Advantage of grounded Y primary for utility 1

[tefc]

Hi,

I’ve be involved in the past on a project with an electrical system transmission on 132kV and distribution on 33kV, with both 132kV and 33kV earthed. We used 2 transformers with the following vector group : Star in 132kV side and Delta on the 33kV side (YNd1)with earthing transformers.

This transformer vector group was required by the local electrical company instead of Dyn11 (but no reversal of power flow at any stage of the project forseen).

Could you explain why grounded Y primary can be of advantage to a utility ? Thanks in advance.

I've read the thread 238-21087 before (Transformer vector group - which one ?).

 

[cuky2000]

Selection of the transformer connection is associated to multiple causes such as economics, legal, operation, reliability and design considerations.

Transmission systems are affected by heavily by lightning activities, switching and other sources of overvoltages that expose the TL, transformers and other power apparatus to abnormal power frequency voltages during fault conditions.

There are some advantages to select the transformer with Y (primary) –D(secondary) as follow:
1- The Delta connection provides path for the third harmonic exciting currents to eliminate 3rd harmonic voltage.
2- The cost of transformers will be less since the YG allow for reduced insulation (BIL) on the primary.
3- Since single L-G fault is the most frequent cause of the fault the secondary side will have better reliability performance. In many cases the primary side protective scheme allow re-closing that successfully clear the fault typically in 90% of the time.
4- Telephone, radio, TV and other communication interference are minimized with the reduction of 3rd harmonic. Transformers Y-Y or GrdY-GrdY connection are less common in the power industry because substantial amount of 3rd harmonic could flow if other ground sources are present in the secondary side. Went this is use, a 3rd winding usually is specified).

 

[prc]

I am afraid many of these connections has a historical perspective.All advantages cuky 2000 has mentioned can be got from Dy connection also except cost advantage.I assume utility has a solidly grounded 132 KV system and you have taken supply for a neutral solidly grounded 33 Kv distribution system .My comments are based on this assumption. If the systems are like that there is no problem for going for Yy trfs saving the cost of NGT . If fault current limitation is required, a neutral earthing resistor on secondary side will serve the purpose.I have seen such arrangement all over Indian power systems.But about 40-50 years back ,atleast in India ,invariably Dy connected trfs were used whenever a supply was taken for Industry use. I dont know why it was done.Dy connection is also essential if the utility 132 KV system is an ungrounded system. With the better grades
of CRGO used in trfs and improvements in interference circuits used by telecommunication systems,harmonics from exciting currents are no more an issue,as it was earlier. So with 3 phase ,3 limbed core type trfs Y y connection can be used with out problem. But thre reason utility insisted for Yd connection may be to limit fault currents.

 

[tefc]

cuky2000, prc, many thanks for your replies. However, if you agree, I would like to get more informations about the following points.

1/I understand that Y at primary side make no sense if the utility 132 KV system is an ungrounded system. I suppose the utility will see only an unbalanced current system in case of line to ground fault.

2/ Considering utility has a solidly grounded 132 KV system. Prc, you say : « But the reason utility insisted for Yd connection may be to limit fault currents ». Could you explain ?

3/ Cuky2000, you say when transformers Y-Y are used, a 3rd winding is usually specified in order to limit substantial amount of 3rd harmonic flow toward the utility. I’ve already seen this type of transformers with a 3rd winding delta. Is it possible to give a percentage regarding the limitation of the zero sequence current at primary side. I’ve found the equations with symetrical components for this transformer, but it is not easy to have an idea. In the other hand, which ratings shall be specified for the 3rd winding of this transformer.

Thanks in advance. Sorry for the english.

 

[prc]

With Yd trfs , fault current for an Lg fault will be limited by the NGT . so the reflection of the same on primary also will be less.
The rating of stabilising delta winding when provided is one third of the MVA of the main winding,

 

[gordonl]

With a solidly grounded system on both sides of the transformer, I would think that it would still be important to isolate the two ground systems using Yd or Dy so ground faults on the utility aren't seen by the customer and vice-versa. As well a YY connection can't supply ground fault current only pass it through, so a source still needs to be established, and counting on the Utility for this is less than ideal from my perspective.

A reason that the Utility may require a Yd is so they establish a new ground fault scource to reduce zero sequence impedance in the area, if there is already a source close by than the importance is reduced. (With the Yd the customer will require the seperate ground fault scource, $$)

 

[cuky2000]

Dear prc, I should clarify what I meant for cost vs BIL.
Consider a LG fault, a typical rule of thumb for maximum overvoltage in the healthy phase is for different transformer connections are as follow:

a- YG max overvoltage = 80% LL Voltage.
b- Delta max. overvoltage= 100% LL Voltage.
[red](For accuracy, the Coefficient of Grounding/ Earth Fault Factor should be determine for each specific system rather than use a fix value as shown above)[/red]
The dielectric strength could be reduced more in the YG connection than in the Delta connection with the adequate MOV surge arresters. For instance a 230 kV winding BIL could be rated as 550, 650, 750, 825 or 900 kV per ANSI (IEC could be similar).

Pass experience evaluating similar power transformer design versus BIL is shown as follow:

BIL(kV) Normalized Cost(PU)
900 100%
1050 110%
1300 115%
1400 117%
1550 120%

Tefc, In reference to your question #3/, I believe that the tertiary winding use for power supply is typical sized about 1/3 of the total transformer capacity MVA. Example:
-Primary (66 kV Star)………15 MVA (100%)
-Secondary (13.2 kV Star)…… 10 MVA (67%)
[red]-Tertiary [/red] (2.3 kV Delta)…… 5 MVA [red](33%)[/red]

For two winding transformers in Y-Y configuration, a small DELTA winding is provided to help eliminate the third harmonic and avoid overheating in the steel transformer tank. Please see the enclose thread238-24305

In regards to your questions 1/ & 2/, see the enclose publication for applications on Independent Power Producers (IPP) systems that show the pros and con for different transformer connections from the protection point of view of the author.

http://www.beckwithelectric.com/powerlines/articles/i37/wayneart.htm