Learnings

Outcomes

The project covered a wide range of activities including the desktop power system studies, market research, equipment procurement and testing, DNO engagement and recommendations. The key outcomes of the project are provided in the following four reports:                                                                                                                                        

·       Desktop analysis and technology assessment report

This report reviewed SPEN’s existing Overhead Line Protection Policy (OHPP) and assessed the performance of legacy ASLs against modern protection technologies. The work included desktop system studies, fault analysis, and market assessment of ASLs, reclosers, and sectionalising technologies from multiple manufacturers. Key challenges identified included low fault current sensitivity, wake-up current limitations, magnetising inrush, and impacts of distributed generation.

·       Laboratory Testing report on three-phase Ganged ASLs 

This report documented laboratory testing of ABB WiAutoLink, Hubbell PRS, and Inael SPIN ASLs at PNDC. Testing evaluated wake-up current and time, pickup current, reclaim time, ganged operation, magnetic inrush discrimination, and earth fault response. ABB WiAutoLink demonstrated the most reliable overall performance, particularly for earth fault operation and inrush discrimination, while issues were identified with communication reliability and ganged operation for other technologies. The report highlighted the importance of live network testing to validate manufacturer claims and identify operational limitations.

·       Laboratory Testing report on NOJA Ecolink

This report documented laboratory and HV network testing of the NOJA Ecolink configured both as a single-phase recloser and sectionaliser. The testing verified key operational characteristics including wake-up current, pickup current, reclosing operation, sectionaliser functionality, and magnetic inrush restraint performance. While the Ecolink demonstrated promising flexibility and operation on low-load networks, testing also identified unexpected maloperations associated with current measurement accuracy and incorrect tripping during network energisation. The report concluded that further firmware and hardware development would be required before wider deployment.

·       Recommendations on protection settings and UK DNO engagement

The report combined lessons learned from testing at the Power Networks Demonstration Centre (PNDC), reviewed overhead line protection practices across UK DNOs, and evaluated technologies such as Siemens Fusesaver and S&C TripSaver II to identify suitable use cases where single-phase reclosers could be deployed in place of ASLs or DOEFs to improve CI/CML performance and reduce short interruptions. A cost benefit analysis was also undertaken using a representative HV network to assess the economic viability of these applications. The study concluded that replacing two-shot ASLs with single-phase reclosers on spurs is generally not economically justified under current regulatory incentives, as the primary benefit is the reduction of short interruptions, which currently have limited financial value on their own. However, significant CI/CML benefits were identified for replacing single-shot ASLs, particularly on heavily loaded or three-phase spurs with high customer numbers, where the operational and customer reliability improvements can justify the additional investment.                                                                                                                                                                                           

Lessons Learnt

Project Methodology 

·       The project reinforced the importance of comprehensive laboratory and live-network testing in building confidence in new protection technologies and verifying manufacturer technical specifications. While manufacturer datasheets and vendor claims provided useful initial guidance, several important operational behaviours and limitations only became apparent during controlled testing at PNDC under realistic HV fault and network conditions. Testing confirmed that some devices performed in line with published specifications, while others demonstrated unexpected behaviours associated with ganged operation, wake-up functionality, communication reliability, and magnetic inrush discrimination.

·       Testing demonstrated the importance of validating manufacturer technical specifications and operational claims under realistic HV network conditions, as some unexpected behaviours only became apparent during live network testing and would not likely have been identified through datasheet review alone.

·       The project demonstrated that close engagement with manufacturers’ technical teams during laboratory testing is essential to support troubleshooting, device configuration, firmware adjustments, and interpretation of unexpected operational behaviour. Direct collaboration with manufacturers throughout the testing programme enabled timely investigation of issues. The project also highlighted that independent testing programmes provide value to manufacturers by exposing operational limitations, firmware issues, and practical deployment challenges that may not become apparent during factory testing, thereby creating opportunities for future product improvement and development aligned with DNO operational requirements.

