Siemens SIPROTEC 7SJ62 Instruction Manual page 167

7SJ62 Manual
C53000-G1140-C121-1
pickup and delay such that operation of the 50-2 element will coordinate with other
protective equipment in the system.
Below is an example of how a 50-2 relay element might be set to protect a power
transformer in a radial distribution system against high magnitude internal faults:
Example: Transformer used to supply distribution bus with the following data:
Base Transformer Rating
Transformer Impedance (Z
Nominal High Side Voltage
Nominal Low Side Voltage
Transformer Connection
High Side Fault MVA
High Side Current Transformer Ratio
Low Side Current Transformer Ratio
Based on the data above, the following are calculated:
Three-phase high side fault current @ 110kV 5250 A
Three-phase low side fault current @ 20kV
Three-phase low side fault current @ 110kV
Transformer full load current @ 110kV
(I )
Base-110kV
Transformer full load current @20kV
(I )
Base-20kV
High side current transformer ratio (CTR-HS)
Low side current transformer ratio (CTR-LS)
The minimum pickup setting for the 50-2 element can be governed by a single inequal-
ity:
If the pickup of the 50-2 relay element is set according to the inequality above, the 50-
2 element will never pickup for a fault beyond the transformer's low-side bushings,
even if changing system conditions increase the high side fault MVA. Using the ine-
quality above as a guide, a setting of 10.00 amperes is chosen for the 50-2 element.
At address , a short time delay should be entered to prevent inrush currents from
initiating false trips.
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 motor starting current is generally on the order of 1.6 times the nominal
motor current, the 50-2 phase element should be set as follows:
⋅
1,6 I
startup
The potential increase in starting current caused by overvoltage conditions is already
accounted for by the 1.6 factor. The 50-2 phase element may be set with no delay
since, unlike the transformer, no saturation of the cross reactance occurs in a motor.
If the reverse interlocking principle is used (see Subsection 6.2.1.3), the 50-2 element
can be employed in a high speed bus protection scheme. With a brief safety delay en-
tered at address  (50 ms), the 50-2 element can be blocked for faults beyond the
16 MVA
) 10 %
TX
110 kV
20 kV
Delta-Grounded Wye
1,000 MVA
100 A / 1 A
500 A / 1 A
3928 A
714 A
84 A
462 A
100 A / 1 A
500 A / 1 A
I
1
Base-110kV
> ×
--------- - --------------------------- -
50-2 Pickup
Z CTR-HS
TX
< <
50-2 Pickup I
φφ Min
–
Functions
6-27