The customer
Elvia is Norway's largest operator supplying power to over 1.8 million people in Akershus, Inland, Oslo and Østfold and has roughly 900 000 customers. Norwegian society expects an uninterrupted supply of energy at all times and it is Elvia’s social responsibility to ensure safe and reliable supply of energy. In 2018, Elvia achieved security of supply equivalent to 99.98 per cent, taking into account both planned and unplanned events.
The challenge
Nevertheless, they are interested in anything that could help reduce cable-related outage time. Elvia decided to start an R&D project to learn more about online quality monitoring of power cables. Monitoring live cables on partial discharge (PD; PD- activity is a good indicator of weak spots in underground cable networks) as well as intermittent and full fault activity can help them preventing outages or shortening the outage time significantly.
Smart Cable Guard
Hafslund learned about DNV’s Smart Cable Guard (SCG) system and started their R&D pilot with SCG early 2018. The goal of the project was to test the SCG technology as a tool to monitor the ageing cable network in Oslo and ultimately reducing SAIDI even further. In this project 6 feeders are monitored with 6 Smart Cable Guard systems where each system consists of two sensor injector units (SIU) and two control units (CU), one of each placed at each cable end.
Example of a 1 system on 1 circuit visualized in the SCG web interface as a single line drawing
The 6 SCG systems monitor a total of approximately 18 km mixed cable (paper insulated cables and cross-linked polyethylene cables) consisting of 31 sections (cable sections between ring main units) with mixed cable types.
Findings
Outage 1: stone penetrating a cable
On 18 September 2018, SCG detected a fault on one of the monitored feeders. Elvia received an SMS and email with the information about the fault straight after the fault was detected by SCG.
Although the system did not pick up any partial discharge (PD) activity before the outage, the fault feature of SCG did its work and indicated the fault and spot in the cable section.
SCG website information details on fault
Fault location indicated on single line diagram
What happened:
A stone punctuated the outer protection layer of the cable. Most probably a high-ohmic fault between phase and grounding was created. The position was 384 m from the last station. SCG positioned the fault in a range of 50 m from the actual fault location. Given the total length of the monitored segment of 4542 m, SCG was 1.1% accurate. “Not bad!”, was the customer reaction. There were many joints close by the spot detected by SCG making it more likely that a joint failed. The SCG operator who was available in this R&D project, analyzed the information and pointed to a joint only two meters away from the fault point.
Dispite to the R&D character, Hafslund’s normal procedures were followed to find the outage and the SCG information was compared with the actual findings only afterwards. This and the low degree of automation (e.g. remote-controlled circuit breakers) in this area of the grid, resulted in an outage time of 1.5 hours before all customers were connected again. If the information from SCG had been taken into account, the outage could have been shortened to about 45 minutes.
The SCG self-test was unintentionally tested after this repair. Normally, when a feeder is energized after a repair, SCG will restart automatically which it did not. The SCG operator contacted Hafslund and after a onsite investigation it turned out that during the repair works the power was disconnected from the SCG sensor in that station, After connecting the sensor to power, the system went live and is monitoring the circuit since.
Outage 2- Excavation work
In August 2019 the same SCG system detected another fault in the cable. The following fault information was sent by email within five minutes after the event happened to Elvia:
Email send to Elvia with fault information
Screenshot of the SCG web interface showing thefault on the single line diagram
In this case the outage was created by excavation works on that site that damaged the cable. Although the system did not pick up any PD activity before the outage, the fault feature of SCG did its work and indicated the fault and spot in the cable section.