Condition monitoring of high-voltage power lines plays an important role in the design and operation of electrical power networks, providing a comprehensive view of the state of the transmission power systems or the electric grid infrastructure. The purpose of condition monitoring is twofold: Firstly, it boosts revenue by reducing installation, maintenance and operating costs. This is especially true when self-powered and autonomous sensors are used, which allow the utility shutdown to be reduced, since battery-powered sensors have a fixed and limited lifespan. Secondly, condition monitoring improves supply reliability since periodic inspections are required, although these are usually carried out by less reliable, traditional, on-ground visual means. The significant advantages condition monitoring brings to the grid operators, can however be outweighed by the operating costs and maintenance requirements of monitoring sensors. Needless to say, this runs counter to the reduction of the reinforcement and maintenance costs grid operators are seeking to achieve these days. Finally, condition monitoring allows predictive rather than corrective maintenance to be done, prolonging the useful life of the assets in the grid at the expense of increased risk of failure. This risk can be mitigated by collecting accurate and real-time information about the performance and operating condition of the grid.
Energy harvesting as power source for condition monitoring is increasingly being investigated as an attractive alternative to batteries, particularly in low power sensing applications. Likewise, the advances in wireless network technologies and the reduction in power requirements of electronic devices have paved the way to independent remote sensing nodes. These features have made condition monitoring a viable reality.
Regarding the energy sources and harvester, there are several alternatives that have been reported in the literature. Wind power [1
], solar energy [5
] or hybrid wind/solar approaches [7
] are used for harvesting power not only in the range of μW and mW but also in medium power range (1–10 W) [7
]. The energy harvester can be an anemometer [1
], flexible piezoelectric devices [2
] or small scale wind turbines with an electrostatic converter [3
] for the wind-based alternative, or solar panels for solar-based energy harvesting [6
] (pp. 1045–1046). Solar panels require, however, regular maintenance, such as cleaning when they are covered with dust, snow or ice, since maximum performance is desired. The mechanical energy of vibration can also be turned into electricity by using one of following three approaches [8
]: piezoelectric, electromagnetic, often modeled as an inductive spring mass system [9
] or electrostatic. The energy harvested from vibration can be improved by implementing several strategies. For example, in [10
] the authors describe an approach of a linear electromagnetic vibration energy harvester with weak magnetic coupling in which the energy harvested is enhanced by implementing the energy localization phenomenon. Likewise, a hybrid piezoelectric-electromagnetic vibration energy harvester is introduced in [11
]. In order to improve performance of the vibration energy harvester, in [12
] an approach based on an array of coupled levitated magnets is proposed. This approach, which is based on the single levitated magnet design proposed in [13
], combines the benefits of nonlinearities and modal interactions. The use of nonlinearity to boost performance is also considered in [14
] where a nonlinear vibration energy harvester based on the concept of high static low dynamic stiffness is proposed. In [15
] with the aim of broadening the bandwidth of the effective harvesting frequency, a multi-frequency energy harvester array consisting of three permanent magnets, three sets of two-layer copper coils and a supported beam, is introduced. Broadening the bandwidth is also a primary objective accomplished in the work presented in [16
]. By using a series of cantilevers with different lengths and resonance frequencies, they manage to widen the overall bandwidth, as well as to increase the generated power. In general, ambient vibrations are random and the vibration spectra vary enormously for different applications, which makes the design of vibration-based harvesters a challenging task. Energy contents in heat can also be scavenged by using thermoelectric generators, which obtain the energy in the form of electricity when there is a difference in temperature -Peltier-Seebeck effect-[17
]. For instance, a thermoelectric generator can be used for human body heat energy harvesting with the aim of powering wearable devices [18
]. Radio frequency-based harvesting approaches obtain the energy from radio waves. According to the RF energy source being considered, there are two categories [20
]: (i) RF signal from dedicated sources; and (ii) RF energy from ambient sources, such as telecommunication towers and mobile devices, in which wireless communication devices can act as RF sources to power their neighbouring devices. The low power conversion efficiency of RF harvesting and the ultra-low voltages that are incident at the receiver antenna constitute a formidable challenge that can limit its applicability [22
