Power swing blocking, device 68, and out-of-step protection, device 78, deal with the electrical oscillations that follow a disturbance on a power system. After a fault clears or a large load changes, the angles of generators across the grid swing against each other, and the impedance a distance relay measures drifts across the R-X plane even though nothing is faulted. Power swing blocking recognizes this slow drift and prevents distance relays from tripping on a stable swing that the system will ride through. Out-of-step protection recognizes when a swing has gone unstable into a pole slip and deliberately trips to separate the system at a chosen point.
Power Swing Blocking (68/78) in one line: Power swing blocking (68) stops distance relays from tripping during a power swing - a slow oscillation of generator angles that makes measured impedance drift across the R-X plane without any fault. Out-of-step protection (78) detects when a swing becomes unstable, or a generator slips a pole, and trips deliberately to separate the network in a controlled way rather than letting it break apart randomly.
A power system holds together because its generators run in synchronism, their rotor angles locked into a common rhythm. When a large disturbance hits - a fault, a line trip, a sudden loss of generation or load - that balance is upset and the machines begin to swing relative to one another, their angles oscillating as the system searches for a new equilibrium. During this swing the voltages and currents at any point on the network change smoothly, and because a distance relay computes impedance from voltage and current, the impedance it measures moves across the R-X plane.
The problem is that this moving impedance can travel right through a distance relay's trip zones. As the angle between two parts of the system opens up, the apparent impedance sweeps in from the load region toward and even into the reactance characteristic of a distance element. To the relay, an impedance point sitting inside its zone looks like a fault, and it may trip a perfectly healthy line in the middle of a swing the system would otherwise have survived. Tripping lines during a swing can turn a recoverable disturbance into a cascading blackout.
The distinguishing feature is speed. A genuine fault causes the impedance to jump into the trip zone almost instantaneously, within a fraction of a cycle, because the fault establishes a new low impedance immediately. A power swing, driven by the mechanical inertia of rotating machines, moves the impedance across the plane comparatively slowly, over many cycles. That difference in the rate of change of impedance is the key the relay uses to tell one from the other.
Power swing blocking exploits the timing difference directly. The relay places a detection zone around its distance characteristics, commonly a pair of concentric zones or a set of blinders, and measures how long the impedance takes to cross from the outer boundary to the inner one. A fault crosses that gap essentially instantly; a swing takes a measurable time - typically several milliseconds or more - to traverse it. If the impedance enters the outer zone and reaches the inner zone only after that transit time has elapsed, the relay concludes the movement is a swing, not a fault, and asserts the blocking signal.
Once blocking is asserted, the affected distance elements are inhibited so they cannot trip while the swing impedance sits within or passes through their zones. This keeps the relay stable through a swing the system will recover from - a stable swing, in which the machine angles oscillate but eventually settle back into synchronism. Blocking is usually applied selectively: elements that must remain available to clear real faults during the disturbance can be exempted, and the scheme is designed so that a true fault occurring during a swing is still detected because it produces the fast impedance jump the swing does not.
The blinder scheme is a common way to draw those detection boundaries. Two or more vertical or angled lines on the R-X plane act as blinders, and the relay times the impedance crossing between them. This gives a clean, geometric way to distinguish the slow lateral sweep of a swing from the abrupt entry of a fault. Getting the blinder spacing and timing right is what makes power swing blocking dependable: too tight and a fast swing may not be caught, too loose and a slow-developing fault might be wrongly blocked.
Not every swing is stable. If the disturbance is severe enough, the machine angles keep diverging instead of settling, the two parts of the system fall out of synchronism, and generators slip poles - their rotors advancing a full pole relative to the rest of the grid. When this happens the system cannot be held together, and the only safe outcome is to separate it into islands that can each survive on their own generation and load. Out-of-step protection, device 78, exists to detect this instability and to trigger that separation deliberately.
The out-of-step element watches the same impedance trajectory but looks for the signature of instability rather than a recoverable swing. In a stable swing the impedance moves in and then reverses back out; in an unstable swing it continues sweeping all the way across the plane and out the far side, indicating the angle has passed the point of no return. By detecting that the impedance has crossed the system completely - passing from one side of the characteristic to the other - the relay identifies an out-of-step condition and initiates tripping at a pre-chosen separation point.
The value of out-of-step tripping is that separation happens where engineers planned it, not wherever the system happens to break first. Splitting the network at a designated location leaves coherent islands with reasonable generation-to-load balance and avoids uncontrolled cascading. For generators, a related out-of-step or pole-slip function protects the machine itself, because slipping poles imposes severe electrical and mechanical stress; it trips the generator before repeated slips damage the shaft and windings. In both cases the theme is the same: recognize when synchronism is genuinely lost and act in a controlled way, while blocking distance elements from tripping on swings that are not.
It measures how fast the impedance moves across the R-X plane. A fault establishes a new low impedance almost instantly, so the impedance point jumps into the trip zone within a fraction of a cycle. A power swing is driven by the inertia of rotating machines, so the impedance drifts across the plane comparatively slowly, over many cycles. The relay times the impedance crossing between detection boundaries and treats a slow crossing as a swing and a fast one as a fault.
In a stable swing the generator angles oscillate after a disturbance but eventually settle back into synchronism, so the impedance moves into the relay's detection zone and then reverses back out. In an unstable swing the angles keep diverging, the impedance sweeps all the way across the plane, and the machines slip poles and lose synchronism. Power swing blocking prevents tripping on stable swings, while out-of-step protection deliberately trips to separate the system on unstable ones.
When machines have truly lost synchronism, the system cannot be held together, so the safe outcome is to split it into islands that can each survive on their own generation and load. Out-of-step tripping does this at a location engineers chose in advance, producing coherent islands and avoiding an uncontrolled cascade where the system breaks apart at random points. It converts an unavoidable separation into a controlled one.
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