Electricity and Control March 2020

TRANSFORMERS, SUBSTATIONS + CABLES

trends or patterns through regular ongoing tests which are conducted over a period of time. These can then be analysed and recommendations made.” He adds that oil testing should be done at least once a year, as a minimum, and more frequently for specialised transformers such as those built for wind farms or solar power plants, or furnaces. An important point is that the type of oil in the transformer must be specified for accurate analysis and correct diagnosis. It may be mineral oil, natural ester oil, synthetic ester or silicone oil in use and each type of oil has its own limits in specific tests. Regularly scheduled oil testing is a cost-effective and sound maintenance practice that serves to extend the life of transformers. Using the knowledge obtained from diagnostic oil testing, customers can optimise their maintenance practices, performing maintenance when and where needed, rather than based on a fixed time schedule. Nel points out that not all the tests need to be performed every time a transformer’s condition is monitored. Some tests need to be done only annually. For example, the PCB (polychlorinated biphenyl) readings rarely change much, so these are tested once a year. Depending on how recently the transformer’s oil was tested, certain tests will be recommended as a starting point. Once the transformer is part of an ongoing condition monitoring programme, the

Some of the typical tests conducted in condition assessment of electrical apparatus are outlined below. All tests carried out by WearCheck are aligned with recognised international standards. Oil quality testing - Colour - insulating liquids generally darken with the presence of oxidation by-products and foreign materials. - Visual inspection - identi es foreign material in the insulating liquid, which may lower its dielectric strength. - Dielectric breakdown voltage - a low value indicates the presence of contaminants such as water, dirt or other conducting particles in the insulating liquid. - Water content - excessivemoisture is one of the primary causes of low dielectric breakdown strength in insulating liquid. High water content may be detrimental to the transformer under a variety of conditions. - Interfacial tension - monitors the progression of oxidation and detects contaminants such as soaps, paints, varnishes and by-products of insulation aging. - Acidity / neutralisation number - monitors the progression of oxidation by detecting acidic compounds which accelerate deterioration of the solid insulation and are precursors to sludge formation. - Power factor at 90°C - a high power factor indicates the presence of contaminants like carbon, metal, soaps and by- products of oxidation. - Speci c gravity - identi es different types of insulating liquids by determining the ratio of the weights of equal volumes of oil and water at the same temperatures. Diagnostic testing - Dissolved gas analysis - the single most important test customers can perform to head-off potential transformer failures. It serves to monitor the generation of gas in transformers to provide advance notice of developing faults and is a good way to detect thermal and electrical problems before failure occurs. - Furanic compounds - furanic compounds form when the cellulose in the paper used in the transformer breaks down. Since the paper is the most important dielectric component of the transformer, having the ability to assess its condition is essential to effective maintenance. - Metals-in-oil - metals such as copper, iron, zinc and lead can be detected and these can be indicators of incipient-fault conditions, potential bearing wear from pumps or other wear metals from vibration of components. Paper quality testing Testing the degree of polymerisation of paper (IEC 60450) provides a measure of paper aging and correlates with important physical properties like resistance to tearing and bursting. This is a critical factor in estimating the real aging of the main transformer insulation.

Taking an oil sample for testing; oil analysis is a critical element of WearCheck’s transformer condition monitoring programme.

22 Electricity + Control

MARCH 2020