Limitations of di/dt technique in DC line protection

Protection issues are major challenges in realising multiterminal (MT) HVDC systems. In this paper, the findings of an investigation carried out on a di/dt based protection technique with the view to ascertaining its suitability for the protection of DC lines are presented. Firstly the main issues regarding the protection of MT-HVDC system are explored and thereafter the di/dt protection technique is evaluated. Some of the limitations of di/dt protection technique were due to the effect of travelling waves and oscillations. These lead to errors in estimating the actual di/dt since the resulting fault current profile may attain its local maximum or minimum before or during the time window set for the measurement. This is particularly the case during directional comparison. Simulations have been carried out in PSCAD and the results presented show that the di/dt protection technique will not be a reliable method for the protection of DC lines, in particular for the protection of DC grid.