Best-in-class grid-protection for next-generation PV inverters

July 30, 2013 //By Steve Drumm
Best-in-class grid-protection for next-generation PV inverters
Steve Drumm, Omron Europe's Market Development Manager outlines the latest standards for grid-connected solar inverters which call for high-performing electromagnetic relays that are designed specifically for grid-protection duties.

As consumers’ dependence on electricity continues to increase, and governments seek to meet carbon reduction targets by increasing reliance on renewable energy sources, an increasing number of small solar plants are expected to supply energy to the grid. These will comprise various classes of de-centralised generators, from sub-3 kW micro generators to 'mini' and 'midi' installations spanning 3 kW to 18 kW and 'maxi' installations up to around 40 kW, and will be located on residential and commercial properties including farms. Central power generation is also expected to rely increasingly on solar energy, which will be collected using large multi-string and central photovoltaic (PV) generators of around 45 kW and above.

By 2015, installed capacity is expected to exceed 35,000 MW. Of this, residential systems of up to 10 kW and small commercial systems in the 10 kW to 10 kW range are expected to account for around 15,000 MW.

Increasing reliance on distributed grid-tied systems in the micro to maxi ranges, as well as central inverters, calls for greater intelligence and improved performance in order to maintain human safety, equipment reliability and power quality. In particular, the design of the inverter grid interface connection is of critical importance. In addition, more and more grid-tied PV systems are being equipped with battery storage, as on-site consumption assumes increasing importance.

As a result, the inverter of a typical consumer or commercial solar generating station is evolving to become a sophisticated energy-management system capable of balancing the flow of energy in real time. Its responsibilities can include throttling the PV system in response to demand-reduction signals from the distribution network operator, which may be issued to prevent grid overload, or to temporarily disconnect from the grid to protect equipment and ensure human safety. Figure 1 illustrates the main functions of a typical transformerless inverter for a grid-tied application.

 

Figure 1. Functional block diagram of a modern grid-tied inverter

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