Medium-voltage automatic transfer: main-tie-main logic

Short answer

Medium-voltage automatic transfer opens the main breaker of the section that lost its source and closes the tie to feed it from the other source, only if the other source is healthy and the main did not open for a fault on its bus. Its logic combines undervoltage, voltage supervision, overcurrent blocking and interlocks between the mains and the tie.

What medium-voltage automatic transfer is

In medium-voltage switchgear with two sources, each source feeds its bus section through a main breaker, and a normally open tie breaker joins the two sections. This is the main-tie-main arrangement, which in a plant corresponds to the secondary selective configuration. The automatic transfer is the logic that, on loss of one source, opens the main of that section and closes the tie so that the other source takes the whole load.

In IEEE C37.2, the function that governs this operation is device 83, automatic selective control or transfer relay. In practice there is no single device: the logic lives in the relays of the mains and the tie, in a dedicated transfer relay or in a bay controller, and it relies on voltage and current protection functions.

This guide covers medium voltage. The transfer of station service and low-voltage loads with a dedicated switch is covered in automatic transfer switches (ATS).

How loss of source is detected

The starting condition is the absence of voltage on the source side of a section. It is measured with undervoltage protection 27 on the VT at the transformer or incoming line side, not on the bus, to tell a lost source from a bus that is dead for another reason.

Depending on the design, 27 is complemented by:

Phase sequence and voltage unbalance (47): detect a lost phase or a source with reversed sequence, which a three-phase 27 may not see.
Supervision of the other source (59 or voltage present): the transfer only proceeds if the section that will take the load has healthy voltage.
Time delay: the undervoltage must last longer than the dips cleared by upstream protection and longer than the utility’s reclosing. If the transfer is faster than the reclose, the plant changes source for an event that would have cleared by itself.
Voltage circuit supervision: an open VT fuse looks like a lost source. The logic must block the transfer on a voltage circuit failure.

Transfer sequence and blocking

The basic open-transition sequence is:

Main-tie-main automatic transfer

01Loss of source 1Sustained undervoltage, with healthy voltage on source 2
02Blocking checkNo overcurrent trip, no VT failure, selector in automatic
03Main 1 opensConfirmed by its 52b contact
04WaitResidual voltage or phase window if there are motors
05Tie closesSource 2 feeds both sections

The blocking conditions are the part that matters most:

Bus fault. If the main opened on its 50/51 overcurrent, or if a bus protection operated, the fault is in the section. The transfer is blocked and a manual reset is required. Without this block, the tie closes onto the fault and the second source trips as well.
Breaker not available. The tie does not close if the main of the affected section does not confirm opening, if any breaker is racked out or in test, or if its springs are not charged.
Local or manual selector. With the switchgear in local control, the logic does not act.
Insufficient backup source. When one transformer cannot carry the load of both sections, the logic sheds load before closing the tie or only transfers essential loads.

Electrical interlocks complete the logic: both mains and the tie must never be closed at the same time except in a planned closed transition, because the two sources would be paralleled with their combined short-circuit level on the switchgear. That case is covered in open versus closed transition transfer.

Motors on the bus: when to close the tie

When the section feeds large motors, they keep spinning after the loss of source and generate a residual voltage that decays and drifts in phase. If the tie closes early and out of phase, the difference between that voltage and the new source produces transient currents and torques that can damage shafts and couplings. ANSI/NEMA C50.41 limits the resultant volts per hertz at the instant of reconnection to 1.33 pu.

That is why the wait before closing the tie is not an arbitrary time. Depending on the process, a slow, residual voltage, in-phase or fast transfer is used. The three methods with phase supervision are explained in the guide on motor bus transfer, and the choice comes from a study of the bus. If the load is mostly passive, a slow transfer is enough.

Retransfer to the normal source

When source 1 returns, there are three options:

Manual retransfer: the operator decides when and how. It is the most common option in process plants.
Automatic open-transition retransfer: the tie opens and the main closes after a stability delay. It causes a second short interruption on the section.
Closed-transition retransfer: the main closes with synchronism check 25, the two sources are paralleled for a short time and then the tie opens. It avoids the second interruption, but the switchgear must withstand the short circuit of both sources during that interval, the utility must allow it and a protection must limit how long the parallel lasts.

Variants by installation

Distribution substations with two transformers: the transfer joins the two medium-voltage buses through the bus coupler. It is often coordinated with downstream distribution automation.
Industrial plants: the logic protects continuous processes and nearly always coexists with motors, drives and emergency generation.
Primary selective: a single bus with two incoming feeders and switching between them. The logic is similar, without a tie between sections; see primary selective configuration.
Relays with built-in transfer: some feeder and transformer relays include main-tie-main logic as a function; the SEL-700BT and SEL-2414 are an example.

What goes wrong in practice

Transfer that does not happen because of a selector forgotten in manual, a breaker in test or an overcurrent block nobody reset.
Unwanted transfer because of an open VT fuse or a time delay shorter than the utility’s reclosing.
Closing onto a fault because the block was taken from the general trip signal and not from the overcurrent element, or because it was never wired.
Motors tripping because of the reconnection torque or because each motor’s 27 acts before the tie closes.
Prolonged parallel in a closed transition without a supervision timer.

How it is tested

The test combines secondary injection and real switching. In general terms, each starting condition and each block is checked separately, together with the total transfer and retransfer times with the real breakers, the behaviour with an open VT and, if there are motors, the residual voltage or the angle at the instant of closing. At commissioning the measured times are recorded to compare them with the study.

We can help with your switchgear

At Tiza Group we do the remote relay configuration of the transfer logic and its blocking, and the testing and commissioning of the switchgear in substations and industry, in medium, high and extra-high voltage, in Colombia or elsewhere on request. If your transfer operated when it should not have, we can quote a relay trip analysis.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (devices 27, 47, 52, 83).
ANSI/NEMA C50.41, Polyphase Induction Motors for Power Generating Stations.
IEEE Std 141 (Red Book): recommended practice for electric power distribution for industrial plants.
NIH, Office of Research Facilities, Technical Bulletin Secondary Selective Power Distribution Automatic Transfer, October 2014.

Frequently asked questions

Why doesn't the automatic transfer close the tie when the main breaker trips?

Because an overcurrent trip of the main points, in most cases, to a fault on the bus of that section. Closing the tie would carry the fault to the other transformer and could take the whole plant down. That is why the logic blocks the transfer until the operator inspects and resets it.

How does a medium-voltage transfer differ from a low-voltage ATS?

A low-voltage ATS is usually a single device, with its switch and its controller, that moves a load between two sources. In medium voltage the transfer is logic shared between relays and controllers that operate power circuit breakers, with protection, interlocks and, often, large motors that impose residual voltage or phase criteria.

Should the retransfer to the normal source be automatic?

It depends on the operating philosophy. Automatic retransfer avoids leaving the plant on a single source by oversight, but it repeats the switching and its risk. Many plants keep it manual or do it as a closed transition, with synchronism check, to avoid a second interruption.