Energization of 150 MVA transformer. Inrush current issue 1

[svenni]

I am looking into limiting the inrush current as much as possible.

The 150 MVA transformer is 33/150 kV and is to be energized from the 33kV side. The transformer is energized for testing purposes only and will never be loaded.

It is being energized from a relatively week grid, through a small(10MVA) 50/33 kV transformer.

I have looked into solutions such as sequential phase switching to hit the optimal point in the voltage curve resulting in reduced inrush.

But I am very interested in a solution of serial connection of resistors, around 100 ohm. This should effectively reduce inrush current. The resistors can remain in series connection as the transformer should never be loaded (only no load losses of transformer). Can anyone see a fundamental error in this setup or advise if components such as 33kV (3-10kW) are readily available?

any hints/tips greatly apreciated.

 

[redfurry]

Just a thought, but have you calculated what the voltage drop will be through the 100 ohm series resistor due to the 150MVA transformer no-load losses. Might be an issue.

 

[svenni]

Im getting about 2 - 5 kV (assuming 1-2% magnetisation current)so it might be wise to reduce the resistor . Otherwise, are there any issues to be worried about?

 

[svenni]

Actually no load current is 0.2% -> 5A and voltage drop over 100ohm resistor will be 500V, so not really an issue for this setup.

Could there be any other reasons for not using a permanent series resistor in this setup?

 

[jghrist]

The weak source itself may limit the inrush enough. Consider that the impedance of the 10 MVA transformer will be about 10 ohms. If this isn't enough, I see nothing wrong with adding resistors. The 10 MVA transformer impedance will be mostly reactive, so it won't add directly to the resistors. The vector sum of the transformer impedance and 100 ohm resistors won't be much more than 100 ohms.

How do you know that 100 ohms is enough?

 

[svenni]

It is true, the weak grid and the 10MVA transformer do limit the inrush current. Regardless the inrush current will stress the 10MVA transformer greatly and cause severe voltage drops through the grid.

An 100ohm resistor was choses a bit arbitrary and a quick EMT simulation shows inrush current is almost completely removed. Resulting in something like 15A peak current that is already dampened after 1 period to the level of transformer no load current. So actually a 500W resistors would be sufficient.

 

[TurbineGen]

You could use a parallel switch with your resistors. Power the unit up with the resistors in the circuit and then have a second breaker short out the resistors to gain full voltage.

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