Research On Novel Method Of Inrush Current Identification Of Power Transformer

It’s an unavoidable problem that how to prevent the transformer differential protection from mal-operation in the case of inrush current and improve the protection sensitivity when no-load switching at internal faults (especially interterm faults). Therefore, new methods are developed to solve the problem mentioned above. The major contributions of this dissertation are as follows.A novel algorithm using waveform singularity factor (WSF) for distinguishing between inrush currents and internal fault currents in transformers is proposed, which is derived from the singularity features of waveforms of currents. First, the definition and the calculation method of WSF are presented and the changing trend of WSF on different conditions is revealed. Then the criterion to identify the internal faults and inrush currents are developed on this basis. Dynamic simulation results show that the proposed method verifies that the WSF calculated in real time is able to clear internal fault even low level internal turn-to-turn fault rapidly, correctly, and reliably. Having low computational complexity, the method has easy way to engineering implementation.A novel scheme based on the sinusoidal proximity factor (SPF) to distinguish inrush currents and internal fault currents in transformers is proposed. First, the SPF is defined and its calculation method is introduced. Then the criterion to distinguish the internal faults and inrush currents are developed according to the changing trend of SPF on different conditions. Dynamic simulation results show that the SPF of the inrush current is very small, but the SPF of the internal fault current is characterized by drastic variation. The experimental results verify the dependability, security, and fast operation of the proposed scheme. Besides, being simple the scheme is easy to be implemented in real time.A new method to calculate transformer leakage inductance that is suitable to the condition of no-load closing of transformer is proposed:so long as knowing the resistance and leakage inductance of transformer under normal condition and utilizing recurrence method in difference equation, transformer leakage inductance under its no-load closing can be rapidly calculated. On this basis, the leakage inductance is normalized to make the setting of protection device more universal. Dynamic simulation results show that the proposed method is not affected by the installation site of TV at bus in the substation; it can be ensured that the transformer can be reliably and rapidly switched off while transformer internal fault occurs, and the proposed method is enough sensitive to slight faults.