In a pilot project, the Swiss energy producer and supplier Axpo has automated the monitoring of pole disconnectors in the power grid – via the new LPWAN radio standard Mioty. Even under unfavorable conditions including test drives at 120 km/h, Mioty has proven to be extremely robust.
In Switzerland, Axpo does not need any special introduction: The company generates and distributes electricity for three million Swiss people - that's more than a third of the population - and is the country's largest provider of energy from renewable sources (mainly hydropower). Like most energy suppliers, Axpo maintains an extensive, high-performance and fail-safe communications network in order to obtain the necessary information on the status of all the components in their energy network at all times. For this task, three years ago the Axpo Group acquired WZ-Systems, a specialist in crisis-proof communications, which now operates the group's communications infrastructure of radio relay links and cloud services.
The experts at Axpo WZ-Systems were approached by their colleagues at Axpo Grid - the grid operator within the group - regarding a proof of concept (PoC) project. Reto Friedl, Product Manager: »Axpo Grid had announced an innovation competition among its employees. One of the winners had the idea of monitoring the pole disconnectors in the grid via IoT.« The initial situation is as follows: the pole disconnector is a mechanical switch located directly on the pole (Figure 1). When maintenance is due in a network segment, the technician on site calls the control center and announces the disconnection of the relevant section. After the manual disconnection, he calls the control center again and reports completion. The dispatcher in the control center then manually changes the overview scheme on the system on the screen.
In order to simplify and automate this process, sensors record the position of the pole disconnectors and transmit them by radio upon request. »Some of the switches are located in very rough terrain without a power supply,« says Reto Friedl, noting one of the problems to be overcome. Signal transmission and energy supply for monitoring must therefore be relatively self-sufficient and low-maintenance, and must also function perfectly under adverse conditions. »In this context, weather influences, e.g. solar radiation in summer and sub-zero temperatures in winter, are also a challenge for the sensors. The sensor technology as such is not too complex, as these only have to transmit the status (on/off) of the switches. In the future, however, other local sensor and environmental data will also be of interest in order to add contextual information or to detect impending failures at an early stage.«
With these requirements in mind, a suitable radio standard had to be selected on the basis of which as many pole disconnectors as possible could be reliably interrogated over long distances. In the search for a suitable technology, Axpo WZ-Systems entered into discussions with Swissphone Wireless AG for the radio technology and with Comtac AG for the sensors. Both companies brought Mioty into play as the radio protocol due to the safety-critical application.
Mioty is a relatively new LPWAN (Low Power Wide Area Network) technology developed by the Fraunhofer Institute for Integrated Circuits (IIS). Based on the ETSI specification TS 103357, Mioty was designed for IoT devices, especially in industrial applications and smart city concepts that have to meet very high requirements regarding robustness and quality of service. The main task of the radio protocol is to establish an interference-resistant radio connection covering long distances. In theory, Mioty is ideally suited for the large-area networking of pole disconnectors. The pilot project has shown that the radio protocol also proves itself in practice.
Mioty is highly resistant to RF interference, due to its integrated telegram splitting technology. »With Mioty, you can send many telegrams in a network simultaneously without them interfering with each other. This is because each telegram is split into many sub-packets, of which up to 50 % may be lost, e.g. due to interference, without actual useful information being lost,« explains Uwe Scholz from Comtac AG's Business Development. The company manufactures wireless sensors using Mioty and other wireless standards. The principle is illustrated in Figure 2.
»This is a real advantage, especially in applications with long ranges, but also with many sensors within reach of each other.« In addition, the radio technology is extremely power-efficient and thus ideally suited for the battery-powered devices required here.
The basic feasibility of the project was quickly demonstrated. For this purpose, two pole disconnectors and sensors were connected via Mioty in the vicinity of an Axpo site using a local base station from Swissphone. The measured values are collected and visualized in an online dashboard for each sensor (Figure 3).
Those responsible at Axpo Grid are satisfied with these initial applications. »The signal transmission is reliable. The essential and constantly updated parameters such as battery status and signal level are determined and displayed - also via Mioty, of course,« explains Reto Friedl and concludes: »Mioty has met our expectations.«
One of the advantages of a radio system is that once the infrastructure is in place, additional sensors can be integrated very easily. Uwe Scholz: »We have also integrated four temperature sensors for network monitoring into the system, as well as monitoring of the gate control at an Axpo site in the radio area. This allows the control center to see whether the gate is open or closed.« In addition, Axpo WZ-Systems also has Mioty handheld transmitters with GPS on trial in vehicles, which can be easily 'tracked' using this platform. They can be used to test radio coverage at potential new sensor locations. »The results are always surprising, because the ranges are higher than you would expect, even in marginal coverage areas,« says Uwe Scholz.
Swissphone Telecommunications, Swissphone Telecommunications GmbH