Corona screen effectiveness in large machines under high voltage, high frequency transient conditions

The slot sections of pre-formed coils used in the manufacture of stator windings for high voltage rotating machines are conventionally finished with a layer of graphite loaded tape, known as a corona screen, for the purpose of suppressing electrical discharge between the coil side and slot wall. The screen resistivity is a compromise between the requirement to limit voltage rise on the screen between points of contact to the slot wall and the need to prevent significant current flow between adjacent stator core laminations. The authors present theoretical analyses and experimental results to show that, at frequencies associated with starting transients and some supply voltage disturbances, the voltage distribution along the corona screen surface may be quite different from that developed in normal operational conditions. Sufficient voltage may be developed at some points to permit discharge to the slot wall even when most of the screen is still intact and effectively earthed. Measurements of discharge current under simulated high frequency transient conditions have shown that the discharge may persist as an arc with much higher potential than normal slot discharge for accelerating surface degradation of coil sides