Power swing blocking 68 (RPSB): how it works

What function 68 (power swing blocking) is

A power swing is a slow, periodic variation of the power flowing between two parts of the system. It appears after a disturbance, such as a fault being cleared, the loss of a line, a load rejection or the connection of a large generator. Generator angles oscillate until the system returns to equilibrium (stable swing) or loses synchronism (unstable swing).

During the swing, voltage and current on a line change, and the impedance measured by distance protection 21 follows a trajectory in the R-X plane. If that trajectory enters a tripping zone, 21 operates even though there is no fault. That can open healthy lines at the worst moment and turn a stable disturbance into a blackout.

Function 68 detects the swing and blocks the selected distance zones while it lasts. In ANSI/IEEE C37.2, number 68 is the blocking or out-of-step relay. In IEC 61850 it is modeled with the logical node RPSB (power swing detection and blocking). It is commonly called power swing blocking (PSB).

Where it is applied

• Transmission lines at HV and EHV with distance protection, especially those linking areas with large power exchanges.
• Long corridors and weak systems, where swings are wider.
• Large generators and their step-up transformers with distance protection as backup.
• Interties between systems and outlets of generating plants connected by few lines.

It is complemented by the out-of-step 78 function, which trips in a controlled way when the swing is unstable.

How it works

The difference between a fault and a swing is speed. In a fault, the impedance jumps from the load region to the fault region in a few milliseconds. In a swing, it moves gradually.

The classic method uses two concentric characteristics (polygons, circles or parallel lines called blinders) around the distance zones. The relay measures how long the impedance takes to cross the band between the outer and the inner characteristic:

• If it crosses in less than a set time, it is a fault and nothing is blocked.
• If it takes longer, it is a swing and the selected zones are blocked.

Other methods analyze the continuous change of the impedance, of the voltage at the electrical center of the swing, or of the powers. Some manufacturers offer methods that need no characteristic settings, because they calculate the swing speed adaptively.

While blocking, the relay must still be able to detect a fault. The unblocking logic releases the zones when negative- or zero-sequence current appears (unbalanced fault), when it detects a three-phase fault from sudden changes, or when a maximum blocking timer expires.

Setting criteria

Setting it requires knowing the dynamic behavior of the system:

• Inner characteristic: outside the largest distance zone to be blocked.
• Outer characteristic: inside the minimum load impedance, so that maximum load does not trigger detection. See protection setting criteria on load encroachment.
• Crossing timer: calculated from the maximum expected slip frequency, which comes from transient stability studies. The fastest swing must take longer than the timer to cross the band.
• Blocked zones: a decision of the utility and the system operator. It is common to block the long-reach zones and evaluate zone 1 case by case.
• Unblocking: negative- and zero-sequence thresholds, and maximum timer.

The IEEE PSRC report on power swing and out-of-step considerations on transmission lines is the most widely used reference for these criteria. The actual values come from the system’s studies and protection coordination.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI) detects the swing from the continuous impedance trajectory, with few settings and a selection of blocked zones.
• SEL (AcSELerator QuickSet) offers the classic concentric characteristic method and, in some relays, a settingless method based on the swing speed.
• Hitachi Energy Relion 670/650 (PCM600) implements detection with concentric characteristics and additional unblocking logic.
• GE Vernova (Multilin, EnerVista) groups power swing blocking and out-of-step tripping in a common element with configurable characteristics.
• Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) includes detection in its distance relays, with its own methods depending on the model.

The detection method changes from one brand to another, and so do the settings. A mixed scheme at both ends of a line requires checking that both relays respond the same way to the same swing.

Why it trips or blocks

• Distance trip without a fault: the function was disabled, the affected zone was not in the blocking list, or the swing was faster than expected.
• Fault not cleared in time: blocking was active and the unblocking logic did not detect the fault, typically a three-phase fault during the swing.
• False detection at high load: the outer characteristic encroaches on the load region, and a load change is read as a swing.
• Open pole: during single-pole reclosing, sequence currents affect the logic.
• Series compensation: it alters the impedance trajectory.
• Loss of potential: it must block distance protection by another path, not by 68.

The first thing to check is the impedance trajectory in the R-X plane from the oscillography, and the state of the detection, blocking and unblocking signals.

How it is tested

A static test is not enough, because the function depends on speed. With secondary injection, for example with an Omicron CMC 356, impedance trajectories are simulated with state sequences or network simulation modules. The test verifies:

• That a slow trajectory activates blocking and a fast one does not.
• That the selected zones are blocked and the others are not.
• That a fault applied during the swing unblocks and trips.
• The maximum blocking timer.

Did your relay trip or block on this function and it is not clear why? At Tiza Group we review the events and settings remotely. See our remote relay configuration service or contact us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
• IEEE PSRC, Power Swing and Out-of-Step Considerations on Transmission Lines (working group D6 report).
• IEC 60255-121, Functional requirements for distance protection.
• IEC 61850-7-4, Compatible logical node classes and data object classes.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions