International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 04 Issue: 10 | Oct -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1097 Neutral Current and Frequency Response Analysis of Single and Multiple Turn Fault in the Winding of a 315 MVA 500 kV HVDC Transformer A.Srikanth 1 , T. V. Sai Kalyani 2 , Dr. M. Surya Kalavathi 3 and Dr. B.P.Singh 4 1,2 Asst.Professor, EEE Dept, St.Martin’s Engineering College, Telangana, India. 3 Professor, EEE Dept, JNTUH, Kukatpally, Telangana, India. 4 Professor, EEE Dept, St.Martin’s Engineering College, Telangana, India. ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - The paper presents the theoretical simulation of neutral current and Frequency Response Analysis (FRA) of HVDC transformer due to fault in the turns of the winding. The HVAC and HVDC (Valve) windings of the transformer are represented by its self and mutual inductances and capacitances. Using these parameters equivalent electrical network is formed. The windings are subjected to standard lightning impulse voltage at its high voltage terminal and neutral currents are calculated. Single and multiple turn faults are simulated at different Coil Depth (CD) of the winding. The paper presents the comparison of neutral current for different turn faults. The neutral currents are analyzed to determine the magnitude and phase difference between them to relate turn faults. The results are complemented by FRA analysis of each neutral current to examine the faults. Comparison between neutral currents of different turn fault and their frequency response establishes the effectiveness of the technique for the detection of minor faults. Key Words: FAR, Transformer, Neutral current, Fault. 1. INTRODUCTION In accordance with the National and International standards [1] a power transformers is tested for lightning impulse voltage to determine the integrity of the windings. In order to conduct the test, the winding under consideration is subjected to impulse voltage at its high voltage terminal while the neutral is grounded. The current in the neutral winding is recorded for reduced and full voltage. A comparison is done between neutral current at reduced and full voltage which gives an indication of failure status. Even a small deviation between two neutral current is regarded as failure. It is however argued that such a costly transformer should not be rejected merely on the basis of neutral current deviation observed by eye estimation since such deviation may occur due to Partial Discharges (PD) also. Difference in neutral current at reduced voltage during calibration and full test voltage is however not construed as failure in case of occurrence of PD. In order to differentiate between the success and failure of transformer during impulse test a diagnostic method based on FRA is proposed by several researchers [2-5]. Many other methods based on Wavelet Analysis (WA) [6,7] as well Coherence Function Analysis (CFA) [8,9] have been proposed in literature. FRA method provides useful information on failure of transformer. However, in certain cases where a single turn or part of turn (minor insulation) is involved in short circuit, ambiguous results are obtained. The results are obscured by noise. The method of FRA is considered complementary to neutral current comparison. 2. MODELING OF TRANSFORMER EQUIVALENT CIRCUIT AND TURN SHORT SIMULATION The HVDC transformer consists of two windings where one is connected to 400 kV transmission line and the other, known as valve winding is connected to HVDC system. The HVAC windings are all-star connected. Fig. 1 depicts the schematic diagram of 400 kV HVAC winding and star (Valve) DC winding. Fig. 1. Schematic diagram of 315 MVA, 500 kV HVDC Transformer During impulse test the AC winding is fully grounded and other end of the valve winding is grounded in accordance with standard test requirement. Fig. 2 shows the equivalent electric circuit comprising inductance and capacitance of the two windings of