24940Y/600Y 3Phase Padmount Transformer Wiring 8

[elmatador82]

Hi Guys,

I am new to the forum. I a WYE-WYE padmount three phase transformer 25kV/600V. The utility company requested our engineering firm to buy Y-Y style transformer. The high voltage utility line is a 4-wire service with the neutral grounded. The utility is only going to run three wires, i.e only connect the primary phase A,B,C and leave the neutral unconnected on the primary side of our transformer. Our secondary has a resistor grounded system (5A NGR). How would the transformer behave if the primary neutral connection is left disconnected? Does the HO bushing need to be connected to the cabinet ground? How would it behave if there was a line to ground fault on the secondary? Theoretically what would you measure for voltages on the secondary side between L-G and L-L? Does the HO bushing need to be tied to the main ground grid at the facility? Our secondary loads mainly consists of delta motor loads.

My apologies for the basic question but I am a junior engineer. Do you guys see any potential problems with the above setup? I kept asking them as to why we can't go Delta-Wye since they are running only three conductors but they wouldn't give me a technical answer and said that their standard requires a WYE-WYE transformer.

Look forward to hearing from you guys.

 

[davidbeach]

Check the transformer, it may not even have an H0 terminal, it may be that H0 and X0 are connected internally.

 

[elmatador82]

The XO and HO bushings are available on the transformer and not internally connected.

 

[kitha]

You do not have to worry about the HO bushing at all. If your worry is regarding "floating neutral scenario" at the primary, it wornt affect you at the secondary as you anyways ground secondary through an NGR.If there's a line to ground fault on the secondary, NGR will limit the first ground fault. There will be a primary zero sequence component but it will not affect even you don't ground the HO bushing.

 

[Mbrooke]

If the secondary wye has the ground being used for any reason,(even if it is for a high/low impedance resistor/reactor), it still requires that the primary wye also be connected to the grounded neutral from the Utility. Leaving the primary wye floating would create a severe voltage imbalance should a phase short to ground on the secondary side. In other words you would have an ungrounded primary wye with a ground wye secondary which is an extremely unstable transformer connection.

I somewhat doubt the Utility would leave the primary wye floating. Standard practice being always to connect both the primary and secondary connections to the Utility grounded neutral. This is done through the concentric shield on the underground HV cables which are used as a grounded neutral. They may leave the primary floating if they think you need the secondary floating and this is not your case.

Your best bet would be to fight with the Utility for a Delta-wye padmount. There is no reason why a primary delta will not work regardless of what the Utility thinks. It is the standard for industrial everywhere and European Utilitys as well. Chances are the Utility is not giving you a technical answer because even the tech does not know why it is specked as the standard. The few who do will bring up cost and ferroresonance. Cost is insignificant and ferroresonance is a rare condition that only occurs in large systems where the transformer is very lightly loaded and a single phasing condition on the primary has occurred. In your case a wye- wye would just cause way more headaches.

 

[jghrist]

I agree with Mbrooke that it would be highly unusual for the utility to not ground the primary neutral. An ungrounded wye - resistance grounded wye connection does not furnish a zero-sequence path or zero-sequence source and would be unacceptable.

Reasons for the utility insisting on wye-wye would be standardization if they owned the transformer. If you own the transformer, the utility should permit a delta primary. Ferroresonance can be a real problem at 24.9 kV, but the installation can be designed to minimize the possibility. Ferroresonance could also be a problem with an ungrounded wye connection.

 

[elmatador82]

Thank you for the response guys. We've already purchased the Y-Y transformer. We are in a bit of a bind here. I am still waiting to hear back from the utility. Would it still be acceptable if we bring 4-wires (A,B,C, and N) to the primary side of the transformer and solidly ground the H0 bushing on the transformer? The X0 would still be resistor grounded. Would there still be concerns in this setup?

 

[Mbrooke]

Your plan sounds OK. Study the wiring diagram of the pad mount carefully though, many pad-mounts will have the neutrals bonded to the tank wall internally. Make sure the neutral from the secondary winding is separate from the primary winding neutral (the two are not connected together internally) and that the secondary neutral is brought out via a fully insulated bushing in the low voltage compartment. Remove any conductive strap from the X0 bushing connecting it to the tank. The low voltage neutral is to remain ungrounded until after the resistor. You might also have to run a grounding conductor from the pad to the facility along with the low voltage phases and neutral if local codes call for it.

As for the primary winding it does not matter if its neutral is connected to the tank wall or brought out via its own bushing; as long as its connected to the utility HV neutral your setup will work fine.

 

[elmatador82]

Thank you Mbrooke for your response. I double checked with our transformer vendor as well as our schematics and the XO and HO bushings are completely isolated from one another. The X0 bushing is brought out via a fully insulated bushing in the low voltage compartment. The XO will be ungrounded until after the resistor.

