TVA Probe Testing ( Tennessee Valley Authority Probe Testing ) was developed in the USA and Canada . The TVA Probe testing ( Tennessee Valley Authority Probe Testing ) was developed to locate areas of localised discharge within the stator slots of high voltage stators.
It was found synthetic epoxy mica insulated windings were showing high discharge levels, due to the erosion of the main wall insulation. This was found to be due to the loss of the corona shields, which comprised a painted graphite coating on the surface of the coils in contact with the stator iron. The loss of the corona shields was due to the poor adhesion of the corona materials used, the fit of the coils within the slots, and the bracing of the windings.
The corona shields are semi-conducting materials, and their purpose is to conduct losses discharged within the insulation to earth. When direct contact is lost between the surface of the stator coil and the iron, causing localised heating and insulation erosion. This form of insulation failure is a recent phenomenon and is associated more with recent insulation designs and can take a few years to develop, but when erosion has started its progression to ultimate failure can be fast, depending on the wall insulation thickness.
In order for the TVA ( Tennessee Valley Authority Probe Testing ) probe testing to be performed, it is normally a requirement that the rotors are removed from the stators so that the electromagnetic probe can be inserted into the stator frame. The stator winding is energised at various voltages up to phase voltage. The TVA ( Tennessee Valley Authority Probe Testing ) probe is moved along each individual slot and the levels of discharge current are recorded from the peak milli ammeter.
The TVA ( Tennessee Valley Authority Probe Testing ) probe consists basically of a ferrite core, half circular in shape over which has been wound a number of turns of insulated conductor, the terminations of which are connected to a meter. When the ferrite TVA probe lead bridges across a stator slot of an energised machine, a small current will be induced into the turns of the probe if discharges are present, either by discharging voids within the cell insulation, or discharges that are external to the main wall insulation. The combination of the probe, the stator iron and the coils within the slots resembles a current transformer, the stator coils representing the primary circuit and the head of the probe, the secondary.