Protecting the last mile – enabling an LVDC distribution network

The report has outlined the potential benefits that LVDC distribution networks will bring to future smart grids. Overall cost and losses are expected to be reduced, and system efficiencies will be improved. The options for utilising existing grid assets in implementing future LVDC systems have been investigated in the second section of the report. Recommendations have been outlined for choosing the appropriate voltage levels, using existing LV cables with different connection arrangements for implementing LVDC networks, interfacing LVDC with AC systems, and choosing earthing arrangements for better LVDC performance. In section three of the report the behaviour of an LVDC network under fault conditions has been characterised in order to ascertain required measures of risk and resilience. Consideration has been given to using IEC61660 for characterising DC short-circuits for an LVDC last mile network, and this has been tested on a simplified unipolar LVDC network. The report has identified which corrective factors within IEC61660 require improvement before application to LVDC grids. The effectiveness of using traditional LV protection with conventional interrupting components has been discussed in section four of the report. The results have shown that this type of protection is unlikely to ensure good performance in the LVDC networks under fault conditions. Slow protection operating times and greater fault impact can be experienced. LV conventional protection increases the risk of physical damage to the converters and other sensitive devices by exposing these components to high transient DC fault currents. Also, the rapid reduction of voltage at converter terminals increases the risk of them unnecessarily tripping for downstream faults, thus resulting in substandard protection selectivity. There is also a risk that local microgeneration fails to ride through remote transient disturbances. Therefore, the report has proposed an effective and more reliable protection scheme concept which can be developed further in order to significantly improve the protection speed and consequent LVDC performance. The concept is based on the combination of AC protection on the AC side and fast acting electronic-based DC protection in addition to internal converter/inverter protection