SELECTING THE RIGHT INVERTER OF YOUR CHOICE

With our ever-increasing demands for electricity, it is necessary not to completely depend on fossil fuels for generation, but to also utilize the untapped potential of various renewable energy sources. Decades ago, solar wasn’t a viable technology to meet our power requirements, but thanks to advancements in PV technology, there is a drastic change in the size of a solar cell and it’s exciting to think that perhaps one day just a few thousand acres of solar panels installed throughout the country might cover all our energy needs.

Everything has changed!

Extensive research and development have happened in the manufacturing of high-efficiency solar cells that are now capable of absorbing more sunlight to convert into electricity. This, in turn, has improved the viability and potential of solar as a mainstay of power generation. Apart from this, initiatives and programs introduced by the government are allowing homeowners, businesses, and utilities to utilize the energy they are creating through renewable energy.

But, no other solar technology component has evolved as constantly as that of the solar inverter. The inverter is a major component in the solar system. It not only converts the DC energy from the solar panels to AC power, but it must also be intelligent, smart-grid compliant, and offer communication and monitoring features, like Wi-Fi, cellular, and power-line communication. Like solar panels, inverters must also comply with a perplexing number of codes and regulations and frequently communicate equally exceptional amounts of system data points providing for an unprecedented level of system monitoring and analysis

Every year, inverter manufacturers face an incredible challenge to create a brilliant, cutting edge piece of equipment at a competitively mandated lesser price.

Selecting the Right Inverter of Your Choice

Typically, solar panel inverters represent a major cost of the solar system. In India, most utility-scale projects have traditionally employed central inverters, although there is an increasing interest in the use of string inverters for these applications.

In solar systems, the solar panels are connected in series to form strings. When using a central inverter, the DC power produced from these strings is combined at a Junction Box where they are connected in parallel with other strings. From there, the DC power is run into the central inverter to convert it to AC.

When using string inverters, there are multiple smaller capacity inverters for several strings, so that the DC power from the strings is run directly to a string inverter rather than to a combiner box and then to a central inverter.

The biggest confusion in the minds of solar engineers, solar designers, solar integrators, and solar project developers is whether they should go for a string inverter or central inverter.

In the case of rooftop or smaller systems, the decision is very simple and clear.  Because of the lower power ratings, it is simply a choice of selecting the closest power rating inverter.  For example, in the case of a 20kW solar panel installation, you can choose a 22kW solar inverter. In the case of a 29kW solar panel, you could choose a 34kW solar inverter.

Another point to note is that 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 levels for consumption within the premises.

But when it comes to larger solar plants of capacities 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.

Conclusion

As a thumb rule for most solar projects, the ideal choice is a string inverter.

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 one of the inverters goes down, the plant continues to operate at high capacity. For examples, in the case of a solar plant with 50 string invertors, if one of the invertors breaks down, the other 49 inverters continue to work. This will provide 98% system availability. If it was a central inverter, 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 better solar energy harvest as compared to central inverters due to the conversion happening in 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. In the case of central inverters, each MPP tracker will usually have about 15 to 20 strings.  Hence in case one of the strings having some technical fault or shading, the performance of 15 to 20 strings will be affected in the case of a central inverter.

From a serviceability point of view also it is easier to maintain spare PCBs or maybe even a complete unit as a spare in the case of string inverters.   But it is very difficult to maintain a central inverter as a spare due to the high cost and large space requirement.

However, in case of very large solar plants such as 50MW or higher, of course, it is much better to choose central inverters such as 1MW or 800kW considering a large number of inverter requirements.