Download XWES Inverter Overview V1.0

BACKGROUND SOLAR INVERTERS
Chances are you never heard of a solar inverter until you decided to install a solar energy system to
replace / substitute / support your grid power (or in some instances, lack thereof).
In order to use electricity from a photovoltaic array in your home, it must first be converted from
direct to alternating current. That’s because most electric loads (i.e. lights, appliances, tools, etc.)
operate on AC. In a solar power system, an inverter performs this vital function. A circuit of on/off
switches manipulates the current polarity so that it changes back and forth, just like the current
delivered by the utility grid. Then the electricity passes through a transformer coil, which adjusts the
voltage to 240volts at 60 cycles (or Hertz) per second.
Technically, a solar inverter is known as a power conditioning unit, or PCU, since it does other jobs
besides changing DC to AC. But that name sounds a little creepy, so most people just call it an
inverter. And just in case you’re wondering, a rectifier converts AC to DC, while an inverter does the
opposite, or inverse. Hence, its name!
Since the majority of modern conveniences all run on 220 to 240 volts AC, the Power Inverter will
be the heart of your Solar Energy System. It not only converts the low voltage DC to the 240 volts AC
that runs most appliances, but also can charge the batteries if connected to the utility grid or a AC
Generator as in the case of a totally independent stand-alone solar power system.
Inverters play a crucial role in any solar energy system and are often considered to be the brains of
a project, whether it’s a 2-kW residential system or a 5-MW residential estate or business power
plant. An inverter’s basic function is to “invert” the direct current (DC) output into alternating
current (AC). AC is the standard used by all commercial appliances, which is why many view
inverters as the “gateway” between the photovoltaic (PV) system and the energy off-taker.
Inverter technologies have advanced significantly, such that in addition to converting DC to AC, they
provide a number of other capabilities and services to ensure that the inverter can operate at an
optimal performance level, such as data monitoring, advanced utility controls, applications and
system design engineering.
There are three categories of inverters for your Solar consideration and we will discuss these
individually.
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MODIFIED SHINE WAVE POWER INVERTERS
This is probably the most popular and economical type of power inverter. It produces an AC
waveform somewhere between a square wave and a pure sine wave. Modified Sine Wave inverters,
sometimes called Quasi-Sine Wave inverters are not really expensive and work well in all but the
most demanding applications and even most computers work well with a Modified Sine Wave
inverter.
However, there are exceptions. Some appliances that use motor speed controls or that use timers
may not work quite right with a Modified Sine Wave inverter. And since more and more consumer
products are using speed controls & timers, the general consensus is that this type of inverter is well
suited for camping and outdoor power generation where the requirement is for smaller power
generation capabilities.
TRUE SINE WAVE POWER INVERTERS
A True Sine Wave power inverter produces the closest to a pure sine wave of all power inverters
and in many cases produces cleaner power than the utility company itself. It will run practically any
type of AC equipment and is also the most expensive.
Many True Sine Wave power inverters are computer controlled and will automatically turn on and
off as AC loads ask for service.
While the Modified Sine Wave inverter (sometimes called a Quasi Sine Wave inverter) is nearly half
the price of a True Sine Wave inverter, it is recommended using a True Sine Wave inverter if you
want to supply automatic power to a normal home using a wide variety of electrical devices. Also,
most appliances run more efficiently and use less power with a True Sine Wave inverter as opposed
to a Modified Sine Wave power inverter.
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GRID TIE INVERTERS
If you are connected to normal Eskom or municipal power and just want to add some “Free Sun
Power” electricity to reduce your electric bill and you do not need a totally independent system, it is
possible that a Grid Tie power inverter will suit your needs.
With a Grid Tie power inverter, whatever electricity that your solar panels produce will reduce the
amount supplied by Eskom or your local authority, in effect lowering your monthly electricity bill.
And, it is envisaged that in future, if you are producing more power than you are using, you can
actually sell the extra power back to the utility company or to your home owner grid in residential
estates! For this type of setup a much smaller battery bank can be installed just to cover short term
outages from a few minutes to an hour or two. In fact, if you don't have frequent long term power
outages and don't need back-up power, then you will not need any batteries at all.
INPUT VOLTAGE
Home Owners are also required to plan the input voltages and this can easily become very
confusing to even the best quasi expert on alternative energy
Inverters offer a range (or window) of voltages to accommodate different designs and sizes of PV
arrays, as well as the constantly changing irradiance through the course of the day (and the year).
As your Solar PV system designer, we will ensure the maximum array voltage possible is less than
the high number in the range. At the same time, the starting voltage of the inverter shouldn’t be so
high that the equipment won’t kick on during periods of low irradiance, or on really hot days when
wire voltage drops regardless of irradiance. Ideally, your array’s maximum open-circuit voltage
should be about two-thirds to three-quarters of the high end on the inverter’s operating range. For
instance, an inverter range of 150-450 volts would be right for an array configured for 300 to 400
DC volts.
The main consideration when deciding on the input voltage (from your battery bank) of your
Inverter is the distance between your solar panel array and your battery bank. The higher the
voltage, the lower the current and the smaller the (expensive) cables need to be. Of course, when
you decide on a system voltage, the Solar Panels, Inverter, and Battery Bank all need to use the
same voltage.
