Transformer insulation testing is conducted to assess the performance of the transformer's insulation system, ensuring it effectively prevents current leakage and short circuits during operation, thereby protecting equipment and personnel. On-site insulation testing of transformers includes measuring insulation resistance and dielectric loss factor.
1. Measurement of Insulation Resistance:
Measuring the insulation resistance of the windings, including bushings, absorption ratio, or polarization index, provides sensitivity in assessing the overall insulation condition of the transformer. It effectively detects moisture in the insulation, surface dampness or contamination, and concentrated defects in the insulation.
However, due to the complexity of transformer insulation structures and temperature effects, the insulation resistance and absorption ratio may have uncertainties in reflecting insulation defects. As transformer capacity increases, improvements in insulation structure can lead to higher winding insulation resistance and capacitance, resulting in an increased absorption time constant. Consequently, the absorption ratio may decrease as winding insulation resistance increases, making the polarization index a more accurate descriptor of transformer insulation.
Measuring the insulation resistance of the core and structural components can effectively reveal the presence of multiple grounding faults in the transformer core. This is because these components have simpler insulation structures and homogeneous insulation media, and under normal conditions, they generally do not experience voltage, serving primarily as isolation rather than withstanding high voltage like winding insulation.
2. Measurement of Dielectric Loss Factor:
Measuring the dielectric loss factor of the windings, including bushings, can assess overall moisture and oil quality deterioration in the transformer. Since transformer insulation comprises insulation paper, insulating oil, and porcelain bushings, along with the impact of surface leakage, the measured loss factor may not effectively reflect localized moisture in the insulation. For instance, when the insulation of the bushing is very low, the overall dielectric loss measurement may fail to indicate this condition; therefore, separate testing of the bushings is necessary.
Similar to leakage current testing of windings, the effectiveness of dielectric loss factor testing diminishes with increasing transformer voltage levels, capacity, and size.
3. Measurement of Winding DC Resistance:
This is a convenient and effective test to examine the condition of current circuit connections. It can reflect the quality of winding connections, inter-turn short circuits, broken strands, broken lead wires, and poor contact at tap switches and junctions. In fact, it is also an effective means to determine whether the DC resistance of each phase winding is balanced and whether the tap switch positions are correct. It serves as a reliable method for assessing the status of current circuit connections. Additionally, measuring winding DC resistance is one of the primary methods to evaluate the longitudinal insulation of the transformer.
By implementing these practices in insulation testing, the accuracy and reliability of results can be significantly enhanced, ensuring better maintenance and operation of transformers.
