Enhancing Reliability of Power Transmission and ...

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Enhancing Reliability of Power Transmission and Distribution Networks with underground cables Hassan Al-Khalidi, Akhtar Kalam School of Electrical Engineering Victoria University of Technology PO Box 14428 MC, Melbourne VIC 8001, Australia [email protected], [email protected] ABSTRACT There is an on-going demand from consumers for more reliable and economical power. Many factors contribute to determining the reliability of a power network: design, construction, operation and maintenance which have their combined input to overall power network reliability. Moreover, deregulation of the electricity sector along with adopting new technology commercially puts great pressure on electricity utilities to operate not only economically but reliably and securely too. This paper will describe the potential benefits of underground cables and how they can enhance power network reliability.

1. INTRODUCTION The main causes of overhead power transmission and distribution networks interruption are in fact unplanned external causes, like storms, bushfire, lightning, trees, animals, vehicle accidents and vandalism. Interruptions can also be caused by equipment and line or cable failure due to overload and ageing. However, overhead networks are more vulnerable to the external causes, adding to that, in general the maintenance works required for overhead networks are about twice the number compared to the underground power networks maintenance works. Based on discussion with a local power distribution company, 70% of outages are attributed to the overhead network and 30% to the underground network. The average duration for an overhead fault is 50-55 minutes with an average duration of ~ 65 minutes for an underground fault; these figures are based on the System Average Interruption Duration Index (SAIDI). The higher duration for the underground network is a reflection of the time it takes to effect repairs. On a per unit basis an underground fault will take ~ 10 times longer to repair (with a similar cost ratio). Hence the need for the interconnectivity on the underground system. For 2004 alone, the company experienced 84 outages on its overhead HV load power feeders and 54 on its underground feeders. The causes for outages on the

overhead network were animals, vegetation, weather and third parties. On the underground system the causes were failures (joints) and third party dig-ins. For many decades overhead lines had proven to be a reliable solution, both technically and economically. Back then, no other alternative and competitive system was available and there was little concern for the environmental aspect when planning and constructing new electrical network. However, modern technology with continuous development of manufacturing and installation makes it possible for underground systems to be competitive with overhead lines, technically, environmentally and economically. Whilst most overhead interruptions occur for a short period of time, mainly less than a second due to surge strikes, other permanent interruptions are usually caused by external factors, therefore the surroundings have a big impact on overhead interruptions, unlike underground cables which are insulated from the surrounding conditions [1].

2. OVERHEAD

POWER INTERRUPTIONS

NETWORKS

AND

Frequent and major power interruptions cause many concerns. Some of them relate to the power systems security and reliability. Aging assets also play a big role in more frequent power interruptions (most of the outages and disruption usually occur on the distribution part of the power network). Figure 1 shows different stages of overhead power network.

Figure1: Power Network with overhead transmission and distribution systems [2].

Patching work has proven to be only a temporary solution. It has time limitations unless major maintenance works are carried out, and the risk associated with this is that it is likely that the entire power network can be disrupted. According to PB Power report [3], 85% of all Customer Minutes Off-Supply (CMOS) results from faults on the 22kV distribution system. The low voltage network to a consumer’s property accounts for only 4% and, although faults at zone substations and on sub transmission circuits impact on a large number of customers, they account for only 11% of customer minutes off-supply. The top three causes of outages account for 50% of all outages (Trees 23%, No Identified Cause 14% and Planned Outages 13%). Lightning, animals or birds and high voltage conductor failures each account for around 7%. In 1999, trees caused more than 80% of faults on the two worst short rural 22kV feeders (Belgrave 24 and Kinglake 2). Figure 2 shows percentage of top causes for outages during 1997 to 1999 by CMOS of local power distribution network TXU [3].

Figure 3: Rating of overhead line versus underground cable [4].

A similar scenario will apply during cold winter days when many of heating appliances are used. Therefore overhead lines are becoming less attractive from a load variation point of view, and underground cables are gaining more reputation in this aspect. In terms of overload capacity for periods of time shorter than 90 minutes, underground cables have much better performance at high temperatures, due to the high thermal mass of the surrounding soil. Another aspect of underground reliability is the potential of reduced network maintenance and losses caused by electricity outages and reduced transmission losses. Yet more benefits include improving public safety by the removal of electricity poles so that the number of fatal or severe road accidents can be reduced, and tree pruning costs and bushfire risks can be lowered, improving amenity hence increasing property values [5]. Figure 4 illustrates parts of an overhead network that have been grounded by using XLPE cables to enhance overall power reliability.

Figure 2: Average CMOS over three years [3].

