Breakdown Voltage vs. Vacuum When using a new, unaged contact, the high-frequency current pulse occurs for a short period of time and disappears quickly after aging. When a switch with a slight drop in vacuum is tested at an AC voltage, the high-frequency current pulse appears only for a few seconds. When the vacuum of the switch drops below a certain critical value, plus the voltage for about 5 minutes, the pulse amplitude and repetition frequency will decrease, but it will continue to exist. Therefore, applying a threshold voltage just below the breakdown voltage to a certain switching tube and finding the relationship between the high-frequency current and the vacuum degree after 5 minutes of applying the voltage (for example) has an estimated vacuum degree. Methods.
When the relationship between high-frequency current amplitude and vacuum degree is actually measured, it is necessary to first find the breakdown voltage of a specific switch. That is, the test voltage is applied from zero and gradually increases until there is a significant breakdown discharge between the switch tube contacts. At this time, the voltage value is the breakdown voltage of the switch tube. Then lower a few test voltages so that the switch tube reaches a steady state with no breakdown between the contacts. After 5 minutes, record the high-frequency current amplitude with an instantaneous recording instrument (such as an oscilloscope) and check the vacuum level.
It should be noted that, when the voltage rises to just below the breakdown voltage, the relationship between the amplitude of the high-frequency current and the degree of vacuum is a single-value correspondence relationship. This is in contrast to the high-frequency pre-breakdown current amplitude measured when the rated power frequency withstand voltage (eg, 42 kV for 1 minute) is maintained during the sophisticated operation of the switch tube. The latter is not a single value corresponding to the degree of vacuum. There is a maximum peak around 10-3 Pa.
At present, the vacuum switch tubes of the photoelectric conversion method are almost all of intermediate suspension potential shields. That is, in the normally closed state of the switch, the dynamic and static ends are high voltage, and due to the geometric symmetry, the middle shield potential is one-half of the high voltage of the dynamic and static ends. The method is to place a ground plate vertically on the side of the switch tube. When the vacuum level is normal, good insulation can be maintained between the dynamic and static ends and the intermediate shield. Electrostatic capacitances are respectively provided between the dynamic and static ends having a high potential and the intermediate shield and between the intermediate shield and the ground plane. However, once the degree of vacuum deteriorates, discharge occurs between the dynamic and static ends and the shield case. As a result, the potential of the intermediate shield rises, causing a change in the electric field between the intermediate shield and the ground plate.
Changes in the electric field can be detected with a Polk element. The Polker element can change the incident linearly polarized light into elliptically polarized light (Pocker effect) by applying an electric field. That is, under the effect of the electric field, the transmittance of the Polker element will increase. Therefore, the electric field intensity corresponds to the luminous flux. The transmitted light is sent to the photoelectric converter with an optical fiber. High-voltage electrical insulation is also achieved through optical fibers, which is safe and reliable.
The magnetron magnetron method has long been used in other fields, but it has just begun to measure the degree of vacuum on the switch. Switching tubes with vacuum levels below 1.33×10-2Pa cannot be used. This is the limit that the switch must be replaced. When the vacuum degree is higher than 1.33×10-2Pa, the remaining life of the switch tube can be calculated based on the changes of the two interval times and the vacuum readings. It is the working principle of the magnetic measurement method of vacuum degree (the switch is in the open state).
The magnetic control principle diagram is in the low pressure state below 1.33×10-2Pa. The number of gas molecules is very small. In the middle of two high voltage electrodes, the probability of collision and ionization of free electrons with gas molecules is smaller, and the current is very weak. After the magnetic field is added, under the joint action of the electromagnetic field, electrons and ions are subjected to the Lorentz force F=qV×B, and a spiral motion is performed between the two electrodes, the motion path is extended, and the probability of colliding with gas molecules is greatly increased. The discharge current can be measured between the electrodes. When the electric field field of the electric field is unchanged, the discharge current is uniquely determined by the degree of vacuum, which is a single-value correspondence relationship. The discharge current is processed by the circuit to measure the degree of vacuum.
The comparative analysis method 1 is further improved based on the power frequency withstand voltage test. Its advantage is that it can use existing equipment (such as power frequency transformers, etc.), but it is known that it is only effective near the critical value of 10-2Pa. Method 2 can realize real-time online monitoring. However, this also increases the complexity of the entire grid system, thus reducing the reliability of the entire system operation, and the need to redesign the switchgear. Method 3 is used for periodic inspection, but the reading accuracy is high. The remaining life of the switch can be calculated through software programming. Specific methods can refer to references.
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