Ferroresonance Occurrence In Power Transformer
Ferroresonance Occurrence In Power Transformer (photo by SNC Manufacturing Co. Inc.)

Worthy of mention //

Ferroresonance occurs when line capacitance resonates with the magnetizing reactance of a core while it goes in and out of saturation.

Ferroresonance is usually associated with potential transformers, which are instrument transformers that are used to develop voltages used by relays; however, it can also occur with power transformers under special circumstances.

Ferroresonance is another occurrence that can cause equipment damage; fortunately, it is preventable by simply avoiding certain types of transformer connections with the types of circumstances that enable it to occur.

Because these connections are routinely avoided in practice, ferroresonance is not encountered very often and there isn’t much information about it in the literature.

Ferroresonance is worthy of mention, however, because it can utterly destroy a transformer.

A situation where ferroresonance may occur
Figure 1 – A situation where ferroresonance may occur. The cable capacitances form three parallel L-C elements that are in series with each other and the source voltages.

The necessary conditions for ferroresonance are established in the system shown in Figure 1.

In the example shown in Figure 1, the ∆-connected tertiary winding of a large three-winding substation transformer supplies a distribution type station-service transformer with a Grd.-Y primary winding. The supply lines to the station-service transformers are through a set of shielded cables. If the cable runs are fairly long, a significant amount of phase-to-ground capacitance may exist.

When a transformer core operates near saturation, the B-H curve is highly nonlinear, and the effective permeability of the core can take on a range of values that vary with the changes in flux density.

Each of the inductances shown as L1, L2, and L3, will have instantaneous inductance values that are proportional to the effective permeability of the core at any given instant in time. These inductances form parallel L-C circuits that are in series with one another and in series with the source voltage.

Since L1, L2, and L3 are constantly varying along with the effective permeability of the core, it is almost certain that a series resonant condition will exist at least part of the time during every cycle.

When a series resonance exists, even for a brief period, this causes very large voltages across the L-C elements. These voltages are capable of destroying the transformer and any other equipment connected to it.

The nonlinear nature of this problem makes mathematical analysis virtually impossible, but the phenomenon has been observed both in the field and experimentally, and the voltages have been measured and recorded.

In the example above, the conditions for ferroresonance can be disrupted by the simple expedient of ∆-connected secondary winding to the station service transformer.

The ∆-connected winding assures that the vector sum of the voltages of all three phases add to zero, stabilizing the neutral point of the Y-connected primary winding and preventing excessive voltage across the windings. The presence of a ∆-connected secondary winding will essentially “snuff out” ferroresonance in this circuit.

Ferroresonance (VIDEO)

Dennis Merchant on ferro-resonance in distribution transformers.

Reference: Power Transformers Principles and Applications – John J. Winders, Jr.
(Get it from Amazon)

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


    Jun 16, 2016

    Ferroresonance is possible with alternators?

  2. Marc Belanger
    May 18, 2016

    We have come across situations that cause a much more likely scenario for ferro resonance than any other, as well as a method to counteract it in just about every case when working in the field to eliminate explosions. Well, it has been said to be called ferro resonance, those who explain this to us may be misnaming it, however, I know it has to do with a resonance of some kind because it causes extreme voltages, far more than enough to cause shielded loadbreak components to fail immediately from vast overvoltages before connections ever take place.

