Underground » Detection and Prevention of Fires and Explosions
The experience of Oceanic Coal (Teralba, NSW) with failures of High Voltage (HV) underground plugs and adaptors led them to approach Pacific Power to investigate the performance of HV plugs and adaptors used in underground mining. The results of the initial investigations were considered of value to the mining industry as a whole and a research proposal was submitted to ACARP for a grant to do a thorough investigation. The investigation resulted in four reports, this being the final overall report.
The study was conducted in two parts. Firstly, a survey was undertaken to gain feedback from the mining industry and to tap into the body of experience of the users. The second, involved a series of electrical tests that challenged the findings of the survey in an environment that would shed light on the most important aspects of the plug and adaptors design, assembly and use.
The survey showed that moisture in the form of Relative Humidity or free ground water was strongly correlated with the failure of the plugs and adaptors. It also brought to light issues to do with,
- Test equipment, techniques and training,
- Standardisation of acceptable pass/fail criteria,
- Lack of high voltage awareness within the mining industry,
- Limited appreciation of Australian and International Standards for HV equipment and tests,
- An increased risk of failure associated with in-house cable termination.
An accelerated aging test was devised to simulate the conditions that the plugs and adaptors would experience in underground operation and give them an electrical duty approximating 10 years of operating life within 3 months. MM supplied the plugs and adaptors and 3M supplied the cable terminating kits, all at no charge.
The first test was carried out on the adaptor components and the tests showed that these components on their own were not a likely source of failure. The test was repeated for a fully assembled plug and adaptor and the results were not as reassuring. The fully assembled units became subject to a number of detrimental, partial discharge sources (similar to sparking). Particular findings are made to the role of the,
- Phase barrier,
- Coupler, its shroud, pin and assembly,
- Insert moulding and component fit,
- Plug cable entry area.
A number of design weaknesses were identified that lead to the generation of partial discharges within the assembled plug and adaptor. Suggestions were made on how to overcome some of the weaknesses whilst others would require monitoring to ensure that the full working life of the plug and adaptor is realised. Difficulties in monitoring the condition of plugs and adaptors in a high relative humidity environment were discussed. These problems have such a profound bearing on the results obtained in underground environments that they need to be specifically addressed. It is highly recommended that a set of guidelines be produced by the industry, that set out methods and criteria for testing to ensure that consistent acceptable standards are reached.
During the course of the investigation a substantially new design of plug and adaptor was brought onto the market and that was also subjected to a similar accelerated aging test as the MM style units. The results were encouraging although (as only one plug and adaptor was tested) the results were not statistically significant. Given the innovative nature of the design it is expected that over time, other manufacturers will also act to incorporate some aspects of this new design into their own plugs and adaptors.
A primary benefit of this research has been to elevate issues associated with high voltage awareness and safety within the industry. Miner's are able to rely on a large body of knowledge encompassed within Australian and International Standards to guide them in the specification, receipt, application and testing of high voltage equipment. The adoption of practices outlined in this research and within the standards will result in fewer failures of high voltage plugs and adaptors and lead to higher levels of safety and increased production within the industry.