Retrofitting the Transmission Grid with Low-cost Sensors

In Statnett, we collect large amounts of sensing data from our transmission grid. This includes both electric parameters such as power and current, and parameters more directly related to the individual components, such as temperatures, gas concentrations and so on.

Nevertheless, the state and behaviour of many of our assets are to a large extent not monitored and to some extent unobservable outside of regular maintenance and inspection rounds. We believe that more data can give us a better estimate of the health of each component, allowing for better utilization of the grid, more targeted maintenance, and reduced risk of component failure.

About a year ago, we therefore acquired a Pilot Kit from Disruptive Technologies, packed with a selection of miniature sensors that are simple to deploy and well-suited for retrofitting on existing infrastructure. We set up a small project in collaboration with Statnett R&D, where we set about testing the capabilities of this technology, and its potential value for Statnett.

Since then we’ve experimented with deploying these tiny IOT-enabled devices on a number of things, ranging from coffee machines to 420 kV power transformers.

To gauge the value and utility of these sensors in our transmission grid, we had to determine what to measure, how to measure, and how to gather and analyze the data.

This blog post summerizes what we’ve learnt so far, and evaluates some of the main use cases we’ve identified for instrumenting the transmission grid. The process is by no means finished, but we will describe the steps we have taken so far.

Small, Low-cost IoT-Sensors

Disruptive Technologies is a fairly young company whose main product is a range of low-cost, IoT-enabled sensors. Their lineup includes temperature sensors, touch sensors, proximity sensors, and more. In addition to sensors, you need one or more cloud connectors per area you are looking to instrument. The sensors transmit their signals to a cloud connector, which in turn streams the data to the cloud through mobile broadband or the local area network. Data from the sensors are encrypted, so a sensor can safely transmit through any nearby cloud connector without prior pairing or configuration.

The devices and the accompanying technology have some characteristics that make them interesting for power grid instrumentation, in particular for retrofitting sensors to already existing infrastructure:

  • Cost: The low price of sensors makes experimental or redundant installation a low-risk endeavour.
  • Size: Each sensor is about the size of a coin, including battery and the wireless communication layer.
  • Simplicity: Each sensor will automatically connect to any nearby cloud connector, so installation and configuration amounts to simply sticking the sensor onto the surface you want to monitor, and plugging the cloud connector into a power outlet.
  • Battery life: expected lifetime for the integrated battery is 15 years with 100 sensor readings per day.
  • Security: Data is transmitted with full end-to-end encryption from sensor to Disruptive’s cloud service.
  • Open API: Data are readily available for download or streaming to an analytics platform via a REST API.

Finding Stuff to Measure

Disruptive develops several sensor types, including temperature, proximity and touch. So far we have chosen to focus primarily on the temperature sensors, as this is the area where we see the most potential value for the transmission grid. We have considered use cases in asset health monitoring, where temperature is a well-established indicator of weaknesses or incipient failure. Depending on the component being monitored, unusual heat patterns may indicate poor electrical connections, improper arc quenching in switchgear, damaged bushings, and a number of other failure modes.

Asset management and thermography experts in Statnett helped us compile an initial list of components where we expect temperature measurements to be useful:

  • Transformers and transformer components. At higher voltage levels, transformers typically have built-in sensors for oil temperature, and modern transformers also tend to monitor winding and hotspot temperatures. Measurement on sub-components such as bushings and fans may however prove to be very valuable.
  • Ciruit breakers. Ageing GIS facilities are of particular importance both due their importance in the grid, and to the risk of environmental consequences in case of SF6 leakage. Other switchgear may also be of interest, since intermittent heat development during breaker operation will most likely not be uncovered by traditional thermography.
  • Disconnectors. Disconnectors (isolator switches) come in a number of flavors, and we often see heat development in joints and connection points. However, we know from thermography that hotspots are often very local, and it may be hard to predict in advance where on the disconnector the sensor should be placed.
  • Voltage and current transformers. Thermographic imaging has shown heat development in several of our instrument transformers. Continuous monitoring of temperature would enable us to better track this development and understand the relationship between power load, air temperature and transformer heating.
  • Capacitor banks. Thermography often reveals heat development at one or more capacitor in capacitor banks. However, it would require a very large number of sensors required to fully monitor all potential weak spots of a capacitor bank.

A typical use cases for the proximity sensors in the power system is open door or window alarms. Transmission level substations are typically equipped with alarms and video surveillance, but it might be relevant for other types of equipment in the field, or at lower voltage levels.

The touch sensors may for instance be used to confirm operator presence at regular inspection or maintenance intervals. Timestamping and georeferencing as part of an integrated inspection reporting application is a more likely approach for us, so we have not pursued this further.

