The present invention relates to the design and development of a novel Gas Density monitor for detection of gas leakage and simultaneous measurement of gas density for effective management of Gas Insulated Switchgear upon evaluation in presence of influential parameters such as Pressure, temperature, molar mass, humidity, and gas concentration.
BACKGROUND:
Here, the development of gas density sensor is given significant importance in industrial and research applications. Sulphur Hexafluoride (SFe) - Gas Insulated Switchgear (GIS) has been the go-to ingredient over conventional transmission substations contemporarily. Dense design, ascending land cost in urban areas and persistent maintenance requirements contributes largely to the use of SF6 switch gear. It is one of the most efficient and reliable architecture of smart transmission sub-stations without which, it is inevitable to structure the Smart Grid. Therefore, it is a must to have smart sensors that can provide robust performances, as far as monitoring of different health parameters of a modern age sub¬station is concerned.
Detection of gas-leakage inside SF6-GIS environment engenders precarious damage to the live conductors and some electrical equipment. Especially, measuring equipment like current, voltage transducers and protection mechanism related actuation system beneath the vessel can critically affect the time of switching where gas leakage is encountered. Abnormal Gas leakage inside the vessel mostly occurs due to sudden breakdown of the SF6 enclosure seals to contain the gas. Besides, internal faults resulting from uncontrolled arcing inside the vessel as well as arc-fire gives rise to abnormal leakage. These actions are to be closely observed by advanced automation system as per IEC 61850 and IEEE 1686 protocols. Hence, it is indispensable to place a high precision Gas Density Sensor that can measure the leakage of SFe Gas inside the vessel at PPM/Trace-level which will, in turn, complement the specifications of IEC 61850 complaints and IEEE 1686 standard to setup SF6 GIS smarter than it actually was. As of now, design, modelling and fabrication of a cost- effective high precision Gas Density Sensor is an area of qualitative research that can

detect the leakage for a maximum gas loss per compartment per annum to be less than 0.5%. In a dense gaseous medium typically for applications like SF6-GIS, several methods to detect gas leakage was tried upon like, ultrasonic method, ultraviolet ionization method, infrared imaging method, laser imaging method, optical detection technology and similar other schemes. However, the research methodology reported in recent past showed that ultrasonic sensor can be the most suitable option to fabricate a cost-effective, high performance gas density sensor. The rest alternatives lag in aptitude due to their high price, inappropriate size, poor sensitivity, inefficient during high wind speeds, etc. Also, mostly their leak detection position and leak imaging appear flat on graph.
Performance of an IEC 61850 complaint sub-station depends on accuracy and efficacy of its embedded sensors for real time measurement purposes and its robust protection mechanisms. In the reported work, the designed Ultrasonic senor measure the time difference between the transmitter and receiver of ultrasonic pulses propagating through the gas. This time difference is termed here as 'time of flight'. If the velocity of the ultrasound is measured, then the time of flight can be measured. The velocity of the
[kP
ultrasound wave through the gas can be described by the equation, C= —, where k is a
A/Po
constant and is also the ratio of molar heat capacity at constant pressure upon molar heat capacity at constant volume of the gas. P is the pressure of the gas at the instant the experiment was conducted, PQ is the density of the gas. The co-efficient ck' does not vary
for a pressure range P = 0.8-1.2 atm. So, keeping all constant if only pQ is varying then the
velocity is inversely related to the density of the gas. If the density increases the velocity decreases, so the 'time of flight' increases. There is no work to the best of our knowledge which is proposed or bought into practice about the direct measurement of Gas density along with detection of leak location inside a Gas Insulated Switchgear.
In this work, an attempt has been made to develop an ultrasonic sensor-based technology for the detection of gas density inside the gas vessel, like shown in Fig.5, and subsequent gas leakage detection. Initially the effectiveness of the proposed method has been evaluated by modelling the measurement system using Finite Element Method and then experiment was conducted to verify the theory. An efficient modelling technique is framed which can address millimeter range of operation and at the same time with high accuracy. Also, it is a dire need to characterize the list of influential environmental parameters that can affect the accuracy.

