B.5.4 8th International Conference on Insulated Power Cables B.5.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

CHALLENGES AT THE PLANNING, DEVELOPMENT AND PERFORMANCE AT THE 275KV XLPE CABLE PROJECT IN THE CITY OF LIVERPOOL

Sebastian EBERT, Horst MEMMER, Andreas WEINLEIN, Gero SCHRÖDER, Südkabel GmbH, Mannheim (Germany), sebastian.ebert@suedkabel.com, horst.memmer@suedkabel.com, andreas.weinlein@suedkabel.com, gero.schroeder@suedkabel.com

ABSTRACT

A 275 kV extra high voltage cable system has been designed, supplied, installed, commissioned and put in operation between the substations Kirkby and Lister Drive in Liverpool for National Grid, UK. This project was demonstrating the successful performance of almost every implication in an EHV direct buried cable project within big city metropolis.

1. DESCRIPTION OF XLPE CABLE SYSTEM

A 275 kV extra high voltage cable, type 2XS(F)K2Y 1x1600 RMS/190 160/275 kV system has been designed, supplied, installed, commissioned and put in operation between the substations Kirkby and Lister Drive in Liverpool for National Grid, UK. The cable route consisting of 15 single sections and 14 joint bays. The cable is rated for a load of 1.600 A respectively 762 MVA. The cable arrangement is seen in figure 2 and 3. The cable is laid in CBS (Cement Bound Sand) with a thermal resistivity of 1.05 Km/W (wet condition) and 1.2 Km/W (dry condition). The standard laying depth of the cables is 1200 mm and the phase distance center to center is 400 mm.

Fig. 1: Cable ducted laying condition

Fig. 2: Cable trench laying condition

The cable manufacturer has recommended the mixture of the CBS to reach the required thermal resistivity values. The civil work company as part of the National Grid Alliance West has installed the CBS under the cable manufacturer l supervision. The cable route followed the main streets across the city of Liverpool Many other services, cables, gas pipes, sewers has to be crossed or laid in parallel to the 275 kV cables. For all these laying conditions rating calculations have to be performed by the cable manufacturer. For some sections directional drilling with laying depths of up to 8 m (where necessary) and phase distances has to be increased up to 2 m in order to reach the required load. The cable route has a total length of approx. 10.3 km, which means a total length of approx. 31 km XLPE cable quantity with a conductor size of 1600 mm² (figure 3). The conductor was designed in segmental construction consisting of oxidized wires to reduce the skin effect in order to increase the transmission capacity [9]. The cable shows an insulation thickness of 25 mm and a composite copper screen wire / lead sheath screen for advanced mechanical strength. A HDPE outer jacket provides the corrosion protection of the cable.

Fig. 3: Cable design

In order to reduce the sheath losses to a minimum, a cross bonding system has been implemented. The link pillars, which contain also the sheath voltage limiters (SVL) are installed along the route at the joint bay positions above ground.

Fig. 4: Pedestrian link pillar

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B.5.4 8th International Conference on Insulated Power Cables B.5.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

The link pillars had to pass special internal arcing and short circuit current tests before National Grid approved the use of the link pillars in the city. The link pillar has been designed as pedestrian model (figure 4).

The cross-bonding design is shown in figure 5. Each cable screen has been crossed bonded in each link pillar. The cable conductor has been transposed in each joint bay entering area (bell mouth).

Fig. 5: Earthing design with cross bonding

Six pieces of factory pre-tested and pre-assembled 275 kV compact outdoor plug-in sealing [6] ends were installed as outdoor terminations (figures 6, 7):

Fig. 6: Design of outdoor sealing end EHFVCS 275

Fig. 7: Installed doutdoor sealing ends

The interface connection between the OSE top bolt and the busbar (earthing spigot) had to be considered as well during this project.

Additionally as part of the 275 kV cable system design both, PD monitoring and a DTS monitoring system via fiber optic cables have been installed and commissioned.

For all activities on site method statements and risk assessments had to be issued for approval prior to the work commence.

Furthermore the cable manufacturer has developed a special earthing design concept for each individual joint bay in order to achieve a required maximum earthing resistance of 5 Ω, which has been measured at each joint bay after installation before and after backfilling with CBS. The size of each earthing system and the numbers of used earthing rods with surrounding conductive material had to be designed individually at each joint bay depend on the thermal resistivity of the ground material (figure 14).

The joint type is three piece composite design, known from [3, 4, 5] which passed both, additional mechanical load tests and water immersion tests of the corrosion protection system according NGTS standards.

Fig. 8: Composite type joint VMEVCB 275

Fig. 9: Joint bay

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B.5.4 8th International Conference on Insulated Power Cables B.5.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

In total 14 joint bays had to be designed and installed with a number of 42 cross bonding joints and associated equipment. During each installation of the EHV cable accessories hold points were implemented as an activity of quality assurance by the cable manufacturer and the National Grid Alliance West.

