The transfer mechanism of the RF emitted from the “defect” to the EMI sensor location is a very complex mechanism to explain and to model. Very little research has been done in that area, and consequently, very little information has been documented about it. There is some research associated with the sensitivity and effect both defect location and HFCT location have on the signature associated with transformers. Most of the data gathered for the generator rules of thumb have been empirical in nature and reflective of information gathered in the field over many years of data gathering.
However, here are the basics:
- Any time there is a flow of electrons, an electromagnetic process is developed (Maxwell’s Equations).
- At the point of generation, the process can be transmitted through the air in the form of radio frequency emissions or through conductors and emitted via an antenna effect.
- At the point of reception (HFCT), the current transformer detects the current flow through a conductor attached to something that can have an antenna effect or a shielding effect. For example, generator windings have an antenna effect while the generator frame has a shielding effect.
Three basic locations of HFCTs have been used over the years for both EMI signature analysis and PD capture:
- Neutral ground (NG) conductor
- RTD leads and slot coupler antennas
- Frame or casing grounds