Overview
What is a Transformer?
A transformer is fundamentally two windings, the primary and the secondary, each wrapped around a magnetic core. The primary windings are connected to the power source, and the secondary windings carry the induced voltage to the load.
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-tyoe 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.
Harmonics (N * 120 Hz, N ∈ R)
The harmonics of magnetostriction contain information regarding the magnetic field of the core. Specifically, I believe that it contains information about the dynamicity of the loads, so while the harmonics for each transformer would differ during the day, we would expect to see the same harmonic patterns for all of the distribution transformers across the city at night.
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