GE BUS1000, Instructions Manual

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3. OPERATION PRINCIPLES
3-6 BUS1000 Busbar Protection GEK-98514B
The R
E
has a fixed value set at 250 , so the R
MAX
value must be such that the below equation is
accomplished in order to avoid false operations with external faults, even in the worst saturation conditions
of the main C.T's
3.4. DIFFERENTIAL SUPERVISION UNIT
The supervision differential unit consists of a level detector with similar characteristics to that of the main
unit, to which is applied the V
D
voltage only, proportional to the differential current. Its operation threshold is
directly adjustable in Amps from 0.2 A to 2 A and independently from the K adjustment of the main unit (see
block diagram in figure 3).
The combination of both units described provides a great security to the protection, thus guarantying that
any failure of a component will not provide a non-desired trip to all the positions associated to the protected
bus. Both units must operate simultaneously so that a trip output is produced. In the case where due to a
failure only one of the units is operating incorrectly, the alarm unit described below will detect it, thus
providing a signalling output and the blocking of the protection.
3.5. ALARM UNIT
The alarm unit associated to the differential protection consists of a very sensitive overcurrent unit (0.027
amps) connected in series to the differential circuit through its own input transformers.
It is aimed at detecting unbalances in the differential circuit due to leaks or accidental disconnection of any of
the inputs to the measure unit. It is also provided with a circuit that detects discordance among the outputs
of the main measuring units and the outputs of the main measuring and supervision units.
The unit provides a timed output (10 seconds).
Figure 5 shows the block diagram.
3.6. LINE OVERCURRENT AND BREAKER FAILURE SUPERVISION UNITS
These units are optional and may belong to a complete BUS1000 system (current supervision only,
breaker failure only or both).
Figures 6 show the block diagrams of a double bus system with both functions for three-phase trip line
protection and single-phase trip respectively.
The units are connected in series to the inputs of every position of the bus differential (one for each position),
through their own input transducers and signal conditioning. Signals coming from each phase are combined
in a selection circuit of the larger before going on to the level detectors of the trip supervision unit (50) and of
the breaker failure unit (BF).
The breaker failure unit picks up its timer only when an external signal comes from line protection relays. In
the case of lines with single-phase trip protection, the level detector receives only signals from those phases
which have been tripped, in order to avoid the operation of the unit with the load current of the non faulted
phases. (See drawing 226B6429F20,21,22). Signals 89AY and 89BY provide information to the bus to which
the line is connected (double-bus systems), in order to lead the trip to the positions connected to the
corresponding bus.