On-line Monitoring of SF6 Density Relay by GIS

1. The necessity of installing monitoring device of the density of gas on GIS

As a main switchgear that has been greatly developed in recent decades, GIS has been widely used in power systems due to its superior performance, safety and reliability, convenient maintenance and long service life.

The SF6 gas pressure and density in GIS play a crucial role in the performance of GIS products.

Density refers to the mass per unit volume of a specific substance under specific conditions. The SF6 gas in GIS is sealed in a container of fixed volume, and it has a certain density value under the rated pressure at 20°C.

The SF6 density of gas monitoring device installed on the GIS, with a pointer and a scale, is called a gas density meter; without a pointer and a scale, it is called a gas density switch; some SF6 gas density meters also have electronic contacts at the same time, which is called a density switch. Density relay, if the density relay has a real-time digital signal output of density, pressure and temperature, it is called a remote density relay, which is a special meter used to measure the density of SF6 gas.

2. The principle of SF6 density of gas monitoring system in GIS

During the operation of GIS, due to the flow of current, the conductor continuously emits heat, so the temperature inside the air chamber will increase, and its air pressure will also increase.

When the heat dissipation of the conductor and the heat dissipation of the GIS shell reach a balance, the temperature of SF6 inside the gas chamber reaches a stable value. This stable value minus the ambient temperature is the temperature rise value of SF6.

Due to the compensation of the double-layer metal belt, the value displayed by the SF6 gas density meter at this time is the pressure value of the SF6 inside the gas chamber at the pressure value of 20°C + the temperature of the temperature rise value. The density meter is generally made into an absolute pressure meter to suit the The device works at different altitudes.

The structure of SF6 gas density relay is a density meter plus an electrical contact switch. The electrical contact switch can convert the mechanical action into an electrical signal. In addition to the display function of the density meter, it also has a relay function, that is, it outputs on-site gas supply alarm and lockout. node signal.

Generally, the density relay has three pairs of points, each of which has a static contact and a movable contact. The static contact is set according to the SF6 gas density value required by the arc extinguishing performance of the equipment.

When the SF6 gas leaks, the pressure will drop, and when it drops to the alarm setting value, the static contact and the moving contact are pulled together, and the SF6 relay outputs a pair of point alarm signals.

At this time, the user is required to supply air to the equipment. If the pressure continues to drop and falls to the blocking set value, the relay outputs another blocking signal for the connection point to block the control system of the equipment.

In addition, when the equipment generates overvoltage due to air supply or other unexpected situations, the relay will also output a contact overvoltage alarm signal to remind the user, so as to realize the safe operation protection of the electrical equipment.

The reliability of the contact conduction of the SF6 relay directly determines the reliability of the operation of the high-voltage switchgear.

Generally, the contact material of SF6 relay is silver-nickel alloy, and the thickness of the surface is gold-plated to 10m to ensure the reliability of the switchgear working in various harsh environments for a long time.

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