Generator Breaker Placement in Circuit

[davidbeach]

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.

 

[Deansharafi]

Generator C/B has one main function: Isolate the generator from the system in case of abnormal SYSTEM or GENERATOR conditions. Tripping the Neutral connection of the generator may interrupt a phase to ground fault of the generator, BUT it will not isolate the generator on other system or generator abnormal conditions. (Consider that the generator could still be motoring sith neutral disconnectd.)
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.

 

[davidbeach]

Breaker would be between the generator windings and the neutral point. Generator motoring would seem to be impossible as no current could flow through the generator windings. With no current flow through the windings, would loss of grounding really matter?

 

[pablo02]

david: to clarify some points, the neutral point is the point where all the phases are connected together and you wish to install a breaker (per phase) at the neutral point (isolating each phase from each other and from the neutral point)... so, try to explain how you would interrupt a phase to ground fault lacking the capability to isolate the phase (generator) from the system? currently, the generator is protected from the system by its relaying scheme and generator breakers, it's usually protected from itself feeding the fault by a high impedance between the neutral point and ground.. your scheme is not feasible because it defeats the protection schemes...

 

[alehman]

If I understand correctly, you are proposing six circuit breaker poles - three at the load terminals and one in the neutral end of each winding. I think pablo hit on the answer - most generators > 600 volts use high resistance grounding of the neutral to prevent the problem you describe. This allows easy detection of faults by monitoring the neutral-ground voltage. Also, this is much less expensive than three more breaker poles.

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.

 

[jbartos]

Suggestion to the original posting: It is not clear which voltage level the generator is for.
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.

 

[busbar]

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.

 

[jbartos]

Suggestion: References:
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.

 

[RajT]

David,
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.

 

[davidbeach]

Thanks to all. It's been an interesting discussion, looks like it wasn't such a swift idea afterall.