Siemens 7SC80 Manual page 212

Functions
2.9 Negative Sequence Protection 46
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.
IEC Curves (Inverse Time Tripping Curve)
The thermal behavior of a machine can be closely replicated due to negative sequence by means of an inverse
time tripping curve. In address 4006 46 46 IEC CURVE, select out of three IEC curves provided by the device
the curve which is most similar to the thermal unbalanced load curve provided by the manufacturer. The trip-
ping curves of the protective relay, and the formulas on which they are based, are given in the Technical Data.
It must be noted that a safety factor of about 1.1 has already been included between the pickup value and the
setting value when an inverse time characteristic is selected. This means that a pickup will only occur if an
unbalanced load of about 1.1 times the setting value 46-TOC PICKUP is present (address 4008). The dropout
is performed as soon as the value falls below 95% of the pickup value.
The associated time multiplier is entered at address 4010, 46-TOC TIMEDIAL.
212
S = 16 MVA
NomT
V = 110 kV
Nom
(TR = 110/20)
V = 20 kV
V
Nom
Dy5
100 A/1 A (CT = 100)
Ι
· TR
V Ι
· TR · 46-1 PICKUP = 95 A can
V
Ι
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
· 46-1