Abstract: Connection symbols of a three phase transformer must be verified. A more complex Dy and Yd type connection is taken for example. Through theoretical analysis, the regularity of ideal calculated values which serves as the verifiable basis of the screening method is obtained. In actual measurement, the influence of asymmetry of the power's three phase voltage should be considered, and calculated values by measuring should be used instead of the ideal ones for verifiable basis. Measured jointly by the hardware and software of a computer, all connection symbols can be verified rapidly. Besides verifying its correctness, it can also be pointed out that which kind of symbol was connected by mistake.

Key words: transformers, screening-methods, asymmetry