PSS damping effect is largely affected by the uncompensated phase frequency characteristic of generatorunit.Theconversionmethodbetweendifferentperunitsystems

incaseofcalculatingvariationofthe uncompensated phase frequency of the generator unit due to the change of additional reactive power adjustment coefficient

is derived. The different adjustment differences are set on one 1 000 MV and one 300 MW generator units respectively and the test verification of the theoretical value of phase frequency characteristic variation due to the calculation of additional adjustment is performed. The method of modifying PSS parameters in case of variation of additional adjustment coefficient is proposed and the real example is given on one300 MW generator unit. The tests results show that the theoretical value of the phase frequency variation of the generator unit due to the change of additional adjustment coefficient is basically identical in agreement with that of test value and can be used in the engineering practice. The method proposed to modify the PSS parameters in response to the change of the additional adjustment coefficient can be used for the modification of PPS parameters after the modification of the additional adjustment coefficient of each generator unit.