What Are Corona Probe Tests For Rotating Machines?

The corona-probe test is intended to be an indicator and locator of unusual ionization about the insulation structure. This test is sensitive to end-winding surface corona, as well as internal-cavity ionization in the insulation structure.

Compared to slot discharge, the discharge energies involved in surface corona or internal-cavity ionization may be of a much lower order of magnitude. The energy in the discharge varies as the square of the voltage across the gap and directly as the effective capacitance at the point of breakdown.

Partial Discharge (Corona) has several undesirable effects, such as chemical action, production of heat, and ionic bombardment. The deteriorating effects of corona are dependent on its intensity and repetition rate as well as the design of the insulation system involved.

Inorganic insulation components such as mica and glass are not affected seriously by corona. Charring or decomposition of organic materials will occur in the vicinity of continued corona activity.

However, surface effects may be limited by insulating finish treatments incorporating pigmentation to resist attack from the weak acid deposits formed by surface corona in the presence of oxygen and moisture.

Corona-probe-test equipment consists of three basic units:

1) Equipment capable of energizing the stator winding at its normal operating line-to-neutral voltage at rated frequency.

2) An antenna or corona probe. For end-winding corona measurement, the antenna usually about 1 in long, surrounded by an insulation housing, and mounted on the end of a long insulating handle.

For internal-cavity-discharge (corona) measurements, these utilize a multiturn coil wound on a ferrite rod approximately 2 in long by 0.25 in diameter and mounted on the end of an insulating handle. Measurements are made by placing the ferrite rod over the teeth enclosing the coil being tested.

3) An amplifier and indicator (for connection to the antenna) or a peak-pulse meter (for connection to the ferrite corona probe). The amplifier is one of the usual type for audio frequencies and must reject 60 Hz and radio frequency signals. The indicator may be earphones, an output meter, or a cathode-ray oscilloscope.

The peak-pulse meter is a broadband instrument calibrated in units of picocoulombs of apparent charge. Measurements may be obtained from the meter itself or by connecting the meter output to an oscilloscope or chart recorder.

The use of the corona-probe test and the evaluation of test data obtained is in relatively early stages of development and study. The ability of the test to distinguish varying intensities of external corona activity and internal cavity corona has been established.

However, the evaluation of data, to permit discrimination between harmful and acceptable levels, has not yet reached the stage where industry standards are established.

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