City Tunnel Projects - Engineering Safety and Quality within a Constrained Environment

Transcription

1 D2001, T&D Conference Brisbane QLD November 2001 City Tunnel Projects - Engineering Safety and Quality within a Constrained Environment L.J. McKerrow 1, W.D. Carman 2, D.J. Woodhouse 3 Keywords Infrastructure Projects, tunnel, fire safety, electrical safety, Earthing System Redundancy Abstract: Experience with a number of major city infrastructure projects utilising underground tunnels has found that significant effort is often required to produce earthing systems that ensure ongoing safety and operational compliance. Projects examined include tunnels dedicated to the provision of power, railway and hydraulic services. Issues discussed include designing and implementing a system of earthing within a constrained environment to ensure correct operation of cross-bonded cable networks and auxiliary utilities, safety of network operators and maintenance staff, and practical management of galvanic and electrolytic corrosion. Cable tunnel auxiliary services, which must correctly operate under inductive saturation conditions, including fire detection and suppression equipment, control, monitoring and communications cabling and equipment, and LV supply cabling. Case studies include a recent project where the high reliability requirements conflict with issues relating to the possibility of transferring hazardous voltages to the tunnel substations during external earth fault conditions, while managing interaction with DC traction systems 1. INTRODUCTION Due to the congestion of services within CBD s and highly urbanised areas, installation of public services, such as power, water, sewerage and telecommunication services as well as road and rail services is becoming increasingly difficult. The demands of local communities as well as the limited availability of suitable real estate, has driven many of these projects underground. As a result underground services tunnels are becoming more and more prevalent in highly urbanised environments. The use of tunnels to supply these facilities provides a unique and challenging task not only to the civil engineers but also to the power system engineers for the following reasons: Tunnels are installed in high strength strata and typically this supporting strata is of very high resistivity which limits the achievable earthing resistance. Large series impedances between earthing nodes BE(Elec) Consulting Engineer, Safearth Engineered Solutions, Engineering Consulting Hunter, EnergyAustralia, 145 Newcastle Road, Wallsend NSW 2287, Ph (02) , Fax (02) lmckerro@energy.com.au BE(Elec)(HonsI) MIE(Aust.) CPEng, Senior Consultant, Safearth Engineered Solutions, Engineering Consulting Hunter, EnergyAustralia, 145 Newcastle Road, Wallsend NSW 2287, Ph (02) , Fax (02) bcarman@energy.com.au BE(Elec)(HonsI) BMaths MIE(Aust.) CPEng, Development Engineer, Safearth Engineered Solutions, Engineering Consulting Hunter, EnergyAustralia, 145 Newcastle Road, Wallsend NSW 2287, Ph (02) , Fax (02) dwoodhouse@energy.com.au

2 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 2 Locations available for earthing electrodes are limited to areas within the tunnel or small easements on the surface. Corrosion issues associated with the electrolytic interaction between dissimilar metals or impressed DC current. Inductive saturation and interference with auxiliary services. Ensuring personnel safety during construction, and for the public and maintenance staff during long term operation. It is a common misconception that an earthing system is confined to the copper conductors and electrodes buried under a substation. In many cases the buried conductor simply provides local voltage gradient control for personal safety, while the fault current is managed by other means including cable sheaths and other interconnections. It is extremely important that a systems approach be taken to earthing system design as the earthing system interacts with other systems contained in the tunnel. The following sections discuss some of the issues that should be addressed in each stage of the tunnel development, from the conceptual design stages, through the construction and commissioning stages as well as the ongoing operational phase. 2. CONSTRUCTION SAFETY RISK The issues that need to be considered during construction are more severe than those expected in the operating tunnel, and are more like those encountered in a mining operation. The typical issues that must be addressed include transfer hazards, circulating currents and induction risks. 2.1 Transfer Hazards Power supplies for construction are often provided from non fault limited urban feeders. The earth potential rise (EPR) resulting from earth faults at the surface substation may be transferred through the tunnel earthing system creating hazardous touch voltages underground. It is also a relatively common occurrence for a trailing cable to be faulted during mine operations. Electrical equipment situated at remote locations, such as the mining machinery at the cutting face, pose the greatest hazard to personnel, as there is typically a solid connection to the above ground substation earth system, there is little or no local earthing, and the operators are essentially in continuous contact with the machinery. 2.2 Circulating Currents Under HV earth fault conditions large currents circulate in power cables and other services. If a plug in a trailing cable is disconnected at the time of a fault, a large arc will be drawn. This is especially critical in environments with potentially explosive atmospheres. This scenario may also result in severe electric shock to the person performing the disconnection. Although the likely voltages may be quite small (less than 60 volts), the currents may be in the order of many hundreds of amps. It should be noted that circulating currents may be present under normal operation of the power system and not just as a result of a fault. They may also be the result of interactions with power systems external to the tunnel. Circulating currents are not restricted to the earthing conductor associated with the power system cabling. Circulating currents may be present in water and gas pipes, newly installed

