4 Common Pilot Protection Schemes for Line Protection

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.

Direct Underreaching Transfer Trip (DUTT)

Channel Requirements

• Operate Time – DUTT applications require typical channel times between 10 and 40ms. These times are not as critical as Blocking or Permissive applications.
• Security – DUTT applications require very high security to prevent the channel from directly causing a false trip output as there is no other supervision required. Typical probability of an unwanted command (P_UC) between 10^-6 and 10^-8.
• Dependability – DUTT applications require high dependability in order to be effective but are also typically backed up by zone 2 protection with a delay. Typical probability of a missed command (P_MC) of <10^-4.

Channel Types

• Audio Tone Channel – Commonly used because they offer reliability due to diverse routing. Channels require medium bandwidth to provide the required operate times. Applications always use dual tones, on a dedicated channel, to provide high security. Currently being phased out by some telco providers.
• Digital & Fiber Optic – Becoming more popular because of the fast operate times, high security, dependability and increasing availability.
• Power Line Carrier – Popular because of a large install base. The power utility has complete control over the communications channel, and the equipment. Unblock trip output is never used in DUTT applications to prevent false operations on loss of channel.

Permissive Overreaching Transfer Trip (POTT)

Channel Requirements

• Operate Time – Permissive applications require channel times between 10 and 20ms. These times are necessary to allow the remote terminal to trip quickly for all internal line faults
• Security – Permissive applications require security to prevent the channel from enabling a trip. Trip is supervised by the local relay so false operations cannot be caused directly by the channel. Typical probability of an unwanted command (P_UC) between 10^-3 and 10^-7.
• Dependability – Permissive applications require high dependability to permit high speed clearing of both terminals. Typical probability of a missed command (P_MC) of <10^-3.

Channel Types

• Audio Tone Channel – Commonly used because they offer diverse routing. Channels require medium to large bandwidth to provide the required operate times. Currently being phased out by some telco providers.
• Digital & Fiber Optic – Becoming more popular because of the fast operate times, and increasing availability.
• Powerline Carrier – Popular because of a large installed base. The power utility has complete control over the communications channel, and the equipment. Unblock trip output is always provided, either programmed into the relay, or the communication equipment.

Direct Comparison Unblocking (DCUB)

• Works just like a Permissive application.
• Unblocking feature provides a “window” for the local relay to trip, unsupervised, during a communications failure. Typically about 150ms.
• Commonly used with Power Line Carrier channels since a line fault can cause a communications failure.

Direct Comparison Blocking (DCB)

Channel Requirements

• Operate Time – Blocking applications require channel times between 10 and 15ms or possibly faster. These times are necessary to prevent the remote terminal from “Over Tripping” on external faults.
• Security – Blocking applications require less security because the communication channel cannot cause a false trip. Typical probability of an unwanted command (P_UC) between 10^-3 and 10^-4.
• Dependability – Blocking applications must be dependable because the relaying system will operate without the teleprotection channel (“over tripping”). Typical probability of a missed command (P_MC) of <10^-3.

Channel Types

• Audio Tone Channel – Rarely used because operate times are too slow. Channels require wide bandwidth for fast operate times. Currently being phased out by some telco providers.
• Digital & Fiber Optic – Becoming more popular because of fast operate times, and increasing availability.
• Power Line Carrier – Most popular because of a large installed base. The power utility has complete control over the communications channel, and the equipment. Large bandwidth On/Off (AM) channels commonly used to meet speed requirements.