Dynamic bending tests-a method to evaluate the ability of stator coil insulation to accommodate 3 phase short circuits

Abstract

The ability of the generator stator end windings to withstand large loading and the corresponding deformations during a severe transient event like a 3-phase short circuit at the generator terminals is important to the reliability of turbogenerators. During such transient events, large fluctuating electromagnetic forces at line frequency are exerted on the stator end windings, in the radial, axial and tangential directions. Mechanical damage to the coil groundwall insulation as a result of these large dynamic forces must be avoided so that the coils can meet the normal maintenance dielectric test requirements as given by IEEE 56-1977. Mica based insulation systems are not mechanically strong. For this reason, a stator end winding support structure is required to absorb the sudden short circuit forces and limit the deflection of the stator coils to levels within the capability of the groundwall insulation. To design an adequate stator winding support structure, the strain to failure of the groundwall insulation at short circuit loading rates must be well known. This can be accomplished with dynamic bending tests of the insulation. The purpose of the dynamic bending tests is to establish low-cycle fatigue properties of the groundwall insulation at loading rates that simulate a sudden short circuit. It is a test that determines the dynamic strain to failure capability of the groundwall insulation. For this purpose, a special dynamic bending test machine has been built and utilized at the authors' company. This paper covers the importance of the dynamic bending tests in determining at what strain a stator coil insulation system will fail during a sudden short circuit. The paper also compares strain to failure characteristics of three different insulation systems and explains that dynamic strain to failure rather than tensile strength or strain is the important property when it comes to evaluating the ability of an insulation system to withstand 3-phase short circuits.