Dead line live bus closing: device 25 modes

When one side has no voltage

The synchronism check 25 compares the voltages on both sides of an open breaker and only allows closing if they are similar in magnitude, frequency and angle. But many closes are made with one side dead: energizing a line from the substation, the first end reclosing after a fault, or re-energizing a bus after a blackout. In those cases there is no synchronism to check.

For that, the 25 function has dead-voltage closing modes. In IEEE C37.2 it is device 25, and in IEC 61850-7-4 its logical node is RSYN, which groups the synchronism check and the energizing modes.

The three modes

• Live bus, dead line (LBDL). The bus is live and the line is not. Closing energizes the line from the substation. It is the mode of the end that starts energizing a line.
• Dead bus, live line (DBLL). The line is live and the bus is not. Closing energizes the bus from the line. It is used to restore a substation that lost voltage, feeding it from another one.
• Dead bus, dead line (DBDL). Neither side is live. It is rarely used, for example in black starts, because the relay cannot tell a dead side from one with no measurement.

Each mode has its thresholds: above the live voltage threshold the side is considered live, and below the dead voltage threshold it is considered dead. Between the two no permissive is given. Thresholds are usually expressed as a percentage of nominal voltage, and their values come from the utility’s philosophy and from the induced voltages measured on each line, not from a general table. The modes are enabled separately, and the same breaker often has different modes for manual closing and for automatic reclosing.

Risks of the dead-voltage modes

A side that looks dead. If the fuse or the miniature circuit breaker of the voltage transformer on one side opens, the relay sees zero voltage. With the dead-voltage mode enabled, it would give a close permissive even though the side is energized, and the close could be out of synchronism. That is why dead-voltage modes must be blocked by the voltage circuit supervision, through an auxiliary contact of the MCB or the relay’s loss-of-potential logic. That same condition should also block other functions, among them undervoltage 27.

An island. If an island with its own generation was left on the other side, for example a solar plant or a cogeneration unit that did not disconnect, the side is not dead: it has voltage at a different frequency and angle. The dead-voltage mode does not enable it, but the synchronism check could, if the differences happen to fall within the windows. In networks with distributed generation, the anti-islanding protection of the plant and the time coordination with reclosing are what prevent that case.

Induced voltage. A de-energized line running parallel to another one in service can carry an induced voltage higher than the dead voltage threshold. The relay sees it as live and gives no permissive. The threshold must be set above that voltage, without going so high that an energized line is taken for a dead one.

Coordination with 79 reclosing

On a line with automatic reclosing 79 at both ends, both breakers open on the fault. If both tried to reclose in dead line mode, they could close almost simultaneously, each onto a line it believes is dead, and the second one to close would have no synchronism check. The usual solution is to assign roles:

• Leader end. It recloses first in live bus, dead line mode. If the fault persists, only this end closes onto it and trips again.
• Follower end. It does not have dead line mode enabled for reclosing. It waits to see voltage on the line and closes with synchronism check.

The choice of leader depends on the network. It is common to choose the end farther from generation, so that a close onto a fault does not subject the generators to a second nearby short circuit, or the end with the stronger source. When the line ends at a generating plant, the plant is usually the follower. In breaker-and-a-half the same idea applies between the two breakers of a line, as explained in reclosing in breaker-and-a-half: leader and follower.

Where it is set and how it is tested

The modes and thresholds are set in the line relay or the bay controller, depending on where the 25 function resides in each design. The voltage selection for the reference in double bus, breaker-and-a-half or ring bus arrangements adds another layer, because the bus side must be measured on the bus the breaker is going to close onto.

During commissioning, with secondary injection, each enabled and each disabled mode is tested, together with the live and dead voltage thresholds, the blocking on fuse failure and the coordination between leader and follower ends, ideally with an end-to-end test.

We review your closing modes

Our team reviews and configures closing modes, thresholds and coordination with reclosing in protective relays of the main brands, remotely worldwide or on site in Colombia, or elsewhere on request, for medium, high and extra-high voltage substations, industry and generating plants. See remote relay configuration or contact us.

Related guides

• Single-pole and three-pole reclosing
• Breaker close permissives: what blocks closing
• Reclosing in breaker-and-a-half: leader and follower
• Substation interlocking: types, levels and logic
• Automatic service restoration after a blackout

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, devices 25 and 79.
• IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
• IEC 61850-7-4, logical node RSYN.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions