In the photovoltaic system, it is the photovoltaic module that converts light energy into electrical energy, and the objectively mismatched components reduce the power generation of the photovoltaic system to some extent. Using multiple MPPT schemes can reduce the impact of component mismatch to some extent.
First, what is the component mismatch?
Each component has its own PV characteristic curve that changes with the light intensity and the ambient temperature, as shown in Figure 1.
PV modules of different manufacturers, different models, and different production batches do not have the same PV characteristic curve. Under different lighting conditions, different temperatures, and different attenuations, the characteristic curves of the components will also be inconsistent. In simple terms, the PV characteristics of different components at the same time in an array are inconsistent, which is the component mismatch. The mismatched components are connected in series or in parallel to form a new combined power curve, as shown in Figure 2. The maximum power output of the new combination curve will be less than the sum of the maximum power output of the power curves before the combination, which is the power loss caused by the component mismatch.
Second, the cause of the mismatch in the photovoltaic system
PV arrays generally use the same batch of models from the same manufacturer, and choose to use the same tilt angle for PV array design and construction. Because the PV characteristics of the components are the same, it can basically be considered that there is no mismatch in the initial stage of power plant operation. After a period of operation, there will be different degrees of mismatch in PV modules. The following three scenarios are discussed in more detail.
The normal attenuation of components
Generally speaking, the attenuation of PV modules in the first year is about 2%, and after that, the attenuation is about 0.7% per year. The national standard stipulates that the attenuation in the 25-year life cycle should not exceed 20%. Only the attenuation of the component does not cause a mismatch. The mismatch that results in the mismatch is the attenuation of each component, that is, the degree of inconsistency of the attenuation. The greater the dispersion, the higher the degree of mismatch. For example, after five years of running a string, most components attenuated by 5%, and only one component attenuated by 7%. Then the mismatch occurs only when the 7% attenuation component is in series with other components. Other synchronizations occur. There is no mismatch between the components that attenuate 5%.
In the same array, generally the same batch of photovoltaic modules is used, the attenuation dispersion is relatively small, and the impact is relatively small. One kind of attenuation is caused by PID (Potential Induced Degradation). Using an inverter with anti-PID function can restore the attenuation of components to a certain degree, further reducing the discreteness of component attenuation.
Component is abnormally damaged
In rare cases, local temperature rise due to the foreign matter sticking to the surface of the module results in hot spots and abnormal damage of the module. Damaged components may cause the entire series component to open circuit, or it may continue to work through the bypass diode to maintain the series string.
Because the abnormal damage is to directly reduce the series component or the battery slice, it does not change the PV characteristic curve of the component, and the characteristics of the components in the work are still the same. Bypass diodes provide a certain loss as a load, and in-line diodes produce small amplitude mismatches.
Uneven light intensity
Due to the accumulation of dust on the surface of the module, shading of shadows, etc., the intensity of light received by each component is inconsistent, resulting in differences in the PV characteristics of the components at the same time, resulting in a mismatch. The degree of occlusion of light intensity is different, and the degree of mismatch formed is also different.
It is noteworthy that although dust accumulation has a great influence on illumination, due to the uniform distribution, the effect of mismatch on the components is rather small; cloud-based illumination blocks shadows, and has a strong randomness that affects coverage, and light The difference in intensity may be greater and is the primary cause of component mismatch within the PV array.
Third, multi-MPPT program can solve which mismatch
MPPT is one of the main functions of the PV system's core equipment photovoltaic inverter. Through constant adjustment of the inverter's own equivalent resistance value, the voltage and current values ​​of the tracked components are affected, and the system is sought and maintained at the highest of the PV characteristic curve. Power point. The impact of MPPT on power generation comes from two aspects: MPPT's ability to dynamically and accurately track complex curves, which depends on the accumulation and patents of inverter manufacturers' tracking algorithms, and the design of multi-MPPT solutions for decoupling PV arrays. It is a power generation enhancement scheme for component mismatch.
Photovoltaic arrays are made up of 21 (or 22) block assemblies in series to form a string, and then consist of multiple strings and parallels. The PV characteristic curve is also a series characteristic curve that is generated in series and then in parallel. Multi-MPPT solution to solve the component mismatch, is to make more MPPT to track separately through array decoupling. The fewer components tracked by a single MPPT, the lower the component mismatch loss.
The decoupling of the array starts with decoupling strings in parallel. The decoupling of the string as the minimum unit can solve the parallel mismatch loss of the string. The smaller the decoupling, the lower the loss of the parallel mismatch. This is also the main difference between centralized and string solutions.
When the decoupling of the strings in parallel is performed to the limit, ie each string is tracked by an MPPT separately, the parallel mismatch can be completely resolved. To solve the problem of component mismatch, it is necessary to proceed from the series mismatch, and decouple the component as the smallest unit. The finer the decoupling, the lower the loss of the series mismatch. Only a micro-inverter solution can solve the tandem mismatch.
Summary: In a large desert power station, the main cause of component mismatch is local light shielding caused by cloud shadows.
By using multiple MPPT schemes, component mismatch within the PV array can be reduced. The use of a string-type scheme can solve the parallel mismatch to some extent, but it has no effect on the series mismatch. The use of a micro-inverter solution can simultaneously solve the parallel mismatch and series mismatch.
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