Date Code 20000421 Applications 5-5
SEL-251, -2, -3 Instruction Manual
• Damage to underground cable and other line equipment is minimized by not reclosing into
faults beyond other protective devices (the substation circuit breaker doesn't trip in the first
place).
• Other distribution circuits fed from the same substation transformer bank see fewer voltage
distortions due to magnetizing inrush or fault current upon reclosure. Fewer trips means
fewer consequent reclosures, resulting in fewer voltage distortions to other distribution
circuits.
Utilities have written papers about converting from fuse to trip saving on many distribution
feeder protection schemes:
"Distribution Overcurrent Protection Philosophy Used at Duke Power Company," R.D.
Melchior, B.W. Jackson, Georgia Tech Relay Conference, Atlanta, Georgia, April 29 -
May 1, 1987.
"Relaying Changes That Will Reduce Blinks," Neil G. Engelman, P.E., Lincoln Electric
System, Lincoln, Nebraska, 33rd Annual American Public Power Association Engineering
and Operations Workshop, Washington, D.C., March 15, 1989.
"Relaying Changes Improve Distribution Power Quality," Neil G. Engelman, P.E., Lincoln
Electric System, Lincoln, Nebraska, Transmission & Distribution, May 1990, pp. 72-76.
In these papers, trip-saving implementations required extra timers for the instantaneous relays to
realize definite-time overcurrent elements. Definite-time overcurrent elements are already
available in the SEL-251 Relay, eliminating the need for external timers.
BLOCKING SCHEMES FOR COLD LOAD INRUSH
Overcurrent elements with low pickup and low time delay settings can operate unintentionally on
cold load inrush. These overcurrent elements need to be blocked from initiating tripping for a
time period after circuit breaker closure if the potential for cold load inrush current exists.
Two overcurrent element blocking scheme implementations are possible. One scheme uses the
50C Relay Word bit; the other uses the 52AT Relay Word bit.
Blocking Scheme for Cold Load Inrush Using the 50C Bit
50C is a phase instantaneous overcurrent element that is not used in the reclosing relay reset
inhibit function. Thus it can be set below load levels for a blocking scheme for cold load inrush.
50C can assert with the presence of load current or deassert in its absence. With this setting,
50C indicates whether the circuit breaker is closed or open. This scheme is not 52A dependent.
To block an overcurrent element for cold load inrush for a time period after circuit breaker
closure, supervise the element with a time delayed pickup 50C condition.
If 50C is used in this blocking scheme, it cannot be used in the internal torque control settings.
The 50C pickup setting would be too low.
To Next Page
Loading...
