In the process of operation, the circuit breaker is not only subjected to mechanical force, electricity, heat, etc., but also subjected to the influence of the natural environment. It is required to work stably for a long time and have sufficient electrical and mechanical life. Under certain conditions, the temperature rise of each part of the circuit breaker is required to not exceed the allowable temperature value that damages the electrical performance, to ensure that the electrical insulation is not damaged, and can withstand the maximum operating voltage of the system, internal overvoltage and external lightning overvoltage. Sufficient ability to close short-circuit faults, and can reliably break rated currents and various fault currents. In a nutshell. The circuit breaker mainly plays two roles in the power grid:
Control function: According to the needs of grid operation, use circuit breakers to put part of power equipment or lines into or out of operation.
Protection function: When the power line or electrical equipment fails, the circuit breaker can quickly cut off the faulty part from the power grid to ensure the normal operation of the fault-free part of the power grid.
In short, the circuit breaker can realize the closing and breaking of various load circuits (including transformers, transmission lines, etc.) under normal working conditions, and it can also open and break the short-circuit faults in the line within the specified time. Achieve automatic coincidence.
The reliability of electrical products has always been one of the important indicators of engineering quality. The reliability of the power grid is largely determined by the reliability of various switching devices in the power grid. Once the switching equipment fails, the entire system will not work normally, or even paralyzed, and the economic losses caused are often far higher than the value of the switching equipment itself. Statistics show that in the power supply and distribution system, unplanned power outages caused by circuit breakers are the most in terms of time and quantity. The economic loss caused by modern power grid accidents is no longer purely based on the harm to the system. Switchgear, especially circuit breakers, are one of the core components of the power grid, and their various electrical parameters will affect their performance and operational reliability.
On the one hand, loop resistance, as one of the most important characteristic indicators in the electrical parameters of switchgear, is the key information for early glimpses of power supply abnormalities, and it is also the most reliable indicator of power system performance and decay. In assessing the reliability and predicting the failure of most types of power connections, including circuit breakers, disconnect switches, load switches, etc., loop resistance stability testing is the most effective, and the size of the loop resistance directly affects the on-load rating of the circuit breaker Temperature rise of working current and dynamic and thermal stability under short-circuit conditions. The change of the contact resistance value will also directly affect the reliability of the circuit breaker's opening and closing and the safety of the circuit breaker's operation. Specifically, if the circuit resistance of the circuit breaker increases for some reason, the original thermal balance will be broken, and the temperature rise of the contact part will increase, which will increase the electrical resistivity of the conductor and further increase the heat generation. Under the condition of high current, it will cause the oxidation of the contact surface and the softening or aging of the supporting insulator, which will cause the failure of the circuit breaker and make it unable to work normally in the short-circuit state.
On the other hand, although the circuit resistance of the switchgear is relatively small, it can be seen from the table that in the case of normal high current operation, the abnormal contact resistance (generally an increase in the resistance value) will also bring about a large amount of energy loss. In addition, the circuit resistance test is also a mandatory item in the circuit breaker factory test, type test, handover test, and preventive test. At the same time, the resistance value of the circuit breaker's conductive circuit is an important part of the circuit breaker's installation, maintenance, and quality acceptance. data.