Transformer operating in parallel with different percentage impedance 1

[rajapon]

Hi Guys ,

3*20MVA S/S

Transformer 1&2 have the same percentage impedance
Transformer 3 have the different percentage impedance

My question can we make parallel opearation of transformer 2&3?


Thansk & Regards


Raja.P

 

[waross]

It depends, maybe,
The short answer is yes.
A more accrate answer is yes but the safe capacity of the transformers in parallel may be less than the safe capacity of one of them by itself.
The transformer KVA ratings must be adjusted to the same %impedance base.
For example, if a transformer of 6% impedance is to be paralleled with a transformer of 3% impedance, the transformer with the higher % impedance should be re-rated to 3%.
A 1000 KVA transformer rated at 6% impedance woud be a 500 KVA transformer rated at 3% impedance. If the other transformer is rated at less than 500 KVA, the safe loading on the combination will be less than 1000 KVA.
What are your KVA ratings and %impedance ratings?
Transformers may have the same %impedance but different %regulation. This may cause slight errors that are generally ignored.

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

 

[rajapon]

Thank You so much WAROSS

 

[tinfoil]

This is a decent guide:

Link

 

[stevenal]

LTC transformers or not, this Link might be a good reference for what happens when unequal transformers are paralleled.

 

[prc]

For perfect sharing of connected loads, %impedances based on their own kVA rating shall be same.With different %z, load sharing by trf of Z2 will be Z1/Z1+Z2 pu of total load.

1000KVA trf of 6 % impedance can be paralled with 500 kVA of 6 % impedance to cater a combined load of 1500KVA.If you parallel with a 500 kVA of 3 % impedance,then each will take 750kVA,overloading 500 kVA by 50%.

Best standard for rules for parallel operation of transformers with examples -Section 6.0 of IEC 60076-8 ed1.0-1997 Application Guide for Power Transformers. For a more comprehensive tutorial, including various types of tap changer paralleling control see IEEE C57.153-2015 "Guide for paralleling Regulating Transformers"

 

[rajapon]

Thank You so much for your rely guys....

 

[waross]

To split hairs, it is best if the X/R ratios are the same.
If the X/R ratios are not the same, the total of the KVA load on each transformer will be slightly greater than the load KVA. The difference is often small enough to be ignored. In most cases of peralleled transformers, the X/R ratio is neither known nor determined.

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

 

[stevenal]

I don't think hairs are being split too finely. The extra KVA mentioned above is due to circulating reactive current between the two transformers that will increase losses and rob capacity.

 

[slavag]

It is possible.
As attached above, see minimal circulating current mode.

 

[waross]

I am away from my texts for a few days, but as I remember, there is no circulating current due to the extra KVA.
The extra KVA derive from the voltage drops through each transformer being at different angles due to differing X/R ratios.
As an example of extra KVA not causing circulating current consider a single phase load on a delta secondary.
For a 2 kW resistive load on a delta secondary, the kW and KVA loading on the in-phase transformer is 1 kW and 1 KVA.
The loading on each of the out of phase transformers is 1/2 kW and 1 KVA.
We have a total of 3 KVA for a 2 kW load.
The power factor of the out of phase transformers is 50% each, one leading and one lagging.
No circulating current.

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

 

[krisys]

I would imagine that the extra kVA will be there when the paralleled transformers are at no-load. When you start loading them, the extra kVA would go away, as un-equal load sharing.

 

[waross]

The extra KVA is NOT a result of load levels.
The transformers will share the load as expected.
Consider: A 1 KVA load across an open delta will result in a 1 KVA loading of each of the two transformers. That's a total of 2 KVA transformer loading for a 1 KVA load.
The extra KVAs are a result of different phase angles of the current in relation to the voltage.
For parallel transformers the effect is very little. For single phase loading of a delta transformer bank the effect is significant.

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