Out-of-step protection 78 (PPAM): how it works

What protection 78 (out-of-step) is

In a stable system, all synchronous machines rotate together and the angle between their internal voltages stays within a range. After a large fault, slow clearing or the loss of an important line, that angle can oscillate. If the oscillation is damped, the system remains stable. If the angle keeps growing and passes 180°, the generator or the area loses synchronism: pole slipping occurs, with high currents, pulsating torques on the shaft and voltages that collapse at the electrical center of the swing.

Function 78 detects that condition and orders the separation of the areas that can no longer stay together. It is named this way:

• ANSI/IEEE C37.2: 78, phase-angle measuring or out-of-step protective relay.
• IEC 61850: logical node PPAM (phase angle measuring), which the standard associates with this function.
• It is commonly called out-of-step protection (OOS, or OST for out-of-step tripping).

Do not confuse it with function 68, which blocks distance protection during swings, or with synchronism check 25, which only permits a close.

Where it is applied

• Synchronous generators, especially large ones or those connected to weak grids through long lines. Each pole slip puts heavy stress on the shaft, the turbine and the step-up transformer.
• Transmission lines at separation points defined by the system operator, to split the grid into islands with a reasonable balance between generation and load.
• Interties between areas or countries, where a loss of synchronism can spread if the separation is not made in time.

How it works

The most widely used principle is tracking the apparent impedance in the R-X plane. During a swing, the measured impedance moves relatively slowly, unlike a fault, which makes it jump almost instantly. When synchronism is lost, the trajectory crosses the line joining the two sources, and the point where it crosses is the electrical center of the swing.

The most common characteristics are:

• Single or double blinder: straight lines parallel to the system impedance. The relay measures how long the impedance takes to pass between them and confirms that it crosses from one side to the other.
• Concentric lenses or circles (mho): an inner and an outer characteristic. If the impedance enters the outer one, takes longer than a set time to reach the inner one and then leaves on the opposite side, out-of-step is declared.
• Methods based on the swing center voltage or on the rate of change of resistance, which some manufacturers use to depend less on impedance settings.

The logic usually counts pole slips and trips after one or several, depending on the system philosophy. It also decides when to open: near 180° between sources the breaker faces the highest recovery voltage. That is why many logics trip when the angle is already decreasing, or the breaker is required to have the out-of-phase switching capability of IEC 62271-100 (IEEE C37.04 in North America).

On generators, the logic also distinguishes whether the electrical center falls inside the zone of the machine and step-up transformer or outside, in the grid. In the second case, grid protection is often allowed to act first.

Setting criteria

Settings depend on:

• The transient stability study, which gives the impedance trajectories and slip frequency for the critical contingencies.
• The impedances of the generator (transient reactance), the step-up transformer and the equivalent grid, which define the reach of the characteristic.
• Maximum load, so that the outer characteristic does not encroach on it.
• CT and VT ratios and the characteristic angle of the system impedance.
• The breaker’s capability to open out of phase.
• The philosophy of the system operator or utility: at which points the system is separated and whether it trips on the first slip or after several.

IEEE C37.102 covers the application on generators. On lines, the reports of the IEEE Power System Relaying and Control Committee (PSRC) on power swings and out-of-step are the usual reference. There are no universal values: settings follow the stability study and the protection coordination.

How each manufacturer implements it

All generator and HV line relays from the major manufacturers include the function, but they organize it differently:

• Siemens SIPROTEC offers it in generator relays and integrates it with power swing detection in distance relays. It is configured in DIGSI.
• SEL combines power swing blocking logic and out-of-step tripping in its line relays, with dedicated zones or blinders and methods based on the swing center voltage. It is set with AcSELerator QuickSet.
• Hitachi Energy Relion 670 names its functions after the IEC 61850 logical node and separates generator protection from line power swing detection. It is handled in PCM600.
• GE Vernova (Multilin) has power swing detection and out-of-step tripping elements in its line and generator relays, configured in EnerVista.
• Schneider MiCOM / Easergy includes it in generator and distance relays, configured with MiCOM S1 or Easergy Studio.

What changes is the shape of the characteristic, how slips are counted and whether 68 and 78 share settings. A setting cannot be moved from one brand to another without redoing it.

Why it trips or blocks

Typical causes of operations or unwanted actions:

• Stable swing entering the characteristic: the setting is too wide or the time between characteristics is too short.
• Heavy load or low voltage: the load impedance approaches the outer characteristic.
• Fault slow to clear: a fault cleared late can drive the machine out of step. 78 acts correctly, but the origin is the slow clearing, often a breaker failure or a poorly coordinated protection.
• Wrong CT or VT polarity or ratio: the trajectory appears in the wrong quadrant.
• VT fuse failure: without voltage, the measured impedance is meaningless; the function must be blocked.

The first thing to check is the oscillography: plotting the impedance in the R-X plane during the event shows whether the trajectory was a stable swing, a real loss of synchronism or a fault.

How it is tested

78 is not tested with a single impedance point. With a secondary injection test set such as the Omicron CMC 356, swing trajectories are generated: voltages and currents whose relative angle rotates at the slip frequency to be simulated, or state sequences that move the impedance from one point of the plane to another.

The test verifies that the function:

• does not operate on stable swings or at maximum load;
• detects slipping at the expected slip frequencies;
• counts slips and trips at the defined moment;
• is blocked on VT fuse failure;
• delivers the correct outputs to the breaker and to the lockout relay 86 logic, if there is one.

The full test is done during secondary injection at commissioning and repeated after changes in the grid.

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 remote relay configuration or contact us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.102, Guide for AC Generator Protection.
• IEEE PSRC, report Power Swing and Out-of-Step Considerations on Transmission Lines.
• IEC 61850-7-4, logical node models.
• IEC 62271-100, high-voltage alternating-current circuit-breakers.
• P. Kundur, Power System Stability and Control.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions