Protection
A negative sequence relay, ANSI device 46N, responds to the component of current that rotates against the machine and heats the rotor. Testing injects currents that carry a known negative sequence content, proves pickup, curve or timing and alarm, and checks the output.
A device 46N relay measures the negative sequence component of the three phase currents. That component is the part of an unbalanced set that rotates in the opposite direction to the system, and it is found by processing the phase magnitudes and angles together. The element is a standard part of generator protection and is also used on motors and transformers. It commonly has a sensitive definite-time alarm stage and an inverse-time trip stage shaped to the thermal capability of the machine. Both electromechanical filter-type units and microprocessor relays exist. The field test injects three-phase currents whose negative sequence content is known from calculation, and observes the pickup, the timing and the state of the alarm and trip contacts.
In a rotating machine, negative sequence current produces a magnetic field that turns against the rotor. It induces currents in the rotor surface at twice the system frequency, which heat the rotor iron, slot wedges and retaining rings quickly because those currents are confined to a thin layer. A generator has a limited tolerance, usually expressed as a thermal capability constant, for this heating. Causes include open conductors, unbalanced loads, unequal breaker poles and uncleared faults. If the element is mis-set, or the current transformers are wired with a swapped phase, it can miss a damaging condition or trip an unharmed unit. Only a test proves the sequence calculation and the curve, since the condition cannot be created safely in a running machine.
Any unbalanced set of three currents can be resolved into three balanced sets: positive sequence, rotating with the system, negative sequence, rotating the other way, and zero sequence, the part common to all three. A perfectly balanced set has only the positive component. A relay computes the negative component either with a network of filters or, in digital units, by arithmetic on the sampled phasors. The setting is expressed against the machine rating, and the inverse curve follows the idea that the heating grows with the square of the negative sequence current multiplied by the time it flows, so a bigger component trips sooner. The element has a minimum pickup below which it never operates, and a reset characteristic that models how the rotor cools. Because the quantity depends on phase angles, a wrong phase rotation makes the relay see the opposite sequence, so correct sequence is part of the test.
With the relay isolated and its trip outputs blocked, the technician records the identity and the settings for the alarm stage, the trip pickup, the curve constant and any maximum time. A three-phase current test set is connected to the relay inputs, taking care that the phase sequence of the injected set is correct, since reversing it swaps positive and negative sequence. The test set is first set to a balanced condition, and the relay should show no negative sequence content. Unbalance is then introduced, commonly with a single-phase current or with two phases at the same magnitude and opposite angle, either of which has a known and calculable negative sequence component. The magnitude is raised until the alarm and the trip stages pick up, and the timing is measured at several levels along the inverse curve so that the shape is verified, not just one point. The reset behavior is checked where it is set. If possible, the contacts are proven through to the circuits they drive.
The record gives the relay identity and settings, the injected currents with their angles, the negative sequence component that each represents, and the pickup found for alarm and trip. It lists the timing measured at each level, the test set and its calibration status, and whether the phase sequence was confirmed. It also notes the as-found and as-left settings, any adjustment, and whether the output contacts were proven through to their circuits.
Results are compared with the settings from the protection study, which depend on the machine thermal capability and the system, not with a universal figure. The shapes of the criteria are a pickup that agrees with the setting inside the manufacturer's stated accuracy, a curve whose timing agrees with the published curve at each level tested, an alarm that responds below the trip level as intended, correct sequence response and clean contacts. Maintenance results are also trended against 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 manufacturer's data.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It is the part of an unbalanced three-phase current set that rotates opposite to the system. In a generator or motor it induces heating currents in the rotor, which is why a dedicated element watches for it.
Rotor heating grows with the size of the negative sequence current and with how long it lasts, so a larger component should trip sooner. The curve mirrors the machine thermal capability.
A single-phase current, or two phase currents of equal size at opposite angles, contains a known negative sequence component that can be calculated. The set injects that pattern and the relay result is compared with the calculation.
Reversing the phase sequence of the injected currents swaps the positive and negative components. A balanced set with the wrong rotation would then look like a severe unbalance to the relay.
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