·       The project demonstrated the value of open and collaborative engagement with other UK DNOs throughout the dissemination phase. Although arranging separate knowledge-sharing sessions with each DNO was time consuming, the one-to-one engagement approach enabled more focused technical discussions and a deeper exchange of operational experience relating to ASLs, reclosers, overhead line protection policies, and lessons learned from different technologies deployed across networks. These discussions provided valuable opportunities to benchmark SPEN findings against wider industry experience, validate project learnings, and better understand practical operational challenges encountered by other DNOs.

Technology assessments and testing

·       Significant variations exist between three-phase ganged ASL technologies and manufacturers regarding wake-up requirements, reclaim times, dead-line detection logic, communication methods, and fault counting behaviour. Market research and testing confirmed that some modern ASLs require considerably higher minimum line currents to remain operational than legacy devices currently deployed on SPEN’s network. 

·       Among all 3 phase ganged ASLs tested, ABB WiAutoLink demonstrated the most technically mature and reliable performance during laboratory testing, particularly in relation to wake-up behaviour, magnetic inrush discrimination, ganged operation capability, and earth fault response. However, considering the minimum wake up current of 5A @90 second, the minimum OHL current will remain one of the key criteria for application of this device.

·       Transformer magnetising inrush current may be a cause of potential ASL maloperation. Testing of Inael SEIN ASLs confirmed that this technology cannot reliably distinguish between transformer inrush current and genuine fault current, resulting in fault counting and unnecessary drop-out operations during circuit energisation and reclosing sequences for faults elsewhere on the network. 

·       Three-phase ASL technologies incorporating ganged operation provide operational and safety advantages by ensuring simultaneous disconnection of all phases during fault conditions. However, testing demonstrated that reliable ganged operation is highly dependent on communication reliability and all individual phase units remaining awake and operational. This demonstrated through ABB WiAutoLink testing that ganged operation may fail if one unit is discharged or not awake. 

·       Testing of the ABB WiAutoLink demonstrated generally reliable ganged operation performance, although it was observed that if one unit was not awake, the ganged operation could fail unless “Partial Trip” functionality was enabled. Testing of other technologies, including the Inael SPIN and Hubbell PRS, identified challenges associated with communication reliability, inconsistent ganged tripping behaviour, and unexpected phase operation during earth faults. 

·       Testing NOJA Ecolink demonstrated flexibility by operating as both a single-phase recloser and sectionaliser using configurable firmware functionality. Testing successfully demonstrated both reclosing and sectionalising capabilities during HV fault conditions. The Ecolink internal energy storage design, based on staged capacitor charging, highlighted the importance of internal energy management for self-powered protection devices operating on low-load networks and ability to one shut lock out if close to a faulted circuit from discharged condition. 

·       NOJA Ecolink incorporates advanced inrush restraint functionality based on both inrush multipliers and second harmonic detection, together with loss of supply detection providing potential operational advantages over traditional ASLs. 

·       At the time of testing, NOJA Ecolink firmware did not yet support three-phase ganged operation, however, NOJA confirmed that they are planning to include the gang operation only through firmware update in near future. 

·       While successful operations demonstrated, testing also identified several unexpected Ecolink maloperations during live HV network energisation. In certain cases, the device recorded unrealistically high current measurements significantly greater than measured network values, causing incorrect tripping behaviour. Instances were also observed where the Ecolink tripped after transient conditions had fully settled and stable load current remained well below the configured pickup threshold, demonstrating that further firmware and hardware development may be required before widespread deployment. After multiple meetings with NOJA, it was concluded that further investigation into current measurement and calculation may be required that may result in future firmware update.  

Operational decisions

·       ASLs remain an effective and economical solution for automatic sectionalising of permanent faults on overhead line spurs; however, their successful operation is highly dependent on their wake-up current and wake-up time characteristics because the devices are self-powered and rely on overhead current to maintain operational readiness. On lightly loaded rural spurs, particularly during overnight low-load conditions, some ASLs may enter a discharged or “sleep” state and subsequently fail to register or respond correctly to fault currents.

·       High-impedance faults and low earth fault current conditions remain major operational limitations for ASLs. On remote spurs, the upstream recloser may detect and clear faults while downstream ASLs fail to register a sufficient fault count because the fault current remains below the ASL pickup threshold. 

