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A solar inverter is an important part of any solar PV installation which is designed to operate appliances that work on alternating current (AC). The diagram below shows all the component parts of such a system. Item Number 4 is the solar String Inverter and it’s connected as close as possible to the solar panels.
A solar inverter works by converting direct current (DC) generated by solar panels into alternating current (AC) needed to power domestic appliances and most industrial machines. Electronic components called IGBTs (integrated gate bipolar transistors) chop up the DC into pulses and other circuits shape the pulses into a sine wave.
- What is a power inverter?
- How does a 3 phase solar inverter work?
- How does a solar inverter work?
- What is the difference between an off-grid inverter and an on-grid inverter?
What is a power inverter?
A typical inverter looks something like this – it has some red and black DC terminals on the back end and on the front end we find some AC electrical outlets that’s because there are two types of electricity there is AC and there is DC. An inverter is used to convert DC or direct current into AC alternating current …
The appliances in our homes are designed to run off of an AC supply and they get that from the electrical outlets which all provide AC electricity.
However, electricity produced by things such as solar panels and batteries produce DC electricity, so if we want to power our electrical devices from renewable sources, battery banks or even our car, then we need to convert DC electricity into AC electricity. We do that with an inverter…
To understand how an inverter works we first need to understand some fundamentals of electricity. Inside a copper wire we find copper atoms.
These have electrons which can move to other atoms. These are known as free electrons because they are free to move around. They will randomly move in all directions but this isn’t of any use to us.
We need lots of electrons to move in the same direction and we do that by applying a voltage difference across the wire.
The voltage is like pressure and will push the electrons. When we connect a wire to the positive and the negative terminals of a battery we complete the circuit and electrons begin to flow.
We call this flow of electrons ‘current’. The electrons always try to get back to their source ,so if we place things such as lamps in the path of the electrons, they will have to pass through this.
They will therefore do work for us such as illuminating a lamp.
An Inverter Convert DC to AC Power
The electricity from solar panels and batteries is known as DC electricity and that’s because this type of electricity flows in just one single direction.
It flows from one terminal directly to the other terminal. If we reverse the battery the electrons flow in the opposite direction.
You can think of DC electricity like a river with the current of water flowing in just a single direction.
Now in these video animations I use electron flow which is from negative to positive but you might be used to seeing conventional current which is from positive to negative.
Electron flow is what’s actually occurring. Conventional current was the original theory and it’s still widely taught today.
Just be aware of the two and which one we’re using. When we use in the oscilloscope to look at the electrical waveform for DC we get this flat line at the maximum voltage in the positive region. If we were to cut the power, then the line will drop to zero.
If we turn the power on and off repeatedly then we get a square wave pattern between zero and maximum, but if we were to post the switch to open and close over different lengths of time then we would get a pulsating pattern.
With AC electricity the electrons alternate by flowing forwards and backwards constantly. That’s how it gets its name because the current of electrons alternates in direction.
You can think of this type of electricity like the tide of the sea. It constantly flows in and out between the maximums of high tide and low tide.
If we followed the copper wires back to the generator the wires are connected to some coils of wire which sit within the generator.
Inside a basic generator we also find a magnet at the center which is rotating. The magnet has a north and south pole, or you can think of it as a positive and a negative half. The electrons in the wire are negatively charged.
As you might already know, magnets push or pull depending on the polarity. So as the magnets rotate past the coils the positive and the negative half are going to therefore push and pull the electrons within the copper coils and also through the connected copper wires.
The magnetic field of the magnet varies in intensity so as the magnet rotates past the coil the coil will experience a change in intensity of the magnetic field.
This will be from zero up to its maximum intensity and then as it passes the coil it will decrease again back to zero.
Then the negative half comes in and pulls the electrons backwards with the same change in intensity. Each full rotation of the magnet will therefore produce this wave pattern known as a sine wave.
The voltage is not constant in this type of electricity. Instead, it repeatedly moves from zero up to its peak, back to zero, then to the negative peak, and then finally back to zero again.
How fast does AC move positive and negative?
In North America and a few other parts of the world we find 60 Hertz electricity, which means the sine wave repeats 60 times per second.
As each wave has a positive and a negative half this means its polarity will therefore reverse 120 times per second.
