optimizer changes the voltage. The voltage will be reduced, will also affect the same MPPT other circuit voltage drop, resulting in power down.
to sum up:
According to the actual situation and scientific design, according to different topography and component obstructions, inverters with different MPPT architectures are selected to reduce the purchase cost and maintenance cost of power station and increase the economic benefits.
(1) In the area with no obstructions and good lighting conditions, it is suggested to choose a single-phase MPPT inverter with single-stage structure to improve system reliability and reduce system cost.
(2) Complicated Hillside Power Station, such as the leader base and other large power stations, has inconsistent and partially obscured phenomena. Different hills have different occlusion characteristics, causing component mismatch. They have to choose multiple MPPT, Then each MPPT2 string input inverter will be a better choice, no fuse wearing parts, fault location accuracy, easier maintenance;
(3) The terrain is not very complex Hill power plant and roof power station, there is no component shelter, it is recommended to choose two MPPT multi-string inverter, which can take into account the string mismatch and high efficiency, more flexible design.
