Q5Protection of Power System
Question
Q.5. (a) Compare among the following relays - (i) Impedance (ii) Reactance (iii) Mho. [8]
(b) What are the types of faults that are likely to occur in a three-phase induction motor? If a motor is not fully loaded, is it necessary to provide protection against single phasing? Explain why. [8]
Answer
Impedance, reactance, and mho relays are the three principal types of distance (impedance-measuring) relays used for transmission line protection, each characterized by a different geometric operating characteristic when plotted on the R-X (resistance-reactance) impedance plane, giving each type distinct advantages and limitations for different line lengths and fault conditions.
| Aspect | Impedance Relay | Reactance Relay | Mho Relay |
|---|---|---|---|
| Operating characteristic on R-X plane | A circle centered at the origin, with radius equal to the relay's impedance setting (reach), operating for any fault impedance whose magnitude falls within this circle regardless of the fault's resistance-to-reactance ratio. | A straight horizontal line at a fixed reactance value (parallel to the R axis), operating for any fault whose reactance component falls below the set reactance value, essentially completely ignoring the resistive component of fault impedance. | A circle passing through the origin, with its diameter along the line representing the protected transmission line's own impedance angle, giving a characteristic that is inherently directional (does not extend behind the relay location) and offers a variable degree of tolerance to fault resistance depending on the specific point on the line being protected. |
| Directionality | Non-directional by itself (the circle is centered at the origin and extends equally in all directions), requiring a separate directional element to be added to make it usable for line protection without maloperation for faults behind the relay. | Non-directional in the resistance sense (extends to infinity on both sides along the R axis), similarly requiring a separate directional element in practical application. | Inherently directional, since the characteristic circle passes through the origin and lies almost entirely on one side, naturally restraining operation for faults in the reverse direction without needing a separate directional element. |
| Sensitivity to arc/fault resistance | Moderate tolerance to fault resistance, since the circular characteristic extends some distance along the resistance axis as well as the reactance axis. | Excellent tolerance to fault resistance, since the characteristic depends only on the reactance component and is completely insensitive to any added fault arc resistance, making it particularly well suited to protecting short lines where fault resistance can otherwise represent a large fraction of the total line impedance. | More limited tolerance to fault resistance than a reactance relay, particularly for faults near the remote end of the protected zone, since the mho circle's diameter (and hence its resistive reach) shrinks as the reach setting is reduced. |
| Typical application | Historically used for simple, non-directional distance protection applications, less common in modern practice given the need for a separate directional element. | Commonly used for protecting shorter transmission lines, where fault resistance can represent a significant fraction of line impedance and its rejection is therefore particularly valuable, and often combined with a mho element for the necessary directional and power-swing-blocking capability. | The most widely used characteristic for phase fault distance protection of medium and long transmission lines, due to its inherent directionality and generally favorable performance during power system swings, since the reduced reach on the resistive axis (compared to a simple impedance circle) provides better security against unwanted operation during stable or unstable power swings that temporarily bring the measured impedance close to the relay's operating characteristic. |
Faults in Three-Phase Induction Motors
- Stator winding faults: including phase-to-phase short circuits, phase-to-ground faults, and inter-turn faults within a single phase winding, each capable of causing severe localized heating and progressive insulation degradation, analogous to the stator winding faults discussed for generators elsewhere in this examination.
- Unbalanced supply voltage (single phasing): a loss of one of the three supply phases (due to a blown fuse, an open contact, or an upstream conductor failure), which does not necessarily stop the motor from running (particularly if it was already running at the moment single phasing occurs) but causes severely unbalanced stator currents, with the two remaining phases drawing substantially increased current in an attempt to continue delivering the required mechanical output torque, risking rapid overheating of the stator winding.
- Overloading: a sustained mechanical load demand exceeding the motor's rated capacity, causing gradual, sustained overheating of the stator winding even without any actual electrical fault being present.
- Locked rotor (stalling): a condition in which the rotor fails to accelerate away from standstill after starting (due to an excessive mechanical load, a mechanical jam, or a supply voltage significantly below rated value), causing the stator to continue drawing the very high starting current characteristic of a stationary induction motor rotor for a sustained period well beyond the normal, brief starting transient, risking rapid thermal damage since the motor's cooling (typically dependent on shaft-mounted fan rotation) is also compromised at zero or very low speed.
- Earth faults: a short circuit between a stator winding and the earthed motor frame, requiring dedicated earth fault protection (typically a sensitive core-balance or restricted earth fault relay) since the fault current magnitude for an earth fault can be substantially lower than for a phase-to-phase fault, particularly on a resistance-earthed or high-impedance-earthed supply system.
- Bearing failure and rotor faults: mechanical faults such as bearing wear or failure, and rotor bar faults (broken or cracked rotor bars in a cage motor), which do not necessarily produce a directly detectable electrical fault current but can be detected through vibration monitoring or, for rotor bar faults, through characteristic current signature analysis of the stator current spectrum.
Necessity of Single-Phasing Protection for a Lightly Loaded Motor
Even for a motor that is not fully loaded, dedicated protection against single phasing remains necessary and cannot be safely omitted, for several important reasons. First, although a lightly loaded motor experiencing single phasing will draw a smaller absolute increase in the remaining two phase currents than a fully loaded motor would (since less total torque, and hence less total current, is required to sustain the reduced mechanical load), the current in the two remaining healthy phases nonetheless still increases substantially above its pre-fault value under single-phasing conditions (the motor attempting to sustain its required torque using only two of its three normal phases, forcing those two phases collectively to carry a disproportionate share of the total load), meaning even a lightly loaded motor experiencing single phasing can still see its stator current rise well beyond safe continuous levels in the affected phases, particularly if the load happens to increase at some later point while the single-phasing condition persists undetected. Second, and just as importantly, a motor's load can change over time during normal operation, meaning a motor that happens to be lightly loaded at the specific moment single phasing first occurs may subsequently be called upon to deliver a substantially higher torque demand later in the same operating cycle, at which point the already-present single-phasing condition would cause a much more severe overcurrent and overheating outcome than if the fault were detected and cleared promptly at its onset; relying on the motor's load happening to remain permanently light is an unsafe assumption that dedicated single-phasing protection is specifically designed to avoid depending upon. For these reasons, single-phasing protection (commonly implemented via negative-sequence current detection, phase-current unbalance monitoring, or dedicated single-phasing preventer relays) is considered standard, essential protection practice for three-phase induction motors regardless of their typical operating load level, rather than being reserved only for motors that are known to always operate at or near full rated load.