Bay interlocking uses only the positions of the devices in the bay itself: breaker, disconnectors and earthing switches. Station interlocking needs positions from other bays, as in earthing a bus, transferring between buses or bypassing. The first lives in the bay controller; the second is hardwired between bays, solved in a station controller or exchanged over GOOSE, and it defines which operations are blocked if that communication is lost.
Two scopes for the same logic
Every interlock is a condition on positions, but not all positions are in the same place. Some conditions look only at the devices of one bay; others need to know what the neighboring bays or the buses are doing. That difference defines where each condition is programmed, how the signals travel and what happens when something is lost.
Automation system specifications call them bay interlocking and station interlocking.
Bay interlocking
It solves the conditions between the breaker (52), the disconnectors (89) and the earthing switches (57) of the same bay:
- Disconnectors operate only with the breaker open.
- A disconnector does not close if the earthing switch of the section it is about to energize is closed.
- An earthing switch does not close if any disconnector feeding its section is closed.
- The breaker does not close with an earthing switch of its bay closed.
All those positions reach the bay controller through its own wiring, so the logic works even if the rest of the substation is out of service. It is the most stable part of the interlocking table: it repeats almost unchanged in every bay of the same type.
Station interlocking
It solves the conditions that depend on other bays or on common elements:
- Bus earth: the earthing switch of a bus closes only if every disconnector connected to that bus, in every bay, is open.
- Bus disconnector: in a double bus, closing the selector to one bus requires its earthing switch to be open and, if the other bus is already connected, the coupler and its disconnectors to be closed.
- Coupler: it is not opened by command while any bay has both of its bus disconnectors closed.
- Bypass and transfer: the bypass disconnector of a bay closes only if the coupler or the transfer breaker is taking that circuit, and normally only one bay at a time.
- Line earth: some philosophies require absence of voltage or confirmation from the remote end.
To evaluate these conditions, the bay needs positions it does not have wired. There are three ways to bring them in:
| Implementation | How the positions arrive | Advantage | Weak point |
|---|---|---|---|
| Hardwired | Auxiliary contacts wired between bays | Independent of the network | Heavy wiring, no supervision, hard to extend |
| Station controller | A PLC or central controller gathers all positions | Logic in one place | Single point of failure if not redundant |
| Distributed GOOSE | Each bay publishes its positions and subscribes to those it needs | Continuous supervision, extension by configuration | Depends on the station network |
The GOOSE version uses, for each device, the IEC 61850-7-4 CILO node, which combines own and received positions and delivers the open and close permissions.
What happens when the station level is lost
When the information from other bays stops arriving (failure of the station controller, the switch or the subscription), station interlocking must fail safe: the terms that depend on that information count as “not satisfied” and the corresponding operations are blocked. What the design must define in advance is the degraded mode: which operations remain available and which require a special procedure.
- Does its condition use only positions from the bay itself?
- YesIt can be operated with bay interlocking, which is still active
- NoIs there an approved operating procedure for degraded mode?
- YesOperate with an authorized bypass, visual check in the switchyard and event logging
- NoWait until the information from the other bays is restored
The interlock bypass, protected by a key or password, is the usual way out in emergencies. Its use is governed by the procedures of the utility and of the person in charge of the installation, and every activation is logged. A bypass does not replace checking in the switchyard that each device is really in the position the operation requires.
How it varies with the configuration
In a single bus, almost everything is bay level; the station only steps in for the bus earth. In a double bus, the coupler, the on-load transfer and the disconnector replica for busbar protection appear, as described in double busbar interlocking. In breaker-and-a-half, the boundary moves: the line disconnector depends on both breakers of the diameter, so the logical unit is the diameter, as shown in breaker-and-a-half interlocking. In main and transfer bus, the bypass of each bay depends on the transfer breaker, as explained in main and transfer bus interlocking.
What goes wrong in practice
- New bays not added to the bus earth condition.
- Station conditions wired through bays that are out of service, left open or jumpered.
- Undocumented degraded mode, which forces improvisation under pressure.
- Central logic without redundancy, which blocks the whole substation on a single failure.
- Control from the switchyard cabinet outside the station logic, which leaves only bay interlocking.
How it is verified
The test goes through each bay condition with the devices of the bay and each station condition with the positions of the neighboring bays, real or simulated. It includes loss of communication or of the station controller, and checks that degraded mode leaves the intended operations available and nothing else. The scope and documentation of these tests are covered in interlock testing.
Do you need to define the boundary between bay and station in a new project or an extension? At Tiza Group we design and configure bay and station interlocking logic, hardwired or over IEC 61850, for substations and industry at medium, high and extra-high voltage. We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. See our secondary engineering service or contact us.
Related guides
- Interlock testing: what is checked at FAT and SAT
- Breaker-and-a-half interlocking: diameter and lines
- Bus coupler interlocking: when it must not open
- Line and bus earthing: interlocking
References
- IEC 61850-7-4, Compatible logical node classes and data object classes (CILO, CSWI).
- IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – AC.
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches.
- CIGRE, Technical Brochure 540, Applications of IEC 61850 standard to protection schemes, WG B5.36, 2013.
Frequently asked questions
Which operations are still allowed if station interlocking is lost?
Those that depend only on the bay itself, if the design provides for it: opening the breaker, isolating it with its line disconnectors and earthing its line side. Those that depend on other bays, such as closing a bus disconnector or earthing a bus, stay blocked until the information is restored or a controlled procedure is applied.
Does station interlocking have to be in a central controller?
Not necessarily. It can be distributed: each bay controller receives over GOOSE the positions it needs and calculates its own conditions. A central controller concentrates the logic but becomes a single point of failure if it is not redundant.
Can opening the breaker be blocked by an interlock?
Opening by protection never goes through the interlock. Opening by command can have conditions in some configurations, such as the coupler of a double bus during an on-load transfer, so as not to open a loop with disconnectors that would then be breaking load.