Ⅰ. What is the necessity of installing SF6 relay?
With the development of electric power industry and the progress of science and technology, the online operation equipment of electric power system is also being updated, and the sulfur hexafluoride gas equipment has gradually replaced the oil-filled equipment in the past.
Sulfur hexafluoride (SF6) electrical equipment with excellent insulation and arc extinguishing properties of SF6 gas as the medium, not only improve the reliability, but also simplify the structure, especially the fully enclosed combination appliances, its volume is significantly reduced, greatly saving the substation footprint.
SF6 gas plays a very critical role in the safe operation of power system with its excellent insulation performance, arc extinguishing performance and stable chemical properties.
The density of SF6 gas plays a crucial role in the insulation of the equipment and the realization of the arc quenching function. The increase of sf6 gas density reduces the average free travel of electrons, thus reducing the freeing and improving the breakdown field strength of the gas.
The decrease in gas pressure of SF6 electrical equipment due to leakage will reduce the performance of the equipment causing it to lose its proper function.
Since pressure and ambient temperature have a certain relationship and are a changing quantity, the change in pressure does not accurately reflect the leakage of the gas.
While the density of sf6 gas in the equipment is a quantity that can reflect the gas leakage, so SF6 electrical equipment must be installed with gas density relays.
According to the results of statistical analysis of the density meter in winter and summer over the past few years, the higher the load current, the greater the error, the maximum error can reach 20%.
When a situation like this is not properly understood, often suspect the density meter poor quality, inaccurate instructions, etc., and treated as a defect, requiring replacement of the density meter. This article mainly discusses the ambient temperature on the gas density relay and the operating considerations.
Ⅱ. The working principle of SF6 relay
First, we understand the working principle of gas density relay, gas density relay is a kind of pressure controller with temperature compensation.
Its main role is to detect the change of sf6 gas density in electrical equipment, and issue alarm and blocking signal, so as to achieve the protection of the safe operation of SF6 electrical equipment.
When the internal gas of SF6 equipment has a leak causing the pressure to drop, the metal expander shrinks downward, driving the contact of the micro switch to close and send a signal or make the circuit breaker for low pressure blocking.
When there is no gas leakage inside the equipment but the gas pressure changes due to the gas temperature deviating from the standard temperature of 20℃, the bimetal temperature compensation device will compensate the expansion of the metal expander and keep the position of the micro switch unchanged.
Therefore, the SF6 relay only sends a signal when a gas leak occurs and the density decreases, truly reflecting the gas leak.
