Ferroresonant resistant transformer

[Mbrooke]

Does such a creature exist? Can manufactures (like Howard industries for example) have the ability to build pole and padmounted transformers with a modified saturation curve?


I am entertaining the idea of a 33kv distribution system, however my only hold back is that engineering literature brings up ferresonance being very likely under single phase conditions.

 

[Skogsgurra]

First, it is resonant only, not resistant. And, yes, it is single-phase in the garden variety execution.
But three single-phase transformers can easily be connected in delta. They will not work well if connected in star.

The cores are fairly simple to modify so you get a leakage path for the loose coupling needed between primary and secondary. And saturation is just a question of flux, which is where the resonant part comes in. So, technically, it should be possible to build such a creature. Or rather three of them.

But they have a lot of losses, due to iron saturation, and may not be worth it. Also, it is not very often that 100+ MVA needs to be stabilized. What application is this?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Mbrooke]

A distribution system with classical loads being run 33kv instead of 11kv. The idea is to bypass the intermediate 33/11kv substations and feed everything directly at 33kv. The 33kv supply substations will be equipped with automatic tap changers to regulate a steady 1.05pu voltage.


11kv is not known for ferroresonance, single phase switching and blown fuses present no over voltage hazard.


Can you explain what you mean by this? "First, it is resonant only, not resistant." By resistant I mean not likely to go into ferroresoance.

 

[Skogsgurra]

I read you completely wrong.

You asked for e ferroresonant transformer and those are also known as magnetic stabilizers. You are obviously not at all asking for that. Sorry.

Is ferroresonance (in the bad meaning) a problem in your grid?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Mbrooke]

Its ok.


The thing is I am evaluating going from 11kv to 22 and 33kv, and I am not sure what to expect since single phase switching and cutout protection of equipment is common.

 

[magoo2]

Can manufacturers have the ability to build pole and padmounted transformers with a modified saturation curve?

They probably do but, when you deal with the volume of distribution transformers in North America, manufacturers tend to standardize on the amount of core steel for a given kVA rating. For three phase transformers, the grounded-wye grounded-wye is preferred over delta grounded-wye for minimizing the likelihood of ferroresonance at the higher voltage systems like 25 kV and 34.5 kV. This grd-y grd-y design was initially thought to be immune from ferroresonance, but the voltage coupling from the adjacent leg caused the overvoltage problem because you then had an open circuit voltage in series with an L-C circuit.

The idea of using three single phase cores in a three phase padmount tank was suggested some thirty years ago to solve the problem with a grounded-wye grounded-wye padmount transformer because, under open phase conditions, the other two phases were not affected as they would be with a five-legged core construction. It didn't prove to be a commercial success even though the cost might have been 10 or 15% more than the standard offering.

If you're looking at 34.5 kV distribution rather than 33 kV, the previous comments apply. My understanding is that the 33 kV (European) distribution systems tend to be three wire circuits so it is a different situation than what I was referring to.

 

[Mbrooke]

All loads will be connected phase to phase- otherwise I would go wye grounded wye grounded with single phase pots.


Worthwhile comments none the less, the same issue comes up with 34.5kv quite a bit.


There is a trend both overseas and in the US to use 35kv class equipment in new builds.

 

[prc]

Manufacturer can make the transformer resistant to ferro-resonance only by adopting a lower working flux density, say 1.0T instead of normal 1.7 T .This is usually done only in potential transformers. In case of power or distribution transformers, it will be prohibitively costly and never done. There are other mitigation measures as explained in IEEE Standard C57.108-R2008 Transformer Connections-Clause 7.3

 

[waross]

Manufacturer can make the transformer resistant to ferro-resonance only by adopting a lower working flux density, say 1.0T instead of normal 1.7 T .This is usually done only in potential transformers. In case of power or distribution transformers, it will be prohibitively costly and never done.

Could you achieve the same result by using 66 kV (over 56 kV) transformers at 33 kV?
Your comment re: cost is still valid, but the extra costs for a special design and build may be avoided.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

So it looks like my only solution to complete avoidance is either grounded wye-wye single pot banks, or eliminating the conditions such as voltages over 15kv.


