Abstract: This article introduces the temperature compensation method and working principle of the circuit breaker SF6 gas density relay, analyzes that the main error of the SF6 density relay in operation is affected by the ambient temperature, and puts forward the method and improvement measures to reduce the impact of the ambient temperature.
Key words: SF6; density relay; temperature compensation
The insulation and arc extinguishing performance of the circuit breaker depends largely on the purity and density of the SF6 gas. To ensure the safe operation of the circuit breaker, the SF6 gas of the circuit breaker must be monitored. The current monitoring method is realized through SF6 barometer and SF6 density relay. However, the reading of the ordinary pressure gauge is only used to monitor the pressure of SF6 gas, and the indication of the gauge pressure cannot distinguish whether the pressure drop of SF6 gas is caused by leakage or the change of ambient temperature. The monitoring of SF6 gas density is realized by the SF6 density relay. Due to the different temperature compensation methods, different structure types, and different installation locations, the SF6 density relays are affected by the ambient temperature during operation.
1 SF6 density relay
The SF6 density relay used in the circuit breaker is a pressure controller with temperature compensation. Its main function is to detect the change of the SF6 gas density in the electrical equipment, and send out the signal of alarm and lock, so as to realize the safe operation of the SF6 circuit breaker. Protect. According to different temperature compensation methods, SF6 density relays can be divided into bimetal compensation indicating type and standard gas compensation non-indicating type.
1.1 Bimetal Compensated SF6 Density Table
The SF6 gas density meter with bimetal compensation shown in Figure 1 combines the density relay and the pressure gauge into one, which can be used for pressure monitoring at any time. components. The hollow elastic metal curved tube is connected with the SF6 equipment, and its internal space communicates with the SF6 gas in the equipment. The end of the elastic metal curved tube is connected with the bimetal belt for temperature compensation, and the double-layer metal belt is connected with the gear mechanism and the pointer mechanism. connect. The double-layer metal strip is compensated according to the difference between the ambient temperature and 20°C, and the density of SF6 is monitored faithfully through the temperature compensation function of the double-layer metal strip. The density meter has 2 pairs of electrical contacts, which are used for signaling and blocking the circuit breaker control circuit when the density of SF6 gas decreases. It is mainly used in GIS, ABB circuit breaker, SF6 CT and other equipment.
1.2 Standard gas compensated SF6 density relay
The standard gas compensated SF6 density relay is shown in Figure 2. The standard gas is used as temperature compensation to monitor the change of SF6 gas pressure converted to 20°C in the equipment, and the drop relative to the rated gas pressure at 20°C is used as the alarm value and blocking value. For example, C1-L1 is the electrical contact for alarm when SF6 gas is lowered, and C2-L2 is the electrical contact for locking the circuit breaker when SF6 gas is lowered. The use of this density relay is mainly early SF 6 electrical equipment, such as LW15, LW14 series circuit breakers.
1.3 Working principle of SF6 gas density relay
The SF6 gas density relay essentially monitors the change of the gas pressure converted to 20°C in the equipment, and uses the drop relative to the rated gas pressure of 20°C as the alarm value and blocking value. The setting value of the alarm is usually when the rated working air pressure drops by 5% to 10%, and the gas supplement alarm signal is sent.
The setting value of blocking is to block the opening and closing control circuit of the circuit breaker when the pressure drops to 8% to 15% of the rated working air pressure. Because the SF6 density relay has a temperature change compensation function, the pressure change should conform to the Pr curve shown in Figure 3 under normal circumstances. When the SF6 gas pressure drops to the Pa curve, the gas density switch sends out an alarm signal, and the SF6 gas pressure drops to the Pc curve. Open circuit The circuit breaker should be self-locked, and a locking signal should be issued, and the corresponding circuit breaker and other equipment are not allowed to operate, so as to ensure the safety of the equipment and system.
2 The influence of temperature on the density relay
The standard gas or bimetal is installed in the SF6 density relay for temperature compensation. The accuracy of the indicated value of the relay is closely related to the temperature compensation, and is affected by the temperature as follows.
