Reivindicaciones

1. A high frequency current transformer, HFCT, type partial discharge sensor device for high voltage equipment and installations, comprising: - a toroidal ferrite core (1); - a first measuring coil (5) wound to the ferrite core (1); and - a first coaxial cable (13) welded to the first measuring coil; wherein the sensor device is characterized by further comprising: - a second auxiliary check coil (8) wound to the ferrite core (1); - an electrical resistance (9) in series with the auxiliary check coil (8); - a second coaxial cable (13) welded to the electrical resistance; and - a U-shaped copper shield (11), which shields the assembly formed by the ferrite toroidal core (1), the first measuring coil (5) and the second auxiliary checking coil (8); and - an outer resin coating (12). 2. Device according to claim 1 wherein the first measuring coil comprises five flat loops and an additional return loop. 3. Device according to claim 2 wherein the additional return loop is fixed on an outer cylindrical surface of the ferrite core. 4. Device according to claim 3 wherein a final terminus of the additional return loop has a U-shaped geometry with dimensions corresponding to a wave impedance equal to that of a 50 Ω measurement cable. 5. Device according to any of claims 2-4, wherein the five flat loops have dimensions of 125 µm thick and 5 mm wide 6. Device according to any one of the preceding claims, wherein the first coaxial measurement cable has a characteristic impedance 50 Ω and is welded to the first measurement coil as follows: a shield of the coaxial cable welded to the terminus of the additional return loop and the active conductor of the coaxial cable to the initial terminus of the measurement coil, connection that is shielded in the U-shaped geometry of the final terminus of the return loop. 7. Device according to claim 6 further comprising an element to be selected from an insulating, adhesive, flexible and transparent sheet of polyamide film and an insulating varnish, covering the first measuring coil, the second self-check coil and the welds of both coaxial cables. 8. A device according to any one of the preceding claims, wherein the second auxiliary check coil comprises two loops. 9. Device according to any one of the preceding claims, wherein the electrical resistance in series with the second auxiliary check coil is a resistance of 300 Ω. 10. Device according to any of the preceding claims, wherein the ferrite toroidal core is a high permeability MnZn ferrite toroidal core. 11. Device according to claim 7 wherein the copper shielding comprises a copper tape with at least 35 µm thickness and U-shaped, which shields in addition to the ferrite core and the measuring and self-checking coils, the welds of both coaxial cables and the insulating sheet or varnish, covering the outer cylindrical surface of the toroidal core and its two circular crowns, which constitute a lower base and an upper base of the assembly, leaving only the inner cylindrical part of the toroidal core without shielding. 12. Device according to any of the preceding claims, wherein the toroidal ferrite core has an inner diameter of 54.3 mm, an outer diameter of 87.0 mm and a thickness of 13.5 mm. 13. Device according to any of the preceding claims, wherein the toroidal ferrite core is split into two equal halves with two air gaps of 125 µm each. 14. Device according to any of the preceding claims, wherein the outer resin coating comprises an epoxy resin layer of at least 3 mm thickness. 15. Device according to claim 14 wherein the resin layer comprises an arrow-shaped mark pointing towards the earth of the installation, which guides the correct installation of the device.