• Electrical India
  • Jan 5, 2017

Transform the potential of your Transformer Reference Materials

To aid in the organization of a corporate or personal library, Megger is introducing a comprehensive series of Transformer Life Management bulletins that broadly cover the fundamentals of managing the life of a transformer...

The Value of a Technical Library

  Enter the office of most engineering types and there, on the shelf, you are likely to find a collection of prized reference materials. When pressed to share, often follows the tale of beloved resources never returned justifying heavy reluctance to lend out again. This valued labour segment (of which I proudly number) are the natural born problem solvers. Often and aptly characterized as incredibly resourceful, when left to their own devices, they figure things out.

  A meaningful part of an engineer’s arsenalis a carefully constructed library of references that are time-vetted supports for doing their job well (even if that support comes only from the reassuring presence and availability of these materials should the need arise to reference them). In many cases, personal financial investment has grown these libraries, of which university text books among others, number. So of no surprise, when these valued employees move on, often these treasure troves follow.

  This highlights an interesting aspect to succession planning; that is, the advantages of establishing and maintaining a protected corporate library, accessible to all. In fact, a corporate library serves important roles, including:

• Spreading knowledge within the organization;employees value education and it will help build individual libraries
• Aiding with succession planning – an additional way to pass knowledge

What you can do:

• While most companies have employees who know a lot, it is unlikely that they have time to write it all down. A better solution may be to build a library and have these internal subject matter experts validate the collection you assimilate and supply what is missing. Annotating key materials as they relate specifically to the company is another invaluable service these employees may provide.
• Materials are all around though they do need to be collected, thoughtfully selected(you may not want to keep everything)and organized.

Introducing Transformer Life Management (TLM) Bulletins – their place in your library

  To aid in the organization of a library(corporate or personal), Megger is introducing a comprehensive series of Transformer Life Management (TLM) bulletins that broadly cover the fundamentals of managing the life of a transformer. These are provided by Megger subject matter experts delivering confidence that the material is current. The bulletins span topics from moisture in transformers to each of the many electrical tests available to assess the condition of a transformer or instrument transformer. Moisture in transformers, for example, is a huge subject so there is value in overview summary publications that introduce the topic. If you collect all publications you can find on atopic, Megger TLM bulletins would be suitable to place at the front of each collection to introduce and frame that subject.

The Mature Library and the importance of maintenance

  Of special note are mature libraries. Their value is already recognized but the risk here is failure to maintain. A champion must nurture it for while some material may be timeless, the most recent understanding of a subject must also be housed therein. Electrical testing, for example, has advanced – if you have material, when was the last time you updated it?

  Megger TLM bulletins include more recent developments in the electrical test domain, including:

• Dynamic winding resistance (the variations of and differences in dynamic measurements)
• Individual temperature correction (ITC)
• Dielectric frequency response (DFR) for transformers and bushings
• How to increase the efficiency of testing through test lead management solutions like “One-time Connection” and a true transformer test van
• Demagnetizing procedures – different approaches

  These advancements are significant. Some examples of what a student subscribing to an out-of-date library would be at risk to miss include the following:

Dynamic measurements on OLTCs

  Assessment of on-load tap changers include traditional static measurements (e.g., exciting current, turns ratio, winding resistance, and SFRA tests) and measurements made during the execution of a tap change, including continuity tests, dynamic current (a.k.a., “ripple”) tests that are sometimes passed off as dynamic resistance tests, dynamic voltage tests (used to calculate dynamic resistance) and finally, true dynamic resistance measurements. Grab a Megger TLM on dynamic measurements to learn the differences between these. For example (but stripping detail):

  A dynamic current test is similar to continuity testing. However, in addition to detecting discontinuity, the test current is measured and the result is presented in a current-time diagram or as a percentage ripple value. Ripple is the magnitude by which the test current decreases during the tap change and is expressed as a percentage of the test current. The challenge with this method is the inconsistency of the results given differing variables (e.g., test currents and connections).

  For a dynamic voltage test, a relatively small test current (0.1 to 1A) is injected through the tap changer using a high-impedance current source and the LV windings (or HV, i.e., opposite windings of that under test) of the transformer are short-circuited. The voltage over the test circuit is measured and resistance versus time can be calculated.

  A new, patented method is to measure dynamic resistance in the tap-changer by simultaneously measuring the test current together with voltages on both HV and LV windings and combine the results with transformer modelling. The difference in this test setup and that for the previous method is that the LV winding is left open.