3. THE EFFECT OF UNDERGROUND CABLES ON OVERHEAD POWER NETWORKS

Adopting new and existing proven underground power cable technology can improve the performance of the overhead power system. Modern underground technology has made great progress in delivering cables and their accessories, with the most significant benefits of reduced cost ratio and high reliable performance compared to conventional overhead lines. Losses per MVA usually are less in underground cables, due to a larger cross-sectional area of conductor, and the rating of overhead lines can decline during high load periods in summer where demand is high on electricity for airconditioning purposes, which can drop the capacity of overhead lines to less that 50% of its full capacity. Figure 3 shows rating of overhead line (OHL) versus underground Cross-linked polyethylene (XLPE) cable, higher power rating can be achieved by underground cable if the daily cycle load is taken into account, this is represented by the dashed line in Figure 3 [4].

Figure 4: Power network with underground subtransmission and distribution system, respectively [2].

There are considerable environmental and amenity benefits from the removal of all the above ground equipment, wires and poles associated with the overhead high voltage distribution lines. The underground cable will suffer less weather and uncontrollable faults. The underground network requires less planned maintenance and therefore an additional reduction in power interruption will be realised. Moreover, the underground network costs less to maintain. This advantage has also not been allowed for in the reliability analysis [3].

A reliability improvement is possible to the controllable unplanned outages such as trees, no cause identified outages and equipment failure. All other fault causes were excluded. Therefore to give a representative picture of the improvements that will be achieved in overall performance the other outages need to be added in. The resulting performance and performance improvements averaged over the three years 1997 to 1999 of local distribution company network, Outage Analysis System (OAS) representation is shown in Figure 5, whereas Figure 6 shows the resulting overall effects of the improvements on the average number of outages encountered by a customer on selected feeder [3].

Figure 5: System Average Interruption Duration Index (SAIDI) averaged over three years [3].

sufficiently to cause a fault. It has been justified on economic and environmental grounds, as it is able to meet fire mitigation standards without excessive vegetation control. In theory this cable can be constructed close to, and even through trees. However TXU found there were problems with this approach. In particular: •

The cable gives possums better access to the exposed high voltage equipment on the feeder main supply routes.



The conductors are more likely in storms to be struck by trees and fall to the ground. This can damage the insulation of the cable and create a personnel hazard.



Faults are more difficult to locate and repair times longer.

Fire areas normally require clearance from trees laterally and overhang and ABC eliminates the requirement for clearing of overhanging branches. Use of ABC has now slowed, as the tree fault rate has not improved overall. Faults have also developed on the ABC cable mainly due to the cable being damaged by trees and subsequent water damage, which has been a subject of further research investigation. The recommendations were to go for the underground cabling system, which is the most effective way to significantly improve reliability in such heavily treed areas [3].

5. CROSS-LINKED CABLE

Figure 6: System Average Interruption Frequency Index (SAIFI) averaged over three years [3].

A largely sound analysis of the causes of outages was reported by TXU in the January 2000 Network Performance Report 21. This analysis shows that there is a correlation (albeit not a terribly robust one) between reliability and rainfall. This report suggests a 60% reduction in the SAIFI and 45% in CMOS are achieved by installing underground cable in selected sections of the low reliability feeder [3].

4. SHORT TERM SOLUTION - AERIAL BUNDLED CONDUCTOR (ABC) In an attempt to rectify the tree problem TXU has installed insulated aerial cables in heavily treed areas, to meet fire mitigation standards and minimise tree and bark related faults. Aerial Bundled Conductor (ABC) eliminates the requirement for clearing overhead branches as the conductor attachments are designed to fail before the conductor insulation is damaged

POLYETHYLENE

(XLPE)

XLPE cable is the major developing technology and has wide industry acceptance at voltages up to 132-154kV. This type of cable uses vulcanised polyethylene insulation, which is solid insulation extruded onto the conductor during cable manufacture. For high quality insulating properties, the raw materials must be free of even minute contaminants and the extrusion and vulcanising process must ensure homogeneity and absence of voids and moisture in the insulation. Compared with oil-filled cable it is considered to be a simplified technology. No other power cables had such a high rate of improvement as XPLE cable technology during the last decade. Improvements were made possible due to the overall cost savings, along with environmental focus and de-regulation of electricity markets which makes XLPE cable systems more attractive solutions where they were not even an option in the past [1,6]. The cables are normally delivered to the site on large drums, typically 4m in diameter and due to height restrictions these drums are transported by low loader. Due to the cable’s high mass and large transport drum size the amount of cable that can be handled in a single length is limited. Therefore, a joint is required at each interval along the route. Jointing employs highly skilled techniques to provide a high quality electrical connection. The insulation is then reconstructed across the joint. The joint is contained within an insulating and waterproof casing for final protection. Joint bay

excavations are wider than for a normal trench to provide a suitable working area; this bay should be kept clean and dry for jointing operations. It is usually lined with concrete. At joint positions a link box is located within 10m and contains electrical equipment associated with earthing of cable sheaths [6].

lines and objects. This system can be utilised to reduce operating and maintenance costs, by reducing unnecessary excavations and lower the risk of damaging other assets [11].

