Eltham-Sydenham

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LPN High Value Project Eltham-Sydenham

132kV Eltham – Sydenham Gas Cables Faisal Khanzada

Justification Paper

Document History

Version Date Details Prepared by

1.0 20/06/13 Justification document Faisal Khanzada

1.1 24/06/2013 Updated Paul Elliott

1.2 24/06/2013 Updated Paul Elliott

1.3 24/06/2013 Final drafting Barry Hatton

Contents 1.0 Business Objective ...... 4 2.0 Background ...... 4 3.0 Fault History ...... 5 4.0 Investment Drivers ...... 5 4.1 Gas cable and known issues ...... 5 4.2 Repair times and network security ...... 6 4.3 Availability of spares and repair costs ...... 6 5.0 Investement Options...... 6 5.1 Recommended Option ...... 6 5.2 Rejected Options ...... 7

1.0 Business Objective

The aim of this project is to minimise the risk of failure of 132kV gas cables and consequential risks to network security if they were to fail.

Drivers- Safety, Network security, Public security Benefits- Network security, Resilience, Reinforcement of current assets.

2.0 Background

The Eltham-Sydenham circuits are part of South London 132kV ring. The circuits are 132kV gas cables running between Eltham Grid to Sydenham substation. The cables are three core 0.6 sq. inch, installed in 1958. Each circuit is approximately 9.5 km in length.

The circuits have a protracted history of gas leakage and in the last few years significant leaks have occurred on these circuits requiring extended outages to effect repair. In one particular event in 2010, the leak was so significant that all gas bottles pressurising the circuit required replacement twice every week. On several occasions even with continuous pumping of gas it has been extremely difficult to hold positive pressure on the cable. Failure to achieve continual positive gas pressure to the cable results in a breakdown of electrical integrity and an electrical fault. This also results in the ingress of water and once this has taken place the extent of the repair or replacement is significant.

Since 2010 the circuits are inspected twice a week in order to ensure positive gas pressure to seek to avoid urgent outages and faults. Feedback from operational staff has also highlighted that the gas pressure monitoring equipment has also failed or provided incorrect information on several occasions which also drives the requirements for very frequent inspections.

The majority of this cable route runs alongside a red route and hence access is severely constrained or even impossible in some circumstances. Any inspection, maintenance or repairs therefore presents significant logistical challenges.

Figure 1 Single Line Diagram of 132kV Eltham-Sydenham Circuits

3.0 Fault History

Table 1 shows the history of 132kV Eltham- Sydenham gas circuits switched out due to gas leak.

Circuit Description 2002 2003 2004 2005 2007 2008 2009 2010 2011 2012 Total ELTHAM 132kV - 1 1 1 2 1 1 3 2 2 14 SYDENHAM NO1 ELTHAM 132kV - 1 1 1 1 1 5 SYDENHAM NO2 Total 1 1 2 3 2 2 1 3 2 2 19

Table 1 Fault History

Circuit Description 2002 2003 2004 2005 2007 2008 2009 2010 2011 2012 Total ELTHAM 132kV - 27 37 8 44 22 27 60 16 72 313 SYDENHAM NO1 ELTHAM 132kV - 1 12 6 13 1 33 SYDENHAM NO2 Total 27 37 9 56 28 40 1 60 16 72 346

Table 2 Outage Duration History

Table 2 shows that on average two leaks every year requiring fault repair have occurred on the circuits. In total the loss of availability of at least one of the circuits is 346 days (Table 2). As stated in Section 2, such is the concern over the performance of the circuits and the associated monitoring equipment that since 2010 the system requires manual intervention twice a week.

4.0 Investment Drivers

4.1 Gas cable and known issues

Gas insulated cables are widely accepted in the electricity industry to carry significant operating costs and risk due to the inherent design flaws that are known to exist. The design flaws associated with gas cables include gas leaks, cable faults, control tape fractures, lead sheath distension and thermal instability of the insulating papers.

In particular, the issue of a double circuit outage is a significant concern, with both circuits feeding a primary substation of a similar gas cable construction.

The failure mode most commonly encountered is the migration of cable impregnate. If the cable load is increased, even within its rating, the accumulated impregnate expands, causing mechanical damage to the cable construction or joints. It is widely accepted within the

It is UK Power Networks policy to withdraw all gas cables from service by the end of the ED1 period.

