Three features combine to make this
instrument unique: dual measurement, load tap-changer testing and
safety
shutdown. The dual set of potential inputs measure the resistance of
the primary and secondary windings of a single- or three-phase
transformer simultaneously. The dual reading characteristic will
speed up the measurement when it is used to test windings on
delta-delta connected windings on three-phase transformers.
Due to circulating currents induced when the
test current is applied to the primary winding, this type of
measurement is countered by the same current on the secondary
winding. This action attenuates the circulating current and the
reading time is improved tenfold.
The Transformer Ohmmeter is extremely
useful when testing the windings and contact resistance on tap
changers with make-before-break contacts and voltage regulators.
The internal shutdown circuit will be
triggered by a voltage kickback of a few microseconds if the
tap-changer
contacts are opened when the tap-changer circuit is operated through
all of the tap positions. This action will check for pitted or
misaligned contacts as the instrument will shut down if either
condition occurs.
Users are protected by the shutdown circuit
safety feature: any inadvertent disconnection of a test lead or loss
of power to the instrument will safely discharge the energy stored
in the test sample.
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Applications
The Transformer Ohmmeter is used:
- To verify factory test readings.
- As part of a regular maintenance
program.
- To help locate the presence of defects
in transformers, such as loose connections.
- To check the make-before-break operation
of on-load tap-changers.
A regular maintenance program that includes
winding resistance measurements is the most effective way to use
this instrument. Once a benchmark is established, subsequent test
results can be compared to determine if changes are occurring in the
transformers, instrument transformers and associated control wiring,
voltage regulators, motors, generators, breaker contacts, all types
of connections (bolted, soldered, crimped, etc.).
Tap-changers are mechanical devices and the
most vulnerable part of a transformer. Tap-changers result in more
failures and outages than any other component and so require
frequent testing and attention to ensure proper, reliable operation.
The Transformer Ohmmeter can be used to
check the make-before-break operation of on-load tap-changers and
also to measure the contact resistance of each tap position.
Schematic comparison of
one-and-two winding measurements |
