New legislation is forcing utilities and manufacturers to reduce sulfur hexafluoride emissions.

Critical condition

Sulfur hexafluoride, more commonly known as SF6, is a key product for the electrical utility industry, providing an inert, non-toxic solution for cooling, insulating and arc quenching operations in potentially high temperature applications, such as electrical breakers and high voltage switchgear installations. 

The gas offers significant advantages in terms of design, compactness and operational effectiveness, but it is also one of the most environmentally damaging gases known to man with a greenhouse gas potency of around 22,000 times greater than CO2 and an upper atmosphere lifetime of 4,000 years.

As a result, SF6 requires handling with extreme care and, as environmental concerns have risen up the corporate agenda, utilities and manufacturers alike have been looking for ways to be even more responsible in their use of the gas. 

This responsibility includes being able to prove to the relevant regulatory bodies that SF6 is being used and stored properly, via an accurate inventory and audit trail of gas use and storage. 

Currently, most utilities use a manual method of periodically reading gauges combined with mechanically switched alarms. This results in, on average, a five per cent gas loss before an alarm is raised. 

In order to meet the new demands driven by legislation, it has become increasingly clear that a system to continuously monitor SF6 gas is required. Any new system must be able to provide the earliest possible warning of gas loss to give any utility the time to respond. 

This requirement resulted in a partnership between Siemens Transmission and Distribution in the UK and one of the UK's biggest users of SF6 gas, National Grid. "Customers want to reduce leak rates below 0.5 per cent per annum which requires a far more precise system than traditional analogue gauges", David Kerr, Siemens project manager, says. "But, utility companies worldwide also want a system that provides them with the earliest possible warning of gas leaks, as well as precise information on the quantity of SF6 gas contained in the equipment to give them a real-time inventory."

The result of the partnership has been the development of
the Siemens Gas Density Monitoring System (GDM) which has been designed to allow the user a greater degree of flexibility for measurements and alarms than currently provided by gauges with contacts. The system uses SF6 density transducers operating with a 4-20mA current loop technology for high noise immunity. The transducers can be mounted directly onto the
GIS chamber via an industry standard gas 'top-up' connection or retrofitted to gas monitoring boxes housing SF6 gas pressure gauges, where ease of access is a requirement.

The transducers are connected via twisted pair, shielded cable to GDM node units, installed within the GIS hall. Up to 45 density transducers can be connected to each GDM Node Unit which houses the data acquisition and storage facilities. The node units are connected via a fibre optic communications cable to a central GDM control cubicle where an HMI is available to provide access to the data and where alarm data is automatically generated. Multiple node units can be connected to the central GDM Control Cubicle to provide adequate coverage for every size of GIS substation.

One of the main advantages of the system is that it can readily identify leaks at a very early stage and predict when alarm criteria will be met. By selecting 'zone status' the current predictions for each gas zone can be viewed by the operator, illustrating when first and second stage alarms are likely to be reached as well as overall leak performance. This data can be used to form the basis of a predictive maintenance programme to minimise the potential for leakages.

The collection and retention of data is also critical in being able to prove compliance with regulations. The system takes ten readings per second and averages the results to produce a real time value. All data is then saved in three separate areas to minimise the risk of lost data.

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