An automatic system is implemented for calibration of voltage ratio devices (e.g. Guildline 9700PL) with standard measurement uncertainty of 2.5´10-7, at rated voltages from 2 V: 1 V to 1000 V: 1 V. The system uses a simple self-calibrating method, which is essentially a counting process. Test voltage is applied from a stable voltage source (Fluke 5720A) to the device under test (DUT) and, with the aid of a programmable low thermal scanner (Dataproof 320A Opt3H), the voltage drops across various sections of the DUT is measured sequentially by a precision digital multimeter (Wavetek 1281). The DUT’s lowest section is taken as the reference section. The voltage drop across the reference section is compared with those across the subsequent sections. The DUT’s voltage ratios are then calculated using a set of mathematical equations based on the volt-drop measurement results. An user-friendly graphical user interface, written in Visual Basic, is developed for test parameter setting, instrument control (including scanner switching), data acquisition, result calculation and real-time event monitoring. To minimise measurement uncertainty for oil immersed DUTs (e.g. Guildline 9700PL), the system is capable of controlling the DUT’s oil temperature within 0.02 °C of the target temperature using a simple control module. |