GE BUS1000, Instructions Manual

Page: 23 / 129

4. APPLICATION
GEK-98514B BUS1000 Busbar Protection 4-3
The auxiliary transformers must be placed preferably as near as possible from the main current
transformers
, in order to reduce the cable resistance seen by the differential protection and thus allow a
lower stabilization resistance value or a lower restraint value. This arrangement also reduces the possibility
for an accidental opening of the main current transformers secondary circuit. Fig 17 represents the
secondary saturation characteristic of the auxiliary transformers.
4.2.3. MEASUREMENT OF K RESTRAINT PERCENTAGE
Figure 1 shows a simplified diagram of the differential unit and its operation with an internal fault.
Figure 3 shows the operation characteristic of the BUS1000 system percentage restraint differential unit,
whose equation is:
I
D
> K I
F
+ 0.1
The K restraint percentage value is defined as the ratio between the current needed in the differential circuit
for the operation of the relay I
D
and the total in absolute values of all the currents in the I
F
relay input circuits
(not the current going through the bus).
This unit's sensitivity for internal faults (differential current equal to restraint current) depends on the K
adjustment selected according to the table:
K SENSITIVITY (A)
0.5 0.2
0.6 0.25
0.7 0.33
0.8 0.5
The selection of the K value is related, as will be shown below, to the R
E
(stabilization resistance in the
differential circuit, 250
Ω
). The value of the K required for a complete stability of protection for external faults,
which may produce saturation in any of the current transformers.
The K value must be selected in order to obtain the required sensitivity for protection according to the fault 's
minimum current.
4.2.4. MEASUREMENT OF R
MAX
Figure 9 shows a simplified diagram of the differential unit and its operation on an external fault that
produces saturation in the faulted line current transformer.
The selection of the adequate stabilization resistance value in the differential circuit and the operation
characteristic of the measuring unit percentage restraint, cause a lack of sensitivity in the protection in these
conditions if the following equation is true:
R
MAX
(1-K)
R
E
>-------------------
2K
R
MAX
being the total resistance seen from the relay terminal supposing the main current transformer is
completely saturated, for the most unfavourable position (greatest resistance value).
The measuring of R
MAX
must be done calculating the value of R for every position:
R = (RsTI + Rp)(NTIX)² + Rs
Where:
RsTI Secondary resistance of the main transformer