The reliability of power system depends on the quality of electrical insulation system of high voltage power equipment. Partial discharge (PD) activity in power apparatus is an indication of insulation degradation due to the presence of defects in the insulation. Partial discharges are ionization processes that take place in voids filled with gas or oil inside the insulation, in dielectric surfaces, and in the proximity of sharp metallic objects. Detection and measurement of partial discharge at the early stage serves as the predictive and preventive maintenance of high voltage equipment in order to anticipate the imminent failure of the power equipment and unplanned power outage. The occurrence of partial discharge on power equipment can be directly an indication of problems in the insulation, but they can also be the consequence of other degradation processes of the insulation. Wide range of detectors including inductive, capacitive, acoustic, and light sensors are used to measure partial discharges. Since the partial discharges occur in extremely short times, usually in the range of nanoseconds, the radiofrequency measurement of the phenomenon in the HF, VHF, and UHF bands is also part of the unconventional methods used for PD detection. Electromagnetic (EM) sensors such as antennas are used for PD measurement due to their ability to perform a complete study on PD measurement such as ability to detect PD pulses, PD localization site, and, to some extent, ability to classify the type of partial discharge online. The Ultra-High Frequency (UHF) method has been an accurate diagnostic technique for many years, which is applied to determine the condition and quality of the insulation system in high voltage (HV) equipements such as power transformer, power generator, electric motor, power cables and gas insulated substation (GIS). Conventional method and nonconventional methods are used to detect PD in high voltage power apparatus by means of PD sensors. A power transformer as one of the high voltage equipment used for power transmission and power distribution among the most expensive high voltage power apparatus in electric power systems. The breakdown of high voltage power transformer can cause significant losses for power utilities as well as blackouts for customers. The survey by CIGRE working group (WG A2.37) conducted on 964 high-voltage power transformers population in 2015 revealed that the weak point of most of power transformers lies in its windings. The most dominant failure causes of the transformer is related to dielectric problem, which is associated to the aging of the paper-oil insulation system. To prevent the 3 premature failure and aging of power transformers, the power transformers need to be routinely observed, tested, and diagnosed. To ensure safe and reliable operation of the power transformer, a PD monitoring system is needed in order to assess the condition of the transformer insulation and to schedule the maintenance and repair plans in the early stage before the breakdown of the transformers. To detect partial discharge online when the transformer is still in service, it requires more sensitive sensors which operate in ultra-high frequency (UHF) range in order to detect the UHF PD signals emitted from the inside of power transformer tank . Recently, many researches have been conducted about designing sensors for PD detection on power transformers. However, according to the literature review, some of the sensors that were designed previously still have some limitations such as low sensitivity to PD signals and a small bandwidth. Moreover, in previous researches about PD sensors design, some PD sensors were not implemented on the actual power transformer in real conditions. Some of the previously designed PD sensors were only tested for PD detection on transformer models, by using a metallic box filled with oil and artificial insulation defects that were located inside the metal box to simulate PD source inside the power transformer. The proposed research in this dissertation aims at designing and implementing the microstrip patch antennae with ultra-wide band characteristics (UWB) operating in the UHF range of 300MHz - 3GHz in order to detect the PD induced electromagnetic signals emitted from the complex insulation system inside the transformer tank. The designed UHF antennas were tested first by using the vector network analyzer (VNA) to measure some basic antenna parameters such as return loss (S11 parameter), VSWR, bandwidth, resonance frequency and input impedance. Later on, the designed UHF antennas were implemented in detecting and locating the partial discharge on power transformer model in high voltage laboratory. In the first phase of this research, the UHF antennas with UWB characteristics namely, circular microstrip patch antenna and rectangular microstrip patch antenna were designed, simulated and optimized to have optimal parameters such as ultra-wide bandwidth characteristics, lower return loss under -10dB, lower VSWR less than 2, higher gain and directivity. The proposed UHF antennas were designed, simulated and optimized using the CST Microwave Studio Software and HFSS Software. After designing