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Ferroresonance and Its Effects On Transformers

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Mike, Thank you very much for the valuable information you send my colleagues and
me are getting from your site. I appreciate the effort you share in the electrical engineering industry. We are also engaged in the design of electrical systems. After completion of any projects, it is a part of our contract to give training and instructions to the operation and maintenance personnel. One project we have is to construct a 4000kVA indoor substation. It is composed of two-2000kVA pad mount transformers.

The main disconnecting means of the substation is a medium voltage switchgear comprising of load break switches and power fuses (one set for the main and two sets for the branches - one of them for the future transformer). Lightning arresters are also provided. Originally, we proposed VCB's but due to budget constraints, the owner agreed to use LBS.

We also supplied and installed a standby generator system (480/277V). The changeover from utility power to generator power is via manual transfer switch. Relays are provided to select and control the loading of the generator system.

The primary (utility) voltage is 34.5kV. It is stepped-down to 480/277V. The transformer connection is wye primary and wye secondary. Both the primary and secondary neutrals are solidly grounded.

The substation was constructed almost two years ago. However, after a utility power interruption, an unexpected thing happened. The operation and maintenance personnel reported that after the resumption of the utility power and as they were about to transfer the power back to the utility (from generator power), they noticed smoke coming from the transformer. This stopped them from performing the changeover. Carbon streaks are noticeable near the two of the transformer bushings.

Immediately, the personnel reported this event to the transformer supplier. A temporary transformer was rented for facilitation.

Upon inspection of the transformer supplier, they reported that the cause of the failure is ferroresonance. We were invited to inspect the transformer as they removed the winding and core. The windings were intact (no visual signs of combustion or insulation damage). I inspected the core and legs and found that there are blackened and burned spots caused by overheating. They also noted that the cause of the smoke noticed by the personnel at that time was the tank's paint.

It has come to my concern that (from some books) ferroresonance is a phenomenon. How can we, applying the codes and standards, protect our system from this occurrence? Of my nine years in the field, this is the only time that a failure was blamed to ferroresonance. Honestly, I have not encountered any provisions dealing with this matter (or I have overlooked it).

Could you please give me some insights, references and/or publications regarding ferroresonance and how to prevent or protect equipment from it? I firmly believe that this is a fact to be considered in the design of electrical systems.
Your reply shall be highly appreciated.
Genaro "Gene" Ferrer,

Response No. 1

Dear Mike and Gene, An article about ferroresonance is available at from Merlin Gerin/Groupe Schneider a European manufacturer of switchgear etc. (They are also the parent of Square D equipment). The article goes into what causes ferroresonance and its effects, and gives some tips on how to avoid it, mostly in the design phase of a project.
Regards, Bas van Duijnhoven,

Response No. 2

I ran into this a few years ago with a wye/wye overhead bank. Primary was at 34.5 kV secondary at 277/480. The primary distribution system was delta connected and the neutral point for the transformers was floating. This was done in an attempt by the owner to utilize a lower cost transformer. The solution was to replace the transformers with primary winding rated for 34.5kV so that a delta wye connection could be made. End of problems.

Ferroresonance can be a more of a problem where a transformer bank is at the end of the line, or at the end of a long tap.

Roger Strand,

Response No. 3

The IEEE has some information at

Response No. 4

Mike, I am a former employee of a utility company where I worked as a Transmission
and Distribution Engineer for several years.

Our distribution system consisted of 2.4kV, 4.16kV, 13.2kV, and 34.5kV voltage levels. Normally, ferroresonance was only a concern at the 34.5kV level, although on rare occasions, it happens at lower voltage levels. At the 34.5kV voltage level, there was greater likelihood that the system capacitive reactance could match the inductive reactance of transformers under certain conditions. This is what causes ferroresonance.

Ferroresonance occurs typically during single-phase switching of a distribution system that has ungrounded primary transformers connected, such as delta or floating-wye (neutral not grounded). It can also occur when single-phase protective devices, such as fuses blow leaving one or more phases still energized. A series R-L-C circuit is formed when this occurs which will resonate if the L and C values are close to the same. It is the nonlinearity of transformer core iron that contributes to the probability that this will occur, hence the term ferroresonance. The probability is also increased the farther the single-phase switching device is located from the transformer.

At our company, we utilized only three-phase gang operated switches and reclosers on 34.5kV distribution systems. We also tried to ensure that only transformers with grounded-wye primaries were installed on the system, although we did have customers with delta-connected primaries. We did use single-phase fuses at the transformer itself, but these were located close enough to the transformer to reduce the capacitance problem.

To recap, the following issues combine to increase the probability of ferroresonance:

1. Higher-voltage distribution systems (such as 34.5kV)
2. Ungrounded primary transformers
3. Single-phase switching or protective device operation remote from the transformer
4. Lightly-loaded transformers
5. Underground distribution (higher capacitive reactance)

You can go to and look under the electrical engineering category in Technical Publications.  A book called Electrical Transmission and Distribution Reference Book by ABB Power Systems/Westinghouse - 1997 covers this topic.

Bryan Thompson,