Overvoltage Protection 59 (PTOV): How It Works

Short answer

The 59 protection trips or alarms when the voltage exceeds a set value for a defined time. It protects the insulation of generators, transformers, capacitors and loads against sustained power-frequency overvoltages, such as a load rejection or the Ferranti effect on long lines.

What 59 protection is (overvoltage)

The 59 protection detects that the power-frequency voltage exceeds a set value and, after a delay, alarms or trips. In ANSI/IEEE C37.2, number 59 is the overvoltage relay. In IEC 61850 it is modelled with logical node PTOV (time overvoltage). IEC documentation usually shows it as “U>” or “V>”, with “U>>” stages of higher setting and shorter time.

Its purpose is to protect insulation and sensitive equipment against sustained overvoltages. Insulation withstands brief overvoltages, but a moderate overvoltage held for seconds or minutes ages it, saturates magnetic cores and damages electronic loads.

It must not be confused with protection against transient overvoltages. Lightning and switching surges are limited by surge arresters, not by a relay.

Where it is applied

Generators: on load rejection, the voltage rises until the regulator acts. In hydro generators, machine overspeed makes it worse. IEEE C37.102 treats generator overvoltage as a recommended protection in many cases.
Long high- and extra-high-voltage lines: an unloaded or lightly loaded line raises the voltage at the open end because of its capacitance (Ferranti effect). The 59 can trip the local end or send a transfer trip to the remote end.
Capacitor banks: overvoltage increases the reactive power and the dielectric stress on the bank.
Solar plants, wind plants and distributed generation: interconnection requirements call for disconnection on overvoltage within maximum clearing times. IEEE 1547 sets this kind of requirement for distributed energy resources; in each project the settings follow the local grid code and the utility’s requirements.
Industry: protection of motors, drives and sensitive loads on low- and medium-voltage buses.
Transformers: together with overexcitation protection 24, which responds to the voltage/frequency ratio.

How it works

The relay measures the voltages from the voltage transformer and compares each one with the setting. The usual options are:

Measured quantity: phase-to-ground voltages, phase-to-phase voltages or positive-sequence voltage. Some relays let you choose true RMS or the fundamental component only.
Phase logic: trip when any phase exceeds the setting, or only when all of them do. The second option gives more security against ground faults and problems with a single VT.
Time characteristic: definite time in most cases, with two or more stages. Some relays offer inverse-time curves.
Reset ratio: the function resets when the voltage falls below a percentage of the setting. A reset very close to the setting can cause intermittent operation if the voltage hovers around the threshold.

IEC 60255-127 defines the functional requirements and how to specify the accuracy and timing of overvoltage and undervoltage protection.

Setting criteria

The values depend on the application, and there is no universal number:

The equipment itself: the maximum voltage the equipment withstands continuously and temporarily, according to its nameplate and product standard.
System studies: load flow and electromechanical transient studies to find the maximum voltage in normal operation and under contingencies.
Utility requirements: for grid-connected generation, the thresholds and times are usually imposed and are not negotiable.
Coordination with the regulator: on generators and tap changers, the 59 time must leave room for voltage control to act before the trip.
Stages: a low stage with a long time for moderate overvoltages and a high, almost instantaneous stage for severe ones.

The VT ratio and the connection of its secondaries also matter.

How each manufacturer implements it

The function is simple, but the options vary between brands:

Siemens SIPROTEC (DIGSI) offers several stages with a choice of measured quantity and phase logic.
SEL (AcSELerator QuickSet) usually separates the elements by quantity: phase, phase-to-phase and sequence.
Relion (Hitachi Energy 670/650; ABB 615/620) (PCM600) offers it as a two-stage overvoltage function, definite or inverse time depending on the model.
GE Vernova (Multilin, EnerVista) groups it with the voltage elements, with definite-time or inverse curves.
Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) offers stages configurable as phase-to-ground or phase-to-phase.
NOJA reclosers include voltage protection designed for distribution networks and distributed generation.

The differences that cause the most trouble are the reset ratio, the default measured quantity and the “any phase” versus “all phases” logic.

Why it trips or blocks

Typical causes of operation in the field:

Real overvoltage from load rejection, a weak grid with high generation, capacitor banks connected at low demand, or an unloaded long line.
Ground fault on an ungrounded or high-impedance grounded system: the healthy phases rise towards phase-to-phase voltage. If the 59 measures phase to ground, it operates even with no phase-to-phase overvoltage.
VT ratio entered incorrectly in the relay, or a 110 V secondary treated as 120 V, or the other way round.
Ferroresonance in inductive voltage transformers on ungrounded systems, which produces high, distorted voltages.
Tap changer away from its normal position, or regulator in manual mode.
Settings group that does not match the network configuration.

If the 59 did not operate when it should have, check whether it was set to “all phases” or whether the measured quantity filtered out the component that rose. Start with the oscillographic record and the three phase voltages at the moment of the event.

How it is tested

With secondary injection, for example with an Omicron CMC 356, balanced voltages are injected and the magnitude is raised until pickup. The test checks:

The pickup and reset values of each stage.
The operating times of each stage.
The phase logic: raise a single phase, then all three.
The measured quantity, for example by injecting harmonics if the relay must respond only to the fundamental.
The assignment of outputs, alarms and records.

At commissioning it is completed by verifying the VT ratio and polarity at the panel.

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 our remote relay configuration service 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.
IEC 61850-7-4, Compatible logical node classes and data object classes.
IEEE C37.102, Guide for AC Generator Protection.
IEEE 1547, Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why did the solar plant 59 trip at peak generation?

When a lot of power is exported through a weak grid, the voltage at the point of interconnection rises. If the inverters' reactive power control does not compensate it, the 59 operates. Check the voltage profile, the reactive power control mode and whether the setting matches the utility's requirements.

Does the 59 protect against lightning or switching surges?

No. The 59 responds to sustained power-frequency overvoltages, lasting from hundreds of milliseconds to minutes. Lightning and switching overvoltages last microseconds or milliseconds and are limited by surge arresters.

Should I measure phase-to-phase or phase-to-ground voltage?

It depends on what you want to protect and on the system grounding. With phase-to-ground voltages, a ground fault on an ungrounded system raises the healthy phases and can cause trips that do not correspond to a real phase-to-phase overvoltage.