BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial view of a transmission tower with a high voltage insulator schematically representing a fast flashover mechanism on the negative dc pole or during the negative half-cycle of the AC voltage.
FIGS. 2
a and 2b are respectively a side section and top views of an open toroidal streamer inhibitor 22 with arcing terminals 24 used as a support structure, according to a preferred embodiment of the present invention.
FIG. 3 is a side view of a high voltage DC transmission tower with a toroidal inhibitor mounted at the tower/ground-end of the insulator string that is supporting the negative polarity power conductor, according to a preferred embodiment of the present invention.
FIG. 4 is a side view of a high voltage AC transmission tower with toroidal inhibitors mounted at the tower/ground-end of the insulator strings according to a preferred embodiment of the present invention.
FIG. 5
a is a side section view of a fiber-reinforced polymer (FRP) hot stick with a toroidal inhibitor mounted at the ground-end of the stick, according to a preferred embodiment of the present invention.
FIG. 5
b is a side section view of an FRP stick with an inhibitor coil wound directly onto the ground-end of the stick, according to a preferred embodiment of the present invention.
FIG. 6 is a side view of an FRP boom with toroidal inhibitor mounted onto the ground-end of the boom, according to a preferred embodiment of the present invention.
FIG. 7 is a side section view of a negative polarity high voltage DC Wall Bushing with a toroidal inhibitor mounted at the wall-end of the bushing, according to a preferred embodiment of the present invention.
FIG. 8 is a partial section view of a transmission tower with an arcing horn located above an insulator string being wrapped in an inhibitor coil, according to a preferred embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown a transmission tower 10 supporting a high voltage conductor 12 via an insulator string 14. This example provides a schematic representation of a fast flashover mechanism on the negative dc pole or during the negative half-cycle of the AC voltage. Negative space charge is generated from the high voltage conductor 12 and hardware that create a negative space charge cloud 16 which can partially settle as negative surface charge 18 on the insulator string 14. As the ground side of the insulator string 14 becomes more stressed, positive streamers 20 are created. If the positive streamer charge gets neutralized, a positive leader can form leading to complete failure.
Referring to FIGS. 2a and 2b, there is shown an open toroidal streamer inhibitor 22 with arcing terminals 24 used as a support structure, according to a preferred embodiment of the present invention. The toroidal streamer inhibitor 22 is shown with its minor diameter d, major diameter D, inner major diameter Di, and outer major diameter Do. These establish the various parameters and dimensions which can be varied for the purposes of the invention.
Referring to FIG. 3, there is shown a high voltage DC transmission tower 26 with an insulator string 14 supporting a negative polarity conductor bundle 28. As shown, a toroidal inhibitor 22 is mounted at the tower/ground-end of the insulator string, according to a preferred embodiment of the present invention. The toroidal inhibitor 22 is provided with space charge producing conductors (not illustrated) wound around it and forming coils for producing space charge and inhibiting a formation of positive streamers. Each conductor has a diameter not exceeding 0.1 mm for reducing a corona inception voltage of the support structure upon which each conductor is wound, in both dry and wet conditions.
Referring to FIG. 4, there is shown a high voltage AC transmission line tower with an insulator string 14 supporting an AC power conductor 32. Similarly as above, a toroidal inhibitor 22 is mounted at the tower/ground-end of the insulator string 14, according to a preferred embodiment of the present invention. The toroidal inhibitor 22 is also provided with space charge producing conductors (not illustrated) wound around it, as described above.
Referring to FIG. 5a, there is shown a fiber-reinforced polymer (FRP) hot stick 34 with a toroidal inhibitor 22 being mounted at the ground-end of the stick, according to a preferred embodiment of the present invention. The toroidal inhibitor 22 is provided with thin conductor coils (not illustrated) having a diameter not exceeding 0.1 mm and is adapted to be grounded.
Referring to FIG. 5b, there is shown an FRP hot stick 34 similar as above, but provided only with an inhibitor conductor coil 36 mounted directly onto the ground-end of the stick 34, according to a preferred embodiment of the present invention. The conductor coil 36 has a diameter not exceeding 0.1 mm and is adapted to be grounded.
Referring to FIG. 6, there is shown an FRP boom 38 having a ground end 40 and a high voltage end 42. As shown, a toroidal inhibitor 22 is mounted at the ground-end 40 of the boom 38, according to a preferred embodiment of the present invention. The toroidal inhibitor 22 is provided with thin conductor coils (not illustrated) having a diameter not exceeding 0.1 mm and being adapted to be grounded.
