GE BUS1000, Instructions Manual

Page: 24 / 129

4. APPLICATION
4-4 BUS1000 Busbar Protection GEK-98514B
Rp Total resistance (wires and other connected circuits)between the main transformer and the
auxiliary transformer
NTIX Transformation ratio of the auxiliary transformer
Rs Total resistance between the auxiliary transformer and the relay
The highest R value found will be considered as R
MAX
R values can be determined through direct resistance measurements from the relay terminals, short-
circuiting the main current transformer secondary. It is recommended to include these measurements in the
protection set up guides.
R
E
= 250
Ω
, then the values of R
MAX
should be:
NTIX 2
R
MAX
< ----------- * 250 * K
X
NTGL
where:
K
X
= 2K/(1-K)
NTIX: Transformation ratio of the auxiliary transformer
NTGL: Global transformation ratio.
As described in the K RESTRAINT PERCENTAGE MEASUREMENT, if the calculation of the required RS
indicates a value above 250
(maximum protection range), the K adjustment value will be increased,
resulting in a modification of the protection sensitivity.
4.2.5. ADJUSTMENT OF THE SUPERVISION DIFFERENTIAL UNIT
The supervision unit included in the differential protection is an overcurrent unit independently adjustable
between 0.2 and 2 amps. Both the main differential unit and the supervision unit described must operate so
that the bus differential relay may trip.
This unit has a double purpose. Firstly, it provides the relay with security, thus avoiding any false trips which
may be produced in the case where one of its components should fail; and secondly, it makes it possible to
limit the sensitivity of the protection (for example, if the protection operation is to be avoided in the case
where the overcurrent transformer secondary circuit opens up) without modifying the restraint characteristics
of the main unit.
Should there be no need to limit the relay sensitivity, the adjustment of this unit must be equal to the
operation value of the main unit, determined be the selected K unit.
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