Siemens siprotec SJ62 User Manual page 77

2 Functions
50-2 Element
75
The pickup value of the relay element 50-2 is set at address 1202, the assigned time
delay 50-2 DELAY at address 1203. This stage is often used for current grading in
view of impedances such as transformers, motors or generators. It is specified such
that it picks up for faults up to this impedance.
Example: Transformer used to distribution bus supply with the following data:
Rated Power of the Transformer
Transformer Impedance
Primary Nominal Voltage
Secondary Nominal Voltage
Vector Groups
Starpoint
Fault power on 110 kV–side
Based on the data above, the following fault currents are calculated:
3-Phase High Side Fault Current
3-Phase Low Side Fault Current
Current flowing on the High Side
The nominal current of the transformer is:
I = 84 A (High side)
NomT, 110
Current Transformer (High Side)
Current Transformer (Low Side)
Due to the following definition
the following setting applies to the protection device: The 50-2 relay element must be
set higher than the maximum fault current, which is detected during a low side fault on
the high side. To reduce fault probability as much as possible even when fault power
varies, the following setting is selected in primary values: I>>/I
1000 A.
Increased inrush currents, if the fundamental component exceeds the setting value,
are rendered harmless by delay times (address 1203 50-2 DELAY).
For motor protection, the 50-2 relay element must be set smaller than the smallest
phase-to-phase fault current and larger than the largest motor starting current. Since
the maximum appearing startup current is usually below 1.6 x the rated startup current
(even with unfavorable conditions), the following setting is adequate for fault current
stage 50-2:
1.6 x I < 50-2 Pickup <I
Startup
The potential increase in starting current caused by overvoltage conditions is already
accounted for by the 1.6 factor. The 50-2 element may be set with no delay (50-2
DELAY = 0.00 s) since, unlike with e.g. the transformer, no saturation of the shunt re-
actance occurs in a motor.
The principle of the "reverse interlocking" utilizes the multi-element function of the time
overcurrent protection: element 50–2 is used as accelerated busbar protection with a
S
NomT
ZTX = 10 %
V
Nom1
V
Nom2
Dy 5
Grounded
1 GVA
at 110 kV = 5250 A
at 20 kV = 3928 A
at 110 kV = 714 A
I
NomT, 20
100 A / 1 A
500 A / 1 A
ϕϕ–Min
= 16 MVA
= 110 kV
= 20 kV
= 462 A (Low side)
= 10, i.e. I>> =
Nom
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015