Referring to FIG. 7, there is shown a negative polarity high voltage DC converter wall bushing 44 mounted on a building wall 46. The wall bushing 44 has a high voltage negative pole 48. As shown, a toroidal inhibitor 22 is mounted at the wall-end of the bushing 44, according to a preferred embodiment of the present invention. The toroidal inhibitor 22 is provided with thin conductor coils (not illustrated) having a diameter not exceeding 0.1 mm and being adapted to be grounded.
Referring to FIG. 8, there is shown part of a transmission tower 50 supporting an insulator string 14 and a high voltage conductor bundle 52. As shown, an arcing horn 54 is used as the support structure for an inhibitor conductor coil 56 being mounted directly thereon, according to a preferred embodiment of the present invention. The conductor coil 56 has a diameter not exceeding 0.1 mm and is adapted to be grounded.
Tests Conducted
A series of tests were conducted with devices and methods embodying the concepts of the present invention. The objective of the tests was to determine the effect that the procedures and devices described herein would have on the flashover voltage of an FRP stick.
Test Object
The test object comprised a 3 m long fibre-reinforced polymer (FRP) stick normally used in work on energized high voltage direct current (HVDC) transmission lines. The flashover voltage was determined, by the technique described below for ordinary sticks as well as sticks whose ground-ends have been provided with the flashover protection device that is the subject of this patent application and which are referred to as Streamer Inhibiting Electrodes or Inhibitor Electrodes.
Test Technique
The test technique has been devised in order to enhance the probability of the occurrence of streamer initiated or fast flashovers on the FRP stick.
Since in previous tests conducted by Manitoba Hydro on FRP sticks a negative polarity voltage proved to be more severe, only such polarity was used. The FRP stick was pre-polluted by a solid layer comprising Kaolin and NACL satisfying IEC Standard 507 to reach a salt deposit density of approximately 2 μg/cm2, which was found to be representative of field conditions in live line work (work under voltage).
The tests were carried out in a large fog chamber satisfying the requirement of IEC Standard 507. The rate of steam injection however was reduced to approximately 0.0025 kg/h/m3 of the fog chamber volume in order to extend the effective testing time.
The test started with the application of −300 kVdc to the FRP stick, which was suspended from a two-conductor bundle situated approximately 10 meters above ground, followed in a few minutes by the start of the steam injection.
The relative humidity in the fog chamber is continually monitored and when it reached 70%, the voltage was ramped at a rate of 10 kV/s to −600 kV or up to stick flashover, whichever came first. The voltage is then returned to −300 kV, held for one minute and the ramp voltage application was repeated until the relative humidity reached 85% or until leakage current measured on the FRP stick showed that a pollution type flashover was eminent.
During the tests the following measurements were taken:
- fog temperature and relative humidity in the test chamber;
- leakage current on the test object by two devices: a normal pollution leakage current measuring system with a sampling rate of approximately 25 kHz and a high speed Tektronix oscilloscope with a sampling rate in the multi MHZ range; and
- discharges on the test object were monitored by a UV camera (30 frames/s) and a high speed video camera (400-1600 frames/s).
The first series of tests were performed with an FRP stick, without an Inhibitor Electrode, where the clear distance between the high voltage and ground electrodes amounted to 2.7 m (i.e. 90% of the insulating length of the stick). In the second test series the lower ground electrodes was replaced with an Inhibitor Electrode while maintaining the air gap clearance at 2.7 m as in the first test series.
Test Results
For an ordinary FRP stick without Inhibitor Electrode the flashover voltage varied between 442 kV and 336 kV corresponding to a mean gradient per unit length of 112-147 kV/m. For the stick equipped with an Inhibitor Electrode (toroid with an overall diameter of 15 cm and a minor diameter of 2 cm) the limit of the test voltage of −600 kV was reached several times consecutively without ever causing flashover of the FRP stick. This means that even at a mean gradient per unit stick length of 200 kV/m, the FRP stick equipped with an Inhibitor Electrode did not flashover. The success of the device subject to the present invention is self evident.
The flashover protection device and methods according the present invention reduce the risk of such fast flashovers by inhibiting the development of streamers under different atmospheric conditions with the insulators only exposed to the system operating voltage without the application of either lightning or switching voltage transients.
Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.
Patent Application
20080020655
