String 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?

Up until recently I worked for a company that almost exclusively installed Gozuk micro inverters. I did the design and system monitoring. We had 170 sites using the technology. They pretty much all outperformed their PV Watts estimate, especially if you were trying to compensate for shade. They DO outperform string inverters even without any shade. If there is no shade, the out performance is not generally worth the cost.The efficiency of converting DC to AC with some string inverters is better; string inverters can only perform MPPT on a string level, while micros perform it at the module level. Since manufacturing tolerances always ensure module mismatch, especially with the cheaper modules, string inverters can't maximize the output of each individual panel, so they can't maximize the output of the system as a whole.

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!

About the risk of system failure due to a string inverter malfunction. It can easily and cost effectively be dealt with. Our own string inverters incorporate double redundancy technology within the one unit [effectively a hot spare auxiliary on standby] and we are about to do our first triple redundancy unit [aircraft employ triple redundancy technology].

Coupled with remote monitoring and control, the DC module + string inverter system can be highly reliable with guaranteed continuity of supply and provide for machine initiated planned maintenance management communications in event of component failure.

I've been in the solar business for almost 30 years. I've installed countless systems with my own two hands so I understand being "in the field." The warranty issue has been carefully thought out. The AC Module is warranted as one product and will be replaced as one product by the panel manufacturer. The other warranty issues you raise apply equally to a string inverter. The answer, as with any product, is to buy from a reputable manufacturer.

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.

MPPT conventionally are located in Inverter that is supposed to track each PV module from an array parallel connected series strings. PP point of all panels are not /cannot be same. Therefore such tracking fails to harvest full power loss could be as high as 15%. Further this demands that all modules have matched MPPT. Further placing MPPT in each module prevented shadow loss in series strings where shadow on one module reduces energy harvest from the entire string of 20 modules or more.. LEECH HV PV modules also boost peak power voltage to voltage as high as 430VDC. That drops ohmic losses and reduces cable costs since module output current drops from 10A to an amp...conversion efficiency is also over 98% peak. All parallel Rooftop Arrays are an advancement that leads to higher energy yields.