No phase shift between systems

[Mbrooke]

Basically I have come up with an idea to eliminate 30 and 60 degree phase shifts between transmission and distribution. The plan is to step down 132kv to 33kv via a Wye grounded delta transformer (YNd11) which produces a 30 degree shift and have a 33kv to 12kv delta wye (Dyn01) with an apposing 30 degree phase shift relative to the 132/33 transformer. The end result would be having the 12 kv distribution system in phase with the 132kv system.

Playing with a Fermat point I think this is doable?


Is this approach practical?

 

[magoo2]

I'm not sure what advantage this offers. Why not simply use a 132 kV to 12 kV, wye-grd wye-grd, two-winding transformer?

 

[Mbrooke]

Isn't always possible in every part of the system as 132kv would need to be extended in many places which isn't practical or feasible such as in small right-of-ways. 132kv is usually brought directly to a distribution substation when the immediate 12kv load pocket exceeds 45MVA.

 

[cranky108]

So how do you propose to effectivly ground your 33 kV?
Delta systems have a tendency to suffere with many interesting problems.

 

[bwen08]

Why not use a Dyn1 transformer for both of them? Roll the A & C phases on the high side of the transmission unit vs the distribution unit, i.e. creating a Dyn11 shift for one unit and Dyn1 for the other to reverse the shift.

 

[Mbrooke]

How does one roll the phases? I think I know what your getting at. If so that is a brilliant concept. In any case something I will consider.


@cranky108: probably a zig-zag fault duty transformer and a 5 ohm resistor.

 

[Marmite]

Large parts of the UK distribution system already follow the approach suggested in the OP, and I'm surprised by the inference that this is somehow a novel concept.

Regards
Marmite

 

[Mbrooke]

Thanks!

Do have more info onto exactly how the UK power companies do this? I apologies for seeming so novel. This concept isn't common around here.

The other option which is has been the normal for now has been 132-33kv Ygr-Ygr with a 33-12kv delta zig zag (Dz0) when trying to maintain phase shift.

How does the UKs approach differ to a Dz0?

 

[Marmite]

There are differences across the UK due to the fact that there were originally 12 area electricity distributors, which were themselves formed by the merger of many smaller municipal distributors, which originally served the local towns generally from small coal fired power stations. This means that there are all kinds of legacy system phasing arrangements across the Country. However, a typical 132/33kV substation would be equipped with 2x Yd transformers operating in parallel with an interconnected star/star earthing transformer on the 33kV side of each main Tx to provide earthing and station auxiliary supplies. At 33/11kV substations you could have star/star, or delta star depending on the legacy system phasing. The 11kV network in the UK is either solidly or impedance earthed via a resistor, reactor , or Petersen coil. It's illegal to operate an unearthed distribution network in the UK. 132/11kV substations could have star/star or star delta plus earthing Tx. The 132kV system in the UK is solidly earthed so the insulation level of the transformers is graded towards the 132kV star point to make for a cheaper transformer. Personally I've never come across a Dz0 on the distribution system, but that doesn't mean it doesn't exist somewhere in the country.
I've attached a copy of the Central Networks primary design manual which may be of interest.
Regards
Marmite

 

[Mbrooke]

Thanks!

Alls good but I cant get the link open. It defaults to a blank page.

 

[ScottyUK]

Try this one:

http://www.eon-uk.com/downloads/primary_network_design_manual.pdf

 

[Mbrooke]

It works!

 

[jghrist]

What is the advantage of having the 12 kV system in phase with the 132 kV system?

 

[Mbrooke]

@jgrist

Good question. Hopefully a UK EE will relieve the secrets behind its application in the UK. I don't believe the UK has my situation at hand but we will see.


The reasoning is twofold.

The first reason has to do with the distribution system itself. Larger load pockets are served directly from transmission, while lesser pockets are served via sub-transmission sometimes called "bulk distribution".

In a some areas 12kv (along with other distribution voltages like 22kv)used to be fed via Ygr-Ygr with a delta tertiary. I believe the reason for this was the insulation could be reduced on the primary and the delta fed ungrounded obsolete distribution voltages where the 12kv acted as sub-transmission before becoming distribution.

As these units were replaced with larger ones zero sequence currents passing into the 132kv system became a concern, especially with units over 30MVA which placed bothersome levels during faults on the 12kv system. A delta primary was chosen, however because of the phase shift older units could not be paralleled with newer ones. Delta-zig zag (Dyn0) were decided upon which allowed for paralleling during maintenance reasons or redundancy.

The 12kv system fed from sub-transmission on the other hand has always had a phase shift due to most 33kv, 46kv and 66kv lines being 3 wire with no neutral run. This lead to delta wye units.

This has never really be an issue, however with automatic load transfer and recloser loop schemes being implemented phase angle and especially transformer phase shift between systems have become a major headache. To increase reliability recloser loop schemes allow distribution to transfer into other neighboring systems fed by different substations. When putting the loop back to normal after the fault is repaired or the substation is put back on line the segment can not be restored via closed transition, the tie point into the neighboring system has to be opened before closing back into the regular system. As a result customers have to experience a second outage. If the first outage was resulting from a sub station outage as many as 12,000 customers have to experience a second outage as each feeder is transferred to the normal feed substation through open transition. Same goes for off loading a substation, 2 outages have to occur in this scenario when closed transition can eliminate both.

The second reason has to do with cogeneration and generation dispatch operations preferences. By having distribution in phase with transmission it is easier to keep an eye on the whole thing, much the same way its a preference (though many, many) times more to have bulk 400kv transmission in phase with 132kv local transmission. Generation feeding into both is in essence at the same rotor angle so to speak.

 

[Mbrooke]

How would one roll the phases? Would this mean taking a Dyn11 and putting the C phase on A bushing, A phase and B bushing and B phase on C bushing?

Would this create a phase offset of a Dyn01?

 

[davidbeach]

Yep, that's one of the two possible rolls. "Normal" connection and the two rolls give three functional vector groups with a 120 degree shift between the groups. Without phase labels they look the same and match the vector diagram on the transformer name plate.

Then if you swap two phases on the high-side and two phases on the low-side you can get (when combined with rolls) another set of three vector groups offset 60 degrees from the previous set. With the right selection of rolls and swaps you get any of the six possible vector groups for that transformer winding. A swap on only one side of the transformer causes the two sides to have opposite phase sequence, one will be A-B-C and the other A-C-B.

 

[Mbrooke]

That is great info So any one role produces a 30 degree shift?

Do any PDFs/excels exist of the various shifts?

 

[davidbeach]

One role is 120 degrees leading, the other one is 120 degrees lagging. In a Dy transformer you can only ever get the odd numbered vector groups, representing some odd multiple of 30 degrees. There's no connection change you can make to change the shift by 30 degrees.

At one point the Basler web site had a collection of technical papers that included one on various transformer connections. I haven't looked for it in several years so I wouldn't know if it's still available.

 

[Mbrooke]

But a Dyn11 will offset a Dyn01, which will produce a 0 degree phase shift? And a role is only 120 degrees?

 

[arrehman1]

Check out "IEEE Standard for Standard Terminal Markings and Connections for Distribution and Power Transformers" IEEE Std C57.12.70

This is "the standard" for understanding transformer phase shifts & vector groups... it took me a while to find this one...

Hope this helps!

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The Wye Wye Transformer Connection video:

http://gpac.link/1GdGRSQ