Beckwith Electric M-3401 Load Shedding Relay

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.

• Asymmetrical voltage depressions occurring from single to phase-to-ground, phase

-to-phase, and phase-phase to ground faults are detected by using a negative

sequence overvoltage element (47S) as a supervision. Typically set to 0.05 to 0.1 pu

of nominal voltage, if the adjustable threshold is exceeded, the undervoltage load

shedding is blocked.

• Three-phase faults that decrease the voltage symmetrically across all three phases,

as well as complete de-energizations of the parts of the power system due to fault

clearing or other sectionalizing, are detected by using undervoltage supervision

(27S) on all phases. The undervoltage supervision is typically set lower than the lowest

survivable undervoltage that could occur from lack of reactive support before the entire

system voltage collapses, typically from 0.9 to 0.7 pu. If any phase voltage is lower

than the adjustable setting, it is assumed that the condition is from a fault (any type,

including three-phase) or complete de-energization of that part of the system.