Electrical Tester online
March 2009
Meeting the transformer challenge

Meeting the transformer challenge

01 March 2009

Dr Peter Werelius - Application and project management

With an aging power transformer population, today’s power utilities face a tough challenge in trying to minimise the loss of revenue and repair costs associated with transformer failures. In fact transformers have become one of the most mission critical components in the power transmission network.

The key to reducing the heavy losses associated with unforeseen transformer failures is reliable monitoring and diagnostic testing. However, traditional methods of transformer testing are usually inconvenient and time consuming, as well as being prone to delivering inconclusive results.

To address these issues, dielectric frequency response (DFR) testing for transformers, which is also sometimes known as frequency domain spectrometry was developed. This test method has been used in laboratories for many years, but only recently with the introduction of the Megger IDAX range of test sets has it become a practical proposition in the field.

DFR testing involves injecting test signals at discrete frequency steps between 1 kHz and 1 MHz into the transformer and plotting the response at each frequency. The resulting profile represents the properties of the insulating material in the transformer and can be subjected to further analysis to provide detailed and accurate information.

One of the most important applications is determination of the moisture content in transformer insulation, as moisture in the insulation significantly accelerates the ageing process and can also cause bubbles between the windings, leading to catastrophic failure. DFR testing is an important aid in helping transformer users to avoid these problems, since it involves only a single test that can be completed in just a few minutes.

An important benefit is that the test can be carried out at any temperature. This is a sharp contrast to conventional methods of moisture assessment that rely on testing oil samples, since the water migrates between the transformer’s solid insulation and the oil as the temperature changes.

For accurate results, therefore, the oil sample must be taken at a relatively high temperature when the transformer is in equilibrium. Unfortunately, this is a rare state in transformers and, in consequence, moisture assessments performed on basis of oil samples are often unreliable.

The full benefits of FRA testing are provided by Megger’s latest IDAX test set, the IDAX300, which is smaller, lighter and faster to use than its predecessors. Designed for use with an external PC for measurement control and data analysis, the IDAX300 can complete a transformer moisture assessment in just eighteen minutes.

The IDAX300 complements the existing IDAX206, which is an ideal option for users that prefer a stand alone instrument.

In addition to transformer moisture assessments, IDAX instruments can also be used measuring and analysing dielectric losses in bushings, cables, motors and similar components, making them a versatile aid for routine maintenance and fault finding in power networks of all types and sizes.

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