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Introduction to Standard Number 1410, Revision 5 Inspection, Reconditioning and Testing of 600-volt Low Voltage Dry Type Transformers
It is fair to assume that the number of distribution transformers in service is inversely proportional to their size and directly relative to the commonality of the voltage configuration. There are more small transformers in service than there are larger ones, and more common voltage units than special voltage units.
This class of transformer has few or no moving parts. Generally speaking, they can be called static devices. You might think there is little to do to a transformer to keep it in running condition. In fact, transformer failures are not at all uncommon and most often the cause of a failure can be traced back to application, environment, or lack of maintenance. All too often, transformers are not treated to the kind of maintenance and regular testing that might greatly increase their lives.
Where ever you go in our country, you will find variations in temperature and humidity, not to mention airborne particulate and contaminants. These environmental factors play a distinct and definite roll in transformer life and performance. Transformer loads are subject to change and they are increased far more often than they are decreased during a lifetime. This also changes the equation about the duration of transformer life.
A typical dry type distribution transformer consists of:
As in most PEARL standards, we treat this as a maintenance procedure rather than a repair procedure. This standard, as all of them do, begins with reference tables identification and identification of the test equipment necessary to complete the standard.
The recondition evaluation section follows with step by step inspection points in logical order to assure all areas are subject to visual observation.
The next section takes a technician through the “as found” testing steps which includes but is not limited to Insulation Resistance, which is looking for moisture in the windings or insulation, debris and dirt that could become a source of short circuits, and insulation breakdown that might point at an impending failure. Next comes the Turns Ratio Test, which allows a technician to simulate the functional performance of a transformer with out inducing a load. This test injects a voltage through the windings and collects input and output voltage data and computes the ratio between the two and compares to what that ratio should be. And finally we recommend performance of a Core Ground Test to make sure that the core is completely isolated from non-current-carrying parts of the transformer.
The visual inspection along with the as-found testing helps determine the actual recondition procedure. The reconditioning procedure specifies disassembly and cleaning of all parts and subassemblies, replacement of parts where necessary, tightening or adjusting where necessary. Certain processes that that are not shown in the standard may become necessary based on as found results and will be addressed in future updates to the standards. These are more repair procedures than maintenance but will be addressed as our standards continue to grow in scope.
The second testing section is intended to evidence that the services performed in the reconditioning process have returned the transformer to optimum operating condition. Once all elements of the standard have been performed and test results confirm the units readiness for service, the PEARL Blue Quality Seal may be affixed completing the PEARL Reconditioning Standard 1410 for Low Voltage Dry Type Transformers.
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Link to the full Standard Number 1410