pplication Notes
P74x/EN AP/N
1, P742, P743 (AP) 6-
2.2.6.1 Application Example
Exam pl e of use of high speed d etectors
I>2 and/or IN>2 to block the 87BB
elem ent before CT saturation
3000/5A
3000/5A
3000/5A
150/5A
25VA
5P10
150/5A
25VA
5P10
I>2 enabled
IN>2 enabled
ph- ph <300A
ph- N 0A
1500/5A
ph-
ph 30 000A
ph-
N 7 500A
I>2 enabled
IN>2 enabled
Blocking order
to 87BB elem en t
TR11
115/13,8K
25 MVA
X = 1 2%
TR12
115/13,8K
25 MVA
X = 1 2%
P3 77 0E Na
FIGURE 1: TRANSFORMER FEEDER EXAMPLE
An example where this facility is required and where there is a high risk of CT saturation, is
shown in the above example.
The problem lies in the transformer feeder circuits TR11 and TR12 both 25 MVA 115/13.8kV
rated and with a reactance of 12%. Both feeders are equipped with 150/5 A CTs. (If rating
is 25 MVA I=125A @115 kV). Maximum busbar short circuit current is 30kA phase to phase
and 7.5kA phase to earth.
The contribution of each transformer feeder under internal fault conditions is as follows:
1. Maximum of 1045A i.e. full load current x 1/X%, assuming infinite 13.8kV bar (which is
unrealistic).
2. 0 A for phase to earth faults.
When an external fault occurs on one of these transformer feeders, the fault MVA will be the
same as that for an internal fault but the feeder will be subjected to an excessively high
overcurrent condition as compared to normal load conditions at rated current.
In the example shown, under the external fault condition, the short-circuit phase to phase
current is 200 times the primary rated current. Taking into account the CT and initial flux
estimated at 80% of that at full load, saturation will be detected at 10 times In, where In is the
CT nominal current – in this case in primary values (150 A x 10 = 1500 A).
With Ι
saturation
= 1500 A and Ι
short-circuit
= 30000 A = 20 x Ι
saturation
.
If the assumption is taken that there is no remnant flux, saturation will be detected 1.4 ms
after the appearance of the fault at which time the current will have reached 0.4 times the
maximum value i.e. 1200 A.
Conclusion: An ultra fast Overcurrent detector in the P742 and P743 when used on HV/MV
transformer feeders makes it possible to pre-empt CT saturation and establish an external
fault condition. The setting used for this detection is Ι>2 for phase faults and ΙN>2 for earth
faults.
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