transformer replacement, different power rating 1

[monjo]

For replacing transformers with spares of same voltage ratio but different kVA rating, what do I need to consider for replacing on the same protection network, ie which side do I need to construct fault currents for and how?

 

[alehman]

That could effect fault current on both sides of the transformer, depending on where generators and motors are located.

 

[jbartos]

Suggestion: Additional information is needed. Specifically, how different kVAs, transformer impedance Z%, description of the power distribution in terms of power sources, rotating machines fault contributions, etc.

 

[njengr1]

The fault current through the replacement transformer will change inversely with the Z% of the transformer. The Z% is generally based on the voltage ratio and is reported on the kVA base of the transformer. That said, usually a lower kVA xfmr will reduce the fault current, while a higher kVA xfmr will increase fault current.
A change in fault current requires checking he capability of the c.t.'s supplying the relays. The CTs must (1)supply enough secondary amps to permit pickup, and(2) not saturate during a fault. Lower kVA ratings will affect (1); higher kVA ratings will affect (2). In addition the fault level on the secondary must be calculated to see that the switchgear can interrupt the fault.

 

[monjo]

For 33000/3300V 3 phase 6000kVA @ 6.8% impedence to replace 4000kVA @ 6.9% impedence. From switchyard feeds a supply line to multiple drives/motors.

 

[sykimk]

Based on the assumption that your existing 3.3kV SGWR has a sufficient rating to endure possible short circuit currents at the previous system, you don't need to worry about SC rating of the existing 3.3kV SWGR.

As recommended by "njengr1", you shoud consider and check the CT ratio of primary and secondary side of transformers and protective relay settings.

 

[SMB1]

What type of protection relays do you have?

 

[jghrist]

I'm not sure that sykimk interpreted your post correctly. If you are replacing a 4000 kVA 6.9% impedance transformer with a 6000 kVA 6.8% impedance transformer, you will indeed have to worry about the SC rating of the existing switchgear.

 

[dede61]

Hi,

I agree with jghirst, adding 2Mva could bring the switchgear out of spec. and become a danger.

regards,

Danny

 

[SMB1]

one more thing

You need to check your CT knee voltage, ratio and burden. with the new short cir. current.

 

[jbartos]

Suggestion: In agreement with postings showing a concern about the higher short circuit current level produced by the new transformer, it can be shown that:
New transformer approximate short circuit current level on its secondary is:
Isec=6000kVA/(sqr3 x 3.3kV) ~ 1050Amps
Isec,sc=(1.0/0.068) x 1050 ~ 15441Amps
Old transformer approximate short circuit current level on its secondary is:
Isec=4000kVA/(sqr3 x 3.3kV) ~ 700Amps
Isec,sc=(1.0/0.069) x 700 ~ 10145Amps
and
6000kVA/4000kVA=1.5 ~ 1050Amps/700Amps ~ 15441Amps/10145Amps

 

[sykimk]

I agree with jghrist's posting. I thought that the original posting was to replace large sized TR with smaller one. If smaller one would be replaced with larger one, in this case, the existing 3.3kV SWGR may not be used due to the excessive short circuit currents.
I think my posting could make you confused.