AORC Technical meeting 2014

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1 http : // B AORC Technical meeting kv Long Distance Underground HVAC Cable Circuit NAVEED RAHMAN Nexans Olex Melbourne, Australia SUMMARY In 2009, the Victorian state government decided to a build an AUD 3.5 billion water desalination plant east of Melbourne on the coast in Wonthaggi, Victoria. The desalination plant was designed to deliver up to 150 billion litres of desalinated water per year and required 165MW of power to operate the plant. After evaluating the possible options, e.g. building a Power Generating Plant, transferring electricity via Overhead Lines, DC Link to supply electrical energy to the desalination plant, it was decided to supply the power using a single 220kV AC underground cable system, which makes it the world s longest 220kV HVAC land cable link. Nexans Olex was awarded the contract to design, manufacture and supply of 220kV underground cable and accessories together with jointing and testing services from Cranbourne Terminal Station (CBTS) to the Wonthaggi Desalination Plant (WDP). The 220kV cable route length is approximately 88km. Based on system design studies; it was decided to build two reactive compensation stations between Cranbourne and the desalination plant, namely Northern Reactive Compensation Station (NRCS) and Southern Reactive Compensation Station (SRCS). This resulted in dividing the entire route into 3 sections as follows: Section Number Section Name Section Length (km) Required Rating (A) Cable Size 1 CBTS to NRCS mm² Cu 220kV Installation Arrangement Laid Direct in Ground in Trefoil 2 NRCS to SRCS mm² Cu 220kV Ducted Trefoil 3 SRCS to WDP mm² Cu 220kV Ducted Trefoil The project works were successfully completed in April 2012 and the desalination plant was commissioned in December Almost all HV underground cable installation projects have unique challenges. This paper presents the Design, Supply & Installation challenges that were encountered during design and construction phase of the new 220kV underground cable circuit delivering power supply to the Wonthaggi Desalination Plant. KEYWORDS 220kV AC Land Cable Link, Underground HV Cable Circuit, System Design, Reactive Compensation, Direct in Ground, Ducted Installation, Cable Installation, Construction Works. naveed.rahman@nexans.com

2 1. INTRODUCTION In 2009, the Victorian state government decided to build a water desalination plant east of Melbourne on the Bass Coast in Wonthaggi, Victoria. The plant was designed to deliver up to 150 billion litres of desalinated water per year to Melbourne. The nearest 500kV and 220kV substation to the desalination plant was approximately 88km North- West in the vicinity of Cranbourne, Victoria. To supply the required power to operate the plant, various options were considered, i.e. Building a dedicated power generation plant which was deemed uneconomical for such a limited power requirement and without a direct energy source. Transmission of electrical power via overhead line (OHL) that was publicly unacceptable. Transmission of AC electrical power via underground cables (UGC) Underground DC cable link was also considered After evaluating the possible options to transfer the required energy to the desalination plant, the State of Victoria decided to supply the power using a single 220kV AC underground cable system. This makes it the world s longest 220 kv HVAC land cable link and it is supplying 165MW of load to the Wonthaggi Desalination Plant. The Victorian government awarded the contract to Aquasure to build the desalination plant and associated infrastructure. Aquasure then subcontracted the construction works to Thiess Degremont Joint Venture (TDJV). TDJV awarded the contract to Nexans Olex to design, manufacture and supply 220kV underground cable circuit from Cranbourne Terminal Station (CBTS) to the Desalination Plant Terminal Station (WDP) in Wonthaggi, Victoria. Nexans Olex contract Scope of Works consisted of: Carrying out the detailed system design Supply and delivery of approx 270km of 220kV XLPE cable Supply and delivery of approximately 235 cable accessories (220kV Joints & Terminations) Supervision of cable installation works Supply and installation of DTS system, which included 3 DTS Units Jointing & Terminating works Carrying out pre-commissioning tests Design verification of as-built cable circuit ratings Project Mile Stones (Key Dates): Contract Signed on: 18 th December 2009 Cable installation completed: August 2011 Jointing works completed: October 2011 Testing completed: November 2011 Cable circuits energised on: o Section 1: 3 rd December 2011 o Section 2: 18 th December 2011 o Section 3: 3 rd March 2012 Practical Completion achieved on: 4 th April 2012 Almost all HV underground cable installation projects have unique challenges, but this being a long underground cable system, special considerations were necessary to design and construct the 88 km of cable system for the desalination plant: Ferranti effect: Cable and accessories insulation design was based on 275 kv to cope with the possible increase in voltage at the receiving end during light load conditions. System design, reactive compensation, losses, harmonics. Supply & logistic issues delivery of 270 km of 220kV cable, 235 joints & terminations, link boxes, bonding & earthing cables, 3 DTS units, optical fibre cables & joints. FAT (cable drums tested at a higher voltage than IEC), sample tests & commissioning tests. Jointing and Termination works. Narrow easement shared with the water transfer pipeline. Constructability and associated civil works. Coordination with major pipeline civil works contractors in restricted corridor widths in widely varying topography. Availability of required resources including qualified jointers to meet the compressed construction timeframes. Project management. 2. PROJECT CHALLENGES 2.1. DESIGN CHALLENGES Due to compressed construction program the original design was carried out as a desk top study based on preliminary cable route survey without carrying out route proving etc. Consequently design modifications & adjustments were required during construction stage as per site conditions. Nexans Olex design team performed these design modifications in a timely manner as required to meet the project completion date. 1

