Zig-zag transformer winding 4

[ppaya]

Hi all:
I need little more info on zig-zag config on Tx winding. I understand it is not uncommon in North America as a secondary winding conn. I need to now why?
My understanding is that it limits SC but how? Is it due to more inductance in winding? What is engineering justification for this more expensive design?
I have Tx 56/67/83 MVA with star solidly grounded on primary (230kV side) and zig-zag through reactanse 1.5OHM on secondary (28kV)
Any info appreciated...

 

[busbar]

 
Voltages might point to a generator-stepup transformer, which is typically Y-[Δ], although Y-ZZ also has 30° shift. The reactor will limit secondary ø-g fault current to (16.2kV/1.5[Ω]) or ~11kA

Maybe it was intended to parallel a Y-[Δ] transformer? Sounds like someone’s big-ticket experiment.

 

[ausphil]

Zig Zag connections are used as earthing transformers where your main power transformer has a delta winding.

It basically gives an earth point so that earth fault currents can flow.

The zig zag arrangement is done so as to reduce the cross section size of the core of the transformer, because the fault current is split between 2 limbs of the transformer, reducing the flux when a fault occurs. It basically sits there happy during normal operation, then kicks in with an impedance and a path for earth fault current to flow.

 

[Deansharafi]

ausphil,
The ratings of zigzag transformers show current values such as 150A. Is this value the maximum earth fault current that is allowed to pass through transformer? What are typical Z% for this type of transformers?

 

[busbar]

ppaya, is the 230kV winding tapped, say at 115kV, as an autotransfomer?

 

[ppaya]

busbar:
this is existing and (typical) DESN station distribution arrangement in Ontario. It is not a autotransformer. Zig-Zag winding is on secondary of the power transformer. Primary is solidly wye grounded winding.

thanks everybody for input, but I wanted to know is this something better then say reactance grounded wye winding on secondary. (engineering justification for zig-zag)?

 

[busbar]

 
The Y-ZZ may have been intended as a substitute for a Y-Y bank with an isolated-[Δ] winding.

The low-side reactor seems kind of odd, for it would limit 28kV ø-g fault duty but not reduce ø-ø fault duty. If for distribution service, the reactor would likely not permit ø-n loads.

 

[GamezBeCJ]

Another use for Zig-Zag transformers (I have designed a few) is to eliminate some of the harmonics generated at the load (those multiples of 5th harmonic).
The ones I've designed are for industrial purposes (no bigger than 10 MVA).
It turns out that having a Zig-Zag connected secondary on a transformer, eliminates the circulation of those harmonics, the same effect can be obtained having a delta-wye in the secondary winding.

Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist

 

[rbulsara]

ppaya:

I beleive the primary objective of use of a zig-zag transformer is to create a neutral point where none exist such as a delts system. It is not intended to use as 'reactive' grounding although it may have some reactance.

 

[rbulsara]

Refer to page 8 in the follwong link..

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

or IEEE green book...for actual desings you may want to talk to manufacturers.

 

[cuky2000]

Part I: Compliance with Local Requirement

In contrast with the standard practice in Canada, the vast majority of power transformers in the US are installed in Delta –Wye configuration with LTC in the LV side instead on the HV and resistance rather that reactance grounded for neutral grounding.

I presume that the local Independent Electric Market Operator (IMO) Rules in Canada and the traditional engineering practice have important influence in the system design and selection of equipment. Perhaps a Canadian colleague could help us to understand better the justification in selecting the ZZ transformer for their local market. As starting point, a typical substation and a summary of the IMO market rules is enclosed.

 

[cuky2000]

Part II: Technical Requirement.

Comparing D-Y and Y-ZZ transformers, the Y-ZZ require approx 15 percent more copper connection and the regulation and efficiency are liable to be somewhat poorer than the D-Y. The D-Y blocks the zero sequence.

Zigzag transformer is preferred where:
- A neutral is required for grounding purposes and at the same time will parallel with existing angular displacement of 30 degrees since the angular displacement of the Y-ZZ will parallel with existing Y-delta connected bank.
- When change of system voltage is contemplated and the transformer may be operated D-ZZ temporarily to be changed later onto Y-ZZ.
- The neutral of the zigzag transformer can be grounded without any third-harmonic voltage trouble. A 3rd harmonic voltage appears in each winding but not from line to neutral on the ZZ side. The unit can also be loaded with single-phase load from the line to neutral of the zigzag.

The enclose figure provide technical information that could help to validate the above comments and perhaps motivate other colleagues to provide new input.

 

[ppaya]

cuky 2000
thanks for post. (star for you)

 

[mc5w]

Mirus International makes zig-zag sets that they call Neutral Current Eliminators to get rid of 3rd and 9th harmonics primarily in 120Y208 volt power systems. The most efficient design would use bifiliar windings if the insulation system would withstand that.

According to something that I read somewhere the Zig-zag set was invented by a Russian EE who was looking for a way to solidly ground delta-supplied transmission lines. In ungrounded systems the system capacitance to ground is in series with the inductance of the supply transformer which either substantially throttles or magnifies the available ground fault current. Either way is a problem. In a solidly grounded system the system capacitance is in shunt with the transformer inductance. In resistance grounded system the resistor both limits first fault current to an easy to interrupt level and acts as a damper when interruption does occur.

By the way, one idea for resistance grounding is to divide the resistor into two parallel banks. One bank would be permanently connected and the other would be amplitude shift keyed so as to produce a line frequency CW telegraph signal. This would be easy to trace with existing ammeters and if you look in the amateur radio literature there are all kinds of digital and switched analog filters and so forth for sniffing out a CW signal. Since Amplitude Shift Keying a.k.a. CW telegraphy is the only communications method that can work with a signal that is below the noise television remotes use amplitude shift keying of an infrared diode. The infrared ASK signal can overcome infrared noise due to light bulbs and so forth.

Mike Cole, mc5w@earthlink.net

 

[mc5w]

Oh, Mirus International make dual secondary and quad secondary transformers for harmonic current cancellation and mitigation by splitting up a 120Y208 volt distribution into 2 or 4 phase shifted segments.

On a quad output transformer one winding wil be zig-zag (zero degrees), the next one wye (30 degrees) and the other two forked wye giving 15 and 45 degrees of phase shift. The dual output transformers can be ordered to act like a quad output slit into two boxes.

Also, back in the days of mercury arc and hydrogen arc rectifiers the transformer feeding the rectifiers would have zig-zag windings so that the direct current would inherently cancel and the transformer would be more tolerant of uneven distribution of the DC between 3 pulse groups.

 

[RalphChristie]

Mc5w, do you have any extra info regarding the invention/history of the zig-zag trsf? (Books, websites etc.)

Just a sidenote:
A zig-zag trsf is also known as a NEC or neutral earth coupler.


Regards

Ralph

 

[mc5w]

Just some things that I have heard and read. A possible resource is to do a patent search and most big city libraries in the U.S. have a complete duplicate of the U.S. Patent Office files on microfilm which makes Patent searches a lot easier. I have not done a patent search in 12 years but when you first do one you need to allocate extra time so that you can do some joy browsing - the first few times are nirvana.

 

[busbar]

 
Try

http://www.uspto.gov

You might do it quickly because predictions are that the "innernet" thing is just a passing fad invented by deviant college kids on a diet of Funyuns and Coca Cola.