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White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment

White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment
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White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment - 1

/ WHITE PAPER Online Dew Point Monitoring of SF6-Gas-Insulated Equipment In rcent years, online monitoring of SF6 dew point has become increasingly common. However, the factors that affect the reliability of online measurement in an environment where there is no gas flow, are not so well known. This paper explains water vapor and moisture transient behaviour and examines their fundamental impact on the installation process, installation design and the selection of connector and sealing materials. of water molecules may increase the moisture level, especially as equipment ages. Traditionally, moisture level has been checked using periodically taken gas samples, but in recent years condition-monitoring systems that incorporate online instrumentation for measuring the dew point of SF6 have become increasingly common. However, it has become apparent that this type of application presents challenges that are quite different to those experienced in more typical industrial dew point measurement or the measurement of basic parameters such as pressure and temperature in SF6-insulated equipment. In particular, the method of installation, the materials used in the measurement system, and the connector types are critical in determining whether the measurements really do provide the intended valuable data for asset management. Furthermore, remote equipment location often sets demanding requirements for stability and the length of the maintenance period of instrumentation used for condition monitoring of high-voltage assets. Water Vapor Pressure and Dew Point Water vapor exists everywhere, and it is always part of total gas pressure -for example, atmospheric (barometric) pressure or system pressure in gas-insulated equipment (GIE). Dew point/frost point (Td/f) is defined as the temperature at which the partial water vapor pressure (pw) of a gas is equal to the vapor saturation In order to maintain the insulation properties of SF6 and to reduce the formation of corrosive by-products from SF6 decomposition, the amount of water vapor in gas-insulated high- voltage equipment must be kept to a minimum. Although initially filled with dry gas and being closed equipment at elevated pressure with no external gas flow, the high penetration ability Figure 1. Example: water vapor pressure 0.3 hPa (mbar) vs. dew point. +20°C SF6 @ 4 BAR AMBIENT AIR Dewpoint -40 °C +9.3 °C Relative humidity 0.6 %RH 50 %RH Vapor pressure (pw) 0.13 mbar 11.7 mbar +35 °C Dewpoint -40 °C +31 °C Relative humidity 0.2 %RH 80 %RH Vapor pressure (pw) 0.13 mbar 45 mbar Table 1. Examples of dew point, relative humidity, and vapor pressure in a gas tank and in ambient air at two different temperatures (+20 and +35°C) and in ambient humidity conditions (50 and 80 %RH). Water molecules (H2O) tend to move from high vapor pressure to low vapor pressure in order to reach equilibrium.

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White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment - 2

pressure (pws). In other words, dew point is the temperature to which a gas must be cooled in order for the water vapor to condense into dew or frost p = p (Tdf) Dew point is not a temperature-dependent parameter, thus it can be measured by taking a gas sample at a temperature different than system temperature. Dew point is, however, highly dependent on pressure, so it is crucial to confirm that it is being measured at the same pressure as the main gas volume, or to know the exact pressure values, in order to be able to carry out the correct conversion for the values - for example, dew...

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White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment - 3

Installation of an Online Dew point Sensor When designing an online measurement setup for a dew point sensor in SF6-insulated equipment, the basic principles of water vapor behavior described earlier should be taken into account in order to ensure correct measurement and thus to enable valid conclusions to be drawn. Traditionally the dew point of SF6 has been measured by taking a gas sample from the tank, meaning that there has always been gas flow to account for during these measurements. Gas flow conceals the effect of very slow diffusion and vapor transients between gas and solid...

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White Paper: Online Dew Point Monitoring of SF6-Gas-Insulated Equipment - 4

the sensor more or less at the same time - assuming that the sensor is sufficiently close to the main tank. Whether the dew point values are exactly the same at the same point in time (100% response) is not really relevant because it is the increasing trend that highlights the fact that corrective action needs to be taken. Any quick, dramatic change in dew point indicates a leak that should be detected by both dew point and pressure measurement. Summary In order to ensure that an online measurement system provides reliable and valuable dew point data with minimized uncertainty, it is...

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