Public Summary
[ Last Updated 13 January 2006 ]
1. Consultant Profile
Integral Energy is one of Australia's keenest competitors in the national energy services business.
As a NSW State-owned corporation incorporated under the Energy Services Corporations Act 1995, we operate commercially, within the terms and conditions of the Electricity Supply Act 1995.
Integral Energy was formed on 1 October, 1995, following the merger of the former Prospect Electricity and the major portion of Illawarra Electricity, as part of the NSW Government's reforms of the electricity industry.
We also distribute and retail electricity and value added energy services to over 1.8 million people in households and businesses across a network franchise area spanning 24,5000 square kilometres in Sydney's Greater West, the Illawarra and Southern Highlands regions, as well as operating a gas business centred on the NSW South Coast.
Integral Energy purchases wholesale electricity in bulk from New South Wales and Victorian generators and a number of independent generation sources, including cogeneration and land fill methane extraction plants.
Technical Experts
The Ministerial Inquiry engaged Integral Energy Australia (IE) as technical consultants to prepare this report.
The Chief Consultant who provided a strategic overview to the consultancy was:
J.N. Allen, BE Grad.Dip.Bus(Mgt), C.P.Eng, FIE(Aust)
General Manager, Integral Energy Networks
The two technical experts who were involved in the Distribution Planning Review were:
Michael Tamp, BE (Hons)
Manager Network Capability, Integral Energy
Ty Christopher, BE (Hons), MIE Aust., C.P.Eng.
Asset Strategy Manager, Integral Energy
The two technical experts who were involved in the Cable Failure investigation were:
Henry Kent, Dip Eng.,
Member CIGRE Aust. Panel 21 HV Insulated Cables
Manager Business and Product Development
Integral Energy Contracting
George Bucea, Master Eng (Romania)
Member CIGRE Aust. Panel 21 HV Insulated Cables
Senior Engineer High Voltage Cables, TransGrid
2. Introduction
In early 1998 a series of failures occurred in the underground cables providing electricity to the central business district (CBD) of Auckland. As a consequence much of the CBD of Auckland was blacked-out for a significant period. It is understood that the loss of power has had a significant impact on the economic activity of Auckland.
Mercury Energy Ltd, the owner of the cables that failed, is an electricity distribution and retail company that services the southern half of the greater Auckland City, an area incorporating the CBD.
A Ministerial Inquiry was established to review the cable failures with special focus on investigating what happened, why it happened and lessons for the future. The Ministerial Inquiry determined there was a need for expert technical assistance to help in its investigations.
Integral Energy, one of Australia's largest energy suppliers, was selected to provide the required expert technical assistance based on its internationally recognised technical expertise in both network planning and utility management, and underground cable installation, operation and maintenance.
This document serves as a Public Summary bringing together the results of both the planning and cable failure parts of the investigations in non-technical terms. This document is supported by two technical reports that provide details of the analysis applied during the investigation.
3. Ministerial Inquiry Terms of Reference
The Inquiry has to report to the Minister of Energy before the end of June 1998 answering the following question:
What caused the power supply and in particular, the power cables into the central business district of Auckland to fail?
The technical reports prepared by Integral Energy investigated two aspects of the power supply:
- A review of the network planning and risk evaluation processes of the high voltage power supply network.
- The technical cause of the cable failures.
4. Investigation Methodology
To answer the questions posed by the Inquiry, Integral Energy used the following investigation process:
- Integral Energy first established the industry standard practice appropriate for supplying electricity to a city the size of Auckland using where appropriate published data and drawing upon industry practice and experience.
- Integral Energy then established the practices applied by Mercury Energy in supplying electricity into Auckland CBD and the events surrounding the power supply failure.
- Finally, Integral Energy compared industry standard practices with those of Mercury Energy, enabling conclusions to be drawn on the cause of the power supply failure.
In determining the facts surrounding the power failure Integral Energy established its findings based on site visits, examination of documentation and technical records, examination of the failed cable components and discussions with relevant people from Mercury Energy and Trans Power.
5. Electricity Distribution Network Planning
5.1 Network Planning Philosophy
In common with other electricity distributors internationally, Mercury Energy is required to plan and develop its electricity distribution system to ensure the needs of customers are satisfied.
One of the roles of the network planner is to anticipate electrical load growth and to develop a network that will meet the requirements of customers in the future. The amount of electricity a network can supply is referred to as the supply capacity.
While achieving this goal the network planner must also consider the security of the electricity supply. Supply security is a term used to describe the ability of the electricity network to continue to provide electricity if certain critical elements of the network are out of service.
Supply security is commonly achieved by providing supply to a load using several smaller capacity cables instead of one large cable. If a single large cable supplies an electrical load and that cable is damaged or taken out of service, all customers supplied by that cable are affected.
However, if a number of small cables are used to supply a load, and one is damaged or needs to be maintained, the remaining small cables can supply the electrical load with minimal disruption to customers.
