Overview
What is a Transformer?
A transformer is fundamentally two windings, the primary and the secondary, each wrapped around a magnetic core. The windings are simply wire that is wrapped in a coil, which allows for the storage of a larger magnetic field than uncoiled wire, wrapped around a steel core, which can support an even larger magnetic field. The windings and the core define the size of the magnetic field that the transformer can support without becoming saturated, which is when an increase in voltage does not produce an increase in magnetic field.
The primary windings are connected to the power source, and the secondary windings carry the induced voltage to the load.
If the primary windings are overloaded, the magnetic field will become saturated, and the induced voltage on the secondary windings will have a more-square waveform, which causes distortion.
Transformer Failures
According to a 2012 survey from the Cigré transformer reliability working group, 45% of transformer failures are due to their windings, and 1 out of every 200 transformers will fail each year. These failures always have warning signs, and Bellwether's goal is to identify those signs for transformer winding failure.
| Location | Percentage |
| Windings | 45% |
| Tap Changer | 26% |
| Bushings | 17% |
| Lead Exit | 7% |
These failures need to be analyzed mathematically to determine what the electromagnetic effect of a failure is and how that would manifest acoustically.
What Do We Expect to Hear?
Magnetostriction (120 Hz)
The primary acoustic manifestation of the electromagnetic field is magnetostriction in the core. The core of the transformer is an iron core that enhances the capacity of the magnetic field and provides a path for the energy to move from the primary magnetic field to the secondary magnetic field. Most distribution transformers use a shell-type core, which involves the core encircling the windings, contrary to the core-type core, where the windings are wrapped around the limbs of the core-type core (which looks like a windowpane).
Magnetostriction is when the individual grains of the core expand and contract with the magnitude of the magnetic field. When the field is maximum positive, the grains are fully aligned and expand to their maximum size. When the field is maximum negative, the grains are fully aligned in the opposite way and are still at their maximum size. Thus, for a time-varying magnetic field of 60 Hz, the grains will reach their maximum expansion twice per cycle, resulting in a 120 Hz tone.
Magnetostriction forces are proportional to the square of the voltage.
Distortion Harmonics (N * 120 Hz)
Distortion is a result of how square a sine wave is and is manifested in the harmonics of the underlying frequency: if the core is saturated, the underlying 120 Hz magnetostriction tonal will become square at points (experiencing what's known as "soft clipping" in the audio engineering world), which means there will be stronger harmonics.
Causes of Transformer Noise
Taken from "What Causes Transformer Noise?":
- Magnetostriction
- Acoustic resonance
- Harmonics distortion
- Core delamination
- Cooling fans
- Load conditions
- Installation conditions
- Weight of transformer
Where Do We Expect to Hear Things?
This project is focused on substation transformers that handle voltages of generally 230-500 kV and a power rating of 5-20 MVA. These transformers are very powerful, very dangerous, and very well guarded — access isn't exactly readily available. Therefore, we are proving ourselves on the pad-mounted distribution transformers that are easily accessible and found on every other street corner in DC. While the distribution transformers are built differently (air-insulated instead of oil-insulated), the underlying physical phenomena are the same, which affords us an easy opportunity to prove our technology on something adjacent to our ultimate goal.
In the DC metro area, load peaks between 5-6pm, and is at its nadir between 3-4am. We therefore will be taking transformer and ambient recordings at both times of the day for 100 distribution transformers in DC.