Central inverter

Both technologies are acceptable. On one hand the string inverter will deliver better performance, easier installation and built in string level monitoring. There is also 20-40 times as many parts as there are in central inverter system. The central inverter is going to require less capital and less O&M cost.

Going by opinions without analysis could lead to false impressions. My guess is central inverter is used if the initial cost of the project is a driving factor, especial if there is low $/kWh. If the O&M company wants to monitor and maintain a much larger fleet of equipment, then the string inverter may be a good choice. What you really need to ask is what happens after 10-years of operation? Are we going to have to rebuild one central inverter or do we need to replace a large percentage of the string inverters?

Luckily, I was able to refer them to an electrician that I knew, and they were able to replace their inverters with reconditioned ones for a reasonable price. However, the reconditioned SMA2500U inverters only had an additional 1 year warranty. Will they be in the same position 2-3 years from now? Ouch! And yes, I understand that a homeowner may have problems getting someone to replace a failed micro that might be in the middle of an array. But at least you know exactly what is wrong through the free lifetime panel level monitoring, and the micros are warrantied for 25 years instead of 10...

According to NEC 2011, the new AFCI requirements are for DC systems above 80V and therefor N/A for micro inverters. The string inverter manufacturers are diligently working on compliance with the new standards - Gozuk included. In fact, we have several in testing around the US. While pricing is always a concern with PV systems, it is too early to tell what (if any) significant ROI impacts there will be. String inverters are typically much cheaper than micros anyway so I do not want to make any speculations on added costs.

I would like some clarification on your opinion that micro inverter "win" as I am always interested in that perspective since we are launching our own this summer.

In terms of money, string inverters will get more and more expensive vs central inverters as the field size increases. It's a heck of a lot of additional labor to hook up 100 string inverters of 10kW vs. 3 or 4 central inverters (which can also be bought in pre-configured 1MW units with some of the connections already done).

Personally, I would draw the line around 250 - 400 kW. Below that string inverters work out well. Above that central inverters have their place (and will save money). Can it be made to work either way, and work well? Sure!

I'm designing a 1MW SPV power plant (Utility Scale). I get confused over choosing inverter. I divided 1MW ito 4x250KWp array for which I've selected 4 Central inverters rated @ 250KW nominal power.
Is the selection is correct or should i Consider string inverters for the 1MW SPV plant? For string inverters, I chose 20KWp arrays with 20KW(AC nominal output) Inverters.

By any means, String Inverters are more efficient and down time is minimal due to its distributed generation architecture. Central System over throws String only by economics (of Inverter to Inverters alone). The other costs like Combiners with Communication / Cable (of high size to minimize DC losses from SPV Strings) may be looked into. In the long run, String will be more economical, this is my humble view.

Using string inverters instead of central inverters for big power plants is a totally different concept, that affects to the whole management of the PV plant in some aspects:

Designing: with multi-MPPT string concept a better management of the shades is obtained. If lower row of the structure is connected to a different MPPT than upper rows, you can place structures closer, obtaining a best ratio of W/m2. Furthermore, cabins are smaller (and provoke less shadow) and marginal areas of the ground can be used (if the inverter is flexible enough), thus more panels can be installed in the same area, peaking down the costs (€/Wp) of LV cables, MV cables, fencing, security perimetral systems, etc. Moreover, it is not necessary to leave special rows for installation, because machinery used is lighter.

Costs of installation: even though string inverters are more expensive than string inverters, they get to save money in some points of the installation. Smart DC boxes can be omitted as long as inverter as inverters have all the protections already included and monitoring is done at string level (this also means that you don't have to use cables to communicate the values from the boxes to the inverter and that you don't have to feed the boxes with power), cabins are smaller, cables are thinner ( thus no use of special machines to install them) and no big cranes are necessary.