The size and number of cables will be influenced by the load they supply and the cost of the alternatives. Finding the best combination is the role of the network planner.
5.2 Industry Standard Planning Practices
Clearly, supply capacity and supply security come at an economic cost to the community. The challenge for the electrical network owner is to find a balance between what is considered an appropriate level of capacity and security and weigh this against the cost of electricity to customers.
While there is no recognised published standard which defines an appropriate level of supply security, it is internationally accepted practice to provide a major load centre, such as the Auckland CBD, with a sufficient range of alternative electricity supply options. The aim in providing alternative options is to enable supply to be maintained should the cable with the largest capacity be taken out of service.
5.3 Mercury's CBD Network Planning Strategies
Auckland's CBD is supplied by a total of four 110kV underground cables (two 110kV gas filled cables and two 110kV oil filled cables).
The electricity supply to the CBD was operated on the basis that a failure of the gas filled cables would result in the electrical load being shared by the remaining oil filled cables, without loss of electricity to customers.
This planning strategy indicates a heavy reliance on the integrity of the oil filled cables supplying the CBD. This reliance was based on the very good service history of the oil filled cables compared to the poor service history of the gas filled cables.
During the late 1980s it was determined that by 1997 there would be a need to enhance supply security to the CBD consistent with electrical load growth projections and city development. It was proposed that a tunnel be constructed capable of carrying additional 110kV cables, and other services, into the heart of the CBD. The tunnel project was delayed three years due to significant difficulties in securing a route for the tunnel and is now due for completion by September 2000.
A Risk Management and Contingency Plan had been developed by Mercury Energy for the cables they expected to be a problem. The plan outlined the steps to be taken to manage the recovery of electricity supply in the event of a limited power failure. The plan did not provide for a response in the event of a total loss of the oil filled cables. This emphasised the reliance placed on the reliability of the oil filled cables.
6. Failure of the Underground Cables
6.1 Gas Filled and Oil Filled Cables.
Two gas filled 110kV underground cables and two oil filled 110kV underground cables were used as the main electrical supply into the Auckland CBD. These types of cables are common throughout the world and the following provides some general industry and common performance problems associated with their use.
Gas Filled Cables
The two gas filled cables used to supply the Auckland CBD are known as impregnated pressure (IP) cable. They contain high-pressure nitrogen gas that assists in the performance of the cable. The purpose of the gas is not to cool the cable but to improve its insulating properties.
IP cables were first introduced in the 1940s but have since gained a reputation of being unreliable.
Common problems associated with IP cables are:
- Gas leaks associated with mechanical failure of the cable caused by vibration or unstable ground conditions; electrical load variations; or poor workmanship during jointing and installation.
- Gas blockages within the cable resulting in under-pressurised sections of the cable which may lead to eventual electrical failure.
- Maintenance or repairs to gas filled cables can result in the cable being out of service for weeks because of the work associated with the de-gassing and re-gassing processes.
Oil Filled Cables
The two oil filled cables used to supply the Auckland CBD are known as self-contained oil filled (SCOF) cable. They contain oil, which is distributed along the length of the cable which assists in the performance of the cable. As with the IP cable, the purpose of the oil is not to cool the cable but to improve its insulating properties. SCOF cable has a very high level of reliability if it is operated and maintained correctly.
Common problems associated with oil filled cables are:
- Oil leaks due to faulty workmanship, faulty installation practices and external damage.
- Damage to the outer coverings of the cables, usually caused by external interference.
- Damage to the cable during manufacture.
- Cracks in the cable due to mechanical fatigue.
6.2 Industry Standards for Cable Maintenance and Operation
From an industry perspective, there are a number of factors which are critical to the life duration of the types of cables used to supply the Auckland CBD. Most notably, these cables must be installed correctly and maintained and operated within the limits for which they were designed.
Soil and geographic conditions can have a major influence on the performance of the cable and for this reason these conditions need to be monitored. In particular accurate data is needed on the soil temperature and soil resistivity. Because these conditions can change over time it may be necessary to also change the supply capacity rating of the cables over time.
Exceeding the design operating temperatures shortens the life of the cable because heat adversely affects the quality of the cable insulation. Ensuring an underground cable performs to its optimum capacity relies heavily on a routine maintenance program which ideally would incorporate the following elements:
- Monitoring oil and gas pressures.
- Carrying out routine cable covering insulation resistance (IR) testing.
- Visual checks including checking for corrosion.
- Monitoring cable temperatures.
- Monitoring ambient and soil temperatures.
6.3 Mercury's Cable Maintenance and Operation Standards
The types of cables used to supply the Auckland CBD were appropriate for the purpose intended and reflect the technology available at the time of their installation. Despite their age, with appropriate levels of maintenance, these cables were more than capable of providing a reliable standard of service when operated within the limits for which they were designed.
There is evidence, however, that the cables supplying the Auckland CBD had been operating at temperatures above their design limits from an early stage after their commissioning.
