Equipment over-specification - you may not need what you think you need.
A client was placing an order for a harmonic mitigating transformer to feed rectifiers in a Telecom application and asked for help specifying the type.
After discussions regarding the particular rectifier, it became apparent that its harmonic signature was unknown. Also, a harmonic mitigating transformer had been specified, thinking it would cover for any harmonics.
This is an unfortunate over-specification response which often happens when specifications are reused. Many engineers are unaware that “harmonic mitigating” is, in actuality, a broad term for a host of transformer designs used with varying performance and application methods.
It was clear, then, that further analysis of the load and the application was essential in order to properly specify the client’s transformer. At this point, by our request, the client supplied a sample rectifier that was evaluated for harmonic content under its expected modes of operation.
Surprisingly, results of the measurements showed that a harmonic mitigating transformer was not even needed in this application. The rectifier utilizes source inverter electronics and produces relatively low levels of problematic harmonics as shown here:
Figure 1: Rectifier current waveform (top) and resulting spectral content (bottom)
From the spectral analysis and additional calculations, it was determined that a K-Factor of 2 would be needed to allow full-load operation in the presence of the rectifier’s low-level harmonics.
The analysis also showed that high-frequency noise from the rectifier (as shown below) was present at all times. Thus, in addition to the K-Factor rating, shielded cables were specified along with an Environmental Potentials EP2000 to absorb the noise.
Finally, a lower- cost standard transformer (much less expensive than a harmonic mitigating unit) was specified as noted and ordered. This analysis not only helped supply the correct equipment and save money, but increased client confidence by knowing that the installation would fully meet the needs of the application.
Figure 2: Rectifier current waveform showing high-frequency noise when sampled at 125kHz