Siemens siprotec SJ62 User Manual page 153

2 Functions
151
Examples:
Motor with the following data:
Nominal current
Continuously permissible neg-
ative sequence current
Briefly permissible negative
sequence current
Current Transformer
Setting value
Setting value
When protecting feeder or cable systems, unbalanced load protection may serve to
identify low magnitude unsymmetrical 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 pro-
tection for low magnitude phase-to-ground and phase-to-phase faults. In particular,
this application is well suited for delta-wye transformers 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 trans-
former as long as the turns ratio "CTR" is taken into consideration.
Consider a transformer with the following data:
Base Transformer Rating
Primary Nominal Voltage
Secondary Nominal Voltage
Vector Groups
High Side CT
= 545 A
I
Nom Motor
I /I = 0.11 continuous
2 dd prim Nom Motor
I /I = 0.55 for T max = 1 s
2 long-term prim Nom Motor
CT = 600 A / 1 A
I > = 0.11 · 545 A · (1/600 A) = 0.10 A
2
> = 0.55 · 545 A · (1/600 A) = 0.50 A
I
2
S = 16 MVA
NomT
V = 110 kV
Nom
V = 20 kV
Nom
Dy5
100 A / 1 A
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
(TR = 110/20)
V
(CT = 100)
I