DCUB scheme: directional comparison unblocking

What the DCUB scheme is

The DCUB (directional comparison unblocking) scheme is, in essence, a POTT scheme with additional logic for loss of channel. It was born with frequency-shift keying (FSK) power line carrier, where the transmitter always sends a guard frequency and shifts to a trip frequency to send the permissive signal.

• With a healthy channel, it works as POTT: each end trips if its overreaching element sees the fault forward and receives the permissive signal.
• If the guard disappears and no permissive signal arrives, the relay assumes the internal fault attenuated the signal and allows tripping during a short window, as if it had received the permissive signal.
• Once the window expires, if the guard does not return, the scheme blocks and the line is left with zone 1 and time-delayed zone 2.

It is a device 85 logic (ANSI/IEEE C37.2), modeled in IEC 61850 with logical node PSCH. The comparison with the other schemes is in the guide on teleprotection schemes.

How it works: logic and signals

The relay receives two pieces of information from the teleprotection equipment: permissive received and loss of guard (channel alarm). The trip logic is:

• Forward overreaching element AND permissive received → trip.
• Forward overreaching element AND loss of guard without permissive, within the window → trip.
• Loss of guard lasting longer than the window → scheme blocked until the guard returns steadily for a set time.

Sequence for an internal fault near B’s line trap that attenuates the signal toward A:

1. The overreaching elements at A and B pick up; both send a permissive signal.
2. A receives neither guard nor permissive: it detects loss of guard.
3. Within the window, A’s overreaching element is already picked up: A trips.
4. B receives A’s permissive signal (or also loses guard) and trips.

The assumption behind the logic, in the words of an SEL technical paper, is that when the channel is lost during a fault it is far more likely to be caused by an internal fault than by an external one. The same paper proposes calling these logics “channel failure trip” (sending a permissive signal on loss of signal) and “channel failure echo”, and estimates the window at around 100 to 200 ms.

Channel and timing

DCUB asks the same of the channel as a permissive scheme (short time and good dependability), plus continuous supervision that tells “no permissive” apart from “no channel”. On power line carrier, the guard frequency provides it; on digital channels, frame supervision and the alarms of the teleprotection equipment provide it. The logic can reside in the communication equipment or in the relay; in the relay it offers more options and leaves a better event record.

Advantages and limitations

Advantages:

• Keeps the security of a permissive scheme with a healthy channel.
• Restores dependability for internal faults that disrupt the channel, the weak point of POTT over power line carrier.
• Limits the risk of a failed channel to a short window, unlike DCB, which stays exposed for as long as the channel failure lasts.

Limitations:

• If the channel is lost for some other reason right when an external fault occurs, the window can produce a false trip.
• It needs the same echo and current reversal logic as POTT.
• The window, the subsequent block and the guard recovery time must be set carefully.

Where it is used

DCUB is common on high and extra-high voltage lines with FSK power line carrier, where a utility wants the security of a permissive scheme without losing dependability for faults near the line traps. It is also applied with digital channels, using the loss-of-channel alarm instead of the guard.

How each manufacturer implements it

GE Vernova (Multilin, EnerVista) offers directional comparison unblocking as a pilot scheme of its own in relays such as the D60. SEL (AcSELerator QuickSet) offers it as a selectable scheme that uses the loss-of-guard input from the communication equipment. Siemens SIPROTEC 4 7SA (DIGSI) includes an unblocking mode in its distance teleprotection. Hitachi Energy Relion 670 (PCM600) treats it as a variant of its permissive logic with channel supervision, and MiCOM P44x relays (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) include loss-of-guard schemes. What changes is the window duration, the subsequent block, and whether the logic is wired from the teleprotection equipment or resolved inside the relay.

Why it fails or misoperates

• Trip for an external fault: loss of guard caused by noise or switching right during the fault, window too long.
• Scheme blocked without anyone knowing: an intermittent guard that left the scheme blocked; no alarm to the control center.
• No trip for an internal fault: loss-of-guard input miswired or inverted.

Compare the records from both ends: guard status, permissive received and time of trip.

How it is tested

Secondary injection verifies the trip with permissive, the trip within the window on loss of guard, the block when the window expires and the recovery. The complete scheme is validated with a GPS-synchronized end-to-end test, which must include a simulated loss of channel.

Did your DCUB trip with the channel in alarm, or stay blocked? Our team reviews the events and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
• IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
• IEC 61850-7-4, logical node models (PSCH).
• Kasztenny, Mynam, Fischer and Guzmán, Permissive or Blocking Pilot Protection Schemes? How to Have It Both Ways, SEL, Western Protective Relay Conference, 2020.
• CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
• Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions