EFFECT OF IMPEDANCE DIFFERENCE FOR PARALLELLING OF 2MVA TERTIARY WINDINGS

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aocalay

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I have a spare transformer and the only difference between the existing and our transformer is the impedance values between HV and tertiary. And the customer wants to operate these transformers in parallel.

Will this impedance difference between transformers effect the parallel operation of transformers ?

Is there any documentation or standard which says that the difference between tertiary impedances of these two transformers ?

The tertiary is loaded and 2MVA.
 
kingpb said:
the difference will cause circulating current between the two transformers. Plenty of papers on that topic.
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If the only difference is the impedance, it should not cause circulating currents in the absence of loads, but it can certainly cause an unequal current sharing under load between the two parallel sources with unequal resistances.
 
As they are fully loaded, especially at low power factors, the secondary voltages will be different and cause headaches when paralleled.

I would guess you would want to add network protectors (reverse power, etc) to be sure to limit damage.
 
GoldDigger said:
If the only difference is the impedance, it should not cause circulating currents in the absence of loads, but it can certainly cause an unequal current sharing under load between the two parallel sources with unequal resistances.
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I guess you missed the part about the 2MVA load in the OP :angel:
 
kingpb said:
I guess you missed the part about the 2MVA load in the OP :angel:
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only the impedances ebtween HV-TV are different so will the only problem overloading of 1 of the transformers and overheating in the future ? Do you please share any technical doxument ? technical paper ?
 
kingpb said:
I guess you missed the part about the 2MVA load in the OP :angel:
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Nope. The size of the load has little effect on circulating currents. Especially if the other windings are not paralleled too. I must admit that the tertiary will either be MV or have very large wires. :)
 
Aocalay,

The impedance-difference between just one pair of windings could materially influence circulating-currents between paralleled 3-winding transformers Furthermore, circulating-current magnitude is not-dependent on load, but very dependent on their No-Load Voltages (or Winding-Turns) ratios!

Can you provide their impedance values? I realize it's available from their Nameplates, but their No-load voltage will be found from their Factory-Acceptance-Test values?

Regards,. Phil Corso [ cepsicon@aol.com ]
 
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GoldDigger said:
Nope. The size of the load has little effect on circulating currents. Especially if the other windings are not paralleled too. I must admit that the tertiary will either be MV or have very large wires. :)
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Thanks for the reply. However, the main windings ( HV and LV ) will also be paralleled and the impedances and ratings of main windings are completely same as existing transformer. So, we have no problem for the parallelling of main windings.
The tertiary is 13,8kV and 2MVA as I said.
The customer is not accepting the proposed HV-TV impedances because they say that since there exist difference between existing transformers impedances, there will be circulating current and there will be overheating when tertiary windings are operated parallel.
How can we prove that cirulating current will be negligible and there will not be an overheating problem ?

Any technical papers or any events you can share with me ?
 
Phil Corso said:
Aocalay,

The impedance-difference between just one pair of windings could materially influence circulating-currents between paralleled 3-winding transformers Furthermore, circulating-current magnitude is not-dependent on load, but very dependent on their No-Load Voltages (or Winding-Turns) ratios!

Can you provide their impedance values? I realize it's available from their Nameplates, but their No-load voltage will be found from their Factory-Acceptance-Test values?

Regards,. Phil Corso [ cepsicon@aol.com ]
Click to expand...

Existing transformer : 2MVA, 13,8kV, and 0,578% @2MVA base
our transformer :2MVA, 13,8kV, and 0,5068% @2MVA base
 
Phil Corso said:
Aocalay,

The impedance-difference between just one pair of windings could materially influence circulating-currents between paralleled 3-winding transformers Furthermore, circulating-current magnitude is not-dependent on load, but very dependent on their No-Load Voltages (or Winding-Turns) ratios!
Click to expand...
I guess I was naive to assume that when the OP stated that the only difference was the % impedance it meant that the open circuit voltages were identical. :(
 
Aocalay,

You misunderstood! You provide data for just tertiary winding data! For 3-winding transformers the following is needed:

HV, MV, and LV winding voltages and capacities (per Nameplate)!

HV-MV; HV-LV, MV-LV impedances (Per Nameplate)!

HV-MV, HV-LV, MV-LV No-Load voltages (Per FAT)!

Regards, Phil
 
aocalay said:
Existing transformer : 2MVA, 13,8kV, and 0,578% @2MVA base
our transformer :2MVA, 13,8kV, and 0,5068% @2MVA base
Click to expand...

I believe that for custom transformers, the manufacturer has an ANSI permitted tolerance on impedance of 7.5% of the "goal".

If the goal was 5.75%, then the impedance of a new transformer might still be 5.32%, even if you specified 5.75% to get a replacement. Some manufacturers will charge a premium if you request tighter than normal tolerances, and even then they will likely only go as tight as 3-4% from the goal.
 
It has been my experience that as long as the HV wdg and TV wdg have the same nameplate voltages, then I would expect little I2 to circulate since the ratios of the two xfmrs are the same. However, as others have mentioned before, there will be uneven loading and care needs to be taken if loading to the capability of the HV to TV MVA rating. The exception is if there are regulators between the xmfr and the load. If the TV wdg is not regulated then you should be okay.

Obviously, the greatest care should be taken during phasing!
 
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