Non-intrusive, self-supplying and wireless sensor for monitoring grounding cable in smart grids

Highlights

• An innovative sensor to monitor the presence of grounding cable is presented.

• Different self-supply alternatives for the sensor were studied.

• The sensor designed is non-intrusive and wireless in order to be introduced in power systems.

• The energy demand for the system’s communications was optimized to make it as efficient as possible.

Abstract

Energy distribution companies have experienced an increase in catastrophic failures in power systems due to the theft of grounding cable. The objective of this study is to develop an innovative technology which is able to detect the absence of grounding cable in medium and high voltage systems. The wireless technology developed communicates periodically with the system’s central control unit to ensure reliable and safe operation of the installation. Because it is not economically profitable to modify power systems’ infrastructure, this novel sensor is non-intrusive as well as self-supplying. Based on the validation step’s results, we conclude that the sensor is ready to be tested in smart grid equipment to monitor the state of grounding cables.

Introduction

In recent years, a wide range of sensors have been installed to monitor traditional electric power networks [[1], [2], [3], [4]]. The emergence of different kinds of sensors in current power systems is based on the development of the concept of the smart grid, which not only allows infrastructures to be modernized and managed more efficiently but also increases their reliability [5,6]. In addition, one of the biggest issues that electric companies need to address in the short term has to do with the monitorization of grounding cable. Grounding cable in middle and high voltage level power infrastructures is made of copper, and due to the high price of this material, thieves steal these cables from power system substations to resell it. This theft of a grounding cable can lead to serious consequences. Not only can material catastrophic failure occur if an overvoltage situation arises and there is no cable to empty out the energy but power system maintenance operators can be killed as the ground cable is used to ensure equipotentiality with the power infrastructure. The technical characteristics usually required by power system owners are easy installation, self-energy supply and sufficient wireless communication capacity to collect and transmit data about the system’s status [[7], [8], [9]].

Research on microelectronic component efficiency has been able to reduce the power consumed by electronic devices in just a few years [10]. Through this progress, it was possible to develop nanowatt sensors whose long lifetimes are limited by the energy storage capacity of the batteries chosen [11,12]. Nevertheless, the smart grid concept tries to use devices based on battery-supply as little as possible, given that metering sensors are widely used in power systems. Power system operating equipment can be located in remote or difficult to access geographical locations, and so recharging or replacing batteries entails high maintenance costs [13]. Because this scenario is not cost-effective for power system owners, some researchers have suggested energy harvesting be used as an innovative technology solution that allows self-supply and lowers maintenance costs [9,10,14].

The energy harvesting concept relies on converting unused energy sources into electricity in order to supply low power demand devices [15]. Different kind of energy sources, such as vibrations [16], frequencies [17], thermoelectricity [18], renewable generation [19] or electromagnetic fields [20], have been used for different applications. To develop vibration or frequency harvesters, piezoelectric materials are used because these materials produce energy when they are excited by a mechanical vibration or an electric frequency. Although the output voltage of piezoelectrics is large, the obtained output current is not enough for all the applications due to the internal resistance of the materials [21,22]. Hence, this current limitation means this energy harvesting technology is not suitable for all applications. Besides piezoelectric technology, triboelectric technology is also a good choice for harvesting energy based on vibration phenomena [23]. Triboelectric technology relies on the periodic contact (vibrations) of opposite charge affinity surfaces to induce the movement of electrons between electrodes through an external resistance [24]. Triboelectric generators have rapidly spread in different monitoring research fields such as railway, roadway or marine equipment due to their low cost, light weight and high power density [25]. With regard to thermoelectric devices, these harvesters are based on certain materials’ internal structures, which are able to convert thermal energy into electricity when a temperature gradient is available [26]. Although some recently developed materials have achieved efficiencies above 20 %, this technology’s main drawback is the necessity of cooling points being available close to the harvester for technical and economic suitability [27].

Other researchers have focused their efforts on developing energy harvesters whose energy is obtained via renewable resources such as solar photovoltaic or wind energy [28,29]. Due to the intermittent nature of renewable sources, these technologies are not suitable for developing energy harvesters for continuous parameter monitoring. In addition, when these technologies are suitable, it is also necessary to develop electronic architectures for energy-band harvesting because of the wide range of operating points [30]. Finally, magnetic field energy harvesters are based on Faraday’s induction law, where a coil exposed to a variable magnetic field produces an induced voltage [14]. The main disadvantage of developing magnetic field energy harvesters is the dimensions of the core where the coil is wound to extract the energy from the magnetic field [30].

The main aim of the project presented here consists of developing an innovative technology which allows the presence of the grounding cable in high and medium voltage systems to be monitored. This technology will help to detect if thieves have stolen the grounding cable, giving electric companies the chance to reinstall it and thereby guaranteeing systems’ reliability and maintenance operators’ security. The sensor must be able to transmit information about the grounding cable’s state to the system’s control unit periodically. The technology developed should be self-supplying, operate reliably and be non-intrusive. The main objectives of this study are to:

• Develop a self-supply system that ensures powering of both detection and information transmission systems. The energy harvester must extract energy and store it properly to provide sufficient autonomy so the sensor does not depend on additional power supplies. Different alternatives, such as obtaining energy from grounding cables or power lines, are analysed.

• Develop sensors to detect the presence of grounding cables for power system equipment. These sensors are based on the integration of radio frequency identification (RFID) tags and their corresponding readers, making this technology suitable for current infrastructures. Sensors, tags and readers must be able to work under the electromagnetic fields generated at high or medium voltage levels.

• Define an alarm system as well as a suitable physical support for a communications and data transmission protocol. An important requirement for the communication system is that the energy demand be reduced as much as possible in order to optimize both the power supply and device’s communication duty cycle with the control unit.

The remaining sections of this paper are organized as follows: Section 2 describes the technical requirements that the developed sensor must meet, Section 3 contains the theoretical analyses and simulation results, Section 4 presents the final prototype and its main characteristics, Section 5 examines results obtained from the prototype, and Section 6 discusses the main conclusions drawn from this study.