Condition monitoring sensors have the responsibility of reducing occupational failures or unscheduled shutdowns especially in power transmission line systems. Existing sensors that are used for condition monitoring are mostly battery-dependent. Powering up these sensors in difficult to access areas where high voltage transmission line usually runs is a challenge because batteries usually have a limited life cycle. Power sources other than batteries such as harvesting from solar energy, magnetic energy, radio frequency energy either produces insufficient energy or not entirely available all the time. Electric Field Energy Harvesting (EFEH) overcomes many of these disadvantages and provides a quality and continuous power source to be used to power up devices especially the monitoring sensors that are used in transmission line monitoring. This paper presents key aspects and drawbacks of six types of energy harvesting methods and a review of existing energy harvesters. The concept of electric field and the usage of EFEH in power transmission line system are explained and a comparison between EFEH with typical energy harvesting methods is discussed. This paper finds that EFEH devices have potential to provide sufficient energy for low powered condition monitoring sensors. Moreover, several improved EFEH approaches are proposed, and future trends are discussed.

<span>Radio Frequency (RF) harvesting seems to be catching up as an alternate energy source whereby RF energy is scavenged from ambient sources and converted into renewable energy in terms of DC power. This converted DC power is then utilized to power up devices that require a low start up power in which eliminates the need for battery replacement. In this paper, a novel RF energy harvesting prototype is presented which consists of two microstrip patch antennas operating on GSM (900MHz) and WIFI (2.4GHz) &amp; WiMAX (2.3GHz) frequency bands with a bandwidth of 220MHz and 10.11MHz respectively to harvest RF signals from ambience. Two matching networks are presented as well to ensure efficient power transfer to load. Rectifiers are designed to transform the RF signals to DC power. The converted DC signals are then combined and fed to a power management circuit which charges a 4.2V NiMh battery and drives a load at a regulated output of 3V.</span>

Radio Frequency harvesting has recently become one of the alternate approaches to power up low power wireless networks. This evolving technology opens the gate for positive energy renewal for wireless components. This paper presents a comprehensive review which includes all the important components in a RF energy harvesting system which are microstrip patch antennas, rectifier modules and power management modules. Different types of microstrip patch antennas and its designs and outputs are discussed. Rectifier modules with Schottky diodes operating under two different frequency bands are also compared and discussed. In addition, different methods of available power management circuits with different methods are also deliberated in this paper. This review also explores various key design issues and envisions some open research directions.

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