TRANSMISSION LINE PROTECTION LENGTH CONSIDERATIONS BASICS

Short lines (SIRs > 4)
Typical protection schemes and communication channels used are as follows:
— Current differential (FO, PW, AT, MW)
— Phase comparison (FO, PW, AT, PLC, MW, R)
— POTT (FO, PW, AT, PLC, MW, R)
— Directional comparison blocking (FO, PW, AT, PLC, MW, R)

Short lines result in a small current magnitude difference and minimal voltage drop difference between close-in and remote faults. Non-pilot overcurrent relays usually cannot be set to discriminate between internal and external line faults. Non-pilot distance relays may be able to discriminate between internal and external line faults for lines with an SIR as high as 20.

However, it may not be possible to coordinate non-pilot distance relays on adjacent long lines with the short line distance relay zones. Therefore, pilot relay schemes utilizing communication channels are used. The most effective forms of pilot protection for short lines include current differential, phase comparison, POTT, and directional comparison blocking schemes.

None of these schemes require distance elements to be set for less than the line impedance. Schemes utilizing Zone 1 distance elements should provide the capability to handle arc resistance or fault impedance, which can be significant compared to the line impedance.

Current differential and phase comparison systems do not provide remote backup for adjacent system elements. However, non-pilot overcurrent and distance relay schemes can be applied as backup protection, provided time delays are adjusted to provide coordination.

Medium lines (SIRs < 4, but > 0.5)
Typical protection schemes used are as follows:
— Phase comparison (AT, PLC, MW)
— Directional comparison blocking (AT, PLC, MW)
— Permissive underreaching transfer trip (FO, MW)
— POTT or unblocking (FO, MW, PLC)
— Step distance
— Step or coordinated overcurrent
— Inverse time overcurrent
— Current differential

Lines with SIRs less than four allow more effective relay discrimination. Relay schemes utilizing under reaching elements can now be set. Zone 1 can be set to underreach the remote end and still protect 80 90% of the line.

The last 10–20% of the line can be protected by a Zone 2 overreaching element. Since selectivity and sensitivity can usually be met, the speed with which a fault needs to be cleared may require the application of pilot relaying. If slow fault clearing is acceptable, simple step distance or overcurrent relays can be applied.

Long lines (SIRs < 0.5)
Typical protection schemes used are as follows:
— Phase comparison (PLC, MW)
— Directional comparison blocking (PLC, MW)
— PUTT, POTT, or unblock (AT, PLC, MW)
— Step distance
— Step or coordinated overcurrent

Long lines have small SIRs, and most long lines are extra high voltage (EHV) or ultra high voltage (UHV) lines. EHV and UHV lines almost always require high-speed tripping of all terminals for stability purposes and to minimize damage caused by the fault.

Long lines can also have other system elements included in the line, such as series capacitors. This makes the total line impedance variable under certain conditions and introduces transient behavior that makes selectivity difficult. Phase and directional comparison protection schemes over PLC or MW are well suited to this type of application.d to provide coordination.