Loss of field protection 40: how it works

What protection 40 (loss of field) is

In IEEE C37.2, device number 40 is the field relay: the one that operates on an abnormally low value or loss of the field current of a synchronous machine. In practice it is called loss of field or loss of excitation protection.

When a generator connected to the grid loses its excitation, it stops producing reactive power and starts absorbing it from the system to stay magnetized. It behaves like an induction generator: it runs above synchronous speed, currents are induced in the rotor and stator current rises. If the grid cannot supply that reactive power, voltage drops in the area and the machine can lose synchronism, putting both the machine and system stability at risk.

In most cases 40 does not measure the field current directly. It measures voltages and currents at the generator terminals and calculates the impedance (or admittance) seen from that point. With normal excitation that impedance is in the load region; with loss of field it moves toward the fourth quadrant of the R-X plane, in the negative reactance region.

Where it is applied

• Synchronous generators in hydro, thermal, gas and industrial cogeneration plants. It is its main application.
• Synchronous condensers, which can lose their field just like a generator.
• Large synchronous motors in industry, where loss of field causes slip and heating.

It does not apply to inverter-based solar plants or to induction generators, which have no field winding like that of a synchronous machine. If you are protecting a complete machine, also review reverse power 32 and negative sequence 46, which are part of the same scheme.

How it works

The relay calculates the positive-sequence impedance with the voltages from the voltage transformer (VT) and the currents from the current transformer (CT) at the machine terminals. It then compares it with one or two characteristics in the R-X plane.

Offset mho characteristic. It is the best known. A circle whose center lies on the negative reactance axis, offset from the origin by a fraction of the transient reactance (X’d) and with a diameter related to the synchronous reactance (Xd). The classic schemes described in IEEE C37.102 and in the technical literature use one or two zones:

• A small, fast zone for loss of field from high load, which is the most dangerous case.
• A large, time-delayed zone for loss of field from light load and to give a stable swing time to pass.

Some schemes add a directional element or undervoltage supervision to speed up the trip only when system voltage drops.

Admittance characteristic. Other manufacturers work in the conductance-susceptance plane (G-B), or directly in the active and reactive power plane. There the characteristic can follow the generator capability curve and the stability limit more closely, and it is coordinated with the underexcitation limiter of the automatic voltage regulator (AVR).

Some relays also accept the measurement of field voltage or current as an additional criterion.

Setting criteria

Settings come from the generator protection study and from its nameplate and test data. They depend on:

• The reactances Xd and X’d in per unit, converted to secondary ohms with the CT and VT ratios.
• The generator capability curve and the setting of the AVR underexcitation limiter. 40 must stay outside normal underexcited operation and outside that limiter.
• The steady-state stability limit, which depends on the step-up transformer and grid impedance.
• The stable power swings shown by the stability study, to set the time delays.
• The voltage circuit supervision scheme.

There is no universal reach or time value. The guides give starting criteria, and the final value is set according to the coordination study and the relay manual.

How each manufacturer implements it

Each brand names, groups and sets 40 its own way, and that often causes confusion:

• Siemens SIPROTEC includes it in its generator relays and sets it in DIGSI. Depending on the series, the characteristic can be defined in the admittance or the impedance plane.
• SEL offers it in its generator relays with mho zones and it is configured with AcSELerator QuickSet; additional logic is built with SELogic.
• Relion (Hitachi Energy 670/650; ABB 615/620) presents it as a function block within the configuration built in PCM600.
• GE Vernova (Multilin) includes it in its generator relays; it is set in EnerVista and combined with FlexLogic.
• Schneider MiCOM / Easergy offers it in its generator relays and it is configured in MiCOM S1 or Easergy Studio.

Names, units (primary or secondary ohms, per unit, admittance), characteristic shape and available blocking change. That is why a correct setting in one brand cannot be copied to another without recalculating it.

Why it trips or blocks

The causes most often seen in the field:

1. Real loss of field: failure in the exciter, the rectifier bridge or the brushes, or opening of the field breaker.
2. VT fuse or circuit failure: the measured voltage drops and the calculated impedance enters the characteristic. If the VT failure block is not active, the trip is a misoperation.
3. Power swing after an external fault or a switching operation, with insufficient time delay on the large zone.
4. Underexcited operation during light load hours, when the AVR limiter is not coordinated with 40.
5. Wrong CT or VT polarity or ratio, or wrong phase rotation setting: the impedance appears in another quadrant.
6. Starting or braking of machines that run as motors (pumped storage, synchronous condenser) with the function enabled in a state where it does not belong.

What to check first: the event record and the oscillography, the state of the exciter at the time, the VT supervision, the active settings group and the impedance trajectory in the R-X plane.

How it is tested

It is tested by secondary injection with a three-phase source such as the Omicron CMC 356. Voltages and currents are injected whose ratio places the impedance at defined points of the R-X plane.

The test verifies:

• The boundary of each zone (reach, offset and diameter) at several angles.
• The operating time of each zone.
• The VT failure block and any undervoltage supervision.
• The trip logic and the signals to the recorder and the control system.

A more complete test simulates dynamic trajectories: the impedance enters the characteristic slowly, as in a real loss of field, or crosses it quickly, as in a stable swing. That confirms the function tells the two cases apart.

Did your relay trip on 40?

Did your relay trip or block on this function and it is not clear why? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us with the brand and model of the equipment.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.102, Guide for AC Generator Protection.
• IEC 60255 (series), measuring relays and protection equipment.
• J. Lewis Blackburn and Thomas J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions