Non-contact condition monitoring of power transmission/distribution cables based on electromagnetic field sensing

Abstract

The electric power system is no doubt a backbone for modern life today. The produced power must be transmitted through transmission/distribution network in an incessant, reliable and efficient way irrespective of what demands are placed in the system. An accurate and reliable monitoring technique/equipment is required to detect the potential risks for taking therapeutic procedures promptly. However, there is an increasing demand of monitoring techniques/equipment because of elevated transmission/distribution voltages (e.g., 1000 kV) and more dynamic power flow. In these years, much attention has been paid to electromagnetic sensing for solving the problems in transmission/distribution networks by many researchers. This is because the measured entity (the corresponding magnetic or electric field) can be safely measured in a non-contact and distant sensing. Moreover, the electromagnetic sensors exhibit a large frequency bandwidth from DC to MHz. Unfortunately, the latest techniques/equipment can neither realise the multi-conductor measurement nor require the prior knowledge of transmission/distribution line’s layout which is very hard to attain in the wild field (or invisible inside the cable). Therefore, these two main problems have to be addressed, and a series of sensing techniques/equipment were developed in this thesis accordingly. In Chapter 2, a sensing technique was introduced to reconstruct the phase currents of a three-phase three-core power distribution cable, and the developed equipment was validated on a 22-kV underground power distribution cable in a zone substation. A DC-component-based fault classification of three-phase power distribution cables was established in Chapter 3 because the dirrect current (DC) can be reconstructed from the magnetic sensing. In fact, the magnetic fields exhibit different patterns and magnitudes around the cable surface in current-, voltage-, and de-energised status. Therefore, an energisation-status identification of three-phase three-core shielded power distribution cable was proposed by observing the patterns of three peaks and troughs around the cable surface regarding the magnetic fields in Chapter 4. To eliminate the limitation of single-core measurement, the non-contact electric-coupling-based techniques for monitoring the voltage of HVAC and HVDC transmission lines were established in Chapter 6, and 7, respectively. The quantity of induction bars is equal to the number of transmission lines, and the voltage of transmission lines can be reconstructed simultaneously. The Chapter 7 then introduced how to identify the fault line during the fault duration of HVDC transmission lines, because a large voltage variation also occurs on the healthy line due to the electromagnetic coupling between lines. Regarding all the proposed techniques/equipment, the spatial configurations of transmission lines or the conductors inside cables are not needed beforehand but attained from magnetic sensing. They are also applicable for the multi-conductor sensing. The equipment can be cheaper by deploying the low-cost magnetoresistive (MR) sensors (come pared to current or potential transformers which cost about US$ 100 k, magnetoresistive sensors only cost tens of US$), and the induction bars can be made of copper. What is more, since the electromagnetic sensing is non-contact and distant, it would be much safer for the serviceman during their maintenance work by deploying these techniques.