High voltage and current metrology at NIST. Precision measurements of the past, present, and the future

The accurate measurement of high voltages and currents is vital to monitoring power flows in electric power systems, metering electric power and energy, and, of course, to characterizing the behavior of electrical insulation. NIST has been making precision electrical measurements since its inception a century ago. This paper discusses metrology that is now used or under development at NIST for ac and pulse high voltages and currents. The compensated current comparator is at the heart of most precision ac measurements made at NIST because it offers the greatest sensitivity and lowest measurement uncertainty even today, some forty years after its development. It is used for characterizing ac high voltage and current transformers in terms of their ratio and phase angles, and also for capacitor ratio measurements. Precision ac high voltage dividers with active electronic compensation have also been recently developed. Measurement systems utilizing the magneto-optic Faraday effect are now being developed for both ac and pulsed currents. The basic principles of these devices and systems, their practical limitations, and measurement uncertainties are discussed in this paper.

Introduction

The National Institute of Standards and Technology (NIST) is now in its centennial year and has been conducting research in electrical measurements for most of that time. Some of the earliest measurement services provided by NIST's predecessor, the National Bureau of Standards (NBS), were for tests of voltage and current transformers used as reference standards by electric utilities. An example of an NBS certificate for a voltage transformer test performed in 1929 is shown in Figure 1. Magnetic instrument transformers are still used for monitoring and metering of electricity in the electric power grid, but the techniques used for testing this equipment have been improved over the years resulting in a reduction in the measurement uncertainties. In addition to the conventional voltage and current transformers, new technologies have been developed that utilize optical techniques to measure voltages and currents in electric power systems. These technologies offer important advantages over purely electrical instruments: their all-dielectric construction makes them much better suited for high voltage applications than magnetic transformers that require large volumes of insulation. The significantly broader bandwidth of optical sensors makes it possible to use the same sensor for metering, protective relaying, and diagnostics. Figure 1: National Bureau of Standards test certificate. The certificate was issued for a test of a voltage transformer performed in 1929.