FIELD OF THE INVENTION:
The present invention relates to a signal indicating device used for monitoring SF6 gas
pressure inside the bushings of transformers in HVDC transmission line. The device
continuously converts pressure signal of density guard for bushings in to electrical
pulses and subsequently to visible signals through LEDs. The LEDs are monitored from
a distance and necessary steps are taken to avoid chances of failure of both bushings
and transformer.
BACKGROUND AND PRIORT ART OF THE INVENTION:
Previously before the invention of the device continuous monitoring of SF6 gas was not
possible and the testing was used to get held up at regular intervals to measure the
pressure through density guards directly mounted on bushings using continuity tester or
multimeter, which was crude and cumbersome resulting wastage of time.
OBJECTS OF THE INVENTION:
Basic object of the invention is to develop a device to monitor SF6 gas pressure inside
the bushings of HVDC transformers used in electrical transmission line.
Further object of the invention is to provide a method of continuous monitoring of SF6
gas pressure for bushings of transformer during impulse testing and after.
Another object of the invention is to prevent insulation failure of the bushings and the

transformer during power service having checked any leakage and pressure fall before
installation.
Also the object of the invention is to save time and cost by way of development of a
novel device of leakage testing and method for SF6 gas inside the bushings of a
transformer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows a circuit diagram comprising 6 nos. of RED LEDs (R1, R2, R3, R4, R5, R6)
and 6 nos. of GREEN LEDs connected to terminal box (TB1), which are also connected
with the test circuit for checking healthiness of LEDs.
Fig. 2 shows connection of terminal box (TB1) with 2 nos. of terminal box of respective
density guard (DG 1 & DG 2)
Fig. 3 shows layout of components along with bill of materials
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF
THE INVENTION:
Before filling SF6 gas in the bushing, the density guard is connected to the device as
per circuit diagram shown in Fig.2. SF6 gas is filled thereafter inside the bushings till a
pressure of 3.2 – 3.7 basis is attained. There are always chances of leakage of the gas
filled at such high pressure during impulse testing, and in case of any leakage there are
chances of failure of the bushing as well as transformer when subjected to such high
voltages during impulse testing. Initially red LEDs glow as there is less pressure inside

the bushing and with increasing pressure range red LEDs get off and respective green
LEDs start glowing displaying the healthy condition of the bushings. In case of any
issues (like bursting of explosion valves during impulse testing or malfunctioning of
valves), the pressure gets reduced, the red LEDs turns on automatically.
In execution of the operation, density guard is mounted on the DC bushing of the HVDC
transformer, wherein density guard has 9 terminals forming 6 contacts, out of which
three are NOs (normally open) and three are NCs (normally close) with the following
sequence.
14-NO-11-NC- 12; 34-NO- 31- NC- 32; 24- NO- 21- NC - 22
The LEDs can be monitored by an operator from distance and take necessary steps
reducing any chances of failure of the bushing as well as transformer when subjected to
high voltages during testing.
The device comprises primarily a circuit as shown in Fig.3 including one way switch
(003) for continuous display, push button (004) for Temporary Display, push button
(005) for LED Test, 6 red LEDs (R1, R2, R3, R4, R5, R6), 6 green LEDs (G1, G2, G3, G4,
G5, G6), 12 UF 1007 diodes housed in a wooden housing (002) with mica sheet atop
(001).
Density guards are mounted on the DC bushing of the HVDC transformer, respective
terminal boxes (DG1, DG2) of which are connected to the terminal box TB1 of the said
circuit of the device as shown in Fig. 2.
Specifications of the device along with bill of materials are as under.

WE CLAIM:
1) A signal indicating device for monitoring pressure of SF6 gas inside bushings of
HVDC transformers, characterized by conversion of pressure difference signals
inside the bushings in to electrical pulse and subsequently electrical pulse to
visible signals comprising, a circuit (007), a one way switch (003), push button
(004), push button (005), red LEDs (R1-R6), green LEDs (G1-G6), UF 1007
diodes (D1- D12) housed in wooden box (002).
2) A method of monitoring SF6 gas pressure inside the bushings of HVDC
transformer, wherein density guards are mounted on the DC bushings, having
respective terminals (DG1, DG2) connected to the terminal TB1 of the circuit
(007) of monitoring device of claim1, while the bushings are filled by SF6 gas.
3) The method as claimed in claim 2, wherein density guards having 9 terminals, to
form 6 contacts (3 NO & 3NC), configured by the sequence, 14-NO-11-NC-12;
34-NO-31-NC-32; 24-NO-21-NC-22 to undertake the test of leakage monitoring.
4) The method as claimed in claim 1, wherein the push button (004) is pressed to
make or break respective contacts to monitor the leakage through indicating
LEDs, wherein green LEDs indicate healthy conditions of the bushings and red
LEDs a failure.