Calculating Transformer Inrush current 2

[cherry2000]

We have a 150kVA Transformer for which we have got the inrush current from the manufacturer. Now we also have 200-400m of cable (cable length differs for each transformer), which should have an effect on the inrush current. Is it possible to calculate the revised inrush current values, if the cable parameters are known?

 

[davidbeach]

Minor impact on inrush current (second or third order). For a given transformer with its given inrush characteristics the two things that change inrush the most are per unit voltage at time of energization and load on the other side of the transformer. The more load the faster inrush subsides. the lower the voltage the less inrush; bring a transformer and generator up together and there is no inrush.

 

[waross]

Hi David. I'm probably going to learn something here! Several transformers at 150 KVA; I'm thinking dry type transformers with cable feeders in a plant. If the feeders are sized for 2% or 3% voltage drop, is it possible that under inrush conditions the second order effect of 1500% or 2000% inrush current may be significant. What am I missing.
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

What I meant is that the addition/subtraction of 200-400m of cable wouldn't have a noticeable affect. Voltage at the instant of energization is the same at both ends of the cable because no current had been flowing. Once the inrush begins there will be some voltage reduction, but undoubtedly not enough to get out of the saturation that causes the inrush currents.

If the cable is on the secondary of the transformers, it will have no affect at all if open circuited. If there is load on the transformer when it is energized, the additional cable will add very slightly to the load and therefore add a extremely slight amount of damping.

The random point on wave closing will have more affect than cable and completely mask any change in inrush characteristics due to the cable.

 

[waross]

Thank you David.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

S&C had an article on the effect of available fault current on inrush current. See

http://www.sandc.com/webzine/2003/012003_1.asp

A long primary cable will increase the source impedance and reduce the available fault current. Inrush will be lower.

 

[cherry2000]

Thanks for all the info. The reason I was looking for the reduced inrush is to set the Breaker instataneous to a lower level. Otherwise, arc flash value exceeds our expectations.

 

[jghrist]

Normally, the instantaneous pickup is limited by the maximum thru fault current to allow coordination of secondary protection. Inrush will be lower than a thru fault. If a maintenance setting is used for arc flash purposes, inrush need not be considered because you will not be energizing the transformer (going from de-energized to energized) while maintenance is being performed.

 

[stevenal]

Incident energy is determined from fault current level, clearing time, and distance. Dropping the pickup on an instantaneous breaker has no effect on any of the above parameters, assuming the fault current exceeds pickup to begin with.

 

[Masbibe]

I only need to correct one thing from davidbeach post (second from top):
"two things that change inrush the most are per unit voltage at time of energization ... the lower the voltage the less inrush;"
Actually it is reverse lower voltage -> bigger inrush. Worst case is switching on zero voltage.
I now this is relativly old post but I am new on forum and now I was read it.

Milovan Milosevic

 

[unclebob]

Have a paper here on Sympathetic Inrush. I'm not a big math fan, there are plenty of equations in this...

Basically, the model used in this paper investigate the inrush phenomenon and some parameters that affect the duration and the amplitude.

 

[cherry2000]

Thanks Miloslav & Bob for your renewed interest on this topic. We ended up adding a three phase current measuring module which can feed the correct 3 phase current value to the digital relay. The locked rotor protection in the relay has been activated to introduce a fast trip in the event of a genuine fault. The breaker has been set at the highest setting since we do not want nuisince tripping while engergizing the transformer. This change cost us a pretty lot. Possibly, if we could have conducted a full blown mathematical study, we could have avoided this change in some of the feeders, which have long cable lengths, which will dampen the inrush. As seen in the paper from Bob, various parameters like system impedence, Transformer residual flux, time of switching etc affects the inrush. It is an interesting topic and good for University thesis papers.