PV inverter

A PV inverter is an electronic device that converts the DC supplied by photovoltaic (PV) panels to AC which is then normally fed to the 230 V mains supply.

PV inverters are usually configured as a string inverter where all the PV panels are connected in series, or they can be much smaller, a micro-inverter, and assigned to each individual PV panel.

PV inverters generally consist of three distinct stages. Firstly the variable DC output from the PV is fed to a DC to DC converter the output of which is then converted to 230 V AC by a DC to AC converter using high frequency Pulse Width Modulation. The performance of the whole inverter is managed by a real-time micro-controller that by executing a number of control scenarios fulfils all the power management and communication functions required, including:

• Continuously varying the input impedance of the DC to DC converter to ensure that the point of maximum power is maintained irrespective of PV operating conditions, for example, panel temperature and shading. This function is called MPPT (Maximum Power Point Tracking).
• Ensuring that the AC output connected to the grid is synchronised and at the appropriate voltage and frequency to enable power to be exported from the inverter to the domestic network or, if not required locally, exported to the grid.
• Managing battery storage if fitted.
• System configuration and communications.

Both string inverters and micro-inverters have their own pros and cons:

• A single larger inverter can be more cost effective than many smaller units.
• If a micro-inverter fails the output from just one panel is lost having little impact on the overall output.
• Micro-inverters are typically mounted on the back of each PV panel often in a harsh and difficult location to access thereby making replacement difficult should they fail.
• If one panel in a string becomes shaded it reduces the output of the whole string whereas with micro-inverters just the output from that one panel is reduced.

A typical domestic string inverter will have a rating of approximately 1.5 to 4 kWp and a maximum DC input voltage of 500 V or above allowing for a number of panels, typically around 30 V, to be connected in series. By comparison micro-inverters, being dedicated to one panel, have a rating of typically around 250 W and a maximum DC input voltage of approximately 45 V. The output of each micro-inverter is typically 230 V AC which is connected to the domestic network via an interface box.

Currently, (2014) inverters can have an efficiency of well over 90 per cent at 20 per cent of rated output rising to over 95 per cent at 40 per cent output and above (European Weighted Efficiency [60]).

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This article was created by --BRE. It was taken from The future of electricity in domestic buildings, a review, by Andrew Williams, published in November 2014.

[edit] Find out more

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• Green Deal.
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[edit] External references

• [60] Brundlinger R et al. prEN 50530 – The New European Standard for Performance Characterisation of PV inverters. EU PVSEC Proceedinngs. Accessed 8 January 2014. Downloaded from http://www.eupvsec-proceedings.com/proceedings?paper=4741