3 Deep crossing designs exact details/long section drawings were developed by the customer and made available to Nexans Olex during construction works. Nexans Olex prepared and verified designs for these deep crossings on a case-by-case basis during construction works. During construction phase, high native soil TR values were measured in sections 1-3 of CBTS to NRCS Circuit, which required adjustment to the system design to maintain circuit ratings. Modifications to other special crossing which needed to meet third party requirements were verified by Nexans Olex in a timely manner, e.g. railway crossings, Melbourne Water crossings, various road-crossing. Adjustments to Joint Bay locations due to site conditions and rebalancing the circuits were also carried out during construction works. Providing Safe Work Method Statements (SWMS) to work under induced voltage conditions in 500kV transmission lines easement SUPPLY CHALLENGES Supply of 270 km of 220 kv cables within 11 months was a challenging task for a single supplier. Nexans Olex supplied the cables from manufacturing Plants in Australia and Belgium. Supply of 235 cable accessories to complete the project in compressed timeframes. Supplying additional cable drums and accessories due to various reasons, e.g. cable damaged during transportation, cable damage during installation, design changes as the project progressed INSTALLATION CHALLENGES Wet weather conditions, site access issues, monitoring multiple sites along 88 km cable route. Verifying cable pulling methodologies and providing pulling tension calculations. Accelerating jointing works by maintaining adequate qualified jointing resources on site to meet the project completion dates. Completing pre-commissioning tests on time, adhering to strict safety requirements and coordinating with numerous contractors. 3. DESIGN CONSIDERATIONS Ferranti effect results in increased voltage at receiving end compared to the sending end voltage in long transmission lines. This occurs when transmission line is either lightly loaded or load is disconnected. Ferranti effect was considered during design. The 220kV cable system is designed to operate at 275kV to cope with the possible increase in voltage at the receiving end during light load conditions. High Voltage Insulated cables are predominantly capacitive in nature. Due to the presence of this capacitance, a current (charging current) will flow through the insulation to the ground via the cable metallic sheath. For a given cable, the total charging current required from the source is proportional to the length of the transmission line. The effect of charging current is generally neglected for short underground transmission lines. However for long underground transmission lines the impact of charging current is considerable and this should be taken into account as it limits the active power transmission capacity. Load flow and system studies suggested providing 2 compensation stations, NRCS & SRCS, along the cable route as per the single line diagram below (Fig 1). Fig. 1 Single line diagram for the 220kV transmission line BP = Booster Pump Station, DPTS = Desalination Plant 3.1. REACTIVE POWER COMPENSATION When designing a transmission or distribution system, designers must take into account the load requirement and the inductive and capacitive elements of the cable circuit, which consume reactive power. This requires the reactive compensation to minimise the charging current component in the cable so the conductor is efficiently used to transfer the power required by the load. To achieve this it is necessary to balance various factors to optimise the management of the reactive power, e.g. optimise capital investment in plant for reactive compensation, minimise system losses to maintain system security and stability. It should be noted that the capacitive charging current will almost overload the cable by itself without carrying any load current. 2

