Phase comparison is a line protection that compares, over a channel, the phase angle of the currents at both ends. For load or external faults the currents entering the line are in opposition; for an internal fault they are almost in phase and the protection trips. It sends only the polarity, not the magnitude, so it works even over power line carrier.
What phase comparison is
Phase comparison protection is a pilot line scheme that measures neither impedance nor direction: it compares the phase angle of the currents at the two ends.
With the convention of positive current flowing into the line at each end:
- Load or external fault: current flows in at one end and out at the other. The two currents are in opposition (about 180°).
- Internal fault: both ends feed the fault. The currents are almost in phase.
The protection only needs to know whether the polarities match. It is a form of unit protection, like differential protection: manufacturers designate it 87PC (GE Vernova lists it that way for its Multilin L60 relay), and the pilot channel it uses corresponds to device 85 of ANSI/IEEE C37.2. Its place among the other schemes is covered in the guide on teleprotection schemes.
How it works: logic and signals
The compared quantity. Classic schemes combine the phase currents into a single composite quantity through a sequence network (for example, positive and negative sequence with set weights), so that one channel detects every fault type. Others compare each phase separately, or the negative sequence.
Half-cycle blocks. Each end turns its quantity into a square wave: it transmits during the positive half cycles and stays silent during the negative ones.
- External fault: since the currents are in opposition, when one end is silent the other is transmitting. There is always a signal on the channel from one of the two.
- Internal fault: the currents are in phase, both ends transmit and go silent at the same time, and the channel is empty every half cycle.
| Condition | Angle between currents | Half-cycle blocks | Channel |
|---|---|---|---|
| Load or external fault | Opposition, about 180° | When one end is silent, the other transmits | Always a signal from one end |
| Internal fault | Almost in phase | Both transmit and go silent together | Empty every half cycle |
Tripping. In the blocking version, the signal received from the remote end acts as a block: if, during a half cycle, there is no local or remote signal for longer than a set angle, the relay trips. In the permissive (or tripping) version, the local and remote blocks must coincide for that angle. The margin against errors is called the stability angle: it covers uncompensated channel delay, current transformer errors and the line charging current.
Fault detectors. Classic schemes use two levels: a sensitive detector that starts transmission and a less sensitive one that enables tripping. That way one end never trips before the other has started transmitting.
Channel and timing
Phase comparison sends little information (one polarity per half cycle), so it was born on power line carrier and today also runs over fiber or digital channels. It requires the channel delay to be known and stable, or compensated, because any delay adds to the apparent phase shift between currents. Modern digital relays compensate the delay and its asymmetry; one commercial GE Vernova relay for this principle, for example, states that it compensates channel asymmetry and line charging current and operates over existing power line carrier or fiber on two- and three-terminal lines.
If the channel is lost: in the blocking version, without the remote signal, gaps also appear during an external fault. As in the DCB scheme, the protection still trips internal faults, but it may overtrip on external ones if channel supervision does not block it; in the permissive version the protection stops tripping by comparison and only the backup remains.
Advantages and limitations
Advantages:
- The principle uses currents only: loss of potential, series compensation and zero-sequence mutual coupling do not affect it the way they affect distance protection.
- Low bandwidth demand: it works over power line carrier.
- Absolute selectivity of unit protection.
Limitations:
- Sensitive to uncompensated channel delay.
- The charging current of long lines or cables shifts the angles and forces compensation or a wider stability angle.
- An end with very little infeed, or with outfeed during an internal fault (external feed on three-terminal lines), may not produce the expected coincidence.
- It is not backup protection: faults outside the line need distance or other elements.
Where it is used
It is applied on high and extra-high voltage lines, especially where the utility has power line carrier and wants unit protection without a high-capacity digital channel, and on lines with series compensation or other conditions that are difficult for distance protection. Where dedicated fiber is available, many utilities now prefer line differential 87L, which compares magnitude and phase.
How each manufacturer implements it
Few manufacturers offer a dedicated phase comparison relay today. GE Vernova has one, the Multilin L60 (function 87PC), for overhead lines, including series-compensated ones, and cables, over power line carrier or fiber. Other manufacturers cover the same need with line differential or directional comparison schemes. What changes is the compared quantity, the stability angle, the fault detectors and how the channel is compensated.
Why it fails or misoperates
- Trip for an external fault: poorly compensated or variable channel delay, reversed polarity on a current transformer, stability angle too narrow.
- No trip for an internal fault: starting or tripping detector not sensitive enough for the actual infeed, weak-infeed end, outfeed.
- Trip on energization: uncompensated line charging current.
How it is tested
At each end, secondary injection verifies the detectors, the block generation and the stability angle. The test that proves the scheme is an end-to-end test with two GPS-synchronized test sets, varying the angle between the injected currents and measuring the actual channel delay.
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References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEC 60834-2:1993, Performance and testing of teleprotection equipment of power systems – Part 2: Analogue comparison systems.
- CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
What is the difference between phase comparison and line differential 87L?
87L exchanges the full currents (phasors or samples) and compares magnitude and phase. Phase comparison sends only the polarity of a current, in half-cycle blocks, and compares the angle. It needs less channel capacity, which is why it can run over power line carrier.
Does phase comparison need voltage transformers?
The comparison itself uses currents only. Some relays add voltage or distance elements for backup or supervision, but the tripping principle does not depend on measuring impedance.
Why does phase comparison need channel delay compensation?
Because it compares the arrival time of the remote end's polarity blocks with the local ones. An uncompensated channel delay looks like a phase shift between the currents and reduces the margin for external faults.