Inverters are central to online and hybrid PV systems and we aim to inspire budding engineers for a better more sustainable world. We will introduce you to inverters.
Number two, we will look at off grid and grid tie inverters.
Number three, we will look at pure sine wave and modified sine wave inverters.
Number four, we look at low frequency and high frequency inverters.
Number five we will look at MPPT and PWM charge inverters and lastly we will look at the efficiency of inverters.
Recently as solar PV systems have come to the fore, there is a great interest in inverters today than there ever was. In almost all PV systems it is the central component that binds the whole system together, therefore having high reliability of an inverter is paramount as it is the component that is most likely to fail, other than the batteries.
An inverter takes the DC voltage from any source, either solar panels, wind turbine or energy storage batteries, and converts it into AC power for use in homes. Household appliances cannot use DC power so a solar inverter is needed to connect a solar panel array, either off-grid or grid-tie.
What types of solar inverters are there?
Video transcript by kind permission on Synergy Files
The functionality of an inverter is much higher today than it was 10 years ago. The main function of an inverter is to convert DC current into AC current. Inverters come in all shapes and sizes.
They are classified mainly on the power rating or the throughput. For example, there are small inverters available that can convert the output from a car battery to run an AC appliance.
On the other hand, there are large inverters that convert the output from a whole solar farm. For domestic consumers inverters are available with power ratings of 500 to 10,000 watts or 10 kilowatts. Similarly, inverters are also classified based on the input that they accept. That is, they can accept 12 volt DC, 24 volt DC, 48 or even 96 volt DC.
Note that 48 volt DC input is the most common type of inverter used for residential solar PV systems, while 12 volt DC input inverters are more commonly used in portable applications.
Power inverters in solar farms can also have input voltages from 300 volts DC to 450 volts DC.Now that we have introduced inverters let’s have a look at the most common question asks about them.
What is the difference between an off-grid inverter and a non grid inverter?
An off-grid inverter is a product that works completely isolated from the grid. It has no provision to tap into the grid electricity or feed electricity to the grid.
Normally, if a PV system is designed with an off-grid inverter, then the panels are connected with a charge controller. The charge controller is connected to the batteries. The batteries are then connected with an off-grid inverter.
Off-grid inverters can also be made for portable use, while grid tied inverter cannot, therefore inverters that are labelled for use in caravans and motorhomes are off-grid inverters. A grid tie inverter, on the other hand, can be directly connected to the solar array and the grid.
There is also sometimes a charge control option in the grid tie inverter and therefore some variants can also be connected to the battery pack.
In other words, a grid tie inverter can become the central component of a PV system. The advantage of using a grid tie inverter is that it can feed excess electricity to the grid and take advantage of net metering.
Grid tie inverters are more expensive because of this additional functionality. Grid tie inverters can also be used without batteries. Some grid tie inverters have the added functionality of shutting down the PV system in case of a power outage.
This is done to prevent ‘islanding’, that is, grid tie inverters ensure that in the event of a blackout it will shut down to prevent the energy it transfers from harming any line workers who are sent to fix the power grid.
Advantages and disadvantages of grid tie inverters
Advantages of grid tie inverters are as follows:
It ensures a smooth power to the load, that is, it has the ability to top-up from either the grid or the battery bank, in case the panels are not producing enough to meet the load.
It can charge the batteries using energy from the grid provided the grid charger option is inbuilt. This feature is very useful when the batteries are drained and the panels are not producing enough
It can feed to the grid when the panels are producing extra amounts of energy
Now let’s have a look at pure sine-wave and modified sine-wave inverters. There are two different kinds of output that an inverter may furnish.
The first one is called pure sine-wave and the second one is called a modified sine-wave. The modified sine-wave inverters are much cheaper than pure sine-wave inverters and that is because they have less circuitry.
What is modified sine wave inverter?
Modified sine wave inverters use transistors that act as a switch and they basically turn on and off the current to create a staircase pulse or a square wave. Appliances that use an output from modified sine-wave tend to over use power and run hotter and thus inefficiently.
What is pure sine wave inverter?
Pure sine-wave inverters, on the other hand, run electrical appliances much smoother. Appliances run without buzz or hissing sound. Now let’s have a look at low frequency and high frequency inverters.
Inverters can be classified into two categories based on the speed of the operation of transistor switches in the commutator circuit. The categories are namely low frequency inverters and high frequency inverters.
Which is better low frequency or high frequency inverter?
A low frequency inverter has several advantages but it is more expensive and because of the presence of massive iron core in its transformer, it is also much bigger and heavier compared to high frequency counterpart.
Often, difficult loads that require high surge at the beginning, such as motors compressors or pumps, are very well managed by low frequency inverters.
Field effect stress and low frequency inverters can operate cooler in part due to the slower frequency of switching required. To produce AC power in a high-frequency inverter there are almost twice the number of components compared to low frequency inverters.
Nonetheless, they are still smaller and lighter overall, because of the absence of a large central transformer.
They are not very well equipped to handle industrial loads and therefore, if a large pump or motor, or an air conditioner is required to be run, then a low frequency inverter is a better option. High frequency inverters application is appropriate for a wide variety of users like tools, battery chargers, small appliances, avian computers.
High frequency inverters make up the large majority of inverters available in the market. High frequency inverters are also available in lower power categories, such as three hundred, six hundred, thousand and fifteen hundred watts, as opposed to low frequency inverters with the power levels normally within thousands, typically 2000 to 3000 watts. Now, let’s look at MPPT and PWM charger inverters.
MPPT inverter vs PWM inverter
A solar inverter is different from a normal inverter in that it has a charge controller built into it. Therefore, inverters used by solar systems also come with either MPPT or PWM option. The MPPT functionality allows more power to be drawn out of the solar panels. This is done by keeping the panel’s output close to the maximum power point of the panels.
Inverters with MPPT functionality are more expensive than the PWM option. It has been noted experimentally that overall MPPT can make the solar energy system up to 20 percent more efficient the PWM option. On the other hand it is a good low-cost solution for small systems.
Only when the solar cell temperature is not too high, that is between 45 degree centigrade to 75 degree centigrade. PWM inverters prefer unshaded irradiance on the panel and tend not to work very efficiently if the panel is shaded.
Efficiency of inverters. Losses are expected whenever we are dealing with energy conversion processes. Similarly, when we convert DC electricity to AC electricity there will be losses.
As of July 2009 most grid tie inverters available on the market have peak efficiencies of over 94% and some as high as 96%. The energy loss during conversion process is mostly heat.