Network Standard Advice No. 1420C 9/6/2011

Transcription

1 Network Standard Advice No. 1420C 9/6/2011 TO: Customers, Service Providers and Ausgrid Staff. Advisory Note on Changes to the Use of 11kV Cable Types. Introduction This Network Standard Advice (NSA) provides information on the use of different 11kV cable types by Ausgrid. Cables continue to be available through Ausgrid s store or approved suppliers as listed in NSA1343. Details mm 2 copper polymeric insulated three core cables shall be used for feeder cables exiting out of zone substations where derating issues dictate. 2. The preferred cables for general feeder use are 300mm 2, 400mm 2 or 500mm 2 aluminium polymeric insulated three core cables in accordance with Ausgrid s current specification mm 2 or 300mm 2 copper triplex cables with 70mm 2 screens (10kA fault duty) are to be used in the following situations dependent on the current rating and earth fault rating required:- Feeder tails out of suburban zone substations, Feeder tails for looping in or tee connecting distribution centres, 11kV UGOH tails, For piece-ins where a 500mm 2 Al tee joint is required, since no 500mm 2 Al tee joint is commercially available, For repairs or replacement of failed joints in mm 2 Aluminium three core cables, to avoid the need to introduce two 3 core joints and very short length of three core cable, Where the conduits are too small to accommodate the three core cables, and Where the internal radii of conduit bends are smaller than the bending radii of the three core cables. NSA1420C 1 of 8 June 2011

2 4. There are also limited situations where 300mm 2 copper paper insulated cable is the only appropriate solution, but these will be identified and detailed in design details for each project mm 2 single core cables with a 35mm 2 screen shall not be used on Ausgrid's network. 185mm 2 single core cables with a 35mm 2 screen shall only be installed in locations where the current rating requires 185mm 2 cables and the earth fault rating does not exceed 5kA, until cable stocks are depleted. A compilation of information on difficulties in using single core cables, and some background information on Triplex cables is attached to this Network Standard Advice. Note: Current network standards, with NSAs and CIAs, are available on Ausgrid s Internet site at Walter Stefani Executive Manager Logistics and Distribution Engineering NSA1420C 2 of 8 June 2011

3 What is wrong with using single core cable for long runs? There are both electrical and installation issues associated with running long lengths of single core cables. There are also commercial issues single core cables are more expensive to buy and also to install than three core cables, so three core cables are always the preferred option on purely commercial grounds, unless the length is very short, in which case the savings in 3/1 joints may offset the added cost of the singles. Electrical constraints Single core cables have individual screens over each core. Current flowing in the core conductor induces voltages (and hence currents, if the screens are earthed at both ends) in the screen wires. The heat generated by the induced screen currents, and the derating of the cable that results, depends on the configuration of the cores. If the three cables are held in touching trefoil arrangement, the circulating currents are minimized, however, the further apart they become, the greater the currents and consequent derating factor. Best Worst With a three core cable, the screen wires of each core are in electrical contact with the wires of the other two cores, so that the out of phase currents can balance each other out resulting in a much smaller total screen current and derating effect. One disadvantage of single core cables for trench installation is the fact that each phase screen needs to be fully rated (a concentric screen means virtually all cable faults will be earth faults), increasing copper content and hence cost. The larger screens required for fault rating also provide a lower resistance path for circulating currents, increasing cable heating/derating. For closely coupled cables though (such as triplex) this is not significant. It is possible to eliminate the screen currents and their derating effects by only earthing the screens at one end of the cable length (single point bonding). However, this then permits the screen to rise to the induced voltage - in the order of 183 volts under fault conditions (5kA with a circuit length of 300m) for the cradled arrangement, or 250 volts in flat formation with 80 mm spacing between cables. However, the biggest risk with single point bonded systems relates to earth potential rise. In populous areas, use of single point bonding without an earth continuity conductor increases the earth fault current entering the ground through the distribution substation earth grid enormously, leading to a much higher earth potential rise (EPR) under fault conditions. In addition to this, the single core cables with a 35mm 2 screen wires provides a earth fault rating of 5kA for 1 second much less than the 10kA on Triplex and CBD cables. The reason they are smaller is that the single core cables were purchased NSA1420C 3 of 8 June 2011

