Arrester voltage rating 4

[SphincterBoy]

I have a situation in which the substation source (12.47 kV, L-L, grounded-wye) is feeding a 500 KVA, 3-phase padmounted distribution transformer (delta-connected primary windings).

There are surge arresters connected phase-ground at the phase connections on the primary side of the padmounted transformer. The current surge arrester ratings are 9 kV
(> 7.2 kV L-G source voltage).

When the transformer is first energized, it is done so one phase at a time.

When the last and final phase is energized, the surge arrester "blows".

My hunch is the transformer inrush current causes a phase imbalance, resulting in a phase-phase overvoltage at the primary terminals of the padmounted transformer, exceeding the 9 kV phase-ground arrester rating.

Should I be increasing the arrester rating to 12 kV?

 

[jghrist]

jbartos,

I can't find references off-hand. My statement is based on 30+ years experience in the industry, 2/3 of it in the US.

 

[dpc]

I am finding this discussion regarding delta-wye vs. wye-wye transformers very interesting.

The use of wye-wye transformers is very popular with some utilities, but I'm not sure that I would agree that "most" three-phase utility transformers are wye-wye. For industrial facilities, my experience has been that the vast majority are delta-wye.

In addition to the telephone interference issue, another concern with wye-wye transformers is the interaction between the primary and the secondary for secondary ground faults. In the wye-wye configuration, the zero-sequence impedance seen on the secondary side includes the primary system impedance. This can greatly decrease the available fault current on the secondary side for ground faults, and this is not always a good thing.

Also, a ground fault on the secondary will produce zero-sequence current on the primary and this can complicate the relay coordination and protection of the primary feeder.

It seems to me that many of these wye-wye transformers are being installed in applications where the risk of ferroresonance is extremely low, so maybe there are other reasons. I suspect that these transformers maybe less expensive than a delta-wye transformer because the insulation on the primary can be graded, although the five-legged core may offset that advantage.

Any thoughts on why utilities have swung toward wye-wye transformers, other than ferro-resonance?

 

[SphincterBoy]

I wish ANSI would thoroughly address these questions in the next C57 update.

Since the advent of the ferroresonance phenomena, the WYW-WYE has made a comeback. Most of the utilities in the northern plains region where I'm located use the DELTA-GRWYE config. when spec'ing padmounted xformers.

Third harmonics, although not creating interference on phone lines any longer, may play havoc with who knows what, in light of the power quality seminars I've attended.
And, correct me if I'm wrong, the third harmonic appears when the neutral of either primary or secondary winding of the WYE-WYE becomes ungrounded. So WYE-WYE would not be concern for third harmonics?

But the ferroresonance problem has occurred more than once on our system, so the WYE-WYE may be the solution.

What are the Europeans using?

 

[jghrist]

I agree that delta-wye is more common in industrial facilities. I still think that there are more Grd-Y/Grd-Y utility-owned transformers. In Europe, I believe you will find almost exclusively delta-wye for three-phase and line-to-line connection for single phase. European distribution systems are considerably different in many respects from the typical North American system. Differences in grounding, sectionalizing, and protection make delta-wye the appropriate choice.

The increase in third harmonics from switched power supplies may make a second look at the Grd-Y/Grd-Y connections desirable. Recent harmonic measurements I have made on a Southeast US utility system indicated that fifth harmonics are higher than third harmonics, however.

The delta connection does not stop all third harmonics either. Where there are phase angle differences among phases, third harmonics are not necessarily zero-sequence. I once measured 13% third harmonic current distorion in the primary of a delta-delta transformer feeding a 25 Ton arc furnace.

 

[busbar]

An interesting collection of IEEE “party-line” verbiage to read is the C62.92 series—specifically C62.92.4: “…Neutral Grounding in Electrical Utility Systems… Distribution.”

It's important to recognize that there will always be the "regional differences and local customs" aspect, and that IEEE products are consensus documents developed by committees comprised of a potentially large geographic domain. In that process, there is a tendency for IEEE documents worded so as to not hurt anyone’s feelings—to address a range of philosophies and contingencies; comparing different practices while trying to least deride any individual committee member’s long-ingrained preferences.

It's worthwhile to remember that we should not become "well frogs"—id est, living in a well and thinking it is the entire ocean.

For instance, in my region grounded-wye primaries are typically avoided, apparently preferring complete zero-sequence isolation of the ∆∙grdY arrangement with, I surmise, ferroresonance generally thought to be a rare occurrence. But then, “upper-range” [>25kV] medium-voltage distribution is not the norm—where economies of graded/single-bushing distribution-transformer primary insulation may be given higher priory.

 

[SphincterBoy]

New twist: I did a little research as to my utility's history and usage of padmounted transformers. It seems my utility *was* purchasing WYE-WYE units up to 16 years ago, when they encountered a severe ferroresonance problem, when the decision was made to switch to DELTA-WYE. In discussion of this issue with another person, it was concluded that *tuned* circuits can occur whether DELTA or WYE connected primary.

I contacted COOPER POWER SYSTEMS, and asked them what type of configuration is most used by their customers. They responded: "Utilities almost exclusively buy WYE-WYE, whereas industrial users buy DELTA-WYE."

I am currently leaning on going back to WYW-WYE transformers with three-phase disconnect switches to solve the problem. I think the decision to switch from WYE-WYE to DELTA-WYE was not a good decision, and certainly more thought should have been placed into finding an optimum solution to the problem.

My thanks to Busbar, dpc, jghrist, jbartos, and bung for all the suggestions.

 

[dpc]

For this particular installation, with the long run of underground cable, I think the wye-wye transformer would make sense. But I guess I'm not totally convinced that this is going to solve your original problem. It may, but if it was ferroresonance, I would think the problem would show up under single phasing conditions, not when you energize the third phase. But I'm not an expert on ferro-resonance by any means.

 

[jghrist]

dpc is correct. Ferroresonance will occur before energizing the final phase. It pays to go back and read the original post. Since the cable run is several thousand feet, though, maybe it is not really known whether the arrester blows before or after the final phase is energized.

 

[SphincterBoy]

The arresters 'pop' before the final switch is closed.

 

[jbartos]

Suggestion and congratulations to jghrist (Electrical) Oct 16, 2002
jbartos,
I can't find references off-hand. My statement is based on 30+ years experience in the industry, 2/3 of it in the US.
///Congratulations to your significant experience! Some publications dare to publish the HV side of distribution transformers ungrounded. E.g.

http://www.usace.army.mil/publications/armytm/tm5-811-1/entire.pdf

Figure 8.1 Cluster Mounted Transformer Bank Installation.
Also, if one notices overhead (pole) transformer in residential and or commercial districts, one can see that the overhead transformers, usually two, are connected on open delta primary (HV) and open delta secondary (LV). There is no grounding on the primary (HV)side.\\

 

[jghrist]

///Congratulations to your significant experience! ///

Thanks. The downside is that it goes with significant age.