For the past year Silver Spectrum Solutions (Pty) Ltd [Silver Spectrum] has been at the forefront of developing innovative, efficient, and cost-effective applications for Remotely Piloted Aircraft Systems (RPAS), better known as drones. Our recent projects focus primarily on the use of drones for the inspection of linear infrastructure such as power lines, railway lines, pipelines, etc. Our initial products relate specifically to high voltage power lines and we are on our way to establishing ourselves as leaders in this emerging field. This particular application of drones is innovative but marred by complex technical challenges. As will be shown in this article, the electrical landscape near power lines has a significant impact on the operation of the drone. In order to better understand this impact, Silver Spectrum has undertaken comprehensive testing using the facilities at the University of Witwatersrand (WITS). These tests provided important information in order to determine the appropriate specification of drones for use in a high voltage setting. These tests have led to the development of two bespoke aircraft which have been proven safe for operation around power lines.
The South African Civil Aviation Authority (SACAA) has set out stringent regulations regarding the use of drones and in-fact, South Africa’s regulatory response to the use and application of drones are widely considered to be some of the strictest in the world. The regulations are set out in Part 101 of the South African Civil Aviation Regulations of 2011 and provide that unless specifically approved by SACAA, the default limitations with regard to the operation of drones are:
- No operation of drones near manned aircraft,
- No operation closer than 50 m to any road or people,
- No operation within 10 km of an aerodrome (viz. airport, helipad, airfield, etc.),
- No operation of a drone weighing more than 7 kg,
- No operation within controlled, restricted or prohibited airspace, and
- No flight above 150 feet (i.e. 45 m) above ground.
Given the vast power grid network, including National and Municipal networks, many of these power lines require operation within the limitations listed above. In order for the SACAA to endorse operations within the default limitations, an exhaustive safety case will need to be provided to them that shows that the drone is capable of operating safely near the power line.
Fieldwork conducted by Silver Spectrum with various aircraft has shown that electrical medium and high voltage power lines (88 kV and above) cause interference with the C2 communications link as well as with the drone’s Inertial Measurement Unit (IMU). This interference causes communication loss with the drone as well as compass-based navigation errors. These negative influences have resulted in forced “return-to-home” and erratic manoeuvring of the aircraft. While none of our tests resulted in crashes, such unpredictable operation is unacceptable while operating within close proximity to expensive and critical power grid equipment. For low and medium voltage lines, the typical size of the drones is such that if a malfunction occurs, it may bridge out the electrical safety clearance, thereby causing a short circuit fault on the power grid.
Silver Spectrum has therefore set out to ensure that our drones will be able to safely operate near electrical grids and are free of any electromagnetic interference (EMI) from the power line.
The two main causes of EMI which may be experienced from a power line are electrical and magnetic field interference. The EMI may couple into the electrical circuits of the radio transmitter as well as the on-board flight controller by electromagnetic induction or electrostatic coupling. Furthermore, the high magnetic field produced by the power line will alter the output of the onboard compass. Apart from the classic EMI coupling into the electronic circuitry of the Global Positioning System (GPS) unit, additional multi-path GPS errors may be caused due to interference with the reception of the GPS signals through the antenna.
Silver Spectrum has contracted with WITS to conduct electric and magnetic field testing on drones. A comprehensive test program was established and field tests were done at WITs’ High Voltage Laboratory in Johannesburg and at the Sky Sales (Pty) Ltd t/a Commercial Drones test field near Centurion. Both indoor and outdoor tests were conducted to ensure that our drones meet the stringent criteria of a magnetic field of 80mT and an electric field of 30 kV/m. A future blog post will delve further into the test programme. We ran further tests to measure maximum flight time (with battery safety reserves), agility and manoeuvrability. These tests revealed that standard off-the-shelf DJI drones may not necessarily have the appropriate electromagnetic compatibility (EMC) for efficient operation near power lines.
As mentioned earlier in this article, our testing and research resulted in two custom made, purpose-built drones, the Transmission Line Inspector (TxLI) and the Distribution Line Inspector (DxLI). These drones are not only accurate but also safe for applications within close proximity to high voltage power lines. TxLI is designed for use where power line voltages are greater than 132 kV and where corona faults are possible. The TxLI carries an ultra-violet corona camera, an infrared thermal camera and high definition video and a still photography camera. The DxLI is developed for use at voltages below 132 kV, where corona is not prevalent. For this reason, the DxLI is fitted with only a thermal camera, high definition video and a still photography camera. An important learning from our testing was that any EMC related investigation must include the payload as well. Initial tests did reveal interference issues between the power connection as well as the telemetry links. Our solutions eliminate these EMC issues and as a further redundancy, both the DxLI and TxLI are supplied with Real-Time Kinetics (RTK) positioning systems.
While we are encouraged by the results of our testing and research, the application of drones for high voltage power line inspections is a fairly novel use of drones in this country and as a result, there is still much testing and research to be done and understanding to be gained from both a technical and regulatory perspective. We look forward to sharing our findings and insights with you in future posts on this blog.