Generator Breaker Placement in Circuit
Generator Breaker Placement in Circuit
(OP)
A thought I've been mulling over for the past few days.
Normally where a generator and unit transformer are installed, and there is a breaker with the generator, the configuration is: Neutral Connection; Generator Windings; Generator Breaker; Transformer. In this configuration, if there is a phase-to-ground fault within the generator tripping the breaker does nothing (much) to reduce the fault current, that is done by tripping the field and prime-mover and waiting for the voltage to collapse to the point that current can no longer flow.
But what if the breaker were instead between the Neutral Connection and the Generator windings? In this case, current, to both the load and to the fault, would be interrupted. No current flow, less total damage. This is assuming that the generator is feeding a delta winding on the unit transformer, so that voltage fed back into the windings after the breaker trips would not have a path through the winding ground fault.
Certainly I'm not the first to think of this, so what am I missing? Why aren't generator breakers on the neutral side of the winding rather than the line side of the winding?
David.
Normally where a generator and unit transformer are installed, and there is a breaker with the generator, the configuration is: Neutral Connection; Generator Windings; Generator Breaker; Transformer. In this configuration, if there is a phase-to-ground fault within the generator tripping the breaker does nothing (much) to reduce the fault current, that is done by tripping the field and prime-mover and waiting for the voltage to collapse to the point that current can no longer flow.
But what if the breaker were instead between the Neutral Connection and the Generator windings? In this case, current, to both the load and to the fault, would be interrupted. No current flow, less total damage. This is assuming that the generator is feeding a delta winding on the unit transformer, so that voltage fed back into the windings after the breaker trips would not have a path through the winding ground fault.
Certainly I'm not the first to think of this, so what am I missing? Why aren't generator breakers on the neutral side of the winding rather than the line side of the winding?
David.






RE: Generator Breaker Placement in Circuit
On the other hand, grounding of the generator is a vlaue that must not change. It is either solidly grounded or grounded through an impedance. To put a breaking device on the neutral will change this fixed value depending on the C/B status.
Hope this helps.
RE: Generator Breaker Placement in Circuit
RE: Generator Breaker Placement in Circuit
RE: Generator Breaker Placement in Circuit
Low voltage generators are normally solidly grounded and are desigend to withstand a phase-to-ground fault until the field trips and prime mover shuts down.
RE: Generator Breaker Placement in Circuit
1. Supposing the generator is low voltage, up to 1000VAC. The neutral of the generator is usually solidly grounded. The ground fault is cleared fast if it is straight or bolted fault. If the ground experiences ground fault currents of lower magnitude, these will be annunciated by the ground fault monitoring system. The circuit breaker in the neutral could result in an interruption of the neutral and the main circuit breaker being closed, posing a potential hazard wherever the fault occurred since there may have been some damages done and area around may still be energized since the generator circuit breaker is still on.
2. Supposing that the generator is medium voltage, up to 100000VAC. This generator will have the high-resistance system grounding, or medium-resistance system grounding, or reactance system grounding. Each system grounding is designed to have the neutral path unopened before the generator circuit breaker trips.
RE: Generator Breaker Placement in Circuit
In ANSI regions, the proposed seems quite in opposition to the philosophy and objectives of IEEE Std C62.92.2-1989 …Application of Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generator Systems §2. Objectives of Generator Grounding, and IEEE Std C37.101-1993 …Generator Ground Protection §6.20 Scheme 20: Generator neutral overcurrent protection.
RE: Generator Breaker Placement in Circuit
IEEE C62.92.1:1987 (R1993)—IEEE guide for the application of neutral grounding in electrical utility systems—Part 1: Introduction.
IEEE C62.92.2:1989 (R1993)—IEEE guide for the application of neutral grounding in electrical utility systems—Part 2: Grounding of synchronous generator systems.
IEEE C62.92.3:1993 (R2000)—IEEE guide for the application of neutral grounding in electrical utility systems—Part 3: Generator auxiliary systems.
IEEE C62.92.4:1991—IEEE guide for the application of neutral grounding in electric utility systems—Part 4: Distribution.
IEEE C62.92.5:1992 (R1997)—IEEE guide for the application of neutral grounding in electric utility systems—Part 5: Transmission systems and subtransmission systems.
RE: Generator Breaker Placement in Circuit
In theory ofcourse you are correct in whatyou say about isolating the gen internal ph-E fault if a CB is installed between the gen star point and the Earth.
However the reason why the practice is not adopted is because there is no real merit in the application itself in that modern relaying practice and fast response of the system isolates the fault anyway (as you say by tripping the excitation and the prime mover)without having to spend money on instaling the CB and the associated cabling.
RE: Generator Breaker Placement in Circuit