Capacitive sensor for fast pulsed voltage monitor in transmission line

To monitor fast pulsed voltage in the transmission line of the fast linear transformer driver, a compact and self-integrating capacitive sensor is developed. A printed circuit board (PCB) with multi-layer ceramic capacitors (MLCCs) serves as a low-voltage arm. Given the reflow structure of the PCB and a parallel arrangement of MLCCs, the residual inductance of the PCB low-voltage arm can be effectively reduced. Moreover, its capacitance can be easily adjusted. The capacitive sensor is designed using a coaxial structure, suitable for installation on the inner high voltage electrode of the secondary water-insulated transmission line. Three matching schemes for the measuring cable are discussed, showing that two-stage division with matching at the end of the cable is more suitable for fast pulsed voltage measurement due to its high dividing ratio and flat frequency response. A square wave with a rise time of ∼1.5 ns and a pulse width of ∼200 ns is used to verify the properties of the capacitive sensor. The results show that the rise time of the designed capacitive sensor can reach 1.5 ns when a short cable is used. Increasing capacitance of the low-voltage arm or resistance at the cable entrance will increase the time constant of the capacitive sensor, which improves its low-frequency properties. A long cable will significantly reduce the high-frequency response and the magnitude of the signal from the capacitive sensor.