Inverters

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Solar Electric DC to AC converter (PV Inverter)

Solar PV panels produce a DC (Direct) current however mains power in the UK is an AC (Alternating) current, and the normal domestic power is in the range 215-250 Volts AC.

 

To use the PV-generated power, the system needs to

convert it from DC to AC and it does this by using an

inverter to take the power generated by the PV panels

and feed it onto the household supply.

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This is a two-stage process comprising DC input to

DC output, which takes the variable DC from the PV

panels and outputs the required voltage for the next

stage, which is the DC-to-AC conversion.  The DC-to-DC

input stage has the MPPT circuitry (Maximum Power

Point Tracking) that optimises the output from the PV

panels for the varying incident radiation levels. The

DC-to-AC circuitry phase matches the power to the household supply and outputs into the house.

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Inverter Variants

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String Inverters

The 230V AC mains power is a 50 Hz sine wave that is about 720V peak-to-peak. A modern 60 cell PV panel outputs around 40V DC and so to minimise the DC-to-DC level change, the panels can be connected in series to increase the string voltage to nearer the 720V thus reducing the electronics required for the DC-to-DC level change.

 

This has the advantage of reducing the cost however, each string of panels is treated as a single panel with only one MPPT, so any module that is in shade, resulting in reduced incident radiation, will adversely affect the MPPT thus reducing the output of the whole string.  Also, if there is a fault in a panel or connector the whole string is taken down and access to all the panels would be required so that the fault can be identified and rectified.

 

String inverters are a good option for unshaded arrays where there is easy access to all the panels, such as a flat roof installation or a ground-mounted array.

 

DC Optimisers with string inverter

If the DC-to-DC level circuitry is placed on each panel and data-over-power communications is used for the simplified DC-to-AC inverter to set the optimiser outputs, then each module can have its own MPPT thus minimising the adverse impact of shading.

 

It is also able to get the instantaneous power output from each panel, which helps any diagnostics and any need for panel cleaning. However, there is a cost implication when compared to a string inverter, and there is still potential for a single point failure (of a connection or the inverter) that can take out the whole string.

 

The optimisers and inverter need to be compatible so limiting spare part options over the 20 to 40 year life of the system.

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Micro-inverters

This is a single complete DC-to-AC converter on the back of each PV module and outputs 230V AC so the panels are connected in parallel rather than series, which means that each panel/inverter combination is a standalone PV system, giving ultimate flexibility in panel positioning and future relocation of panels. A data-over-power communication module can collect individual panel outputs, and this allows for monitoring for diagnostic and cleaning purposes, but as the 230V mains is a universal standard, individual panel/inverter combinations can be easily replaced with different manufacturer components, should the original manufacturer parts no longer be available.

 

Hybrid Inverters

These combine PV and Batteries in a single inverter.