What about a higher X/R ratio, or lossier transformer?



Basically would you feel comfortable with this: A 33kv system with fused cutout protected transformers consisting of 50kva single phase, 3 50kva banks and mostly 150-250kva 3 phase 3 limb units. What would the odds be of ferro? I am hoping the load will dampen it out- at least it should on the 3 phase units from the amount of load being served.

 

[prc]

No waross, when you use a 66 kV unit on 33kV, the secondary voltage will come down by half !
Mbrooke,As others said, Yn/Yn transformers are not resonance proof. You will find that mistake in literature before 1990.There are several ferro-resonance mitigating measures mentioned in above standard, Main is measures to avoid single phasing during switching.

 

[Mbrooke]

But if I go Y-Y, 3 single phase units are not susceptible.

Can you post the main points of Clause 7.3, if permitted? I am not getting it here:

http://odysseus.ieee.org/search?sea...findstds/standard/&search=Search&history=true

 

[prc]

The main point is avoiding single phasing- this can be achieved by using 3 pole switches and fault interrupters so that single phasing is avoided. Use MOV arresters on both HV &LV sides.

 

[Mbrooke]

In other words if you want a delta line, it must be 15kv and under. 25 and over GrY-GrY.


I still don't get how Europe does it. I've heard of 22 and 33kv being routine with delta units and fused cutouts. I am starting to think thats why 11kv exists in the first place...

 

[MatthewDB]

I still don't get how Europe does it. I've heard of 22 and 33kv being routine with delta units and fused cutouts. I am starting to think thats why 11kv exists in the first place...

Remember that part of the ferroresonance risk includes the cable capacitance. In Europe with denser cities, cable lengths tend to be shorter with a lower capacitance.

 

[Mbrooke]

Perhaps- but I am willing to gently challenge that. There is far more underground distribution in Europe, and 33kv can travel much greater distances under all comparable scenarios. All in all I think that given the typical load densities encountered by most utilities 33kv has much greater capacitive reactance as the practice is to load feeders to the maximum thermal limits or run the feeder the furthest voltage drop will allow, usually a combination between the two with final numbers multiplied by 0.5 to 0.9 to allow for picking up load from other circuits. Further consider that most 15 to 35kv class equipment is rated 10kaic. 33kv requires a larger transformer to achieve 10 kaic then 11kv- so where a 40MVA max unit would be at 11kv; 80 or 100MVA would be at 33kv.




Given the efficiency gained by bypassing an entire voltage class and eliminating a half dozen substations for every subtranmission substation, its worth weighing the risk.

 

[Mbrooke]

Would they mean ferro here? I am confused why 22kv is the cutoff when it can happen at any voltage given the right conditions.

https://www.sandc.com/globalassets/...all-documents/descriptive-bulletin-811-30.pdf

A Note on Single-Pole Switching
In single-pole switching of ungrounded-primary
three-phase transformers or banks (or single-
phase transformers connected line-to-line), circuit
connections or parameters may, in some cases,
produce excessive overvoltages. In particular, for
the following applications above 22
kV, single-pole
switching by any means—including the Loadbuster
tool—should be performed only under the conditions
stated in italics:


* For unloaded or lightly loaded delta-connected
or ungrounded-primary wye-wye connected
three-phase transformers or banks (or line-to-
line connected single-phase transformers), rated
150 kVA or less three-phase, or 50 kVA or less
single-phase—or of any kVA rating when combined
with unloaded cables or lines—where maximum
system operating voltage exceeds 22 kV.
Single-pole
switching should be performed only if each phase
is carrying 5% load or more, or if the transformer
or bank is temporarily grounded at the primary
neutral during switching.


*For loaded or unloaded ungrounded-primary
wye-delta connected three-phase transformers or
banks—alone or combined with unloaded cables or
lines—where maximum system operating voltage
exceeds 22 kV.
Single-pole switching should be
performed only if each phase is carrying 5% load
or more and if the lighting-load phase is always
switched open first (or switched closed last); or if
the transformer or bank is temporarily grounded
at the primary neutral during switching.