Abstract : A novel method for suppressing transformer inrush current is proposed. A low-power voltage source PWM inverter is connected in series with other transformers via a matched transformer. When the PWM inverter is resistive, no inrush current occurs. In a single-phase circuit, the rated power of the PWM inverter, acting as a damping resistor, is 14% of the main transformer's power during current inrush, while in a three-phase circuit it is 19%. Digital computer simulation using PSCAD/EMTDC technology cannot determine the effectiveness and practicality of the proposed method. A prototype was developed for testing, and the results confirm that the proposed method can completely avoid inrush current.
Keywords : inrush current; secondary inrush phenomenon; core saturation; series voltage source PWM inverter; transformer model; PSCAD/EMTDC
1. Inrush Current Suppression Method
1.1 Transformer Model Constructed Using PSGAD/EMTDC
PSCAD/EMTDC considers the issues of transformer core saturation and inrush current generated when the power is turned on. Table 1 lists the experimental results of the transformers mentioned in this paper. Based on these experimental results, a transformer model was constructed on the basis of PSCAD/EMTDC. Table 2 shows the parameters of the transformer model constructed on the basis of PSCAD and EMTDC. PSCAD and EMTDC technologies can consider the saturation flux and hysteresis of the core, but for simplicity, the hysteresis is negligible, and only the saturation voltage, inrush current decay time, and core impedance are considered. The transformer model does not consider the number of turns and core cross-sectional area of the transformer; the flux is generated by voltage integration. Therefore, core saturation may be considered according to the set saturation voltage. Here, the saturation voltage is 1.25 pu. For power transformers, it is generally assumed that the rated flux density of the oriented silicon steel sheets used is 1.5 T, and the saturation flux density at the saturation voltage is 2.1 T. On the other hand, when the transformer is turned on at voltage phase 0, the maximum flux density of the core is 3.0 T. Therefore, it is appropriate to use the set saturation voltage to study the saturation phenomenon of transformer core. The air-core reactance depends on the reactance of the coil. Even when the core is fully saturated and the transformer core is in an air-core state, there is still an impedance of the air-core reactance. For simplification, the air-core reactance value is set to twice the leakage reactance in PSCAD/EMTOC. On the other hand, the literature [3] gives the measured value of the air-core reactance, which is about 20% to 30% larger than the leakage reactance. Considering this, this paper uses the set value of PSCAD/EMTDC and regards the air-core reactance as twice the leakage reactance. In Table 2, the leakage reactance is 0.114Ω and the air-core reactance is 0.228Ω. It is necessary to give the air-core reactance a unit "pu" in PSCAD/EMTDC. The rated voltage of the transformer used in the study is 200V, the rated current is 30A, and the rated impedance is 6.67Ω. Therefore, the air-core reactance is set to 0.034 pu.
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