Photovoltaics

Making a bigger inverter investment upfront may save costs later on in grid-scale PV projects.

The PV industry is no stranger to concerns about reliability. Module makers are frequently taken to task over how long their products will last out in the field.

Which is strange, because project owners have a pretty good idea about which part of the PV plant will be the first to fail… and it is not likely to be the modules.

No, the weakling of the PV industry, the component that is pretty much guaranteed to fail within the 20-plus lifespan of the plant, is the inverter.

“It really varies depending on the system, but within a small system the inverter is relatively low value,” comments Sam Wilkinson, PV analyst at IHS. “People will expect to replace the inverter once in the 20-to-25-year lifetime of the system.

“In a residential system we believe that 80% of inverters will fail between nine and 15 years after installation.”

Matthew Feinstein, of Lux Research, agrees. “Reliability is always an issue,” he says. “It is pretty well covered as there are a lot of good suppliers out there, but what it really comes down to is the price pressure in a given market. Inverters always need to get more reliable.

Scheduled maintenance

“If you are talking about your average inverter you’ve usually got two replacements. They are the component that will need to be replaced. That is why there is always scheduled maintenance.”

Paying more for an inverter is less about getting one that will last longer and more about being sure the company you bought it from will still be able to honour its warranty when a breakdown happens, he adds.

This ability to honour warranty terms is particularly important with grid-scale PV, since a central inverter failure will take your entire production offline. Fortunately, most inverter suppliers have pretty good pedigrees.

The residential market is dominated by the German maker SMA Solar Technology, which is the market leader with a €1.5bn turnover in 2012, and the Danish company Danfoss, a diversified manufacturer within interests spanning cooling, air conditioning and heating.

Neither is about to go bust any time soon, although SMA this month announced 700 job cuts in its solar division following a 50% drop in sales since 2010.

At the grid-scale end of the industry, meanwhile, suppliers include major conglomerates, such as ABB and Parker, which again offer reasonable reassurance in terms of longevity.

Central inverters

Traditionally, these companies have provided large central inverters serving an entire project, backed by a service-level agreement that could last for the lifetime of the plant and may include regular site visits and remote monitoring, with regular servicing and repairs.

However, the value of such setups is being challenged. “When you get into these bigger systems there are a couple of approaches,” says Wilkinson. “You can have very big, centralised inverters, say 1-2MW scale, and you are not going to replace those. They last a lifetime.

“But the other approach, which you do increasingly see, is using very small inverters. The approach there is you have a supplier that replaces inverters on site and if one of them goes down then you simply swap it.”

Danfoss pioneered this approach at an 80MW plant in Eggebek, Germany, and a 12MW facility at Busenwurth, using TLX Pro Series string inverters. 

At Eggebek, strings of 23 or 24 modules have been connected individually to each input of a 15kW or 10kW inverter, so each string has individual maximum power point (MPP) tracking.

For monitoring and communication, the plant has 136 master inverters, each with a daisy chain Ethernet network of followers for redundancy. Danfoss says the system is able to reroute communication in 30 milliseconds in the event of a fibre-optic cable breakage.

Cable losses--how to keep them low

The company adds that using a large number of small inverters in this way can simplify DC cabling and keep cable losses below 1% over a 300m span.

Also, having more granular MPP tracking can reduce shading loss, which at Eggebek improved output from 54MW to almost 84MW. Individual MPP tracking furthermore means project owners can mix and match module types, power rates and cell technologies.

Finally, Danfoss believes it can improve operations and maintenance. “With a decentralized concept, inverter uptime is maximised,” says the company. “TLX inverters need minimal servicing over an extended period.

“Additionally, in the event of inverter failure, replacement can be done in as little as 15 minutes with stock on hand. Otherwise Danfoss will ship a new inverter within 24 hours, minimising downtime.”

What Danfoss does not mention is that investing in multiple small trackers is likely to be more costly upfront than buying a single central unit. Over the lifetime of the project, though, the investment could pay off through factors such as improved plant efficiency, Wilkinson believes.

“It’s a bit of an industry trend, to be honest,” he says.