The Role of Intelligent RTUs in Low Voltage Networks

7 May 2019

Jaguar EV Plug Into Low Voltage Networks

The UK’s electricity system is developing rapidly to support a low carbon economy. There is rising demand for electricity from electric vehicles (EVs), heating, lighting, IT network services and a growing and migrating population. In the UK our traditional distribution model is moving from one of centralised control to a decentralised one, with multiple injection points from renewable power sources and energy storage systems. At the same time, the UK power grid is ever more exposed to adverse weather conditions (created by climate change). The growing demand and availability of generating assets also means that the ability to meet unexpected demand has fallen below 5% of overall capacity during the last decade.

Electricity Network Resilience

Even so the UK’s electricity network enjoys high levels of resilience. There is a high degree of confidence in its ability to meet future demand. There are several factors for this, one of which is the use of intelligent remote terminal units (or intelligent RTUs) throughout the distribution network.  The network itself is complex and includes a mixture of generating stations, substations and other sources of power in addition to demand side response (DSR) programmes. The network includes a transmission section above 230kV (referred to as extremely high level or EHV), a primary distribution level from 230 to 35kV (referred to as high voltage or HV) and a medium voltage (MV) level that runs from 35 to 1kV. Below 1kV is referred to as secondary distribution and the low voltage or LV network.

Intelligent RTUs are programmed to hold a virtual map of a substation and provide complete monitoring and automatically controlled responses to power outages and other electricity related substation events. If there is a network fault and/or a substation issue, the RTUs are set to automatically reroute power (if it is safe to do so) to ensure downtime is limited. Not only does this benefit the end users but the electricity supply companies who are regulated by Ofgem and fined for long power outages. More info:

Callisto Intelligent Remote Terminal Units

The Callisto NX is one of the UK’s most installed remote terminal units and is used by several UK district network operators (DNOs) and overseas utilities. The RTU is very cost-effective and compliant to IEC 61850. The Callisto RTU has measurement accuracy to within 0.1%, a wealth of protection functions and two independent microprocessors with a powerful digital signal processor (DSP) to ensure consistent high performance. The RTU has built-in Ethernet and serial ports and an optional GPS mode for remote communications, as the unit will report to a master station and invariably one owned and operated by the DNO or supply company.

Intelligent RTUs like Callisto NX can be installed across a distribution network to provide substation automation and control via programmed logic, measurement and test routines. Applications can run from EHV to LV networks.  The LV network provides a 400Vac three phase supply to local substations (from which 230Vac single phase supplies are derived) and therefore provides the power sources to which we already plug-in or hard wire a range of devices and systems. The scale of connections is therefore far greater than on the primary network. As demand  and connections to the network increase so does the need for substation control and automation.

Whilst remote terminal units have been installed successfully for many years on  the primary side of the distribution network, the secondary or low voltage network is relatively new territory. Many DNOs are now looking at how best to deploy intelligent RTUs like the Callisto NX and other power electronic devices in order to maximise capacity on their low voltage networks and improve reconnection times.


Society in general is becoming more reliant on technology and there is a greater interdependency between the networks we connect with each day. Migration from rural to metropolitan areas is also increasing the economic impact of downtime whether its cause is weather related, a failure within the network infrastructure or from an exposed cyber security weakness.

Overall demand on national electricity grids will continue to increase. To keep the ‘lights on’ requires complete network monitoring and automation of the type that can only be provided by intelligent RTUs and SCADA systems. These devices will continue to develop to meet future demands. They will include ever more advanced algorithms and potentially in the future some form of artificial intelligence in place of the programmed logic.