Effects of the Electromagnetic Structure of the Power Transformer Core on Hysteresis Squareness Ratio, Noise and Vibration Characteristics

Abstract

1.Study Objective and MethodTransformer magneto-mechanical structure deformation due to lightning impulse effect may cause damage of the transformers in terms of the noise and vibration performance. This study proposed a single-phase transformer with a voltage specification of 345 kV (high voltage)/115 kV (low voltage) and a capacity of 120 MVA [1,2]. The results of the continuous lightning impulse tests revealed that the transformer designed as parameter of K-factor, as shown in Figure 1. After testing results, there is generated significantly low noise (<65 dB) and vibration (<10 μm), which are remarkably lower than the IEEE standard of 85 dB.2.Experiment Result and DiscussionThe pattern of the fault currents depends on the fault type and location along the winding length. Figure 2(a)-(b) shows the transformer after the lightning impulse full-wave and chopped wave for high voltage bushing level. The Figure 2(c) shows the thermal imaging results of the transformer after the lightning impulse testing, indicating that high (102 °C) and low (31.2 °C) thermal conduction were obtained at the two sides of the top collier. These thermal imaging measurement results suggest that the transformer can operate effectively because it exhibits satisfactory insulation performance after the lightning impulse and alternating current insulation tests [3,4].The sweep frequency response analysis is a useful method to detect and diagnosis of the defects for the power transformers. The results of the frequency spectral tests are conducted using SFRA are shown in Figure 2(d). Based on the SFRA theory, the core properties affect the SFRA characteristics only at low frequencies (typically up to 1 kHz). Therefore, the noise results can be considered as correction values between the values obtained before and after the application of the SFRA. The k-factor between 0.4-0.6 for transformer insulation is maintained better performance.  Figure 1 Lightning k-factor: (a) turn-to-turn insulation and overshoot voltage, (b) k-factor distribution.  Figure 2. Lightning impulse results: (a)-(b) full wave voltage and current, (c) thermal imaging, front-back side, (d) SFRA at 10 Hz–2 MHz before and after the lightning impulse test.