Individual Temperature Correction (ITC)

  Line frequency power factor1 has been one of the preferred tools used to evaluate the condition of the insulation in substation electrical equipment for a long time. This testing tool relies on comparison with benchmark or previous results, whereby deterioration or contamination is detected by a change in power factor. The success of a comparative analysis is based on assurance that the results being compared are representative of the state of the insulation and not changeable test variables present. Otherwise, a change in power factor may be attributed to a test variable(s) and not an actual change in the condition of the insulation.

  One notable test variable in power factor testing is (top oil) temperature. Therefore, for a meaningful comparison between tests, all power factor results need to be converted (or corrected) to equivalent 20° C power factor values that adjust for the influence of temperature. However, IEEE standards now acknowledge that the long-used corrections tables provided for this purpose are not reliable and no longer endorse their use. Dielectric frequency response (DFR) testing, which involves performing approximately 20 measurements from 1 kHz down to typically 1 mHz, has given operators a new alternative to determine the Individual Temperature Correction (ITC) factor for the line frequency power factor value measured at any temperature between 5 and 60° C, so that it can be accurately normalized to 20° C for trending analysis. Megger’s TLM bulletin on ITC describes how this is done.

Dielectric Frequency Response (DFR) for transformers and bushings

  Dielectric Frequency Response (DFR/FDS) measurements (narrow band, NB, and wide band, WB) are techniques/ methodologies for general insulation testing and diagnostics. It is recommended to routinely perform narrow band DRF measurements (1 – 500 Hz) in addition to single (line) frequency power factor tests. In comparison with 50/60 Hz power factor(PF) measurements, DFR measurements provide the following advantages:

• Capability of performing individual temperature correction of measured 50/60 Hz dissipation factor at various temperatures to values at reference temperature, 20 °C (NB and WB)
• Capability of estimating temperature dependence in an object; from a measured dissipation factor at a certain temperature calculate the dissipation factor at a different temperature (WB)
• Capability of estimating the moisture content of oil-immersed cellulose insulation in power and instrument transformers and bushings (WB)
• Capability to verify if seemingly good PF values actually are, and reveal when seemingly good PF values actually are not, thereby providing for earlier detection of problems (NB and WB)
• Capability of generally investigating causes for increased power factor in power components (NB and WB)

  Megger TLM bulletins will describe how DFR fits into a transformer and bushing test program, including for example, why reliance on a line frequency power factor alone does not provide a full picture of insulation health.

Increasing the efficiency of testing through lead management solutions

  Lead management, including quality of connection and correct lead placement, is an important part of testing. The time required to perform electrical testing is mostly influenced by the time involved for test set-up and breakdown.2 Therefore improving test efficiency largely becomes a matter of improving lead management. The one-time connection concept reduces the number of connections required to execute electrical tests, such as transformer turns ratio and winding resistance tests, and eliminates the possibility of incorrect test connections. Another innovative solution is a true transformer test van. Megger’s TLM bulletin on lead management solutions explains these concepts.

  With improved lead management solutions, in addition to the time savings, safety hazards are minimized. Less required connections and connection changes means less ladder climbs to the top of the transformer. For some tests particularly, touching each bushing terminal with a grounding stick before removal of a test lead prevents risk of shock. Minimizing the number of disconnects lowers the chance that a tester becomes complacent and neglects this recommended safety measure.

Demagnetizing procedures

  Though not necessarily a failure, a magnetized core may result in potentially damaging in-rush currents (which exceed the transformer’s rated current by an order of magnitude and more) when the transformer is energized. This carries multiple implications but to the transformer itself, the high mechanical forces and resulting vibrations due to these currents may cause increased wear on the insulation of transformer windings. For the power system, protective relays cannot distinguish between causes of high current (e.g., a transformer fault or a magnetized core) and will trip until this condition is corrected. A magnetized core may also influence certain off-line, diagnostic test results such that meaningful conclusions about the condition of the transformer cannot be accessed.

  Residual magnetization leads to lower magnetizing inductance. Results from tests in which none of the transformer windings are short-circuited may be notably impacted, particularly exciting current and SFRA tests.

  There are a couple of demagnetization approaches. With an alternating direct current method, the magnetic alignment of the core iron is neutralized by applying a direct voltage of alternate polarities to the transformer winding for decreasing intervals. A variant of this is a Constant Voltage Variable Frequency method (CVVF). The effectiveness, efficiency, and safety of automated demagnetization features can vary; Megger’s TLM bulletin on core demagnetization will aid in your understanding of this topic.

Accessing Transformer Life Management (TLM) Bulletins

  In appreciation for your support through the years and because a more informed individual is our best customer, Megger invites you to download a newly featured TLM bulletin each month at no charge to you – every month you will receive an alert as soon as the latest bulletin becomes available on megger.com/TLMbulletin.