Development of Field-Mill Instruments for Ground-Level and Above-Ground Electric Field Measurement Under HVDC Transmission Lines

The electric environment under HVDC transmission lines is characterized by electric field, current density and space charge density. The measurement of electric field at and above ground level is described in this paper. A field-mill instrument has been developed for ground-level measurements and has been used extensively under the bipolar dc test line at IREQ. The same principle was applied to develop a cylindrical field probe for measurements above ground. Details of the operating principle, design and construction of the two instruments are given in this paper together with an account of the calibration procedure and operating experience.