Sustained inrush of 400 amp at 34.5 kV on transformer
Sustained inrush of 400 amp at 34.5 kV on transformer
(OP)
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?
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?






RE: Sustained inrush of 400 amp at 34.5 kV on transformer
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.
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
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/...] 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..
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
The classical 12XFLA for 0.1sec does not fit well for your case.
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
Was this a Hoax or what? It first appeared to be phenomena of unprecedented proportions, but, yet it died with only a cursory review.
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
http://www.dstar.org/P_R_Summary_Ferr_2.htm
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
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
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
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.
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
RE: Sustained inrush of 400 amp at 34.5 kV on transformer
Chapter 2 will be the ungrounded 12kV-system madness.