AC/DC Maintenance Overpotential
performed to determine whether
serious Insulation weakness exists and to verify insulation strength adequate to normal operation.
Dielectric Absorption -
performed prior to Hi-Pot testing for evaluation of stator insulation (e.g., clean, dry, contaminated)
and suitability for Hi-Pot testing. Most windings with satisfactory levels of dryness will have
polarization indices between 2 and 5.
Leakage Voltage -
time stepped voltage Hi-Pot testing
that may be evaluated along with a series of previous results to determine possible trends in
Slot Discharge used to
check adequacy of coil surface grounding. Outer surface of the stator coils is painted with
conducting varnish and should make effective electrical contact with the core to prevent voids that
may become ionized at high voltage, resulting in high energy capacitive discharges and in potentially
serious degradation of insulation.
detect and locate areas of unusual ionization or partial discharges in the stator
windings. This test can be used in qualitative analysis of insulation condition.
of Turn Insulation uses
a high energy surge voltage with an evenly distributed oscillatory wave form to detect short circuited
or weakened turn insulation in multi-turn stator coils, which if left unchecked, could result
in significant coil and iron damage.
Insulation Resistance of
Each Phase Megger
tests performed to indicate a general state of dryness and cleanliness of the insulation system.
A polarization index of less than 2 is interpreted as an indication of the need for cleaning the
stator winding or applying heat to remove moisture contamination on the winding surfaces.
RTD or T/C Insulation
to measure the insulation integrity of the RTD or thermocouple system within the generator.
This system must possess sufficient dielelectric strength to withstand the high voltages
associated with normal operating conditions.
Through Bolt Insulation
through bolt insulation to assure isolation from the iron sufficient to prevent circulating
currents and possible overheated sections of the core and bolt. Through bolt difficulties
that involve core damage invariably require restacking of the core and rewinding of the stator
to effect a satisfactory repair. Considering the potential for significant damage,
insulation resistance should be maintained at conservatively high levels.
Thermocouple Continuity Test
ohmeter test conducted
to verify that T/C elements are not open.
detect abnormally high or low resistance between the ventilating tubes on hydrogen inner-cooled
coils. Shorts between the tubes may lead to damage from circulating currents.
Abnormally high resistance between the tubes may lead to insulation damage between tubes during
high voltage testing.
performed to detect abnormally high
or low resistance between the ventilating tubes and the copper of the winding on hydrogen
inner-cooled coils. Low resistance or shorting may lead to damage from circulating
currents if there are multiple shorts. High resistance can lead to insulation breakdown
during high voltage testing.
RTD Resistance Measurements
operation, this detector is positioned: (1) between the top and bottom coil sides
in the stator slot (2) in hot and cold cooling gas or (3) in the coolant discharge for the
stator coil to measure the temperature of the stator winding. Trending these
temperatures over time can be useful in early detection of internal coil problems, such
as high resistance transposition joint, unusual circulating currents within a phase group
or a change of current distribution in coils in the phase group.
Because generator monitoring
involves small temperature changes, accurate and consistent readings are required.
should be taken at all maintenance inspections or repairs so that actual measurements
can be compared to factory calculated values and for phase balance for evaluating potential
Transposition Voltage Comparison Test
Generator Service Specialties, Transposition Voltage Comparison Test.)
Group Short Testing
(Short Program) (See: Generator Service Specialties, Transposition Group Short
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