Page 43 - Basic PD Theory
P. 43

PD Investigations


               compare the results of one machine to those of similar machines.  Due to the influence of the test arrangement on the results,
               the test setup (sensors and test instrument) must be the same for all comparisons.  Based on statistical analysis, the type of
               machine does not appear to  have a great impact to PD levels and though insulation types have different types of failure
               mechanisms, in these gross comparisons there does not appear to be much difference in the overall levels of PD activity.  Load
               and temperature can also greatly influence test results, but these are dependent upon the type and condition of the insulation
               system, and thus, cannot be used in these gross comparisons.  The remaining operating variables – voltage and gas coolant –
               must be similar to compare results [Section 5.1.3].
               A further caution that can have an impact on comparison of PD data from similar machines and the Iris statistical database
               needs to be highlighted [9].  Like all current pulses, the high characteristic frequency partial discharges are greatly affected by
               impedance mismatches in the path they traverse.  If accessibility issues cause the PD sensors to be located away from the
               machine terminals or winding, and there are locations of impedance mismatches between the sensor and machine or winding,
               then the pulse magnitudes will be affected.  However, this will have no impact at all on the Trend evaluation, which is the
               primary purpose of the partial discharge test, but it could influence the ability to compare results to the Iris statistical database.
               Because of the great variety of design features, production conditions and insulation systems, it is not possible
               to directly compare the absolute values for different types of machines or to establish absolute acceptance
               limits [10].  The statistical comparison suggested here is based on gross comparisons only.

                                                                  Comparison to similar
                                  Current PD                            machines
                                    Levels







                                   High PD?       No                          No specific problems –
                                                                              monitor in 6 months


                                 Yes


                                  Further
                                investigation            B
                                  required





















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