Selection of Dielectric Loss Factor tgδ Test Equipment and Test Methods

Dielectric loss factor tgδ test: The principle of this test has been explained earlier. tgδ is the ratio of IR to IC, and it can reflect the amount of energy loss per unit volume within the dielectric material. It is only related to the properties of the dielectric material and has nothing to do with its volume size. Therefore, the purpose of the tgδ test is also to effectively detect general aging, moisture, dirt, and other overall defects in the insulation of equipment. For small capacitance equipment, such as bushings and transformers (capacitive type), it can also detect whether there are air gaps and fixed insulation cracks in the internal structure, as well as concentrated local insulation defects. However, it should be noted that when measuring tgδ for large-capacity equipment such as transformers and cables, only their overall distributional defects can be detected, while local concentrated defects may not be detected: For small-capacity equipment such as bushings and transformers, measuring tgδ can effectively detect both local concentrated and overall distributional defects. This is why large transformers not only need to test the tgδ of the lead-out bushings separately but also the dielectric loss tgδ of the bushings together with the windings, because if there is a defect in the bushing, it cannot be reflected when the overall insulation is good. The selection of equipment and conventional test methods: Due to the reasons of accuracy and sensitivity, when measuring the dielectric loss of transformers and general bushings (including capacitive CT), a dielectric loss tester should be used. When testing the capacitance and tgδ of capacitive PT, an alternate frequency dielectric loss tester can be used, which introduces the test method for the medium-voltage capacitance C of CVT, which is relatively convenient (self-induced method). The principle of the former is to compare the amplitude and phase difference between the internal standard circuit current and the current of the tested item, while the latter is based on the bridge principle and discrete Fourier algorithm. The main wiring forms are two types: Positive connection method: Suitable for measuring equipment with insulation between two phases and ground, with higher test accuracy, such as tgδ of the main insulation of bushings and capacitive CT, tgδ of the coupling capacitor, etc.; Reverse connection method: Suitable for measuring equipment with one end grounded, the instrument’s casing must be reliably grounded, such as tgδ of the transformer together with bushings and windings, tgδ of the terminal screen of bushings and capacitive CT, etc. There is also the self-induced method, and the delta connection method, etc. Different test equipment has different wiring forms, depending on the on-site environment and standard equipment. It should be noted that during on-site testing, conditions should be created to strive for test accuracy, such as the tgδ test of the bushings of the high and low voltage sides of the main transformer must use the positive connection method, and the installation unit should be required to make a test platform to achieve the condition of two-phase insulation. For the tgδ test of the medium-voltage capacitance of CVT, it is necessary to fully understand the operation procedures of the instrument and follow the instructions to conduct the test.