Transformer Inrush - Limited by short circuit availability?
Transformer Inrush - Limited by short circuit availability?
(OP)
Hi all,
I'm aware that transformer inrush is a phenomenon caused by energizing the transformer at a non-zero crossing of the voltage waveform that causes the remanent flux to interact with the energizing flux such that you get large increases in magnetizing current, often leading to terminal currents around 12xFLA. Is this limited at all by the short circuit availability of the source energizing the transformer? I thought perhaps the saturation of the transformer core reduced the impedance of the transformer so much that it was primarily winding resistance that limited the current flow. I also know that there are all sorts of off-nominal harmonic currents such as 2nd and 5th involved that I have not been able to reconcile with this possible interpretation.
My specific scenario is I'm doing a protection study on a system that involves a 115/34.5 kV two-winding, delta/grounded-wye transformer protected by a numerical differential relay. A instantaneous overcurrent element has been set in the high-voltage winding protection to back up the differential for winding faults. Typical setting philosophy is to set it above 12xFLA (restrain in inrush) or 1.25% of the secondary bus fault current, whichever is higher. This ensure that 1) the relay does not operate on inrush and 2) only operates for transformer faults. This relay has been set according to that principle, but when running my short circuit studies it is apparent that the system cannot supply enough short circuit current for the element to ever operate when trying to set above 12xFLA.
My first thought was to reduce the pickup to operate for transformer faults (while avoiding secondary faults) and time delay it 0.1 to 0.2 seconds to ride through the inrush. However, if the inrush current is dependent on the short circuit availability of the system this is a moot point. I have been perusing the forum topics for the last hour and have not seen this explicitly addressed amongst the other transformer inrush topics, so here we go.
I'm aware that transformer inrush is a phenomenon caused by energizing the transformer at a non-zero crossing of the voltage waveform that causes the remanent flux to interact with the energizing flux such that you get large increases in magnetizing current, often leading to terminal currents around 12xFLA. Is this limited at all by the short circuit availability of the source energizing the transformer? I thought perhaps the saturation of the transformer core reduced the impedance of the transformer so much that it was primarily winding resistance that limited the current flow. I also know that there are all sorts of off-nominal harmonic currents such as 2nd and 5th involved that I have not been able to reconcile with this possible interpretation.
My specific scenario is I'm doing a protection study on a system that involves a 115/34.5 kV two-winding, delta/grounded-wye transformer protected by a numerical differential relay. A instantaneous overcurrent element has been set in the high-voltage winding protection to back up the differential for winding faults. Typical setting philosophy is to set it above 12xFLA (restrain in inrush) or 1.25% of the secondary bus fault current, whichever is higher. This ensure that 1) the relay does not operate on inrush and 2) only operates for transformer faults. This relay has been set according to that principle, but when running my short circuit studies it is apparent that the system cannot supply enough short circuit current for the element to ever operate when trying to set above 12xFLA.
My first thought was to reduce the pickup to operate for transformer faults (while avoiding secondary faults) and time delay it 0.1 to 0.2 seconds to ride through the inrush. However, if the inrush current is dependent on the short circuit availability of the system this is a moot point. I have been perusing the forum topics for the last hour and have not seen this explicitly addressed amongst the other transformer inrush topics, so here we go.






RE: Transformer Inrush - Limited by short circuit availability?
My feeling is that inrush would be limited by upstream short circuit capacity, but should you ever have a system where the upstream infrastructure can't provide 12xFLA to downstream equipment?
RE: Transformer Inrush - Limited by short circuit availability?
If you are using an "Available Short Circuit Current" value be aware that the first cycle peak current may be over twice the calculated (or measured) available short circuit value. We have some gurus here that can do the math, but I would not be surprised to see a peak inrush in excess of the calculated available short circuit current.
Note: when switch gear is tested and rated, the discrepancy between actual peak fault current and available short circuit current is taken into account. The initial peak fault current decays to the available short circuit current.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Inrush - Limited by short circuit availability?
For what it's worth, I generally set transformer instantaneous elements at 130% of the fault current on the secondary terminals, and have never had an issue. Admittedly, though, the short circuit strength of our system is relatively low, and as you noted, weaker systems will - generally - not to provide as high inrush.
RE: Transformer Inrush - Limited by short circuit availability?
This has been discussed a few times before. Weaker systems will result in lower transformer inrush. There was an old S&C paper that discussed this, but the link in the old thread seems to be broken.
RE: Transformer Inrush - Limited by short circuit availability?
RE: Transformer Inrush - Limited by short circuit availability?
waross, so with that in mind I'm looking at a 1st cycle peak current of 1200A, which is still less than the 1500A for 0.1 seconds that can be assumed for a strong system.
mgtrp, agreed on the points about harmonic filtering in relays. This particular digital relay's overcurrent elements only operate on the fundamental component. In the SEL-387 manual (different relay from what I'm dealing with, but similar operation) they even state that you can ignore the inrush current directly and just set based on load and fault conditions. That didn't sit right with me as all the older guys here have never ever done that with a modern relay.
Dpc, somehow I missed that post. If I had found that topic this thread would not needed to be posted.
stevenal, I was aware that inrush is a curve but I only typically look at the 12x at 0.1 point and ensure my overcurrent elements (be they instantaneous, definite time, or inverse time) do not operate at that point. As to your second point my intention is to also use a 51P to back up for system through faults that would fit between the inrush and damage curves and coordinate with downstream devices.
I appreciate everyone's contributions, it looks like my suspicion was correct!
RE: Transformer Inrush - Limited by short circuit availability?
The short circuit impedance has a definite affect on the peak inrush current. High inrush currents occur when the core saturates during the energization. Once the core saturates the current is the voltage divided by the impedance ohms of the transformer.The point at which the core saturates is a function of the residual magnetizim of the core,The crossing of the wave form voltage, and the rated flux density of the core
RE: Transformer Inrush - Limited by short circuit availability?