Solar PV System Sizing: Over Sizing & Under Sizing of Inverters

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Inverter is the heart of a solar Power project. The purpose is to convert the DC power generated from the solar power plant in to the AC power and feed it to the grid as per the required protocol. Inverters are generally available in various sizes ranging from 1KW capacity to 1 MW capacity and irrespective of plant size any inverter can be selected, however the decision of inverter selection is a trade of between the cost, and efficiency.  The operating efficiency of an inverter ranges between 50%-98% depending on its loading, hence the selection should be based on the consideration of the loading pattern of your inverter. The operational range of the inverter is the deciding factor for the operation of inverter in the most efficient range.

Inverter selection is a trade off between the cost, efficiency and the reliability.

Inverter selection is a trade off between the cost, efficiency and the reliability.

Most of inverter manufacturers specify their efficiency at Standard Test Conditions (STC), this is given as 1000W/m2 at 25ºC and specified in the range of 97-98.5% efficiency. It is unlikely that the inverter will operate at this efficiency range, hence it becomes important to evaluate the inverter efficiency at different loading conditions.

Inverter efficiency is best when it is loaded between the 80%-90% loading conditions.

Inverter efficiency is best when it is loaded between the 80%-90% loading conditions.

If an n inverter is undersized, this will tend to overload the inverter and hence may affect the operation life and reliability of the inverter. However the inverter oversize up to 10% is a general practice, which allows the inverter to operate at the high efficiency range in most part of the day as the PV array produce typically 80% of its peak power rating during most of the day hours. Inverter suppliers provide the efficiency v/s load curve which is used while selecting inverter for a project.

The field measurements of inverter efficiency reviel that there is a wide variation in the inverter efficiency due to the varying operating conditions at the same site. While the STC inverter efficiency is about 98.5%, the actual measurements show that the efficiency is far lower than what specified by the manufacturers.

Inverter Efficiency actual

For the owner of a properly designed PV system in the UK, a higher yield over the course of the year will be achieved, maximising financial return, and the initial expense incurred by installing an unnecessarily large inverter is avoided.

 

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