How are HPS transformers designed to shield against voltage transients?

Frequently Asked Questions Transformer Design 

Electrostatically shielded transformers may help minimize or limit the effects of voltage transients. Common Mode noise is measured from line to ground and is usually the most troublesome. Transverse Mode noise is measured from line to line. Attenuation is the difference of an incoming transient on the primary of the transformer to the secondary side.

An electrostatic or Faraday shield is simply a thin piece of grounded non ferrous metal (generally copper foil) placed between the primary and secondary windings of a transformer. The shield extends from the top to the bottom of the windings. Some manufactures use shields that don’t extend the full length of the coil face. While less expensive, they will not offer as much protection as a full shield.

There is no national standard that gives test methods for measuring CMNA and TMNA. Hence, in the industry, various companies have different claims that they have succeeded in getting into customer or consultant specifications. A lot of the confusion for shielded transformers results from differing claims made by various manufacturers and experts. Some recent reports indicate that and electrostatic shield may have little to no benefit where the secondary is grounded which is in most applications. The difference in the claims results from many variables:

  1. Standard single shielded distribution and drive isolation transformers may theoretically provide typical values of CMNA =60 dB and TMNA = 10 dB.
  2. A single shielded transformer with a low capacitive coupling of less than 30 pF may theoretically provide typical values of CMNA = 100 dB and TMNA = 40 dB.

A manufacturer should be willing to share their testing procedures and test circuit to verify their
claims.

Note attenuation ratings vary by frequency. As the frequency increases, dB ratings go down. The ratings given above may be best case for a wide range of frequencies from 100Hz to 1MHz. Actual attenuation might be significantly higher at the lower frequencies. Some manufactures may claim higher dB’s attenuation by using much lower frequency ranges.

There may be a large difference between calculated dB and actual dB due to real-life inconsistencies in material and manufacturing. Manufacturers should have actual test data to back up their attenuation claims.