Sustained inrush of 400 amp at 34.5 kV on transformer 4

[KWman]

Do you have any case studies or equipment solutions that might be effective on correction of a severe resonance initiated by transformer inrush? The application is a commercial data center with two incoming 20 mVA transformers 34.5 kV / 12 kV, and distribution network consisting of 20 area subs 12 kV to 480 V sized 2500 to 4000 kVa.

Extended inrush as long as 25 to 40 seconds is observed as 400 amperes at the 34.5 kV service, when energizing one of the transformers, with or without minor load pickup. There is a very loud audible signature during the event... very extended inrush sound. It even occurs when energizing one of the area subs. There is voltage sag of 8% or so, and significant voltage harmonic distortion.

The 12 kV Wye secondary may be ungrounded (due to a screw up), which is one enable for ferroresonance. I'm also looking into the short circuit MVA of the utility and if there are any cap banks on their lines (none on the subject site.)

After the extended transient, system is normal and can operate critical load brought on line, such as chiller VFD and computer UPS loads.

Ever seen anything like this? Any papers or references you can direct me to? Any good literature search services you recommend?

 

[alehman]

TWENTY 2500-4000 KVA transformers!?! To answer one of your questions, I've never seen anything like that. This surely is the "mother of all data centers".

25 to 40 seconds doesn't seem totally unreasonable for transformers this size, particularly where there are two steps of transoformation. I'm guessing the utility system capacity is comparable to (or may be less than) your 20 to 40MVA. The utility source would be considered "soft" in comparison to the size of your system which will prolong energization inrush.

You could have a ferroresonance issue with ungrounded neutrals, although it seems unlikely. Is the sound when energizing a steady tone or does it vary in pitch? If it varies randomly, that typically indicates ferroresonance. Search this forum for several recent threads on that topic.

 

[busbar]

 
Offhand, full-load current is 335A for the 34.5kV primary.

One classical reference is §7 of ANSI/IEEE C57.105-1978. [TOC:

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

] Accordingly, a single-phase switching event {late or early pole operation?} is required with higher-than-normal zero-sequence capacitive reactance [typcially from shielded cable.]

The reference states, “Frequently the transformer is described as rattling, rumbling, or whining when ferroresonance is present,” and lists various three-phase transformer-winding and -grounding configurations most and least susceptible to ferroresonance..

 

[GOTWW]

Maybe the utility has a reactor in line to limit the inrush (or fault current)?
The classical 12XFLA for 0.1sec does not fit well for your case.

 

[GOTWW]

Was this a Hoax or what? It first appeared to be phenomena of unprecedented proportions, but, yet it died with only a cursory review.

 

[KWman]

This is not a hoax (I posted it). The numbers are real, including the size. I know of several data centers that are this big, electrically speaking. I am new to this forum, so if I am not following protocol, I would appreciate any coaching you can give. I have not answered the replys thus far, as none contained questions.

 

[KWman]

Reply to the sound question: I have not heard it personally, but it has been described as "sustained inrush..." I have seen the oscillo of the event, which shows an initial offset for a few cycles, then sustained high symetrical current for the duration. So I interpret that the sound would be constant, like an inrush but much longer.

 

[alehman]

Here's link with audio recordings of ferroresonance in some different types of transformers.

http://www.dstar.org/P_R_Summary_Ferr_2.htm

 

[KWman]

Thanks to all for the info so far. I checked out the audio download from DSTAR, which was helpful. I don't think we have classic ferroresonance in this case. It may be some kind of parallel resonance, and the possibility of utility side reactors or cap banks is the first to be investigated, along with the possibly ungrounded 12 kV system. Any more insights, no matter how many days or weeks out will be appreciated. Will post more data and observations as they become verified.

 

[RalphChristie]

KWman

From the J&P Transformer Book:

As in any network consisting of inductances and capactances, transformer windings are capable of oscillatory response to certain incident disturbances. When the disturbance has the appropriate properties severe dielectric stresses and, on occasions, failure can result.

Since in most cases the max. voltage developed in the transformer windings occurs during the first one or two oscillations, the natural frequency and damping of the oscillatory circuits are of only secondary impotance. In contrast, however, certain switching transients may consist of an initial peak voltage followed by an ascillatory component. If the frequency of this oscillation coincides with a natural frequency of the winding a resonance can develop which can take several cycles to reach its maximum amplitude.

Resonance always requires a passive structure, namely the transformer winding, and an active component repressented by the various sources of oscillating voltages in the electric system.


In 1979 CICRE set up a Working Group to deal with resonance problems and to report on the state of art, including the provision of a description of the response of transformers to oscillating voltages and making a survey of the possible sources of oscillating voltages in electrical systems. The Working Group's findings were presented in 1984:
WG 12.07 Part-winding resonance
"Resonance behaviour of high-voltage transformers"
CIGRE Report 12-14, 1984

The working Group considered that a very detailed analysis was necessary to get presicise info about the resonance of a particular trsf. Three different approaches are possible - calculation, measurement or a combination of the two.
They also reported the sources of oscillations in networks
The existence stems from one of tree possible sources:
* Lightning
* Faults
* Switching
The Working Group investigated 21 categories of incidents and their analysis revealed that in only three of these catagories was their a likelihood of oscillations which might coincide with a natural frequency of the trsf:
* Polyphase close-up faultson a single line
* Energisation of a short transformer-terminated line from a strong bus
* Repetitive re-ignitionsduring the de-energisation of a trsf loaded with a reactive load.


Regards

Ralph

 

[KWman]

Thanks RalphChristie for the reverences and summary. I will obtain the reference and review in detail over the next few weeks.

 

[DanDel]

I would imagine that the Utility contribution has a significantly high X/R ratio, which would help explain the extended 'inrush' current.
Even with 20 2500kVA transformers, initial inrush current (per IEEE 12x FLA) is over 10kA. This would almost appear as a fault to the Utility. A high X/R ratio would cause an extended DC offset, as described on the oscillography.
400A for 25 to 40 seconds certainly may be caused by a reactive system, but may also partially be voltage recovery from the Utility.

 

[KWman]

Thanks to DanDel for the reply. I would like to clarify that the 400 ampere x 20 to 40 seconds is for ONE transformer being energized... either a 33/12 kv utility interface (one of two) or a single area sub 12kv/480 (one of 20). In tests thus far, when a utility interface main transformer is energized by closing the 33 kV breaker, the cables to the area subs are energized, but not the area transformers, as their incoming breakers are open.

 

[alehman]

For ONE transformer in the 2500kVA range, that's definately not normal. Your original post was not clear about that. Voltage recovery from the utility side also would probably not be as much of a concern. I would try a voltage/current recorder get some waveforms and harmonic analysis. That may at least allow you to rule out ferroresonance.

 

[busbar]

 
Chapter 2 will be the ungrounded 12kV-system madness.