Partial discharge sensor for a high voltage insulation monitoring device

1. A partial discharge sensor for a high-voltage insulation monitoring device; comprising a housing and located in the housing a measurement circuit for measurement of partial discharges in a high voltage system to be tested and a coupling capacitor having one electrode connected to the measurement circuit and the other electrode to a first high voltage conductor to be connected to a high voltage line of the system to be tested,

wherein it further comprises a calibration circuit comprising a calibration capacitor having one electrode connected to the calibration circuit and the other electrode connected to said first or a second high voltage conductor to be connected to a high voltage line,

wherein the coupling capacitor and the calibration capacitor are realized as one dual capacitor,

wherein an electrode of the dual capacitor connected to the high voltage conductor is a common electrode to both, the coupling capacitor and the calibration capacitor, and

wherein the measurement circuit comprises a signal transmission line and the electrical power for operation of the measurement circuit and the calibration circuit is supplied through the signal transmission line.

2. The partial discharge sensor according to claim 1 , wherein the calibration circuit is located in the housing.

3. The partial discharge sensor according to claim 1 , wherein the dual capacitor comprises a hollow cylindrical shaped isolator having at least one high voltage electrode located on an inner wall of the isolator and two electrodes respectively connected to the measurement circuit and the calibration circuit and located on an outer wall of the isolator.

4. The partial discharge sensor according to claim 3 , wherein the electrode connected to the calibration circuit is shaped as a disk and located at a bottom of said cylindrically shaped isolator.

5. The partial discharge sensor according to claim 3 , where the electrode connected to the measurement circuit is mainly shaped as a cylinder and mainly located on a side wall of said cylindrically shaped isolator.

6. The partial discharge sensor according to claim 1 , wherein the coupling capacitor and the calibration capacitor are realized as two individual capacitors.

7. The partial discharge sensor according to claim 1 , further comprising a converter supply circuit coupled to the low voltage electrode of the coupling capacitor, for supplying the calibration circuit with a necessary supply voltage.

8. The partial discharge sensor according to claim 1 , wherein the measurement circuit comprises an amplifier with differential output and where the signal transmission line is realized as twisted cable pair.

9. The partial discharge sensor according to claim 1 , wherein the calibration circuit comprises a pulse generator synchronization line that is connected between the coupling capacitance and the calibration circuit.

10. The partial discharge sensor according to claim 9 , wherein the calibration circuit includes two branches connected in parallel, a first branch connected on the one hand to earth and on the other hand to a switching unit and a second branch comprising in parallel a capacitor and a signal generator, one end of the second branch being connected to earth and the other end being also connected to the switching unit, the switching unit being connected to the calibration capacitor.

11. The partial discharge sensor according to claim 10 , wherein the switching unit has two positions, a first position connecting the calibration capacitance to the first branch and a second position, connecting the calibration capacitance to the second branch.

12. The partial discharge sensor according to claim 10 , further comprising a serial communication channel for adjusting a height and a sequence of calibration pulses and/or to control the switching.

13. The partial discharge sensor according to claim 1 , further comprising a serial communication channel for adjusting a height and a sequence of calibration pulses and/or to control a switching unit.