power distribution

By Morné Bosch, general manager for sales and marketing, ArmCoil

The importance of implementing effective asset management plans that improve the lifespan of electrical infrastructure equipment is highlighted in best practices around the world. However, from an asset management point of view, these commonly practised maintenance strategies are considered to hold excessive portions of redundant costs. As such, many distribution power network users and owners are using strategies that simply don’t produce the results they are searching for, which then leads to unplanned costs for corrective maintenance or replacement of the distribution transformer. Subsequently, the question arises as to why these preventative maintenance policies/programmes did not identify the corrective actions required ahead of schedule. The answer might be that the necessary actions were identified at a critical point and, due to lack of skills, were not communicated properly; or the maintenance required was delayed due to a decreased maintenance budget. At this point, it is relevant to examine the difference between preventative maintenance, corrective maintenance, strategic maintenance and a replacement strategy. In many instances, these are commonly mistaken for preventative maintenance measures, which unfortunately leads to flimsy maintenance practices. Preventative maintenance: sometimes referred to as preventive maintenance, is performed regularly on equipment that is in good working order to lessen the likelihood of failure. Corrective maintenance: is a task performed to identify, isolate and rectify a fault so that the failed equipment, machine or system can be restored to an operational condition within the tolerances or limits established for inservice operations. Strategic maintenance: is the strategy that defines the rules for the sequence of planned maintenance work. It contains general scheduling information, and can therefore be assigned to as many maintenance task lists and maintenance plans as required. Replacement strategy: is an investment strategy that attempts to mimic the returns of a certain asset or group of assets by using a combination of different derivatives rather than buying the individual products in the market. Comprehension of these differences will lead to applying cost effective, reliable and conditionally based maintenance policies and programmes. Having certain and comprehensive condition assessment of electrical equipment supports the selection of the appropriate maintenance plan.

 

 

Analysing the situation in three steps

 

ArmCoil advocates that distribution transformer users and owners investigate the current condition of their assets by using their existing suppliers or through ArmCoil’s On Site Condition Analysis (OSCA) team.

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An initial three-phase strategy used by ArmCoil’s OSCA programme assists in determining the needs of existing distribution transformers and associated infrastructure.

This three-phase initial strategy may be used as phase dependent or as a holistic approach for proven long-term cost saving, which enhances preventative maintenance policies (refer Figure 1).

 

Using this strategy has a beneficial impact on long-term cost reduction; assists in early detection of transformer and associated infrastructure replacement; and possibly a cost saving on insurance prem

 

iums, not to mention all the benefits enjoyed from minimum environmental impact rewards. Strategic maintenance strategies always come with associated start-up costs and it is advisable to implement maintenance strategies for a minimum of two to three years in order to accurately monitor and record the savings from start-up to current levels.

This is when the maintenance strategy delivers the results being sought and thereby supports the optimisation of networks, which in turn has positive results on increasing production and minimising downtime.

It is advisable to utilise the services of specialist service providers, such as ArmCoil, to assist in choosing the correct strategic maintenance plan for the network. Specialist engineers and service providers can accurately pinpoint the unique requirements necessary for maximum output production and service provisions from plant to plant. Replacement strategies are not that complicated once all the information required is gathered.

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The return on investment enables strategic decisions based on accurate information of the distribution network equipment, which derived from the preventive maintenance plan that the overall strategic maintenance plan had in turn supported.

 

 

Examining a hypothetical ArmCoil OSCA scenario

A 2000kVA, 33kV-415V, manufactured in 1960 and currently in operation. Unfortunately, there are no past maintenance records or historical staff knowledge of the equipment – a common occurrence as staff are promoted, retire or resign and so on. Currently, from a visual point of view, the following is noticed:

  • Oil is leaking from the unit, which is evident from looking at the plinth.
  • Heavily discoloured, there is no evidence of what colour the unit should be. Rust is evident on the radiators, skids, conservator, fasteners and piping.
  • There is a continuous ‘buzz’ noise emanating and increases on peak loads.
  • When standing close to the unit, an ominous odour is also noticed.
  • The unit trips randomly due to high temperatures during operation. Staff report that the tap changer has not worked for the past few months.
  • There is a general concern that this unit is unsafe and could ‘explode’ at any time. An inspection of the unit is undertaken by a specialist to establish the corrective maintenance action required.
  • The inspection covers the full range of oil sampling, thermal imagery of the unit and cable joints, and full visual inspection of the unit whilst in operation – culminating in the submission of a comprehensive inspection report.

Based on the inspection results, which can take up to two weeks to be made available, in the worst case scenario the unit is found to have the following defects:

  • Furan tests are below the minimum requirement (insulation levels indicate overloading),
  • Moisture content is above the acceptable levels,
  • Di-electric results are below the minimum requirement,
  • PCB content is high (this has no impact on the unit’s performance),
  • The unit’s oil level is very low, and
  • Tap-changer, breather, Buchholz relay, auxiliary equipment, radiators and gaskets are in need of replacement.

Armed with the inspection report, over a period of 1-2 days, the corrective maintenance, as put forward by the experts, could include the following:

  • Instead of oil purification, the oil is replaced. The reason is that the Furan results are so poor, the purification procedure could weaken the insulation and cause the embedded sludge to loosen up and settle on other undesired areas, thereby possibly creating a failure condition during energising.
  • All recommended parts including gaskets are replaced except for the tap changer due to shut down time restrictions. The tap changer is therefore temporarily short-circuited in the attempt to halt the fluctuation of supplied voltage.
  • Since thermal imagery shows blocked radiator fins and channels, it is decided to remove them and have them ‘flushed out’ on-site only. Replacement of radiators is delayed due to shut down time constraints.
  • A follow-up inspection is scheduled to be conducted in a month to start the preventive maintenance strategy to predict failure or replacement of the unit.

The above corrective maintenance strategy and documentation received places the unit’s owner in a proactive position to evaluate the unit in operation in terms of reaching its ‘end of service’ criteria. It is advisable to consult internally or externally on the requirements for the replacement unit, as there are many variables to consider, which are unique to each network/s specifications.

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Furthermore, before taking the route of rewinding transformers from 50kVA to 3.5MVA, time should be taken to compare repair costs with locally manufactured costs. Also, compare the repair delivery period versus the manufacturing delivery period: both options have advantages and disadvantages for your network’s specifications. The cost saving potential that can be achieved if the measures of maintenance strategies discussed herein are implemented is, in general, not quantifiable. It depends on company-specific factors such as input to increase efficiency/production, current maintenance strategies in practice, technical design of equipment and design of plant, network structure, topographic conditions and legal framework.