MikeDB,
Thanks for your input and share with us your concern. This is motivating us to address some of the potential problem. So far the project is proposing three alternatives: Pipe type cable (HPFF), Self Contained Fluid Filled (SCFF) and Solid Dielectric (XLPE). In this face of the project we do not have all the answers however, there are some of your question have been discussed and I would like to share with all of you in this forum:
- Q1] The cost to put 230kv underground must be horrible. A-1] The objective is to relocate the existing overhead line underground. The developer performed a cost/benefit analysis and offering the utility to pay all cost associated with TL relocation.
- Q-2] My catalog only lists cable up to 138kV, how many times has this been tried at 230kV? A-2] This is one of the main reason of our post do to the lock of large amount of cases in the US
A2-a. The power industry is moving toward the acceptance of solid dielectric cable above 138 kV. Utilities such as FP&L, PG&E, PJM, NYPA and may other are using XLPE. The Neher & McGrath method for calculation is commonly accepted and IEC is also used as an optional reference since national standards such as AEI std. CS7-93 2. goes up to 138 kV.
A2-b Even thought in the US there are limited installations (~2000 ft) of solid dielectric cable operating at 230 kV, outside the US there were 170+ circuit miles in service in 1993. There are excellent reliability performance reports from France were more than 50 per cent of the in-service cables in the field 63 kV-400 kV are polymeric cables (LDPE-HDPE and XLPE).
A2-c- In the US there is 4000+ circuit miles of pipe type cable (HPOF) with 3 conductors encased on 8” and larger steel pipe with many section of hwy and RR crossing use encase pipe with solid dielectric cable.
A2-d 230 kV cable may be available from: Pirelli, Nexan (Alcatel), ABB(Kabel), Siemens, Mishubishi, Toshiba….(there limited case in service up to 500 kV and test up to 1000 kV).
- Q-3] Using XLPE instead of EPR. A-3] We choose XLPE instead of EPR because its comparative dielectric losses and possible better compatibility with the engineering practice in the US.
- Q-4] What size conduit is required? >12"?. A-4] The preliminary pipe size is expected to be around 8 inches.
- Q-5] Can they use directional boring on conduits that large? A-5] With Horizontal Directional Drilling (HDD) it is possible. Initially small pilot hole is drilled follow by drilling a bigger size hole.
- Q-6] Putting single phase cable in steel or reinforced concrete will cause heating problems of the steel due to induction -- a definite no-no. A-6] Pipe type cable is normally routed in steel encased pipe. There are fair number of HV underground solid dielectric cable in service using encased in steel pipe for RR or HWY crossing application. We run a preliminary calc. that may us believe that this could be resolved.
- Q-7] Putting all three phases in one conduit may limit power flow due to the temperature rise in the cables and poor dissipation of heat. A-7] It is expected that the cable will be routed as symmetrical as practical to cancel most of the EMF field. that produce the heat. The presence of water and the high thermal conductivity of the steel pipe will help to remove some heat. A completed detail study will be performed to address those potential cases. Worst case scenario will be using pipe type cable encase in steel pipe on the entire underground portion.
- Q-8] What is the maximum length of cable for that size, you may need several splices to go a mile. A-8] The project will be highly satisfied if could get up to 3 splices.
- Q-9] Pulling tensions for the installation of the cables is also a concern on cables this big, even on straight runs. A-9] There are experience using 3000+ ft of cable for pulling purposes. However this case should be analyzed in the case-by case basis depending on the cable route, friction and other characteristics.