3 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 3 earthing conductors, cable sheaths and structural components. Care should be taken during installation when disconnecting and connecting cabling and services, especially those that span large distances within the tunnel Circulating Currents Example - Underground Mine Figure 1 shows a current distribution under a 66kV earthfault condition at a mine substation. A total of 37% of the fault current flowed into the 11kV drop-down bores and cable sheaths. For a 66kV earth fault at the surface substation, currents flowing in the sheaths of the Longwall and Development feeder cables were 260 amps and 493 amps. These could generate a spark with energy of 67,600 joules and 243,000 joules respectively. For an 11kV earth fault, the maximum spark energy would be 25 joules for the same clearing time and circuit conditions due to fault limitation on the 11kV circuits. In tunnelling applications where atmospheres are generally not explosive, the ignition risk is not a major concern, however the potential hazards associated with high currents must still be addressed. If the construction earthing system is designed appropriately it is possible to significantly reduce the circulating currents. 2.3 Induction risks Figure 1: Circulating current distribution Currents may also be the result of magnetic coupling with HV transmission lines running parallel to the tunnel system. This coupling may induce currents in the cable sheaths, earthing conductors, services and structures. These issues are particularly difficult to manage, as the source of the induction is generally outside the control of the tunnel power system engineers Induction Risks Example The following example reviews a site with a number of 330kV circuits passing 17kA on a throughfault condition, and 132kV circuits passing a total of 28kA. The configuration was modelled to determine the interactions between the transmission lines and the tunnel below

4 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 4 using a software package specially developed with safearth. The results were later confirmed by current injection testing. A 400m long cable on the surface, earthed at one end, had 871 volts induced. A cable in the same location but 200m underground had 150 volts induced under a 330kV earthfault condition. This demonstrates that although the conductor is below ground an induction risk still exists. These voltages can be reduced on structures and water pipes by installing earthing along their length, however, this is accompanied by a corresponding increase in circulating currents. 3. CONSTRUCTION RISK MITIGATION To successfully manage construction risk it is essential to have a clear understanding of the performance of the construction supply earthing system. This involves understanding how it interacts with other system components in the tunnel under likely fault scenarios. Injection testing allows the key parameters to be determined, so that the correct mitigation measures can be put in place. Mitigation measures include: Installing additional earth electrodes to reduce the local earthing impedance. This option is dependant on the soil resistivity in the area, is not always cost effective, and does not resolve the issue of circulating currents. Installing low impedance return paths to source substations. Care must be taken that this does not create transfer hazards as a result of other fault scenarios. Isolation between earthing systems. This is a common approach in mine earthing systems. The incoming HV supply to the above ground substation is provided with a separate earth grid to that of the below ground earthing system. If designed correctly, this arrangement will greatly reduce the transfer hazard within the tunnel resulting from faults at the surface substation. However, large currents may still circulate as in the example given in Section It is often necessary to install fault limitation on the supply feeding the underground workings. Although this may be quite costly, it may provide the most robust solution, especially in high resistivity areas. It is important to note that this will not address transfer voltage issues associated faults external to the construction supply. In one case where a wharf was built to allow the extracted material to be removed by barge, it was found that it was more cost effective to use earth electrodes in the salt water to provide an extremely low earth impedance. 4. OPERATIONAL SAFETY Many of the hazards associated with the construction stages are equally valid for the completed system. The main differences being that the earthing and power systems are no longer in a state of constant modification, however, the systems must operate correctly over a much longer period of time. Some of the key issues include: Electrical equipment installed at remote locations requires special attention, similar to transfer hazards associated with the mining case. Transfer hazards from an external substation.

