The Key Elements to Field Testing Power Transformers

7 April, 2020 | Blog

Gregory Sines, P.Eng.

Electrical Engineer

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With a little practice, transformer testing can be a simple task. However, interpreting the results is an important factor often overlooked when training field representatives. The process to energize or re-energize the equipment will be expedited if these representatives understand what the results mean. This will make it easier to identify and resolve issues with the transformer or the test equipment being used. Many papers have been written for engineers on evaluating test results; however, field representatives often only focus on the procedure to acquire data.

A single visit

Field service representatives who arrive on site with the appropriate knowledge, training and equipment can ensure that required EHS procedures are respected, that isolation and grounding protocols are in place, that the application of specific equipment is within its operational parameters and that measurements are strictly and systematically performed in accordance with the appropriate standards.

During the measurement processes, it is imperative to understand the importance of good grounding, accurate connections, the condition of the test equipment, the influence the condition of test leads can have and the impact of the ambient weather. Test setup and equipment configuration will also influence the readings; thus, it is not only important to be fully conversant with the limitations and functionality of the test equipment but also the characteristics of the results based on the connections applied.

From the system perspective, if the transformer cannot be fully isolated locally at the terminals, it is necessary to ensure that there are no measurement or protection devices connected to the cables or overhead busbars; for example, a local voltage transformer, as this will directly affect specific measurements made on the power transformer and may return erroneous results.

From the transformer perspective, the subtle differences, which may occur between the results of each phase or tap position, depending on which of the many tests are being carried out, require experience to understand whether the result returned is consistent with the internal arrangement of the core and windings or requires further evaluation before leaving the site.

Other conditions could also affect test results. For example, performing HV excitation testing of transformer windings can yield unexpected results if the core of the transformer is magnetized with a DC static voltage. So, if the winding insulation was tested with a DC high-resistance meter before HV excitation, the normal patterns are not observed. If the pattern mismatch is recognized right away, it can save time and confusion from erroneously suspecting a winding excitation issue. De-magnetizing the core with a designed demagnetizing unit or repeating the excitation test several times can correct this issue.

Another abnormality that can arise from excitation testing of a magnetized transformer is that the centre phase can show capacitive results. This is counterintuitive, since the transformer winding is an inductor wrapped around a magnetic core that induces voltage and current on another inductor. However, the DC static voltage in the core causes the testing equipment to measure the abnormal phase angle.

The value of such expertise provides a structured and accurate analysis of the transformer’s condition at a point in time and will often negate the need for return visits to site in order to verify or validate previous measurements, which were subsequently challenged during data analysis activities. This one-stop service can be invaluable and highly cost effective, particularly in remote access areas where return visits are expensive and often difficult to schedule.

Equipment handling

Being clear on the operation and expected outcome of tests will help identify whether the transformer has been evaluated correctly. More often than not, the test setup needs to be corrected when erroneous results are observed. Unfortunately, test equipment that has been mishandled quickly becomes unreliable. Cables and test leads have the highest failure rate for test equipment used in the field. Constantly unpacking and repacking the leads can create fatigue stress over time if they are not wrapped properly. Coaxial testing cables are prone to kinking and failing from being handled roughly or wrapped incorrectly in cold weather. Often, transformers must be tested during the winter, and this is when test equipment is most susceptible to damage. Internal damage will be prevented by ensuring test equipment is kept within specified operating temperatures before using it. Wrapping the leads properly will prevent cable damage.

Wrapping cables properly

Regardless of the season, cables should be wrapped with care to ensure test equipment will deliver accurate and consistent results. To do this, all cables, regardless of type, should be wrapped in a clockwise fashion with no kinks or tight bends. The thicker the lead or cable, the wider the loop. Coaxial cables and fine test leads should never be forcibly wrapped around hands or forearms, as this will start the fatigue stress process. Copper and aluminum are extruded in a clockwise fashion when crafted into wires. Groups of wires are twisted clockwise into bundles. The bundles are wrapped in shielding, which winds its way down clockwise around the wires. Braided ‘wire wrapping’ is the only shielding that is bidirectional; however, the conductors within are extruded and bundled clockwise.

Knowing this, it seems reasonable to foresee that forcing a cable or test lead to wrap counter clockwise, and being stuffed into a leads bag, will cause the conductors to unwind within the insulation, hence kinks. Constantly twisting and untwisting the wires this way will cause them to fail over time. This process is expedited by cold weather. Additionally, forcing the cable can crack the freezing insulation and create short circuits between conductors, which can lead to internal failure of test equipment. This is especially dangerous with high-voltage equipment!

Conclusion

When offering an optimized and highly effective  field testing service,  it is extremely important to have not only a clear understanding of the site protocols, the limitations of the test equipment to be deployed and the influence from climatic conditions, but also the internal geometry of the test object and the knowledge base to fully evaluate and understand in real time the results obtained from each of the critical tests performed. Understanding what to expect from test results, understanding and following the correct test procedures and proper test equipment handling are the three key elements to field testing power transformers.

Feel free to call on BBA’s experts for more information about transformer field testing.

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