Sub transmission voltage regulators

Growling makes me think ferroresonance- and it would make sense if the wye point is not connected to the earthed system neutral. Even if ferroresonce is not an issue, I HIGHLY recommend that you never operate a bank as floating wye since the voltage will be all over the place. Always use the correct delta regulators if the line is 3 wire.

I've seen and done this myself; if you have a solidly grounded Y point you should be fine, although you now have a 200A choke on one phase....If you were in delta, ungrounded Y or impedance grounded Y I'd say stay away from it for the reasons you highlight.

Sorry I should have noted I'm from Australia and we run a HV delta system ( no O/H earthed system neutral). Star connection is a common arrangement on the national grid for 33kv regs as to lower costs and use 22kv regs ( 19.1kv individual phase ratings) . Each S/L connection of each tank was taken down to the earth ring of the site before bonding to create star. I have not come across ferroresonance issues in an overhead network before ( under ground HV yes), but I don't have that much experience with regulator sites either, is it a common issue with regs...?

If you have three distribution transformers on hand that can be used temporarily, connect the primaries in star and the secondaries in delta. The secondary voltage is unimportant as long as all three are equal and on the same tap.
The primary wye point will give you a stable neutral reference for the regulators.
50 KVA or 75 KVA will probably be large enough.
100 KVA will be better.

Bill
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"Why not the best?"
Jimmy Carter

Why would 2 33kv regs in open delta cost more than 3 22kv regs in wye?

Our distribution transformers are per wired from the manufacturer primary delta, secondary Wye, and are unable to be changed..
We seem to be getting away from the original question, I will follow up on the earthing situation tomorrow ( they were meant to test earth continuity from each tank to earth ring and resistance to earth of the ring) and ferroresonance suggestions, thank you very much for these. But if someone could explain what the effects would be of putting the 1 x 200a reg into service with 2 x 300a on a locally grounded ( local earth ring at site , no O/H earth system neutral) in Wye configuration,and why it would or wouldn't work, I would be forever grateful...

I know this might not be what you are looking for, but you need 33kv delta regulators. Grounding the regulators via a single grounding system will probably get rid of the ferroresnaonce, but unless you have a stable, very low earth resistance with fully redundant electrodes you can get someone killed.


My understanding is that the NESC requires a neutral run back to the supply substation for any wye connected loads and I would imagine the IEC requires the same.

The earthing system of this site incorporates an earth ring ( 4.5m width x 5.5m length) of 6 x 1.2m CU electrodes installed at .6m below ground line connected by 120mm CU cable this also has 1 x additional earth stake off each end of the ring. The regulator is bonded to a earthed steel frame with this being taken down on at two points on the diagonal of the ring.. The concrete slab the frame sits on has the re-enforcement cage fully welded with this also bonded to the earth ring at two seperate points on the diagonal. From earth tests we are expecting sub 8 ohm readings I think, would need to refer to the report. I can't remember the expected fault current at site or x/r ratios off the top of my head. We do have SEF protection equipment on the feeder. We work from ASNZS7000 and it doesn't have anything about running back to zone substations on primary voltage Wye connected loads..

In other words ASNZ7000 lets you use the earth as a sole conductor?

My suggestion was for three single phase transformers. If you don't have any single phase transformers in stock then forget my suggestion. Sorry.

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

Please provide the impedance of the new and old voltage units. Perhaps this will allow some one to help you with concrete answer if there is not issues connecting the voltage regulating transformers.
In the meantime, let us know the following background information:
[ul][li]Why the 200A units were replaced by the 300A rated units? [/li][/ul].
[ul][li]There is any issue with overvoltage or large harmonics (3rd & 5th harmonics could be an issue for Y connection)[/li][/ul].
[ul][li]Is the connected load greater than 11 MVA?[/li][/ul].
below are a few remark and extra information that I hope could be helpful to advance this thread from good opinion to more concrete technical answer:

MBrook - yes all our substation site have a local earth system for each site, it can be either a seperate earth which is generally used on rural single and 3 phase subs( hv and Lv Brough down on to individual earth systems seperated by a distance calculated in our earthing program), single earthing which is used on our urban subs and when 2 or more subs are interconnected it is classified as an MEN connected system. We also has SWER ( Single wire earth return) on our long rurals, if you want to see a scary earthing scenario have a look at this stuff at 19.1 kv.

Waross - sorry mate didn't even think of single phase subs. Ours are only made up to 25kva in singles, so went straight over my head.

Cuky2000 - sorry was out of the office today will chase up impedences tomorrow, will try get some photos of name plates uploaded.
They are being upgrade due to a significant load increase on the line in the next couple of months. They were nearly maxed as it was and will nearly be maxed out again once the new loads are applied ( this is what I have been told by planning dept) and will continue to be heavily loaded until a new additional feeder can be built.
The issues are with under voltage due to the length of the feeder/ the amount of load on the line. It would be approx 60km long with large inductive loads (Chicken sheds and pumping loads) at the extremities and many SWER feeders off the main line, conductor is 3W 19/3.75mm AAAC (Neon). This feeder is also a back feed for another town about 120km away if the 132kv radial for that town goes down...

Hi guys, been trying to chase impedences but no luck as of yet, nothing on tanks, name plates, tech specs we have or online will have abit more of a poke around tomorrow, even contact manufacturer. Found out the line will run close to 20MVA or 350A at 33kv so should nearly be dragging on the ground in the middle of summer. Yay

Hi MADNCE,

The large unit appear to run above the 20MVA load current rating. If one of the larger unit is substituted by one of the smaller regulators, the situation to be at risk to operated at overloaded conditions even without considering the effect of potential circulating current and the impact of the 3rd & 5th harmonic derating effects.

Did you considered to run temporarily in open delta with a reduced capability?

Could you please clarify the rating provided earlier "200A regs @ 22kv rated and 300A regs @ 22kv rated" connected to a 33 kV delta system (Phase-phase-to-phase).

See a typical voltage regulator rating below.

Provided you are not overloading the replaced 200 A unit ( ie keep all 3 phase units to 200A only) you can test run with 1 number 200 A unit and 2 nos 300 A units as a bank. Believe these are auto connected units with solidly grounded neutral. When 300 A unit is working at 200 A , the % impedance reduces to that proportion. Up to 5-10% impedance variation will not create any problem. You can easily check the impedance at site. Apply a small voltage on primary with shorted secondary. Extrapolate voltage for rated current of secondary. That applied voltage as a % of rated primary voltage is the % impedance.

FWIW, (and yes I know I sound like a stuck record player) Cooper specifically discourages ungrounded wye in their manuals.