2. COMPLEXITY OF INSTALLATION WORKS

As the route led straight through the city of Liverpool key facts like traffic management, SHE management had to be challenged during all stages of the project, which ran through more than 2 years due to specific requirements of the city council and the highway agency not to open too many trenches in one time because Liverpool was the European capital of culture in 2008 and a lot of events took place at this time.

Figure 8 shows a drum offloading as part of one delivery within the city center of Liverpool at the main road between the main Irish ferry port of Liverpool and the motorway which is mainly used by all trailers for the heavy transport through Great Britain. Each cable drum had a weight up to 50 ton, a diameter of 4.5 m and a width of 3.7 m. Also not negligible in terms of decisions for a football town like Liverpool is to mention that roads like this are the main roads on the way to the Liverpool football ground “Anfield Road” for supporters from outside to watch their Liverpool Football Club. These effects the complexity of the necessary permanent project management with many years experiences in this business between all contractors at all times. That means in details that route survey had to be carried prior to each cable drum delivery in order to clarify details like the required street furniture, ground load tests, police escort services etc. Once all details are known permit applications had to be forwarded to the authorities for their approval. Furthermore method statements and risk assessments had to be issued for approval. Once the approval had been given to the contractors only appointed qualified persons could be allowed to be involved in the delivery process. As an escort vehicle was supervising the transport from the factory in Germany to the port and from the ship to the storage yard also the last stage between the motorway in front of Liverpool up to the designated offloading point had to be escorted by an escort car and additionally by several police cars. Qualified persons had to supervise the key points of the final delivery in order to avoid local problems up to the road closures area.

Fig. 10: 50 t cable drum offloading liverpool city within main road closure & police escort

A general cable pulling schematic drawing is shown in figure 11 which shows the position of drum stand, winch and main rollers. Ground loading tests had to be carried at each joint by prior to drum offloading.

Fig. 11: Cable pulling overview

Another main challenge were specific cable accessories installation methods in accordance with NSI 5 (working under induced voltages and isolating working) due to parallel working in different joint bays and at the sealing ends within the National Grid Substations. This caused a combination of grounded and isolated way of working.

This work had to be carried out in accordance with the National Grid specification "Guidance Note 11 MAKING OR BREAKING DOWN SEALING ENDS ON HV CABLES "of UK BP / SE / NSI 5 CABLE SYSTEMS" (NSI 5 Working Procedure). Special Permits, risk assessments and method statements had to be issued. Furthermore special trainings were required for all involved engineers, fitters and labours [8].

Fig. 12: Earthed working at the sealing ends

The cable manufacturer had to follow these guidelines as almost the whole length of cable (approx. 10.3 km) was already installed, the cable had entered the National Grid substation which means that the cable is automatically under the client rules. Due to time schedule restrictions a parallel working was unavoidable. This means that a joint installation and a sealing end installation were running in parallel. Both working groups had to be in constant communication for all working steps via radio. For an

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B.5.4 8th International Conference on Insulated Power Cables B.5.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

earthed working method at the sealing ends, the conductor and the screen had to be earthed at all times (figure 12). For an isolated working procedure, a combination of at least 2 isolation barriers between man and electrically conductive parts of the cable system or installation scaffold had to be used (figure 13). The scaffolding had to be earthed every 5 m.

Fig. 13: Isolating working at the sealing ends

This means isolated mats (suitable for 20 - 25 kV), isolated working boots (suitable for a insulation strength of 7.5 kV per shoe) had to be worn. Alternatively isolated gloves could be worn but this would not allow a high standard working quality during the jointing due to handling problems. For an isolating working insulating transformer had to be used in order to allow a galvanic isolation between the generator and the working tool.

For the isolated jointing work in the joint bay isolated mats and boots had to be worn as well as a combination of at least 2 isolation bareers between man and electrically conductive parts of the cable system or installation scaffolding.

Fig. 14: Isolating platform during jointing

For the using of chain hoists for lifting operations of joint parts like the jointing body isolated gloves (suitable for a insulation strength of 7.5 kV per glove) had to be used as the chain of the hoists are a conductive connection with the scaffolding beam on the the top. For reasons of difficult practicability in terms of manual handling lifting slings were used between chains of chain hoists in order to avoid the using of isolated gloves at some stages of the working process. Another difficulty was the connection of the cable screen with the lead sheath of the cable. Once this connection was made, an isolated tape had to be

wrapped around in order to avoid a contact with the screen and the at this moment isolated conductor at the same time with the hands of the workers.

Fig. 15: Isolating working in a joint bay for jointing of EHV 275 kV XLPE cable joints

The difficulty of the working under these circumstances was the combination of different working procedures (mixture of earthed and isolated working at the sealing ends and isolated working in the joint bay). It had to be avoided that both working teams are working on the same phase at the same time. Therefore a constant radio communication was unavoidable. If the sealing end installation team had to work on one phase, this phase had to be earthed at the conductor and the screen by the sealing end installation team.

