SF6 leak detection with NDIR Principle

SF₆ gas is used as the insulating medium for AIS / GIS Substations for its superior dielectric strength property. Besides, SF₆ is colourless, odourless, non-flammable, chemically inert in nature and highly stable.  SF₆ leakage is one of the most common problems for a power utility where a GIS or AIS circuit breaker is installed. The leakage can occur in any part of the Circuit breaker/GIS where two parts are joined together such as valve fittings, bushings and flanges but in rare cases, it can leak straight through the aluminium as a result of poor casting. These leaks can be found by using gas leak detectors or thermal imaging. Leak testing also minimizes maintenance and reduces environmental emissions.

Importance of SF6 Leak Detection

In HV, EHV or UHV Circuit breakers / GIS chambers SF6 gas is stored in high pressure to quench the arc during the fault. If SF₆ leakage starts happening then the pressure reduces rapidly inside the SF₆ chamber which reduces the arc quenching capability. Also Pure SF₆ gas is a colourless, odourless, toxic and flammable halogen compound but due to its high Global Warming Potential (23000 times that of CO₂) & exceptionally high atmospheric life (Approx. 3200 years), SF₆ leakage is not at all suitable as it depletes the ozone layer which is harmful to a human being. 

Also SF₆ in GIS can be harmful especially when the breakdown maintenance is going on. Under the action of high power arc, spark discharge and corona discharge, the SF₆ gas can decompose and form toxic by-products such as SO₂ & H₂S. Additionally, every utility has been restricted SF₆ usage up to a certain limit as per the environmental guidelines. Usage of excess SF₆ beyond the certain limit will only cost an additional penalty for an individual utility and this is why SF₆ leakage has to be minimum and rectification is required at the earliest.  

NDIR Working Principle

SF6 leak detection is done with two methods:

1. Non-Dispersive Infrared (NDIR) Principle
2. High Voltage Ionization

NDIR Principle uses a dual-wavelength NDIR (Non-Dispersive Infrared) sensor to monitor SF₆. Most molecules can absorb infrared light, causing them to bend, stretch or twist. The amount of IR light absorbed is proportional to the concentration. The energy of the photons is not enough to cause the ionization, and thus the detection principle is very different from that of a photoionization detector (PID). Ultimately, the energy is converted to kinetic energy, causing the molecules to speed up and thus heat the gas. Each molecule absorbs infrared light at wavelengths representative of the types of bonds present.

SF₆ has a strong absorbance at 10.7 µm (947 cm-1, Figure 2), and C-H bonds absorb in the range 3.3 to 3.5 µm depending on the structure of the rest of the molecule, and H₂0 absorb in the range 5 to 8 µm and below 3 µm. The SF₆ absorbance band is unique and therefore highly selective. By contrast, many compounds have similar C-H bonds, and this absorbance is suitable to detect a range of hydrocarbons non-selectively.

Now if we compare Figures 2 and 3, it is observed that there is no interference by water absorbance in the SF₆ measurement because water has no absorbance at 10.7 µm (947 cm-1).

The absorbance of the gas is directly proportional to its concentration, following the

Lambert-Beer law:

A = -log (I/ I₀) = εlc

I = I₀10^-A = I₀10^-εlc

Where,

I₀ = Initial light intensity emitted from the lamp (measured in pure air)

I = Intensity of light reaching the end of the detector (with sample gas present)

A = Absorbance in units of length-1

ε = Molar extinction coefficient in concentration-1 length-1

l = Path length (sensor cavity depth and c is the concentration.

Because l and ε are fixed, measuring the light intensity before and after the sample allows a measurement of the concentration.

From figure 1, Non-dispersive refers to the fact that all the light passes through the gas sample and is only filtered immediately before the detector. The filter in front of the detector removes all the light except that at 10.7 µm, corresponding to SF₆. 

Advantage of SF6 leak detection with NDIR Principle

1. With NDIR principle leak detection facility circuit breaker and GIS leakage can be checked while it is online, no shut down is required
2. Compared to the traditional leak detection techniques NDIR is having quite higher detection efficiency
3. NDIR sensor is having a faster response time and higher sensitivity
4. The NDIR technology is suitable for monitoring the equipment which is difficult to reach by the contact measuring tool and can detect the potentially dangerous leakage from a few meters away
5. The measurement would not get affected in the presence of Moisture or Common Volatile Organic Compounds (VOC)
6. The measurement would not get affected by moderate wind flow
7. NDIR sensor is having a higher life than that of the conventional sensors

Conclusion

To know more about SCOPE marketed SF6 leak detector, please write to us at marketing@scopetnm.com