AC OUTPUT
This is the maximum load your home appliances and other electric devices can expect to get from
the inverter at one time. For normal grid-tied systems, this spec is not a big deal, since the grid is
always there to supplement any power generated by a PV array. For a standalone system, the AC
output rating is a much bigger deal and usually requires that a load analysis be conducted. SolarX
offers a calculation service to assist you to make the correct decision regarding the input voltages of
your solution
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INVERTER STACKING
Inverter Stacking refers to the use of multiple inverters. Two or more inverters can be installed in a
configuration known as stacking that can provide more power or higher voltage. If two compatible
inverters are stacked in series you can double the output voltage. This would be the technique to
use to provide 120/240 volts AC. On the other hand, if you configure them in parallel, you can
double your power. Two 4000 watt inverters in parallel would give you 8000 watts (8KW) of
electricity.
For Off-Grid Solutions you can stack up to 10 Inverters. For Grid-Interactive solutions you can stack
up to 2 Inverters. For a 3 Phase configuration, you can stack up to 1 inverter per phase.
INVERTER EFFICIENCY RATING
Like solar modules, an inverter is rated for its efficiency in processing the available energy. The
rating takes into account current losses due to circuits heating up, low voltage levels, changes in the
way the current flows or gets stored, and the unit’s own internal energy consumption. Beware,
however, of a single efficiency rating listed on a product spec sheet. The actual efficiency of an
inverter should vary with the different loads it encounters (or even no loads at all). For this reason,
an inverter advertised as having 90 percent “peak” efficiency may not be better than one with 85
percent peak efficiency. A better spec (if you can find it) is the “weighted efficiency”. You can
sometimes find a graph on the spec sheet, known as the efficiency curve, which provides a more
detailed picture of the inverter's capacity to produce power.
POWER INVERTER CONSIDERATIONS
The Power Inverter is connected directly to the batteries and the main AC breaker panel to supply
power from the batteries to the loads (appliances). The Power Inverter converts the low voltage DC
to 120 volts AC. Power Inverters are available for use on 12, 24, or 48 volt battery bank
configurations.
Most Power Inverters can also charge the batteries if connected to the AC line. Alternatively, the AC
line input could be your own AC Generator in the case of a stand-alone solar power system. When
using an AC Generator to charge the batteries, the Power Inverter transfers the AC Generator
power to the loads via a relay. This way the AC Generator not only charges the batteries but also
supplies your AC power while it is running. If your Generator is at least 5000 watts, you can charge
your batteries and have extra AC power at the same time.
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TYPES OF RENEWABLE SYSTEMS
There are 5 types of inverter systems / designs; we will cover 4 of them in this discussion. The 5th
design is called AC Coupled and this system is where customers are moving from a grid tie to a
battery backup grid tie
i. Battery back-up with no Solar
This is the most basic system useful for occasional grid failures. The batteries are kept at full
charge at all times from the grid. This system uses a sub ‐ panel to power critical loads during a
black out. A generator can also be installed with a transfer switch to recharge batteries and run
the critical loads. This system does not require a grid tie inverter. This system does not require an
agreement with the utility company because it cannot sell back to the grid. It is incapable of
selling back to the grid when installed properly.
Backup
Battery
Storage
Fuse Box
Generator
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Lighting &
Electricity
ii. Basic Off-Grid System
This does not require a grid tie inverter. It cannot sell back to the grid, as there is no grid. This
system has an inverter/charger so it can re ‐ charge batteries from a generator. The primary
charging source here is solar. The solar requires a solar charge controller. This system can also have
wind and hydro as a charging source, but they would each require their own controller.
Backup
Battery
Storage
Fuse Box
Lighting &
Electricity
AC / DC
Inverter
Generator
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iii.
Battery Backup Grid-Tie
This system is very much like an off ‐ grid system with a few important differences. This
system has a utility connection and that is the primary source of power. This system has a
battery based grid tie inverter and solar panels. When the sun is shining, the solar panels try
to push the voltage of the batteries up. The grid tie inverter is programmed to only allow the
batteries to go to a pre ‐ set voltage, so all excess power is sent out the ac input terminals
and off to the utility. This makes the utility meter spin backwards. The MPPT solar charge
controller does two functions here. It greatly improves the output of the solar array over a
PWM controller and it is the fail-safe in case the grid goes down. You still need a regulator to
protect the batteries from over charge in case the utility goes down. You can add a
generator to this system to extend the time where you have power during an outage.
Some battery based grid tie inverters will not charge batteries from a generator, but they
will all keep the lights on for the critical loads panel during an outage regardless if they can
charge or not. You can also have wind and hydro to charge the batteries in a system like this,
but each charging source does require its own charge controller.
Backup
Battery
Storage
Buy from
Grid
IMPORT
METER
Fuse Box
Sell to
Grid
EXPORT
METER
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Lighting &
Electricity
AC / DC
Inverter
iv.
Straight Grid-Tie
This is the most popular type of solar system for residential estates and gated communities.
When the sun shines, the grid tie inverter sends power to the utility. Most of these systems
will have a separate utility meter so they can tell how much power you are buying vs. selling.
This system has no batteries. This system will shut down during a power outage and cannot
by law produce any power. The lights in the house will be out during an outage such as load
shedding.
Buy from
Grid
\
IMPORT
METER
Fuse Box
Sell to
Grid
EXPORT
METER
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Lighting &
Electricity
AC / DC
Inverter