Automatic Transfer Switch (ATS): Logic and Timing

Short answer

The automatic transfer switch (ATS) monitors the voltage of the normal source and, if it fails, moves the load to the alternate source, starting the diesel generator when that is the alternate. It uses undervoltage (27), phase sequence (47) and frequency, delays so that it does not act on short sags, and interlocking so that sources are never paralleled; when the normal source returns, it retransfers.

What it is and where it is found

Automatic source transfer is the automation that keeps a switchboard energized when its normal source fails. It can be an automatic transfer switch equipment (ATSE) with its controller, or a set of motorized breakers operated by a relay, a PLC or a bay controller. It is a key part of AC station service. It is found in the AC station service board of substations, in the main industrial switchboards and in essential-load boards with an emergency diesel generator.

IEC 60947-6-1 covers low-voltage transfer switching equipment and distinguishes PC class, based on switch-disconnectors, from CB class, based on circuit breakers with protection. In North America the listing reference is UL 1008.

Parts and how each one works

Switching elements: two breakers or contactors with mechanical and electrical interlocking, or a double-throw transfer switch.
Voltage sensing of both sources: undervoltage and overvoltage per phase, frequency, phase sequence and unbalance. A commercial controller monitors all of this on both sources.
Controller: runs the logic and timers, issues the start command to the generator and records events.
Generator start contact: goes from the controller to the diesel generator control.
Indication and control: position of each source, automatic/manual selector, test buttons.

The logic step by step

Logic of an automatic transfer

Detection. Undervoltage 27 detects a voltage drop or loss on any phase of the normal source; phase sequence monitoring 47 and frequency monitoring confirm that the alternate source is usable
Start delay. Prevents starting the engine or transferring on momentary outages, such as utility recloses. In a commercial controller this delay is adjustable from 0 to 6 s
Generator start. If it is the alternate source, followed by the stabilization delay so voltage and frequency settle before taking load
Transfer. Opens the normal source and closes the alternate one, with interlocking so that both are never closed when the design does not allow paralleling
Retransfer. When the normal source returns and remains stable for its delay, the load goes back. Some controllers let you choose whether a transfer, once started, is completed even if the normal source returns during the start
Cooldown. The generator keeps running unloaded for an adjustable time before stopping

The timing of each step comes from the design: what the load tolerates, the utility reclosing times and coordination with upstream protection.

Types of transition

Open (break before make). The most widely used. The load is without voltage during the changeover.
Delayed. It pauses for an adjustable time in a neutral position, with no source connected, so that the residual voltage of motors decays. One manufacturer recommends it for older drives, rectifier banks and load management.
With phase monitor. It transfers at the moment the angle between sources is small, to reduce the shock on motors.
Closed (make before break). The sources are paralleled for an instant. In a commercial design the overlap lasts less than 100 ms and is only allowed if the sources are within acceptable parameters. The same manufacturer asks to consult the utility about the required protection. It includes protection for loss of synchronism and for prolonged paralleling.
Bus-tie schemes in split-bus switchboards: the logic handles two incomers and a tie, so that only two of the three breakers can be closed at the same time when the sources must not be paralleled.

How it changes by voltage level

The context for each level is in station service by voltage level.

Compact medium-to-low-voltage substations and industry: a commercial ATSE between the utility and the diesel generator.
High and extra-high voltage: logic between two station service transformers, an external supply and a diesel generator, often in a relay, a PLC or a bay controller, with motorized breakers.
Solar plants: transfer between the internal medium-voltage network and a backup for SCADA and critical auxiliaries.
Medium voltage: in industry there are also transfers between medium-voltage feeders, with their own relays and synchronism criteria. That main-tie-main logic is covered in automatic transfer at medium voltage, and the case of buses with large motors in fast bus transfer. This guide focuses on low voltage and station service.

Setting and selection criteria

Voltage and frequency thresholds for each source, according to what the loads tolerate.
Delays coordinated with utility reclosing and with the protection.
Type of transition, according to the motor loads and what the utility allows.
Short-circuit withstand and closing capacity of the equipment, consistent with the fault current of the switchboard.
Number of poles: whether the neutral is switched or left solid, according to the grounding scheme.

Connection with protection, control and alarms

The controller or IED uses the undervoltage 27 function (logical node PTUV in IEC 61850) from the VTs or from direct measurement of each source, and operates the incomer breakers. SCADA receives the position of each source, the automatic or manual mode, the transfer failure and the generator status.

Common faults and what to check first

Does not transfer: selector in manual, control supply lost, interlock open or the alternate source out of limits.
Transfers unnecessarily: start delay too short compared with the reclosing times.
Does not retransfer: the normal source does not meet the return thresholds or the delay is too long.
Motors that trip on retransfer: delayed transition or a phase monitor is missing.
Reversed phase sequence on one source after maintenance.

Testing and maintenance

Test with a real outage of the normal source: timing, interlocks, generator start, retransfer and cooldown. The 27 and 47 relays are verified by secondary injection. During maintenance, exercise the transfer periodically under load and review the event log.

The guide Generator set protection shows how to protect a generator set.

We support you with your transfer scheme

Does your transfer not act, act too often or not return? Send us the events and we will quote a trip analysis. If a site visit is needed, we do the tests and commissioning in Colombia, or elsewhere on request; write to us.

References

IEC 60947-6-1: transfer switching equipment.
UL 1008: transfer switch equipment (North America).
IEEE 1818: guide for the design of low-voltage auxiliary systems for substations.
NFPA 110: emergency and standby power systems.
IEC 61850-7-4: compatible logical nodes.
IEEE C37.2: device function numbers.

Frequently asked questions

What is the difference between open-transition and closed-transition transfer?

In open transition, the load is disconnected from one source before it is connected to the other, with a brief interruption. In closed transition, the two sources are paralleled for an instant so the load is not interrupted; it is only allowed if they are within limits of voltage, frequency and phase, and the utility may require additional protection.

Why did the transfer wait several seconds before starting the diesel generator?

Because it has an adjustable engine start delay, so that the engine does not start on momentary outages, such as a utility reclose. It then waits for the generator voltage and frequency to stabilize before transferring.

Why do motors trip when the transfer returns to the normal source?

Because the motor keeps spinning and generating residual voltage. If it is reconnected out of phase, the current and torque can be very high. The solution is delayed transition (neutral position), a phase monitor or closed transition.