Measuring Transformer Distributed Capacitance

By Kirby Creel, Engineering Manager, Datatronics

This article is a general discussion of distributed capacitance, Cd, in transformers with emphasis on measurement. We will discuss how capacitance occurs, references to formula for calculating, how capacitance is modeled, how Cd is measured (with three cautionary notes) and guidelines to aid in evaluating / measuring Cd.

Capacitance in a transformer winding cannot be avoided. The voltage difference between turns, between winding layers and between windings to core create these parasitic elements. In general, part of the designers' task is to keep capacitance to a minimum. On rare occasions winding capacitance is increased to reduce ringing in switching transformers.

There are several capacitive elements in a transformer. Capacitance occurs winding to winding, turn to turn, windings to core and stray capacitances between terminals and case. To better understand the capacitance phenomenon in transformers. Reference is an article aimed at Cd of coils for higher frequency applications with several test methods. This discussion will be aimed at the measured capacitance that appears across a winding in a transformer. Each winding in a transformer will add capacitance to this total. Transformer action will cause the sum of reflected capacitances to appear at the terminals of each winding.