Excerpt from Calibration of Voltage Transformers and High-Voltage Capacitors at Nist

In the past nist had used a comparative null method to calibrate voltage transformers. The un known transformer was compared to a nist refer ence transformer using a voltage comparator consisting of a variable resistive divider and a mu tual inductor. Reference transformers were avail able with ratios ranging from up to Measurement uncertainties in the comparison of the unknown transformer with the reference trans former were to.ol% for ratio and t0.3 minutes for phase angle. The ratio and phase angle of the reference transformers were known to about the same accuracy. There are several disadvantages to this approach. Since the comparator has a limited range, several reference transformers must be available to cover the anticipated users' needs. The ratio and phase angles of each one of these trans formers must be carefully determined over the secondary voltage range of interest. These trans formers then have to be rechecked at regular inter vals to determine if the ratios and phase angles have changed.

If a direct measurement method were available that was sufficiently accurate and straightforward to make the calibration of these reference trans formers a simple task, then that method could be used to measure the client's transformer directly. At nist, the direct null method in use origi nally involved balancing the secondary of the ref erence transformer against the output of a resistive divider used in conjunction with a variable mutual inductor to provide phase angle balance. Such a measurement was difficult because the resistive di vider ratio changed with heating. Since the late 19605 a direct null method has been available that is straightforward and accurate and is now used at nist in place of comparative methods us ing reference transformers.

Capacitors are invariably measured by balancing the unknown capacitor against a known standard using some type of bridge arrangement. There are a variety of such bridges described in the literature The one most used in high-voltage applications in the last 60 years is the Schering bridge (fig. The two high-voltage arms of this bridge consist of the standard and unknown capacitors. The two low voltage arms are resistors (one has a parallel capacitor for phase angle balance).

About the Publisher

Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com

This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.