Reducing Maintenance Costs for Circuit Breakers

Abstract: Previous techniques for estimating servicing intervals for
high-voltage high-current circuit breakers are inaccurate when based
solely on the number of operations (N) or the number of operations
times the average interrupted current (NI). New techniques for
measuring RMS and I2T values for each arc (breaker operation) have
shown to correlate better with breaker wear when accumulated over a
number of breaker operations. The TACTUS ® (patented) monitor
utilizes these RMS and I2T accumulated values to warn of impending
breaker failures. This provides a means to more efficiently schedule
breaker maintenance operations, thereby reducing maintenance costs for
the breakers.

    The Problem: High-voltage high-current circuit breakers used by
international electric power utilities are expensive to purchase and
maintain. Unanticipated failures of these devices mean customers
without electric service and utilities with lost income. Previous
maintenance regimens measured the total number of operations (N), and
sometimes included a factor based on an estimate of the interrupted
current (I). This resulted in the so-called "NI" method of determining
the need for breaker maintenance.
    When accumulated "NI" factors for a breaker crossed a given
threshold, breaker maintenance was scheduled, regardless of necessity.
Some breakers would fail before scheduled maintenance, while others
were serviced unnecessarily. Much time and manpower were spent
inefficiently.
    The Solution: Power Solutions, Inc. determined that a better
measurement of accumulated wear was required. RMS (root mean square)
current measurements could be made on the interrupted currents, but
breakers could generate arcs lasting longer than one cycle of the
primary power frequency (usually 60 Hz.), thereby invalidating RMS
measurements (made over only one cycle).
    A favorable alternative was obtained by integrating the square
of the instantaneous current during the period of the arc. This
so-called "I-squared-T" (I2T) measurement gave a better indication of
the energy dissipated by the destructive arc passing through the
breaker.
    The product: The TACTUS ® breaker monitor was designed to
measure both RMS and I2T values for arcs during breaker operations.
These values are accumulated on a per-phase basis (all breakers are
3-phase) and compared against programmable trip-points. An alarm relay
is activated when these per-phase accumulated values exceed their trip
points. Separate trip-points for RMS and I2T are maintained by the
TACTUS ® monitor.
    Additionally, the monitor diagnoses itself during powerup and
displays a special heartbeat-pulse pattern on its front-panel to
indicate proper operating conditions. TACTUS ® provides a standard
RS-232 interface to a terminal or host computer, allowing full
programmability for the vast majority of breaker installations found
worldwide. Maintenance personnel can visually determine breaker wear
from the TACTUS ® via several programmable functions.
    The advanced technology embodied in the TACTUS ® monitor
allows operations supervisors to better schedule preventative
maintenance operations on breakers, thereby avoiding costly premature
breaker failures and unnecessary maintenance operations.