Polaris solar PV net news: photovoltaic systems, converting solar energy into electrical energy is photovoltaic module, and an objective component mismatch, PV system’s output was reduced to some extent. Multi MPPT scheme is used, to a certain extent, reduce the mismatch effect.
What, component mismatch
Each component has its own p-v curve, the curve will change with the changes of light intensity, temperature, as shown in figure a.
Different manufacturers and different models, as well as the different production batches of PV modules, p-v curve is not entirely consistent in different lighting, different temperatures and different attenuation, characteristic curve of each component are inconsistent. In simple terms, different components at the same time within an array do not match the p-v curve, that is, component mismatch. By mismatched components in series or in parallel, the formation of new combinations of the power curve, as shown in Figure II. New combinations of curves for maximum power output, will be less than the maximum power output before the combination of the power curve, and this is the power component mismatch losses.
Second, the PV system mismatch causes
Photovoltaic array models generally use the same manufacturer in the same batch, select the same tilted angle of photovoltaic array design construction, because the component p-v curves, basic power plant operational stage may be considered not mismatch occurs. After a period of operation, PV modules with varying degrees of mismatching, discussed more in the following three cases.
Normal attenuation of discrete components
Generally speaking, attenuation of the PV modules in the first year is about 2% every year thereafter, attenuation is about 0.7%, GB provides 25 decay during the life cycle of not more than 20%. Merely attenuation did not lead to mismatch components, lead to mismatch is the attenuation of discrete components the pieces, the attenuation level of inconsistency, discreteness of larger, higher degree of mismatch. For example, a string running after 5 years, most component decay of 5%, only 1 component decay of 7%, then the mismatch is only produced in the 7% decay components with other components in series and other synchronization attenuation of 5% no mismatch between the components.
At the same array generally uses the same batch of PV modules, attenuation of discrete is relatively small, is relatively small. An attenuation of them by PID (PotentialInducedDegradation) caused by use of PID function of inverter can be some degree of recovery components of attenuation, further reducing attenuation of discrete components.
Damaged components are not functioning properly
In rare cases, due to foreign body surface adhesion component causes local heating, forming hot spot resulting in abnormal damage of the components. Damaged components may cause the component circuit breaker in series as a whole, may also bypass diodes to keep the concatenation of string or continue to work.
Due to fair wear and tear is reduced directly in-line components, or cell, is not changing the component p-v curve, the characteristics of its components are still the same. Bypass diodes as load caused some damages in series will produce minor mismatch.
Light intensity is not uniform
Due to accumulation of dust on the surface of a component, shadows and other reasons, the components received light intensity are inconsistent, resulting in each component under the p-v curve at the same time there difference form mismatch. Different levels of light intensity is obscured, the degree of mismatch is formed by different.
Noteworthy is that the dust accumulated on the lighting effect, but due to distribution, component mismatch effects instead of small to cloud-oriented lighting, occlusion shading affect coverage has a lot of randomness, and light intensity difference may be large, is the main reason components within the photovoltaic array mismatch.
Third, multiple MPPT scheme can solve the mismatch
MPPT is the core equipment of PV inverter PV system is one of the main functions, by varying the inverter’s own equivalent resistance value, affected components tracked by the voltage and current values, to find and keep the systems working in the p-v curve of maximum power point. MPPT impacts on electricity from two aspects: the MPPT dynamic static precision tracking capabilities for complex curves, depending on the inverter manufacturer on tracking algorithm of accumulation and patents; on the multi MPPT of photovoltaic array for decoupling scheme, this is for components mismatch power upgrade programme.
Photovoltaic array is made up of 21 (or 22) components in series form a group, made up of multiple groups and parallel, p-v curve is also the first series and parallel arrays of characteristic curves. Multi MPPT mismatch solution components, through decoupling array allowing more MPPT to each track, components of an individual MPPT tracks less components lower mismatch losses.
Array of decoupled beginning with decoupled set of series-parallel. With string as the smallest unit of a decoupling can solve sets of parallel and mismatch losses, decoupling the finer, parallel lower mismatch losses. This is focused on the main difference between type and string type.
When parallel decoupling for a group to the limit, when each string from an MPPT tracks separate, parallel mismatch can be completely solved. Mismatch requires further components from in-line mismatching, with components as the smallest unit of a decoupling, decoupling the finer line lower mismatch losses. Only a micro-inverter can achieve settlement series mismatch.
Summary: in the large desert plants, component mismatch was the main reason local illumination of shadow in the clouds visible.
By using multiple MPPT programmes can reduce component mismatch effects in the PV array. Using string-programmes can be solved some parallel mismatch, but no effects on the series the mismatch using micro-inverter can also resolve the mismatch in parallel and series mismatch.