Tee Connection Interlocking: Tapped Line and Remote Ends

Short answer

In a tee-connected tap, the local interlocking only covers the tap: the tap disconnector operates with the transformer breaker open, and the tap earthing switches with that disconnector open. The line has no breakers at the tap point, so earthing it depends on both remote ends being open and locked out, something no local controller can see and that is coordinated with the control centre.

Tee connection interlocking has a feature no other arrangement shares: the devices that really isolate the line are in other substations. In a tee connection or tapped line a transformer is connected to an existing line at an intermediate point, with no breakers sectionalizing the line there. The local logic protects the tap; the line is protected by coordination between substations.

What is interlocked at the tap

From the line towards the load, the tap has a tap disconnector (89D), an earthing switch for that stretch (57-1), the transformer HV circuit breaker (52), the HV terminal earthing switch (57T), the transformer and its low-voltage breaker (52LV). Summary of typical logic:

Device	Opens if	Closes if
Tap disconnector 89D	the transformer 52 is open	the 52 open and earthing switch 57-1 open
HV circuit breaker 52	free	earthing switches 57-1 and 57T open
Stretch earthing switch 57-1	free	the 89D and the 52 open
HV terminal earthing switch 57T	free	the 89D, the 52 and the 52LV open
LV circuit breaker 52LV	free	the 57T open
Line earthing switch 57L at each substation	free	that substation’s line terminal open

The tap disconnector never energizes the transformer: the transformer is energized with its HV breaker, with the 89D already closed. That way the inrush current is made by a device built for it.

The full table for this arrangement, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.

The line depends on the remote ends

To work on the line, both ends, one at each substation, have to be opened and locked out, and it must be certain that the tap cannot back-feed voltage. No controller sees all of that:

the controller at substation A sees its breaker, its disconnector and its 57L, not those of B;
the tap controller sees its 89D, not the line ends;
the earthing switch at substation A can close with its end open, even if B is still energizing the line.

That last case is the most serious: closing the 57L with only one end open is a deliberate bolted fault, with the operator next to the switch. That is why the line earthing switch is operated with permission from the control centre, after confirming the state of both ends.

Releasing a tapped line for work

01Open the tap HV circuit breakerThe load goes out of service
02Open the tap disconnectorIt separates the tap from the line
03Open both line endsEach substation opens its breaker and line disconnector, on orders from the control centre
04Lock out and confirmThe line ends and the 89D are locked out and the control centre confirms the state
05Verify and earthLine earthing switches at both ends and portable earths at the work site

If the tap has generation, it is no longer a passive load and becomes a third line terminal: its isolation enters the same conditions as the other two.

Why the tap disconnector is the critical device

The 89D is the only device that separates the tap from a line that is normally energized. If it is opened on load, the arc appears on the line and is cleared by the distance protection at the line ends, taking out the whole line. That is why its interlock with the HV breaker is treated with the same care as in a substation: open and closed position contacts with discrepancy supervision, as in any substation disconnector, and a padlock during the work. If the tap has a bay controller, the positions of the 89D and the breaker are also sent to the control centre, which needs them to authorize the line earthing.

Transfer trip

When the tap has no breaker of its own, or when its breaker fails, a transformer fault must open the line. The line protection at the ends does not always see it with enough sensitivity, so the transformer protection sends direct transfer trip to the breakers at both ends. It is protection logic, but it shares two things with control:

its enabling may depend on the 89D position: with the tap separated from the line, there is no point tripping the ends;
the communications channel must be supervised, and its failure is reported as an alarm.

Bay level and station level

At the tap everything is bay level. What in other arrangements would be station level is here between substations: line earthing, transfer trip and, if automated, the close permission of each end with the other end earthed. Those conditions are solved by procedure with the control centre or with a communications scheme between substations, never by assuming the local controller can see them.

Medium and high voltage

Medium voltage: the tap usually has a load-break switch or a fused switch instead of a circuit breaker. A medium-voltage switch-disconnector can break the transformer load current, but not a fault.
High voltage: motor-operated tap disconnector, HV circuit breaker and, often, a teleprotection channel to the line ends. Many utilities restrict tapped connections on transmission lines; whether a tap is allowed, and with what protection, is set by the utility’s interconnection requirements.

Typical mistakes

Line earthing switch with only one end confirmed.
Energizing the transformer with the 89D. The disconnector is closed with the HV breaker already closed.
Transfer trip without channel supervision.
A tap with generation treated as a load, with no isolation or earthing at that third terminal.
Earth forgotten at one end before re-energizing the line.

How it is tested

At the tap the local matrix is covered. For the line, transfer trip in both directions and the signalling of positions to the control centre are tested together with the substations at the line ends. End-to-end tests are the method for that verification.

We can help with your interlocking logic

At Tiza Group we design tap logic as part of secondary engineering: local interlocking, transfer trip and signals to the control centre, and we program it through remote configuration, for medium and high voltage and for industry. Testing with the line ends is done in Colombia, or elsewhere on request.

References

IEC 61850-7-4: logical nodes CILO, CSWI and XSWI.
IEC 62271-102:2018: alternating current disconnectors and earthing switches.
IEC 62271-103: switches for rated voltages above 1 kV up to and including 52 kV.
EN 50110-1: operation of electrical installations (five safety rules: disconnect from all sources, secure, verify, earth and protect).
IEEE C37.113: guide for protective relay applications to transmission lines, including lines with more than two terminals.

Frequently asked questions

Why can't the tap disconnector be opened with the transformer in service?

Because a disconnector has no arc-quenching chamber and would break the load current or the transformer magnetizing current with an arc, on an energized line. The transformer breaker is opened first; the interlocking requires it.

Who gives permission to earth the line at a tap?

The control centre, after confirming that the line breakers and disconnectors at both ends are open and locked out, and that the tap cannot back-feed voltage. The tap controller cannot see the remote ends, so that condition is handled by procedure or by a communications scheme between substations.

When does a tap need transfer trip?

When a fault at the tap must open the line and the line protection cannot detect it with enough sensitivity: typically a tapped transformer without its own breaker, or a failure of the tap breaker. The signal travels over a communications channel to both ends.