In the rest of the world we mostly find 50 Hertz electricity, so the sine wave repeats 50 times per second, therefore the current reverses 100 times per second.
The inverter consists of a number of electronic switches known as IGBTs. The opening and closing of the switches is controlled by a controller. These can open and close super-fast in pairs to control the flow of electricity.
By controlling the path which the electricity takes and how long it flows in the different paths we can produce AC electricity from the DC source.
I’m going to animate these using some simple switches to make them easier to visualize. Remember AC is where the current reverses direction.
We can reverse the direction of current by reversing the battery. We could very quickly reverse the battery to produce a rough AC supply but an easier way would be to connect four switches or IGBTs across our load such as a lamp. If we open and close these in pairs then we can produce AC electricity.
So if we were to close one and four then the current flows in one direction and if we then open these and close two and three, we get current flowing in the other direction. We can use the controller to automatically do this again and again and again.
If we did that 120 times per second, then we would get 60 Hertz electricity and if we did that 100 times per second, we would get 50 Hertz electricity.
As we have a low voltage input we’re going to get a low voltage output. To reach the 120 volts or 230 volts required to power our appliances we will also need a transformer to step up the voltage to a useful level.
When we look at this through an oscilloscope we get a square wave in the positive and the negative regions.
This is theoretically AC because it reverses direction but it doesn’t really look much like an AC sine wave, so how can we improve this?
Do you remember earlier in the video when I said we can open and close the switch at different speeds and duration to change the waveform? Well we can do that for this too.
What we do is to use a controller to rapidly open and close the switches multiple times per cycle in a pulsating pattern, each pulse varying in width.
This is known as pulse width modulation. The cycle is broken up into multiple smaller segments. Each segment has a total amount of current that could flow but by rapidly pulsating the switches we control the amount of flow occurring per segment.
This will result in an average current per segment which we see increases and decreases thus giving us a wave.
The load will therefore experience a sine wave the more segments we have than the closer it mimics a smooth wave. We can control the output voltage by controlling how long the switches are closed for.
We could for example output 240 volts or 120 volts just by trimming the opening and closing times. We can also control the frequency by controlling the timing of the switches.
We could output 60 Hertz, 50 Hertz or 30 Hertz – whatever is needed for the application.
That’s how we can take a 12-volt DC battery and convert this into a 120 volts or 230 volts ac supply by using some IGBTs, pulse width modulation and a transformer but what if we needed more power?
How does a 3 phase solar inverter work?
Most homes around the world use single-phase electricity. Large commercial buildings, as well as some homes especially in Europe, will use three-phase electricity.
Homes in North America use split-phase electricity where a center tap transformer splits a single phase into two which provides two hot wires and a neutral.
With three-phase electricity we have a connection to each of the three phases. The phases are coils of wire which were inserted into the generator 120 degrees apart from the previous.
This means the coils experience the peak of the rotating magnetic field at different times and it’s this that gives us our three phases, each with a different sine wave that is slightly out of sync from the previous.
Remember, electricity wants to get back to its source in a complete circuit. As the current is flowing forwards and backwards at different times in each of the phases we can essentially connect the phases together and the current will move between the different phases.
As the polarity of each phase moves forwards and backwards at different times, any excess will flow in the neutral back to the source if needed, but that’s only if the load on any of the phases is unbalanced.
With single phase we have these large gaps between the peaks but with three-phase these can be combined to fill in the gaps and therefore deliver more power.
Larger applications require a three-phase inverter,or example, to run the compressors in a large cooling system the DC supply. In this case will be a rectified three-phase AC supply.
That means that three AC sine waves are combined together and pass through some diodes which prevent the electrons from flowing backwards.
This turns it into a ripple DC waveform. We then use a capacitor to smooth the ripple out into a constant DC supply.
To turn the clean DC into three-phase AC, we use a three-phase inverter. For this we use GBTs again. I’ll animate these as simple switches for simplicity and also number these as follows.
To get our three phases we need to open and close witches in pairs to direct the flow of current from our supply and return paths.
That way the connected motor will experience alternating current. For the three-phase supply we time the switches to simulate the three phases. Let’s see how this works.
First we close switches one and six – this will give us phase 1 to phase 2. Then we close switches one and two – this will give us phase 1 to phase 3.
