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Topic Title: 132/33kV YNd transformers in petrochem
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Created On: 16 May 2011 08:06 PM
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 16 May 2011 08:06 PM
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makisman

Posts: 9
Joined: 05 October 2007

Hi,

I recently came across YNd1 and YNd11 transformers at 132/33kV (or similar) in new petrochemical installations. The 132kV system is solidly earthed at the star points of the generator transformers. The 33kV system is resistance earthed using zig zag transformers.

Question 1: What are the benefits of using this configuration instead of the more common one with Dyn11 or Dyn1 transformers?
Question 2: If the 33kV system supplies transformers only, does it have to be resistance earthed or solidly earthed is acceptable?

Thanks in advance...
 17 May 2011 08:42 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

Originally posted by: makisman

Hi,

I recently came across YNd1 and YNd11 transformers at 132/33kV (or similar) in new petrochemical installations. The 132kV system is solidly earthed at the star points of the generator transformers. The 33kV system is resistance earthed using zig zag transformers.

Question 1: What are the benefits of using this configuration instead of the more common one with Dyn11 or Dyn1 transformers?

Question 2: If the 33kV system supplies transformers only, does it have to be resistance earthed or solidly earthed is acceptable?

Thanks in advance...


Hi,

I'm not a practising T&D engineer, (those who are please correct any errors I may have included) but my understanding of the star/ delta/ zig zag arrangement is this:

Question 1: What are the benefits of using this configuration instead of the more common one with Dyn11 or Dyn1 transformers?

A star winding is utilised on the HT/ primary side because:
- is more economical for a high-voltage winding (see tapping's);
- has a neutral point available for neutral CT monitoring;
- permits direct earthing or earthing through an impedance to provide voltage stability;
- permits reduced insulation level of the neutral (graded insulation);
- permits the winding taps and tapchanger to be located at the neutral end of each phase;
- permits single-phase loading with neutral current
- allows a flexible vector relationship (YNd1 or YNd11) to be selected via links (on the HT side)

For Delta secondary
- is more economical for a high-current, low-voltage winding;
- in combination with a star-connected winding, reduces the zero-sequence impedance in that winding.
- Presents a path (trapped in delta) for triplen harmonics (not transferred to the primary winding)

When you are working with transformers of this size (60/ 90MVA) cost is an important factor to consider....

Question 2: If the 33kV system supplies transformers only, does it have to be resistance earthed or solidly earthed is acceptable?

The Zig Zag transformer could be an earthing & auxiliary transformer with a 400V star secondary for substation supplies. A three phase downstream load (transformers) could be capable of creating a phase to earth fault which requires an earth path back to the source, the delta winding. Without the return earth path to the delta winding via the 'earthing' transformer (using the zig zag winding) the only earth path would be via the tank of the transformer which is not good for the transformer (especially if 3 limb type, heating effect) or an effective way in which to control the phase to earth current. Utilising the zig zag transformer allows the designer to control the earth fault current to a level which does not stress the equipment and is easily detected to remove the faulty circuit. 1000A is a typical figure quoted by the DNO's to limit the earth fault current of the delta winding.
 20 May 2011 11:37 AM
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williamjohn

Posts: 178
Joined: 22 November 2010

Hi,
I agree with MickeyB. I have seen step down transformers with star primaries so that there was no hgh tension connection between the bushings and the bottom of the winding. However these transformers had a third winding in delta that was not brought out of the transformer tank. Thus the transformers were star/delta/star. The delta winding allowed single phase currents to flow in the secondary although the primary star point was not connected.
Earthing resistors are often fitted in high voltage systems to limit the fault current to full load. This reduces the damage in the event of an earth fault but sensitive earth fault protection must be fitted. Also a solidly earthed neutral means that faults to earth on the secondary can exceed the three phase fault value. However this will not happen in your case with a separate earthing transformer which is presumabely much lower MVA than the main transformer.
With an isolated neutral on the secondary and no direct connection between the transformer tank and the winding, faults to earth return through the system capacitance. The fault may not be cleared leaving a very dangerous situation. Any protection on the zig zag earthing transformer should trip the main transformer.
Best wishes, John
 21 May 2011 04:46 PM
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alancapon

Posts: 5805
Joined: 27 December 2005

Originally posted by: makisman
. . . The 132kV system is solidly earthed at the star points of the generator transformers. The 33kV system is resistance earthed using zig zag transformers. . .

Question 2: If the 33kV system supplies transformers only, does it have to be resistance earthed or solidly earthed is acceptable?

