OVERCURRENT RELAY RATCHETING

Effects of autoreclosing on disk type overcurrent relays (ratcheting)

Electromechanical disk type time overcurrent relays respond to current above their pickup level in a time inversely proportional to the current level. When the current level is above the pickup, the disk begins to turn and continues to turn until the rotating contact meets the stationary contact to cause a breaker trip or until the current drops below the pickup level, as would occur if the fault were cleared by downstream protection.

Disk reset will occur when the current has dropped below the pickup level for a period of time. The disk resets to its original position at a relatively slow rate by the action of a coil spring.

This relatively slow reset action of the disk should be taken into consideration when autoreclosing is applied to any breaker or circuit recloser downstream of this breaker.

The most common application that requires special care is the circuit recloser operating to clear sections of a feeder downstream of a circuit breaker having disk type overcurrent relays applied. The circuit recloser can autoreclose the faulted section multiple times, resulting in several periods of fault current flow through both the circuit recloser and the upstream circuit breaker.

In a coordinated system, the circuit recloser trips the fault and removes the fault current flow before the disk type overcurrent relays reach their trip point. A minimum safety margin is normally provided to assure good coordination. At the point the overcurrent condition is removed, the disk has rotated some percentage of the amount required to provide a trip.

If the disk is not fully reset by the time the circuit recloser operates to re-energize the faulted section, the disk will not have as far to travel as during the original fault. This ratcheting effect could lead to a loss of coordination resulting in the unnecessary tripping of the circuit breaker.

Another example of ratcheting is an application in which two or more breakers having disk type overcurrent relays are in series. When both encounter an overcurrent condition, different time-dial settings or unequal currents due to load could cause the percentage of disk travel of the upstream relay to vary considerably.

Once again, if autoreclosing of the downstream breaker occurs before the upstream device is reset, a loss of coordination could occur.

Solutions for these loss of coordination events include use of larger safety margins to allow for partial reset of electromechanical relays, application of inverse time relays with fast resets on the upstream breakers, or longer dead times for autoreclosing.

Microprocessor or static relay designs may have an instantaneous reset of the timing function as well as other selective reset characteristics, which is normally considered in the coordination process.

AUTO RECLOSE BLOCKING OF POWER LINES

Blocking or disabling of autoreclosing on distribution circuits may be required for various circumstances. Requirements will vary depending on specific design features incorporated into the distribution system. The following are several conditions for which autoreclosing is blocked:

a) Line-side voltage supervision. 
Autoreclosing can be blocked if voltage exists on the line. Such supervision is usually provided if large motors, generators, or other sustained sources are connected to the line.

Autoreclosing is blocked if these downstream sources are maintaining voltage on the line in order to prevent possible damage to the associated rotating equipment due to being energized out of phase or to the establishment of an undesirable system operating condition.

b) Bus faults.
Autoreclosing of lines can be blocked for bus faults.

c) Underfrequency/undervoltage load-shedding schemes. 
When distribution circuits are tripped by action of load-shedding schemes, autoreclosing is blocked to avoid undesired load restoration during system overload conditions. Autoreclosing may be re-established when the frequency/ voltage condition has returned to normal and maintained that condition for some time.

d) Downed conductor protection.
Autoreclosing can be blocked when protective systems designed to protect for a downed phase conductor operate. A downed conductor represents a permanent failure and a safety concern––autoreclosing is undesired for such a condition (Rockefeller et al. [B13].)

e) Voltage unbalance.
Autoreclosing can be blocked if a voltage unbalance condition is detected at the station. A source-side open circuit can cause such unbalances. Restoring service under unbalanced conditions could cause damage to customer equipment.

f) Manual trips.
Autoreclosing is blocked if the breaker is opened manually at the station or by remote control. When a breaker is opened in this manner, it is desirable that it be under operator control for closure.

g) Breaker failure protection. 
Breaker failure protection typically trips all breakers connected to the bus directly or through the bus differential protection. Restoration of the bus is usually under an established procedure that includes isolation of the failed breaker. Autoreclosing of the healthy breakers are blocked until the failed breaker is isolated and the bus is restored.

h) Breaker failure to close.
If an attempt is made to autoreclose, and the breaker does not close (based on 52 auxiliary contact) or fails to close within the expected close time, or if a discontinuity (open circuit) is detected, then further attempts to autoreclose are blocked.

i) Hot-line maintenance.
Improve safety by disabling autoreclosing while line crews perform hot-line maintenance.

j) High-current faults.
A high-set instantaneous element can be used to block autoreclosing for close-in, high magnitude faults. This type of blocking is typically applied where these faults are likely to be permanent or in the substation equipment or exit cables, or exceed the damage rating of the source transformer or other equipment.

k) Breaker failure to trip or trip circuit monitor alarm.
If a relay trip has occurred, but the breaker stays closed longer than the expected breaker trip time (e.g., 6–10 cycles), block autoreclosing and initiate tripping of the backup breaker. Also, if trip circuit logic is available, then use this logic to block autoreclose if an open trip circuit is detected.

l) Cumulative operations lockout.
Used in locations with fault duty approaching adjusted circuit breaker rating to block autoreclosing after a predetermined number of operations until inspection and necessary maintenance can be performed.

POWER DISTRIBUTION SYSTEM DELAYED RECLOSING

Delayed autoreclosing may need to be considered when the upstream protection is provided by electromechanical relays or fuses and the circuit protection is provided by microprocessor-based relays, unless the microprocessor-based relays can be set to mimick the reset characteristic of the electromechanical relays.

Without this time-delay reset feature on the microprocessor-based relay, it is possible to have the upstream device operate incorrectly, resulting in an overtrip. As an example of this, the low-set instantaneous trip on a distribution feeder is eliminated to improve power quality by eliminating momentary service interruptions.

If an instantaneous autoreclose is used after a timedelayed trip, an additional time margin needs to be used between the operating times of protective devices in order to maintain coordination of the feeder overcurrent relays and an upstream electromechanical relay or fuse.

By delaying an upstream protective device to coordinate with the back-toback operation of the feeder relay, coordination is maintained with the instantaneous reclose. Delaying the autoreclosing eliminates this problem by allowing all devices time to rest before the next fault.

Delayed autoreclosing is used on circuits that have automatic sectionalizers to allow proper coordination with the distribution circuit breaker. The time-delay autoreclosing of the distribution circuit breaker needs to be set to match the programmed time intervals of the sectionalizer switches to allow successful isolation of the faulted line section.

Distribution circuits that have customer-owned generators connected to them present a special problem. In most cases, it will be necessary to delay autoreclosing to allow the customer generator to be disconnected before the circuit is re-energized from the utility source.

Removal of the customer generation is normally accomplished by the operation of an underfrequency, undervoltage, or reverse power relay, which tend to have longer tripping times. As the operating time for these devices may be slower to remove the connected generator than the relays that detected and cleared the fault, autoreclosing times could need to be extended to allow these devices to operate or the function be disabled.

In cases where the connected generator is comparable to the load, it may be necessary to provide additional security against energizing the generator out of synchronism. This additional security can be provided by dead-line autoreclosing logic, synchronizing check, or transfer trip protection.