]. Electric field energy harvesting is another alternative presented in many papers in literature [23
] based on the principle that an energized conductor creates a radial electric field. The advantage of this technique is that the energy can be harvested from a no-load AC power line without current flowing through it. The electric field is always present around the high voltage power lines and it is stable and predictable since the voltage is regulated. In general, the energy is obtained from the electric fields around the power lines by using the principle of a capacitor divider. However, for the same amount of power, the size of the electric-field-based harvester must be greater than that of the magnetic-field-based alternative. The energy density obtained from a magnetic field is considerably higher compared to an electric field [23
]. In addition, the high voltage involved with electric fields, challenges the technical feasibility of its implementation, increasing the associated cost: the installation of this type of harvesters requires a major power shutdown since they are usually either attached under the line or coiled around the cable [25
]. The energy harvesting from the magnetic fields surrounding high-voltage lines has received considerable attention in recent years. There are two approaches as a function of the way the harvester is located [28
]: (i) free-standing when the harvester is located in close proximity to the conductor without enclosing it; and (ii) the current-transformer-based inductive harvester which encloses the conductor to obtain maximum magnetic coupling [28
]. Current transformers (CTs) are generally used to measure AC amperage by sensing the magnetic field generated by the primary current flowing in the conductor around which the CT is installed. However, CTs can also be used for inductive energy harvesting. The performance of the inductive harvester greatly depends on the magnetic core material characteristics, such as the magnetization curve, permeability, power losses and saturation [29
]. Although saturation is nearly always avoided, in [31
] the authors revealed that for any given core, regardless of the application, power harvest is maximized when the core is permitted to saturate at a particular time window within the line cycle. An alternative approach is presented in [30
], where a miniaturized linear permanent magnet synchronous generator is utilized. The magnetic field is first converted into mechanical vibration by using a permanent magnet and then the vibration energy is turned into electricity by using a synchronous generator.
As far as the power and storage management circuit are concerned, power electronics are required for several reasons. Firstly, high power extraction involves impedance matching between the energy harvester, and the load, i.e., when the load impedance equals the complex conjugate of the source impedance [28
]. Secondly, some form of voltage regulation is essential owing to the fact that the output current and voltage generated by the harvester do not usually match with the load requirements. Finally, the intermittency of the energy source should not affect the continuous operation of the system. Therefore, some form of energy storage is generally included. In [34
] the impedance matching is achieved by a compensating capacitor along with a buck-boost converter which assure constant load impedance. The energy is stored in a rechargeable battery. In [36
] the authors introduced an adaptive approach, which achieves automatic power optimization by using a dc-dc converter thereby maximizing the power stored in a battery. Other works have used dc-dc converters to achieve maximum power transfer [37
]. Batteries are the most frequent storage strategy for energy harvesting systems [5
], Supercapacitors have also been used to buffer energy so that the system can operate in a power outage [7
]. The physical deterioration of the rechargeable batteries constitutes the limiting factor of the lifetime of the energy harvester. The premature aging of the cells occurs when the battery is subjected to repeated charge/discharge cycles. In order to prolong the lifetime of the battery, in [44
] is presented an implementation that uses a two-stage storage system consisting of supercapacitors and a lithium rechargeable battery.
This paper presents a simulation-based strategy for characterizing a CT-based inductive electromagnetic energy harvesting system, in terms of the core material through its magnetization curve, number of turns in the secondary winding, magnetic path length and cross-sectional area of the core, copper cross section of the winding, primary current, length of the copper wire, load resistance and compensating capacitor, inter alia. This work introduces a new energy harvester design strategy aiming at extracting the maximum power from a high voltage line. An experimental setup with different configurations validates this strategy. The MATLAB/Simulink tool is used for modeling the energy harvesting system and for data visualization. The system is able to accurately estimate the matching impedance, the optimal dimensions of the coil and the power harvested for different primary current conditions and different loads. As a result, a rapid prototype design is developed.