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There are four common pilot protection schemes for line protection used today: (1) Direct Underreaching Transfer Trip, (2) Permissive Overreaching Transfer Trip, (3) Direct Comparison Unblocking, and (4) Direct Comparison Blocking. We will examine the communications channel requirements and channel types used for each of the schemes.

The Scenario

Telephone companies will no longer support audio leased lines starting as early as 2015. This presents an issue for utilities currently operating their RFL™ 9745 equipment with an analog communications interface. In some scenarios, full or fractional T1 leased digital data service lines are available for a nominal monthly fee. Utilities are looking for a path forward that minimizes operational impact, requires little engineering, and assures comparable security & dependability for their protection signalling.

Pilot Protection Relaying uses a communications channel (pilot channel) to provide coordination between the line protection relays. The relaying system is an advanced alternative to Step Distance Relaying which requires no communication and therefore manual coordination between relays.

Communications assisted methods are unique in that they effectively eliminate the non-detection zones (NDZ) found in many local methods used for islanding detection. As a result, local protection elements may be set less sensitively, reducing false operations during system disturbances.

Power system islanding occurs when distributed generation becomes isolated from the power system grid and continues to provide power to the portion of the grid it remains connected to. Islanding can occur through the operation of switching devices such as breakers, disconnects or reclosers. Utilities can also experience islanding with system faults, switching operations, environmental causes and equipment failure. For example, a fault causing a recloser to open and lockout causes the generator to become islanded from the source station.