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We measure high voltage for you

using instrument transformers for both outdoor and indoor applications, special transformers, sensors and power transformers

Ferroresonance is still relevant

KPB Intra s.r.o.
Ing. Josef Stejskal

The phenomenon called “ferroresonance” has already been described in many publications. For the uninitiated, it may be briefly stated that this involves a phenomenon found in electrical networks, where resonance occurs between an induction transformer and a capacity main. We shall leave aside a more detailed explanation for now (for those interested, the necessary information may be obtained on the Internet). It must be said, however, that resonances are harmful to high voltage equipment and often lead to permanent damage. As manufacturers of metering transformers, we therefore feel obligated to draw attention to the relevance of the given problem.

Experience from recent years has been referenced in frequent responses to customers at which interruption of a current metering transformer has occurred in connection with the replacement of older electromechanical electrometers with new electronic ones. In principle, such an exchange should not cause any difficulties, but when we think more deeply about this, unwittingly, greater susceptibility to ferroresonance may occur. The essence is that the new electronic electrometers have an internal load substantially lower than the original electromechanical system. This leads to so-called “stress reduction” of the transformer and subsequently to a decrease in the dampening effect of possible resonances.

To this actuality, often this leads to a frequent psychological error by designers. It has long been the experience of the human subconscious where professional dimensioning is unknown. In the field of current metering transformers (CMT), this occasionally leads to a requirement for higher outputs in relation to the degree of precision without any justification. Here, however, it is necessary to note structural links. Dimensioning of a transformer for higher output may be none other than increasing the dimensions, both the primary as well as the secondary conductors and by decreasing both active resistances. The result is evident in the decrease in the dampening effect of ferroresonance.

Another reason for the susceptibility to ferroresonance is the lack of communication from the side of the customer, i.e. designer, users… The origin of this situation is again in human psychology. Because, for example, if this leads to damage to the CMT even after several replacements, according to business logic, the transformer is always to blame. According to lay reasoning – how could it be otherwise, particularly if the equipment is still within the warranty period? According to experience with business rules, most dealers therefore select the method in which “it is necessary to submit a claim without describing the relationships of the equipment and with the conviction that if the manufacturer refuses the claim we will turn elsewhere”. But the question is put forth as to whether such a procedure actually resolves the problem. The opinion of our manufacturers of this equipment is that this isn’t entirely certain. Many customers, after numerous interruptions of the CMT under specific circumstances, admit that this or that installation point is actually problematic, that there were already problems with overvoltage and the breakdown occurred in other components (absence of safeguards). I write about this primarily to emphasize that technical problems must always be resolved by only technical means. For a successful resolution, a manufacturer needs to be informed about all the circumstances, on principle in advance.

If we notice that problems have already occurred somewhere, which are marked by the occurrence of ferroresonance, we recommend several solutions:

1. One is the universally known connection of dampening resistance in the perimeter of an open triangle helped by coiling, see details at www.kpbintra.cz.
2. In other cases (particularly at railways), we propose transformers with a lower saturation of the magnetic perimeter. Such instruments are larger and more expensive, but their resistance to resonance compensates for the costs related to removal of defects from previous designs.
3. In other cases, particularly in open pit mines, reinforcing the insulation of the primary coiling is beneficial after adapting the other parameters of the machine.

From the aforementioned, it is apparent that the problem of ferroresonance is still relevant in electrical networks.

Experience shows that designed technical solutions have demonstrated their reliability over the years. The solution, however, has always been reached after mutual communication with the manufacturer.