Date of Award

Summer 2022

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Mohammod Ali

Abstract

Nuclear power plants (NPPs) contain myriad power, control, instrumentation, and other types of cables. The polymer insulation and jacket materials of such cables degrade over time due to operation and environmental conditions e.g., heat, humidity, and radiation. Since the life span of NPPs may extend beyond 40-50 years regular monitoring of cable insulation and jacket polymers is critical to ensure safe and reliable operation. The agingrelated degradation of cables causes changes in the relative permittivity or dielectric constant of the insulation and jacket materials. Capacitor sensors, if properly designed and developed can measure this change and thus can provide an estimate of cable insulation health. Such low-cost sensors can be attractive for cable insulation aging detection because they can be deployed in large numbers and could potentially be wireless enabled for ease of data telemetry. Since real-life aged cable specimens are normally not available for testing cable specimens are aged under an accelerated aging environment in an oven the condition of which is governed by the modified Arrhenius equation to simulate real-life aging condition.

This dissertation focuses on the study, design, and application of capacitor sensors like the interdigital capacitor (IDC) sensor and the serpentine (SRC)capacitor sensor. Typically, such a sensor applies a low frequency (kHz) AC signal on a set of driving electrodes which create localized electric fields within a material under test (MUT) e.g., cable jacket or insulation. A set of sensing electrodes that are connected to a circuit or chip measures the permittivity change in the MUT in the form of an appropriate interelectrode capacitance. The challenges with capacitor sensor design and development include achieving high sensitivity, electric field penetration depth, effects of air-gap mitigation, conformability on cylindrical surface, and conductor integrity. Furthermore, for cables containing both the jacket and the insulation it is currently not possible to measure the aging related permittivity variation of both the jacket and insulation with a single sensor because of electric field penetration depth being dependent on sensor geometry and material characteristics.

This dissertation is motivated to address the above challenges. First, insights are gained from analytical model review and studies of sensors using analytical methods to understand the influence of sensor design parameters on sensitivity and electric field penetration depth. Unit-cell IDC sensors are analyzed using full-wave finite element electromagnetic (EM) simulations using Ansys Maxwell that reveal that the presence of a conducting backplane is highly beneficial in achieving both high sensitivity and electric field penetration depth. Analyses also demonstrate that extremely thin substrates are conducive from both performance and installation point of views.

Experimental sensor design, fabrication, and testing are conducted considering a variety of sensor substrate materials and cables. Cable specimens with and without jackets that had undergone accelerated aging testing are measured using IDC sensors demonstrating their feasibility and applicability. To allow sensor electrode conformability, electrode integrity, and effects of airgap reduction a flexible fabric-based IDC sensor is built and tested on Okoguard Okolon and Okoguard aerial jumper cables. Okoguard Okolon cable specimens aged at 140°C show capacitance more than doubling when a sensor is placed on the CPE jacket of a cable specimen that had undergone accelerated aging from zero to 840 hours. This aging amounts to about 52.5 years of real-life field aging considering 70°C operating temperature. Tests conducted on the EPR insulation of this cable show a capacitance increase by 33% from its original state. The effects of airgap on the measured capacitance due to aging related material surface degradation is also studied that reveal the need for airgap reduction when sensors are installed on curved surfaces.

Finally, the challenges of measuring thru-the-jacket insulation only permittivity variation of a cable a novel reconfigurable capacitor sensor is designed, developed, and tested. The electric field penetration depth for this sensor was changed by activating and deactivating PIN diode switches. In one instance, the sensor measures the permittivity variation of the jacket while in the next, it measures the permittivity variation of both the jacket and the insulation. By leveraging previously developed permittivity estimation models from large scale finite element simulations these two sets of measurement data are then used to evaluate the aging related permittivity variation of the insulation.

Rights

© 2022, Md Nazmul Al Imran

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