5 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 5 Circulating currents and induced voltages. Ongoing inspection and testing to ensure correct long term operation. The earthing system must be designed to ensure that transfer hazards as well as touch and step voltages are within acceptable limits. This should be confirmed as part of the commissioning process, and remedial measures implemented where compliance to the specified safety criteria is not achieved. 4.1 Transfer Hazard Example In a recent tunnel project, to ensure high reliability of supply as well as meet necessary power requirements the supply for a new tunnel substation was obtained from a major 132/33kV distribution substation. The 33kV supply to the tunnel was fault limited to less that 3kA, and was supplied by two, three core cables with individually screened conductors. This arrangement reduced the touch voltages associated with a 33kV earth fault to well within acceptable limits. However, the fault scenario of concern was a 132kV earth fault at the major substation. This condition could result in an earth potential rise in excess of 1500 Volts, and a large portion of this could be transferred to the tunnel earthing system posing a hazard to personnel. The solution involved distributed earthing along the length of the tunnel with careful arrangement of earthing electrodes to reduce interaction with steel reinforcing. Special isolation configurations were designed for areas where a DC traction system passed over head. To ensure correct long term operation of the earthing system an inspection and test plan should be developed and implemented. Earthing systems unlike other systems generally do not result in plant outages or obvious problems when their components fail. It is not until there is a major fault that the uncontrolled flow of fault current and large voltage rises cause damage to equipment or injury to personnel and the problem is identified. The foregoing issues should be addressed in the design phase. Conductors and electrodes should be arranged to facilitate testing and inspection without major disruption to the operation of the system. 5. CORRECT OPERATION OF UTILITIES Circulating currents and induced voltages not only present hazards to personnel, but may also result in damage to, or malfunction of sensitive equipment. 5.1 Correct Operation of Power System Cable Network The operational and safety risks created by induction from earth fault current flowing in power cables into auxiliary services can be difficult to mitigate. The continuous earthing system provided for the power system does not always provide adequate shielding for magnetic interference. Cross bonded cable designs may not be available if the real estate required for the cross bonding pits is to difficult to obtain. In s uch instances single point bonding arrangements may be used to prevent circulating currents in the cable screens. It is often the case that the firing voltage of the sheath voltage limiters (SVL s) is set too high to fire under fault conditions and therefore the sheath will remain open circuit during a fault. In one case the maximum sheath voltage when in single point earthing configuration was 50 volts, with a SVL firing voltage of thousands of volts. While 50 volts may seem a harmless sheath potential, because the SVL s don t fire the sheath cannot conduct the fault current and therefore the inductive interference into auxiliary services was some 90% higher, (than the

