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 insulation strength.

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.

Corona Probe  developed to 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.

Surge Test 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 Resistance  used 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 Resistance  checks 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.

Tube-to-Tube Resistance   performed to 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.

Tube-to-Copper Resistance  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   monitored during 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.

Phase Resistance Measurements  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 problems.

Transposition Voltage Comparison Test   (See:  Generator Service Specialties, Transposition Voltage Comparison Test.)

Transposition Group Short Testing (Short Program)  (See:  Generator Service Specialties, Transposition Group Short Testing.)


Keystone Power Services
526 Braddock Avenue - Turtle Creek, PA. 15145   
Ph: (412) 823-0365 Fx: (412) 823-3769

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