Long term induction voltage test（ACLD） is the important method to check the insulation strength of converter transformer

and the calculation of its entrance capacitance plays a key role in the design of long term induction test.In this paper

the circuit and math model is established on the basis of analyzing the structure of the single-phase and three-winding converter transformer so to calculate the equivalent capacitance between windings or between the windings and the earthed parts.The entrance capacitance of the applied end is calculated by the use of each equivalent capacitance so to obtain appropriate amount of compensation inductance.The entrance amount of the calculated capacitance is verified by the test data at site so to verify the reasonableness of the model.Finally

the variation rule of load characteristic of converter transformer during the test period is summarized in accordance with the voltage application experience for the long term induction voltage test which is performed for many times at site.