4 for use solely as substation cables feeder or transformer tails, where the individual screens were connected to the screens over the cores of the three core cables typically around 35mm 2 (total 105mm 2 due to constant contact of screen wires of each phase in three core cable designs). Single core cables should not be used for long lengths without proper consideration of the fault ratings. Installation constraints Problems can arise when pulling singles through ducts due to their tendency to bunch up and jam when going around conduit bends. When the combined diameters of three cables roughly equal the interior diameter of the conduit (jam ratio ~ ), the cables can line up linearly as they are pulled around the bend. The cables then wedge against the conduit wall as they are forced towards the inside of the bend, causing them to jam. To pull jammed cables with enough force to get them through a bend usually ruins the cable by ripping off the jacket or crushing the insulation. Copper 300mm 2 singles have a diameter of 40mm, giving a jam ratio of 3.2 in 125mm conduit. Running single core cables in a single duct also tends to increase pulling tensions, and may require the use of a coefficient of friction of anywhere between 10 and 100% higher than normal for a 3 core cable. What should we be installing now? Where possible, 300, 400 or 500mm 2 Al 3 core cable is the preferred option depending on required rating and number of cables in proximity. Where installation constraints make use of 3 core Aluminium cables impractical, Copper triplex cable is the preferred alternative. Note that in locations where fault level is likely to exceed 10kA (e.g. in Newcastle, or on front line feeders in substations with paralleled transformers), the screen rating may not be adequate. Note that some of the constraints of 3 core Aluminium cables may be overcome with careful design or revised installation practices. The normal problems are the larger bending radii and cable size compared to internal diameter of the smaller conduits on our network. The bending radius of a 400mm 2 Al 3 core cable is 2100mm while for 500mm 2 Al 3 core it s 2300mm (the bending radius of 300mm 2 copper triplex by comparison, is mm (depending on supplier), smaller even than that of the old 300mm 2 copper 3 core PILC cable (1600mm)). Pulling around corners through an open pit (rather than using conduit bends) will reduce side wall pressures, pulling tensions and provide greater flexibility in setting up for the pull. This simplifies installation where road crossings (with one or two right angle bends) must be undertaken. It may also be possible to avoid the large bending radii during design by locating your joints adjacent to any major bends. This will permit the cable to be pulled straight and then set into its final position using the smaller installed bending radii (1300/1400mm for 400/500mm 2 Al 3 core respectively). This is comparable to the pulling bending radius of 300mm 2 copper singles/3 core PILC. Limiting pulls to predominantly straight runs in this manner also permits 500mm 2 cable to be run in the older 125mm duct lines. NSA1420C 4 of 8 June 2011

5 What is a Triplex cable? TRIPLEX CABLE INFORMATION A triplex cable is a cable composing of three single core cables laid-up together. A triplex cable is similar to high voltage aerial bundled cable without the catenary wire. What are the sizes of the triplex cables Ausgrid are currently buying? Ausgrid are currently buying 185mm 2 and 300mm 2 copper triplex. Each triplex cable consists of three single core cable. The three single core cables can be either 185mm 2 or 300mm 2 circular stranded compacted copper conductor, Ethylene Propylene Rubber (EPR) or Tree Retardant Cross Linked Polyethylene (TR-XLPE) polymeric insulation, 70mm 2 stranded copper wire concentric screen, composite PVC/HDPE sheath. What is the cable diameter of the Triplex cable? There are two diameters to consider, the diameter of the circumscribing circle over the laid-up bundled cable and the diameter of each single core cable. The diameter of the circumscribing circle over the laid-up bundled cable for the 185mm 2 triplex cable is 76mm and for the 300mm 2 triplex cable is 85-89mm (depending on supplier), and the diameter over each single core cable is 35.3mm for the 185mm 2 triplex cable and mm (depending on supplier) for the 300mm 2 triplex cable. NSA1420C 5 of 8 June 2011