During this work the jointing team in the joint bay was not allowed to work on this phase because they had to follow the isolating working procedure. If they had to work on one phase, the earthing connection at the sealing ends had to be removed, so they were isolated from the conductive parts of the cable system. The earthing connection at the sealing ends on the far end (other end of the cable circuit) had to be removed prior to all works commence and the cable screen at the nearest joint bay was earthed in order to reduce the cable length and bring the earthing connection closer to the working place to reduce the risk of induced voltages. The 275 kV cable system was in fact not in operation at this time but due to so many other services crossing the new installed 275 kV cable circuits and due to so many parallel services a risk of induced voltages from outside was present resulting in all the precautions to allow an absolute safe working process which is the aim of all working contractors.

This process has been designed and well planned by the cable manufacturer together with all other project contractors, which is mandatory. The design is the foundation for reducing risks for involved personnel. Therefore, the cable manufacturer endorse the safety by design concept. This system has been proofed by daily briefings, talks, constant communication which each other, issuing workable documents as risk assessments and method statements which all parties had to understand and sign prior to the work commence.

Every working step change had to be documented and signed which was controlled by the quality assurance team in order to save the health of the workers against induced voltages and to comply with the client health and safety and working rules and requirements.

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B.5.4 8th International Conference on Insulated Power Cables B.5.4

Jicable’11 – 19 – 23 June 2011, Versailles - France

All these steps were helping to reduce incidents and to perform all works on the highest quality level.

This project is demonstrating the successful management of challenges comprising of almost every implications in a EHV direct buried cable project within big city metropolis.

3. COMMISSIONING OF XLPE CABLE SYSTEM

After successful installation a wide number of commissioning tests had to be carried out. The cable manufacturer had to perform SVL operating voltage tests, over sheath DC voltage withstand tests, insulating AC voltage withstand test acc. to IEC 62067 with a voltage of 210 kV at all phases and partial discharge tests at all accessories (figure 16). Further the cable manufacturer performed a positive and zero sequence impedance measurement, a verification of the cross-bonded sheath system, a cable conductor resistance measurement, a measurement of the sheath system contact resistances and a measurement of the insulation resistance at all installed 275 kV power cables. All tests have been passed successfully.

Fig. 16: On-site AC test set-up

All tests had to be carried out during a short time outage as the cable manufacturer had to replace an existing oilfield cable system. This means that the cable manufacturer had to manage all replacement works without any gap, which had to be approved and confirmed by all involved parties including the city council before the fixed outage time had to be applied officially to the client for their final approval.

4. SUMMARY

A 10.3 km 275 kV XLPE cable circuit was installed in the heart of the city of Liverpool after passing all relevant customer requirements (type registration).

The installation works has to handle several special challenges during installation as limited space along the route, limited time for work activities (route access), and working under induced voltage conditions.

The 275 kV cable circuit was put in service in December 2009 after passing successfully all required commissioning tests and has operated continuously without any abnormalities since then.

REFERENCES

[1] National Grid Transco Technical Specification TS 2.5

[2] "Guidance Note 11 MAKING OR BREAKING DOWN SEALING ENDS ON HV CABLES "of UK BP / SE / NSI 5 CABLE SYSTEMS"

[3] “GUIDANCE NOTE 24/24a MAKING OR BREAKING-DOWN SEALING ENDS ON H.V. CABLES” of UK BP/SE/NSI 5 CABLE SYSTEMS”

[4] J. Kaumanns, E. Plieth, R. Plath, 2003, On-site AC testing and PD measurement of 345 kV / 2500 mm2 XLPE cable systems for bulk power transmission, Jicable’03, paper A8.4

[5] S. Sutton, R. Plath, G. Schröder, 2007, The St. John´s Wood - Elstree experience – testing a 20 km long 400kV XLPE-insulated cable system after installation, Jicable’07

[6] J. Kaumanns, G. Schröder, A. Weinlein, V. Stroot, J. Lehnhäuser: 400 KV XLPE-Insulated cable systems with dry plug-in outdoor terminations, Jicable’07, paper A.1.2

[7] A. Weinlein, G. Schröder, S. Ebert, H. Geyer, 2011, On-site testing with compact a.c. test-system at the first 500 kV XLPE cable project in South America, Jicable’11, paper C.4.1

[8] G. Schröder, "Working under induced voltage conditions at installation of EHV cable systems", S1-3.9, ICOLIM Conference 2004, Bucharest, Romania.

[9] G. Schroeder, J. Kaumanns, R. Plath, “Advanced measurement of AC resistance on skin-effect reduced large conductor power cables”, Jicable’11, paper C.8.2

GLOSSARY CBS Cement Bound Sand EHV (Extra) High Voltage SHE Safety Health Environment NSI National Safety Instruction XLPE Cross-Linked Polyethylene SVL Sheath Voltage Limiters AC Alternating Current DC Direct Current PD Partial Discharge(s) DTS Distributed Temperature Sensing OSE Outdoor Sealing End

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