The Load Center Principle - GE AKD-10 Application Manual

The Load Center Principle

Pioneered by General Electric, load center unit substations
provide reliable equipment for power distribution in industrial
plants and commercial buildings, power station auxiliaries
and other applications requiring continuity of service.
GE offers a complete line of load center unit substations for
indoor or outdoor installations. The unit consists of an
incoming line, a transformer and low voltage sections. Load
center unit substations are handled as a single packaged
system, simplifying engineering coordination and application.
Standard design eliminates unnecessary purchasing and
engineering details. Mechanical and electrical coordination
results in greater reliability. Expert field engineering is available
to ensure proper application, installation and operation.
How to select switchgear
The application tables on the following pages provide a list of
low voltage power circuit breakers available for load center
unit substation applications. The air power circuit breakers
are coordinated with transformers and system capacities
(electrically, thermally and mechanically). For analysis
procedures on motor starting fused breakers, overcurrent
trip details, short-circuit ratings, etc., refer to ANSI C37.13
and ANSI C37.16.
These tables should be used only as guidelines, taking into
consideration voltage, temperature, power factor, altitude and
other service conditions that may affect application on a
particular power system. For instance, under certain circuit
arrangements, the total running motor short-circuit current
contribution may be greater than that shown in the motor
contribution tables. This condition might exist for unit sub-
stations having a high ratio of running motor nameplate
horsepower to actual demand, such as may occur in heavy
machining or stamping press operations. This condition could
also exist when a secondary selective system operates with
one main breaker open and one main and one tie breaker
closed so that the feeder breaker can see "twice" the normal
motor contribution to a short circuit.
For these types of systems, the use of higher-rated or WPF
fused circuit breakers may be required to stay within the
short-circuit rating of the feeder breaker.
Power circuit breakers are available with various combinations
of long-time delay, short-time delay and instantaneous trip
elements. Care should be taken to specify the combination of
trips that will provide the balance of selectivity and protection
required by the power system.
A selectively coordinated substation uses main and tie
breakers with long-time and short-time trip characteristics
(LS) to delay the opening of the main circuit breaker allowing
the faulted feeder an opportunity to clear. This provides service
continuity for all but the faulted circuit and generally allows
coordination of main and tie breakers with the various trip
characteristics (LS) (LSI) (LI) available on feeder circuit breakers.
™
WavePro circuit breakers do not require instantaneous trips
to achieve their short time ratings. Therefore, there are no
hidden instantaneous trip functions that would cause a loss
of selectivity in this type of arrangement.
Selectivity can be carried a step further in the substation by
specifying selective feeder circuit breakers that have long-
time and short-time trip characteristics to allow downstream
devices to clear faults within their area.
A refinement of the selective feeder incorporates the long-
time, short-time and instantaneous characteristics to provide
selectivity without sacrificing instantaneous fault protection
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