THE CHOICE OF SUITABLE FREQUENCIES FOR MEASUREMENTS BASED ON FSM

The measurements which are related to the high-voltage substation grounding system are inevitably affected by electromagnetic interference caused by power frequency and its harmonics. In order to avoid the problem due to interferers, traditional measurements are based on heavy test current injection. In most cases this approach is impractical. In this paper, we evaluate experimentally an effective method for eliminating the effect of interference at power frequency. This approach based on test signal at three frequencies, is slightly different from the power frequency. For that matter, there are two problems: 1. How to choose adequate set of test frequencies, and 2. How to determine optimal intensity of the test current. In this paper, we solve the mentioned problems based on the results of extensive field test measurements at substations 110/X kV, 220/X kV and 400/X kV. We show that the set of suitable test frequencies are: 40 Hz, 60 Hz and 75 Hz. Also shown is a method for finding the optimal intensity of the test current