Understanding and Preventing Ferroresonance

Ferroresonance is a term and condition not often heard about in electric utility work. Nonetheless, it’s important to know about ferroresonance because it is an immensely hazardous condition that can result in arc flash injuries and damaged equipment.

The Electrical Engineering Portal offers this scientific definition of the word at https://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/ferroresonance: “Ferroresonance is a non-linear resonance phenomenon that can affect power networks. The abnormal rates of harmonics and transient or steady state overvoltages and overcurrents that it causes are often dangerous for electrical equipment. Some unexplained breakdowns can be ascribed to this rare, non-linear phenomenon.”

In the remainder of this installment of “Voice of Experience,” we will explore the conditions that often lead to ferroresonance as well as prevention techniques.

Personal Experience
Many of you know I worked for a large utility for 40 years. When I began my employment, the system was either 4 kV or soon to be converted to 12 kV. Ferroresonance was not really an issue. As I progressed from apprentice to journeyman, working overhead and underground distribution, I never experienced ferroresonance working on the primary voltage systems that were 12 kV or less. However, other parts of the utility were upgrading the system from 12 kV to 25 kV to manage load, and they couldn’t build substations quickly enough to manage the increased system load. The higher primary voltage and the method of switching to re-energize equipment (i.e., transformers) are factors that contribute to ferroresonance.

Large three-phase, constant-voltage underground distribution transformers are subject to ferroresonance conditions. The steady voltage becomes transient through circulating current in partially energized three-phase coils. Also, because of long lines in between the points of energizing the phases, overhead transformers that are energized with intervals of time from first to last transformer can go from the steady constant system voltage to a high voltage, potentially causing severe damage to workers, tools and equipment.

Here’s a simple example. When switching a three-phase transformer back into service, the coils in the transformer are energized at primary voltage with a delay between the A phase and the C phase. A circulation current develops in the coils, resulting in a much higher nominal system voltage on the last phase to be connected. If that is a UD cable/elbow parked on a feed-through or parking bushing rated at the nominal system voltage, when the elbow is removed to plug back in on the transformer busing, the voltage at the probe can exceed the rating on the elbow and arc to the outer jacket, burning up the elbow and the stick. If an employee is switching with a 6-foot hot stick, this could result in arc flash injuries.

A typical three-phase loop may have a single-phase transformer, and the employee must go to one or more locations to energize all three phases. The time lag in between energizing all three phases is the most significant contributing factor. That lag allows time for the two phases of the three-phase transformer to develop a transient over-voltage through the coil of the transformer. By the time the third-phase elbow is removed from the parking stand, the voltage has exceeded the rating of the equipment, and an arc occurs. This was an example of my first ferroresonance incident. Again, I had never experienced this type of incident on a 12-kV system. It can happen, but I had not seen anything like it until I was on a 25-kV system.

Preventing Ferroresonance
To prevent this from happening, the bayonets must be removed from the three-phase transformer and then inserted once the primary cables have been energized. Another method is to park all three elbows in the transformer, switch the cables and energy, and then go back to the transformer and quickly energize all three phases before the transient voltage rises to a dangerous level. A breaker between the high and low sides of the three-phase transformer will not isolate the coil, so caution must be exercised when switching the cables. The bottom line is that the coil of the three-phase transformer cannot be delayed when energizing.

The same results can occur on overhead three-phase lines feeding large banks of transformers if there is a delay in energizing the phases on a three-phase wye bank. If jumpers are being energized at a remote location of the overhead three-phase single transformers, the fused cutout should be opened on the bank to prevent the circulating current.

A three-phase gang switch on overhead was one of the best actions developed to prevent ferroresonance on overhead systems. A three-phase gang-operated switching cubicle helps to eliminate the ferroresonance on UD systems. There is no delay in energizing each of the three primaries.

Single-phase switching should be the first red flag to recognize when switching three-phase UD transformers. The first time I experienced ferroresonance was when switching at Hartsfield-Jackson Atlanta International Airport years ago. I drove up when a lineman was about to pull the last of the elbows off a parking stand. He removed the elbow with an 8-foot hot stick, and when the elbow cleared the parking stand, there was an instant fireball at the end of the hot stick on the elbow. The lineman immediately plugged the elbow back onto the parking stand, and the fire went out. The system voltage had circulated in the coil of the three-phase transformer. The transient voltage exceeded the rating of the 25-kV elbow. An arc occurred to the jacket and neutral on the cable termination, and the fault current increased. When the elbow was reinstalled, the voltage was back in control.

I am also familiar with another incident on a 46-kV radial UD pad-mount transformer. With no bayonets in the transformer, the riser pole switches had to be closed one at a time to energize the transformer. After closing two switches, the third switch blew a fuse each time for the first two attempts. Once everything was verified normal, three long sticks were used to successfully close all three switches simultaneously, and everything held.

Conclusion
There are other situations not mentioned in this article that may result in ferroresonance. If you have questions, seek more information from your company’s engineering staff or contact Incident Prevention magazine at 815-459-1796.

About the Author: Danny Raines, CUSP, is an author, an OSHA-authorized trainer, and a transmission and distribution safety consultant who retired from Georgia Power after 40 years of service and now operates Raines Utility Safety Solutions LLC.