We will run 4-wires from the HV utility to our primary and then tie the HO to the ground grid.

What would be the draw backs or concerns be with such a setup? Are there any potential safety concerns? Please let me know your thoughts.

 

[waross]

Ground versus neutral; The terms tend to be used interchangeably in the distribution field, but you may want a connection to the system neutral. That is a connection to the distribution neutral conductor (which is grounded) rather than to a ground grid.
With the HO tied to the ground grid, any neutral currents resulting from any unbalanced loading or faults will flow to ground, through the earth and then through the system neutral grounds back to the neutral. Although this will work some would prefer to make a direct connection to the neutral conductor rather than using the earth as a conductor.

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

 

[elmatador82]

Thank you for your response Bill. Your point does make sense. Would the following method work better to prevent neutral currents from flowing back to the H0?

- Bring 4-wires (A, B, C & N) from the high voltage line (system) and terminate them straight to the H0, H1, H2, H3 bushings on the primary side of the transformer
- Once terminated to the H0 bushing is it okay to solidly ground the H0 but keep it's solid ground/electrode independent of the plant ground grid? This way you you can prevent neutral currents from flowing back to the primary neutral? I am not even sure if this is even feasible in reality.
- OR Would it be better to just terminate to HV neutral wire to the H0 bushing and NOT solidly ground it and rely on the power line's neutral ground? The run from the pole to the primary of the TX is approx. 200meters. The only concern would be a break in connection in neutral, although unlikely.

Look forward to hearing back.

 

[davidbeach]

If the transformer is just a two winding wye-wye transformer (as opposed to having a delta tertiary) then the current into/out of the H0 bushing is limited by the grounding resistor connected to the X0 bushing. Look at the zero sequence diagram of a wye-wye transformer.

 

[VTer]

How do you plan to bring neutral to the transformer HO if the utility comes in with 3-wire conductors?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[stevenal]

Since this is a padmount, perhaps they are running three UG cables as Mbrooke stated. The forth conductor is the three concentric neutrals.

 

[elmatador82]

davidbeach - Thanks. I looked at the diagram and it makes sense. The amount of current circulating back to the primary will therefore be very small taking into account the turns ratio, 0.12Amps in this case.

VTer - I managed to convince the utility engineer to run 4-wires out to our primary. They will confirm if they are going to run concentric neutrals or a 4th conductor. Either way, they will run the neutral over to the primary.

 

[VTer]

So I am assuming you are going to have some sort of primary equipment for your transformer protection where the utility will terminate? Will this equipment also have a neutral bar in it or will the utility just land their fourth conductor or conc. neutral to the ground bus?
If the primary equipment sits on the same ground grid as the transformer, why couldn't you just solidly ground the HO to the grid as well as your ground bus in the primary equipment? This way you would not have to run a separate neutral conductor between them.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[elmatador82]

VTer: Our transformer is 200 meters from the Utility Pole. The Utility Pole will have a gang operated switch with fused protection for the conductors leaving the pole. The pole is actually a 4-wire service. They have a neutral wire running on their high voltage line. The utility will land the neutral conductor onto their neutral line, not just to ground. On the transformer primary that neutral will land on the H0 bushing. The utility pole or in your words primary equipment does NOT sit on the same ground grid as the transformer due to the 200 meter distance.

 

[Bronzeado]

elmatador82,

If your Y-Y transformer is a three-limbed core transformer, it will burnout some time after one of the HV fuses blowup. This happens due to the "zero sequence flux" created by the voltage imbalance that will induce high eddy currents in the tank, heating it up so the oil.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[elmatador82]

Bronzeado:

We have wired the high high winding temperature back to our control system. We shut down all motor loads upon detecting this. Would this buy anytime on saving the transformer? I don't know the internal design of the transformer core. Is there something we could implement to prevent this from happening? Or perhaps generate an alarm for phase loss and shutdown the facility?

 

[waross]

It is normal for secondary neutral currents to be reflected onto the primary neutral of a Y:Y transformer.
It is a characteristic of a three legged core that it forms a "phantom" delta and acts as if there was a delta winding.
It is a characteristic of a 4 wire Wyeelta transformer or 4wY bank of single transformers that the delta will transfer power from healthy phases to a missing or low phase. In a transformer loaded 67% or more the current on the healthy phases will often be over the rated current. Leakage flux of a three legged core make may this somewhat inefficient, but you have replaced winding heating with tank heating, and heat is the killer.
I suspect that in the event of a primary phase loss the primary neutral current may approach the line currents of the healthy phases. Actually the vector sum of the line currents, but phase shifts and zero sequence impedance issues during an event may make a simple calculation based on pre-issue values less than 100% accurate.
BUT, any primary neutral current substantially greater than a reflection of the expected neutral grounding resistor current may be an indication of a primary phase loss.
Comments??

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