Ferroresonance

[Jackson1]

I worked for the Southern Company at one of the southeastern utilities (now retired). I was in charge of their Transformer test department. I believe that we were getting some sort of ferroresonant condition at times...while we were doing an excitation test. I worked in this department for over 20 years and can recall three transformers that seemed to go into resonance. Most every time this happened, some of our equiptment was severly damaged. Let me give you a little background. The first piece of equiptment was a 560 amp 3 phase circuit breaker, then a 0 - 13 KV step voltage regulator (9000 KVA), then a 11 /44 KV step up transformer (5000 KVA), then to a HV voltage and current metering section and finally to the transformer under test. I had full control of each one these in the control room. Whenever one of these transformers went into resonance it was an all of a sudden thing without any warning. We even had a voltage transient study run. The study showed the possibility of voltages above the the insulation level of the sensors. The HV sensors seemed to be the weak point in the chain of equiptment. We were in the process of putting some overvoltage protection in the chain between the step-up transformer and and the metering section. My question is, does anyone know of any other protection that could be added or any way to predict this happening. Thanks for your replys, Jack B.

P.S. - The step voltage regulator could be run from 0 volts up to approximately 6200 volts. The system was de-energized and the regulator was manually changed to 6000 volts to 13000 volts. All of these resonant conditions occured when engizing the regulator at 6000 volts.

 

[jbartos]

Suggestion: The ferroresonance often occurs when the transformer and associated transmission line downstream are open at the downstream end of the transmission line. The Potential Transformers (PTs) upstream are experiencing effects of ferroresonance. Resistors are usually added on the PT secondaries to limit ferroresonance voltages on the PT secondaries. However, the ferroresonance effects can extend beyond the PT secondary and added resistors, depending on the amount of energy entering the ferroresonance phenomena.
Books dealing with protection and protective devices are often addressing the ferroresonance, e.g. Blackburn, IEEE Std 242 Buff Book, etc.

 

[busbar]

You might start with the §9.2 bibliographic citations for IEEE C57.105-1978 §7.

 

[jghrist]

It seems likely that your problems are with the VT's. There has to be some capacitance in the circuit for ferroresonance to occur; perhaps eliminating capacitance is a solution. See thread238-26675 for a good discussion of the issue.

 

[Bung]

Schneider have a very good "cahier technique" (in English) on ferroresonance. See their website.



Bung
Life is non-linear...

 

[Jackson1]

Hello,
I really appreciate your replys and references. Let me explain this a little deeper. These sensors (3) that I was referring to are special designed for a Multi-amp transformer loss test system. The sensors themselves are approximately 10" in diameter and approximately 4 ft.long. They contain a capacitance-divider voltage sensor and a wound multirange current transformer. They are insulated with SF6 gas at a pressure of approximately 15 PSI (the best I can remember). The voltage from the capacitance divider is fed to a multirange network. The machine also calculates watts from the volts, amps and pf of the system. What most of these articules deal with are lightly loaded PT's. What I was trying to prove (before my retirement), would it be possible to get some "series ferroresonant" condition, between the varible exciting inductance of the transformer under test and the inherent capacitance of the windings plus the capacitance of the sensors plus the capacitance of the step-up transformer (Delta connected). This thing has bugged me for years. Another thing, whenever these transformers were in a resonant condition, it sounded like a bunch of people inside the transformer banging away with hammers. I have already read the articule on the "cahier technique", which by the way is an excellant paper.

 

[jbartos]

Suggestion: Reference:
1. J. Lewis Blackburn "Protective Relaying Principles and Applications," 2nd Edition, Marcel Dekker, Inc., 1998,
indicates in Section 7.4.1 on page 197: "Ballast resistors are used to reduce the shift of the neutral from either unbalanced excitation paths of the voltage transformers or from ferroresonance between the inductive reactance of the voltage transformers and relays and the capacitive system."
What is needed to be added that the ferroresonance can occur in nonlinear systems with hysteresis only. The linear systems including inductance and capacitance experience the regular textbook type linear system LC resonance.