Use of Fronius string inverters in largescale PV systems
and their effect on the system as a whole
© Fronius International GmbH
Version 1.0, 7/2020 , Jürgen Wolfahrt, Volker Haider, Jasmin Gross
Solar Energy
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TABLE OF CONTENTS
INTRODUCTION .............................................................................................................................. 4
1
2 INVERTERS FOR LARGE-SCALE SYSTEMS ............................................................................... 5
2.1 The string inverter ............................................................................................................................. 5
2.2 The requirements .............................................................................................................................. 5
2.2.1 Flexibility in planning ......................................................................................................................... 5
2.2.2 Simple installation and commissioning ............................................................................................. 5
2.2.3 Maximum performance for high yields .............................................................................................. 5
2.2.4 Prompt service for dependable yields............................................................................................... 6
2.2.5 Savings potential............................................................................................................................... 6
3 DESIGN OF LARGE-SCALE SYSTEMS WITH STRING INVERTERS .......................................... 7
3.1 Decentralised system design ............................................................................................................ 7
3.1.1 Decentralised individual string inverter configuration ....................................................................... 7
3.1.2 Decentralised grouped string inverter configuration ......................................................................... 8
3.2 Centralised system design ................................................................................................................ 8
3.3 Mixed and special types of system design ....................................................................................... 9
3.3.1 Decentralised design with AC daisy-chaining ................................................................................. 10
3.3.2 Centralised design with AC daisy-chaining .................................................................................... 11
3.4 Criteria for deciding which system design to select ........................................................................ 11
3.4.1 Accessibility in the case of roof-installed systems .......................................................................... 11
3.4.2 Required structural measures ......................................................................................................... 11
3.4.3 Distances and dimensions .............................................................................................................. 12
3.4.4 Conditions at outdoor sites ............................................................................................................. 12
4 EXAMPLE OF A MULTIPLE MW SYSTEM WITH STRING INVERTERS .................................... 13
4.1 Key data .......................................................................................................................................... 13
4.2 The system design .......................................................................................................................... 14
4.3 Installation & commissioning .......................................................................................................... 15
4.4 Advantages of the system concept ................................................................................................. 15
5 Summary ........................................................................................................................................ 17
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1 INTRODUCTION
There is no one single aspect that determines the optimum design of a large-scale PV system, as in every
project there are numerous technical and commercial considerations that have to be compared and modified
to reflect the individual wishes of the customer and the local circumstances. Apart from selecting the most
appropriate inverter, the choice of system design is pivotal to the project.
This paper examines the operational capability of string inverters in large-scale PV systems. The inverters are
represented in varying system designs and the technical and commercial product requirements studied. The
benefits and effects on the commercial system as a whole are discussed. Finally, the practicality of string
inverter concepts is exemplified with the help of an example system.
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2 INVERTERS FOR LARGE-SCALE SYSTEMS
When considering large-scale PV systems, a basic distinction is made between two system concepts: those
in which string inverters are used and those that favour a central inverter. The following sections look at the
broad operational capability of string inverters and consider their application potential in large-scale
commercial systems.
2.1 The string inverter
The modern definition of a string inverter is an inverter whose output ranges from a few kilowatts to around
100 kW. It can be fitted directly to a wall, to a vertical support, or even installed horizontally. A string inverter
is generally compact, making it easier to transport and requiring only 1 to 3 people for its installation.
They can be found in residential settings as well as in commercial projects. In large-scale PV systems, string
inverters are used in roof-mounted systems and in outdoor installations.
2.2 The requirements
Because large-scale PV systems need to be implemented in a cost-effective manner, factors such as flexibility,
ease of use, functionality, performance, and price/performance ratio are crucial when choosing the best
inverter for the job.
2.2.1 Flexibility in planning
String inverters cover a broad application area. They can be installed on a roof, inside or outside a building,
right next to the PV modules, directly at the transformer station, on a support or horizontally on the ground.
Their relatively low weight and compact dimensions make the whole planning operation considerably more
flexible, as no modifications to infrastructure have to be made or heavy lifting gear arranged in advance. String
inverters are particularly advantageous for installations where the strings within a PV system have varying roof
or slope orientations. If the project requires, string inverters of differing sizes and functionalities can be mixed
within a PV system, and any revisions can be carried out much more easily and with a greater degree of
flexibility.
2.2.2 Simple installation and commissioning
The fact that string inverters are comparatively low in weight and usually have an extremely compact footprint
makes them very straightforward to transport: no heavy-duty machinery, cranes or HGVs are necessary.
Fronius string inverters can be installed and commissioned very quickly, an added benefit being that this can
be done directly by an installer on site. The advantages of compact string inverters with regard to handling and
transport come to the fore in locations that are difficult to access, such as open spaces with no metalled access
roads or on roofs that are only accessible using ladders.
2.2.3 Maximum performance for high yields
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The yield of a PV system depends on a number of factors, such as insolation, efficiency, the cooling system
or even partial shading of the solar modules. String inverters boast a high level of conversion efficiency.
Climatic conditions, such as the effect of heat or direct sunlight on the device, have a marked influence on the
yield of the PV system as a whole. Fronius string inverters have an active cooling system and a professional
power derating behaviour. These combine to deliver maximum yield for longer, even at temperatures as high
as 50°C, which has a positive impact on yield figures. In warmer regions in particular, devices featuring active
cooling technology deliver considerably higher yields, resulting in shorter payback periods.
2.2.4 Prompt service for dependable yields
The dependability of the yield of a PV system utilising string inverters is guaranteed by two important factors.
Firstly, the string inverter concept increases supply reliability, as only a small part of the system is affected by
a failure (e.g. 27 kW in the case of the Fronius Eco). This means that a string fault or downtime due to servicing
activities has less of an impact on the overall yield. As string inverters can be equipped with sophisticated
string monitoring features, any faults that do occur can be limited in advance to a particular module array
without the need for someone to have to drive out to the site. Secondly, thanks to their compact design, string
inverters can be replaced quickly and easily. With Fronius inverters, some specific components can be
replaced by a single person, offering the system operator flexibility over the entire service life of the PV system,
as spare devices or components can be held on site to ensure a rapid repair in the event of a fault. This process
keeps service costs down, guarantees yields and means that the PV system will only be offline for a short
period, as servicing can be carried out quickly and easily by a local installer.
2.2.5 Savings potential
The fact that most types of string inverter can usually be mixed very easily within a PV system not only
simplifies the planning (compared with central inverters), it can also keep the initial costs down. For example,
the system planner can design the bulk of the system to use string inverters equipped with 1 MPP tracker,
while reverting to a string inverter with multiple MPP trackers for complex or shorter strings.
The active cooling of string inverters not only ensures higher yields, even at higher temperatures, it also
extends the service life of the device through improved cooling of the power electronics. This also lowers costs
and shortens the payback period.
Moreover, PV systems that use string inverters can in most cases be designed to operate at the low-voltage
level, resulting in lower outgoings for transformers. In situations where string inverters – operating at 1500 V,
for example – are installed at the medium-voltage level, a second low-voltage transformer will be required as
soon as the PV system is used for self-sufficiency in a 230V/400V grid. Unless specified otherwise by the grid
operator, the string inverter concept also allows the required measuring system to be integrated into the low-
voltage level, once again lowering costs due to the significantly cheaper measuring equipment and sensors
available for this level.
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