Protection
A sync-check test confirms that a device 25 relay permits breaker closing only when the two sides are close enough in voltage, phase angle and frequency, and that it blocks closing outside those limits, including the live-line and dead-bus options.
Device 25 is the synchronism-check relay. It supervises the closing of a breaker that joins two energized systems, such as a generator to a bus, a tie between two sources or a line to a bus after a trip, and it allows closing only when the voltages on the two sides are close enough in magnitude, phase angle and frequency. The test uses a relay test set that provides two independent voltage sources, one for each side of the breaker, with adjustable magnitude, angle and frequency. The relay is checked for its closing window and its blocking behavior, and for the dead-line and dead-bus options that permit closing when one side is deliberately de-energized. The relay may be a stand-alone electromechanical or solid-state unit or a function within a microprocessor-based protection and control relay. The test covers the relay and the close-permit output it controls.
Closing a breaker between two systems that are out of step produces a sudden surge of current and torque. On a generator it can twist the shaft, stress the windings and couplings and shorten the life of the machine, and on a system tie it causes a disturbance that can trip other equipment. A synchronism-check relay is the last guard against a mistaken close by an operator or an automatic scheme, so a relay that permits closing too widely, or never permits it, matters equally. Faults that testing finds include wrong voltage references, swapped phases or incorrect transformer connections, windows set too wide, and mistaken dead-line or dead-bus logic that would allow a close onto a live source. Testing confirms the supervising function before the first real close.
Two energized systems can be paralleled safely when the voltage magnitudes, phase angle and frequency on the two sides are nearly the same, so that little current and torque result when the contacts meet. The relay compares a voltage from each side, usually through voltage transformers, and measures the magnitude difference, the angle between them and the slip, which is the frequency difference that makes the angle change with time. When all three fall inside the set limits it issues a permit. Some schemes add a hold-off interval so that the conditions must persist before the permit is given, and others apply the angle window to account for the breaker closing time. The dead-line and dead-bus options check that one side has no voltage and the other is live, so a close can be allowed onto a de-energized part of the system. Because a wrong phase connection looks like a valid angle, correct wiring and phase assignment are part of what is verified.
The relay is isolated from the breaker close circuit, or that circuit is blocked, following the owner's lockout and work-permit procedures, so the test cannot close a breaker. The voltage inputs are disconnected from the voltage transformer secondaries and connected to the two sources of the test set, and the secondaries are never back-fed. The relay's settings are read and recorded. Starting with both sources matched, the test set moves one quantity at a time, the magnitude difference, the angle and the frequency difference, until the permit output drops out, then returns until it picks up again, establishing each edge of the window. The angle and slip limits are checked together as well, since the permitted angle may depend on the slip. Dead-line and dead-bus conditions are checked by removing voltage from one source and confirming that closing is permitted or blocked as the settings require, and that it is blocked when both sides are dead if the scheme says so. Any hold-off timer is timed. Afterward the connections are restored and the settings compared with the protection study.
The record holds the settings the relay was tested against, the edge of the closing window found for magnitude, angle and slip, the dead-line and dead-bus results, the timing of any hold-off, the relay identification and the breaker and bus it supervises, and the test set with its calibration status. As-found and as-left results are kept separately, with any adjustment noted.
The edges of the closing window are compared with the settings in the protection study, within the accuracy the manufacturer states for the relay, and the permit should be present inside the window and absent outside it. The dead-line and dead-bus logic must permit or block exactly as the scheme requires. Comparison with earlier tests shows drift. ANSI/NETA ATS-2025 describes acceptance testing of protective relays, and for maintenance see ANSI/NETA MTS-2023. Refer to your own licensed copies for the values and tolerances.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It compares the voltage on the two sides of an open breaker and permits closing only when the magnitude, phase angle and frequency difference are all close enough. It does not close the breaker itself; it supervises the close command.
A large current and torque surge occurs as the two systems pull into step. On a generator that can damage the shaft, couplings and windings, and on a tie it can disturb the network and trip other equipment.
These options let a breaker close when one side is intentionally de-energized. A fault in that logic could allow closing onto a live source as if it were dead, so the test confirms the permit appears only in the conditions the scheme intends.
A swapped phase or incorrect voltage transformer connection can look like a valid angle at the relay, so the relay might permit a close that is actually out of step. Verifying the phase assignment is part of the function test.
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