Some of the influential parameters chosen as part of the Ultrasonic Method are Medium Temperature, Humidity, Atmospheric Pressure and Change in concentration whose variation analysis is shown in Table-I. For millimeter range of operation, instantaneous speed of sound is considered to be more effective rather than average speed. In a longitudinal wave, particles move parallel to the direction of wave whereas, in a transverse wave, particles move perpendicular to the direction of the wave. An isentropic process is an ideal thermodynamic process that is both adiabatic and reversible.
• From Newton-Laplace Equation;
lyx RX 273.15 L , e
c= I1 x 11 +
M \J 273.15
where;
c = Speed of sound
Specific Heat of Gas (at constant Pressure)
y
' Specific Heat of Gas (at constant Volume)
R= 8.3145 J/mol
M= Molar mass of ideal Gas
0= Temperature (in °C)
For a given altitude, the pressure variation is taken to be negligible and hence maybe
neglected while analyzing.
For calculation of change in concentration, following things are to be monitored;
. Mass , n
Density = —- and c = -
J Volume V
For perfect Gas;
XP
c = —
RT
where, X= Mole Fraction
Molar Mass
Also, Molar Density =
Molar Volume
Solving the equations empirically, we found that;
Concentration
Density of the Gas =
Molar Mass

• Object distance from transducer is;
_ (c x T.O.F)
2
where, c= Velocity of Sound in that medium
• Due to moisture, the value of y is changed to;
_ 7+h ^ ~ 5+h
where; h = fraction of water molecules in air
. 0.01 X RH X 133.322 x e(6)
h = —
p
again where; RH = Relative Humidity
e (0) - Water Vapor Pressure P = Ambient Pressure
• Molecular Mass of Water Vapor is 18 g/mol.
J f^jxRx 273.15 / i~~
28.969 x [l-(0.378 X h)] \J 10.15
where, CE = Exact Speed
By Applying Power Series Expansion with first order approximation, it was found out that;
CE= [331.296 + (0.606 x 0)] x [1 + (0.1607 x h)]
Further applying series expansion to the equation of water vapor pressure, it was obtained;

B/Cx(!-S-S-)
8
10
e(e)=10(A-^e)=-4/
10 'C
Now the instantaneous value of the speed is given by;
Cs = [331.296 + (0.606 x 0)] x [1 + (RH x 9.604 x 10"6 x 100032 x (0-ooo492))j
where;
Cs = Instantaneous Speed of Sound 0 = Temperature (in °C) RH = Relative Humidity

• So, after studying all parameters, it is quite evident that the instantaneous speed of sound in gas, particularly SFe, is only and mostly dependent on the change in Relative Humidity and Temperature of the media and least on change in concentration and Pressure.
These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following detailed description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
The above and other features, aspects, and advantages of the subject matter will be better understood with regard to the following description, and accompanying drawings and graphs where: -
Figure 1 shows Experimental Arrangement for density measurement of SF6 Gas.
Figure 2 shows Curve showing Density of SF6 Gas v/s Time of Flight.
Figure 3 shows Curve showing Weight of SF6 Gas v/s Time of Flight.
Figure 4 shows Wave response showing attenuation and parameters for ultrasonic sensor in
WAVE2000 software
Figure 5 shows Ultrasonic sensor mounted on gas chamber for testing
DETAILED DESCRIPTION:
Considering a rectangular vessel of size 100 mm x 30 mm 2 mm made of PLA as shown in Fig. 1, on the left side of the chamber, an ultrasonic sensor consisting of a transmitter and receiver of frequency 40 kHz is fixed with a 10 mm gap. Then, the size of the transmitter and receiver is taken for both as 5 mm. The transmitter transmits the acoustic wave and the wave propagates though the gas filled in the chamber as shown in Fig.4. The wave gets reflected from the opposite wall and received by the receiver. Time of flight of the acoustic wave varies with the variation of gas density. Simulation was carried out using a Finite Element software (WAVE2000). In normal air the time of flight of the signal required to travel 200 mm distance through the gas medium is 1270 microseconds. Simulation was then conducted