Mercury Energy does not have a detailed maintenance policy, specific to pressurised cables. Based on Integral Energy's investigations this critical area of maintenance appears to have been reliant on maintenance staff undertaking the maintenance based on their past experience, rather than on a formal set of procedures.
Furthermore, contrary to manufacturer's instructions Mercury did not carry out routine cable sheath insulation resistance (IR) testing. These tests are most important as they are designed to highlight defects in the anti-corrosive cable covering. If ignored, such a defect can lead to corrosion resulting in gas or oil leaks.
There is no evidence to suggest that cable, ambient and soil temperature monitoring procedures have been adopted by Mercury.
There is evidence that Mercury's maintenance and operating staff were not aware of the existence of all of the cable low gas alarms fitted to the gas filled cables even though they have a sophisticated alarm display facility. There is a possibility that had all the existing facilities been fully utilised one or more of the electrical failures to the gas cables could have been averted.
7 Conclusion
Based on the investigations and the technical assessments undertaken, Integral Energy has concluded the following:
- The load forecasting methodology used by Mercury Energy was not highly sophisticated but is consistent with internationally accepted practice.
- Mercury Energy's approach to systematic reviews of the electricity supply network is in accordance with industry practice.
- The general approach to the planned evolution of electricity supplies into the CBD is consistent with industry practice and has led to an electricity supply network appropriate for a CBD area the size of Auckland.
- Delays were experienced in securing a route for the cable tunnel which would carry additional 110kV cables into the CBD. Based on recent load forecasts the fifth cable from a planning perspective will now not be needed until the year 2000, however, if the project had been completed to the original timetable the power supply to the CBD would have been more secure.
- The problems associated with the reliability of the gas filled cables did draw the focus of Mercury's network planners away from a more comprehensive understanding of the total supply security issue. This distraction is not considered to be a direct contributor to the loss of supply.
- Mercury Energy's network planners made judgements based on the information provided by the cable asset managers. If the network planners had been aware of the actual loading capability of the underground cables the network planning and contingency planning approaches may have been different. This may have avoided the loss of supply to the CBD. This is considered to be one of the central issues to the failure in supply.
- The risk management and contingency plans established by Mercury to manage the supply security risks appropriately reflected the reliability concerns of the gas filled cables. Mercury in recognition of previous service reliability with the oil filled cables did not have a specific risk management and contingency plan to cover the failure of these cables.
- There was insufficient technical expertise related to cable technology and insufficient appreciation of the importance of soil conditions within Auckland Electric Power Board (AEPB) at the time of preparing the specifications for the gas filled cables and the oil filled cables, and later during installation.
- Cable manufacturers supplied, and AEPB accepted, cables which were in accordance with AEPB specifications. Both the gas and oil cables were installed in soil conditions which did not allow the cables to achieve their specified rating.
- The two gas filled cables were installed contrary to good engineering practice. In at least one location ground stability was inadequate for the purpose of properly supporting the joints and the cables leading into them. A number of electrical faults subsequently occurred at that location.
- Insufficient investigation was undertaken by AEPB/Mercury in assessing the causes of repetitive electrical faults on the gas filled cables. Most of these faults were as a result of poor installation.
- Not all of the "low gas pressure" alarm functions provided as part of the gas filled cable installations were appreciated nor exercised by Mercury at the time of Integral Energy's investigation. No automatic cable disconnection facilities were in place in case of "extremely low gas pressures" within the cables.
- Mercury assigned a low reliability to the gas filled cables and did not place emphasis on improving the condition of these cables.
- Mercury had a view that the two oil filled cables were fully reliable - up to their full nominal rating of 60MVA. (MEL113, page 23) In fact the rating of these cables was much lower due to the ground conditions in which they were buried. When they were loaded to more than half their nominal rating they would have started to overheat.
Once the cables were installed most of these issues would not be detected by conventional routine cable maintenance practices and there are grounds to believe that Mercury Energy may have been lulled into a false sense of security.
Mercury's false sense of security is confirmed by an apparent lack of precautions that other operators may have taken following the failure of the two gas filled cables. For example:
- securing these now critically important oil filled cables from external damage by arranging regular route patrols, and,
- monitoring for possible overheating by installing and interrogating temperature monitoring devices at known, or suspected, hot spots along the oil cable routes.
- When the two gas filled cables failed, additional load was placed on the two oil filled cables. The first oil filled cable failed due to thermo-mechanical reasons. This means that the higher than allowable cable temperature facilitated the metallic conductors to move with respect to their insulation and metallic sheath. This movement caused a joint to be compressed internally resulting in the electrical failure of that joint.
- The remaining oil filled cable then took on additional load and became overheated, to the extent that it failed under "thermal run-away" - meaning that the cable generated more heat than its environment could dissipate causing the insulation to break down and the cable to fail electrically.
- Mercury does not have an adequate maintenance policy for 110kV gas and oil filled cables. It did not comply with manufacturers recommendations in regard to the routine testing of gas pressure and oil pressure alarms and accuracy of their initiating devices, and electrical checking of the integrity of the outer coverings of the cables.
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