Undervoltage Protection 27 (PTUV): How It Works

What 27 protection is (undervoltage)

Function 27 compares the measured voltage with a lower threshold and, if the voltage stays below it for the set time, alarms or trips. It is one of the simplest functions in principle, but also one of those that cause the most unwanted trips when the voltage circuit has problems.

Low voltage can come from a nearby fault, a system overload, loss of a source or the start of large motors. The function cannot tell the cause, so the logic around it matters as much as the threshold.

• ANSI/IEEE C37.2: device 27, undervoltage relay.
• IEC 61850: logical node PTUV (time undervoltage).
• IEC 60617: symbol U<.

Where it is applied

• Motors: it prevents a motor from staying connected at low voltage, when its current rises and it heats up, and from restarting uncontrolled when the voltage returns.
• Substation and industrial buses: it detects loss of supply to start automatic source transfers or to open breakers before re-energization.
• Undervoltage load shedding: it disconnects loads in stages to prevent voltage collapse.
• Solar, wind and storage plants: it is part of the interconnection protection together with overvoltage protection 59 and frequency protection 81, with curves that must respect the required voltage ride-through capability.
• Generators: backup and supervision of other functions.
• Station service in substations and power plants.

How it works

The relay measures the fundamental voltage, normally on all three phases, and applies one or more stages:

• Definite time: the most common; each stage has its own threshold and delay.
• Inverse time: some relays offer curves in which the time decreases the lower the voltage falls.

Logic options that change the behaviour completely:

• Phase mode: it operates on any phase, on two or on all three. Requiring all three makes the function more secure against a single blown fuse.
• Measured voltage: phase-to-neutral or phase-to-phase, or a positive-sequence component.
• Minimum voltage blocking: it inhibits the function when the voltage is almost zero, so it does not trip with the breaker open or the equipment de-energized.
• Supervision by current or breaker position, for the same purpose.
• Fuse failure (loss of potential) blocking: it prevents a trip when the problem is in the secondary circuit and not on the network.

In generating plants, the characteristic is usually expressed as a voltage-versus-time curve the plant must ride through without disconnecting. The 27 is set below that curve.

Setting criteria

• Threshold below the minimum normal operating voltage and above the value at which the protected equipment is damaged or stops working properly.
• Time long enough not to trip on sags from faults cleared by other protection, or on large motor starts, but short enough for the application.
• Coordination with transfers and reclosing: the 27 for motors and transfer schemes must coordinate with the network reclosing time to avoid out-of-phase reconnection.
• Interconnection requirements of the utility and of standards such as IEEE 1547 for distributed generation, where applicable.
• Voltage stability studies for load shedding.
• Voltage transformer ratio and connection type.

The final values come from the coordination study and the utility’s requirements for each point.

How each manufacturer implements it

• Siemens SIPROTEC with DIGSI: undervoltage stages with a choice of measured voltage and current supervision criteria.
• SEL with AcSELerator QuickSet: phase, phase-to-phase and sequence undervoltage elements, with supervision built in SELogic.
• ABB Relion with PCM600: undervoltage blocks with definite and inverse curves and a required number of phases.
• GE Vernova (Multilin) with EnerVista: phase and auxiliary undervoltage elements, with configurable curves and supervision.
• Schneider MiCOM or Easergy with MiCOM S1 or Easergy Studio: stages with “any phase” or “three phase” logic and open-breaker blocking.

The same threshold can behave differently depending on whether the brand refers it to phase or line voltage, whether it requires one or three phases, and whether fuse supervision is enabled by default.

Why it trips or blocks

Causes of trips we see in the field:

• Loss of potential: open VT fuse or miniature circuit breaker, loose terminal or wiring error. It is the number one cause of unwanted 27 trips.
• Voltage sag from an external fault with a 27 time shorter than the fault clearing time.
• Large motor starts that depress the bus more than expected.
• Trip with the equipment de-energized because supervision by minimum voltage, current or breaker position is missing.
• Wrong settings group or VT ratio entered incorrectly.
• Voltage polarity or sequence wired wrong, which affects positive-sequence measurements.

The first step is to compare the voltage in the relay record with that of another device connected to the same bus. If only one relay saw the drop, the problem is in its secondary circuit.

How it is tested

With secondary injection and a test set such as the Omicron CMC 356, the test generally checks:

• The pickup and reset of each stage, per phase.
• The operating times.
• The one, two or three phase logic.
• Blocking by fuse failure, minimum voltage and open breaker.
• The assignment of alarms and trips to the outputs and to IEC 61850 messaging where applicable.

For load shedding and transfer schemes it is completed with functional tests of the scheme. The settings must come from a protection coordination study.

Did your relay trip or block on this function and the reason is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or write to us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEC 60255-127, Functional requirements for over/under voltage protection.
• IEEE C37.96, Guide for AC Motor Protection.
• IEEE 1547, Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces.
• IEC 61850-7-4, Compatible logical node classes and data object classes.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions