TransGrid – Distributed Acoustic Sensing

1. Executive Summary

TransGrid manages a high voltage power transmission network consisting of 112 substations, over 13,000 km of transmission lines, and 84 km of underground cables.  External interference is a major threat to the continuity of high voltage cable installations and to alleviate this threat TransGrid has installed Distributed Acoustic Sensing (DAS) on an underground cable.  Cable corridor monitoring using DAS provides 24×7 monitoring of the total length of the cable with that helps identify unauthorised adjacent activity.

This innovative system has achieved benefits for its stakeholders, including stakeholders and the public, by reducing risks to electricity supply and public safety while at the same time reducing the costs.

2. Description of Project

2.1 TransGrid’s Journey in Distributed Acoustic Sensing

Our Cable Network Risk and DBYD Controls

The introduction of DAS has created significant benefit to all its stakeholders. It has enabled the reduction of route patrols by monitoring the entire asset continuously whilst reducing network management costs.

TransGrid’s high voltage underground power cable network is 84 km long, predominately running through suburban streets that external parties often have to cross when augmenting or replacing their own infrastructure.  These external works present a risk to TransGrid’s assets with studies showing that third party damage is three to five times higher risk than that of cable failures.  Insufficient information exchange between cable operators and construction companies is a factor in approximately 40% of mechanical works failures[1].

A cable strike can cause injury or death of the plant operator and result in major blackouts to commercial hubs such as the Sydney CBD.  The repairs involve excavating public areas causing significant disruption to the community.  It can take 6-8 weeks and cost millions of dollars to repair 330 kV cables, with cost recovery not guaranteed and legal processes taking multiple years and significant resources.

Current Controls

The state governments in Australia attempt to address this risk by requiring power utilities to be members of ‘Dial Before You Dig’ (DBYD), a referral service for notification of the presence of underground assets to third parties.  A construction company will put in an enquiry over an area of interest to DBYD that will then notify the members who have assets in that area.  The utility will respond to the construction company with their asset locations and requirements for working around those assets.  Despite provision of this information, and utilities offering spotters at nil or nominal cost, some construction companies choose not to utilise these services or ignore the specified requirements, thereby running the risk of making contact with the asset.

Figure 1 – Typical Urban Location of Cable

The traditional method of intercepting these rogue operators is to conduct patrols of the cable route.  TransGrid conducted patrols of major cables on a six days a week basis (Sundays and public holidays excluded), patrols would normally start after 10am to align when road occupancy licences were granted.  These inspections are a significant cost to the network and efficiencies are always being investigated.  For example, as there is no significant differences in work volumes of construction happening on a Tuesday or Wednesday, conducting patrols less frequently can be undertaken, but this risks missing an unauthorised excavation.  Due to the high consequence, the savings from reduced patrols may not be proportionate to the risk introduced and therefore lead to a breach of the Electrical Safety Act requirement to reduce risk As Low as Reasonably Practicable (ALARP).

A Need for Innovation

To achieve cost efficiency by reducing patrols whilst maintaining vigilance, TransGrid has engaged a DAS provider to provide monitoring on a critical 330 kV cable supplying the Sydney CBD.  DAS systems are able to detect activity that is in close proximity to the cable, providing early warning of impending threats.  This capability has enabled the TransGrid to reduce security patrols whilst providing the benefit of 24/7 monitoring of the entire cable length.

The DAS system uses Rayleigh backscattering to measure strain on the fibre caused by vibration from objects in motion near the cable.  This technique involves rapid pulsing of a laser down the length of a dark fibre core and measuring the strain on the fibre caused by external forces; this poling can occur thousands of times a second.

From the scattering, the DAS system can identify the signature and accurately localise the event, which assists in determining the best course of action to be taken to reduce the chance of a strike or partial strike.  The DAS system TransGrid has deployed to monitor and protect its 330 kV power cable has been set up and calibrated to operate in a high noise urban environment, delivering low rates of false positives.

A single dark fibre can protect multiple assets in the vicinity.  Due to the high-speed protection and communications requirements of circuits at transmission level, most modern cable installations will have fibres included in the installation.  An interrogator unit can monitor up to 50 km of asset, depending on the optical loss of the fibre.

Figure 2 – DAS using Rayleigh backscatter

The Process

The DAS service employed uses the following process:

Event activity in the form of alerts and alarms are displayed in the DAS client portal in real-time.  A screenshot of the portal is shown in Figure 3.  The portal provides a view of the monitored assets overlaid with the ‘areas of interest’ (from DBYD enquiries).  This allows matching of detected activity with any known third party locations.  If these companies contact the utility to discuss the works, the system will in a further upgrade to be performed allow the results of the discussion can be included on the area of interest in the portal, e.g. ‘spotter required, night works, booked 14-15th May’ or ‘Surface resheeting only, no risk to asset’.  This would provide useful information to the operator in the event of an alarm occurring over an area of interest.

Figure 3 – Typical DAS System Screen Shot

2.2 Benefits of Solution

Figure 4 visualises the changes to network risk and operating expenditure for a Business as Usual (BAU) case and with the implementation of the DAS system.

Figure 4 – Risk Cost of Unauthorised Excavation Detection and Cost of Mitigation Measures (Normalised)

Improved Public Safety and Environmental Management:

TransGrid’s safety obligations are stated in the Work Health and Safety Act 2011, Electricity Supply Act 1995 and the Electricity Supply (Safety and Network Management) Regulation 2014 (Regulation 2014).  We demonstrate compliance to our safety obligations by designing our Health and Safety and Electricity Network Safety Management Systems, to demonstrate achievement of the below safety objectives are controlled ALARP or SFARP as required by the Acts.

TransGrid conducts patrols as these have been determined to be reasonably practical to do.  Given the risk costs, removing routine patrols without substituting with another control measure, such as DAS, would go against TransGrid’s ALARP principles.

It is noted that cables are oil filled and a puncture of the aluminium sheath will result in oil leakage.  This oil supply must be kept up until the cable can be appropriately choked, so the consequence is not limited to the electrical capacity. 

Operational Expenditure and Resource Efficiency:

The provision of DAS has allowed the reduction of cable route patrols by 66%, from six to two days a week, net OPEX costs are unchanged.  This is due to an older cable, without accompanying fibre, following a similar route still requiring patrols at the original frequency but this will reduce in the future. 

The provision of DAS on this circuit has allowed the reduction of cable route patrols freeing up resources to be deployed elsewhere in the business.

Improved fault finding:

During a fault, with DAS the noise and discharges are detected the same way that construction can be detected; pinpointing the fault location to within 5 metres is possible for a low resistance fault.  Thankfully, we have not yet needed to use the fault locating functionality of DAS as there have been no insulation failures on their EHV underground network for over 20 years.

Many fault location techniques for primary faults require signals be applied to the primary conductor.  If both ends of the conductor are terminated into Gas Insulated Switchgear (GIS), then significant works are required in order to provide access.  Concerns exist that the GIS components may have a limited number of mating cycles and when coupled with the very real possibility of introducing contaminates while accessing the GIS, a large incentive exists to find the fault location by other means if possible.

3. Program and Project Management

3.1 Project Management

The delivery of the DAS was first implemented on a trial basis.  At that period of time the DAS provider was an early start-up and systems were not entirely developed and the DAS provider leased a dark fibre on the communication cable installed with the high voltage cable.  Over the trial period, TransGrid asset management worked with the DAS provider to ensure that the project would meet the requirements to build a sound business case meeting TransGrid’s asset management objectives.

After successful completion of the trial, approval was obtained to implement the system on a permanent basis.  This was initiated within existing operational expenditure budgets.  The justification of the project was based on the savings of reducing risk whilst keeping inspection works cost neutral.  The decrease in routine patrol costs would be taken up in the DAS service and interdictions required as a result of the DAS alerts.

3.2 Stakeholder Engagement

After successful completion of the trial, various stakeholders were engaged:

Asset Manager:

The strategist developed a sound business case to the Head of Asset Management to approve full implementation of the system.  The progression from trial to live project was approved by demonstrating no net change to the operational budget, improved risk management and the requirement to have a process in place before the future Powering Sydney’s Future (PSF) cable was commissioned.  Having PSF without a functional DAS would require excessive route patrols.

Works Delivery

A before go-live meeting was held with the Asset Monitoring Centre (AMC) and Works Delivery.

Works Delivery are currently responsible for performing the inspection and maintenance specified in the Maintenance Plans including the route patrols.  It was important to have their feedback as the implementation of the DAS system as their regular work volumes would decrease but unplanned volumes would increase.  Handling of out of hours callouts and the impact of works the next day were discussed and the resourcing of non-regular tasks.  Fatigue provisions could result in the planned work next day being short staffed (and then cancelled) if someone was called out overnight.  An interdiction during the day could divert resources on works already in progress.

Asset Monitoring Centre (AMC)

TransGrid needs to be able to respond to alerts from the DAS system at all hours.  The AMC operate adjacent to our control room and monitor asset conditions that may cause reliability of safety issues in the near term (within three months).  They work during the day and whilst there is an on-call roster, disturbing them unnecessarily overnight will affect day operations.  TransGrid System Operators control room is staffed 24×7.  During the day they can have very high workloads whist they manage maintenance switching.  Phone calls and emails from the DAS provider could go unnoticed.  It was decided that the AMC would manage the alerts and interdictions from 7am to 7pm, whilst the System Operators would handle any alerts received overnight.

3.3 What’s next for Distributed Acoustic Sensing

TransGrid’s flagship ‘Powering Sydney’s Future project is due to be commissioned in FY22.  This is a 20 km installation installed through city streets with DAS to be included as part of its construction and operation.

4. Summary

DAS is delivering significant benefits in line with the TransGrid’s corporate themes and its asset management objectives.  The rollout of DAS is a further step in TransGrid’s asset management journey by delivering against its objectives.  The following table shows the corporate and asset management objectives that are being met by being a successful project.

Table 1 – Alignment to Asset Management Objectives

Corporate themeAsset Management Objective
Safe/reliable powerManage Network Safety Risk
Create an efficient high performing networkManage assets efficiently without compromising security holder and consumer value.
Seek new innovations and technologiesLeverage Asset Management to support new technologies and innovations that improve or grow our business

[1] Electra No.247-398

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