Transformer inrush current -> voltage dip in offshore network 1

[LJL]

I'm trying to find a way to calculate the voltage dip when a transformer is connected to weak network.

The idea is that when a transformer is connected, it may take up to ten times the nominal current. This causes a voltage dip in the network because the generator reactances are fairly high.

There are a lot of info/softwares about calculating inrush currents and also the transformer manufacturers can always provide the inrush values. The problem is that these are usually made assuming that the network is rigid. When the network is weak, the inrush current is lower because the voltage drops right away.

I haven't been able to find any material about transformer inrush currents in weak networks. That is why I ask if anybody has seen any test results, books, articles etc. about the subject? I would be very pleased to hear about them.

It seems to be a complicated phenomena. It is obvious that the inrush current gets smaller and smaller. But it is not obvious that the voltage dip gets smaller and smaller. Actually I believe that the voltage dip is at its highest usually at the 3rd cycle. This is because the generator reactance gets higher (Xd''->Xd'->Xd) faster that the inrush current gets smaller.

The biggest problem in the calculation seems to be now that how does the generator/network X (which is time dependent) effect the inrush current.

 

[RajT]

Can you state the following
System voltage
System capacity in MVA
System sc capacity in MVA
The size in MVA of the transformer and its Z%

 

[Modula2]

I assume you are referring to an unloaded transformer. Is that correct?

 

[LJL]

to RajT:
This is not just one particular case. So the values you asked for can be "anything". But normally we are talking about 4kV-11kV. Transformer 8-20MVA. Generators 5-10MVA each (xd''~12%). There are usually 4-6 generators, but in the situations that I'm interested in, they are not all connected because the dip is higher at that time.

to Modula2:
Yes, we are talking about an unloaded transformer.

 

[RajT]

You should have no problem energising a 20 MVA Tx with a 10MVA gen noting that you will not be able to load the Tx as it stands.

Large Txs of 20MVA ratgs will typically have a inrush of about 14xflc with a duration not exceeding 1 sec. In your case if you had 1x 10 MW gen on line at 11kV and if you try to energise a 20 MVA Tx then the resultant effect would be for the Tx inrush to take say 2 secs or so. The system voltage would dip ofcourse but the drop would not be severe enough to drop out the motor contactors, though any sodium floods may go out.
Ideally ofcourse you would want to arrange for say atleast a 20 MW gen available on line to energise an equally rated transformer to be switched as it is not the switching which is a problem but the automatic and sometimes fast system loading on the transformer which could be a problem as the gen would soon be overloaded with obvious results.

 

[stevenal]

The usual approximation is to assume inrush is voltage indepedant. The next level would be to use the 10X stiff source value to find an inrush impedance. Use this value with your system parameters to find a better approximation of dip. The time dependant solution you seek is too much for my lunch break. Don't forget that your solution will be phase dependant also, assuming the switch poles aren't staggered to close at the same point in the cycle.