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Diagnostic testing of electrical equipment

Comprehensive diagnostic tests for power transformers, auto-transformers and shunt reactors

Scientific and Production Association (SPA) "Technoservice-Electro" has gained significant experience in complex diagnostic tests for power transformers, auto-transformers and shunt reactors (the transformers). Currently, more than 100 transformers per year are examined. The total number of surveyed transformers is more than 1,000 units. Reliability of the results is confirmed with autopsy (repair) of dozens of transformers, as well as normal, trouble-free operation of other transformers.

SPA "Technoservice-Electro" has surveyed 6-500 kV voltage transformers with capacity of 0.4 - 1000 MVA. Operation of transformers has longed from 0.5 to 62 years. About 70% of transformers had been in operation for 25 years or more. Transformers were manufactured in Russia, Ukraine, as well as in Sweden, Belgium, the Czech Republic and Austria. The surveys were conducted at power stations and substations (including industrial substations) in about 45 regions of Russian Federation and Belarus, Latvia and Serbia.

Surveys conducted by the staff of SPA "Technoservice-Electro" repeatedly allowed eliminating the risk of accidents with transformers (usually as a result of defects in oil-filled bushings, as well as in the active part). In addition, they could provide technically based plans of repairs and necessary replacements. Economic efficiency of these surveys is achieved through timely resolution of dangerous defects, optimization of planning of the necessary volume of repair work, and in the case of absence of significant defects of transformers even with a long service life - an increase in the turnaround time, while maintaining a sufficiently high reliability of electrical machines.

Results of complex diagnostic tests of transformers, which have worked for 20 or more years show, that 30% of them can be used without any restrictions and only 2% have to be replaced. Approximately 15% of transformers require prompt overhauls, and other transformers require relatively small and inexpensive renewals (23%), palpitations or diagnostic monitoring (30%) (see chart).

Goals and objectives

The main purpose is to provide an objective assessment of the transformer, to identify defects in all systems and components of electrical machines, including:

- solid insulation (moisture, pollution, destruction);

- transformer oil;

- winding (level compacts, deformation);

- magnetic system (pressing, loss of insulation design elements, etc.);

- cooling system, cleaning and protection of oil;

- bushings;

- voltage regulators and contact system;

and work out recommendations to eliminate defects, hold repair work to continue the operation of transformers. 

The basic methods of examination

With comprehensive diagnostic examination using both traditional methods, widely used for monitoring, and the methods which are rarely used in the operation, as they require expensive equipment, supplies, training for personnel, etc.

To reduce the volume and the cost, the results of operational measurements are usually used.

The main stages of complex examination of transformers are:

1. Preparatory stage.

• Analysis of accidents and characteristic defects of transformers of similar design (including the results of surveys and repairs).

• Collection and analysis of operational information (modes of transformer, fault current levels, the results of measurements of the electrical parameters, physical and chemical analyzes of oil from the tank, fittings, tap changer, etc.).

2. Phase of field work.

Field surveys are conducted in the biggest possible load (preferably at least 50% of par, including measurements on de-energized transformer (if electrical tests are planned).

Duration of field surveys - 4-10 hours. On the de-energized transformer the field surveys duration is increased to 9-18 hours.

Measurements on energized transformer include:

• insulation characteristics of the windings and bushings;

• no-load losses;

• short circuit resistance;

• DC resistance of the windings;

• other measurements (if necessary).

In the load mode of the transformer the following measurements are made:

• location of the partial discharges, spark and arc discharges in the tank with acoustic instruments;
  

• location of the partial discharges, spark and arc discharges in the tank and inlet with electromagnetic method;

•thermal imaging examination of tank inlets, coolers, heaters, filters, adsorbers acquirement, contacts, hardware clamps, etc. ;
 
   

•vibration survey to determine the status of pressing, winding magnetic circuit, general condition of the transformer, oil pumps and cooling systems;

•measurement of magnetic field near the walls of the tank.

sampling of the oil from the tank, oil-filled bushings and switching contactors for analysis in the chemical laboratory.

3. Stage of laboratory research

Includes analyzes of oil from tanks, bushings, contactors:

• highly sensitive chromatographic analysis of 11 dissolved gases;

• moisture content of transformer oil by coulomb-metric titration with Karl Fischer reagent;

• analysis of aging and degradation of solid insulation of electrical equipment on the content of furan derivatives in transformer oil, using high performance liquid chromatography;

• measuring the dielectric loss tangent tgδ ,bulk conductivity of oil at different temperatures during heating and cooling;

• assessment of the operational properties of transformer oils for presence of defects by controlling the fractional composition, the amount and nature of pollution (solids) in oils, by a class of industrial purity and using the method of membrane filtration;

determination of antioxidizing additives, aromatic hydrocarbons, products of aging in oil using infrared spectroscopy;
• identification of breakdown voltage, acid number, flash temperature and other parameters.

4. Stage of drawing up a technical report.

As the outcome of the examination, customer gets a report that shows the results of the survey, analysis and conclusion on the power transformer (autotransformer or shunt reactor), which includes:

• a list of of defects in the active part, bushings, cooling system, RPN, and others, identified through a survey;

• documentary evidence of the possibility of further normal operation of the transformer (autotransformer, shunt reactor) or a statement of the necessity to withdraw it from operation;

• recommendations for further use with the list of possible restrictions in operation regimes and the additional diagnostic monitoring;

• a list of necessary works in maintenance or program of transformer’s overhaul

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