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To reduce the risk imposed by use of defected composite insulators on the operation of power grids, this paper introduces a nondestructive ultrasonic phased array (UPA) technique that allows effectively test such insulators. The method offers a great potential by reducing inspection time as well as allowing for analyzing components characterized by a complex geometry. The UPA inspection system utilizes an open-ended rectangular waveguide sensor, operating at frequency of 2.5 MHz. The system is simple, safe and relatively inexpensive. In this work, samples of silicone rubber composite insulators with various types of detects are studied and the obtained results show that void defects in the bulk of the insulator housing are easiest to be detected. Holes under insulator sheds can also be detected by the edge of scanning range. For the defects near the core-shed interface, the detection becomes possible by comparisons with sample without defects.