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According to the principle, SF6 gas density relays are divided into four categories: mechanical non-indicating density relays, mechanical indicating density relays, digital indicating density relays, and transmitting output density relays.
In the early days, the typical use of mechanical non-indicating density relays was flat-height LW6 type (without pressure gauge), and the use of mechanically indicating density relays was Xigao LW14 and 15 type circuit breakers. Its SF6 pressure monitoring system was composed of an SF6 relay.
Among them, the transmission output density relays are mostly imported products, which have the characteristics of high precision and superior performance. There are two ways to detect whether SF6 gas leaks, one is to use a high-precision pressure gauge, and the other is to use a gas density relay.
The commonly used density relay is basically a density relay of pressure gauge type, which integrates the pressure contact and the gauge, which is convenient for observation and detection.
The SF6 gas density meter can be divided into oil-filled (shockproof) SF6 gas density meter, which is suitable for energy storage and circuit breaker equipment with large operating vibration. The common supporting mechanisms are hydraulic and spring mechanisms. Another oil-free type is used on occasions with low vibration such as busbars and transformers.
In the actual emergency repair work, SF6 relay alarms are often encountered. According to statistics, SF6 pressure alarm failures account for about 60%-70% of SF6 equipment failures.
The treatment of this kind of fault is generally relatively simple. First, check whether the SF6 gas density meter indicates whether it is in the alarm area, and then deal with it according to the site conditions and working standards. There are four situations in which the SF6 relay sends a low-pressure alarm during operation.
1) Low-pressure alarm of SF6 equipment caused by low pressure
The seals of all SF6 equipment have leakage, which is only a problem in large quantities and small quantities. The annual leakage rate of the equipment is controlled at ≤ 0.5%. In addition to the annual SF6 micro-water test, the cumulative SF6 gas consumption is relatively large.
For the troubleshooting of this reason, as long as the qualified SF6 gas is charged to the rated pressure, the alarm fault signal of the SF6 relay can be eliminated, but the monitoring needs to be strengthened.
Due to the long-term operation of the SF6 equipment body and pipeline, the temperature difference of the external environment changes greatly, the sealing parts of the equipment will definitely age and cause poor sealing, which will increase the leakage of SF6 gas in the equipment, resulting in the frequent occurrence of SF6 alarms in the equipment.
In the work, it has been found that the metal bellows of the SF6 relay have a leak point in the welding place. Due to the large temperature change, the bellows often move back and forth, and the welding process is not in place, resulting in air leakage at the welding place. Therefore, it is necessary to check the power failure of the equipment that frequently alarms, and deal with the SF6 leakage point in time.
In winter, the outside temperature of the site is low and some SF6 pressure gauges do not have temperature compensation, which leads to a decrease in pressure, and the SF6 relay issues a low-pressure alarm. For this kind of failure, as long as the judgment is made according to the temperature curve and the on-site temperature, the air is properly supplemented.
2) Low-pressure alarm of SF6 equipment caused by high pressure
For low-pressure alarms, it is often considered to be caused by low pressure, but it is rarely caused by high pressure. For this, the general method is to depressurize the circuit breaker, and the problem can be solved, but why does the pressure increase during operation?
For this phenomenon, we should understand the equipment operation. First of all, it is necessary to promptly rule out whether the SF6 relay equipment is overloaded, and whether the main contact resistance exceeds the standard.
If the equipment is overloaded or the main contact resistance exceeds the standard, the contact point of the equipment will be heated, the temperature of the SF6 gas will rise, and the SF6 pressure will rise, which will cause serious consequences when running for a long time. Therefore, we must pay attention to the phenomenon of pressure rise during the operation of SF6 equipment and deal with it in time.
At the same time, on the basis of excluding the above-mentioned pressure gauge rise, the SF6 gas density meter should be calibrated. If there is a problem with the density meter, it should be replaced in time.
If the SF6 gas density meter is calibrated correctly, it should be checked whether the outside temperature is too high, and the SF6 density should be adjusted to the normal working pressure in time.
