Change in Transformer Inrush
Change in Transformer Inrush
(OP)
We are having issues with one of our step-up transformers tripping on inrush almost every time we energise it.
The transformer is 3/5 MVA and is a star-connected autotransformer with delta tertiary. It is a step-up between a four generators and the grid, and has been in-service and trouble-free for almost 10 years.
We are able to energise it from the grid without issue, but when we try to energise from the generators, it almost always trips immediately due to "switch-on-to-fault" instantaneous overcurrent. I am confident that the relay setting is not a problem because it has not changed, and we are also seeing the high currents on the generator protection relays (pick up of overcurrent elements on all four units, and one has seen sufficiently high current to trip).
I presume that an increase in transformer inrush would be caused by the core/windings shifting, or alternately by the transformer becoming magnetised. We will look to have magnetising current measured at some stage, although I'm not sure that we'd have past test results to compare against on this unit.
Any other suggestions of possible causes, and methods to diagnose/repair?
Thanks!
The transformer is 3/5 MVA and is a star-connected autotransformer with delta tertiary. It is a step-up between a four generators and the grid, and has been in-service and trouble-free for almost 10 years.
We are able to energise it from the grid without issue, but when we try to energise from the generators, it almost always trips immediately due to "switch-on-to-fault" instantaneous overcurrent. I am confident that the relay setting is not a problem because it has not changed, and we are also seeing the high currents on the generator protection relays (pick up of overcurrent elements on all four units, and one has seen sufficiently high current to trip).
I presume that an increase in transformer inrush would be caused by the core/windings shifting, or alternately by the transformer becoming magnetised. We will look to have magnetising current measured at some stage, although I'm not sure that we'd have past test results to compare against on this unit.
Any other suggestions of possible causes, and methods to diagnose/repair?
Thanks!






RE: Change in Transformer Inrush
RE: Change in Transformer Inrush
David Castor
www.cvoes.com
RE: Change in Transformer Inrush
RE: Change in Transformer Inrush
I'm sure that if we were to slowly energise the transformer, we would be fine. But that is not a long-term solution, especially as there are other loads off the generator bus that normally should be in service. We are able to back-energise from the grid, and then synchronise across the transformer circuit breaker, but that doesn't help us when we have a grid outage, which is generally when we would be using the local generation. The town in question is supplied by a single transmission line which is affected relatively often by lightning.
RE: Change in Transformer Inrush
RE: Change in Transformer Inrush
The energization of the first generator in the transformer may cause an inrush current so high that could damage the generator.
The solution adopted was the same proposed by Smallgreek. In our case the slope of the automatic voltage regulator, has changed during starting.
RE: Change in Transformer Inrush
I guess that we'd measure "as-found" excitation current, try ramping up slowly the voltage from ~0 to 4.16 kV +10%, then slowly ramp back down to zero. After demagnetising, measure the excitation current again and hope that it's decreased.
Does this sound reasonable?
Providing a soft-start on a permanent basis is not feasible, given the other loads connected to the same bus.
RE: Change in Transformer Inrush
Neil
RE: Change in Transformer Inrush
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Change in Transformer Inrush
Is this associated with all phases or one phase? Same phase each time? Does more than one relay trip? If single relay, could be relay malfunction or problem with low-side CT's (including residual magnetism, shorted turns).
Looking at the transformer itself, transformer single phase excitation test should shed some light if there is a residually magnetized core. Results are often compared to previous results, but you can also do a quick check that you have two high similar readings and one lower reading.
There are a wide variety of tests geared toward detecting transformer movement (leakage reactance, FRA), but to my understanding they rely heavily on detecting changes, so not very useful without a baseline.
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(2B)+(2B)' ?
RE: Change in Transformer Inrush
I understood that residual magnetism was more or less limited by the hysteresis curve and was much much less than the normal working flux. Further to this, the flux density during normal operation of the transformer is more than enough to overcome and remove any residual magnetism. I understood that any residual magnetism was a result of the de-energization and did not persist from one operation to the next.
