Novel piezoelectric transducers for high voltage measurements

A prerequisite for safe and stable operation of an electric power system is the accurate and reliable measurement of the system parameters, in particular, current and voltage. Conventionally, this has been achieved on High Voltage (H.V.) systems by expensive, bulky iron-core transformers and also by capacitor transformers. Both of them are increasingly coming under review (chapter 1) in modern power system due to their cost, safety implications for personnel and surrounding plant if failure occurs, installation time and indeed substation land requirements. Research effort into viable alternatives to instrument transformers has been ongoing for many years to reduce the cost and improve the safety and accuracy of this devices. In the last years, this research has accelerated due to the new requirements of the modern metering and protection systems based on electronic and microprocessor devices. This trend in the modern systems has allowed the development of novel transducers where the accuracy, the reliability and the safety has been significantly improved. The main alternative incorporate optic fibre viewed as the most realistic method of providing cheap and safe isolation between the chosen sensor at phase potential and earth. The sensor itself is, in the case of current measurement, based on the Faraday effect while in the case of the voltage measurement is generally a Pockels or Kerr effect device measuring a fraction of the actual phase voltage by employing a capacitive voltage divider. In parallel to the development of the optical alternative other different transducers have been studied based on the properties offer for new materials and technologies developed in the last years. A clear example of new emergent technologies is found in the piezoelectricity (chapter 2). In just over 100 years piezoelectricity has moved from a laboratory curiosity to big business. During this period, several technologies have been developed to utilise the piezoelectric effect. In turn, each of these technologies has become an essential component in many kinds of electronic products. The U.S. piezoelectric industry, good mirror for analyse the international technologic development, has gone through several boom and bust cycles. However, new applications are emerging for piezoelectricity because of the developments in piezoelectric ceramics and polymers. Even though piezoelectric quartz crystal still holds the largest market segment, several new piezoelectric ceramic and piezoelectric polymer materials are being developed that represent good, attractive market segments in terms of growth, competition, and investment. This thesis proposes an alternative for the measurement in High Voltage environments by using the piezoelectric technology. The thesis was initially concentrated in the alternative of the voltage measurement, but the successfully results in this field have open the way to propose alternatives for measuring current. Accordingly, the present thesis develops a novel instrument transformer based on the mechanical forces developed for the piezoelectric materials under the influence of an electric field. The transformer consists of an actuator piezoelectric column and a sensor piezoelectric column. The actuator column is connected to the a.c. High Voltage to be measured. This electrical voltage produces the generation of mechanical forces within the material. The forces can be measured by means of the piezoelectric sensor and convert it back into a low electrical signal which can drive an electronic protection or measuring system. The novel piezoelectric transformer herein treated is of wide application in electrical networks for measuring, providing both advantageous accuracy and large transformation ratio. Furthermore, It is expected to reduce the volume, weight and manufacturing cost of the traditional electromagnetic transformers.