4 Figure 2 shows the typical variation of active power transmission capability against the length of the circuit PRELIMINARY DESIGN Due to the extremely fast track nature of the project, the preliminary design was carried out as a desktop study based on preliminary cable route survey without carrying out route proving and verifying other actual installation conditions. Fig. 2 Variation of active power transmission capability vs circuit length for a given circuit System studies carried out by TDJV had identified two worst case circuit operating scenarios. First scenario was 110% of system voltage at CBTS (sending end) with no reactive compensation at both NRCS and SRCS. Second scenario was 90% of system voltage at CBTS (sending end) with reactive compensation at both NRCS and SRCS at 100% (See Table 1 for details). Studies done by TDJV suggested 500mm² copper cables for CBTS to NRCS section and 400mm² copper cables for cable sections between NRCS & WDP via SRCS to carry both active and reactive power under various cases. Description System Voltage at CBTS Item (Unit) Case 1 Case 2 Comment Un (%) % of Un Load at NRCS BP (MW) Power factor 0.95 Reactive Compensation at NRCS Reactive Compensation at SRCS Load at Desalination Plant Q (%) % of Q Q (%) % of Q DP (MW) Power factor 0.95 Table 1: Most Onerous Cable Operating Scenarios Nexans Olex then carried out detailed load flow studies to verify that the proposed cable system design fulfils the most onerous cable operating scenarios. Load flow studies confirmed that the cable circuits will have the following maximum currents in the cable. Cable Maximum Designed Continuous Current Carrying Capacity, A 500 mm² Cu 220 kv XLPE Cable mm² Cu 220 kv XLPE Cable 475 Consequently design adjustments had to be made during construction works to suit site conditions, e.g. deep crossings, high native soil TR values were found, design of special crossings gas pipes, water mains road crossings, creek crossings, relocation of joint bays OVERALL SYSTEM DESIGN The cable route length between the CBTS and WDP is approx 88 km, which is divided into 3 cable sections / circuits as follows: SECTION 1 - CTBS to NRCS Route length of approximately 13 km 500mm² Cu 220 kv cable Cable supplied by Nexans Belgium plant 3 major cross bonding & 9 minor sections. 3 minor sections (or 1 major section) have 2 sub-sections connected by straight joints Total of 12 sections and each section length of approx 1000 metres. SECTION 2 - NRCS to SRCS Route length of approximately 37 km. 400mm² Cu 220 kv cable Cable supplied by Nexans Australia Plant 5 major cross bonding & 15 minor sections. Each minor section has 2 sub-sections connected by straight joints Total of 30 sections and each section length of approx 1235 metres. SECTION 3 - SRCS to WDP Route length of approximately 38 km. 400mm² Cu 220 kv cable Cable supplied by Nexans Australia Plant 5 major cross bonding & 15 minor sections. Each minor section has 2 sub-sections connected by straight joints Total of 30 sections and each section length of approx 1270 metres. Each section was economically optimised by utilising straight through joints in minor cross bonding sections. This eliminated the necessity of link boxes / pits and concentric bonding cables by half. 3

5 3.4. THERMO-MECHANICAL FORCES Excessive thermo-mechanical forces on cable joints and sealing ends during service conditions can lead to pre-mature failure of accessories during service life of an underground cable system. Thermomechanical behaviour of a high voltage cable system basically depends on thermo-mechanical properties of the cable, type of installation (rigid, flexible or a combination of both) and temperature variations. Table 3: Maximum EMF at ground level The thrust force generated by a cable can be calculated by the formulas presented in Cigre publication TB194. To reduce these thrust forces, Nexans Olex designed and optimised the snaking and clamping arrangement outside the joint bays to minimise the thermal expansion / thrust force of cable on the joints. See below typical snaking arrangements. Fig 4: Maximum EMF level for 500mm² Cable Trefoil Touching Fig. 3 Typical Snaking arrangement at Joint Bays 3.5. INDUCED VOLTAGE AND ELECTRO- MAGNETIC FIELD CALCULATIONS The cable system was designed to keep the induced sheath voltage within acceptable limits. Detailed analysis and calculations were performed for both normal operation and fault condition as follows. Fig 5: Maximum EMF level for 400mm² Cable Trefoil Touching ARPANSA is the Australian Radiation Protection and Nuclear Safety Agency 3.6. CABLE TEMPERATURE MONITORING Table 2: Typical Induced voltages To minimise the EMF levels a trefoil arrangement of cable/ducts installation method was utilised. EMF calculations were carried out for the various cable arrangements used for the cable route. Calculations were based on the design continuous loads. Distributed Temperature Sensing (DTS) system was designed and installed to monitor the cable temperatures between CBTS and WDP. Nexans Olex installed 3 DTS units at NRCS, SRCS and WDP and a dedicated single mode fibre cable to measure the temperature of entire cable route. Fig. 6 DTS Unit arrangement 4

