Central inverters in demand for PV system installers for large and medium size PV power plants


Central inverters in demand for PV system installers for large and medium size PV power plants

Central inverters are
the a most popular choice of PV system installers for large and medium size PV
power plants. Grid inverters are continuously undergoing reform due to
technological development, competitiveness and changing market preferences in
different regulatory environments. This article highlights the salient features
of ‘REFUsol 333K – a new generation central inverter’ which helps reduce
the balance of system cost of the entire project; it also discusses the other
benefits of using REFUsol 333K over the conventional central inverters
delivered as a technology advantage.  
Need of innovation

Most of the inverter
manufactures have developed a range of highly competitive and cutting edge
technology products in their product portfolio. Conventionally, key focus of
development was:

     * To
develop transformer less and compact inverters

     * To
improve efficiency of the product using high efficiency fast switching
devices and

     * Developing
suitable capacity units and providing effective cooling system to the
inverter units

     * Developing
compact devices along with fast and precise MPP tracking, integrated
data logger, monitoring facilities etc.

Apart from the above
considerations, REFUsol 333K has been developed after careful
understanding of entire PV application and prevailing PV plant development
trends worldwide. Clear understanding of specific requirements of the product
during different project development phases has helped designing the product
with improved reliability for overall PV plant operation.

REFUsol 333K
offers several benefits to the customer throughout the PV plant project life
cycle. The use of REFUsol 333K in the project changes the entire project
economics; it mainly saves the cost by reducing Balance of System (BoS)
requirement of the plant. Other design features implemented while developing
the REFUsol 333K those are mainly responsible for cost reduction and
improved reliability are discussed below;

BoS components

A reduction of system
costs was in the main focus since the beginning of developing the innovative
REFUsol 333K concept; however it inherently facilitates the PV designer
to build the entire PV system with higher conversion efficiency. The Balance of
system required for a PV power plant depends on selected PV and inverter
technology together. The major factors responsible for the changes in project
configuration and hence BoS are:

    * System
voltage and rating of the selected PV modules and inverters

     * No
load voltage of the PV modules

     * Range
of maximum power point voltage window of the inverters

     * Minimum
and maximum ambient temperature at the PV plant location

DC side configuration is
very important with the view to optimize the plant configuration in terms of
most efficient and cost effective design. Generator combiner boxes, mounting
structure frame design and DC cabling design  depending indirectly on the
selected inverter. Design attributes of the inverter should offer maximum
flexibility to optimize the plant configuration to reduce overall costs
although it is very difficult to compare actual cost benefits of any particular
attribute implemented.  


Quantity of combiner
boxes required for any PV plant mainly depends on the PV array design for the
selected PV modules and inverter for the ambient condition where the plant is
located. Specific to the inverter it all depends on maximum number of modules
that can be connected in a string, more the number modules per string lesser
the combiner boxes will be required (assuming rest of the parameters and
project component remains same). Maximum open circuit voltage (Voc) and maximum
power point voltage (Vmp) of the modules at minimum temperature are critical
and should fit in the inverter specifications strictly. Minimum 20 percent of
the combiner boxes can be reduced by use of 333K inverter if compared with the
plant of similar capacity designed with conventional central inverters.

and DC cable

Higher AC and DC
voltages of REFUsol 333K have the advantage that the line losses can be
reduced almost everywhere in the system. Firstly, in the inverter, which forms
the part of efficiency gain, secondly the cable losses in the PV installation
will be significantly lowered. In addition, the installation costs for cabling
and the associated costs are reduced by designing the plant with REFUsol 333K.

    * The
high output voltage of 690 volts decreases the overall system losses to
58 percent compared with a 400 volt system.

     * The
higher output voltage also leads to a reduction of transmission losses by
76 percent for the same cross section.

     * The
losses in the cables between the Inverter and medium-voltage transformer
can be reduced by one third.

Compact size REFUsol
333K can be installed outdoor in the PV array field strategically to reduce the
DC cable overall lengths.

REFUsol 333K
central inverter is designed with the view to do decrease specific drawbacks
encountered in the installation and performance of conventional central

Unlike conventional
central inverters, REFUsol 333K is a transformer less inverter and
multiple devices can be connected to a transformer to operate in parallel,
which allows cheaper installation with reduced losses.

IP65 protected
electronic unit REFUsol 333K is a compact unit which can be installed
outdoor and hence the cost associated to building control rooms required for
the inverters can be saved.

If compared to a system
designed with conventional central inverters, the system designed with REFUsol
333K inverter would save typically around five percent of Balance of System
(BoS) cost. However, saving of two percent can be estimated in overall capital
cost of the installation.

  1. Other
    benefits to the customer

Besides saving on BoS
component cost REFUsol 333K unique feature which offers better utility
to the costumer are highlighted below.

     * Suitable for outdoor areas and
industrial applications

     * Lower Cost with higher

     * Lower weight approximately 850
kg. Hence logistic and infrastructure cost will be reduced considerably

     * No special qualification is
required for installation of the inverters

     * Higher
system voltage design enables 690VAC enables the parallel operations of
several devices on just one transformer what results in lower system
losses and lower balance of system cost

     * Higher output voltage leads to a reduction of the
conveyance losses of 76 percent compared with 400VAC with same cable cross

     * Input
voltage of up to 1500V it is possible.

     * No
auxiliary power supply is required.

     * It
DC-DC booster allows to start power in feed to the grid at 600W hence
contributes to generate additional units for the plant when the sun is

     * Effectiveness
of whole system will increase due to lower losses in the string, reduced
cable length and optimized plant layout.

     * Robust and low maintenance ‘temperature controlled air cooling’ system allows the operation under
extreme climatic conditions.

     * Integrated data logger to
record and forward relevant data to analysis and control.

     * REFUsol ULTRAEtaPlus circuit topology

     * Very high peak
efficiency of 98.5 percent

     * Wide DC Voltage

     * Especially high
efficiency at low DC voltage link

     * Fixed N-Potential
(low EMC disturbances, low leakage current)

Use of REFUsol 333K improves the overall
profitability of the solar PV plant as by minimizing electrical losses in the
system, reducing BoS components and improving the performance ratio of the

By Nitin Bhosale, Refu Solar Electronics