Solar on-grid inverters are today the most popular inverter topology for solar systems. They are basically solar inverters which have a solar controller (usually MPPT nowadays) plus a conversion section and then a synchronizing and control section to regulate and manage the AC Output power being generated.
Solar power system installers and owners often face the difficult situation of choosing whether to opt for string inverters or central inverters.
String inverters are usually referred to as grid tie inverters with power ratings starting from as low as 2kw and going upto around 80kw. They generate AC output in a directly usable Low voltage grid
Whereas central inverters are typically inverters of higher power ratings. Nowadays most central inverters start from power ratings of 250kw. In fact many inverter manufacturers offer central inverters in power rating 500kw to 870kw with Low voltage 3 phase output. For higher power rating such as 1MW or higher, many manufacturers have started offering a solution which contains not only the inverter , but other options such as a step-up transformer to convert the low voltage to medium voltage or various kinds of circuit protection switchgear. Even some inverter manufacturers are offering central inverters in a containerized solution which is ready to deploy at site and contains the necessary protection switchgear and transformer.
Which one to choose?
The biggest confusion in the minds of solar engineers, solar designers, solar integrators , solar project developers is whether to go for a string inverter or central inverter.
In case of rooftop or smaller systems the decision is very simply and clear. Because of the lower power ratings it is simply a choice of selecting the closest power rating inverter. For example in case of a 20kw solar panels installation you can choose a 22kw solar inverter. Or in case of a 29kw solar panels one could choose a 34kw solar invertor.
Also the string inverters are most suitable for locations where the solar power is intended to be used for a captive or industrial application consuming power inside at low voltage levels. The string inverters save the loss of converting power to a higher voltage level and then again stepping it down to lower voltage level for consumption within the premises.
But when it comes to larger solar plants of capacity 1MW or higher, it becomes a difficult decision.
String inverters offer various advantages such as modularity, better redundancy, reduction in space requirements, reduction in DC cabling losses, etc.
Central inverters offer the advantage of reducing the number of inverters and making it a centralized approach, hence perceived to be easier to manage.
As a thumbrule, for most solar projects the ideal choice is string inverters.
First of all for rooftop or most captive solar power projects string inverters are definitely a better choice.
Further, String inverters offer a high level of redundancy and plant availability due to the fact that in case 1 of the inverters goes down the plant continues to operate to a high capacity level. Eg. In case of a solar plant with 50 string invertors. In case 1 of the invertors breaks down, then also the other 49 inverters continue to work, hence providing a 98% system availability. Whereas if it was a central invertor, then the solar plant may have had 100% or 50% of the plant down.
With the advent of higher efficiency string inverters the DC-AC conversion losses are nearly similar to central inverters.
String inverters have another advantage of having a better solar energy harvest as compared to central inverters due to the conversion happening at a lesser number of strings. For example, a 34kw string inverter will typically have 4 to 6 strings. These solar strings will be connected to 2 independent MPPT inputs. Hence each MPP tracker is operating with 2 to 3 strings. Whereas with central inverters usually each MPP tracker will have about 15 to 20 strings. Hence in case of 1 of the strings having some technical fault or shading the performance of 15 to 20 strings will be affected in case of a central inverter.
From a serviceability point of view also it is easier to mantain spare PCBs or maybe even a complete unit as a spare to replace in case of string inverters. But it is very difficult to mantain a central inverter as spare due to the high cost and large space requirement.
However, in case of very large solar plants such as 50MW or higher capacities, of course it is much better to choose central inverters such as 1MW or 800kw, etc considering the large number of inverter requirements.