System and design studies

Establish the actual current carrying requirements for the particular identified application.

Perform a system design study under one or more of the following conditions:

 Continuous loading

 Emergency (short time) loading

 Peak cyclic loading

The cable system would be designed and optimised to fulfil the particular requirements. For example, if the peak load is only required for one or two hours in each period of twenty four hours, with a lower load for the rest of the time, then the resulting maximum conductor temperature would be less than that calculated for a continuous load. In this case a cyclic ampacity calculation is recommended to be performed in place of the continuous ampacity calculation with the objective of quantifying which of the following would benefit the application most:

 Installation of the 2,500 mm²cable size at a greater depth.

 Reduction in the likelihood that three cables per phase would be required for the whole cable route to allow for a deep crossing under one or more major route obstructions.

 Installation of the 2,500 mm²cable in a narrower width of trench and swathe at the proposed depth of 1.3 metres.

 Selection of a smaller conductor size than 2,500 mm².

then not have to demonstrate the performance of their joints at such low design temperatures.

The following information should be obtained:

From prospective suppliers:

 More detailed experience lists including those at lower system voltages and lower ambient temperatures.

 The minimum ambient temperatures that their cable systems have encountered.

 The types and numbers of accessories that they have supplied over a wider range of lower system voltages (i.e. 220-500 kV) for use at lower ambient temperatures.

 Any national or special tests that have been undertaken, which demonstrate low temperature performance.

 Any private developments that they have undertaken, which demonstrate low temperature performance (e.g. tests performed on 345 kV XLPE cable systems^[3][4]

in Montreal at the IREQ laboratory).

From utility companies, particularly those operating in regions of low ambient temperature:

 Details of their engineering practices and experience of low temperature operation.

 Details of National or in-house cable specifications concerning (a) limiting design temperatures and (b) tests to validate low temperature performance.

From electrical test houses

 If test houses have performed tests at low temperature on cable joints (e.g. tests performed on 345 kV XLPE cable systems in Montreal at the IREQ laboratory).

 Whether they have low temperature test facilities.

 Whether they would be interested in performing low temperature tests.

If the time and uncertainties associated with proving the performance of the joint designs for low winter temperature operation are deemed to be unacceptable, then consider alternative actions:

 Evaluate the feasibility of externally heating the joints in winter.

 Select a tunnel installation with a controlled air temperature.

1.8.3.2 Obstruction studies

To perform more detailed designs of how the cables could cross the obstructions on the route without compromising the power carrying ability of the circuit. In particular consideration should be given to:

 The depth of all identified obstructions.

 The way that the owner or operator of the obstruction would permit the cables to cross.

 Calculation of the ampacity if any heat is emitted from the obstruction (pipelines may contain fluid at temperatures above ambient)

 If directional drilling can be performed with sufficient positional accuracy.

1.8.3.3 Power system studies

Perform more detailed study of how the transmission lines would operate within the wider electrical transmission network. Recommendations for further studies are given in Appendix, Section 5.

1.8.3.4 500 kV studies

 Seek from the manufacturers further supporting evidence, if available, on electrical service and test experience for joints at 500 kV.

 If insufficient electrical service and test experience is available upon which to make a decision, then either:

 Sponsor a long term prequalification test.

 Await results of the first two years of service experience from the 500 kV Shanghai project, following its (proposed) commissioning date in 2010.

 Install one or more short cable lengths, including at least one set of joints, on the

 Instigate discussions with cable manufacturers with regard to possible future development tests to demonstrate 500 kV operation and low temperature performance:

 Draft new test schedules to prove low temperature performance for consultation with the manufacturers.

 Draft detailed schedules which would form the basis for enquiries to be sent to prospective suppliers at the project bidding stage, comprising a) special low temperature proving tests and b) pre-qualification (PQ) proving tests and type approval tests (TAT) based on the IEC 62067^[1]specification.

 Draft and agree detailed specifications for manufacturing and jointing quality control (QC) tests, noting that IEC (International Electrotechnical Commission) and AEIC (Association of Edison Incorporated Companies) specifications do not give complete coverage for all QC aspects.

1.8.3.7 Select how contracts should be divided between different suppliers

For the large size and duration of the 500 kV Study Project construction work, and to reduce the risk of common mode failure, it is recommended that contracts be awarded to more than one supplier. If possible, each group of parallel cables should be supplied and installed end-to-end by a different manufacturer. One possible selection of supplier numbers for each scenario is given in Table 9:

Detail Scenario Number Route length

Stage Number of trenches Number of suppliers

1A.10 10km - 4 2

Table 9. Number of suppliers for each undergrounding scenario

The above is only typical; in the project schedule (Appendix, Section 4) a maximum of two suppliers was assumed. The eventual number of suppliers used would depend on their technical capability and ability to supply and install the volume of cables and accessories needed. One advantage of using multiple suppliers is the reduction in the likelihood of multiple outages because of common mode failure. The disadvantages are the difficulties of managing multiple subcontractors on the same project and the need to hold a wider range of spares.

1.8.3.8 Evaluate and identify resources necessary for the project

Experience has shown that the most successful EHV (extra high voltage, i.e. 220 kV to 500 kV) cable projects have been those in which the utility took an active interest and so obtained a reciprocal response from the manufacturer.

It is recommended that the Transmission Facility Owner’s (TFO’s) engineers be actively involved at the manufacturers’ works in:

 Auditing the factory prior to bidding and after awarding the work.

 Monitoring design progress.

 Witnessing the proving tests, prequalification tests, type approval tests and routine