Analysis Of The Causes Of Heating Of 10kv Insulating Contact Box And Its Countermeasures

Abstract: As an important equipment of the power supply and distribution system, the circuit breaker is responsible for the conversion and transmission of electric energy by connecting the moving and static contacts between the switch cabinet. The static contact, connecting busbar, moving contact arm, plum blossom contact finger and circuit breaker conductive arm are the main heating elements in the high-voltage switch cabinet. If the contact temperature of the high-voltage switch cabinet is too high, it will cause the insulation of the Insulation Contact Box in the switch cabinet to be reduced, resulting in single-phase grounding or phase-to-phase short circuit, causing various safety accidents, causing power system paralysis or casualties.
Keywords: 10kV switch cabinet; contact heating; cause analysis; countermeasures
1 Main causes of contact heating
1.1 The static contact is in poor contact with the busbar connection surface or the fixing bolts are loose
The static contact of the central cabinet in the 10kV switch cabinet is a round copper body connected to the busbar by bolts. The capacity of the switch cabinet is different, and the size of the static contact copper body and bolts are different. If the contact area and contact pressure between the static contact and the busbar do not meet the specification requirements, and the overlap surface is less than 1.5 times the width of the busbar, the actual resistance of the busbar connection point will increase due to the lack of sufficient overlap pressure. If the static contact and busbar cut-off current is too small, it can cause abnormal temperature of the high-voltage switch contact. Silver or tin plating of the connection surface between the busbar and the static contact can improve the conductivity and reduce the heat effect of conductive conduction. The busbar connected to the static contact is generally made of copper to avoid heat generated by the transition of copper and aluminum. The torque wrench is used to tighten the bolts. The tightening torque should not be too loose or too tight. Too loose will cause poor contact, while too tight will cause the metal surface to creep, causing the metal around the bolt to warp up and reduce the contact area.
1.2 Insufficient tension of the moving contact firm spring or improper material selection
The 10kV handcart cabinet adopts a sliding contact structure, and the moving contact of the circuit breaker is composed of multiple trapezoidal contact fingers. The trapezoidal contact finger is installed on the contact finger bracket. The plum blossom contact bracket is composed of two circular rings and a connecting rod. Generally, the trapezoidal contact finger is fixed to the bracket with a spring. When the circuit breaker is in working state, the contact resistance between the contact finger and the static contact and the conductive arm is determined by the tension of the solid spring. When the tension of the solid spring is insufficient, the contact resistance of the moving and static contacts will increase. According to the thermal effect formula of the current Q = I2Rt, it can be known that the heat will increase, and the heat of the contact is transferred to the spring, making the elasticity of the spring spiral body larger. If the tension drops further, the heating of the contact will be aggravated. The inspection of the heat-damaged contacts found that there were obvious arc burn marks on the surface of the static contact. The trapezoidal contact finger has discharge during operation. If it is not discovered and handled in time, it will cause single-phase grounding or short circuit accidents. The size of the fixed spring tension is related to the material of the spring, the thickness of the spring, and the tightness of the spring spiral body. The circular fixing ring and spring of the trapezoidal contact finger must be made of non-magnetic materials. If it is a magnetic material, eddy current will be formed when the current passes through, causing the elastic coefficient of the spring to decrease, resulting in a decrease in the tightening force of the contact finger. The circuit breaker capacity is different, and the spring model is also different. When replacing the spring, the spring model should be checked to detect whether the tension of the spring meets the use requirements.
1.3 The connection of the circuit breaker conductive arm is too loose
The circuit breaker conductive arm and the circuit breaker body contact seat are connected with bolts. If the tightening force of the connecting bolt is too small, the contact resistance between the conductive arm and the contact seat will increase under the action of the electric force, and the heat may cause damage to the vacuum circuit breaker. Therefore, the fixing bolts need to be tightened regularly. The copper quality of the static contact does not meet the requirements, and the current carrying capacity of the busbar and static contact is too small. Qualified busbars and static contacts need to be replaced.
2.3 Add cooling fans to switchgear with high heat generation
High-voltage switchgear has good sealing performance and heat is not easy to dissipate. The effect of forced convection cooling can reach 3 to 8 times that of natural cooling. Therefore, cooling fans are generally added to the top of the circuit breaker room and busbar room of the main incoming line cabinet and busbar cabinet with large loads to evacuate the generated heat in time and reduce the thermal pile effect of the switchgear. The fan and temperature are interlocked and the fan is stopped when the temperature is low. The configuration of the fan is not less than two, and they are controlled separately so that when one fan fails, the other can still operate normally. For switchgear with heavy loads, the manufacturer is required to install fans on the top of the circuit breaker room and busbar room when ordering. Dust-proof facilities are set at the air inlet of the fan, and a stainless steel baffle is used on the contact surface between the fan and the switchgear to prevent dust and foreign matter from entering the switchgear. 2.4 RFID wireless passive power online temperature measurement technology is used for switches with large loads. Wireless temperature measurement technology uses wireless communication to transmit temperature signals, has excellent insulation performance, and can isolate the high voltage of the switchgear. The temperature sensor is directly installed on the heating point, which can accurately measure the operating temperature of the monitoring position and realize online monitoring of the operating temperature of the switch cabinet. Schematic diagram of temperature measurement principle The temperature data received by the collector is transmitted to the communication management machine via the RS-485 communication line. The background temperature measurement software collects and stores the uploaded data, and then the background software configuration realizes the functions of real-time temperature display, temperature limit alarm, report query, etc. A collector can receive four antenna input signals, and the effective distance of the radio frequency line does not exceed 5m. The 10kV switch cabinet currently used by a company is the KYN28C-12 mid-mounted switch cabinet. According to the load and daily inspection records of each switch in the #1 substation, 10 switches are selected to install temperature sensors on the circuit breaker contacts. Each circuit breaker moving contact is equipped with 6 temperature sensors to measure the temperature of the upper and lower contacts of the three phases of the circuit breaker. The collector transmits the signal to the communication management machine, and then introduces it into the monitoring system of the #1 substation. By monitoring the temperature of the switch contacts in real time online, it is convenient for operators to monitor. Set the corresponding temperature alarm value, and send out an alarm through the alarm system to prompt the inspection personnel to inspect the problem area. When the switch cabinet with a large load exceeds the alarm temperature value, start the cooling fan in time. Due to the corrosion of ammonia to copper in the synthetic ammonia substation, the busbar is made of aluminum busbar and the static contact is made of copper contact. After investigation, it was found that the circuit breakers of two switches are prone to heat, so a cable tie fixed passive wireless temperature sensor was installed on the static contact of the switch cabinet to monitor the temperature of the static contact.
3 Conclusion Heating is the ultimate manifestation of all equipment failures. Online temperature monitoring can timely detect equipment hidden dangers and defects. At the same time, RFID wireless passive power online temperature measurement technology has outstanding advantages in reliability, scalability, cost, installation and maintenance. It has been successfully applied to various types of high-voltage switch cabinets in substations, and the data can be transmitted to cloud platforms, computers and mobile phone apps.