6. INNOVATIVE

This paper presented the ways of enhancing the reliability of power networks using unground cables. There is ongoing demand for reliable electricity in terms of supply and distribution. Underground cables have the potential to reduce outages, maintenance cost and transmission losses in the best and most effective environment-friendly way possible. In general transmission losses are lower with underground cables compared to overhead lines. Another important benefit is to reduce the number of fatal accidents and minimise the severity of an injury. Underground cables can deliver big savings in tree pruning for local councils, yet reduce the risk of bush fires and increase public awareness of negative impact on the environment of overhead lines. Modern technology makes underground cables a more practical solution to improve power network reliability where they were not an option in the past.

IDEAS ENHANCING NETWORKS RELIABILITY

POWER

New ideas which enhance reliability, efficiency and lower the cost of installation and maintenance are contributing significantly in the development of underground systems. These ideas cover various phases of underground systems. A few ideas are identified as follows: 6.1.

CABLE TUNNEL

Although underground cables are almost isolated from external causes of disruption, they are vulnerable to third party dig-in damages. Most urban underground cables run in congested streets, which require being closer to part of the traffic for a long time, causing disruption to traffic flow, parking and pedestrians in the area. Using a tunnel has several advantages over conventional cable burial technique. Impact on traffic flow and other utilities would be minimal; short outages are required to connect; there is ease of access for maintenance and testing; the cable environment would be secure for 100 years; and a direct tunnel route will result in reduction in cable length. Naturally ventilated tunnels with cables are laid in vertical snaking on saddles with spacing of 7.2 m in horizontal and 0.6 m in vertical direction; tunnel temperatures are permanently monitored using fibre optic distributed temperature sensor (DTS) [7,8]. 6.2.

FIBRE OPTIC SENSING SYSTEM

Although some methods are already in use to locate a fault, the cable needs to be removed from service and connected to detection equipment, but locating a fault in an underground system can take extensive time and effort. The new method is integrating Fibre Optic Distributed Temperature (FODT) sensor into cable. This sensor can find the fault immediately by applying fault detection of XLPE installed underground cable in a resistance grounded system. The maximum detection distance, distance resolution and processing time for fault location are 10km, 1m and 30s respectively [9,10]. 6.3.

UNDERGROUND OBJECT RADAR

Underground assets maps, if they exist, are often inaccurate, incomplete or out of date, and the use of metal detectors to find these assets often proves disappointing. Ground-Penetrating Radar (GPR) system technology, GPS devices can produce an underground image which has a great detail of the subsurface features. Schlumberger Corporation, in conjunction with the electric Power Research Institute (EPRI) and Gas Research Institute, has developed a new GroundPenetrating Image Radar (GPIR) system, which creates sharp, three-dimensional (3-D) images of underground

7. CONCLUSION

8. REFERENCES [1]

J. Karlstrand, G. Bergman and H.A Jonsson, “Cost-efficient XLPE cable system solutions,” AC-DC Power Transmission, Seventh International Conference on (Conf. Publ. No. 485), 2001.

[2]

U.S. Department of Labor, O.S.H.A., “Electric Power, Generation Transmission Distribution eTool,” [Available online] http://www.osha.gov/SLTC/etools/electric_pow er/illustrated_glossary/ 2005.

[3]

J.T.S Mutton, B. Simpson and R. Mann, “Investigation of Low Reliability Urban and Rural Feeders Phase Two of Two TXU Networks,” [Available online] http://www.esc.vic.gov.au/PDF/2000/txurelinve stpbpower.pdf, 2000.

[4]

G.S. Markus B, Klaus T, and L. Nils, “ABB Review Special Report,” ABB Ltd: Zurich/Switzerland. [Available online] http://www.abb.com/global/seitp/seitp202.nsf/9 9ad595c32e0c2d9c12566e1000a4540/68bf266d 373cf81a48256e06007cb4bc/$FILE/ABB_Revi ew_Special_Report_2003.pdf, 2003.

[5]

Putting Cables Underground Working Group, “Putting Cables Underground Report,” [Available online] http://www.dcita.gov.au/cables/index.htm, 1998.

[6]

W. Peter, “Environment Effects Statement,” Melbourne. 1989.

[7]

R.G. Schroth and D. Obst, “Long distance tunnels for the installation of 400 kV XLPE cables,” Cables in Tunnels (Ref. No. 2000/070), IEE Seminar. 2000.

[8]

R. West, and S. Larsen “Kentish town project [power cable laying],” Cables in Tunnels (Ref. No. 2000/070), IEE Seminar. 2000.

[9]

X. Zhang, X. Jiang and H. Xie “The application of fiber optic distributed temperature sensor to fault detection of XLPE insulated underground cable,” Properties and Applications of Dielectric Materials, 2000. Proceedings of the 6th International Conference on. 2000.

[10]

S.J.M. Mendez and J. Monteys, “Design of power and communication cable installations to minimise interference in tunnels,” Cables in Tunnels (Ref. No. 2000/070), IEE Seminar. 2000.

[11]

R. Bernstein, “Imaging radar maps underground objects in 3-D,” Computer Applications in Power, IEEE. 13(3): p. 20-24. 2000.