Automatic switching sequences in substations

Short answer

An automatic switching sequence executes, with a single operator command, the series of commands that takes a bay or a bus from one state to another: for example, moving a bay from one bus to the other or taking it out of service and earthing it. Each step is a normal command, subject to the same interlocks, and the sequence stops as soon as a step is not confirmed.

What an automatic switching sequence is

Many substation switching operations are fixed series of steps. Moving a bay from one bus to the other in a double bus requires closing the coupler, closing the disconnect towards the new bus, opening the one to the old bus and, if desired, opening the coupler again. Taking a bay out of service and earthing it requires opening the breaker, opening the disconnects, verifying the absence of voltage and closing the earthing switches.

An automatic sequence executes that series with a single operator command. The operator chooses the sequence, the system checks the start conditions and executes the steps one by one, waiting for each to be confirmed before giving the next.

What is usually automated

Transfer of a bay between buses in double bus, triple bus or sectionalized double bus.
Taking a bay out of service: opening, isolation and, depending on the owner’s philosophy, earthing.
Putting a bay into service: the reverse sequence, with earths removed and closing with synchronism check where applicable.
Moving a circuit to the transfer bus, with the protection transfer as an intermediate step.
Clearing a bus for maintenance: transfer of all its bays to the other bus and opening of the coupler.

Operations that require physical checks the system cannot make, such as visually checking a blade or applying portable earths, are left out or included as manual confirmation steps.

How it coexists with the interlocks

The design rule is simple: a sequence is not a shortcut. Each step is a normal command that follows the same path as an operator command: selection, check of interlocks and conditions, execution and confirmation. In IEC 61850, each step is a command to the device’s CSWI, which checks its CILO before executing.

Execution of each step of a sequence

01Condition checkRemote mode, no blocking alarms, device available
02Device selectionSelect-before-operate command, when the design requires it
03Interlock checkThe device's open or close permission must be active
04ExecutionThe command reaches the device operating mechanism
05ConfirmationThe final position is received within the supervision time

If any of these points fails, the step is rejected and the sequence stops. That way the automation never leaves the substation in a state an operator could not have reached with individual commands.

Where the sequence runs

Bay controller: single-bay sequences. They remain available if the station network or the station controller is lost.
Station controller or gateway with logic: sequences involving several bays, such as bus transfer or clearing a bus.
Dedicated PLC: in some industrial substations or projects where the owner prefers to separate automation from control. It is programmed per IEC 61131-3, as explained in PLCs in substations.
SCADA or control centre: some systems run sequences from the upper level, sending individual commands to the substation.

The location decides what happens if a level is lost: a station sequence stops if the station controller goes down; a bay sequence does not.

Why it stops

Interlock without permission: the most common cause. It can be legitimate (the real state does not allow the step) or an information problem (an invalid position or a lost GOOSE message).
Step not confirmed in time: the disconnect did not reach its final position within the supervision time, because of a problem with the motor operating mechanism or its auxiliary contacts.
Intermediate or invalid position during or after a step.
Protection trip in any affected bay.
Change to local control at the bay or at the device.
Operator cancellation.

In every case, the sequence must report clearly which step failed and why. A generic «sequence aborted» message forces the cause to be reconstructed by hand. The causes of a rejected command are reviewed in command rejected from SCADA.

What the design decides

Start conditions: which states allow the sequence to start.
Supervision times by type of device.
Manual confirmation steps, if any.
Behaviour on stopping: remain in the state reached or allow a reverse sequence started by the operator.
Logging: each step, its result and its time must be recorded in the event log.
Access: who can run each sequence and from which control level.

Sequences and personnel safety

A sequence commands devices that may be near people. That is why many operating philosophies require earthing or earth-removal sequences to include an operator confirmation step, and require the absence of voltage to be verified by means accepted by the work rules, such as the five safety rules of EN 50110-1. The sequence speeds up switching, but it does not replace the isolation and lockout procedure or the permit to work.

How it is tested

Sequences are tested at the factory and site acceptance tests (FAT and SAT). In general terms, each complete sequence is run with the real or simulated devices, and then each cause of stopping is provoked at each step: interlock without permission, timeout, intermediate position, trip, change to local and cancellation. It is checked that the sequence stops, that it reports the correct cause and that the event log is complete.

We can help with your sequences

We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. At Tiza Group we do the secondary engineering of bay and station controllers, with their interlocks and automatic sequences, and the FAT and SAT of the control system, for medium, high and extra-high-voltage substations and for industry. Site work is done in Colombia, or elsewhere on request.

References

IEC 61850-7-4, Compatible logical node classes and data object classes (CSWI, CILO, XCBR, XSWI).
IEC 61850-7-2, Abstract communication service interface (ACSI) (control services).
IEC 61131-3, Programmable controllers – Part 3: Programming languages.
IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1.5 kV DC – Part 1: AC.
EN 50110-1, Operation of electrical installations – Part 1: General requirements.

Frequently asked questions

Can an automatic sequence bypass an interlock?

No. Each step of the sequence is a command like any other and goes through the same interlock check. If an interlock does not give permission, the step is rejected and the sequence stops. A sequence that needs to bypass an interlock is badly designed.

What does the sequence do if it stops halfway?

Usually it stops in the state reached, reports which step failed and why, and leaves the decision to the operator. Undoing the steps automatically can be worse than staying put, because whatever caused the stop may also affect the way back.

Where should the sequence run, in the station controller or the bay controller?

Sequences that only affect one bay can run in its bay controller and remain available even if the station level is lost. Those involving several bays, such as a bus transfer, usually run in the station controller or a PLC, which sees the whole substation.