IoT for Critical Infrastructures

STARTSEITE News from Swissphone News IoT for Critical Infrastructures

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.

Pilot project: Remote monitoring of pole disconnectors

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.

Figure 1. The pole disconnectors are located in the open field directly on the power pole. During maintenance in a network segment, the maintenance technician switches the mechanical switch manually.

Query switching status by radio

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® guarantees high transmission security

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.

Figure 2a. Traditional LPWAN radio protocols transmit the data in a payload. If a disturbance occurs during transmission, the entire payload data is lost.

Figure 2b. Mioty® redundantly splits the payload data into subpackets (telegram splitting), so that reconstruction is possible if some subpackets are lost.

»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.

Feasibility quickly demonstrated

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).

Figure 3. A dashboard shows the status of the static sensors in terms of data to be measured and connection status, and in the last line the reception strength per sensor and the loss rate of transmitted messages during the last 24 hours. Green = RSSI ≥ -120 dBm, orange = -120 dBm > RSSI ≥ -130 dBm, red < -130 dBm.

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.«

Easily expandable infrastructure

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.

mioty® or LoRa?

Back to the question of why a comparatively new radio system was chosen. According to Reto Friedl, the company was primarily looking for a robust system that could communicate reliably even in difficult terrain conditions. »We have to ensure this due to our business-critical use case. Here, mioty® convinced us first in theory and then also in practice.« According to Uwe Scholz, the necessary range would also have been possible with the more established LoRa. However, LoRa would likely not have provided the robustness necessary for operators of critical infrastructure to be future-proof with radio sensors in an unlicensed frequency bath.

»In the future, there will be more and more sensors transmitting simultaneously in the same public radio frequency band. This can, with greater node density, lead to collisions and information will be lost if telegrams are jammed during transmission.« Mioty® addresses this problem by providing redundancy in telegram splitting. When telegrams are split into small ‘packets’ with pieces of information placed in several packets, information can be reconstructed in the receiver even if individual packets are lost. »Especially in borderline conditions, when long ranges are involved or many sensors are activated within reach of one another, this advantage of mioty® becomes noticeable. We see a lot of potential for us here,« says Uwe Scholz.

Test drives show high radio range

The feasibility study showed that mioty® can cover a large coverage area with a few powerful base stations. The test area chosen was a rural, slightly hilly plain, closed off on two sides by elevations with valley cuts. »A base station in the plain can cover an area of around 300 km²,« Swissphone’s CTO, Harald Pfurtscheller, concludes from the evaluated test runs. »The radio links for the static sensors on the radio masts or in switchgear are 3.5 km to more than 12 km long and usually without direct line of sight. They usually work even with a lot of reserve to the typical system sensitivity of –138 dBm.

The mobile mioty® transmitters, showed reliable radio communication, with some reserve capacity, at distances over 20 km, even without line of sight. Under line-of-sight conditions, radio distances of 30–35 km could be demonstrated without special directional antennas. The stability of such long indirect links will now be further investigated over the course of the year.«

Figure 4. Visualization of radio coverage levels. Geographical coverage with redundant radio coverage makes it possible to avoid dead spots even in very challenging topographical conditions.

According to Harald Pfurtscheller, detected radio gaps due to topographical restrictions could be easily compensated for with a second or third neighboring base station dozens of kilometers away. In overlapping zones, this increases transmission reliability. This is the type of redundant network planning that Swissphone strives for in mission-critical applications (Figure 4).

Stable data connection at 120 km/h

Figure 5. Graphical summary of the results of a test drive at around 120 km/h: Good radio connections were measured even in zones with poor simulated radio coverage (red) (green and yellow marked drive sections in the red map area). For measured values, see Figure 6.

Thanks to the Telegram splitting method, mioty® can be used to transmit data very robustly with a long range and at high speeds. »The robustness of the protocol has also been confirmed during test drives on the motorway at over 120 km/h relative speed between the vehicle and the base station,« says Swissphone’s CTO Harald Pfurtscheller. A key finding for future mobile mioty® applications.

Even in zones where simulations suggest poor radio coverage, good radio links were measured during a test drive with transmitters in moving vehicles.

Further expanding mioty’s® performance

Currently, most typical LPWAN scenarios can be addressed with mioty®. In many use cases only an uplink is required (sensor/radio node can reach a base station). mioty® use cases with downlink (base station can reach sensor/radio node) are currently still limited and require a corresponding prior notification on the part of the radio node (Class A). A broad-based industry alliance is working on the further development of the ETSI specification to enable true bidirectional applications with defined latencies (Class B), in addition to further mechanisms for efficient and low-power communication in highly scalable and demanding radio applications.

They are marked in green and yellow driving sections in the red zones in Figure 5. Even in extreme areas with high distances of the vehicle to the base station and low RSSI values, signals can still be received with mioty® at high relative speeds of the vehicle to the base station (Figure 6).

Two terrain cross-sections with base station and measurement point illustrate the challenging measurement conditions (S1 and S2 in Figure 5, 7 and 8). The fact that the functionality has also been demonstrated for mobile or portable scenarios is relevant, among other things, for future use cases in the alerting and monitoring context.