Protection
A differential relay, device 87, compares the current entering and leaving a protected zone and trips when they differ. Testing injects current into the restraint and operating paths to verify pickup, slope, harmonic restraint and the high-set element.
A differential relay, ANSI device 87, protects a defined zone such as a transformer, generator, motor, bus or short line by comparing the currents that flow into the zone with those that flow out. For a healthy zone the two agree, and for an internal fault they do not, so the relay can trip quickly and without needing to coordinate with downstream devices. The test confirms that the relay picks up on a small difference, that its restraint slope behaves as set, that the restraint against inrush and overexcitation works where the relay has it, that the high-set element operates without restraint, and that the trip output is correct. It applies to electromechanical, solid-state and numerical relays, and to transformer, generator, bus and line differential schemes. The current transformer ratios and the compensation for winding connections are checked as part of the same exercise.
Differential protection is the fastest and most selective protection on the most valuable equipment, so a hidden error costs a great deal. A relay that is too insensitive leaves winding faults burning until a slower backup acts, and a relay that is too sensitive trips on magnetizing inrush, through faults or current transformer saturation and takes healthy equipment out. Errors that matter include wrong tap settings for the ratio of the current transformers, a missing or wrong compensation for the transformer phase shift, reversed polarity on one current transformer, and a restraint circuit that does not restrain. Many stay invisible until energization or a through fault. Secondary injection exposes them while the zone is out of service.
The relay forms two quantities from the currents it receives. The operating quantity is the difference between the currents entering and leaving the zone, scaled for the ratios of the current transformers and corrected for any phase shift in the protected equipment. The restraint quantity reflects how heavily the zone is loaded. The relay trips when the operating quantity exceeds a minimum, and exceeds a percentage of the restraint quantity, which forms a sloped characteristic. The slope tolerates small errors from current transformers and tap position as load rises. Transformer relays add a block based on harmonic content, since magnetizing inrush and overexcitation contain characteristic harmonics that an internal fault does not. A high-set unrestrained element operates on a very large difference without waiting for the restraint logic, because a severe fault can saturate a current transformer and distort the restraint signals.
The protected equipment is de-energized and the relay is isolated from its current transformers and trip circuits, under the owner's safety plan. Identification, settings, tap values and the connection of the transformer windings are recorded, and the insulation between the relay circuits and the frame is measured at the level the standard assigns. A relay test set then injects current, usually through a separate channel for each winding or restraint input. To find the minimum pickup, current is raised in one winding alone until the relay operates. The slope is checked by injecting balanced currents at a chosen restraint level and then increasing the difference until the relay operates, at several restraint levels, to confirm that the points fall on the characteristic. Compensation is checked by injecting currents with the phase relationship the transformer connection would produce and confirming that the relay sees no difference. Harmonic restraint is tested by adding the specified harmonic content to the injected current and verifying that the relay blocks. The unrestrained element is tested by a sharp increase in current, and the operating time is measured. Each phase is tested in turn and the trip output is observed at the contact. Settings are restored afterward, and the polarity of the current transformers is verified in the field before the equipment is energized.
The record describes the relay and the settings it was tested against, what was found for each protective element, and the insulation condition of its circuits, along with the test equipment used and its calibration status. As-found and as-left conditions are kept separately, so that a later reviewer can see what the relay did before and after the work and can repeat the test the same way.
Results are compared with the relay's settings and the manufacturer's published tolerances, and the measured points should fall on or near the configured characteristic with the margin the manufacturer states. Minimum pickup, slope and high-set operation are each judged individually. Phases are compared with each other, since a single phase that differs points to a wiring or input problem. The harmonic restraint should block where it is meant to and release where it is not. Differences from earlier tests of the same relay are investigated even if inside tolerance. The insulation reading is read against the standard's table for the circuit voltage. The differential function is among the protective relay tests in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance. For the actual values and criteria, refer to your licensed copies of both and to the manufacturer's instructions.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Current transformer errors and tap changer movement create small false differences that grow with load. A slope lets the relay stay stable through heavy through current while remaining sensitive when the load is light. A single fixed pickup would either be too insensitive for light faults or too touchy under load.
When a transformer is energized, magnetizing inrush looks like an internal fault to the differential element, but it has distinct harmonic content. The relay uses that content to block tripping. Testing confirms that the block works and releases for a real internal fault.
It trips on a very large differential current without waiting for the restraint logic, because a severe internal fault can saturate current transformers and distort the restraint signals. It is tested by a sharp injection and by timing the response.
It proves the relay and its settings, but not the field wiring to the current transformers. Polarity, ratio and connections are checked separately, and a final check with load current, reading the relay's differential value, confirms that the zone really balances.
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