Voltage sags tolerance and responsibility curves

Voltage sags being inherent disturbances to power systems, equipment should be characterized by tolerance curves and grid operators are compelled to report voltage sags. Furthermore, voltage sag responsibility curves are currently being discussed by regulators. This paper condenses relevant information on these subjects, and presents three case-studies. The description and reporting of voltage sags are addressed. The voltage sag possible causes are identified for every cell in the EN50160 reporting table. The corresponding fault location and fault clearance process are identified. Reasons for the operation of back protection are also pointed out, thus explaining the long duration sags. Case studies correspond to transmission network (220 kV) sites, representative of urban, urban/rural and rural locations. Recorded sags were classified according to EN 50160, and their possible causes identified. Results are discussed, and recommendations proposed as regards implementation of the responsibility curves.