Protection
The power factor relay, device 55, operates when the angle between voltage and current passes a set value. Testing injects voltage and current with a controllable phase angle to find the pickup, the reset and the trip response on the lagging and leading sides.
Device 55 is the power factor relay, an element that responds to the phase angle between the voltage and the current of a circuit, and not to their magnitudes alone. It is not the same thing as the insulation power factor test, which checks dielectric losses in a bushing or winding; this one is a protective element. It is applied mainly to synchronous motors and generators, where a sudden change of power factor reveals loss of field or loss of synchronism, and in some capacitor bank and power-correction controls where it supervises switching. The test confirms that the element picks up at the angle that was set, for the lagging side, the leading side or both as the application requires, that it resets properly, that it needs enough voltage and current to be reliable, and that its output reaches the trip or alarm circuit.
A synchronous machine that loses its field or slips out of step draws very different reactive power from the system, which shows up as a swing in power factor long before the machine reaches thermal limits. Left alone, such a condition can cause severe torque pulsation, rotor and damper winding damage, voltage disturbance on the supply and a trip of the whole bus. The power factor element is a way to see that condition quickly and remove the machine. A wrong phase relationship in the connections, a reversed current transformer, a setting entered with the wrong sign or a dead contact will keep the relay silent when it matters, and none of these is revealed by looking at the settings alone. Testing proves the entire measuring chain, polarity included.
Power factor is the cosine of the phase angle between voltage and current, so a relay built on it is really an angle detector. In an electromechanical design, a voltage coil and a current coil act on a disk or a cup, and the torque depends on the product of the two quantities and the cosine of the angle between them, so the relay operates when that torque passes the restraint at a particular angle. In a digital relay, the firmware measures the two phasors, calculates the angle and compares it with the set value, with separate logic for lagging and leading conditions. Because the angle only means something if both quantities are large enough to measure, the element is supervised by minimum voltage and current. The sign of the angle also depends on polarity, which is why the connection convention, the direction of the current transformer and the phase being measured must all agree with the diagram.
The relay is isolated, the current transformer secondaries are shorted before any lead is changed, and the voltage transformer secondaries are isolated so nothing is back-fed. A three-phase or single-phase relay test set supplies voltage and current to the relay inputs in the connection the relay expects, with the phase sequence matched. Healthy normal values of voltage and current are applied first with the angle set to a comfortable operating condition, and the angle is then moved slowly to find where the output operates, first toward the lagging side and then toward the leading side. The angle is swept back to find the reset. Operation is repeated with a step change of angle to measure the response time, where the manufacturer specifies one. The minimum current or voltage at which the element still works is also checked, because the supervision is part of the function. The trip or alarm output is confirmed against the circuit it drives. As-found settings are left unchanged unless the work order directs otherwise.
The record states the relay type and settings, the voltage and current applied during the test, the angle at which pickup occurred on each side, the reset angle, the response time where measured, and the result of the supervision check. It notes the test connection and phase sequence used, the instrument and its calibration status, the as-found and as-left settings and the state of the trip or alarm circuit, so that the angles can be compared with earlier tests.
The measured pickup angles are compared with the settings from the protection or machine study, allowing the tolerance published for the relay, and the lagging and leading sides are checked separately because they are often set differently. Reset behavior and supervision thresholds are judged against the manufacturer's data, and results are compared with earlier tests of the same relay to detect drift. A result that is right on one side and wrong on the other usually points to a polarity or connection problem and not to calibration. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 include the power factor relay among the protective relay tests. For the tolerances and values, refer to your own licensed copy of those standards and to the relay instruction book.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
No. The insulation power factor test measures dielectric losses in bushings, windings and cables. The device 55 test checks a protective relay that responds to the phase angle between system voltage and current.
The two sides often protect against different problems and are frequently set at different angles. A relay can be correct on one side and wrong on the other, so each is proven on its own.
An angle is meaningless when the quantities are too small to measure reliably, so the relay blocks the element below a floor of voltage and current. Testing confirms the block works, to avoid false operation at light load or during a start.
The angle depends on which voltage is paired with which current and on current transformer polarity. A swapped connection can reverse or shift the angle, so the relay may be silent for a real event or may operate at normal load.
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