4.2 Repair times and network security

In comparison to other cable types, the repair of a gas cable takes a considerable amount of time. A typical repair time could easily be 16 weeks due to the length of time taken to de-gas the cable, locate the fault, obtain spares, effect the repair and depressurise the system.

During this time, network security is severely compromised, because often the remaining healthy circuit is of an identical construction. Gas cables are prone to failure when the load on them is suddenly increased, as is the case for a single circuit outage. That means the second gas cable circuit is most likely to develop a fault when the first circuit is out of service. In terms of the Eltham – Sydenham circuits there are 15,000 customers at risk for a second circuit outage.

UK Power Networks had first-hand experience phenomena of this during the replacement of the Laleham – Twickenham gas cable circuits in the SPN area. One circuit had been switched out for decommissioning when the now more heavily loaded second circuit faulted as a result of the additional load. As UK Power Networks were aware of this potential failure mode, additional reinforcement works had been carried out prior to this work taking place to mitigate this possibility.

4.3 Availability of spares and repair costs

Spare parts for the gas pressurisation equipment, sealing ends and joints are now very difficult and expensive to source due to the lack of support offered by current cable manufacturers. There is also lack of suitable transition joints for use with modern XLPE cables. The typical cost for a gas cable repair is £200-£250k with a significant proportion of this cost being the excavation to locate and then fix the fault.

The expertise to carry out the necessary jointing work is also very difficult to obtain. UK Power Networks has been forced on at least one occasion to call on the services of elderly, long retired jointers to provide the necessary technical advice.

5.0 Investment Options

5.1 Recommended Option The recommended option is to replace both gas circuits with XLPE cable. As detailed in “Asset Stewardship Report for Underground cables”, it is UK Power Networks strategy to replace all gas filled cables before the end of ED1. Replacement of gas filled cables have been phased throughout ED1 in order to make the programme deliverable in a practical sense. This particular circuit has been phased to fit with other asset replacement work taking

UK Power Networks (Operations) Limited. Registered in England and Wales. Registered No. 3870728. Registered Office: Newington House, 237 Southwark Bridge Road, London, SE1 6NP 6 132kV Eltham-Sydenham Gas Cables Working draft v0.1 Justification Paper Version 1.3 place in the vicinity at the same time. It is the largest gas cable replacement project in the LPN plan and has been phased mid period. This allows the necessary time for the negotiation of the consents necessary to carry out the work and to allow assessment of risks and potential contingencies to pick up load if the second circuit failed during the replacement works.

The forecast cost profile in the ED1 period for this option is shown in the table below in £k.

2015 2016 2017 2018 2019 2020 2021 2022 /2016 /2017 /2018 /2019 /2020 /2021 /2022 /2023 0 0 0 0 5,649 7,531 7,531 5,649

5.2 Rejected Options Do Nothing

The known failure mechanisms with this type of installation present significant network risk, particularly the outage duration following a fault (up to 16 weeks), the inability to obtain spares and jointers capable of repairing the cable and the fact that the backup circuit is usually of the same design. The cost of repair is usually between £200-250k.

As already discussed in this paper, following a fault the backup circuit is usually required to carry twice the load. This causes heating and expansion of the cable and increases its likelihood to fault just when it is needed to operate reliably.

A do nothing approach puts UK Power Networks at odds with industry best practice and exposes customers to unplanned outages running into months due the difficulty and complexity of the repair of this cable. Once de-energised to repair a fault this cable also has shown to deteriorate rapidly due to the change in load and gas pressure potentially leading to further faults. This option is therefore rejected.

Replacement of a faulty cable section with XLPE

The replacement of specific sections of gas cable with more modern XLPE is not a viable option because:

The issues associated with these circuits are not limited to any one particular cable section.

Replacing a cable section may require identifying a new secure re-gassing point and joint location to enable a trifurcating transition joint of approximately 10 x 30m area.

The re-pressurised cable can fail after pressure test and commissioning.

Purchase of land for new re-gassing point may involve issues due to planning permission which may result long time to purchase a land.

Although above mentioned challenges can be addressed it requires considerable cost and time and will increase the network risk in the event of cable failure. This option is therefore rejected.

Replacement of gas cable with cable of similar design

The replacement of the gas cable with a cable of similar design is not possible because the cable is no longer available. This option is therefore rejected.

UK Power Networks (Operations) Limited. Registered in England and Wales. Registered No. 3870728. Registered Office: Newington House, 237 Southwark Bridge Road, London, SE1 6NP 8