Winding Diagnostic by On-line Transfer Function Determination via Power Transformer Bushing Tap

Transfer function is unique for each system and it changes with a variation of the system parameters. Due to a short circuit in the power network near the transformer a displacement of windings can occur. On-line measured transfer function with a suitable transformer model and the estimation of parameters can support the diagnostic of this problem. Condenser bushings rated 115 kV and above are usually provided with series main and tap capacitors. Bushing voltage tap can be used for on-line monitoring of transfer function. In this method one of the high voltage windings of the transformer was excited with a sinusoidal signal from 10 kHz to 5 MHz from the bushing tap and the voltage of the neutral point of the high voltage winding was measured as output. To evaluate the feasibility of this method, the transfer functions of a 220 V / 10 kV, 200 kVA distribution transformer was monitored in off-line condition. The transfer functions from the winding terminals and from bushing model with bushing tap in the normal and fault conditions were compared. A 220 V / 100 kV, 5 kVA oil immersed laboratory high voltage transformer was considered to evaluate the feasibility of on-line monitoring of transformer. A 110 kV high voltage bushing model with 40 layers and 123 pF was used as the transformer bushing. Results show that monitoring of transfer function through bushing tap is possible.