Q5Protection of Power System

As discussed in relation to another question in this examination, the impedance, reactance, and mho relay characteristics each occupy a distinctly shaped region on the R-X impedance plane, and their differing shapes give each type markedly different behavior specifically with respect to two important practical considerations: tolerance to fault arc resistance, and stability (security against unwanted operation) during power system swings.

Behavior with Respect to Arc Resistance

Fault arc resistance (the resistance of the actual arcing fault path itself, which can be a significant, non-negligible addition to the pure line impedance up to the fault point, particularly for lower-voltage lines or for faults involving a high-impedance path such as through vegetation or a tower structure) adds a resistive component to the total apparent impedance seen by a distance relay, shifting the measured impedance point on the R-X plane away from the pure line impedance locus (a straight line at the line's own characteristic impedance angle) toward the resistance (R) axis. A reactance relay, whose characteristic depends only on the reactance component of the measured impedance and is completely insensitive to the resistive component, provides the best possible tolerance to arc resistance among the three relay types, correctly identifying an in-zone fault regardless of how much arc resistance is present, provided the fault's reactance component alone still falls within the relay's set reach; this makes the reactance characteristic particularly well suited to protecting short lines, where a given absolute amount of arc resistance represents a much larger fraction of the total line impedance (and hence poses a much greater risk of causing an under-reaching relay to fail to detect a fault) than the same arc resistance would represent on a longer line. A mho relay, whose circular characteristic passes through the origin and has a diameter along the line's own impedance angle, tolerates a moderate amount of arc resistance (since the circle does extend some distance into the resistive region), but its resistive tolerance diminishes as the fault location approaches the relay's remote reach boundary, since the circle's width in the resistive direction narrows as the fault point on the characteristic angle line approaches the circle's edge; this makes the mho characteristic less tolerant of arc resistance than a pure reactance relay, particularly for the most remote parts of its reach. A simple impedance relay, whose circular characteristic is centered at the origin, offers an intermediate level of arc-resistance tolerance, generally better than a mho relay for faults near the reach boundary but without the pure reactance relay's complete insensitivity to resistance.

Behavior with Respect to Power Swings

A power swing (an oscillation in power flow, voltage, and current occurring across the network following a system disturbance, distinct from an actual short-circuit fault, arising from the dynamic interaction of the connected generators' rotor angles as they swing relative to one another following a disturbance) can cause the apparent impedance measured at a given relay location to move substantially across the R-X plane over a period of a second or more, potentially passing through or near a distance relay's operating characteristic even though no actual fault exists, risking unwanted (and potentially very disruptive) tripping during a power swing condition unless the relay is specifically designed or supplemented to avoid this. A mho relay generally provides the best inherent stability against power swings among the three characteristics, since its circular characteristic (being comparatively compact, particularly along the resistive axis, and inherently directional) presents a smaller target area on the R-X plane for a slowly moving power swing trajectory to intersect, and because the rate of change of impedance during a genuine fault (a sudden, near-instantaneous jump to the fault impedance) is very much faster than the comparatively gradual, continuous impedance trajectory traced out during a power swing, allowing supplementary power-swing-blocking logic (which specifically measures the rate of impedance change and blocks relay operation if the impedance trajectory is moving too slowly to be a genuine fault) to be readily and effectively added to a mho-based distance protection scheme. Impedance and reactance relays, having characteristics that extend much further across the R-X plane (particularly the impedance relay's characteristic, which extends equally in all directions from the origin, and the reactance relay's characteristic, which extends without limit along the entire resistance axis), present a correspondingly larger target area for a power swing trajectory to intersect, making them inherently less secure against power-swing-induced unwanted operation than a well-designed mho characteristic, one of the key reasons the mho characteristic has become the dominant choice for phase fault distance protection in modern practice.

Scheme of Distance Protection for a Three-Phase Line

A complete distance protection scheme for a three-phase transmission line requires multiple distance measuring elements at each line terminal to correctly detect and respond to every possible fault type: three phase-to-phase distance elements (measuring the impedance between each pair of phases, AB, BC, and CA, to detect phase-to-phase and three-phase faults) and three phase-to-ground distance elements (measuring the impedance from each individual phase to ground, using an appropriately compensated impedance measurement that accounts for the difference between positive-sequence and zero-sequence line impedance, to detect single-line-to-ground faults), since a fault involving ground requires a different impedance measurement and compensation than a fault involving only phase conductors, due to the differing positive-sequence and zero-sequence impedance of the transmission line itself. Each of these six measuring elements is provided with the three-zone (Zone 1, Zone 2, Zone 3) stepped reach and time-delay arrangement discussed in relation to another question in this examination, and the complete scheme additionally incorporates a starting (fault detection) unit that identifies the general presence and type of fault to select which of the six specific measuring elements should be enabled to respond, a directional element ensuring correct discrimination against reverse-direction faults, and, in modern practice, power-swing-blocking logic (particularly important for mho-characteristic-based schemes, as discussed above) to prevent unwanted tripping during genuine system power swings that are not actual faults, together forming a comprehensive protection scheme capable of correctly identifying and responding to every type of fault that can occur on a three-phase transmission line while remaining secure against non-fault system disturbances.

This complete treatment of relay characteristic comparison and the distance protection scheme fully satisfies the requirements of this examination question as originally set out in the paper text.