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OVERVIEW OF SOLAR INVERTER USED IN
SOLAR MODULES
1
N.Prasidha Devi, 2 Dr B.Murali Babu.,PhD.,
1
M.E(PED) Student , 2 Professor
Paavai Engineering College, Namakkal, Tamil Nadu-6370 018, India.
2
Paavai Engineering College, Namakkal, Tamil Nadu-6370 018, India.
1
Abstract : This article will provide an overview of the different types of inverters used in solar power systems, including the
advantages and disadvantages of each inverters with its suitable applications.
IndexTerms - Solar Inverter,Application.
I. OVERVIEW
A solar inverter or PV inverter, is a type of electrical converter which converts the variable direct current (DC)
output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial
electrical grid or used by a local, off-grid electrical network. It is a critical balance of system (BOS)–component in a photovoltaic
system, allowing the use of ordinary AC-powered equipment. Solar power inverters have special functions adapted for use with
photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
Solar inverters use maximum power point tracking (MPPT) to get the maximum possible power from the PV
array. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a nonlinear output efficiency known as the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and
determine a resistance (load) to obtain maximum power for any given environmental conditions.
The fill factor, more commonly known by its abbreviation FF, is a parameter which, in conjunction with the
open circuit voltage (Voc) and short circuit current (Isc) of the panel, determines the maximum power from a solar cell. Fill factor
is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc.
There are three main types of MPPT algorithms: perturb-and-observe, incremental conductance and constant
voltage. The first two methods are often referred to as hill climbing methods; they rely on the curve of power plotted against
voltage rising to the left of the maximum power point, and falling on the right.
The inverter is a fundamental component of any solar photovoltaic system, since it is the device that converts
the normal DC output of solar modules to an AC supply which can be used by electrical devices such as lamps, home appliances,
office equipment, motors, etc. While the function of all inverters is basically the same. In solar power systems they can be
classified into three main types, according to the way in which they integrate with the photovoltaic array:



String inverters(standard inverter)
Micro-inverters
Power optimizer systems
II. STRING INVERTERS
The name of string inverters comes from the fact that photovoltaic modules are connected in a series circuit, or
string, before connection to the inverter. All PV modules in a series circuit carry the same current, and their voltages add up
directly.
Some inverters allow the connection of multiple strings in parallel, instead of using a single circuit for the entire
PV array. This setup is beneficial when the array is divided into sections with different orientations and production profiles: PV
modules connected in series achieve optimal performance when all modules in a circuit have approximately the same output and
operating conditions.
Single-phase inverter with MPP tracker for typical outputs ranging from 0.5 kW to 5 kW for a string of solar
panels connected in series. These are widely used in rooftop photovoltaic systems on private dwellings..
Fig 1. Single phase PV string inverter
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2.1 Advantages of String Inverters
String inverters are the most affordable of the three options. Maintenance is greatly simplified, since there is
only one point of conversion from DC to AC. Individual modules are simply connected with junction boxes. Since the solar array
provides an aggregated DC output, backup batteries can be charged. It is just a matter of connecting the charge controller and
battery before the inverter, to use the DC supply.
2.2 Disadvantages of String Inverters
Solar PV modules are connected in series. This means that, if a single module is experiencing performance
problems, the entire string will suffer a drop in energy output. For this reason, string inverters are better suited to locations where
there will be no shades on top of the PV array. It is also necessary to keep the surface of PV modules clean - an array covered
with fallen leaves is as unproductive as a shaded array. If the inverter itself fails, the entire solar PV array will be unusable - DC
output can't power most home appliances or synchronize with the utility grid.
III. MICRO INVERTERS
This technology is the complete opposite to string inverters in terms of connection and operation. Instead of
using series-connected circuits of solar modules and a central inverter, a micro-inverter is installed on every module that
composes the PV array. Then, the output of all micro-inverters is connected in parallel to an electric circuit which works with
alternating current.
Micro inverters are small inverters rated to handle the output of a single panel. Modern grid-tie panels are
normally rated between 225 and 275 W, but rarely produce this in practice, so micro inverters are typically rated between 190 and
220 W (sometimes, 100 W). Because it is operated at this lower power point, many design issues inherent to larger designs simply
go away; the need for a large transformer is generally eliminated, large electrolytic capacitors can be replaced by more reliable
thin-film capacitors, and cooling loads are reduced so no fans are needed. Mean time between failures (MTBF) are quoted in
hundreds of years. More importantly, a micro inverter attached to a single panel allows it to isolate and tune the output of that
panel.
