By Arthur G. Zuch
High accuracy testing of current transformers (CTs) at high primary currents under specified load conditions has always been a challenge. Computer controlled testing systems designed for this task traditionally have been large, heavy and expensive.
Optimized Devices Inc. of Pleasantville, NY has developed a line of solid state instrument transformer test systems that exhibit significant improvement over conventional products. Some of the new developments and features are described below
THE CURRENT GENERATOR
It has been standard practice to derive the applied high primary currents by means of a variable transformer connected to the AC power line. This has now been replaced with a solid state synthesizer and switchmode power amplifier. Advantages are lower waveform distortion, 50/60 Hz operation, better repeatability, and high ramp up and ramp down rates for very fast test speed with low heating.
THE REFERENCE TRANSFORMER
An instrument transformer test system is only as good as its reference. A low loss toroid core is used operating at very low flux density. This allows high accuracy calibration with excellent repeatability. An exclusive innovation allows the reference transformer to act as a universal reference (patent pending) so as to test CTs with standard and non-standard ratios.
An all electronic comparator compares the instrument transformer under test to the internal reference. Recent advances in operational amplifier technology allow comparisons to within 0.001%.
PC or laptop controlled, test screens are very easy to use and provide test program storage, test result reports, and network connection for data transmission or remote control. CT test systems are available to test over the primary range of 20 – 5,000 A. Standard IEC and ANSI burdens are included. Options include 5 A and 1 A secondaries and overload testing to 20 P. PT test systems cover the range of 120 -14,400 vac.
All electronic current generation and comparisons result in improved measurement repeatability. This in turn allows higher accuracy calibration accuracy. Typical measurement accuracy is 0.01%, satisfactory for certification for the 0.1% metering class.
High speed electronic control means less heating. Less heating allows the use of smaller components, resulting in less weight and lower cost.