Deployment on Transformer

Our first pilot deployment (not counting the coffee machine) was on three transformers and one reactor in a 420 kV substation. The sensors were deployed in winter when all components were energized, so we could only access the lower part of the main transformer and reactor bodies. This was acceptable, since the primary intention of the deployment was to gain experience with the process and hopefully avoid a few pitfalls in the future.

Moreover, the built-in temperature sensors in these components gave us a chance to compare readings from Disruptive sensors with the “true” inside temperature, giving us an impression of both the reliability of readings from Disruptive sensors and the ability to estimate oil temperature based on measurements on the outside of the transformer housing. We also experimented with different types of insulation covering the sensor, in order to gauge the effect of air temperature variations on sensor readings.

Deployment in Indoor Substation

Following the initial placement on the transformer bodies, we instrumented an indoor GIS facility, where we deployed sensors on both circuit breakers and disconnectors; plus one additional sensor to measure ambient temperature in the room. Since the facility is indoors and all energized components are fully insulated, this deployment was fairly straightforward. Our main challenge was that the cloud connector had a hard time connecting finding a cellular signal, but with a bit of fiddling we eventually found a few locations in the room with sufficient signal strength.

Concrete buildings can make it hard to find a good signal for mobile broadband. In this case we raised the cloud connector to the ceiling in search of a signal.

Deployment on Air Insulated Breakers

Finally, we took advantage of a planned disconnection of one of the busbars at a 300 kV facilty to instrument all poles of an outdoor SF6 circuit breaker. As mentioned above, disconnectors and instrument transformers were other instrument transformers. However, due to the layout of the substation, these were still energized so the circuit breakers were the only components we could gain access to.

Apart from monitoring the breakers, this deployment enabled us to test how the sensors reacted to being placed directly on uninsulated high-voltage equipment, and to check for any negative side-effects such as corona discharges.

Developing a Microservice for Data Ingestion

The sensors from Disruptive Technologies work by transmitting data from the sensor, via one or more cloud connectors, to Disruptive’s cloud software solution. The data are encrypted end-to-end, so the sensors may use any reachable cloud connector to transmit data.

As a precautionary measure, we opted to maintain an in-house mapping between sensor device ID and placement in the grid. This way, there is nothing outside Statnett’s systems to identify where the sensor data are measured.

Disruptive provides various REST APIs for streaming and downloading data from their cloud solution. For internal technical reasons, we chose to use a “pull” architecture, where we download new sensory readings every minute and pass them on to our internal data platform. We therefore developed a microservice that:

  1. Pulls data from Disruptive’s web service at regular intervals.
  2. Enriches the data with information about which component the sensor is placed on and how it is positioned.
  3. Produces each sensor reading as a message to our internal Kafka cluster.

From Kafka, the data are consumed and stored in a TimescaleDB database. Finally, we display and analyze the data using a combination of Grafana and custom-built dashboards.

The microservice runs on our internal Openshift Container Platform (PaaS).

We wrote a custom adapter that ingests data from Disruptive’s web services and produces messages on our internal Kafka cluster. From here, the data flows to Timescale and dashboards. The data are anonymous on Disruptive’s web service, so the adapter also adds contextual information to the messages.

The Value of Data

Do these newly acquired data help us take better decisions in the operation of the grid, and hence operate the grid in a smarter, safer, and more cost-effective way? This is really the litmus test for the value of retrofitting sensors and gathering more data about the components in the grid.

In this pilot project, we consulted field personnel and component experts regularly for advice on where and how to place sensors. However, it was the FRIDA project, a large cross-disciplinary digitalization project at Statnett, that really enabled and inspired the relevant switchgear expert to analyze the data we had collected in more detail.

Once he looked at the data, he discovered heat generation in one of the breakers, with temperatures that significantly exceeded what would be expected under normal operation. A thermographic imaging inspection was immediately ordered, which confirmed the readings from the Disruptive sensors.

The temperature sensors made us aware of high temperatures in one of the breakers, indicating a possible incipient failure of a critical component. As a result, the control centre changed the operating pattern on the substation and planned for mainentance of the unhealthy component. The figure shows the temperature on each phase of the breaker, with busbar A in the top panel and busbar B in the bottom one. The room temperature is shown as a dotted orange line.

Based on the available data, the breaker and thermography experts concluded that the breaker, altough apparently operating normally, shows signs of weakness and possibly incipient failure. This in turn lead to new parts being ordered and maintenance work planned for the near future.

While waiting for the necessary maintenance work to be performed, the operation of the substation has been adapted to reduce the stress on the weakened equipment. Until maintenance is performed, the limits for maximum amount of power flowing through the switchgear are now updated on a regular (and frequent) basis, based on the latest temperature readings from our sensors. Having access to live component state monitoring has also made the control centre able to make other changes to the operating pattern on the substation.