    What happens in the most common situation is that 3 phase power company padmount transformers and banks of unmatched transformers being switched in to energize them one phase at a time instead of using a 3 gang switch to close all 3 phases simultaneously when loads are not present. The systems are normally wye primary, delta secondary transformers of different kVA’s on the same circuit that will come back on without any load on them. When the first phase is thrown in, nothing usually happens. The second phase may act up a bit and show small signs of corona discharge while closing in, then, the 3rd phase will draw a massive arc and explode from a distance as getting closer to the 200 amp junction. The explosion is devastating, absolutely destroying the 200 amp loadbreak elbow even though no load is on the secondary, not allowing the delta secondary to “snuff” it out before the overvoltage and resonance happens. I think this is more likely due to saturation than resonance on it’s own, but by adding a simple load such as lighting circuits or a small resistive load to all 3 output phases of all transformers can eliminate the problem. The best form of elimination is to always close in at a 3 phase gang switch, however, this can’t always prove to be easy when there isn’t one available to switch from. This phenomenon has been known to kill many linemen over the years who choose to close in single phase solid blade disconnects instead of doing so at safer locations that offer protection, or 3 phase gang switches, and this normally happens because the circuit’s mapping is either not even looked at, transformer sizes and configurations that exist within the circuit are ignored or the phenomenon is not known by the worker so it isn’t important to the linemen or cable splicer to close in with any specific manner other than just energizing the circuit at the easiest place possible. It is due to lack of knowledge that these circuits can cause massive fireballs or devastating explosions alike to those circuits that are being opened while under full load without knowing about it.

    We normally open and close into circuits under a slight load to eliminate any issues, as said, to snuff the excitement of the core which changes the optimal frequency of the inductor, which may very well end up being a point of resonance while the high voltage shielded cable is a capacitor and causes the resonance phenomenon to happen by creating a tank circuit.

    Under a no load situation, voltages may climb up to 10 times instantaneously and cause the oil filled transformer to ignite internally, making a massive bomb out of the rapidly heating and nearly explosive gases created by the oil heating up. The arc flash internal to the transformer will ignite killing pretty much anybody in the vicinity of the transformer if the fuses allow a couple hundred to 500 amps. At this level, the high voltage circuit will feed up to nearly 7 to 10 megawatts in an instant. That’s a very large magnitude of power released in one flash creating an instant expansion of flaming oil, molten copper or aluminum, and shrapnel from the steel can and core making the outcome like a battle in war.

    I haven’t personally seen any close ups of this happening, but I have seen recreations of the accidents happen on video and at test labs to confirm the situation, and it isn’t pretty. If anybody has any questions that may involve your safety while working in this field or switching of circuits like this, feel free to drop me a message. I will be more than happy to assist you, however, I can’t be responsible for any actions you may take doing anything for your project or job.. That is your own responsibility. A lot of mistakes are made by the untrained or poorly trained. If Ferro Resonance isn’t brought up in your line training classes, something is wrong and you wouldn’t be a good candidate for the switching of these circuits. It may be a deadly job for you to take without the knowledge and know how of what to do . Electricity is dangerous. Commercial and industrial wiring and cabling can be lethal, however, high voltage distribution circuits are downright deadly. Work safe and use thy brain to defend your safety. Always wear the proper PPE and pay close attention to ensure you always work with another person that is trained in the same field as you are, as well as in electrical rescue, poletop rescue, confined space rescue, and a clear participation of OSHA 10 T&D is a plus if the instructor is knowledgeable about the field, the safety aspect of it, and finally all aspects of electrical transmission and distribution circuits, in the overhead and the underground, in vaults and manholes especially. There isn’t a way out unless you protect yourself the correct way.

  3. Dhiraj Tiwari
    Dec 22, 2015

    Thanks for the explanation but there is one thing i need to ask is that we place a resistor in front of relay to avoid any ferroresonance, can you please how can this help to cope up with this effect.

  4. […] considerations and protection methods against the ferroresonance phenomena, discussed with traditional voltage transformers, are not applicable with voltage […]

  5. jignesh
    Jun 12, 2014

    we face the above mentioned condition exactly currently. Also got the solution as you described over here ( Yep Exactly the same solution ) even though it run for 4 to 5 yrs and same failure happening again. Now what will be solution other than that Delta Connected Second winding to PT.
    Your input are highly valuable.

  6. […] Reducing the risk of ferroresonance […]

    May 12, 2013

    Thanks EEP for registering me as amember. I want to ask a question.
    In hollow type CT, it is mandatory to connect the primary with the shield over the secondary winding.
    What is the reason ?

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