Undervoltage Load Shedding

Area undervoltage can occur when there is a lack of reactive support for the

load. This lack of reactive support manifests itself as an undervoltage condition,

with the undervoltage most severe at the load area requiring the reactive support.

As load in the transmission systems are approximately symmetrical on all three

phases, the undervoltage condition is seen on all three phases. If the power

system voltage profile over the system is viewed as a plane, the undervoltage

event from lack of reactive support can be conceptualized as a depression from

the 1.0 pu level nominal) to some lower level, with the lowest point the area of

highest reactive support requirement.

A method of gaining selectivity for load shedding is to employ multiple time

undervoltage elements. In that manner, the localized areas with the lowest voltage

(highest reactive power support requirements) are shed first.

When implementing an undervoltage load shedding scheme, it must be secure

from asymmetrical voltage depressions occurring from unbalanced faults; single

phase-to-ground, phase-to-phase, and phase-phase

to phase-ground faults as well as three-phase faults and complete system

de-energizations.