Page 10 - PD Experience on 3-5kV
P. 10

insulation discoloration   visual inspection               (Asphaltic mica)

                   3.5  COIL MOVEMENT FAILURE

                   3.5.1  Impact:
                     If properly installed, the wedges and side packing should prevent winding looseness.  However, some
                     insulation resins shrink when they are cured or thermally aged, coils may get smaller and so become loose in
                     the slot.  Also, some of the wedging and packing materials may become brittle and shrink over time, allowing
                     the coils to become loose.  In the presence of oil, side packing and ripple springs will soften faster because of
                     the lubricating medium.
                     When windings become loose in the slot, the immediate problem is that, if left unattended, the looseness and
                     vibration will quickly allow the laminated rough stator core surface to damage the semi-conductive coating on
                     the surface of the coils.  Damaged coil surfaces create discontinuities on the surface and allow voltage stresses
                     to build up across these isolated locations, or between these and the stator core.  If the voltage stress exceeds
                     the electrical breakdown point of the gas medium, a discharge will occur.  See Case Study 5.3.

                   3.5.2  Probability:
                     In a global VPI, coil movement normally has a low probability; however, when deterioration of the semi
                     conductive coating occurs the probability increases due to the chemical attack on the resin securing the
                     wedges in place.

                   3.5.3  Risk to the winding:
                     Eventually, a ladder-effect develops where the groundwall is thinner at the point of contact with the core, but
                     maintains normal thickness at the core air vents.  Though the absolute time between detection and failure is
                     unknown, it can be as short as two years in many thermoset (hard) windings, especially those with a high
                     electric stress across the groundwall.


                                      Bipolar C2                                 Bipolar C2
                            0 to 3.16 pps  3.16 to 10 pps  10 to 31.6 pps  31.6 to 100 pps  0 to 3.16 pps  3.16 to 10 pps  10 to 31.6 pps  31.6 to 100 pps
                            100 to 316 pps  316 to 1000 pps  > 1000 pps  Subset 8  100 to 316 pps  316 to 1000 pps  > 1000 pps  Subset 8
                        750                              750       750                             750
                        500                              500       500                             500
                        250                              250       250                             250
                        Pulse Magnitude [mV]
                                                                   Pulse Magnitude [mV]
                        -250 0                           0 -250   -250 0                           0 -250
                        -500                             -500     -500                             -500
                        -750                             -750     -750                             -750
                           0  45  90  135  180  225  270  315  360    0  45  90  135  180  225  270  315  360
                                    Phase Angle [deg]                          Phase Angle [deg]
                           High Load with Positive Predominance                  Low Load



                   3.5.4  PD Pattern
                     Failure mechanism     Polarity       Load Effect   Temperature Effect   Phase Location
                      Coil Movement         Positive        Direct           Inverse           225°

                           Symptoms                   Detection Tests                     Machine types
                    Partial discharge, ozone,   PD, visual inspection, wedge tap, ozone  Hard groundwall systems – epoxy and
                    loose wedges             monitoring                        polyester




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