Inrush current for two transfomer in series

[ctpt]

There is a system where two transformers are connected in series. There is an existing transformer of 150 kVA (480V/4.16kV) in an offshore platform. This transformer feeds a new platform at 4.16kV through subsea cable and connects to a new transformer of 200kVA ,4.16kV/480V. (I have no idea why the new transformer is of bigger capacity than the upstream). Now the question is whether the inrush current for the 200kVA transformer will be multiple of its own IFL (say 12times @0.1 sec.) or it would be 12 times the IFL of 150kVA transformer. Is inrush current dependent only on its own rating irrespective of the upstream system capacity?

 

[tinfoil]

Without going through the math, the inrush current of any transformer is limited by how much impedance is represented by the source.

Even your 150kVA transformer will not experience its full IFL, because the system to which it is connected has a non-infitite capacity (non-zero impedance).

But the conservative engineering assumption to make is that there IS an infinite bus there, so that the installation is immune to future upgrades made to the source. For example, the utility could decide to build a major substation right next door, causing your actual supply impedance to drop dramatically.


In your case, the 200kVA 'downstream' xmfr might have been chosen because there is a chance that the upstream system (i.e. your 150kVA unit) may be upgraded. In that event, if you had chosen to 'downgrade' the equipment for the 200kVA xmfr due to lower fault levels, it may become inadequate.


On the other hand, you DO have to coordinate the tripping levels. Do do this, you have to deterine the fault and inrush levels, which can be done by building a MODEL of your system (There's lots of threads on this forum that discuss modelling)