Experimental studies on digital protection systems under unbalanced and non sinusoidal conditions

Modern digital protection systems incorporate functions that were not possible to implement on traditional electro-mechanical relays, or were not implemented on many commercial static, and even early digital relays. An example is negative sequence detection and the resulting relay operation. Most of the current literature, even when it mentions the use of the negative sequence component to detect unbalanced conditions, does not go deep into the analysis of its characteristics and advantages over phase quantities, and hardly present practical studies. This contribution presents the results of some tests carried-out on a new type digital relay, where negative sequence response is compared with phase response under unbalanced conditions. The impact of the harmonic distortion is in addition evaluated.