How does an inverter and MPPT of a PV system Work? – Sustainable Energy – TU Delft

The solar inverter is one of the most important
power electronic devices that makes the modern PV system widely usable. It makes the DC power from the modules usable
for all the appliances in our house. Although these appliances might internally
either work with DC or AC, all of them are used to an AC grid. To know what an inverter is, we must first
understand its precise function in the PV system. The output of a PV module is Direct Current
or DC. What is meant by that? DC, or direct current is the unidirectional
flow of electric charge. DC is produced by power sources like batteries,
laptop chargers, and solar cells. It is common to talk of quantities like DC
voltage, power and current. The symbol for DC power is this. AC, or alternating current on the other hand,
is the flow of electric charge such that it constantly reverses direction. The usual form of an AC power is a sine wave. The symbol for an AC signal is this. So why do we need AC power? After a brief 'war of currents' in the 19th
century, AC power was chosen as the standard for central power generation, transmission
and distribution.

Until this day, almost all national electric
grids are AC based. Most households have what is known as the
"AC mains socket". Consequently, most household electric appliances
expect to be fed AC power, even though sometimes the internal circuitry of the appliances might
use DC. Nevertheless, the fact remains that solar
power produced needs to be converted to an AC form, so that it is more "usable" in the
electricity framework we have today. How do we make this conversion from DC solar
power to the easily usable AC power? By using an inverter! The inverter converts a DC electric signal
into an AC one. Let’s talk about the features of the solar
inverter and its applications in the modern PV system. Inverters are classified based on their mode
of operation, their size, or implementation topology. Let’s first discuss the classification based
on the mode of operation; there are inverters for grid-connected systems, ones for stand-alone
systems and inverters that can be applied in situations that require both connection
types, called bimodal inverters. The first two types are the most common ones. Here, a typical grid-connected PV system is

The DC PV modules are connected to the regular
AC electricity grid via a grid connected inverter, sometimes called grid tied inverter. In this case, the PV system is grid connected,
this means that the load can be supplied either by the PV system or the grid, depending on
the available irradiance and the demand. The inverter latches on to the grid’s frequency
and voltage. So the inverter that supplies AC power to
the grid acts as a current source, while the role of the constant voltage source in the
system is fulfilled by the grid. The grid connected system topology can only
be applied when there is an electricity grid available. In some places there is no grid availability. Here we have to apply a stand alone system
topology. Take a look at the following diagram. Here, a typical off-grid or standalone PV
system is shown. Consequently, the inverter is a standalone
inverter. In this case, the PV system is not connected
to the grid, which means the load can only depend on the PV system for power.

So the inverter that supplies AC power to
the load has to appear as a voltage source with a stable voltage and frequency, supplying
power at 230Vac or 110Vac, whichever is the standard at the location. In an off-grid system excess energy has to
be stored in order for the system to still be able to operate, even at night, when there
is no PV power generation. Which types of storage are available, and
which are most suitable for PV system applications will be discussed in the next video. Now we already know the basic application
of an inverter in the PV system: power conversion from DC to AC. Can the inverter perform any additional function? The answer is yes! Thanks to the advances in power electronics,
it is common to have inverters that implement an maximum power point tracking or MPPT mechanism
before inverting the voltage, thus ensuring that the PV modules or arrays are operating
at their maximum power point or MPP.

Apart from the modes of operation, inverters
are also classified on the basis of the implementation topology. There can be 4 different categories under
this classification. Central Inverters, which are usually around
several kW to 100 MW range. Module inverters, or microinverters, typically
rated around 50 to 500W. String inverters, typically rated around 500W
to a few kW. A string is nothing but a number of PV modules
connected in series. And finally, Multi string inverters, typically
rated around 1kW to 10kW range. Let's start with the central inverter. This is the most traditional inverter topology
in use. As seen in the figure this is a simple implementation
with one central inverter catering to all the PV modules in a PV system. While this inverter topology increases the
ease of system design and implementation, it suffers from several drawbacks.

In large systems, large amounts of DC power
will be transferred over long cables to reach the central inverter. This increases DC wiring costs, and also decreases
safety, as DC fault currents are difficult to interrupt. An MPPT implementation inside the central
inverter will only cater to the entire system as a whole. If the various modules, strings are mismatched,
let's say due to partial shading, the overall system output is drastically reduced. Also, the system is usually designed for a
fixed power. There is little scope for extendibility of
the system if more strings and modules need to be added. Next, let's look at micro inverters or module
level inverters. As the name suggests, each module has a dedicated
inverter with an MPP tracker. Therefore the topology is more resilient to
partial shading effects as compared to the central inverter topology. Clearly, the micro inverters provide the highest
system flexibility, since extending the size of a system under this topology is far simpler.

Furthermore, the DC wiring costs are greatly
reduced. However, the investment and maintenance costs
tend to increase, especially if the cost per Wp are compared. Then we have the string inverter concept,
which seeks to strike a balance between the module level inverter and the central inverter
topologies. The string inverter topology is more resilient
to mismatch than the central inverter, because each string is independently operated at its
MPP, thus guaranteeing a higher energy yield. String inverters are smaller than central
inverters. However, the implementation is more complex
than the module inverter. Also, the partial shading will have a greater
influence over the string inverter topology than over the micro inverter topology. Finally, Multi String inverters. This concept seeks to combine the higher energy
yield of a string inverter with the lower costs of the central inverter.

Each of the strings is pre-power-processed
using low power DC-DC converters. Each string has its own MPP tracker implemented
alongside the DC-DC converter. All the converters are connected via a DC
bus to the inverter, and ultimately to the grid. Within a certain power range, only a new string
with a dedicated DC-DC converter has to be included to expand the system size. We now have an overview about the solar inverters
and their topologies. Of course, the choice of your topology for
implementation would depend entirely on the system needs, size, and budget. But in general, while choosing an inverter
for your PV system, what are the requirements for a good solar inverter? There are several characteristics expected
from a good solar inverter. As every power processing step expends power
itself, the solar inverters are expected to be as efficient as possible. This is because we wish to deliver maximum
PV generated power to the load or the grid. Typical efficiencies are in the range of more
than 95% at rated conditions. Depending on the topology, it is expected
that the inverters have built in MPP trackers. Grid-tied inverters are expected to have active
islanding detection capability.

Islanding refers to the situation in which
the inverters in a grid-tied setup continue to power the system even though the power
from the grid operator has been restricted. Due to safety issues, islanding needs to prevented. Therefore, inverters are expected to detect
and respond by immediately stopping from introducing power into the grid. This is also referred to as anti islanding. Since in a lot of situations, the solar inverters
are exposed to ambient conditions, they must comply with the temperature and humidity conditions
of the location. Since grid-tied inverters pump power into
the grid. They are expected to maintain very high quality,
so as to not corrupt the power flow in the grid. Thus inverters are expected to have very low
harmonic content on the line currents. It is a work in progress to increase the lifespan
of the inverter, the crucial power electronic device in the modern PV system. A good inverter will probably reach, under
favorable conditions, around 10-12 years of lifetime. This is a bottleneck in the modern PV system's
lifetime, especially considering the fact that PV modules can last over 25 years.

So now you know what an inverter is, what
it does, and how it can be applied in the different system topologies. The following exercises will make you further
understand the inverter..

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