Then we close switches three and two – this will give us phase two and phase three. Then we close switches three and four – this will give us phase two and phase one.
Then we close switches five and four – this will give us phase 3 and phase one. Then we close switches five and six and this will give us phase 3 and phase two.
This cycle repeats again and again. So if we check this with an oscilloscope we now have a wave pattern that looks something like AC, except it’s still a little bit square. this will work fine for some applications but not all, so again we need to use pulse width modulation to create the sine wave.
We’re going to use a controller to rapidly open and close the switches to vary the output frequency and voltage, and that way we get our three-phase AC supply.
How does a solar inverter work?
What is a solar inverter?
A solar inverter is a device that converts the DC electricity received from your solar panels into a form of electricity that can be used by appliances and other electronics in your home.
Let’s understand how it works and the different types of inverter available for solar. During the day when the sun shines on the solar panels installed in your homes electrons within the solar cells start to move around which produces dc energy.
The energy then goes straight into an inverter which converts it into ac energy which is the standard electrical current used to power appliances at home.
Now … let’s look at the various kinds and their functions. There are three common types of solar inverter:
- grid tie inverters
- off-grid inverters and
- hybrid inverters
Grid tie inverters are used with the solar pv system that is integrally connected to the utility grid power. They must connected to the grid to function.
The electricity produced by the solar system which is not consumed in the property is automatically injected into the grid via a bi-directional meter.
In case of a power cut, the inverter stops functioning as a safety feature, which is known as anti-islanding.
This system doesn’t have battery backup, so during power cut you will not have access to power. Hence this system can be used in area where there are rare or very few power cuts.
It can alternatively be used if you have a normal inverter battery system at home for power backup during power cut.
Off grid inverters on the other hand work just like the normal inverters used at home. In this system dc power generated by a solar panel is used to charge the solar battery.
How does a hybrid solar inverter work?
When there is power cut the inverter will draw stored energy from the battery, converting dc power from the battery to usable ac power for running electrical appliances.
This system can be used in area where there is frequent or long power cuts. Combining the capabilities of both grid tie and off-grid inverters into one hybrid inverter system can be used in both high power cut areas or areas where there is rare or very few power cuts.
Under normal operating conditions it can supply power to the home, charge the batteries and excess power can be fed into the grid.
In case of a power cut the unit will automatically switch over to battery supply and continue to operate independently from the electricity grid.
Solar systems provide several advantages for their users. They help save money, are easy to install and maintain, and most importantly reduce your carbon footprint.
If you’re looking to go solar you should talk to experts to know more about the range of grid tie, off grid and hybrid inverters.
We will look at the following topics:
- introduce you to inverters
- off-grid and grid tie inverters
- pure sine wave and modified sine wave inverters
- low frequency and high frequency inverters
- 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 system 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.
The functionality of an inverter is much higher today than it was 10 years ago.
How does an off-grid solar inverter work?
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.
What is the difference between an off-grid inverter and an on-grid inverter?
Well 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 labeled for use in caravans and motor-homes 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.
How does a solar grid-tie inverter work?
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 of grid tie inverter 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 amount of energy.
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.
Modify sine wave inverter use transistors that act as switches and they basically turn on and off the current to create a staircase pulse or a square wave.
Appliances that use a output from modified sine-wave tend to over use power and run hotter and thus inefficiently. Pure sine-wave inverters on the other hand run electrical appliances much smoother and they run without a buzz or hissing sound.
What is a high or low frequency inverter?
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.
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 its 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 inverter 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, and 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 300, 600, 1000 and 1500 watts, etc, as opposed to low frequency inverters. Low frequency power levels are normally within thousands, typically 2000 to 3000 watts.
What is an MPPT solar inverter?
Now let’s look at MPPT and PWM charger inverters. 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 a MPPT or PWM charge control 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 with 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, PWM is a good low-cost solution for small systems, only when the solar cell temperature is not too high that – between 45 degree centigrade to 75 degree centigrade.
PWM inverters prefer direct irradiance with no shading on the panel and tend not to work very efficiently if the panel is shaded.
Losses are expected whenever we are dealing with an energy conversion process. 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 the conversion process is mostly heat.
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