Fusing Lateral Tap-Offs in an Ungrounded Wye System 1

[Chaps]

What are the advantages and disadvantages of installing a fuse(s) on a single phase tap-off from a three phase line to prevent the recloser in a substation from unnecessarily operating for faults on the single phase tapoff in an ungrounded wye system?

Because it is an ungrounded wye system, there is no neutral wire and for "single phase" customers the utility needs to run a second phase. Therefore, one disadvantage I can think of off the bat is for a single phase tap-off you need 2 fuses. If the system was a grounded system with a neutral wire, you would only require 1 fuse at the tap-off. Are there are any more advantages/disadvantages?

 

[rbulsara]

Is this a hypothetical quesiton or a real situation?

Single phase phase L to N is not same as single phase between two 'phases'. The L-L voltage is 1.73 times L-N voltage. So you can't substitute L-N supply with a L-L supply.

In theory, you can still provide a L-N supply wiht one fuse even if the Wye system is ungrounded. It is not necessary to ground a system to derive a neutral!

Why would you even have a ungrounded wye system is a totally different matter and not the subject of you question. Fusing is not even a criteria to compare a grounded and an ungrounded system.

 

[Chaps]

Rbulsara, First of all, this is a real situation. Secondly, to clarify the siutation, at this utility, there is no neutral wire, however, the pole-mount transformer casing is grounded with the use of a ground rod in the earth. Because there is no neutral, a second phase line is run and the transformer is connected phase to phase. My question on all of this, is for these 2 phase lines being tapped from a three phase "ungrounded wye" system, are there any advantages/disadvantages with fusing these 2 phase tap-offs, versus fusing a 1 phase tap-off, besides the fact that you need to introduce a 2nd fuse because f the 2 phase setup?

 

[busbar]

For ANSI regions, a checklist of sorts, is ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding...Distribution

Assuming distribution voltages, the basic philosophy of overcurrent-device coordination [e.g., “fuse saving” versus “trip saving”] does not change markedly comparing ungrounded to grounded systems.

OTOH, insulation coordination is treated quite differently, as implied by rbulsara. Ungrounded systems can have potentially serious ground-overvoltage problems.

 

[jbartos]

Suggestion: Incorporation of two fuses, one in each line, will protect the single phase tap-off transformer better since the single fuse protection of the tap may lead to a situation when the single fuse protection clears and the second line phase voltage will be impressed on the equipment making it a potential hazard against the ground (which may be passing the ground leakage currents).
The 3phase 4wire power distribution system would require one fuse in the line conductor only.

 

[busbar]

Chaps, I need to clarify my first post. Your second post showed up before my first post, and I had yet to see your second post.

Your description seems more like it is a C62.92.4 three-wire unigrounded-wye system and not an ungrounded-wye system. In a typical utility setting, the transfomer-secondary neutral point [XO] serving the distribution circuit is often solidly grounded through a substation ground grid, but a neutral conductor is not routed with phase conductors [as in a multi-grounded system.]

As you stated, with a three-wire unigrounded-wye system, loads are served phase-to-phase. Also, earth return is relied upon to sense ground faults, and it is typical for ground-overcurrent relaying to be set at relatively low values.

 

[Chaps]

Guys, i thnak you so far for this useful information. it does make sense, and you are right busbar, th system is considered a uni-grounded system as the power transformer at the substation is grounded to a ground grid, but this is not extended to the distribution line.

 

[jbartos]

Comment on the previous posting. The neutral is not necessarily routed with phase conductors in multigrounded systems. The neutral is usually routed with the line conductors with one system ground at the source, e.g. transformer, generator, etc. Multi-point grounded neutrals are sometimes used in special power distribution locations.

 

[jghrist]

What kind of earth fault protection is provided at the substation? If it is sensitive, then the fused taps will not coordinate properly because they have to be sized for full phase current.

 

[busbar]

jghrist — I agree. At 50 amperes (primary) coordination is not possible with larger fuses. It is a limitation in the design.