For example, in the same 10-panel array used as an example above, with micro inverters any panel that is underperforming has no effect on panels around it. In that case, the array as a whole produces as much as 5% more power than it would
with a string inverter. When shadowing is factored in, if present, these gains can become considerable, with manufacturers
generally claiming 5% better output at a minimum, and up to 25% better in some cases. Furthermore, a single model can be used
with a wide variety of panels, new panels can be added to an array at any time, and do not have to have the same rating as existing
panels.
Sometimes, as much as two solar panels are attached to the same micro inverters (duo micro inverter). The
power that inputs the micro inverter is then ≥600 W and 24 V (i.e. as said, two 12 V solar panel can be tied together). Micro
inverter then converts power provided by solar panel(s) into standard AC voltage, typically 230 VAC / 50 Hz or 240 VAC / 60
Hz. The typical size of this micro inverter is: 22x16.4x5.2cm/ 8.66x6.46x2.05".
As said, micro inverters produce grid-matching power directly at the back of the panel (i.e. 220 V). Arrays of
panels are connected in parallel to each other, and then to the grid. This has the major advantage that a single failing panel or
inverter cannot take the entire string offline. Combined with the lower power and heat loads, and improved MTBF, some suggest
that overall array reliability of a micro inverter-based system is significantly greater than a string inverter-based one.
This assertion is supported by longer warranties, typically 15 to 25 years, compared with 5 or 10 year warranties
that are more typical for string inverters. Additionally, when faults occur, they are identifiable to a single point, as opposed to an
entire string. This not only makes fault isolation easier, but unmasks minor problems that might not otherwise become visible – a
single under-performing panel may not affect a long string's output enough to be noticed.
Fig 2. Singe phase PV Micro inverter
3.1 Three-phase micro inverters
Efficient conversion of DC power to AC requires the inverter to store energy from the panel while the grid's AC
voltage is near zero, and then release it again when it rises. This requires considerable amounts of energy storage in a small
package. The lowest-cost option for the required amount of storage is the electrolytic capacitor, but these have relatively short
lifetimes normally measured in years, and those lifetimes are shorter when operated hot, like on a rooftop solar panel. This has led
to considerable development effort on the part of micro inverter developers, who have introduced a variety of conversion
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topologies with lowered storage requirements, some using the much less capable but far longer lived thin film capacitors where
possible.
Three-phase electric power represents another solution to the problem. In a three-phase circuit, the power does
not vary between (say) +120 to -120 V between two lines, but instead varies between 60 and +120 or -60 and -120 V, and the
periods of variation are much shorter. Inverters designed to operate on three phase systems require much less storage. A threephase micro using zero-voltage switching can also offer higher circuit density and lower cost components, while improving
conversion efficiency to over 98%, better than the typical one-phase peak around 96%.
Three-phase systems, however, are generally only seen in industrial and commercial settings. These markets normally install
larger arrays, where price sensitivity is the highest. Uptake of three-phase micros, in spite of any theoretical advantages, appears
to be very low.
3.2 Advantages of Micro-inverters
Thanks to the parallel connection, the operation of every single PV module is completely independent on the
rest of the array. This means that problems with an individual module will not affect the rest. Micro-inverters allow PV arrays to
operate effectively even with the presence of shades, or when sections of the array face in different directions. Monitoring
functions can be integrated directly into individual micro-inverters, which allows problems with individual modules to be detected
and solved. Grounding is very simple, since it can be integrated directly into the circuit which collects the output from microinverters. Increased energy output over a traditional string inverter system, assuming the same installed photovoltaic capacity.
3.3 Disadvantages of Micro-inverters
This type of inverter system is the most expensive of the three. Micro-inverters are unsuitable if the user plans to
implement battery storage. The DC output of each module is immediately converted to AC, so there is no aggregated DC output
which could be used to charge a battery bank. Maintenance is complex, since micro-inverters are installed on the racking directly
below the solar modules: if the array isn't very accessible, micro-inverter technology might not be the best option. For example, a
ground-based system with micro-inverters can be serviced with ease, but in a roof-mounted system there would be complications
due to small size of the gap between the modules and the roof.