6 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 6 fully cross bonded case) stressing the electronic equipment and compromising safety for maintenance staff. 5.2 Correct Operation of Utilities It is critical that auxiliary systems such as fire detection, communication systems and control systems operate correctly under earth fault conditions. Magnetic coupling between the faulted phase conductor and auxiliary circuits can result in the generation of significant voltages or induced currents, that may cause damage to, or temporary malfunction of equipment, or present hazardous voltages to personnel. It is often difficult to mitigate issues in the auxiliary circuits, as each type of circuit will have different operational requirements, requiring individual considerations of how best to provide effective protection. There are often a number of systems within the tunnel that cannot be earthed, due to either operational requirements (eg LV power, communications cables), or current carrying limitations. These systems are usually earthed at one point. In addition to the voltage of the 'earthed point', a voltage is induced into the exposed 'unearthed' section of the conductor. These issues are highlighted in the following examples. LV Power Cable - Single phase LV cables up to 4km in length are often used to supply the fire water systems. In one case a maximum of 3000 V/km of exposure was induced, the voltages far exceeding the allowable limits. Therefore, some form of isolation was required to limit the 'exposed' cable length. Analysis was undertaken for various cable lengths and the maximum distances between isolation transformers determined to be 400m in this particular case. Communication and Fire Sensor Cabling - In the same case the main communication cables were run in RS485 full differential signalling and were terminated at control nodes every 150m. From these nodes conductive cabling was used to link to the fire detectors. Two areas of concern were crosstalk and voltage withstand. Analysis showed that as the differential repeaters could withstand up to -10dB power frequency crosstalk, then 750m of unshielded cable could be installed. The voltage withstand level of the input circuits on each repeater was required to be rated to 1kV, as the induced voltage levels were calculated to be 600V between repeaters. 5.3 Practical Management of Galvanic and Impressed DC Corrosion The common causes of corrosion associated with tunnels are listed as follows: Dissimilar metals foundations, pipe, earth conductors Varying soil types along the length of the tunnel Impressed DC current from traction system The most common cause of electrolytic corrosion is the use of dissimilar metals. To minimise this effect, it is common practice to attempt to provide electrical isolation between the earthing system and structural foundations. Experience has shown that it is practically impossible to guarantee this isolation. The most common points where the electrical isolation is defeated includes the hold down bolts securing large electric drives or switchboards to concrete foundations as well as low voltage equipment such as distribution boards and light fittings bolted directly to steel work. Although the attempted isolation may not be one

7 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 7 hundred percent successful, the increase in resistance between the two systems may reduce the rate of corrosion to an acceptable level. Electrolytic corrosion may also be a result of varying soil types along the length of the tunnel. This demands careful consideration regarding where earthing electrodes are to be installed, as well as the type of material chosen. It may also necessitate isolation sections in the tunnel structural steel work, to ensure that, the steel work is not electrically continuous along its length. This requires close coordination with other services including pipe work and the electrical system. The other major cause of corrosion is stray DC currents from traction system leakage. The location and amount of corrosion is dependant on the magnitude and the path of the leakage current. The worst case scenario is when the DC traction system substation is located in close proximity to the tunnel steelwork, where the current densities flowing out of the tunnel steel work maybe quite high. The basic means of corrosion control can be summarised as follows: Coating or removal of either anodes or cathodes Breaking the electrical connection between anode and cathode Removing the electrolyte or increasing its electrical resistance Application of a protective current (Cathodic protection) As outline above, there are many mechanisms that may result in corrosion, and in many cases the application of a single method will not be the most effective way of providing a robust solution. It is also important that these issues be addressed early in the design as they will require close coordination between a number of engineering disciplines including civil, structural and electrical. 6. DESIGN AND COMMISSIONING PROCESS As outlined in the previous sections, it is extremely important that a systems approach be taken to earthing system design. The following section summarises the key steps in designing an earthing system. The first step in the process is information gathering to determine: The design fault currents and ground return currents. Soil conditions (resistivity, moisture content, seasonal variations, geological and corrosive properties). Site area restrictions, alternative site possibilities, corrosion issues. Services and facilities nearby (or further away) that may be affected by ground currents/voltage rises, or by induced voltage or currents due to fault currents flowing in lines. Site testing requirements. This information gathering step is critical, in that the significant factors must be appraised before simplifying assumptions can be made with confidence. To ignore parameters, such as soil resistivity layering or voltage transfer paths, can lead to unsafe conditions or over expenditure.