·       Partial phase operation of non-ganged ASLs potentially introduces operational and safety concerns, including risks associated with backfeeding faults through transformers windings.

·       Distributed generation connected downstream of ASLs introduces additional protection coordination challenges. Downstream generation may contribute fault current towards upstream faults, causing ASLs to incorrectly register fault counts and potentially operate unnecessarily during transient upstream fault events.

·       Single-phase reclosers can be alternative or complementary technology to traditional ASLs in several network applications, particularly were reducing short interruptions and improving customer reliability performance are priorities. Unlike ASLs, single-phase reclosers can clear transient faults locally without requiring upstream recloser operation, thereby reducing the number of customers affected by short interruptions. 

·       Single-phase reclosers are particularly beneficial on poorly performing overhead line circuits with high transient fault rates, heavily loaded spurs, low fault current locations. 

·       Single-phase reclosers can also provide effective protection where upstream multi-shot auto-reclosing facilities are unavailable and technically unjustified. Similar application of single-phase reclosers may be justified after technical and economical comparison with pole-mounted switches or pole mounted auto reclosers.

·       Practical operational considerations including device size, installation complexity, commissioning procedures, communication systems, firmware configuration, maintenance requirements, and field operability were shown to be critical factors when assessing suitability of ASL and recloser technologies for deployment across SPEN’s network.

·       Using IDMT protection characteristic on the first PMAR autoreclose shot could provide operational benefits by allowing enough time and fault current to alleviate the wake up downstream ASLs, particularly modern three-phase ganged ASLs, to wake up and register fault counts correctly. This approach could improve ASL coordination performance on lightly loaded rural spurs where wake-up current and wake-up time limitations were identified during PNDC testing. The use of IDMT on the first shot, potentially combined with a high-set instantaneous element for higher fault currents, would provide longer fault duration for low-current faults while still limiting fault energy for severe faults. However, the study also identified several associated risks, including increased fault clearance times, higher fault energy, greater likelihood of secondary damage to overhead line equipment, increased risk of permanent faults developing from transient events, and potential coordination challenges with upstream and downstream protection devices. Consequently, the project recommended that any future adoption of IDMT-based first-shot protection should be subject to detailed protection grading studies, assessment of network-specific fault levels, and careful evaluation of the trade-off between improved ASL operation and the increased risk of equipment damage and customer interruptions.

·       Single-phase reclosers are commercially comparable in cost to three-phase reclosers (e.g. NOJA OSM) once communications and monitoring functions are included, therefore, in a smart HV network where monitoring is required, the technico-economical choice can be three-phase recloser. 

UK DNO Dissemination and engagement

·       The engagement with UK DNOs suggested that overhead line protection policies demonstrated that there is no single standardised approach adopted across UK, with each DNO developing protection philosophies based on historical network design, spur protection strategy, operational experience, and customer performance priorities. Nonetheless, most DNOs are continuing to rely on existing single-phase ASLs with only one or two approved suppliers may be available.

·       Single-phase reclosers, Siemens Fusesaver and S&C TripSaver II, have been trialled and are now generally being deployed in a targeted manner rather than as a wholesale replacement for ASLs, mainly on spurs with high transient fault rates, long rural circuits, or high customer counts where the operational and CI/CML benefits are greatest.

·       A common practice among DNOs was to prioritise fast first-shot protection to minimise damage from transient faults, with most utilities either avoiding reclosing on SEF altogether or limiting it to a single shot for safety reasons associated with grounded conductors.

·       DNOs shared operational experience that wake-up current limitations and low fault current performance remain significant challenges for ASLs, particularly on lightly loaded rural spurs.

·       It was recognised that reducing short interruptions may become increasingly important under future regulatory incentives, making smart reclosers more attractive in future RIIO periods.

·       The discussions reinforced that close coordination among protection philosophy, autoreclose settings, fault levels, and operational practice is essential, as no universal solution or “silver bullet” application currently exists for overhead line protection.