Protection
A loss-of-field relay, ANSI device 40, protects a synchronous generator when its excitation fails. Testing injects voltage and current to move the apparent impedance across the mho circle, proves where the zone picks up and how fast it times out, and checks the output.
A device 40 relay protects a synchronous generator against loss of excitation. It is an impedance relay: from the machine terminal voltage and current it calculates the apparent impedance and asks whether that impedance has entered a zone on the impedance plane. The zone is usually a circle, a mho characteristic, offset from the origin so that it sits mostly in the region that the machine enters when it absorbs reactive power from the system. Many relays use two zones, one for a severe loss of field with a short delay and a larger one for a milder case with a longer delay. The element appears in microprocessor generator relay packages and, on older machines, as a dedicated electromechanical or solid-state unit. Field testing replaces the generator with a three-phase test set that supplies voltage and current at controlled magnitudes and angles.
When excitation is lost, a generator runs past synchronism and begins to act as an induction generator, drawing a large amount of reactive power from the system while it still delivers real power. The rotor surface heats from induced currents, the stator end region overheats from the high current, and the drain on the system can depress voltage over a wide area. Machine damage can develop quickly, so the protection must recognize the condition and trip, yet it must stay stable during recoverable swings and nearby faults. Incorrect offset, diameter, angle or timing leaves the unit unprotected or exposed to nuisance trips. A real loss of excitation cannot be staged just to check the relay, so secondary injection is the practical proof.
On the impedance plane, resistance runs along one axis and reactance along the other, and the relay converts each voltage and current pair into a point on that plane. A healthy, loaded generator operating at normal power factor keeps that point well away from the zone. As excitation falls, the machine absorbs reactive power and the apparent impedance moves into the lower region, drifting toward the circle. The circle is defined by its diameter, its offset and sometimes a directional supervision line. Reaching the boundary starts a timer, and the output operates only if the point stays inside for the whole delay. A voltage supervision element may also be used to accelerate or restrain operation. Because impedance equals voltage divided by current, the same location on the plane can be reached with many combinations of voltage and current, and the test uses that freedom to stay inside the capability of the test set.
With the relay removed from service, the trip outputs blocked and the circuits isolated from the generator, the technician records the relay identity and the zone settings taken from the study: diameter, offset, angle of any directional line, and delay for each zone. A three-phase voltage and current test set is connected to the relay inputs, preserving polarity and phase sequence, with the output contact timed by the set. Starting from a healthy operating point, the test point is moved along a line toward each boundary by changing the magnitude or angle of the current relative to the voltage. The boundary is found by searching for the point at which the output operates, and the search is repeated along more than one path so that the shape, the offset and the diameter of the characteristic are all verified. For timing, a fault state is applied suddenly from a point well inside the zone, and the interval to contact operation is read. Supervision elements, such as a voltage block, are exercised where they are set. If the scheme permits, the contact is proven through to the trip circuit.
The record gives the relay identity, the settings for each zone and the injected states that were used to prove them. It lists the boundary points found, the timing for each zone, and whether supervision features were exercised. It also names the test set and its calibration status and the as-found and as-left settings. Any adjustment made, and whether the output contact was proven through to the trip or alarm circuit, are noted for the next test.
Results are compared with the settings produced by the protection study, since the right zone depends on the machine reactances and the system. The shapes of the criteria are a boundary that agrees with the setting for diameter, offset and angle inside the manufacturer's stated accuracy, no operation outside the zone, a time delay that matches each zone's setting, and a clean contact. Maintenance results are also trended against the previous test and the acceptance baseline. The scope of relay testing is described in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance. For tolerances and clause detail, consult your own licensed copy of those standards and the relay manufacturer's data.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It detects loss of excitation on a synchronous generator. The machine then absorbs reactive power from the system and heats its rotor and stator, so the relay trips it before damage develops.
The apparent impedance of a machine that has lost excitation moves into the region that corresponds to absorbing reactive power. The offset places the circle where that movement occurs and keeps it away from normal loading.
One zone trips quickly for a severe loss of field, and a larger zone with a longer delay covers a milder case while riding through stable swings. Each zone is tested for its boundary and its timing.
Yes. A test set stands in for the machine and supplies voltage and current that place the apparent impedance at chosen points, so the characteristic can be proven while the unit is out of service.
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