I understood that energization surges were dependent on not only the amount of residual magnetism but also on the point on the wave that the switch closes. The result being that even with a high residual magnetism, 50% of the time this would tend to reduce the inrush rather than increase it.
Have I been wrong? It has happened before.
My understanding notwithstanding, I seem to remember some discussions regarding a circuit that would de-magnetize a transformer so as to avoid residual flux from pushing a transformer into saturation on energization. Does anyone remember those threads?
Bill
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"Why not the best?"
Jimmy Carter
RE: Change in Transformer Inrush
A loose CT connection can create a high resistance causing saturation. I've seen some poor crimp connections come back to cause problems after a few years of oxidation and thermal cycling.
I would be more suspicious of the CTs, CT secondary wiring, and relay.
Is it always the same phases that are high?
RE: Change in Transformer Inrush
1. Generator frequency is "probably" stable during inrush. Certainly, we are not picking up our underfrequency protections, set at 58 Hz. Furthermore, I doubt whether the frequency could drop much in 4-5 cycles, and pre-energisation frequency is generally fairly close to 60 Hz.
2. While we do have a relatively weak system, the generator total size is approx 4.3 MW @ 0.8 pf, in comparison to 3/5 MVA transformer size. Furthermore, this site has been in service for almost 10 years under the same configuration without issue until recently.
3. I don't have waveform captures from many energisations (none others available to share at this present time, although I have looked at them and they are similar), but from the relay event log file and other wave forms that I have seen, the phase with highest current is not consistent.
4. We are taking an outage on the unit later this week or early next week, and will be measuring excitation current @ 20% rated kV, and hopefully short-circuit current @ 600V. I also have the factory test report and some field test data from the middle of last year (excitation current only).
5. My understanding of residual magnetism in power transformers is not as good as I'd like! Certainly in instrument transformers (e.g., CT's), magnetising a core is a possibility. I had previously located a thread about demagnetising a transformer: h
6. I'm fairly confident that it's not a CT or relaying issue because A/ the settings in question have remained consistent for at least 3 years (I have past test reports etc), and B/ the upstream generator protection is also picking up the high currents and, in one case, operated. The generator protection oscillography also shows high currents and saturation (I can upload this oscillograph if it's useful, although it is in Beckwith *.osc format rather than COMTRADE). It is not always the same phase.
A further development:
Site staff have reported today that the transformer is now "growling" - making a lot more noise than previously.
As mentioned above, we will be measuring excitation current and short circuit impedance (both at reduced voltage), are there any other checks that might be useful?
RE: Change in Transformer Inrush
You mentioned that things have been going okay for years. Has anything changed that you know of that could give rise to these trips?
Has the transformer recently been tested? Meggar, gas analysis, etc.? I'd be wary of a "growling" transformer!
RE: Change in Transformer Inrush
RE: Change in Transformer Inrush
dpc
RE: Change in Transformer Inrush
RE: Change in Transformer Inrush
Also when transformer is denergized,there is a standard battery of electrical tests performed with a Doble tester or similar device. This includes transformer turns ratio, CH, CL, CHL, power factors, single phase excitation currents, etc.
Less standard denergized tests to look at (as mentioned above) include leakage reactance and FRA, depending on what test capabilities are available.
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(2B)+(2B)' ?
RE: Change in Transformer Inrush
Transformer saturation under normal voltage/frequency conditions could be the result of the core's magnetic path opening up. Perhaps this unit needs to be untanked and hardware tightened up.
RE: Change in Transformer Inrush
The transformer is no longer tripping when energised. A technician travelled out to site, and checked the transformer. The transformer was tested and came back with virtually identical results to when it was commissioned, so no concerns there. He also reported that the transformer noise was nothing out of the ordinary.
So, with the transformer working correctly and no further testing, we are calling this case closed, albeit with reservations.
RE: Change in Transformer Inrush
Does it no longer trip when you energize from generator?
Have you formed an opinion as to the cause?
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(2B)+(2B)' ?
RE: Change in Transformer Inrush
All I can think of is that we had some remenance, and that now it is gone.
RE: Change in Transformer Inrush
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Change in Transformer Inrush