8 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 8 Once the information is gathered, the design procedure is structured to minimise complicated analysis. Decisions on whether complex modelling will be necessary may be judged from the results of the data gathering investigations and preliminary calculations. For example, detailed analytical modelling is only recommended in order to: Find appropriate designs if simple empirical formulae cannot be acceptably applied to the particular installation. Investigate a range of remedial measures. Minimise manpower and material costs by more accurate modelling when appropriate. The preliminary design stage should be closely coordinated with other disciplines as design constraints (e.g. those associated with cable sheath bonding or corrosion issues) may conflict with earthing requirements. In a recent project, isolation sections installed in steel reinforcing to mitigate corrosion issues imposed limitations on the placements of earthing electrodes. In most of the cited investigations, injection testing was incorporated throughout the design process. A high level of interaction between testing and design is required in complex earthing configurations, where there are a significant number of unknowns. Identification and quantification of unknown earthing parameters allows a more robust design to be undertaken. More detail is available in a previous paper by Woodhouse [3] on testing in highly urbanised environments. In a recent project where soil resistivity data was not easily available, an earthing electrode was installed and tested and the results were used to refine the computer models developed for the remainder of the system. Additional modeling and testing was also carried out throughout the installation to allow design engineers to refine their designs as the installation progressed. The feedback to the design group provides confidence that the complete system would meet the design requirements, as retro fitting additional earthing within the tunnel would be extremely expensive if not practically impossible. Commissioning of the installations involves a two stage process of verification and hazard assessment. Verification requires that the earthing system installation be checked against what was required by the earthing system design. This also establishes initial performance figures which form a basis for future comparison. Hazard assessment requires that sufficient injection testing be conducted to ensure that the most significant hazard scenarios may be determined and performance assessed. 7. CONCLUSION In light of the results of the case studies discussed it may be seen that it is essential that a Systems Approach be taken in the development of earthing systems associated with tunnels. The earthing system design must be integrated with many components of the tunnel including the civil, structural, power reticulation and auxiliary services. The key issues involved in the earthing system must be addressed in the conceptual design to ensure that the conflicts between disciplines are managed. This close coordination must be maintained at all stages throughout the project and must include designers, project managers, installation personnel as well as commissioning engineers. A correctly designed and implemented earthing system reduces hazards to personnel and equipment, eliminates the need for difficult and expensive rectification works, and ensures high reliability and long term safety and operational compliance of the earthing system.

9 City Tunnel Projects Engineering Safety and Quality within a Constrained Environment Page 9 8. REFERENCES [1] ESAA EG-1:1997 ESAA Substation Earthing Guide, Electricity Supply Association of Australia, September. [2] Carman, W.D., Mine lightning protection and earthing system related hazard assessment and management ERA Earthing 2000 Transcripts. [3] Carman, W.D., Woodhouse D.J., Poon P.W.Y., Measuring the Performance of Earthing Systems in Cable Fed Systems in Highly Urbanised Environments, 10th CEPSI Conference, 1994, Vol. III. 9. BIOGRAPHIES Lachlan J McKerrow was born in Sydney, Australia on September 30, He received his BE(Elec) (1993) from the University of Newcastle, Australia. Following graduation he worked in heavy industry where he gain experience in the design, installation and commissioning of power and control systems associated with large bulk material handing plants. He joined Energy Australia in 1999, were he is a Consulting Engineer in Safearth Engineered Solutions, a specialist engineering group focused on earthing including research and development. William D Carman was born in Newcastle, Australia on June 14, He received his BE(Elec)(HonsI)(1982) from the University of Newcastle, Australia. He joined Energy Australia, then Shortland Electricity, in 1977 as a cadet engineer. Since 1982 he has been closely involved in earthing systems design and testing, R&D projects, and training throughout Australia and the Asian region. He is active in standards committees and chairs the ESAA earthing committee. He is currently working towards his PhD in the area of earthing system risk quantification and mitigation. Darren J Woodhouse (M'2000) was born in Maitland, NSW, Australia, on May 25, He received his B.E.(Elec.)(Hons I) (1993) and BMaths (1994) from the University of Newcastle, Australia. He joined Energy Australia, then Shortland Electricity, in 1988 as a cadet engineer. Since 1993 he has been Development Engineer in Safearth Engineered Solutions, a specialist engineering group focussed on earthing, including research and development. He is currently working towards his PhD in the area of power system earthing system testing.