USE OF STRING INVERTERS IN UTILITY-SCALE PV SYSTEMS

REFUsol GMBH 02/2013

Depending on the

application, it is

also worthwhile to

use string inverters

in many utility-scale

PV systems.

REFUsol TECHNOLOGY2

STRING INVERTERS IN UTILITY-SCALE SYSTEMS

In large-scale PV projects worldwide, central inverters are

mainly used. Switching cabinets are usually combined in large

concrete stations and installed as central units in the PV park.

A detailed examination of this practice is worthwhile, however,

as it also makes sense in many cases to plan specific large

projects with string inverters.

As well as very high transport and installation outlay,

switching cabinets in these concrete stations also require costly

cooling and on-site servicing, and put a large section of the

whole PV system out of action if they break down.

Instead of a central inverter, it can definitely be advan-

tageous to install smaller inverter units in the PV park on a

decentralized basis. This significantly reduces transport and

installation outlay, eliminates the need for costly cooling sys-

tems, and the maintenance-free string inverters can be quickly

and easily replaced if they fail. This also considerably reduces

the risk of a breakdown, as string inverters can be easily re-

paired while a replacement inverter ensures that yields are

maintained.

Examination

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Reduced investment costs with string inverters

Q QQ Simplified project planning or re-planning compared to cen-

tral invertersQ QQ Partial delivery of inverters possible, or partial commission-

ing of the park (depending on progress of construction)Q QQ Easy transportation to the construction site, no 40-ton truck

or provision of special routes for a truck or a crane required

in the park, therefore better use of spaceQ QQ Outdoor inverters, easy installation on the module frame,

no housing and no external cooling neededQ QQ Integrated data logger, no external string boxes/generator

junction boxes or monitoring systems requiredQ QQ Easy installation (by 1 person) and commissioning with few

settings, no commissioning by manufacturer requiredQ QQ Cheap standard transformers instead of double-wound

transformersQ QQ Flexibility for east/west-oriented PV systemsQ QQ Lower cabling costs on the DC side fully compensate for the

increased cabling work on the AC side

Increased yield with string inverters

Q QQ Higher efficiency (incl. DC supply)Q QQ Individual MPP tracking for each individual string inverter in-

stead of central MPP tracking for the central inverter (shade/

clouds, snow, dirt, module mismatch etc.)Q QQ Detailed string monitoring due to high number of inverters

without cost-intensive generator junction boxes safeguards

yield at string level and enables module faults to be isolat-

ed to one of 4-5 stringsQ QQ More efficient control and longer feed-in duration thanks to

broader voltage range than with central inverterQ QQ Flexible system planning on sites with rough terrain, differ-

ent gradients and non-rectangular boundaries

Reasons for string inverters in utility-scales PV projects

Advantages of string inverters with regard tobreakdown risk and servicing

Q QQ In the event of a breakdown, only a small area of the system

is affected (e.g. 20 kW)Q QQ Higher availability of the whole system thanks to simple re-

placement process Q QQ Easy replacement of string inverters safeguards yields dur-

ing a repairQ QQ More flexibility throughout the whole service life of the

PV system (e.g. variable module degradation, replacement

modules, replacement inverters etc.)

Quality and durability

Q QQ Convection cooling of the string inverters prevents contam-

ination inside the devices and minimizes wearing parts so

that no maintenance whatsoever is requiredQ QQ Exhaust air and heat are expelled directly from outdoor

string inverters because of their compact design, so there

is no need for a fragile and high-maintenance cooling systemQ QQ With their IP65 housing and condensation protection, REFU-

sol string inverters are designed for all climate zones (dust,

sand, salt mist, air humidity and temperature cycles)Q QQ All REFUsol string inverters are built to last over 20 years,

as verified by MTBF calculations in accordance with EN/IEC

61709Q QQ Experience from almost 50 years in the inverter manufactur-

ing business and over 15 years in the photovoltaics industry

guarantees quality at the highest level.