Why Should the SF6 Gas Density Relay Be Checked Regularly?

The influence of temperature difference on the reading deviation of running SF6 density relay:

The temperature value T of the switch body (arc-extinguishing chamber) and the temperature value T relay of the installed density relay (possibly the mechanism box) are not the same, that is, T body ≠ T relay, which results in temperature difference. The temperature in the substation is not the same everywhere, some places may be hotter (facing the sun), some places may be colder (facing the shade), and the temperature of each switch body (arc-extinguishing chamber) is not completely equal.

In fact, there is a certain temperature difference between the temperature value T of the switch body (arc-extinguishing chamber) and the temperature value T relay of the installed density relay (mechanism box). When the temperature difference between the two is 6-8℃, the abovementioned deviation will occur. When the temperature difference is serious, it will also cause misoperation.

The reasons for periodic verification of SF6 density relay:

Like other pressure gauges, SF6 density relay used to measure SF6 gas pressure will produce certain drift in its alarm and lockout values after being used for a period of time. In addition, the temperature compensation material will cause large errors in the alarm and lockout values due to material aging and deformation after a certain period of use. Moreover, because the density relay contacts do not act frequently, the contacts may become insensitive or invalid. This hidden danger can directly cause a major accident when the SF6 switch cannot alarm or lock due to gas leakage, causing the pressure inside the body to drop to the alarm or lockout value. Therefore, regular verification should be carried out. The quality problems of this kind of gas density meter are mainly due to:

• The poor seismic performance itself, after the strong impact of switch separation and closing, faults such as pointer jamming, contact never working (not working or always working), and deviation exceeding the standard may occur.

• Due to oil leakage, its seismic performance has declined. After the strong impact of switch separation and closing, faults such as pointer jamming, contact never working (not working or always working), and deviation exceeding the standard may occur.

• Because the commonly used relays have magnetic assisted electrical contacts, the closing force of the touch head itself is small, and the time is slightly longer. When the touch head is oxidized, the contact will be interrupted or unreliable. For oil-free types, the magnetic-assisted electrical contact touch heads are exposed to the air, so they are very susceptible to oxidation or dust accumulation, and their contacts are prone to bad contact or interruption. For oil-filled types, although the magnetic-assisted electrical contact touch heads are immersed in silicone oil, after a long time and several actions, their contact performance will decline, and the oil film has insulating effects, and their touch points may also be bad.

• Due to its own manufacturing quality problems, there are inaccuracies in temperature compensation, and the error is seriously biased.

• Due to insufficient aging or material problems, after a long time, drift will occur, and the accuracy will be seriously biased, causing the density relay to be unusable.

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