Protection
A directional power relay, ANSI device 32, operates when real or reactive power flows in a chosen direction beyond a set level. Testing injects voltage and current at set angles, proves the direction and the pickup, and checks the delay and the output.
A device 32 relay measures the direction and amount of power flowing at a point in the system. It needs both a voltage and a current, taken from potential and current transformers, because power is the product of the two and the angle between them. The element can be set to respond to forward or reverse flow, to real power or reactive power, and to power above or below a setting. Typical homes are generator breakers, where it detects the machine motoring from the system, and utility interconnections, where it limits or blocks export or import. Both electromechanical and microprocessor versions exist. The field test supplies a three-phase voltage and current set whose angle is controlled, and observes where the output operates, how quickly, and in which direction.
When a prime mover loses its input while the generator stays connected, the machine keeps spinning as a motor and draws power from the system. A steam turbine can overheat its blades, and an engine can be damaged or catch fire on unburned fuel, so reverse power must be detected and the unit separated. At an interconnection, unintended export may violate an agreement or back-feed a circuit that crews believe is dead. These conditions involve very small power levels compared with the equipment rating, so the relay is set sensitive and slow, and small errors in angle or wiring can make it blind or let it operate the wrong way. A test is the only way to confirm polarity, direction and sensitivity before the machine runs.
The element multiplies the voltage and current phasors and uses the component of that product that lies along a chosen axis. Power flowing in the forward direction produces a positive result and power flowing the other way produces a negative one, so the relay needs only to look at the sign and size of the result. Because the measured quantity depends on the cosine of the angle between voltage and current, a flow of real power at a given level appears when the two are nearly in phase, while reactive flow appears when they are far apart in phase. At very low power factor the instrument transformers' own angle errors become a large part of what the relay sees, which is why sensitive reverse-power elements are judged with that error in mind. Most devices add a definite time delay so that a brief swing following a disturbance does not cause a trip.
With the relay isolated from the live circuits and its outputs blocked, the technician records the relay identity, the connection convention and the settings for direction, quantity and angle. A three-phase voltage and current test set is connected to the voltage and current inputs, observing polarity and phase order exactly as in service, and the output contact is monitored by the set's timer. Voltage is held at a normal level while current is applied at an angle that represents forward power, and the relay should stay quiet. The angle is then turned to represent flow in the operating direction, and the current is raised slowly until the output operates, giving the pickup, then lowered to find the reset. Direction is confirmed by repeating at angles on both sides of the boundary of the characteristic. Timing is measured by stepping the current from zero to a chosen multiple of the pickup. Where possible the contact is proven through to the trip circuit with the breaker in the test position.
The record gives the relay identity and settings, the injected voltage and current levels with their angle, the pickup and reset found for each direction tested, and the delay as timed. It notes that the relay stayed quiet in the non-operating direction, which test set and calibration status were used, and the as-found and as-left settings. It also notes whether the output was proven through to the trip or blocking circuit, and any adjustment made.
Results are judged against the settings issued by the protection study, because the right pickup depends on the machine or interconnection involved. The shapes of the criteria are a pickup that agrees with the setting inside the manufacturer's stated accuracy, operation in the intended direction and no operation in the opposite direction, a time delay that matches the setting, and a clean contact. Maintenance results are also compared with earlier tests and with 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, refer to 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.
Most often a generator that has lost its prime mover and starts motoring, which can damage turbines and engines. It is also used at interconnections to limit or block power export or import.
Power depends on voltage, current and the angle between them. With only one of the two, the relay could not tell the direction of flow, so the test set must supply both with a controlled angle.
Motoring draws only a small fraction of rated power, so the pickup is low. A definite delay keeps brief power swings after a disturbance from tripping a healthy unit.
A device 67 is a directional overcurrent element that responds to fault current direction. A device 32 responds to the direction and amount of power, usually at far lower levels and with longer delays.
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