ABB REG316 4 Series Operating Instructions Manual page 681

DEMANDS ON MEASURING TRANSFORMERS FOR RET 316 / RET 316*4
Introduction
The operation of any transformer protection is influenced by distortion in the
measuring quantities. The current to the protection will be heavily distorted when
the current transformer is saturated.In most cases it is not possible to avoid
current transformer saturation for all fault conditions, therefore measures are
taken in the transformer protections to allow for current transformer saturation with
maintained proper operation. RET 316 / RET 316*4 can allow for heavy current
transformer saturation but not an unlimited one.
Requirements on current transformers
Choice of current transformers
The current transformer should be to type TPS,TPX or TPY with accuracy class
5P20 or better. The use of the linearized current transformer type TPZ leads only
to a small phase angle shift and they can be used without problems, if the same
type is on both sides of the transformer. Possibly ABB Switzerland Ltd, Utility Auto-
mation can be contacted for confirmation that the actual type can be used.
The current transformer ratio should be selected so that the current to the protec-
tion is larger than the minimum operating value for all faults that shall be detected.
Minimum operating current for the transformer protection in RET 316 / RET 316*4
is 10% of nominal current.
Conditions for the CT requirements
The requirements for RET 316 / RET 316*4 are a result of investigations performed
in our network simulation program. The tests have been performed with a digital
current transformer model.
The setting of the current transformer model was representative for current trans-
formers type TPX and TPY.
The performance of the transformer protection was checked for internal and ex-
ternal both symmetrical and fully asymmetrical fault currents. A source with a time
constant from 40 up to 300 milliseconds was used at the tests. The current require-
ments below are thus applicable both for symmetrical and asymmetrical fault
currents.
Both phase to ground, and three phase faults were tested.
Released:
ABB Switzerland Ltd
Department:
UTAST
CH-ES 30-32.10 E
02-07-02
Version 3.10
and higher
Rev.: E
1/3