Out-of-Step Protection Testing (78): Trip on an Unstable Swing

Out-of-step protection testing proves that the 78 element distinguishes a stable power swing from an unstable one and trips the correct breaker at the set angle. It verifies the swing detection, the trip characteristic, the blocking of distance zones, and the trip output.

Key takeaways

• 78 is not 68. The 68 element blocks distance protection during a swing; the 78 element trips when the swing is unstable and the system cannot recover.
• The trip angle is a system decision. The element trips at a controlled angle so that the breakers separate the system at a point where the transient recovery voltage is acceptable.
• Two-machine equivalent. The test reproduces the impedance locus between two equivalent sources; the swing center and the angle are the quantities that matter.
• Separation logic. The element may trip one breaker, both breakers, or a transfer trip; confirm the scheme, not only the element.
• Prove the non-trip case. A stable swing must not trip. Test both outcomes.

What does an out-of-step test prove?

An out-of-step condition occurs when two parts of a power system lose synchronism. The angle between the equivalent sources increases without limit, and the impedance locus passes through the system. The 78 element detects the unstable swing and trips the breakers to separate the system before the poles slip.

The test proves that the element detects the unstable swing, measures the angle or the impedance correctly, trips at the set angle, blocks the distance zones during the swing, and does not trip for a stable swing that recovers.

How do you test a 78 element?

1. Record the settings. Note the impedance characteristic, the trip angle, the timer, the separation logic, and the distance zones that are blocked.
2. Connect three currents and three voltages. Confirm the relay measures the two-source equivalent impedance.
3. Set the pre-fault point. Start with a load point that is outside the characteristic and stable.
4. Move the impedance along the swing locus. Ramp the angle between the two equivalent sources and record the impedance trajectory.
5. Confirm the stable swing. Reverse the ramp before the trip angle and confirm that the element resets without a trip.
6. Confirm the unstable swing. Continue the ramp through the trip angle and confirm the trip output.
7. Measure the trip angle. Record the angle at the trip and compare it with the setting.
8. Test the separation logic. Confirm which outputs operate and whether a transfer trip is required.

What settings and test points should you use?

How do you wire the VAE-430 to the relay?

What is the pass criterion?

• Stable swing: no trip when the locus reverses before the trip angle.
• Unstable swing: trip at the set angle, within 5 degrees.
• Timer: trip within 10 ms of the setting.
• Distance blocking: the distance zones do not trip during the swing.
• Separation: the correct breakers or transfer trip outputs operate.
• Reset: the element resets after a stable swing.

Why does the element trip on a stable swing, or fail to trip on an unstable one?

Trip on a stable swing: the trip angle may be too low, the timer may be too short, or the distance zones may not be blocked. Check the event log for the angle at the trip.

No trip on an unstable swing: the impedance locus may not cross the characteristic, the trip angle may be too high, or the separation logic may be disabled. Confirm the characteristic and the output matrix.

Wrong breaker trips: the separation logic may be mapped to the wrong output. Confirm the scheme drawing and the relay configuration, not only the element settings.

Frequently asked questions

What is a pole slip?

A pole slip occurs when the rotor angle of a generator or the equivalent angle between two systems passes 180 degrees. The system loses synchronism, and the 78 element trips to separate the parts before the slip damages equipment.

Why trip at an angle instead of at the impedance boundary?

The trip angle determines the point in the swing where the breakers open. It is chosen so that the transient recovery voltage and the mechanical stress on the breaker are acceptable. The impedance boundary alone does not define the opening point.

What is the difference between 78 and 68?

The 68 element blocks the distance protection during a swing. The 78 element trips when the swing is unstable. Some relays use a single swing-detection function with both block and trip outputs.

Can I test 78 with one test set?

Yes, by simulating the two-source equivalent at the relay terminals. The test set does not need to be at both ends of the line; it needs to reproduce the voltage and current that the relay would see.

Do I need to test the transfer trip?

Yes, if the scheme uses one. A transfer trip is part of the separation logic; a correct 78 element with a failed transfer trip does not separate the system.

Where VA-TEK test sets fit

The VAE-430 provides three currents and four voltages for the two-source equivalent and the distance zones. Use the fourth voltage for a polarizing or sync-check quantity if the scheme needs it. Start with the relay protection testing hub and the relay testing whitepaper.

Related reading: power swing blocking relay testing for the 68 element, and distance protection relay testing for the zone tests.

Standards and references

• IEEE C37.113-2015 — guide for protective relay applications to transmission lines.
• IEEE C37.90-2005 — surge withstand capability of relay systems.
• IEC 60255-121:2014 — functional requirements for distance protection.
• Relay manufacturer’s technical manual — out-of-step logic and trip angle.