Why Grid-Tied Inverters Have High Input Voltage

GA. Using high voltage DC leads to less DC current, so thin cables and low I²R losses. Recently I build a solar converter that uses 760VDC from solar panels to run 5HP 3-phase AC pump motor. This is independent system that runs the pump from above 1/4 Sun to full Sun and pumps water in proportion with Sun energy available at more than 90% efficiency.

This is related to the size (kW) of off-grid solar system.

I believed the off-grid inverters you've seen is meant for low power off-grid solar system with batteries back-up.

The quantity of batteries (normally in 12V) required for back up in this type of low power system is normally 1 block, 2 blocks or 4 blocks.

This is why off grid inverter is designed to with 12V, 24V, and 48V input.

There is no reason to get 25 blocks (300V) when 2 blocks of batteries are already good enough for the job.

Of course, high power off-grid system will definitely use high voltage inverter since large quantity of batteries are involved.

Try -

http://www.solarpaneltalk.com/

http://www.wind-sun.com/ForumVB/

Thanks to all your responses

Please note, my question was not why panels are connected in series in grid-connected systems

My question is why we don't make a series connected in off-grid systems. I have seen 5KW 48V inverter

and 72 Volts too.

How can you get there WITHOUT series connections.

In fact one panel is already a series connected group of single cells. Typically 36.

Aims inverters makes a 12 kw inverter off grid

as well sonny island from SMA can be used in pairs and quads for offgrid

The downside to series connected panels is that if only one of them is shaded, it affects the output of the whole chain.

While I know a lot about power electronics, I am not an expert at solar installations so take this suggestion as a simple theory.

It is a lot harder to control a grid connected inverter than an off grid one. The synchronisation and control schemes are complex. Therefore, the grid connected inverters use higher (and up to date) technology. The off grid inverters can get away with a few switches, a signal generator and a transformer.

Old semiconductors didn't do well at high voltage. That is why old designs were all low voltages. Many years ago, only large industrial drives could afford the high voltage technology because they had no choice.

Therefore, the newer grid connected inverter use newer technology in the controls and power electronics designed by younger engineers who are used to seeing 600V and 1200V transistors. They are not afraid of running a DC bus at 300V or even 800V if needed.

The off grid inverters can get away with old (and simpler) designs that have been in use for 10-20 years. They are probably maintained by the older engineers who have seen many "high voltage" transistors and capacitors blow in their faces. They might not be comfortable with high voltage. Also, why re-design them if they work properly and the cost is ok? There is a lot of risk in re-designing a product.

This is why, as a company and its staff matures, we have to hire young people to bring in new ideas and ways to do things. Otherwise old engineers become too conservatives and the technological advances slow down.

I believe that low voltage parallel systems are preferred do to reliability, cost and safety.

Reliability: We all know that if one element in a series circuit fails that the whole circuit fails. Therefore paralleling solar arrays provides more reliability than putting them in series.

Cost: If low voltage solar arrays are put in series and they feed a high voltage battery bank when a low voltage series element fails it sees high voltage across its terminals. For example: If ten 24VDC arrays are put in series and one fails, the failed array will see 216 VDC (9 x 24VDC) presented across its terminals. This means that all elements that could fail need to rated to sustain this high voltage. High voltage DC fuses are expensive. High voltage disconnects are expensive. ETC.

Safety: Since many of these low voltage systems are intended to be installed on roof tops which are exposed to the elements, a failure due to a tree limb combined with rain would present a safety hazard if the array was part of a high voltage series array.

Other: It would be interesting to investigate what building codes and home owner's insurance companies have to say about high voltage vs. low voltage.

(I am not an expert in this area, but have considered the pros & cons of installing one on my home.)

It's cheaper and more efficient to go from 300 to 220v using using smaller guage wire and higher voltage/lower current transistors which are common and cheap these days.

Batteries are expensive, that's why mobile systems are normally 12 or 24 volt and a non grid tie system or Remote Area Power System are normally 48v (Suitable for user servicing) although large R.A.P.S. systems and industrial systems can be 110v to 440 or higher but the cost is justified by the technical requirement, payed for by the business and serviced by a qualified technician.

The other reason is Regulations and safety. Regulations often don't allow unlicenced installation of systems over 24 or 48v and messing around with 300v from a battery pack is very dangerous.

Regards

David