Updated Dec 5, 2022
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Inverters are a very important part of the transition to renewable energy. They are necessary because solar panels give a direct current (DC) power output, which basically means the current flows one way. However, nearly all of our homes and businesses use alternating current (AC) power, where the current flows in both directions at a given frequency. Inverters sit between the solar array and the house or business, converting the DC output from the solar panels into useable AC output. An inverter may feed electricity directly into the power grid, to household appliances, or into storage facilities like deep-cycle batteries.
Development of inverter technology has been a key part of the explosion in renewable energy. Early inverters were expensive, inefficient (throwing power away and heating up) and problematic. Even now, problems with inverters are the most common type of problem experienced by owners of solar arrays. Modern inverters are more efficient, cheaper, smaller, smarter and much more reliable than their earlier counterparts.
DC power is pretty self-explanatory. The current runs one way only. In the case of solar cells, the current will vary fairly slowly through the day as the suns’ intensity changes, but the current will always flow the one way. If we plot current vs time, we get the DC graph shown below.
AC power is different. The current not only flows both ways, but it’s intensity changes rapidly. When current is plotted against time, the curve forms a ‘wave’. There are all sorts of different types of waves for AC power. However the type of wave that we use in our homes and businesses is called a ‘sine wave’. The AC curve in the figure below is a sine wave.
The inverter’s job is to take the DC power and convert it to an AC power curve.
Early inverters used mechanical switches to create simple versions of AC power, and there are some (cheap) inverters using mechanical switches still available today. The simplest version just switches on and off, producing the ‘chopped’ waveform shown below. For higher frequency, the switch turns on and off more rapidly.
The next step up is instead of turning the current off, the switch is more complex and actually reverses the current. This converts to the DC current to an alternating ‘square wave’ current. Again, the frequency can be adjusted by changing how fast the switch operates.
Some types of equipment without sensitive electronics can run on this type of power. However homes and businesses need their AC power to be more like a sine wave.
Changing DC current to sine wave AC current requires more complex electronics. The figure below is a circuit diagram for a ‘do-it-yourself’ sine wave inverter.
Sine wave inverters work in three stages: the oscillator stage, the booster or amplifier stage, and finally the transformer stage.
The oscillator stage does what the title says it does: changes the DC current to an oscillating AC current. The oscillating current can be set to a particular frequency: for the United States the frequency is 60 Hz. This means there are 60 full waves per second. The DC current is converted to this type of AC current using integrated circuits. However at this stage the oscillations, or wave heights are quite small, too small to power anything useful. The wave heights need to be increased, hence the next stage.
The booster stage simply takes the signal from the oscillator stage and amplifies it. This creates waveforms with much higher wave heights, high enough for useful power. However there is one thing left to get right before the power can go to a home or business: the voltage.
The final transformer stage gets the voltage right. A typical residential array may have DC voltages up to about 600V. Commercial arrays can have even higher voltages, for example 1000V or even higher. In the United States, AC power is delivered at 120 V. Stability of this voltage is very important for stability of the grid and equipment that runs off the grid. Hence voltage control is a very important part of an inverter.
Sine wave inverters are available in two basic types: pure sine wave inverters and modified sine wave inverters. The difference is basically in the electronics. Modified sine wave inverters use simpler and cheaper electronics to produce a wave that is not quite a smooth sine wave. Pure sine wave inverters use more expensive electronics to generate a wave that is very close to a pure sine wave.
The figure below compares outputs from a modified sine waver inverter and a pure sine wave inverter.
Modern inverters have many functions and play a key role in getting the most power and energy from your solar array, and also minimising disruption to the grid.
For starters, both the current and voltage output from a solar array change as the intensity of the sun changes through the day. Inverters are smart enough to take in different currents and voltages, and still give the same output current.
The inverters are even smarter than that. Solar panels like to operate at particular values of voltage and current. The optimum value of voltage and current is called the ‘maximum power point’. Modern inverters can alter the impedance of the circuit (for electronic buffs, impedance is a combination of resistance and inductance) to make sure the solar array is operating close to its maximum power point. This is called ‘maximum power point tracking’ or MPPT for short.
But wait, there’s more! Different panels in the array may be located on different areas of a roof, for example, and therefore get different amounts of sunlight. The amount that the impedance needs to change to get the panels operating near the maximum power point will be different for the panels in the different areas. This means it is not possible to optimise the different areas, if there is only one controller available. Modern inverters have multiple inputs, or multiple MPPTs. Thus panels on one area of a roof can be combined in a ‘string’ and use one MPPT on the inverter. Panels in a different area of the roof can be combined in another ‘string’ and use a different MPPT on the inverter. Thus panels in the different areas can be controlled separately and can operate close to their maximum power points.
This concept is taken all the way by inverters called ‘microinverters’. Microinverters sit on each panel, and allow each panel to operate at their maximum power point. Microinverters are particularly useful when shade is a problem.
Power quality is a very big issue for solar power. In countries like Australia, where solar uptake on houses has been huge, there is so much solar power going into the grid that the grid power can become unstable which creates huge headaches for network regulators.
Inverters can play a critical role here. Pure sine wave inverters generate a great sine wave and good inverters can regulate frequency very well. However there is another element that must be controlled and that is the power factor.
The power factor defines how well the alternating voltages and current match in time. The figure below illustrates this, and shows a matching power factor, a lagging power factor and a leading power factor.
For best grid stability, the power factor for the power coming from the solar array needs to be as close as possible to the power factor from the grid. Modern pure sine wave inverters can apply power factor correction to the output power. This is a very important step forward for inverter technology and a big help in the transition to green power.
Modern pure sine wave inverters are sophisticated electronic devices that play a crucial role in any solar power system. Their output power is much higher quality than modified sine wave inverters.
The basic function of an inverter is to convert DC power output from the solar array into AC power output that we can use in our homes and businesses. However they do much more than that, including optimising the output from the solar array, and minimising disruptions to the grid.
The inverter is definitely not the place to skimp on quality!
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