Functions
2.6 Negative Sequence Protection 46
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
116
Examples:
Motor with the following data:
When protecting feeder or cable systems
, unbalanced load protection may serve to identify low magnitude un-
symmetrical faults below the pickup values of the directional and non-directional overcurrent elements.
Here, the following must be observed:
A phase-to-ground fault with current I corresponds to the following negative sequence current:
On the other hand, with more than 60% of unbalanced load, a phase-to-phase fault can be assumed. The delay
time 46-2 DELAY must be coordinated with the system grading of phase-to-phase faults.
For a power transformer
, unbalanced load protection may be used as sensitive protection for low magnitude
phase-to-ground and phase-to-phase faults. In particular, this application is well suited for delta-wye transform-
ers where low side phase-to-ground faults do not generate high side zero sequence currents (e.g. vector group
Dy).
Since transformers transform symmetrical currents according to the transformation ratio "CTR", the relationship
between negative sequence currents and total fault current for phase-to-phase faults and phase-to-ground
faults are valid for the transformer as long as the turns ratio "CTR" is taken into consideration.
Consider a transformer with the following data:
Nominal current I
Nom Motor
= 545 A
Continuously permissible negative
sequence current
I
2 dd prim
/I
Nom Motor
= 0.11 continuous
Briefly permissible negative se-
quence current
I
2 long-term prim
/I
Nom Motor
= 0.55 for T max = 1 s
Current transformer I
Nomprim
/I
Nomsec
= 600 A/1 A
Setting value 46-1 Pickup = 0.11 · 545 A · (1/600 A) = 0.10 A
Setting value 46-2 Pickup = 0.55 · 545 A · (1/600 A) = 0.50 A
Base Transformer Rating S
NomT
= 16 MVA
Primary Nominal Voltage V
Nom
= 110 kV
(TR
V
= 110/20)
Secondary Nominal Voltage V
Nom
= 20 kV
Vector Groups Dy5
High Side CT 100 A / 1 A (CT
I
= 100)
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