2.1 The influence of filling with SF6 gas
In the process of filling SF6 gas to the equipment equipped with bimetallic compensation SF6 density meter, due to the sudden expansion and depressurization of SF6 gas, the temperature of the spare parts of the related gas filling pipeline, including the bimetal used for SF6 density meter compensation, generally drops from the ambient temperature to 0°C Below, the double-layer metal used for compensation is always compensated at 20°C, and the SF6 in the equipment is basically maintained at the ambient temperature. At this time, the indicated value of the density meter cannot truly reflect the density of SF6 gas at the actual temperature.
2.2 Influence of current heating effect during equipment operation
When the circuit breaker is in normal operation, the joint will have a certain temperature rise through the load current, making the temperature of the SF 6 gas in the equipment higher than the ambient temperature. The bimetal or standard gas used for compensation is far away from the primary conductive circuit, so it cannot be corrected. Compensating for temperature differences will cause errors in the action value of the density relay. The size of the density relay error depends on the temperature rise caused by the load current of the circuit breaker and the loop resistance, which is negligible during operation.
2.3 Influence of the mechanism box heating and tide driving device
In order to prevent the mechanism box from being damp, a heating and damping device is installed in the mechanism box. Normally, the temperature of the temperature controller in the mechanism box is set to be 3-5°C higher than the ambient temperature, and the temperature control device should be activated when the temperature is lower than the set temperature. The temperature of the mechanism box is higher than the ambient temperature, and the SF6 density relay installed in the mechanism box, the accuracy of the indicator's indication will be affected.
(1) The standard gas used for compensation of the SF6 density relay of LW14 and LW15 series circuit breakers is installed in the mechanism box. Since the temperature of the mechanism box is higher than the ambient temperature of the circuit breaker body, the temperature compensation of the standard gas used for compensation is still based on the temperature compensation of the mechanism box. The temperature inside is carried out, resulting in an error in the SF6 density value. special
Especially when the temperature is lower than 20°C in winter, the probability of SF6 air pressure alarm for this type of circuit breaker increases greatly.
(2) The domestic outdoor circuit breakers that have been put into production in recent years also use SF6 density meters, but the density meters are still installed in the circuit breaker mechanism box. Due to the effect of the heater in the box, the ambient temperature of the bimetal used for meter compensation The external ambient temperature is too high, and the influence on the indicator value of the meter density cannot be ignored.
3 Reduce the influence of temperature on SF6 density relay
According to the "Measures for Preventing Accidents of AC High Voltage Switches" in "Measures for Preventing SF6 Switchgear Accidents", the SF6 pressure gauge and density relay should be checked regularly, and the temperature sensor used for the density relay should be in the same temperature environment as the circuit breaker body. Therefore, in the equipment operation and maintenance test, the SF6 gas in the equipment should be kept at the same ambient temperature as the bimetal and standard gas used for temperature compensation to reduce the error of the SF6 density relay.
(1) When filling the equipment with SF6 gas, it should be carried out slowly, so that the liquid gas can be fully vaporized before entering the electrical equipment, so as to prevent the inflation pipeline from cooling down too quickly and causing condensation. After the equipment is filled with SF6 gas, wait for a period of time to increase the internal temperature of the SF6 electrical equipment to reach equilibrium with the external temperature before reading the SF6 pressure.
(2) The following modifications can be made to the influence of the heating and tide-dispelling device of the mechanism box: move the standard gas or density meter used for compensation to a place with the same temperature as the SF6 operating environment in the equipment to reduce the influence of the heating and tide-dispelling device. It is recommended to move To the outside of the mechanism box, such as A B B circuit breaker, install the density meter on the circuit breaker frame; design the standard gas for compensation in the equipment, so that the ambient temperature of the two is the same, such as SIEMENS 3AP1FG circuit breaker. way, and use the compensation
The standard gas is exposed outside the mechanism box, as shown in Figure 4.
(3) When calibrating the density relay, try to measure the density or pressure of SF6 gas accurately after the SF6 equipment is out of operation and the temperature inside and outside the equipment reaches equilibrium.