6 installation to ensure that curing temperatures and pumping pressures were controlled so as not to damage the PE ducts. Figure 7: Typical conductor temperatures versus fibre temperature Rise (established using Nexans Olex finite element program Sirolex Fig 8: Typical Micro-Tunnel arrangement 4. INSTALLATION CONSIDERATIONS Civil & cable installation works were carried out by TDJV & their contractors. Nexans Olex monitored the cable installation works to ensure that the cable was installed as per design requirements. Installing 88 km route of 220kV cable required optimal utilisation of the available resources and innovative project management. The typical cable section length between two consecutive joint bays was between 1100m to 1300m. Pulling tension calculations were carried out for each cable section and combination of nose (winches) and bond pulling (caterpushers) techniques were utilised to ensure that the maximum cable pulling tension and the maximum side wall pressure did not exceed the recommended limits. The direction of pulling was determined after giving due consideration to the specific route profile and location of joint bays. During cable installation the pulling tension was monitored and recorded for future reference. Fig 9: Deep crossing under bore 4.1. DEEP CROSSINGS There were a number of locations where 220kV power cables had to cross an existing service or a river. These crossings were classified as deep crossings. Five deep crossings were done by utilising the micro tunnel installation technique. Micro-tunnelling is the special installation technique utilising a small horizontal bore (approx 1.5 to 2.0m diameter) sleeved with a concrete pipe and containing the cable ducts in the desired arrangement. The micro-tunnel was filled with grout encasing the ducts, this was done by filling the void between the bore sleeve and cable ducts with a specially designed thermal backfill material to maintain cable ratings. The material in nature was low shrinkage and pumpable and of low viscosity making it easy to fill. Appropriate measures were taken during Fig 10: Deep crossing under bore filled with grout Due to the depth, special considerations were given to the thermal properties of the surrounding soils. At each crossing, core samples were taken (max depth & half depth) to establish TR values including moisture content to finalise design ratings / installation details / trench cross-section WORKING UNDER EXISTING 500KV OHL The 220kV cable section between CBTS & NRCS shared the same easement with an existing 500kV overhead transmission line, which caused safety concerns. In some cases the induced voltages were 5

7 measured up to 120V, which was considered as a safety hazard for the jointers and the other project site staff. A safe working method statement (SWMS) was prepared to outline the additional safety precautions required during jointing/terminating works, e.g. isolating the far end of the cables during jointing, cables earthed during jointing works, using insulating mats, use of appropriate (7.5kV) insulating gloves during jointing works JOINTING AND TERMINATION WORKS Nexans Olex installed approx 235 joints and terminations within 18 months utilising 4 jointing teams. Special consideration was given to fatigue, health and safety issues when performing repetitive work for a long time, such as the repetitive installation of the same type of joint PRE-COMMISSIONING TESTING The following tests were performed as part of the Pre-Commissioning Tests: Visual Inspection. Sheath Integrity Test (Sheath IR). Main Insulation IR Test. Phase sequence check. Cross Bonding Verification test. SVL test. Positive and Negative Sequence Impedance test. High Voltage AC test 24 hour soak test at system voltage was carried out CONCLUSION LESSONS LEARNT On successful completion of this project, we believe that there are some valuable lessons learnt during the design and project execution, which should be considered when planning such large projects with compressed programs. Allow adequate time for design approvals from all stakeholders. Maintain adequate design resources during project execution to provide solutions for unforeseen events and services found during construction. Maintain adequate site resources to conduct QA Audits of multiple sites during construction works. Cable installation methodologies and installation equipment should be tested prior to commencement of cable installation works on site. Implement a project management system, which controls and records all critical aspects of the project and achieves better communication between various contractors and client staff. Acknowledgment The author would like to acknowledge that this project using the latest developments in cable and installation design technology was successfully implemented through the significant co-operation between various contractors and the client s project management team. REFERENCES [1] Nexans Olex, Design Report for 220kV Underground T/L Cranbourne Terminal Station to Victorian Desalination Plant, 2010 [2] Technical specification for 220kV Underground T/L Cranbourne Terminal Station to Victorian Desalination Plant, 2009 [3] Nexans Olex, System Design and Verification Report for 220kV Underground T/L Cranbourne Terminal Station to Victorian Desalination Plant The successful completion of this project demonstrates that underground HVAC cable system is also a viable option for long distance power transmission. It is important to establish the requirements of reactive power compensation for long distance transmission system. Site access requirements should be clearly understood to avoid delays during project execution. The safety regulations and the legal requirements should be carefully considered by the contractors as they can vary from one state to another in Australia and in other countries. 6