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Micro-inverters versus conventional (string or central) inverters

A company with over five decades of experience in various sectors, Spitzenberger & Spies GmbH is a firm believer 
in the mantra: “If there is a better way, we will find it”. The company has underlined this with its range of PV applications – as we discover in this technical feature.

Testing possibilities
One of the main advantages of modern micro-inverters in comparison with conventional string – or central-inverters 
is the advanced efficiency. This efficiency is the result of a different solar system concept: In conventional installations of solar fields the panels are connected in series (as a string) with special DC safety connections. A string-inverter is connected to this panel field. To achieve the maximum available energy harvesting, the inverter is searching for the optimum working point.
This function is called the MPP-Tracking algorithm. The dynamically changing load condition is caused by the variation of the solar position during the day, by cloudy weather conditions and the resulting shadowing and by pollution of the panels. 
A power loss of the whole system is the result. Even in the case of a partial disturbance, the whole panel string is affected by the power loss.

Each micro-inverter harvests optimum power by performing maximum power 
point tracking for its connected panel
This brings us to the two main disadvantages: the initial costs – which are significantly higher than with conventional installations – and the maintenance problem – micro-inverters are mounted on the roof. On the other hand, no special DC safety connections are necessary for the interconnection between panels and inverter.
The inverter industry meanwhile reacts on the higher initial costs and produces micro-inverters with two independent panel inputs (one micro-inverter for two panels and independent maximum power point tracking on each connected panel). The calculation of such a system gives nearly the same initial costs than with conventional string inverter installations. One more difference between conventional inverters and micro-inverters is the power rating of the inverter.

Fast response time
Due to the fast DSP based regulation system, the response time to load changes is very fast.
For the IEC/EN 50530 and the specified MPP tracking algorithm this fast response time is absolutely necessary.

Calculation example
A string of 10 panels with 250W power each delivers a string power of 2500W. The power loss of 15% of one panel reduces the overall power to 2125W which is 15% total power reduction.
The same system with micro-inverters gives a different result:
The power loss of 15% of one panel reduces the power of this influenced panel to 212.5W, the others are delivering full power. The system overall performance is reduced only to 2462.5W – this is a power reduction of only 1.5%.


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