Specifications

A3
GE Limitamp
®
Medium Voltage Motor Control
General
A
3. Transformer overload
These functions are basic only and are not intended to be
comprehensive. Ground fault, differential, fault pressure,
undervoltage, etc., are often required and may also be added
to a given control. In addition, a transformer controller must
allow for transformer inrush current and not cause a nuisance
trip-out from a momentary line-voltage dip.
Transformers must be protected from primary and secondary
(winding or downstream) faults. In Limitamp controllers,
current-limiting fuses are applied to protect the transformer
from a primary winding fault, as well as faults in the conductors
from the controller to the transformer. The fuses are selected
to clear high-magnitude fault currents at the first fault half-cycle
and allow the contactor to energize a transformer without
operating on inrush currents. (Inrush currents occur when
transformer is energized, typically 8-12 times rated amperes
for 0.1 seconds). GE Type EJ-2 current-limiting fuses may be
applied when used with an overcurrent relay that is chosen
to coordinate with the EJ-2 fuse and protect the transformer
from damage as a result of a fault in its secondary circuit.
Prot
ection
To determine a basis for protection, refer to ANSI transformer
short-circuit ratings, which define the magnitude and duration
of downstream faults that a transformer can withstand
without damage. A relay would have to be set to operate
before the damage point is reached. Base ratings, impedance
and the connection of the primary and secondary windings
of the transformer must be supplied in order to arrive at the
relay setting. The relay for this purpose can be an electronic
overload relay.
A common problem with single-phased transformers is a
phenomenon known as ferroresonance, which can occur
when an unloaded or lightly loaded transformer sustains an
open conductor in its primary circuit. Ferroresonance causes
system overvoltage as a result of the transformer core
inductance forming a “tuned” circuit with the system distributed
capacitance. To avoid ferroresonance, all three lines must be
switched simultaneously as with a medium-voltage contactor.
However, if one line fuse blows, then single-phasing will occur.
To prevent this, the medium-voltage contactor may be supplied
with a contactor tripping mechanism that operates from a
striker pin located in the fuse. When the fuse element burns
in two, the spring-loaded striker pin is released. It projects
upward and operates a contact that trips the contactor. This
feature, known as blown fuse trip, would provide positive
transformer protection from single-phasing due to blown fuses.
Transformer feeders typically are applied on critical process
applications where it is important to maintain continuity of
operation through a system voltage disturbance. Mechanically
latched contactors allow the contactor to remain closed during
a disturbance. Like circuit breakers, latched contactors are
opened either manually or by means of a shunt trip solenoid.
Capacitor Feeders
Limitamp 400 amp contactors are ideally suited for capacitor
switching applications. (Note: 800 Amp is not rated for
capacitor switching.)
Capacitors may be switched with the motor, but maximum
rating for this function must be determined by motor design.
When the capacitors are provided in Limitamp control, they
are normally mounted in an auxiliary enclosure beside the
Limitamp controller. A capacitor rated up to 200 KVAR can
be mounted in the top of a two-high CR194 enclosure with
the controller in the bottom.
Transformer & Capacit
or Feeders
Table A.4 is a listing of switching capacities for both transformer
and capacitor loads.
Table A.4 CR194 Vacuum Switching Capacities (One-high)
Future Starters
Future squirrel-cage, full-voltage non-reversing starters
can be installed in two-high construction only when factory-
prepared space has been purchased with the original
Limitamp equipment.
8-hour Open
Rating (Amperes)
System
Voltage
3-Phase
Transformers (kVA)
3-Phase
Capacitors (kVAr)
400
2400 1600 1200
4160 2800 2100
4800 3200 2400
7200 4800 3600
800
2400 3200
N/A4160 5600
4800 6400