Principle and Operation of Insulation Resistance Testing Before Closing:
Use an insulation resistance tester (megohm meter) to measure the insulation resistance of the circuit breaker. Connect the “L” terminal of the insulation resistance tester to the conductive part of the breaker and the “E” terminal to the breaker’s enclosure or grounding part. After applying a DC voltage, read the insulation resistance value. For circuit breakers rated at 10kV and above, a 2500V insulation resistance tester is typically used.
Purpose and Significance:
The insulation resistance test can determine whether the insulation performance of the circuit breaker is good. A low insulation resistance may indicate issues such as moisture in the insulation, aging of the insulation, or damage to insulating components. This testing helps to avoid insulation breakdown failures after closing the circuit breaker and ensures that the breaker can operate in a normal insulation state.
Principle and Operation of Circuit Resistance Testing:
The direct current voltage drop method is adopted. A direct current (typically 100A or more) is applied across the terminals of the circuit breaker, and the voltage drop is measured across its terminals. The circuit resistance is then calculated based on Ohm’s law. Specialized circuit resistance testing equipment is used, and the test leads are clipped onto the circuit breaker input and output terminal.
Purpose and Significance:
Circuit resistance mainly reflects the conductive performance of the circuit breaker. Excessive circuit resistance can lead to severe heating during operation, affecting its lifespan and performance. Conducting a circuit resistance test before closing the circuit breaker checks whether the moving and stationary contacts are making good contact, ensuring normal conduction after closing and reducing energy loss.
Principle and Operation of Mechanical Characteristic Testing:
A specialized mechanical characteristic testing device is used to measure parameters such as the open and close times, synchronism, and speed of the circuit breaker. The tester collects mechanical signals during the operation of the breaker, such as displacement and speed, via sensors, and converts these signals into electrical signals for analysis.
Purpose and Significance:
Mechanical characteristic parameters are important indicators of circuit breaker performance. For instance, if the open and close times do not meet requirements, it may lead to the circuit breaker failing to cut off fault currents promptly. Poor synchronism can cause imbalance currents during three-phase operations, affecting the stable operation of the power system. Conducting mechanical characteristic tests before closing the breaker ensures that the operational performance after closing meets the required standards.
Principle and Operation of Dielectric Strength Testing After Closing:
For circuit breakers after closure, either an AC dielectric strength test or a DC dielectric strength test can be conducted. The AC dielectric strength test involves applying an AC voltage higher than the rated voltage (generally 1.3 to 1.5 times the rated voltage) between the phases of the circuit breaker and between phases and ground for a specified duration (usually 1 minute). The principle of the DC dielectric strength test is similar, but a DC voltage is applied. During the testing process, close observation is required to check for phenomena such as breakdowns or flashovers in the circuit breaker.
Purpose and Significance:
The dielectric strength test is the most direct method for checking the insulation strength of the circuit breaker. Conducting the dielectric strength test after closing can further verify the insulation performance of the circuit breaker in the closed state, ensuring it can withstand various overvoltages during system operation and guarantee the safe operation of the power system.
Current Carrying Capacity Testing (In Some Cases) - Principle and Operation:
In some special circumstances, such as when there are doubts about the circuit breaker’s current carrying capacity or after significant repairs, a current carrying capacity test is necessary. This can be done by loading the circuit breaker with the rated current or slightly above the rated current for a certain duration while monitoring the temperature changes of the circuit breaker and the condition of the contacts.
Purpose and Significance:
Verifying the circuit breaker’s current carrying capacity after closing is crucial to ensure it can normally carry the operating current of the system, preventing damage due to overload during operation and ensuring the reliable supply of electricity in the power system.
Operating Mechanism Action Test - Principle and Operation:
After the circuit breaker is closed, the operation mechanism is subjected to open and close operation tests to check whether the mechanism operates flexibly and reliably. It is essential to observe whether the open and close coils of the operating mechanism function correctly and whether there is any jamming in the drive components, while also checking that the auxiliary switch toggles correctly.
Purpose and Significance:
The operating mechanism is a critical component for the normal operation of the circuit breaker. The operation mechanism action test ensures that the circuit breaker can accurately perform open and close operations in response to control signals from the system after it is closed, promptly responding to system faults or normal operational commands and maintaining the orderly operation of the power system.
