Why SF6 Appliances Use SF6 Relays Instead of Ordinary Pressure Gauges?

1. Overview of SF6 gas density

(1) The so-called density refers to the mass per unit volume of a certain substance under certain conditions. The SF6 gas in SF6 appliances is sealed in a fixed container. Under the rated pressure at 20℃, it has a certain density value. Within the range of various allowable conditions for electrical operation, although the pressure of SF6 gas changes with temperature changes, the SF6 gas density value remains unchanged. Because the insulation and arc extinguishing performance of SF6 circuit breakers depend to a large extent on the SF6 gas density and purity, the monitoring of the detection of SF6 gas density and purity is particularly important. In order to achieve the purpose of constantly monitoring the SF6 gas density, the SF6 circuit breaker should be equipped with a density relay or a gas density meter. The SF6 gas density meter is for monitoring, and the SF6 relay is for control and protection.

(2) The symbol of density is represented by ρ, and the unit symbol is kg/m3 or g/L. According to the definition of density, the indicated value of the SF6 gas density meter should be the unit of density. However, the indicated values of the density meters currently produced and used are all MPa, which is a unit of pressure. The indication is not the real pressure, but the pressure converted to 20°C.

(3) According to the ideal gas state equation pv=nRT, it can be seen that the pressure p is a linear temperature relationship. When the gas temperature in the SF6 relay changes, the value of the pressure gauge changes with it, and it cannot be prepared to show the state of the equipment.

2. Structure principle of the SF6 relay

(1) SF6 relay (meter) is an indispensable and important accessory for SF6 circuit breakers and SF6 composite apparatus (Gas Insulated metal-enclosed Switchgear, GIS). The basic function of the SF6 relay is to monitor the sealing condition of the SF6 electrical appliances in operation and whether there is air leakage.

(2) SF6 relay (meter) is mainly composed of spring metal curved tube, gear mechanism and pointer, double-layer metal belt and other parts. In fact, it is based on the structure principle of spring tube pressure gauge. The variable and stretchable double-layer metal belt is used as a temperature compensation device to make the values of spring tube and the double-layer metal belt expanding or contracting with the temperature change equal and reversely superimposed, so that the density (pressure) reading does not change with the change of the ambient temperature.

3. Structure temperature compensation principle of the SF6 relay

(1) SF6 relay is actually a pressure relay with the function of temperature compensation, and its density value is expressed by the corresponding pressure value of 20℃. When the temperature changes, the elastic Bourdon tube and the double-layer metal belt change at the same time, so that the pointer display remains unchanged.

(2) In the case of constant volume, any pressure change caused only by temperature change can be compensated by the corresponding change of the elastic Bourdon tube and double-layer metal at the same time, so it will not cause the deflection of the pointer and ensure the indicated value of pressure is correct.

(3) The response of the SF6 relay pressure element to any pressure changes that are not caused by temperature changes will be converted into the SF6 relay pointer action and reflected on the dial, and the pressure value exceeding the minimum or maximum pressure allowed by the system can be monitored.

Related Article for Reference