Necessity of On-Site Verification of SF6 Density Relay

Abstract: SF6 gas has excellent insulation properties, arc extinguishing properties and stable chemical properties, and has been widely used in power systems, especially in various high-voltage switchgears. Since the SF6 gas density relay operated on site is located on the outdoor high-voltage switchgear, the influence of the climate change all year round will cause the rubber seal of the SF6 gas density relay to be damaged to varying degrees, and the density relay will cause contact due to infrequent action. Point inflexibility or poor contact and other phenomena, from the actual operation situation, it is very necessary to regularly check the SF6 density relay and pressure gauge in operation.


1 Calibration principle of SF6 gas density relay

Density relay is a special instrument for controlling pressure in the industrial process measurement and control system. Its function principle is to control or alarm when the input pressure reaches the set value. The on-off function controls the upper and lower limits of the controlled pressure. The gas pressure in the airtight container changes with the temperature. For the convenience of statistics and comparison, the relative pressure value of SF6 at 20°C is usually taken as the standard value. During on-site calibration, the SF6 pressure value measured at a certain ambient temperature must be converted to its corresponding equivalent pressure value at 20°C, so as to judge the working performance of the density relay.


2 On-site verification method of density relay

2.1 The main items of detection of SF6 gas density relay
(1) Alarm (air supply) starting pressure value.
(2) Alarm (gas supply) returns the pressure value.
(3) Locking start pressure value.
(4) Locking returns the pressure value.
(5) When the SF6 gas density relay has a pressure gauge, the display value of the pressure gauge should be checked.

2.2 Calibration method of special instrument
At present, the density relay calibration mainly adopts the special instrument calibration method.
A special calibrator is used to calibrate the density relay. The calibration principle is to collect the temperature and contact action value of the density relay, and automatically convert it into an equivalent pressure value of 20°C through a single-chip microcomputer, so as to realize the calibration of the density relay. During the calibration, close the valve 1 to realize the isolation from the air chamber of the body, open the valve 2 to connect with the special instrument interface, and the instrument can automatically complete the calibration of the relay.


3 Problems existing in the inspection and verification of SF6 density relay

(1) Disassembly and assembly of the density relay may destroy the original gas seal, and it is not easy to ensure the original sealing performance.
(2) The newly installed density relay has undergone long-distance transportation and bumps, which may affect its accuracy and stability.
(3) The on-site installation and maintenance time is generally tight, and it takes a long time to remove it from the equipment and send it to the laboratory for inspection.
According to the actual experience of our work, the inspection of SF6 gas density relay and gas pressure gauge is usually carried out on site, and the effect is good.


4 Precautions for on-site calibration work

(1) Safety measures must be taken, and corresponding inspection procedures and operating procedures must be formulated.
(2) The inspection work should generally be carried out after the equipment is powered off. Before the inspection, the DC power supply connected to the density relay must be cut off, and the corresponding connection wires of the alarm and locking contacts should be disconnected from the terminal block to prevent them from being connected with the secondary circuit and sampling. The signal line constitutes a loop that affects the inspection and ensures that it does not affect the normal operation of other equipment.
(3) Pay attention to protect the sealing surface of the pipe joint, the sealing gasket should be replaced after calibration, and use a leak detector for air leakage detection.
(4) Pay attention to the cleaning of the pipe joints before and after the calibration to prevent impurities and unqualified gases from entering the body. Flush with a small amount of qualified SF6 gas if necessary.
(5) Pay attention to the isolation valve between the equipment body and the gas circuit of the density relay. After calibration, it must be restored and confirmed by re-inspection.
(6) The JD11 SF6 gas density relay calibration platform can only be used after passing the inspection. The displayed pressure value is a relative pressure value. When the tested density relay is expressed by an absolute pressure value, it should be uniformly converted to a relative pressure value. For example, the density relay of HPL420B2 circuit breaker of ABB Company, its alarm and blocking pressure values are represented by absolute pressure values.
(7) Carry out SF6 gas recovery. Because during the operation of electrical equipment, SF6 gas will decompose a variety of toxic and corrosive gases and solid decomposition products, which not only affect the electrical performance of the equipment, but also endanger the safety of operating and maintenance personnel.
(8) In the process of checking the density relay of SF6 circuit breaker, some density relays were found to be unqualified. The reason for the unqualified density relay may be that the SF6 standard gas package has a long thin copper tube, During the installation or maintenance process, the pressure in the standard SF6 gas bag increases due to the bending or collision of the copper pipe.


5 Conclusion

SF6 circuit breaker is an important protection and control element in power system, and density relay is an important part to ensure the safe operation of SF6 electrical equipment. Its performance directly affects the safe operation of electrical equipment. Therefore, it is very necessary to test its reliability and accuracy regularly. The cycle, items and requirements of density relay inspection shall be carried out in accordance with relevant regulations and standards. Years of practical experience show that the inspection should combine the actual situation of the equipment, abide by the relevant safety regulations, formulate practical methods and steps, and pay attention to relevant matters, so as to ensure the safety of the equipment and the correctness of the inspection work.
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