Q3Power System Protection

Power Swing and Its Effect on Distance Relays

A power swing is a relatively slow (compared to a fault transient), large-amplitude oscillation in the power flow and relative rotor angle between generators (or groups of generators) at different points in an interconnected power system, arising from a significant system disturbance (such as a major fault, sudden loss of a large generator or line, or other large disturbance) that causes rotor angles to swing, as discussed via the swing equation and equal area criterion in the corresponding subject's related answers, without necessarily indicating an actual electrical fault on the specific line being monitored by a given distance relay.

Effect on apparent impedance: a distance relay computes the apparent impedance seen at its location as the ratio of measured voltage to measured current; during a power swing, as the angular difference between the sending-end and receiving-end system voltages varies slowly over time (following the swing dynamics), the apparent impedance measured by a distance relay located along the swinging path traces out a slowly-moving trajectory on the relay's R-X (resistance-reactance) impedance plane, which — for a sufficiently severe swing (a large angular separation approaching or exceeding 180 degrees between the two ends) — can pass directly through, or very close to, the relay's own Zone I and Zone II tripping characteristics, exactly as if a genuine fault were present on the line, even though no actual fault exists.

Risk of unwanted tripping and its consequences: if a distance relay is allowed to trip in response to this power-swing-induced impedance excursion (mistaking the swing for an actual line fault), the resulting unwanted tripping of a healthy transmission line during a system-wide stability disturbance can itself worsen the overall disturbance — removing a line during an already-stressed swing condition reduces the remaining transmission capacity available to help arrest the swing, potentially precipitating a more severe cascading disturbance, or even directly causing loss of synchronism (an actual out-of-step condition) that would not otherwise have occurred, making unwanted power-swing tripping a serious risk to overall system security and stability, well beyond the consequence of a simple unwanted single-line outage under normal conditions.

Power Swing Blocking

To prevent this unwanted tripping, distance relays are typically equipped with a power-swing blocking (or power-swing detection) function, which distinguishes a genuine fault from a power swing based on the fundamentally different rate at which the apparent impedance changes in each case: an actual fault causes an essentially instantaneous (step-change) shift in apparent impedance (occurring within a fraction of a power-frequency cycle, as the fault is initiated), whereas a power swing causes a comparatively very slow, gradual movement of the apparent impedance trajectory (occurring over a timescale of several cycles to seconds, governed by the much slower electromechanical swing dynamics of the connected generators' rotor inertia). Power-swing blocking logic typically uses two concentric impedance-detection characteristics (an outer detection zone and an inner zone corresponding to the relay's actual tripping characteristic) together with a timer — if the measured impedance trajectory takes longer than a set minimum time to travel from the outer detection boundary to the inner tripping zone boundary, the relay concludes the impedance excursion is due to a slow power swing rather than a fast fault, and blocks tripping for the duration that the impedance remains within the swing-detection region; conversely, if the impedance moves from the outer to inner boundary faster than this set time threshold, the relay concludes a genuine fault has occurred and permits normal tripping.

Out-of-Step Tripping

While power-swing blocking prevents unwanted tripping during a stable swing (one that will naturally damp out and return to a stable operating condition without requiring any line disconnection), a genuinely unstable swing (an actual loss of synchronism, or 'out-of-step' condition, in which the angular separation between system areas continues to increase without bound rather than settling back to a stable value) does eventually require deliberate, controlled system separation at a pre-selected, suitable location, to prevent the unstable oscillation from causing widespread equipment damage and uncontrolled, more extensive cascading outages elsewhere in the system — this is provided by a separate out-of-step tripping function (distinct from, and specifically designed to operate independently of, the power-swing-blocking function described above), which specifically detects the characteristic signature of a genuinely unstable swing (typically by monitoring whether the impedance trajectory continues progressing all the way through the full R-X plane, completing a full swing cycle characteristic pattern, rather than merely entering and then reversing back out of the blocking detection zone as a stable swing would) and initiates deliberate, controlled tripping at the specific, pre-designated location best suited to safely separate the system into stable islands, distinguishing this deliberate, planned out-of-step tripping action from the unwanted, unintended tripping that power-swing blocking is specifically designed to prevent during ordinary stable power swings.