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Non-Destructive Diagnostics of Transformer Coils

#1873


Developing Hardware, Software and Methodical Support for Diagnostics of High-Power Transformer Windings without their Disassembly

Tech Area / Field

  • NNE-EPP/Electric Power Production/Non-Nuclear Energy
  • INF-SIG/Sensors and Signal Processing/Information and Communications
  • INS-MEA/Measuring Instruments/Instrumentation

Status
3 Approved without Funding

Registration date
12.05.2000

Leading Institute
VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Supporting institutes

  • Moscow Power Engineering Institute, Russia, Moscow

Collaborators

  • Doble Engineering Co., USA, MA, Watertown

Project summary

High-power systems are a main constituent in the economic development of many countries. Reliable operation of equipment comprising such systems is possible in the case of timely repair and due maintenance. Critical and expensive in high-power systems are high-power transformers.

Russian standards prescribe to check more than 10 parameters of transformers technical status including winding geometry monitoring. During transformers servicing, aging, insulation shrinkage, impact from pulsed electrical loads, significant vibration and compaction system relaxation cause the reduced winding compression strength, and this, in its turn, results (in the case of short circuit) in the deformation of windings, their shorting and eventually in the transformer failure.

Existing methods of winding geometry diagnostics, i.e. sounding by low-voltage pulses and short-circuit impedance measurement, only state significant changes in geometric dimensions of windings due to their deformation, but do not allow timely detection of their occurrence reason, i.e. reduced winding compression strength. Assessment of the winding residual compression strength allows one, if necessary, to precompress windings beforehand, thus preventing possible short circuit-induced deformation of windings. There are good reasons to state, that it is possible to create a more perfect (if compared with the above-mentioned ones) procedure for diagnostics of windings deformation on the basis of electromagnetic frequency characteristics of a transformer.

Previously, the group of experts at Russian Federal Nuclear Center - RFNC-VNIITF (Snezhinsk) has performed investigations aimed to assess the effect of compression strength and (or) deformation of high-power oil transformer windings (60MVА and more) on transformer electromagnetic and mechanical frequency characteristics. In particular, this group has developed the procedure of the effective disassembly-free assessment (i.e. without disassembly) of the winding compression residual strength /1/. When a de-energized transformer with disconnected buses is loaded by a mechanical pulse, excluding any damage to the design, changes in the electrical voltage which characterizes free damped mechanical vibrations of the core elements are registered on the transformer lead-ins. The winding compression strength is assessed on the basis of certain regularities in voltage spectral composition changes. Along with the winding compression strength assessment this procedure allows detection, in some cases, of unfixed elements of the design inside a transformer (e.g. magnetic shunts on tank walls). By now the procedure is tested with more than 50 high-voltage transformers and there was observed satisfactory coincidence between diagnosed compression strengths and actual ones. Based on test results transformers 600МVА and 400МVА were disassembled and repaired two times, and this prevented large accidents.

Using obtained results (within ISTC framework) RFNC-VNIITF:

· Runs researches aimed to improve methods of winding compression strength diagnostics/1/, what can significantly increase diagnostics accuracy and expand area of application up to its application as a means to monitor winding compression at the manufacturer’s;

· Runs researches aimed to develop the disassembly-free procedure to monitor winding geometrical parameters using electromagnetic frequency characteristics of a transformer;

· Uses the above-mentioned procedures as the basis for the development of the independent small-sized diagnostics facility compatible with IBM PC intended for tests in production areas and in field.

These procedures are based on original technical approaches and have no analogs. These procedures are interrelated by the used physical phenomena and are applied jointly. They can be of interest both to institutions servicing high-power transformers, and to transformers manufacturers. During manufacturing the latter acquire possibility to enter detectable electromechanical characteristics in passport data and in their database. In future this will allow efficient and low-cost assessment of the technical status of windings during servicing and will enhance this product competitiveness.

In future it is possible to use the above procedures, after some improvement, to diagnose generators, electric motors, etc. Actually, these methodical and hardware solutions can be used to evaluate electric and mechanical characteristics of any structures incorporating wiring and core.

SS VNIIE activities include:

- analyzing the mechanism of physicochemical destruction of the winding and barrier insulation in high-power transformers during servicing;


- detecting the most heated zone of a winding;
- developing the procedure how to select the winding insulation sampling;
- developing methodical instructions how to assess the residual service life of the transformer insulation through measuring polymerization.

SS VNIIE procedure used to assess the residual service life of the transformer insulation together with the method of the disassembly-free diagnostics of the compression strength and the method used to assess dangerous deformations of high-power transformer windings being developed at RFNC-VNIITF is the basis for the decision whether it is possible or not to pursue the transformer operation, i.e. it tests the status of the transformer most critical element, and namely, windings.

The total cost of the project is cheaper than the cost of an average-power transformer. Proposed methods have the following advantages:


· minimal cost and minimal time for diagnostics;
· no need in transformer disassembly.

This work results can give significant economy in the area of power production and transmission. This small-sized diagnostic facility can be widely used at enterprises and institutions involved in monitoring and production of transformers.

1. RF Patent № 2117955. – “Transformer diagnostics method”.


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