GE STD15C Instructions Manual page 10

GEK-45307
hA Repeat
-
Percent Ratio Error
1. Burden on CTs
Line A, Z
=
0.192
+
0.205
+
0.568
=
0.965
Line B, Z
0.156
+
0.188
+
0.568
=
0.912
Line C, 7
=
0.096
+
0.226
+
0.568
=
0.890
2.
Impedan ce Ohms
0.965 0.912
0.890
3. 8 times Tap, Ampere s
36.8
25.6
69.6
CT voltage required (17)
35.6 23.4
61.9
5.
'C required , from excitatio n curve
1.1 0.25
0.17
5. % of Ratio Error
3.1%
1.0% 0.3%
Percent error is less than 20%, so CT taps and relay taps are satisfact ory.
III Percent Slope Setting
1. Assume load ratio control maximum range
10.0%
2. Relay tap mismatc h, from IA above (Lines A-B)
4.6%
Use 25% setting
14.6%
RATI NGS
MODELS 12STD15C AND 12STD16C
Continu ous Rating
The through -current
transfor mer
and differen tial-curr ent
transfor mer will stand
twice tap value for any combina tion of taps, or they will stand twice tap value if
all
but one
of the restraint winding s
carry 0 current, and the
full restraint
current
(equal to twice
tap value) flows
through the differen tial—cu rrent
transfor mer.
Short Time Rating (Therma l)
220 amperes for 1 second, measure d in the primary of any transfor mer of the type STD
relay. Higher currents may be applied for shorter lengths of time in accorda nce
with the followin g equation :
=
48,400
where
=
current amperes
t
=
time in seconds .
Short Time (Electric al
For both the STD15C and STD16C the sum of the multiple s of tap current fed to the
relay from the several
sets of current transfor mers should not exceed 150.
These
multiple s should be calculat ed on the basis of RMS symmet rical fault current.
This
limitatio n
is a result of the voltage rating of the
rectifier s in the
through —
current restraint circuit.
Note that in Figure 1 external
fault current can flow
through circuit breakers 52—1
and 52—2 without being
limited
by the transfor mer
i in peda n ce.