Transformer Riddle No.77 - Transformer Bushing tan delta values
In a Transformer Bushing, there is a mention about the tan δ, that represents the loss factor of the bushing. What does the two values of tan δ indicate? What are the capacitance involved for the performed tests of tan delta?

#1
Sun, September 25th, 2011 - 13:11
Bushing may be further classified as having capacitance taps or PF taps. Bushing rated at 69 kV and above have capacitance taps where as bushing 23 to 69 kV have PF test taps. A condenser-type bushing is usually used on transformers above 50 kV. The condenser-type bushing is made up of equal capacitance layers (concentric capacitors) between the center conductor and ground flange. These concentric capacitors provide equal voltage steps; that is a uniform voltage gradient. As shown in Figure below, the condenser-type bushing consists of 10 equal capacitance layers and each layer has a capacitance of 1 pico-farad (pf).

Therefore the total capacitance (C) is equal to 0.1 pf. For example, if one capacitance layer breaks down then the capacitance of the bushing would be one-ninth or 0.11 pf; or an increase in bushing capacitance of 0.01 pf, or 10% increase. Figure above also shows the various insulations associated with the condenser-type bushing. The main insulation C1 is the insulation from conductor to the tap electrode or shield layer; the tap insulation C2 is between tap electrode and ground or flange; and C3 is the overall insulation to ground of the upper and lower porcelain. PF tests on bushing are usually performed by energizing the bushing conductor and measuring the test current and loss for the insulation system between the conductor and grounded flange. Many modern bushings have capacitance taps or PF test taps. Test on bushings with taps can separately determine the core insulation (C1) from the tap insulation (C2). The various insulation systems of the bushing with taps are shown in Figure below.