Understanding degradation and failure

What causes failures in HV electrical assets and how can they be prevented?

Forensic science is best known for identifying the causes of aircraft crashes or gathering criminal evidence, but it is increasingly applied to the understanding of degradation and failure mechanisms which lead to electrical equipment failure and cause network outages.

by Anne McIntosh, Managing Director, EA Technology Analytical

Lessons learnt through forensic investigations can reduce the occurrence or repetition of expensive and dangerous failures, and ultimately lead to an increase in the safety and reliability of power system assets. Forensic investigations may also provide key legal evidence, leading to prosecution under health and safety laws and charges of corporate manslaughter, as well as apportioning responsibility for the financial consequences.


The forensic investigation process
The main role as forensic investigators is to determine the condition of electrical equipment, identify the primary degradation and failure mechanisms, and present recommendations to enhance the performance of the assets. The ultimate aim is to prevent future failures from occurring.


Background Information
A forensic investigation starts with all relevant background and historical information. This includes manufacturing histories, service records, operational conditions and details of any repairs, maintenance or modifications, and a reconstruction of events leading to a failure.


Retrieving the Samples
Making sense of the often charred and twisted remains of an equipment failure relies on using two approaches in tandem: sophisticated laboratory analysis of carefully preserved samples, and expertise in interpreting the evidence, results and patterns of events. This means that samples must be retrieved from the failure site and handled with the utmost care to avoid damage and loss of evidence. Then they must be transported without damage or contamination to the analytical laboratory.


Off to the Lab
In the laboratory, we perform comprehensive testing of equipment and samples, including non-destructive testing, electrical testing, materials analysis and mechanical testing. The tests employed are unique for each investigation, and must be carefully targeted to gather all relevant information. Low-powered optical microscopes are used to examine the condition of samples, while higher-powered microscopy with magnification up to x1250 are used in conjunction with image analysers to identify such things as the presence of material defects, impurities, degradation, inappropriate selection of materials, poor quality control and errors in the manufacturing processes. Scanning electron microscopy raises magnification as high as x100,000, enabling x-ray elemental analysis quickly and accurately, while quantitative chemical analysis is carried out on features as small as 0.001m3. Using these techniques, we can study the surface of a sample to determine its elemental composition, identify any contamination, corrosion and degradation products and find and study fracture surfaces. A variety of mechanical tests can be carried out to determine if the mechanical performance of the components contributed to the degradation and failure process.


Putting It All Together
EA Technology Analytical has 40 years’ experience in forensic investigations specifically within the electricity industry. Our collective know-how is supported by a database of records of how thousands of assets have deteriorated and failed in the past.


Laboratory examination and testing produces a mass of detailed information for each investigation, but the most important part of our work is the ability to interpret such data, to present an accurate picture of how and why the degradation and failure occurred. Drawing on our accumulated past experience is an important part of our forensic analysis process.


The Business Outcomes
Prevention is always better than cure – and generally more cost effective! Recommendations from a forensic investigation should be practical and cost-effective, and initiate or support measures to ensure the future safety and reliability of individual assets and the electrical network as a whole. Typically, recommendations are to implement improved condition assessment techniques and maintenance practices to identify and measure degradation progress prior to failure. In some instances, changes in the design of equipment or the materials used are identified, which of course is important for manufacturers. Several case studies demonstrate how forensic investigations have been used to improve the performance and operation of electrical network assets.


Case study – Cable failure
EA Technology investigated the failure of an 11kV XLPE insulated armoured cable from a UK industrial site. The failure disrupted production at the site causing significant financial losses. The cable was 15-20 years old and was manufactured to BS6622, a common specification for industrial cables. The microscopic examination of the XLPE insulation showed a high density of large contaminants and extensive water treeing. Data from the failed cable was compared with data from ageing tests on over 50 XLPE cables from a variety of sources. The ageing test represents 16 years of normal service life, giving results comparable with the real service of the failed cable. The longest water tree in the failed cable was compared with the longest tree in each cable from the ageing tests: the tree length in the failed cable was far greater than the majority of cables and approximately ten times longer than tree length in many of the good quality cables. The trees were all centred on large contaminants several of which were larger than would be found in a good quality modern cable. It was concluded that the failure had been caused by excessive treeing activity, influenced by the large contaminants in the cable. As this was an issue for the whole length of cable, it was recommended that the cable be replaced as the risk of further failures was significant. It was also recommended that the new cable specification should have a limit on contaminant size to reduce the risk of a similar mode of failure in the future.


Case study – CT failure
EA Technology investigated the failure of a cast resin current transformer (CT) from a 33kV circuit breaker at an industrial site in Australia. After carefully removing sections of the remaining resin from the CT, a foreign body was seen on the outside of the winding. The foreign body was removed and examined under a microscope. It was wrapped in layers of the binding tape, apparently trapped there while the tape was being applied during manufacture. The foreign body was enclosed in an air pocket forming a void in the insulation, resulting in partial discharge causing erosion of the surrounding insulation and a build up of heat, leading to catastrophic failure of the insulation. Three other intact CTs from the same location were also destructively examined: one showed early onset of corrosion in a similar location, also putting it at high risk of partial discharge failure. It was therefore recommended that partial discharge monitoring be implemented on the remaining population of CTs to provide early warning of impending failure. It was also recommended that CT replacement should be actively considered to reduce risk of failure in the longer term.


Case study – Substation failure
In 2005 a fault occurred in a 22kV substation on a generation site. The substation contained a 19-panel oil and compound insulated switchboard rated at 33kV. The fault initiated a sequence of tripping events which lasted for 16 minutes and eventually resulted in loss of all generation on the network. A fire initiated within the substation completely destroyed the entire switchboard and severely damaged an adjacent substation. As a result the company involved sustained millions of dollars worth of damage to their site, loss of supply to all of their customers, and system stability issues and rotating outages after emergency works to restore supplies. The investigation into the cause of failure involved collation and analysis of SCADA records, fire system records and generator output records as well as a detailed examination of the damaged switchboard. This revealed a number of areas of electrical arc damage, and a study of the protection operations confirmed that the initiating fault occurred within a compound-filled busbar chamber. Further investigation revealed that the compound insulation had been poured in two layers, one in the factory prior to shipping and the second added after erection on site. Voids or contamination trapped between the layers after installation had resulted in partial discharge, finally leading to flashover. A protection study revealed that the extent of the damage resulted from inoperable protection which was unable to clear the fault. As a result EA Technology has been working with the client to retrofit busbar and circuit breaker fail protection to all of their key substations.


Conclusion
Forensic investigations of failed electrical equipment can assist in working out what went wrong after the event. But much more valuable are targeted recommendations which lead to effective preventive actions, ensuring that failures do not reoccur – a far better outcome than the prospect of a day in court.