Bus Disconnector Interlocking: Conditions

What the bus disconnector does and why it is interlocked

The bus disconnector, designated 89B following the device numbers of IEEE C37.2, connects a bay to the substation bus. In configurations with two buses it also works as a selector: the circuit is on the bus whose disconnector is closed. Its function is to provide a visible isolating distance, not to switch load.

The International Electrotechnical Vocabulary (IEV 441-14-05) defines the disconnector as a device that opens or closes a circuit when the current is negligible or when no significant change of voltage appears across its terminals. That definition is the basis of all its interlocking: each condition placed on it aims to guarantee one of those two situations before the blades move.

If the disconnector opens under load current, an arc forms between the blades that does not extinguish on its own. On the bus disconnector the risk is greater than on the line disconnector, because an arc that spreads to earth or between phases becomes a bus fault, and the bus differential protection 87B trips every circuit connected to that bus.

Usual interlocking conditions

In a conventional bay with a single breaker, the close and open permissions of the 89B are built from these conditions:

1. Bay breaker (52) open. This is the main condition: with no breaker closed, there is no current to interrupt.
2. Bus earthing switch open. Closing a disconnector onto an earthed bus means bringing the earth to the circuit, or short-circuiting the bus if the circuit is live.
3. Bay earthing switches open on the section between the disconnector and the breaker, where they exist. For the same reason: the disconnector must not join an earthed section to one that may be energized.
4. Valid position of every device involved. If any reports an intermediate or invalid position, the permission is withdrawn.

These conditions apply to both the open and the close operation. Some designs add that the line disconnector must be open before the bus disconnector closes when energizing a bay, but that is a switching-order rule rather than a matter of device safety, and it belongs to the line disconnector logic.

The exception: on-load transfer between buses

In a double busbar configuration a circuit can move from one bus to the other without interruption. The sequence is well known: with the coupler closed, the disconnector to the new bus is closed and then the one to the previous bus is opened. During that operation the bay breaker stays closed, so condition 1 is not met and the interlock needs an alternative path.

That path is bus-transfer switching. If the coupler is closed and both of its disconnectors are too, the buses are at the same potential and the bay disconnector only commutates the part of the current shared between two parallel paths. IEC 62271-102:2018 recognizes that duty and assigns the disconnector a bus-transfer current and voltage (clause 5.108 and Annex B). The capability depends on the transferred current and on the size of the loop between the coupler and the disconnector.

That is why the alternative condition is usually: coupler closed, both of its disconnectors closed, and the other bus disconnector of the same bay closed. The National Grid interlocking specification (TS 3.01.01) lists, among the purposes of interlocking, ensuring the correct sequence of on-load bus transfer. While the transfer lasts, the coupler must be blocked against opening, because if it opens with both bay disconnectors closed, all the exchange current between buses flows through those disconnectors.

Variants by configuration

• Single bus and sectionalized single bus. Only the breaker-open and earthing conditions exist. There is no on-load transfer.
• Main and transfer bus. The main bus disconnector behaves as in a single bus. The disconnector to the transfer bus has its own logic, which depends on the transfer breaker and on no other circuit already being transferred.
• Double bus and double bus with transfer. The bus-transfer condition described above is added.
• Breaker-and-a-half and ring bus. Each breaker has its disconnectors, and the permission depends on the breaker the disconnector belongs to, not on the line. The bus disconnector is conditioned by the bus-side breaker and by the earth of that bus.
• GIS. Many bays use a three-position switch (closed, open and earth) in which the mechanism itself prevents going from closed to earth without passing through open. The electrical interlock is kept for the conditions involving other devices.

Where it is implemented

In older substations the interlock is hardwired with auxiliary contacts of the devices in series with the coil or the motor of the disconnector motor drive. In current ones it is solved in the bay controller. In IEC 61850-7-4 each device has an instance of the CILO logical node with two outputs, EnaOpn and EnaCls, which enable opening and closing; the algorithm is defined by the implementer. The positions of other bays, such as that of the coupler, arrive in GOOSE messages when the condition is station level.

A point that is often forgotten: the disconnector replica used by the 87B takes the same auxiliary contacts. A badly adjusted contact affects the interlock and the busbar protection at the same time, as explained in disconnector replica for the 87B.

How it is verified

During commissioning, each condition is tested positive (with the condition met, the device operates) and negative (negating a single condition drops the permission). The test also checks the behavior with an invalid position, the loss of the signal coming from another bay, and control from each level: switchyard, bay, station and control center. See interlock testing for the full test matrix.

How we can help

At Tiza Group we design, configure and test bay and station interlocking logic in controllers of the main brands, for substations at medium, high and extra-high voltage and for industry. We prepare the secondary engineering, configure the controllers through our remote relay configuration service and test on site in Colombia, or elsewhere on request. Contact us with the configuration of your substation.

Related guides

• Line disconnector interlocking: conditions
• Line and bus earthing: interlocking
• Bus coupler interlocking: when it must not open
• Double busbar interlocking: coupler and transfer
• Disconnect replica for bus differential protection

References

• IEC 60050-441, International Electrotechnical Vocabulary, 441-14-05 (disconnector).
• IEC 62271-102:2018, High-voltage switchgear and controlgear – Part 102: Alternating current disconnectors and earthing switches, clause 5.108 and Annex B (bus-transfer switching).
• IEC 61850-7-4, logical nodes CILO, CSWI and XSWI.
• IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16 (interlocking).
• National Grid, TS 3.01.01 (RES), Substation Interlocking Schemes, issue 1, 2018.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers.

Frequently asked questions