Preventing Catastrophic Transformer Failures

Oct 29, 2019 10:25:34 AM

Transformers are vital for asset integrity: A guide on what to consider and the questions to ask to maximise the life of your transformer.

By Josh Lorraway 

Asset integrity management, in particular maintaining the integrity of electrical equipment, is an area that Bluefield is regularly asked to provide advice and assistance on by clients.  (Read more here and here).

Some of the most critical assets for any site to manage are its transformers.  When a transformer fails, it fails catastrophically. The impact of a transformer failure is significant as it can lead to serious interruptions for a business and damage to other property.

Source: Engineering World Channel

Transformers play a critical role in ensuring the objectives of a business are met through delivering a safe and reliable supply of electricity to various areas on a site or its supporting infrastructure. The cost of replacing a transformer following a failure in service is extremely high, so managing it effectively to maximise its life is essential.  Condition monitoring, which enables a planned change out of a transformer instead of a breakdown, also helps maximise the value of the asset.

The life of a transformer is essentially determined by the condition and performance of the insulation system. Therefore, the condition of the cellulose paper products plays a massive role regarding the life of a transformer. We believe there are four key considerations when looking to maximise the life a transformer:

  1. Heat Management
  2. Moisture Control
  3. Oxygen Management
  4. Life Cycle Management

Heat Management

The first key factor is managing the temperature. Operating a transformer beyond the rated temperature results in an increased degradation of the internal components (particularly the paper). An effective cooling system can provide an economic solution by managing the dissipation of heat.

Some of the cooling options that are available for transformers are as follows:

Dry Type Transformers:

  • AN – Air Natural whereby the natural air flow provides cooling for the transformer.
  • AB – Air Blast, which involves a filtered air supply using blowers or fans.

Oil Immersed Transformers:

  • ONAN – Oil Natural Air Natural uses the principle of convection to circulate the oil and dissipate the heat.
  • ONAF – Oil Natural Air Forced is an improvement on the ONAN method, which introduces a forced air arrangement instead of natural air flow.
  • OFAF/ODAF – Oil Forced/Directed Air Forced utilises a pump to circulate the oil through a heat exchanger whilst air is forced to flow over the heat exchanger.
  • OFWF – Oil Forced Water Forced is like the OFAF method but instead, water flow is used to dissipate heat.

Bluefield recommends that sites consider the following heat-related questions:

  1. Do you understand how temperature is measured for your transformers?
  2. Have you reviewed the setpoints for any fans or pumps associated with the transformer?
  3. How is your transformer affected by ambient temperature?
  4. Have you considered fibre optic probes to measure hot spots inside the windings of your transformers?
  5. Do you know the condition of all components and that they are not contributing to heat rise?

Moisture Control

Second is the control of moisture.  Ageing and catastrophic failures of transformers are more likely when excessive moisture accumulation occurs. Particularly important is the moisture content in the paper wrapping of the windings, which is an indicator of the performance of the insulating medium. It is important to understand that over time moisture will be present, however, the severity of the moisture content will depend on the quality of the initial installation and the control of external sources of moisture.

We recommend sites consider the following questions with respect to moisture control:

  1. Do you know what the moisture content of your transformer was when it was new?
  2. Do you know what the moisture content of your transformer was after it was assembled at site?
  3. What are you currently doing to ensure your transformer is kept dry?
  4. Are you using any online drying systems or self-drying breathers?
  5. Does your current transformer maintenance strategy include a mid-life dry out and re-clamp?

Oxygen Management

Next is the management of oxygen. Oxygen contributes to an accelerated rate of paper ageing, an accelerated rate of oil degradation, increased acids in oil and an overall reduction in the life expectancy for the transformer dielectric system.

The challenge lies in reducing the exposure of oxygen and moisture to the transformer. How can this be achieved? One method is the use of Conservator Oil Preservation Systems (COPS), which ensures the oil level surface is at constant atmospheric pressure and effectively separates transformer oil from air.

Oxygen-related considerations:

  • How do you manage oxygen in your Transformer?
  • How do you check for leaks?

Lifecycle Management

The final consideration and arguably the most valuable is the overall lifecycle management of transformers. Lifecycle management encapsulates the first three key considerations and builds on these through an asset management lens.

The criticality of each transformer asset plays an important role in determining an effective maintenance strategy. A criticality assessment should be carried out on all transformers and understood by key maintenance personnel onsite to ensure priorities are retained.

Transformer diagnostics, real time monitoring, planned maintenance and overhaul activities are all useful strategies that can be employed to assist with understanding the condition of transformers, improving reliability and reducing costs. Understanding the health of your transformers and having a robust management standard is key to maximising the life of transformer assets on site.

We recommend that sites consider the following lifecycle management questions:

  1. Do you know what transformers are “critical” to your business?
  2. Do you have Asset Management Plans for all critical transformers?
  3. Do you have a strategy for critical spares for your transformers?
  4. How are spare transformers maintained on your site? Are they ready to go straight into service in the event of a failure?
  5. Do you currently have a robust management standard for your transformers with the aim of maximising the life of your transformers?
  6. Do you have a baseline of the condition of all your transformers on your site?

Pulling it All Together

Managing transformers, like all parts of asset integrity management and electrical compliance, still relies on getting the basics right.  Bluefield recommends applying the TCDC principle to the integrity of electrical equipment:

  • Tight – ensure the equipment is always tightly mounted, free of looseness that can cause vibration and/or hot joints and bolts are correctly tensioned.
  • Clean – ensuring the equipment is clean goes hand in hand with keeping the equipment cool. Need to ensure there is no build-up of contaminants and/or foreign materials. No oil leaks.
  • Dry – maintain the IP rating of enclosures is integral and utilising moisture absorbents/breathers where possible.
  • Cool – enclosures need to be designed and installed such that the heat generated by electrical current can be removed. May require positive pressure or a ventilation method.