IV. POWER OPTIMIZER SYSTEMS
This technology can be considered a hybrid of both systems mentioned above. The system uses a central string
inverter, but the PV modules are not connected directly in a series circuit. Instead, an individual power optimizer is installed in
every module to stabilize the output voltage, and then the series connection is carried out. In terms of cost, this system is
intermediate between micro-inverters and traditional string inverters.
4.1 Advantages of Power Optimizer Systems
Voltage conditioning allows independence between modules, similar to what is accomplished with microinverters: a malfunctioning module will not affect the rest. Monitoring functions can be included with the power optimizers,
allowing problems to be identified and addressed with ease. Voltage optimization creates stable operating conditions for the
inverter, and longer circuits can be connected. An aggregated DC output is available, which allows the use of batteries or DC
loads. Increased energy output over a traditional string inverter system, assuming the same installed photovoltaic capacity.
4.2 Disadvantages of Power Optimizer Systems
More expensive than traditional string inverter systems. Maintenance requirements are also higher, due to the
presence of a power optimizer in each module.
V. WHEN TO USE EACH TYPE OF INVERTER
The best type of inverter for a PV array is greatly determined by two factors:
 Site conditions
 Use of batteries or DC loads
The following are some examples of scenarios that favour a specific type of inverter technology:
5.1 String Inverter
 Site with ample space and no obstacles that would cast shadows, allowing the entire array to be faced in the optimal
direction.
 A battery bank will be used.
 The budget is limited
5.2 Micro-inverters
 The array will be divided into sections with varying production profiles, due to the presence of shades and differences in
orientation.
 The system will be grid-tied, and will not use energy storage.
 Maximum system performance is required, even if the price is higher.
5.3 Power Optimizers



The array will be divided into sections with varying production profiles, due to the presence of shades and differences in
orientation.
An aggregated DC output is required for charging batteries and powering DC loads.
Maximum system performance is required, even if the price is higher.
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VI. MARKET
S.No.
Type
Power
Efficiency
1
String inverter
up to 100 kWp
98%
2
Micro-inverter
module power range
90%–95%
3
Power optimizer
module power range
98.8%
VII. COMPARISON OF INVERTERS
S.No.
String Inverter
1
Rigid Design
 String sizing
 Minimum system size is 8
panels
 DC and AC design required
Flexible Design
 No string sizing Multiple
configurations
 All-AC design
Rigid Design
 String sizing
 Minimum system size is 8
panels
 DC and AC design required
Complex Installation
 Added complexity with DC
and AC electrical work
Easier Installation
 Simple all-AC installation
(no DC electrical work)
 No mounting a string
inverter
Complex Installation
 Added complexity with DC
and AC electrical work
Greater Productivity
 In independent studies,
Enphase produced up to 3.1%
more energy
Higher Reliability
 No single point of failure
Lower Productivity
 Productivity varies by string
length
Less Durable
 Inverter has IP14 enclosure
 Avoid installation in direct
sunlight
Greater Durability
 IP67 enclosure
 Ultra-reliable component-ts
Less Durable
 Inverter has IP14 enclosure
 Avoid installation in direct
sunlight
Not as Safe
 Up to 600-1000 volts DC on
roof
Increased Safety
 Low voltage
exceeds 60 volts
Not as Safe
 Up to 600-1000 volts DC on
roof
2
3
4
5
6
 Design changes in field are
difficult
Lower Productivity
 Productivity varies by string
length
Less Reliable
 Inverter is a single point of
system failure
Micro Inverter
DC never
Power optimizer
 Design changes in field are
difficult
Less Reliable
 Inverter is a single point of
system failure
VIII. COMPARISON OF INVERTERS
For the most affordable option – we need a centralised string inverter. They function best if you have your solar panels
installed on your roof.
If solar panels are installed on a shaded part of your roof, or we have many angles to roof and the panels are spread out
across the angles then a micro inverter or power optimizer will give the best results.
Micro Inverters and power optimizers are more efficient than a centralised string inverter and if we have either of these
types installed then we will receive slightly higher amounts of energy from our system than if we have a centralised string
inverter.
If one of solar panels in the system is underperforming, a Micro Inverter or a Power Optimiser device will ensure that the
system still produces a substantial amount of power. If one panel fails, the others will still continue to produce electricity if we
have a Micro Inverter or Power Optimiser Inverter installed.
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Because Micro Inverters and Power Optimisers are both relatively new technology, they also have the ability to allow to
monitor the performance of individual solar panels, through smart phone or via a web portal.
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