This article provides information on how to monitor gas and the different alerts you can set. The article discusses the various types of defects that can lead to partial discharges. It also describes various methods for measuring partial discharges, including electric, acoustic, and chemically specific methodologies.
SF6 Gas Monitoring and Alarming Practices in Gas-Insulated Switchgear (GIS) Systems
Other monitoring includes the observation of circuit breakers and gas density.
GIS monitoring has numerous applications. There are many other components in switchgear, such as breaker components that are similar to GIS. Gas detection is the most common monitoring method. Gas monitors should be installed on nearly every piece of equipment that uses SF6 gases, like GIS or breakers.
Circuit breakers are widely used in monitoring systems. Many commercial products are available from OEMs and third-party vendors for monitoring circuit breakers. GIS isn’t the only solution.
Most breakers have some form of monitoring, even if it’s basic (such as counting how many times they have been used). More sophisticated monitors are available, and they’re installed widely, even though some users might not utilize all the features.
In the last ten to fifteen years, partial discharge monitoring (PD) has gained increasing attention due to its potential to warn of emerging concerns about insulation in GIS. Few commercial solutions are available because data analysis is usually a human-based process.
1. SF6 Gas Monitor
GIS employs SF6 for electrical insulation, arc quenching, and circuit breakers. The density of SF6 gas affects both characteristics. Gas pressure is often used instead of density when discussing gases. In written material, the terms “normal operating pressure” and “fill pressure” are frequently used. Gas density is critical, but pressure is used because it is easy to measure and understand.
Pressure is temperature-dependent. If a state does not change, the pressure will remain constant. Gas monitoring is primarily used to ensure that there is sufficient SF6 to operate the machinery. This usually requires measuring the gas density.
There are some caveats when measuring gas density. The enclosure is the spot most often used in GIS systems to measure gas density (see Figure 1). Although gas pressure is assumed to be uniform throughout the chamber, there could be density differences due to temperature gradients.
In operation, gas temperatures in a GIS enclosure may be higher where the density meter is usually installed. A density measurement within the enclosure can artificially increase estimates of the population in the GIS’s livable areas.
The temperature and density of equipment can be affected by other factors, such as convective motion within the enclosure or solar gain for outdoor installations.
Changes in the density of SF6 are usually treated as incidental changes, and safety margins have been built into the SF6 density criteria for them.
Gas density monitoring systems, depending on their configuration, can provide two useful outputs.
Output Type #1 – Continuous Output Signal:
These can be used for diagnostic purposes to track trends. Historical data can tell you, for example, if a leak is the result of a gradual or abrupt increase in severity. It can be used to monitor SF6 emissions and help meet environmental standards.
Since SF6 is a potent greenhouse gas, it is important to keep an eye on emissions.
Output Type #2 – Threshold Alarms:
An alert is sent when the gas concentration drops below a preset level. Two cutoffs are used in most cases. First, there is a Low Gas Light (used to trigger corrective action).
The second level involves a control signal. This can be used in switchgear to halt a process or, if necessary, completely shut down the machine. The equipment user’s policy may determine the second level. The second signal is usually associated with a Critical Density Level below which the apparatus cannot function.
These goals can be achieved using a wide range of technologies. Here are some examples:
Function #1: Simple Pressure Switches:
This technique is used only in devices with large safety margins. The threshold pressures are high enough to ensure adequate gas density despite temperature differences.
These switches are calibrated according to the values above and used in a variety of medium-voltage devices.
Function #2 – Temperature-Compensated Pressure Gauges:
A separate temperature signal can be used to change the response of a sensor. The gauges are calibrated to show true pressure or compensated pressure. However, signaling is done by relays and switches set at two different density threshold levels.
Function #3 – Gas Monitor with Reference Gas:
A mechanical bellows interfaces the reference gas with the measured gas in a sealed chamber. The temperature effect on the measured gas is the same for both the reference gas and the measured gas. Therefore, when pressure is increased instead of temperature, this effect is canceled.
The bellows will activate microswitches in response to pressure differentials caused by density.
Function #4 – Direct Measurement of Density:
The resonant frequencies of sensors based on tuning forks are affected by changes in gas density. These sensors produce a continuous signal that measures the density. The signal is also interfaced with relays for threshold alarms.
Sophisticated monitoring systems can use state equations to calculate density by measuring temperature and pressure independently. Thermal models can also provide more accurate gas density indications at different GIS apparatus components.
These systems can be used to monitor and quantify minor gas leaks more accurately. However, this technique is not commonly used in commercial systems.
This post was written by Justin Tidd, Director at Becker/SMC. For nearly a half a century, Becker Mining has been at the forefront of safety, producing the best electrical equipments in the industry. Becker/SMC is the industry’s leader in increasingly more sophisticated electrical control systems. Most of the major innovations, design features and specialized electrical components have been developed by Becker/SMC