Abstract—An age-old battle has been raging since the first
electrical distribution system was installed. How sensitive do we
set protective relays to be assured all faults are detected and
isolated without risking overtrips? Overtrips isolate parts of a
system more often than necessary, causing increased outages and
potentially risking system stability. Conversely, an overly secure
protection system may not detect some faults, leading to
equipment damage. Typically, when a utility experiences a
failure to trip, the relay sensitivity is increased. Several years
may pass without incident, and then the line experiences an
overtrip. After an initial analysis of the events, there is a
tendency to undo the decision to increase sensitivity in favor of
more security. This scenario begins yet another cycle of
dependability versus security.
With today’s multifunctional protective relays, powerful
protection schemes can be realized that provide the relay
engineer with the capability to achieve both dependability and
security without compromising either. However, the typical
practice when these new systems are installed is to copy the
electromechanical protection settings and schemes, especially if
these are the standards for that utility. There is a prevailing
mindset to not change these schemes and standards; however, to
achieve increased dependability and security, we must step
outside of our protective box.
When properly designed, today’s protection systems provide
better performance than electromechanical protection systems.
For example, line protection schemes such as POTT (permissive
overreaching transfer trip) and DCB (directional comparison
blocking) have evolved into hybrid versions. With high-speed,
inter-relay communications combined with many new and
advanced relay elements, the relay engineer has an opportunity
to further improve these schemes, which was not possible with
electromechanical relays. Improvements include combining the
best features of several schemes, adding direct tripping for closein
faults, setting separate timers for fast and delayed tripping on
DCB schemes, and adding additional supervisory permissives,
such as undervoltage elements, to allow the signal to echo back.
This paper delves into applying new modifications to age-old
proven schemes and then analyzes the potential benefits,
enhancing dependability, security, or both.