The phenomenon of partial discharge has been long studied in various power components and in recent times, it has been agreed that the occurrence of such discharges can be considered as a vital parameter of component health. Partial discharges in power cables is a pervasive phenomenon that over time may lead to failure. For this reason, their monitoring has become increasingly sought after by asset owners. Nevertheless, the symmetry in the structure of a coaxial cable makes the isolation of the discharges in power cable more complicated when compared to other power components. In addition to this, there are added limitations to a given system with regard to the sensitivity of measurements and the maximum length of cable monitored, due to the increased attenuation towards high frequencies. Therefore, this thesis investigates the possible methods that could be used for online-monitoring of sensitive partial discharges (10 pC) in long cable systems ( >10 km), by means of new techniques or new applications of existing techniques. In this thesis a comprehensive feasibility study is followed by a thorough evaluation of the most promising methods and a demonstration of proof of concept of the parts thereof. The prime focus of this thesis is to investigate distributed measurement over a long cable system with the help of built-in sensor units. A built-in capacitive sensor type is modelled analytically, and simulated to check its performance before being tested experimentally in the last phase of this project. The final results indicate the feasibility of its implementation. Furthermore, the possibility of communication between such distributed sensor nodes is explored. One such method of communication proposed in this thesis is the wireless communication inside a cable using antennas. This thesis proves and quantifes to a certain extent the performance of an antenna (transceivers) inside a coaxial power cable paving the way for other tailor-made applications using `RFID-like' (Radio Frequency Identication) technology in the future.