The control centre now continuously monitors the heat development in the substation, using the sensors from this pilot project. The new data has thus not only helped discovered an incipient failure in a critical component, it also allows us to keep operating the substation in a safe and controlled way while we are waiting for an opportunity to repair or replace the troublesome components.

Lessons Learned in the Field

Under optimal conditions, the wireless range, i.e. the maximum distance between sensors and cloud connectors, is 1000 meters with line of sight. Indoor, signals are reliably transmitted over ranges of 20+ meters in normal mode when the conditions are favorable. A weak signal will make the sensor transmit in “Boost mode”, and this quickly drains the battery.

High-voltage power transformer are huge oil-filled steel constructions, often surrounded by thick concrete blast walls. We quickly learned that these are not the best conditions for low-power wireless signal transfer. When attaching the sensors to the main body of the transformer, we observed that the communication distance was reduced to less than 10 meters and required line-of-sight between the sensors and the cloud connector.

One reason for the short transmission distance is that the metal body of the transformer absorbs most of the RF signal from the sensor. Disruptive therefore adviced us to use a bracket so that the sensors could be mounted at a 90 degree angle to the surface when mounting the sensors on large metal bodies. We used Lego blocks for this purpose. Disruptive have since developed a number of range extenders that are arguably better-looking.

Lego bracket vs. surface range extender. If you roll your own, keep in mind where on the sensor the actual temperature measurement is made, and rotate it accordingly. On the sensors in our Pilot Kit, this was the upper right corner of the device (not in the corner where the small orange dot is located).

Although we did experience an improvement in signal transmission range when mounting the sensors at an angle to the transformer body, sending signals around the corner of the transformer still turned out to be very challenging.

Disruptive have yet to develop a ruggedized cloud connector. The need to position the cloud connector such that line of sight was maintained, limited our ability to place the cloud connector inside existing shelters such as fuse cabinets. We therefore developed a weather-proof housing for outdoor use, so we could position the cloud connectors for optimal transmission conditions.

A weather-proof housing allowed us to position the cloud connectors for optimal transmission conditions.

All sensors have their device ID printed on the device itself. However, the text is tiny and the ID is long and complex. We opted to put a short label on each sensor using a magic marker in order to simplify the deployment process in the field. This simplistic approach was satisfactory for our pilot project, and required minimal support system development, but obviously does not scale to larger deployments.

As mentioned above, we have deployed a number of sensors directly on energized, unisolated, 300 kV components. We were quite curious to see how the sensors would cope with being mounted directly on high-voltage equipment. So far, the measurements from the sensors seem to be unaffected by the voltage. However, we have lost about 25 % of these sensors in less than a year due to high battery drainage. We suspect that this may be related to the environment in which they operate, but it may also be bad luck or related to the fact that our sensors are part of a pre-production pilot kit.

Finally, sticking coin-sized sensors onto metal surfaces is easy in the summer. It’s not always equally easy on rainy days or in winter, with cold fingers and icy or snow-covered components.

Other Things we Learned Along the Way

So far, our impression is the the data quality is good. The sensor readings are precise, and the sensors are mostly reliable. The snapshot from Grafana gives an impression of the data quality: as can be seen, there is very good correspondence between the temperature readings from the three different phases on busbar A after switching, when it has been disconnected.

However, both the cloud connectors and the SaaS-based architecture are weak spots where redundancy is limited. If a cloud connector fails, we risk loosing data from all the sensors communicating with that connector. This can to some extent be alleviated by using more cloud connectors. The SaaS architecture is more challenging: downtime on Disruptive’s servers sometimes affects the entire data flow from all their sensors.

Deployment is super easy, but an automated link to our ERP system would ease installation further, and significantly reduce the risk of human error when mapping sensors to assets.

The sensors are very small. This is cool, but if they were 10x larger with 10x stronger wireless signal, this would probably be better for many of our use cases.

Finally, communication can be challenging when dealing with large metal and concrete constructions. This goes for both the communication between sensor and cloud connector, and for the link between the cloud connector and the outside world. This is in most cases solvable, but may require some additional effort during installation.

Next Step: Monitor All Ageing GIS Facilities?

This pilot project has demonstrated that increased component monitoring can have a high value in the transmission grid.

Our main focus has been on temperature monitoring to assess component health, but the project has spurred quite a lot of enthusiasm in Statnett, and a number of other application areas have been suggested to us during the project.

One of the prime candidates for further rollout is to increase the monitoring of GIS substations, either by adding sensors to other parts of the facility, by selecting other substations for instrumentation, or both.

A further deployment of sensors must be aligned with other ongoing activities at Statnett, such as rollout of improved wireless communication at the substations. Nonetheless, we have learned that there are many valuable use cases for retrofitting sensors to the transmission grid, and we expect to take advantage of this kind of technology in the years to come.

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