Q3Protection of Power System

A synchronous generator (alternator) is subject to a distinctive range of possible fault and abnormal operating conditions, extending beyond the simple stator winding short circuits common to other power system equipment, due to its rotating field structure, its role as an active power source rather than a passive load, and its mechanical coupling to a prime mover.

• Stator winding faults: including phase-to-phase faults (a short circuit between two stator phase windings), phase-to-ground faults (a short circuit between a stator phase winding and the earthed stator frame or core), and inter-turn faults (a short circuit between adjacent turns within the same phase winding, particularly difficult to detect since it may initially produce only a small circulating current without significantly disturbing the overall phase currents), each capable of causing severe localized heating and progressive insulation damage if not rapidly detected and cleared.
• Rotor (field) winding faults: including a single earth fault on the rotor field winding (which, being an ungrounded DC circuit under normal conditions, does not immediately cause any fault current or abnormal operation from a single earth fault alone, but creates a dangerous condition in which a second, independent earth fault at a different point on the same field winding would short-circuit a portion of the field winding, causing severe unbalanced magnetic forces on the rotor and potentially serious mechanical damage), necessitating dedicated rotor earth-fault protection to detect and alarm on the first fault before a second fault can occur.
• Loss of excitation: a failure of the generator's field excitation supply (due to exciter failure, a field circuit breaker inadvertent trip, or a field winding open circuit), causing the generator to fall out of synchronism with the system and to draw excessive reactive power (behaving somewhat like an induction generator) from the system instead of supplying it, risking both generator overheating (particularly in the rotor and stator end regions) and system-wide voltage instability.
• Unbalanced loading (negative-sequence currents): unequal loading of the three stator phases (from an unbalanced external system fault, unequal single-phase loads, or an open-circuited phase) produces negative-sequence stator currents, which in turn induce double-frequency currents in the rotor body and damper windings, causing severe localized rotor surface heating that can rapidly damage the rotor if the unbalanced condition is not cleared promptly.
• Prime mover failure: a loss of mechanical driving power from the turbine or engine driving the generator (due to steam supply failure, governor malfunction, or mechanical failure of the prime mover itself), which, if undetected, would cause the generator to begin motoring, drawing mechanical power from the connected electrical system to continue driving its own now-unpowered prime mover, a condition that can cause damage to certain prime mover types (such as steam turbines, where motoring can cause overheating of the last-stage turbine blades due to windage losses in the absence of adequate steam flow for cooling).
• Overspeed: a sudden loss of electrical load (such as from an external system fault causing the generator breaker to open) without a correspondingly prompt reduction in mechanical input power can cause the generator-turbine set to accelerate to a dangerously high speed, risking severe mechanical damage from the resulting excessive centrifugal stress.

Generator Differential Protection for Stator Protection

Differential protection is the primary, most sensitive scheme used to protect a generator's stator winding against phase-to-phase and phase-to-ground faults occurring within the protected zone, based on the fundamental principle of comparing the current entering each stator phase winding (measured at the neutral end of the winding) against the current leaving that same phase winding (measured at the line (terminal) end), under the normal, healthy condition that these two currents must be equal (Kirchhoff's current law applied to the winding as a whole, since no current should be lost or gained within a healthy winding with no internal fault).

In the standard current balance (Merz-Price) differential scheme applied to a generator stator, a matched pair of current transformers is installed at each end of every stator phase winding: one CT at the neutral end of the phase winding, and a second, identical CT at the line (terminal) end of the same phase winding. The secondary windings of each matched CT pair are connected together in a loop, with a sensitive overcurrent relay (the differential relay) connected across this loop; the CT polarities are arranged so that, under normal, healthy (through-current) conditions, the secondary currents from the two CTs circulate around the loop and cancel each other out at the point where the relay is connected, meaning the differential relay sees essentially zero current under all normal load and even external through-fault conditions, since in either case the current entering and leaving the winding remains equal. When an internal fault occurs within the protected winding (between the two CT locations), the current entering and leaving the winding at the two CT locations is no longer equal (since a portion of the current now diverts through the fault point rather than flowing straight through the winding as in healthy operation, or additional fault current may be contributed from the fault itself, depending on the exact fault type and location), causing a net differential (out-of-balance) current to flow through the differential relay, causing it to operate and trip the generator's main circuit breaker (and, simultaneously, to trip the field circuit breaker and to activate any other necessary shutdown sequences, since simply opening the stator circuit breaker alone does not remove the generator's own internally generated fault-feeding EMF, which continues to drive current into a stator winding fault as long as the field excitation remains present).

Because the differential scheme responds only to the net difference between the two CT currents rather than to the absolute magnitude of load or through-fault current, it provides extremely fast, highly sensitive protection that operates essentially instantaneously for internal faults while remaining completely stable (non-operating) for any external fault or load condition, however severe, making differential protection the standard, first-choice primary protection scheme for generator stator winding protection in virtually all but the smallest generating units.

It is further worth noting that all of the generator fault types listed above ultimately fall into two broad protection categories: those detected through direct electrical fault current measurement (stator winding phase and ground faults, primarily via the differential scheme discussed above), and those detected through indirect, condition-specific measurement techniques targeted at a particular abnormal operating state rather than a direct short-circuit current (loss of excitation, unbalanced loading, and prime mover failure, each addressed by a dedicated protection function discussed in relation to another question in this examination), illustrating that comprehensive generator protection necessarily requires a coordinated suite of multiple, complementary protection functions rather than any single scheme alone.