Transformer Inrush Current/ Line Inrush Current

[GJJ]

Gentlemen,

I am planning on feeding an offshore platform via a #2,15kV subsea power cable. A factor I would like to consider is whether the combination of the magnetizing inrush current (Inductive) of the transformer and the line inrush current (Capacitive) of the subsea cable will present a coordination and/or protection problem if both are energized at the same time. Only the step-up transformer and subsea cable will be energized. The step down transformer and loads at the receiving platform will be added after energization. The net current appearing at the energizing (sending) platform should be lower as the two components compensate mutually. I would like to prove this conclusion by long hand calculation if possible. If any one knows of any technical papers and/or mathematical tutorials that may help me get started I would appreciate it very much. Please advise. Thanks.

Regards,
GJJ

 

[Alex68]

You are right to study this important transient but I don't think that a simple hand calculation exists.
You're going to analyse a typical case of electro-magnetical transient and the correct tool is ATP.
You need ATP and an expert user who can decide what model to be used for each component of your system.

 

[davidbeach]

GJJ, Alex68 is pointing you in the right direction. A thorough analysis of the situation may suggest that you never want to energize just the transformer and line. You may need load at the receiving end or insertion resistors at the sending end to limit transient currents and voltages. The length of the cable will be a factor in the analysis.

 

[matwong]

Just want to remind that single-pole switching is to be avoided. We have experience once that the arrester at receiving end exploded due to the ferroresonant effect; though we are lucky that the transformer is OK.

 

[raisinbran]

GJJ,
In the mining industry, we routinely energize transformers (unloaded) at the end of long cable runs. We do not experience any troubles with transients and/or cordination. Maybe the size and type of the equipment in your world and mine are entirely different. No pun intended.
Typical cable - 4/0 SHD G-GC
Typical cable length - 15,000 feet
Typical voltage - 12470 VAC
Typical transformer size - 5 MVA (dry).
I am curious what equipment types that you deal with.
Regards,
Raisinbran

 

[hvcad]

This type of calculation can be undertaken using the PSCAD-EMTDC program.
See

http://www.ipst.org/TechPapers/2005/IPST05_Paper026.pdf

for inrush calculations for windfarms. We have also sucessfully applied this methodology to offshore interconnecions.

Ken

Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.