the antennas, the next step was to fabricate the proposed UHF antennas and these antennas were fabricated on the printed circuit board (PCB) with FR-4 Epoxy substrate. The optimal designs of the proposed circular microstrip patch antenna and rectangular microstrip patch antenna have a bandwidth of 3.3GHz and 1.24GHz, respectively. Both designed circular and rectangular microstrip antennas have a lower return loss (below-10dB) at all resonance frequencies, lower VSWR less than 2, and an ultra-wide bandwidth (UWB) characteristic. In the second phase of this research dissertation, to investigate the detection sensitivity performance, the designed antennas were implemented preliminarily for PD detection in air- insulated systems before being applied for PD detection in oil-insulated systems such as on power transformers. The fabricated UHF antennas with UWB characteristics were implemented to detect PD in air insulation by using two types of insulation defects namely, needle-plate electrode model and plate-plate electrode model to generate corona discharge and surface discharge, respectively 4 in air insulation. Based on PD measurement results obtained by using the designed UHF antennas, it was investigate that these antennas have high sensitivity to detect PD activity in air-insulated systems. In the third phase of the research, the designed UHF antennas having UWB characteristics were implemented for partial discharge detection in transformer oil insulation through laboratory experiments by using the needle-plate electrode model, used as an artificial insulation defect to generate corona discharge in oil insulation. In the laboratory experiments conducted in this research phase, four applied voltage levels namely, 10kV, 12kV, 14kV and 16kV were used to investigate the effect of the applied voltage to the partial discharge features. To evaluate the sensitivity performance of the designed UHF antennas in detecting PD pulses, four antenna positions namely, 50cm, 70cm, 100cm and 200cm were used and varied respectively in the experiment. Based on PD measurement results obtained by using the designed UHF antennas, it was investigated that the designed antenna has a high sensitivity to detect the PD-induced electromagnetic waves originating from the PD source in oil even when the antennas are positioned at a distance of 2m from the PD source. To validate the PD measurement results obtained by using the designed antennas, these antennas were used to detect PD pulse signals simultaneously with the commercial HFCT sensor used as a comparison PD sensor. The fourth phase of research dissertation has focused on the implementation of the designed UHF antennas (rectangular microstrip antenna and circular microstrip antenna) for partial discharge detection through laboratory experiments on the oil-filled transformer tank model. Two types of insulation defects namely; the needle-plate electrode model and plate-plate electrode model were used as artificial insulation defects to generate corona discharge and surface discharge in oil insulation. The oil-filled metal tank model having a dimension of 50cm x 50cm x 50cm was used to simulate an actual oil-immersed power transformer. The thickness of the metal tank wall is 5mm was used to simulate the power transformer. In the laboratory experiments conducted in this last phase of research dissertation, four applied voltage levels namely, 10kV, 12kV, 14kV and 16kV were used to investigate the effect of the applied voltage to the detected partial discharge features. To evaluate the sensitivity performance of the designed UHF antennas, four antenna positions namely, 50cm, 70cm, 100cm and 200cm were used and varied respectively in the experiment. In addition, the PD measurement results obtained by using the designed UHF antennas (circular patch antenna and rectangular patch antenna) were compared with the existing PD sensors, namely commercial HFCT sensor, loop antenna and Hilbert fractal antenna, available in the laboratory. It was investigated that the designed circular antenna and rectangular antenna are more sensitive than loop antenna and Hilbert antenna in detecting corona PD and surface PD emitted from inside of the oil-filled transformer tank model. Therefore, in conclusion, the designed UHF antennas (circular microstrip antenna and rectangular microstrip antenna) presented in this research dissertation have potential to be valid candidate UHF PD sensors that can be used for the diagnosis of both air-insulated and oil-insulated HV and MV power assets, where irradiated UHF PD signals can be potentially sensed.. 5 Overall, this dissertation has proposed cost effective UHF sensors, with optimal capabilities, that can be used for PD detection and measurement on high voltage power apparatus such as on power transformers. The proposed UHF antenna designs in this research will help electricity companies to use them as alternative sensors to commercial PD sensors for the diagnosis of high voltage and medium voltage power equipment based on UHF PD detection technology. This research will also contribute as a reference material regarding UHF antenna design for partial discharge measurements using UHF sensors.