Answer / k.prakashchandra

Isolate all phase windings at both ends and measure IR of
each phase to body and between windings

Answer / subash

A. Purpose and Theory

The IR test measures the resistance of the electrical
insulation between the copper conductors and the core of
the stator or the rotor. Ideally this resistance is
infinite, since after all, the purpose of the insulation is
to block current flow between the copper and the core. In
practice, the IR is not infinitely high. Usually, the lower
the insulation resistance, the more likely it is that there
is a problem with the insulation.

PI is a variation of the IR test. PI is the ratio of the IR
measured after voltage has been applied for 10 minutes
(R10) to the IR measured after one minute (R1), i.e.:

PI = R10/R1

A low PI indicates that a winding may be contaminated with
oil, dirt, insects, etc. or soaked with water. In the test,
a relatively high DC voltage is applied between the winding
copper and the stator or rotor core (usually via the
machine frame). The current flowing in the circuit is then
measured

Answer / nithin

Insulation resistance of alternator is checked using a megger and before measuring it the connections to the protective devices like CT,PT and also the connection to the AVR to be disconnected. Then the star connection of the winding to be disconnected ,then we can check IR value

after parallaing two generators current is shooting up in both the generators.for ex: load is 400 kw..after paralling and load sharing current in running gen is approx 900 amps and incoming is 750 amps which is too high when trying toshift the load current in incoming gen is coming more than 1300 amps and both getting tripped and making blackout..

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