Galvanically Isolated Clamp-on Medium-Voltage Sensing

Voltage sensing is often necessary to control and monitor power converters, electric drives, and power distribution systems. Low loading and wide bandwidth are typical requirements for these sensors, and medium voltage systems pose special challenges due to the need for insulation and isolation. Capacitive sensing can offer a solution with low loading and inherent isolation, but low-frequency and dc signals are challenging to measure. Capacitive voltage sensing techniques are briefly reviewed, and a clamp-on ac voltage sensor utilizing capacitive coupling through a cable's insulation is examined and tested experimentally. This sensor is capable of capacitively isolated differential and three-phase line-to-line measurements. A very wide bandwidth of 31 mHz-107 kHz is obtained by using a high impedance amplifier, and two shielding schemes to limit capacitivitely coupled noise are tested and compared. It was found that a grounded shield yields better performance than a driven shield, as the driven shield introduced a high-frequency resonance in the sensor response. A low noise floor of 3.4 $\mathrm{V}_{\text{rms}}$ with a range of 4.5 kV is obtained.