Siemens SIPROTEC 4 User Manual page 141

Nominal current
Continuously permissible negative
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:
[formel-i2-058-260602-kn, 1, en_US]
A phase-to-ground fault with current Ι corresponds to the following negative sequence current:
[formel-i2-033-260602-kn, 1, en_US]
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 trans-
formers 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 relation-
ship 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:
Base Transformer Rating
Primary Nominal Voltage
Secondary Nominal Voltage
Vector Groups
High Side CT
The following fault currents may be detected at the low side:
If 46-1 PICKUP on the high side of the devices is set to = 0.1, then a fault current of Ι = 3 · TR
PICKUP = 3 · 110/20 · 100 · 0.1 A = 165 A for single-phase faults and √3 · TR
be detected for two-phase faults at the low side. This corresponds to 36% and 20% of the transformer nominal
current respectively. It is important to note that load current is not taken into account in this simplified
example.
As it cannot be recognized reliably on which side the thus detected fault is located, the delay time 46-1
DELAY must be coordinated with other downstream relays in the system.
Pickup Stabilization (definite-time overcurrent protection)
Pickup of the definite time elements can be stabilized by means of a configurable dropout time. This dropout
time is set in 4012 46 T DROP-OUT.
SIPROTEC 4, 7SJ62/64, Manual
C53000-G1140-C207-8, Edition 08.2016
= 545 A
Ι
Nom Motor
/Ι = 0.11 continuous
Ι
2 dd prim Nom Motor
/Ι = 0.55 for Tmax = 1 s
Ι
2 long-term prim Nom Motor
/Ι = 600 A/1 A
Ι
Nom prim Nom sec
46-1 Pickup = 0.11 · 545 A · (1/600 A) = 0.10 A
46-2 Pickup = 0,55 · 545 A · (1/600 A) = 0,50 A
S = 16 MVA
NomT
V = 110 kV
Nom
V = 20 kV
Nom
Dy5
100 A/1 A
2.7 Negative Sequence Protection 46
(TR = 110/20)
V
(CT = 100)
Ι
· TR · 46-1 PICKUP = 95 A can
V
Ι
Functions
· TR · 46-1
V Ι
141