Comparison of DC Ramp and Polarization and Depolarisation Tests on Hydrogenerators

Hydro-Quebec has started carrying out Polarization and Depolarization Current measurements (PDC) more than 20 years ago. A Direct Current Ramp Test (DCRT) apparatus has also been built in house in the early 2000's and since 2012 systematic measurements with both tests are prescribed to be done every 6 years on every one of our 350 generators. An extensive comparison of both types of electric tests has been done to determine the influence of factors such as temperature and humidity, type of ground wall insulation and the nature of the stress grading coating on the measured current. The DCRT and PDC test procedures are such that they are not always giving the same diagnosis and sometimes this can easily be explained, but in some case more in depth analysis is required. A comparison of the results of both types of tests was done and the main features and limits are presented herein.