Introduction; Application; Description - GE STD15C Instructions Manual

Relays of the STD type are transformer differential relays provided with the features of percentage
restraint. A static decision unit controls a small telephone type relay
Percentage restraint permits accurate discrimination between internal and external faults
while harmonic restraint enables the relay to distinguish, by the
current caused by an internal fault. and that of transformer
Each Type STD relay is a single phase unit. The Type
of two-winding power transformers and has two through-current restraint
current circuit.
The Type relay is designed for use with three-winding power
ent restraint circuits and one differential current circuit.
protection (see Figure 9) when only three circuits require through current restraint, while the
th circuit. being the weakest, needs no through-circuit
The current transformer ratios and relay taps should be selected to obtain the
overload of the relay or current
out
transformer
efor
the points in mind:
The lower the relay tap and the lower the CT
1.
the lowest CT ratio and the lowest relay tap may not be compatible with
following restrictions.
it is desirable to increase the CT ratio in preference to the
Since the relay burden is likely to be small compared to the lead burden. increasing the
ratio tends to improve the relative
secondary fault current and increasing the accuracy of the
The CT
3. The relay current corresponding to
twice tap value, the thermal rating of the relay.
4. The CT ratios should be high enough that the secondary currents will not
internal fault conditions (refer to RATINGS).
5. The relay current corresponding
should not exceed the relay tap value selected (magnetizing inrush might operate the instantaneous
avercurrent unit).
the transformer manufacturer should be consulted for the "equivalent self-cooled rating
the rating of a self-cooled transformer that would have the same magnetizing inrush
as the transformer being considered.
Courtesy of NationalSwitchgear.com
GEK-45307
TRANSFORMER DIFFERENTIAL RELAYS
PERCENTAGE AND HARMONIC RESTRAINT
AND

INTRODUCTION

DESCRIPTION

APPLICATION

in the various windings of the power transformer should be
a choice is available of increasing either the CT
should not exceed the continuous thermal rating Of the CT
KVA (on a forced cooled
rated KVA of the power
If the transformer under consideration does not have a
3
is designed to be used for the
and has three through
It may also be used for four
or the
the higher will be the sensitivity.
CT's as a result Of
should not
(On Self cooled basis)
the
high
between
Inrush.
and One
sensitivity
Of
of the
Or the
the
the relay under
that is