Protection Engineering Basics and Schemes Protection Engineering | Page 186

Load shedding example
No:
Connected
Frequency (allowable Under and
Generation
Feeders Load
overfrequqncy is 5%)
1.2MW
10
1MW
50HZ
Event occur(load increases) by 40%(Demand is now 1.4MW), frequency is 46Hz
Frequqncy is now 48, Stage 2 tripping will be
initiate
1st Stage
Underfreque
ncy Relay
operates
pg. 186
10% load cut
2nd Stage 25% load Cut Frequqncy is now 47 .5 Hz, Stage 3 tripping
will be initiate .load shedding situation
3rd Stage 30% Load Cut Frequqncy is now 47 Hz, Stage 4 tripping will
be initiate ,load shedding situation
4th Stage 35% load cut Frequqncy is 46.5 Hz, all feeders need to be
isolate ,complete breakdown situation
Status
Frequqnc
y
normalize
d, no
need for
further
stages
Frequqnc
y
normalize
d, no
need for
further
stages
Frequqnc
y
normalize
d, no
need for
further
stages
necessary
steps
taken to
normalize
the
breakdow
n