Protection Engineering Basics and Schemes Protection Engineering | Page 117

will flow to other normal loops and increases the chances
of equipment damage or normal loop isolation.
We can see one example of time grading here, Time
grading is done basically mainly by TMS and PSM.
Section Rating (MVA) Pick up Protection(TMS) Trip Time
Power Grid
Transformer 1(T1)
Transformer 1(T1)
Transformer 2(T2)
Transformer 3(T3)
Transformer 4(T4) 20
20
20
1
1.5
0.5 1.137238435
1.137238435
1.137238435
1.137238435
1.180978375
1.137238435 0.14
0.12
0.1
0.09
0.035
0.025 7.610561231
6.523338198
5.436115165
4.892503648
1.47040965
1.359028791
Here we changed the TMS and PSM and get different
tripping time responses of relays in protection scheme,
thumb rule of protection stated that tripping time of
downstream protected equipment(Feeders, substations
etc) must be less then upstream protected
equipment(Power Grid, Generator etc. ).
5.3.2.2 Very Inverse TCC Relay:
It is required where moderate trip current vs time
characteristics required, as you seen in SI relay the power
on PSM is 0.02, here the power on PSM is 1.
pg. 117