Line disconnector interlocking (89L): conditions

What the line disconnector does

The line disconnector, 89L in IEEE C37.2 nomenclature, sits between the bay breaker and the outgoing line or cable. When open, it separates the line from the rest of the substation with a visible isolating distance. The line earthing switch (57L, a grounding switch in IEEE usage) is usually mounted on its structure and earths the outer section so work can be done on it.

Like any disconnector, it is not built to interrupt load. The International Electrotechnical Vocabulary (IEV 441-14-05) defines it as a device that operates only with negligible current or with no significant change in voltage across its terminals. Its interlocking exists to guarantee that condition and to prevent an operation from bringing voltage onto an earthed section.

Usual conditions

In a line bay with a single breaker, the open and close permissives of the 89L are built from these conditions:

1. Bay breaker (52) open. No current through the disconnector.
2. Line earthing switch (57L) open. Closing the disconnector with the earth applied brings the earth into the bay, or short-circuits the voltage coming from the bus as soon as the breaker closes.
3. Bay earthing switches open in the section between the disconnector and the breaker, if any.
4. Valid position of every device involved.

The reverse relationship also exists: the 57L may only close with the 89L open. That is the earthing switch condition, covered in detail in the guide on line and bus earthing switches.

The line disconnector interlocks do not depend on the bus the bay is connected to. That is why, in a double bus, the 89L logic is the same as in a single bus, while the logic of the bus disconnector changes with the transfer.

Switching sequence

The interlocking prevents operation under load, but it does not always impose the order in which the disconnectors of a bay are opened or closed. That order is an established operating practice:

The reason for the order is where a possible arc ends up. If one breaker pole did not open and the disconnector draws an arc, a fault on the line side is cleared by the line protection; one on the bus side is cleared by the bus protection, which trips every circuit on that bus. Energizing follows the reverse order. Some controllers turn this order into an interlock, for example by requiring the 89B closed before the 89L can close. That is a decision of each utility’s philosophy.

Breaker-and-a-half and ring bus

In the breaker-and-a-half arrangement each line sits between two breakers: the bus-side breaker and the center breaker of the diameter. The line disconnector does not have one breaker of its own, but two. Its main condition is therefore that both breakers are open, or at least that the line cannot receive current through either path.

Opening the line disconnector has a use of its own in that arrangement. With the line isolated, the two breakers of the diameter can be closed again to keep the tie between buses while work is done on the line. That operation requires the 89L to be proven open, and the breaker interlocking takes it into account.

In a ring bus something similar happens: each circuit connects to a node between two breakers, and its disconnector depends on both. Once the line disconnector is open, the ring can be closed again with its breakers so continuity is not lost.

In both arrangements the number of conditions grows, and with it the likelihood of a design error. A typical case is interlocking the 89L only with the bus-side breaker and forgetting the center one. The logic of the whole diameter is covered in the guide on breaker-and-a-half interlocking.

The remote end

A line has two ends and each has its own interlocking, with no knowledge of the device positions at the other end. The line disconnector does not need it: with its breaker open it cannot interrupt load, even if the line is energized from the other end.

The difficulty lies in the earthing switch. Closing the 57L with the line energized from the remote end is a close onto voltage. That is why the line earthing switch is conditioned on absence of voltage, measured with the line voltage transformer or a voltage detector, and on an operational confirmation that the other end is isolated. EN 50110-1 sets this out in its working rules: disconnect completely, secure against reconnection, verify absence of voltage, then earth.

Where it is implemented

In hardwired designs, the conditions are built with auxiliary contacts in series with the control circuit of the motor-operated disconnector. In numerical designs they are solved in the bay controller, with one instance of the IEC 61850-7-4 logical node CILO per device and its EnaOpn and EnaCls outputs. In breaker-and-a-half, the position of the center breaker is usually in another controller and arrives by GOOSE. If that message is lost, the permissive must drop.

How it is verified

Each condition is tested positively and negatively: with all of them met the device operates, and negating any single one drops the permissive. In breaker-and-a-half and ring bus the test is repeated with each of the two breakers. The invalid position, the loss of the GOOSE message carrying the other breaker’s state and the behavior from each control level are also checked. The full test sequence is in the guide on interlocking tests.

How we can help with your bay

Our team designs and configures the interlocking logic of line bays in bay controllers of the main brands, for medium, high and extra-high voltage substations and for industry. Field testing is done in Colombia, or elsewhere on request. See secondary engineering and remote relay configuration, or contact us with your substation single-line diagram.

Related guides

• Bus disconnector interlocking: conditions
• Line and bus earthing switch interlocking
• Breaker-and-a-half interlocking: diameter and lines
• Interlocking tests: what is verified at FAT and SAT

References

• IEC 60050-441, International Electrotechnical Vocabulary, 441-14-05 (disconnector).
• IEC 62271-102:2018, Alternating current disconnectors and earthing switches.
• IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16 (interlocking) and 8.4 (measures for working de-energized).
• EN 50110-1, Operation of electrical installations, 6.2 (dead working).
• IEC 61850-7-4, logical nodes CILO, CSWI and XSWI.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers.

Frequently asked questions