A guide to choose inverter: Central or String?

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Mr. Sunil Rathi, Director- Sales and Marketing, Waaree Energies Limited

The solar PV module is known to produce DC power. In order to convert this DC power into AC power, an inverter is used in the power plant. However with increasing demand in conjunction with up-gradation in technology, the role of an inverter has expanded. Considered to be the brain of the power plant, an inverter is also responsible for communication, grid management, safety, monitoring of energy to name few responsibilities. While such responsibilities have increased, the category in which an inverter can be classified has remained more or less the same i.e. a central inverter and a string inverter. With the continuous tumbling down of solar energy prices, the plant designer has to be cautious on what components actually go into the power plant. This is because the product in place should both be technically compatible and commercially feasible as changes in any of these conditions could render the plant’s viability. This makes their selection crucial. With the two available categories, the selection between central or string inverter in a power plant is always a challenge to the designer.  Given the advantages and disadvantages of each of this type of inverter, we believe that such challenge would not end soon. Thus for a novice, it becomes crucial that he understands these different types of inverters and further also the considerations which go into selecting them. This blog hence aims to educate its reader the basics of and comparison between central & string inverter. It would also act as a guide for a novice in the solar field who intend to set up a PV power plant.

Central Inverter

A central inverter is the one where DC output from numerous PV strings are taken into single combiner box and fed into it. This DC power is converted to AC power. This power is further converted to grid compatible power with the help a medium voltage transformer. A central inverter is generally placed in protected environment (conditioned environment to be specific) which is also near the main electricity service panel. While the operating voltage range of central inverter are comparable (to string inverter), their input current would range to around 4 kA. Central inverter are known to be installed in utility scale power plants in ranges from few 10’s of MW to GW scale plant.

String Inverter

A string inverter as the name suggests is usually connected to fewer strings of PV array. This AC power from various inverters are pooled at the AC combiner box which may be grid compatible or a transformer may be used to convert it to low voltage grid compatible power. String inverters can be installed almost anywhere near their subsequent strings. Their power ratings start from as low as 2 kW and go all the way up to around 80 kW. Such inverters could be used both in small, medium and utility scale power plants.

With a basic understanding of these inverters, let us now move forward to points to be considered while selecting an inverter.

  1. Capital cost: With the market being inclined more towards highly efficient technology at comparable cost, its consideration in selection of inverter is crucial. It is known that the price on inverter depends on its power capacity. This means that the string inverter should be more costly than a central inverter (which was a case before years). However the cost of string inverters have been falling and it is expected that they would match the prices (or be slightly costlier) with the central inverters by 2020. Such reasons can be attributed to economies of scale of manufacturing along with increased demand of string inverters.
  • Operation & Maintenance (O&M) and replacement: The next most important consideration is O&M and replacement. A central inverter needs a separate room for its smooth operation. The power modules which handle such huge DC power and convert it to AC power needs active cooling (with dust free air). The fans and filter used for such operation required periodic cleaning. Additionally, the power module along with certain boards may need immediate replacement after end of its life or in case of failure. All such spares are required to be maintained by the O&M team which could add to cost. The much smaller string inverter eliminates the need for cooling and other components required. Deemed to be maintenance free, a string inverter leads to direct replacement in case of failure. However, if not maintained in stock, a slight deviation in technology of replaced inverter (say communication technique) could require certain up-gradation to power plant.
  • Plant performance: The performance of power plant (if all the other parameters are kept same) may be gauged by efficiency of the inverter used. Both central and string inverter have efficiency well between 98~99%. Thus the energy output of both the plant ideally remains same. However when in operation slightest variation in any parameters (say variation in tilt angle, orientation between different strings, shadow on few strings, etc.) may lead to mismatch in power output within strings. Such variations when experienced in plant housing central inverter would expedite the power loss at plant level. Whereas in case of plant housing string inverter, only a portion of power plant would experience such loss. Additionally with failure in inverter itself, a central inverter would take down the entire plant with it however only a portion of plant would be non-performing in case of shutdown in string inverter.
  • Interaction with modules: String inverters are generally known to be transformer-less while central inverters have galvanic isolation transformer built in them. Such isolation in central inverter gives a chance of negative grounding of PV strings. Such negative grounding is not possible when using string inverter which may lead to PID-s (as we explained to you in our article “What’s and why’s of PID!”). However with PID free modules available in market today, such situations could be offset.

While with all these and other factors give upper hand to one technology over other, the fact that the industry was dominated by central inverter cannot be ignored. However with the advancement in technology, the share of string inverter over central inverter is poised to increase in next few years.

Waaree Energies along with supplying PV modules of international grades is also supplying inverters. They, similar to our modules are certified by various agencies for quality and safety standards. The inverters range from single phase 1 kW to 75 kW three phase inverters. You can find more details of our inverters at https://www.waaree.com/inverters.

Let us all pledge to make solar energy the primary source of energy in the near future.

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