Design and Implementation of Precise Thermal Converters for the Highly Accurate AC Voltage Applications

Abstract

There are several designs of instruments and various techniques for measuring the a.c voltage. The a.c-d.c thermal transfer standards represent the common method for measuring the a.c voltage accurately in terms of the d.c voltage. The single junction thermal converter (SJTVC) is widely used as a.c-d.c thermal transfer standard. It is to be noted that the SJTVC is mainly an active electric resistor (heater) whose temperature is sensed by a standard thermocouple element. This device has an input which is either a.c current or voltage and an output signal represented by the output voltage of the thermocouple. This thermal converter suffers from many drawbacks such as, it has low d.c output voltage requiring accurate milli-voltmeter to measure it which its accuracy and resolution at this range of d.c voltage is very small. This leads to high uncertainty and not very high accuracy of a.c voltage measurement. Also, the SJTVC is limited in usage at the high frequencies which is higher than or equal to 100 kHz.

These drawbacks are studied and treated in this thesis through fabricating of modified designs TVCs. The suggested solution depends on three different electrical connections of a number of new single junction vacuum thermocouples their heaters leads gathered in three groups; Series, Parallel and Series/Parallel arrangements. The modified TVCs are implemented entirely in National Institute for Standards (NIS), Egypt to improve the capabilities in ac voltage measurements. Referring to the above description of the SJTVC, these designs cover a range of voltage from 200 mV to 1000 V at frequencies from 20 Hz to 100 MHz but they are calibrated to 100 kHz only due to technical limitations. As a result of the steadily increasing demand for high accuracy a.c voltage measurements at high frequencies, single junction vacuum thermocouples (UHF-type) will be used in these new designs. This type of thermal elements will be used to replace of the conventional type of the thermalelement.

Also, in this thesis a trial is made to fabricate a.c voltage standard sources which benefit from the advantages of the thermal element and depend on its theory. The a.c voltage is obtained by designing two a.c voltage standard sources one at 1V and the other for the 10V a.c. The higher voltage level of 10V can be theoretically used but due to certain technical limitation it is used in the range of 5V only. This is in a way similar to the d.c voltage standard sources (the solid-state voltage source and the saturated standard cell) which have a 1V and 10V d.c voltage standard sources and the other ranges are obtained by using voltage dividers.

This thesis consists of seven chapters, In chapter 1 an overview on the details about the field of metrology and its importance in the modern industry is introduced. This chapter concentrates on the electrical metrology. Chapter 2 discusses the present different designs and techniques for measuring and sourcing the a.c voltage and a comparison between these techniques is made. In chapter 3 the modified designs of TVCs for measuring the a.c voltage and the new a.c voltage standard sources are discussed. This chapter introduces their theory of operation, their constructions and their characteristics. The characteristics are experimentally and automatically determined. Such characteristics include the steady state values, short-term stability, long-term stability, settling time, d.c response time, d.c reversal error and n-factor. In chapter 4, we calibrate the modified designs for measuring and sourcing the a.c voltage automatically by building a new program using the LabVIEW package. Also, we evaluate the uncertainty statement. In chapter 5, the equivalent circuits of the modified designs are developed and simulated by using LT-Spice program and the simulation error between the practical and simulated results are determined. Chapter 6 confirms the modified designs locally by comparing the characteristics of the modified design TVC with another existing TVC and internationally through bi-lateral international comparison with traceable organization which calibrates their equipment in Fluke, USA. The results are analyzed by using the X-Bar chart and the efficiency test and the final conclusion is shown. The modified design TVC (The series/parallel arrangement box, 1Volt) is also calibrated in PTB, Germany. This international calibration assures that the modified designs are very accurate, precise and dependable in the measurement of the a.c voltage at different frequencies from 10 Hz to 1 MHz. Finally in chapter 7, the conclusions of this thesis and the future work are represented.

This thesis presents an analytical study for the new method for measuring and sourcing the alternating voltage using a very accurate method. These modifications achieve the level of accuracy and uncertainty which are required in the primary standards laboratories in the field of the electrical a.c voltage measurement and sourcing.