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Photovoltaic inverter leader MPPT and power generation
- May 16, 2018 -

From the perspective of the initial input cost analysis, long-term maintenance cost analysis, long-term reliability, zero-voltage ride through, and transmission loss perspectives of the distributed inverter system, the author will talk about it, and the centralized solution will not change the traditional focus. In the case of a stable topology with energy aggregation and centralized grid connection, the MPPT function is pre-inverted from the inverter to the combiner box, making each “distribution-type convergence box” (centralized solution) a professional name for DC convergence equipment. In front of the “confluence box”, a “centralized type” is added to indicate the difference.) Up to 2 MPP traces per 2-way string can be achieved. This provides a 16-way incoming and outgoing hub with 8 MPPT traces if 1 megabyte. The tile square array needs 12 types of 16-way feeder hubs, so the 1MW square array has almost 100 MPP traces. Compared to the centralized 1MW, there are only 8 MPP traces at most, and the hub-and-spoke type significantly increases the number of MPPTs. , which greatly reduces the impact of parallel misconnections on PV power generation.


MPPT


MPPT (Maximum Power Point Tracing), ie maximum power point tracking. Because the utilization of photovoltaic cells is not only related to their internal characteristics, but also by the environment such as sunlight, temperature and other factors, its output characteristics and the panel temperature and light, intensity has a strong correlation, and has a non-linear characteristics.


According to the parameters of the photovoltaic cell, the typical I-V and P-V characteristics of the photovoltaic cell are shown in FIG. 1 under the same temperature and different light conditions.

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According to the parameters of the photovoltaic cell, the typical I-V and P-V characteristics of the photovoltaic cell under the same light intensity and different temperature conditions are shown in Figure 2:

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When the temperature is the same, with the increase of irradiance, the open circuit voltage of the photovoltaic cell is almost constant; when the irradiance is the same, with the increase of the temperature, the short circuit current of the photovoltaic cell is almost unchanged. It can be seen that the temperature change mainly affects the output voltage of the photovoltaic cell, and when the irradiance changes, it mainly affects the output current of the photovoltaic cell. When the light and temperature do not change much, the photovoltaic cell is approximately regarded as a direct current source.


From the P-V characteristic curve of the typical photovoltaic cell mentioned above, it can be seen that for maximum utilization, the photovoltaic cell needs to operate at different and unique maximum power points (MPP). Therefore, for all photovoltaic power generation systems, the optimal operating state of the photovoltaic cell array should be sought in order to maximize the conversion of light energy into electrical energy. The maximum power point will uniquely correspond to a working voltage. This is the maximum power point voltage because of the The maximum power point operating voltage of the battery array will be changed due to factors such as sunshine intensity, device junction temperature, and external load, so the MPPT function is to track the maximum power point voltage in real time, so that the photovoltaic cell module always operates at the maximum power point voltage, making the maximum Limit the use of photovoltaic panels to generate power.


Multi-channel MPPT increases power generation


Because the photovoltaic cell array is composed of unit photovoltaic cell strings, each string has its own operating voltage, and the voltage is largely inconsistent. If the traditional centralized scheme is adopted, the strings will be connected in parallel when the energy is collected in the combiner box. A parallel mismatch occurs, and a parallel mismatch occurs when the combiner box and the inverter are connected in parallel, which seriously affects the power generation efficiency of the photovoltaic modules. As shown in Figure 3 below, in the case of single-channel MPPT, the photovoltaic cell module is affected by various factors, resulting in the occurrence of two or more peaks. Tracking to any one of the peaks will cause loss of power generation. In the case of multi-channel MPPT, the characteristics of each string can be accurately traced to form a plurality of MPPT curves, so that the maximum performance of each string of photovoltaic battery strings is exerted.

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There are many factors that lead to parallel mismatch of PV arrays, such as shading, inconsistencies in attenuation and inconsistencies in the manufactured products of PV modules, inconsistent tilt angles of PV modules due to topography, and inconsistencies in temperature and light in large PV power plants. and many more. If only one MPPT is tracked using a MW matrix, the parallel mismatch effect can be very large. If shadow shading, inconsistent temperature/light, and inconsistent performance of components are all probabilistic factors, the parallel losses caused by inconsistent tilt angles of photovoltaic panels in mountainous terrain are basically deterministic factors.

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The distributed multi-channel MPPT technology is designed to be accurate to one MPPT tracking for every two PV strings, and the regulated output is obtained after the distributed convergence box is boosted again to solve the problem of parallel mismatch. As shown in Figure 4 below, multiple MPPTs can form multiple MPPT curve traces, and each MPPT curve is formed by two clusters. This refinement makes the maximum performance of every two strings play a role in solving the parallel mismatch problem of photovoltaic power plants. , greatly increase the power generation of photovoltaic power stations, thereby greatly enhancing the benefits of the power station.

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Some research institutions have used software to actually measure the effects of multi-channel MPPT and selected a photovoltaic power plant site in Inner Mongolia. The test concluded that multi-channel MPPT can indeed increase a lot of power generation.

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Figure 5 shows a screenshot of the monitoring software. The figure shows that the output powers of power unit 2 and power unit 3 are the same, but the input voltages of power unit 2 and power unit 3 are quite different (input voltage of unit 2 is 621V, unit 3). The input voltage of 586V) fully proves that even the configuration of the same battery string may cause the maximum power point voltage to be inconsistent due to the complex environment of the power station.


Summary


With the rapid development of photovoltaic grid-connected power generation technology, from the original traditional centralized 1MW square array up to only 8 MPPT to the distributed 1MW square array with 200 MPPT, is a true portrayal of human excellence and excellence. The hub-and-spoke solution has increased the number of MPPTs from an order of magnitude. The ultimate refinement of the maximum power point tracking has greatly reduced the parallel mismatch of the solar cell battery modules. From the data obtained from many institutional tests, it can be improved by at least three. % of electricity generated. If the mountainous area, front runner base, and roofed steel tile project are used, the increased power generation capacity will be higher, and it will be more suitable for complex terrain. The tilt angle of the PV panel cannot be installed in the same project site application.


It is believed that the distributed scheme will occupy a very important place in the photovoltaic grid-connected power station.