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Increasing Mine Productivity by Optimising Electrical Protection

Underground » Mining Technology and Production

Published: November 17Project Number: C25069

Get ReportAuthor: Richard Griffiths and Peter Stepien | ResTech

This project sets out to examine and re-evaluate some key mining electrical design and protection techniques on a first principles basis, with the aim of improving both productivity and safety. It is based on the proposition that changes in the technology and scale of mining electrical systems over the past few decades have produced problems which are not fully addressed by regulations, standards and industry practices that are mainly based on older technologies and electrical environments.

 

If we do not fully understand why our electrical systems have been safe, then as mining environments and technologies change, we may eventually stumble into danger and, in addition, we are losing production unnecessarily due to nuisance trips on protection systems. Regulations and standards have been prescriptive on the subjects of the earth fault tripping ratio, earth fault limitation level and earth continuity and earth leakage trip levels, without providing much technical justification for the rules they contain. While some changes in standards have occurred recently, we believe that more changes are needed.

 

The protection systems of interest are primarily those related to managing touch voltages and arcing under earth fault conditions and the key electrical system design issues examined are related to system capacitance, tripping ratio requirements and the effects of increased frequency.

 

The work completed has led to the following main outcomes and recommendations:

· There are advantages in having key electrical system design rules embedded in mining regulations and Australian standards however due to large scale changes in the mining industry, there are now many situations where the prescriptive nature of such rules could actually decrease safety and productivity in some cases. Our recommendation is that the relevant sections of mining regulations and Australian standards need modification to allow for alternative approaches that are backed by engineering analysis rather than fixed rules.

· Historically, Australian standards have required a minimum earth leakage tripping ratio of \10\. Even AS/NZS 4871:2012 effectively continues this general requirement. This prescriptive approach is shown to actually decrease safety in some situations and contribute to nuisance tripping of various protection systems. We have researched and analysed literature on the subject in order to identify the source of this 10:1 tripping ratio rule and examined the requirements in the U.K. and U.S.A, where much lower tripping ratios are used (\3\ and \2.5\ respectively). The work undertaken suggests that the value of \10\ is a somewhat arbitrary figure that is probably based on tests carried out on old electromechanical protection relays when exposed to faults flowing through rectifying devices (DC faults). It can be shown that, for particular combinations of circuits and relays, a tripping ratio of \10\ is not large enough while, on the other hand, it is far too large for most common situations. With the use of modern electronic relays and the recent introduction of DC sensing earth leakage relays, most systems would only require a tripping ratio of \4\ . The majority of the benefit would be gained even if AS/NZS 4871 was simply altered to allow a tripping ratio of \5\ without specific conditions other than requiring an engineering report for systems that include directly connected rectifiers. We recommend that the relevant standards be modified to allow an engineered approach to determining the optimum tripping ratio on a given electrical installation.

· Over past decades there has been a move to higher power system voltages and an increase in the size of mine electrical systems. This means that system capacitance to earth is now much more important than it was when many of our standard golden rules for protection were developed. Our examination suggests that the insulation on some 11 kV underground systems could be at risk during some types of earth faults. This may increase the risk of dangerous cross country earth faults. The normal way to mitigate this risk is to increase the earth fault current limitation level however regulations and standards often prevent this approach. A perceived concern is that higher earth fault limitation current levels may increase the energy in arcs at the fault location, however we have demonstrated that this effect is minor on most 11 kV systems because the fault arc power is already dominated by the capacitive charging current which greatly exceeds the NER current.

· In contrast to the situation on 11 kV systems, the risk to insulation on 3.3 kV systems and 1 kV systems without large EMC filters, during some types of earth faults, is well below industry standards so there is scope to reduce the earth fault limitation level on these systems. This can lead to reduced risks from touch voltage and arc fault energy. It can also increase noise margins on earth continuity relays (by allowing higher trip settings) and so reduce nuisance tripping.

· Electrical systems and key protection parameters were analysed using a frequency response approach. This has provided insights into the effects of capacitance and the frequency of the excitation source. It is particularly relevant to understanding how the introduction of variable frequency drives can effect protection system performance and change the touch voltage seen on equipment during earth faults, switching transients or normal operation.

· Calculations and detailed simulations of actual 11 kV, 3.3 kV and 1 kV systems suggests that sympathetic earth leakage trips due to system capacitance are likely to be very common on 11 kV systems and less common, but certainly possible, on 3.3 kV and some 1 kV systems that have large ground referenced EMC filters. An improved earth leakage protection approach is evaluated to mitigate this problem.

· Known issues with earth leakage relays and earth continuity relays are examined and some possible improvements in their design suggested. In order to achieve these improvements, alterations to the relevant standard (AS/NZS 2081) will be required. An earth continuity noise immunity test is recommended for addition to AS/NZS 2081.

· The concepts of adaptive protection are investigated. The proposal is that an intelligent system receives information from protection relays to detect changes in the system. An automated analysis of the electrical system then allows the detection of situations which don't comply with the touch voltage requirements of AS/NZS 4871. The system can then alert the mine engineer to this issue and recommend an adaption of protection settings that can achieve compliance.

 

The work undertaken has been presented at various seminars, and an 'industry forum' and reviews have been invited from a group of experienced engineers representing manufacturers, industry and regulators. The aim of this report is not to claim any authority but primarily to promote discussion, closer examination and serious consideration of the detailed technical foundation for these important electrical protection systems.

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