In the British Isles, the public electricity system must be referenced to earth to prevent danger. There are two main methods used, one is an earthing resistor between the transformer star point and earth, the other is impedance earthed using a zig-zag connected earthing transformer. You usually would not use an earthing resistor with a delta winding, so the only option is an earthing transformer.

A resistance or impedance is often introduced into the earthing connection at HV to limit the earth-fault current that can flow. This reduces the cost of the cabling, particularly as a lot of companies are using an XLPE cable with a copper screen.

Although it is possible to earth one phase (either directly or through a resistor), this is rarely done. Without the resistor, the configuration suffers from a very high earth-fault current. Either way, the remaining two phases will sit at line-line voltage from earth - in your instance 33kV, instead of 19.2kV. This obviously increases the insulation (and therefore cost) of the connecting cables, as well as having insulation (cost) implications on anything you choose to connect to the system.

Regards,

Alan.
 21 May 2011 05:38 PM
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ArthurHall

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Joined: 25 July 2008

''In the British Isles, the public electricity system must be referenced to earth to prevent danger. There are two main methods used, one is an earthing resistor between the transformer star point and earth, the other is impedance earthed using a zig-zag connected earthing transformer. You usually would not use an earthing resistor with a delta winding, so the only option is an earthing transformer.''

Alan
Dont forget direct earthing which is extensively used at 11KV and below. The star point is solidly connected to earth.
 21 May 2011 09:14 PM
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alancapon

Posts: 5805
Joined: 27 December 2005

Arthur,

Well spotted. I should have mentioned direct connection to earth as well. I did clarify it in my second paragraph as referring to HV. We are introducing resistance earthing at 11kV to limit earth fault currents, however the LV neutral is always solidly earthed and will always remain so.

Regards,

Alan.
 22 May 2011 01:04 PM
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makisman

Posts: 9
Joined: 05 October 2007

Thanks for your enlightening replies...

In conclusion, am I right to say that a YNd transformer with resistance earthed secondary via a zigzag transformer offers offers the best solution in 132/33kV industrial systems? In addition to MickeyB's points, I was reading that a YN primary reduces the probability of ferroresonance...
 22 May 2011 07:29 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

Specifying a zig zag/ star or delta tertiary is 'good practice' for various reasons and a 'safe choice', however, star/ star transformers have been utilised by the old CEGB/ UK DNO's from the 1970's (apparently the delta can be omitted if the transformer is designed to present a sufficiently low Zero sequence path for earth fault protection).

The J+P Transformer Book discusses the 'pros & cons' of the delta tertiary winding bus does not appear to give clear direction on whether the star-star option is a 'win-win' choice.

Personally I would have done the same as you have found if I was specifying the equipment and didn't want to deal with any 'unexplained' problems in a few years time.... The use of a star-star transformer would need to be carefully selected especially when considering the downstream loads and the impact triplen 3rd harmonics may have on the power quality of the output voltage from the transformer.

This thread touches on some of the 'issues'
http://www.theiet.org/forums/f...atid=226&threadid=2359
 22 May 2011 10:36 PM
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williamjohn

Posts: 178
Joined: 22 November 2010

MickeyB
Thanks for the interesting thread.
With a star/star transformer and no tertiary delta, does the star point of the step down transformer primary have to be earthed?
The neutral earthing point for the primary may be fifty miles away. Is there current in this circuit with an earth fault on the secondary circuit?
John
 23 May 2011 02:54 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

Originally posted by: williamjohn

MickeyB

Thanks for the interesting thread.

With a star/star transformer and no tertiary delta, does the star point of the step down transformer primary have to be earthed?

The neutral earthing point for the primary may be fifty miles away. Is there current in this circuit with an earth fault on the secondary circuit?

John


I'm confused as to why you would have such a distance? Can you please explain why you would have such a distance between the transformer and the NE point of the transformer? Wouldn't the NE point of the primary be close to the transformer? If the star/ star has both neutrals earthed then there will be a link for phase-earth, phase to phase etc.. faults of 1:1 (not taking into account the turns ratio).
 23 May 2011 04:34 PM
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ArthurHall

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Joined: 25 July 2008

I think he is refering to the neutral earth connection at the source transformer which could be a fair distance away.
Transformers with a star connected primary normaly have the star point connected to earth, a CT will be around the connection as part of the transformer differential protection.
As it happens I am currently working on an offshore substation for a windfarm. we have two 240MVA YNd1 132/33 transformers with ZNyn1 earthing transformers
 23 May 2011 05:24 PM
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williamjohn

Posts: 178
Joined: 22 November 2010

Thanks Arthur.
With a delta tertiary, I do not think the primary star point is normally connected so there are no primary earth currents with an earth fault on the secondary. Perhaps a point in favour of a delta tertiary.
Best wishes, John
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