6 What is the minimum internal bending radius for triplex cable? Nominal Minimum Internal Bending Radii for Specified Cables (Guide Only) Cable type 11kV EPR insulated cables: Minimum Internal Bending Radius During Installation (mm) After Installation Bundled Cable(mm) After Installation Phase Cable(mm) 185 CU1 EPR 70 CU(WS) Z YQ / TX CU1 EPR 70 CU(WS) Z YQ / TX kV TRXQ insulated cables: 300 CU1 TRXQ 70 CU(WS) Z YQ / TX Are there any differences with terminating the single core cables associated with the triplex cables as opposed to our current single core cables? For 300mm 2 triplex cables only, Yes. As detailed in NS177 and in the installation instructions supplied with the cable termination, the outer composite (HDPE) sheath will need to be removed for a distance of 120mm below the gland plate to decrease the cable diameter to ensure the cable gland will fit correctly over the inner composite (PVC) sheath. A 80mm length of rejacketing tube (stockcode ) is then shrunk centrally over the outer composite sheath cut. For both 185mm 2 and 300mm 2 triplex cables, a different screen wire lug (stockcode H95851) and screen wire gland (stockcode ) is required to terminate 70mm 2 screens. All new switchgear that come with the gland plates predrilled, have a 26mm diameter hole to suit the screen wire gland (stockcode ). Does Triplex cable have a memory, making it difficult to set in place when terminating? No see photo. The Triplex cable is effectively the same as a single core cable, except the triplex cable has a composite sheath, and uses a 70mm 2 screen instead of a 35mm 2 screen. Are Triplex cables comparable electrically with single core cables with 35mm 2 screen wires? Not exactly. They are slightly lower rated, but the difference is negligible. More importantly, they use 70mm 2 screens per phase, so they are fault rated at 10kA for 1 second, NSA1420C 6 of 8 June 2011

7 compared to the 5kA that is the maximum the single core cables with 35mm 2 screen wires achieve. Are there any differences with jointing the single core cables associated with the triplex cables as opposed to single core cables with a 35mm 2 screen? No. The mechanical screen wire connector currently supplied with Ausgrid s single core polymeric joint kits is suitable for screen wires up to 70mm 2. What is the length of cable on each drum? For 185mm 2 triplex cable the standard length is 500m, and for 300mm 2 triplex cable the standard length is 320m. What is the conductor code? Code Full Description Old Description kv/v Nickname KV 185 CU1 EPR 70 CU(WS) Z YQ / TX / EA1400/10A KV 300 CU1 EPR 70 CU(WS) Z YQ / TX / EA1400/10A KV 300 CU1 TRXQ 70 CU(WS) Z YQ / TX / EA1400/10B What are the stockcode numbers? Full Description Stockcode number 11KV 185 CU1 EPR 70 CU(WS) Z YQ / TX KV 300 CU1 EPR 70 CU(WS) Z YQ / TX NSA1420C 7 of 8 June 2011

8 6.35/11kV UNDERGROUND POLYMERIC INSULATED ELECTRIC CABLES TRIPLEX CABLE CONSTRUCTION 185 Cu 1 EPR 300 Cu 1 EPR 300 Cu 1 TR- Triplex Triplex XLPE Triplex 5. CONDUCTORS 185 Cu 300 Cu 300 Cu (b) Shape Circular Circular Circular (d) Maximum allowable continuous conductor temperature ( o C) (e) Overall dia. (mm) CONDUCTOR SCREEN Semi-conductive polymer Semi-conductive polymer Semi-conductive compound 7. INSULATION (a) Type of insulation EPR EPR TR-XLPE (b) Nominal radial thickness (mm) (d) Dia. over insulation (mm) NON-METALLIC INSULATION SCREEN (c) Dia. over ins. Screen (mm) METALLIC INSULATION SCREEN (a) Number of Wires (b) Nominal C.S.A of a wire (mm 2 ) (d) Details of materials applied over the core screen wires prior to application of the polymeric sheath Polypropylene Tape Polypropylene Tape Polypropylene Tape 10. WATER SWELLABLE TAPE N/A N/A N/A 12. NON-METAL INNER SHEATH (a) Material 5V-90 PVC 5V-90 PVC 5V-90 PVC (c) Dia. over sheath (mm) POLYMERIC OVERSHEATH (a) Material HDPE HDPE HDPE (c) Dia. over oversheath (mm) COMPLETED CABLE (a) Average overall diameter (mm) (b) Min. bending radius-(mm) (i) during installation (ii) set of bundled cable (iii) set of phase cable (c) Mass of cable kg per km (d) Max. pulling tension, stocking grip (kn): (i) for each core (3 stockings req) (ii) for total cable (1 stocking req) (e) Continuous current rating (Amps)*- (i) cable laid direct (70 o C) (iii) cable laid in duct (70 o C) (iv) cable laid direct (90 o C) (vi) cable laid in duct (90 o C) * Based on buried depth of 0.8metres, soil temperature of 25 o C and soil resistivity of 1.2 o C.m/W. NSA1420C 8 of 8 June 2011