by filling SF6 (Sulphur-hexafluoride) gas inside the vessel. Density of SFe gas was gradually increased Time of Flight of the signal corresponding to each density value was measured. Figure 2 shows the variation of time of flight with increase in gas density. Also, the curve pertaining to variation of time of flight with increase in weight is shown in Fig.3. Simulated results show that with the increase in the density the time travelled by the wave is increases. The response is linear in nature with a R2 value of 0.9.
For second physical implementation, The HC-SR04 ultrasonic sensor was placed on the top of a rectangular shape sealed acrylic test chamber of volume 7220 cc. The acrylic box is mounted on an electronic weight balance. The room temperature was 23 °C ± 2 °C and the relative humidity was 40%. The SR-04 ultrasonic sensor was connected to an Arduino uno, which was interfaced to a PC through a data acquisition system. Measurement of density of SF6 gas is carried out by introducing sound in the frequency range 40 kHz in a test chamber. This work proposed to build an efficient module that consists of ultrasonic sensor HC-SR04 with Arduino UNO Board for the measurement of time required for the sound wave to travel from the transmitter to the receiver. This device makes the use of Arduino UNO microcontroller board for calculation of time of flight of the sound wave and displaying the obtained results on the PC. The time of flight is calculated for the different amount of SF6 gas. It was found that time of flight of the ultrasound wave increases with the increase of amount of SF6 gas in the chamber, which was relevant to our theoretical knowledge. The vessel with sensor was placed on electronic balance with 0.1 g resolution. Mass of the vessel without SFe was measured and was set to zero value. The chamber was then connected to the SF6 gas cylinder through a manually controlled valve. The chamber is sealed and the inlet of the SFe gas is connected with a one-way valve so, there is no leakage of SFe gas in the environment, which ensures the increase of SF6 gas density in the chamber as the weight of the chamber increases. The chamber was filled in SF6 gas through a one-way valve. The mass SFe gas was measured using the balance. From the reading of mass, density of SFe gas was calculated (density = mass/volume). At particular density of gas, time of acoustic wave was measured. Experiment was then repeated for increase in gas density. The complete experimental flow can be illustrated from Fig.6. This is to note that several readings of time of flight were noted at each gas density, as shown in Table-I, and the average value was plotted. There is a clear indication of the increase of time of flight with the density of the gas. The response is linear in nature with a R2 value of 0.96. Readings are not exactly repeating.

This may be due to the mismatch of the impedance between the gas and the sound wave oi due to the non-uniform distribution of the gas in the entire volume. By optimizing the size oi the vessel, it may possible to improve the consistency of the reading. However, readings are reliable and there is an increasing trend of time with density.

We Claim:
1. A novel method involving ultrasonic sensor for gas density measurement , using simple and highly accurate sensing mechanism. The sensor is based on ultrasonic principle. When a wave traverses in an empty chamber, the velocity and time of flight hence calculated for the wave will have a value different than that of the wave within a chamber containing a significant amount of gas within it.
2. A novel method as claimed in claim 1, relates to the use of SF6 Gas as the medium where the ultrasonic wave traverses and thereby the gas monitoring methods are employed.
3. A novel method as claimed in claim 1, relates to the direct measurement of gas density as part of monitoring by the sensor using the sensing mechanism deployed.
4. A novel method as claimed in claim 1, relates to the simultaneous detection of leakage position and leak size along with the density measurement.
5. A novel method as claimed in claim 1, relates to the simultaneous measurement of pressure and temperature inside the chamber using manometer and temperature sensor relevantly with other measurement techniques.
6. A novel method as claimed in claim 1, relates to the use of a novel ultrasonic microphone for the detection of acoustic waves coming out from the leak detected from the chamber and hence incorporating the leak sensing mechanism of the system.
7. A novel method as claimed in claim J, relates to the use of data acquisition system, where the data obtained from the respective sensors will be mirrored to the local data centres where exhaustive analysis will be carried out.
In the present work, a novel method involving ultrasonic sensor for gas density monitoring has been developed. The sensor prototype has been tested for different volumes of gas at different values of physical parameters. The achieved characteristics are suitable for employing the sensor for ultrasonic detection of gas leakage. The response characteristics of the presented sensor are significant, fast, highly reproducible and accurate.

Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the present invention should not be limited to the description of the preferred embodiment contained therein.