ROTATING
MACHINES CORONA PROBE TESTS BASIC INFORMATION
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.
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