Negative Sequence Overcurrent Protection; Definite Time Negative Sequence Overcurrent Protection - NR Electric PCS-9611 Instruction Manual

3 Operation Theory
16 50/51SEF3.En
17 50/51SEF3.OutMap
18 50/51SEF4.3I0_Set
19 50/51SEF4.t_Op
20 50/51SEF4.Opt_Dir
21 50/51SEF4.En
22 50/51SEF4.OutMap
23 50/51SEF4.Opt_Curve
24 50/51SEF4.TMS
25 50/51SEF4.tmin
26 50/51SEF4.K
27 50/51SEF4.C
28 50/51SEF4.Alpha

3.7 Negative Sequence Overcurrent Protection

3.7.1 Definite Time Negative Sequence Overcurrent Protection

The application of negative sequence overcurrent protection to motors has a special significance.
Unbalanced loads create counter-rotating fields in three-phase induction motors, which act on the
rotor at double frequency. Eddy currents are induced on the rotor surface, which causes local
overheating in rotor end zones and the slot wedges. This especially goes for motors which are
tripped via vacuum contactors with fuses connected in series. With single phasing due to
operation of a fuse, the motor only generates small and pulsing torques such that it soon is
thermally strained assuming that the torque required by the machine remains unchanged. In
addition, the unbalanced supply voltage introduces the risk of thermal overload. Due to the small
negative sequence reactance even small voltage asymmetries lead to large negative sequence
currents.
This relay provides a two-stage negative sequence overcurrent protection with definite time delay
characteristics. Each stage can be enabled or disabled by scheme logic settings independently.
3-32
The logic setting of the stage 3 sensitive earth
fault protection
The output matrix setting of the stage 3 sensitive
earth fault protection
The current setting of the stage 4 sensitive earth
fault protection
The time setting of the stage 4 sensitive earth
fault protection
The setting is used to select the directional mode
for the stage 4 SEF protection, see Section 3.6.3
The logic setting of the stage 4 sensitive earth
fault protection
The output matrix setting of the stage 4 sensitive
earth fault protection
The setting is for selecting the inverse time
characteristic curve for SEF4 protection
The time multiplier setting of the IDMT sensitive
earth fault protection
The minimum operation time setting of the IDMT
sensitive earth fault protection
Constant "k" for the IDMT sensitive earth fault
protection, see Section 3.6.2
Constant "C" for the IDMT sensitive earth fault
protection, see Section 3.6.2
Constant "α" for the ID MT sensitive earth fault
protection, see Section 3.6.2
Date: 2014-04-08
0~1 1
0x00000 ~
1
0xFFFFF
0.005~1.0A 0.001A
0~100s 0.001s
0~2 1
0~1 1
0x00000 ~
1
0xFFFFF
0~13 1
0.05~100.0 0.001
0~100s 0.001s
0.001~120.0 0.0001
0.00~1.00 0.0001
0.01~3.00 0.0001
PCS-9611 Feeder Relay