Fault Analysis and Treatment of High Voltage Vacuum Circuit Breaker

1 Introduction

The superiority of vacuum circuit breakers is not only that they are oil-free equipment, but also that they have long electrical life, mechanical life, large breaking insulation capacity, strong continuous breaking capacity, small size, light weight, frequent operation, fire prevention, and less operation and maintenance. They were quickly recognized by the operation, maintenance and technical personnel of the power department. The quality of high-voltage vacuum circuit breakers produced in China in the early days was not stable enough, the current-carrying overvoltage was high during operation, and some vacuum arc extinguishing chambers still had air leakage. By the time of the Tianjin Vacuum Switch Application Promotion Conference in 1992, my country's vacuum circuit breaker manufacturing technology had entered the forefront of similar products in the same industry internationally, becoming a new historical turning point in the application and manufacturing technology of high-voltage vacuum circuit breakers in my country. With the widespread application of vacuum circuit breakers, failures also occur from time to time. The author analyzes the common failures of vacuum circuit breakers and provides treatment methods.

2 Common abnormal operating conditions of vacuum circuit breakers

2.1 Circuit breaker refuses to close or open

It manifests itself as follows: after the circuit breaker receives the closing (opening) command, the closing (opening) electromagnet is activated, the iron core push rod pushes the closing (opening) latch open, and the closing (opening) spring releases energy, driving the circuit breaker to close (open), but the arc extinguishing chamber of the circuit breaker cannot be closed (opened).

2.2 Circuit breaker mis-tripping

The circuit breaker trips in normal operation for unknown reasons.

2.3 After the circuit breaker mechanism stores energy, the energy storage motor keeps

It manifests itself as the energy storage motor of the operating mechanism starts working after the circuit breaker is closed, but after the spring energy is fully stored, the motor continues to run.

2.4 Increased DC resistance of circuit breaker

It manifests itself as the contact resistance of the arc extinguishing chamber contacts increasing continuously after the circuit breaker has been running for a certain period of time.

2.5 The circuit breaker closing bounce time increases

It manifests itself as the closing bounce time of the circuit breaker increasing continuously after it has been running for a certain period of time.

2.6 Discharge from the CT surface of the circuit breaker intermediate box to the bracket

It manifests itself as the current transformer surface discharging to the intermediate box bracket during the operation of the circuit breaker.

2.7 The arc extinguishing chamber of the circuit breaker cannot be disconnected

It manifests as the circuit breaker cannot be disconnected or not all phases are disconnected after the circuit breaker is opened.

3. Failure Cause Analysis

3.1 Circuit breaker refuses to open or close

When the operating mechanism refuses to move, the cause of the refusal is generally analyzed first, whether it is a secondary circuit failure or a mechanical failure, and then it is handled. After checking that the secondary circuit is normal, it is found that the clearance of the universal shaft head connected to the main crank arm of the operating mechanism is too large. Although the operating mechanism operates normally, it cannot drive the circuit breaker opening and closing linkage to operate, resulting in the circuit breaker not being able to open and close normally.

3.2 Circuit breaker mis-tripping

When the circuit breaker is in normal operation, the circuit breaker cannot be opened without external operating power supply and mechanical opening action. After confirming that there is no wrong operation, check the secondary circuit and operating mechanism. It is found that the auxiliary switch contact in the operating mechanism box is short-circuited, and the opening power supply is connected to the opening coil through the short-circuit point, causing the wrong opening. The reason is that the top of the circuit breaker mechanism box is leaking, and the rainwater flows down along the output arm and falls on the auxiliary switch of the mechanism, causing the contact to short-circuit.

3.3 After the circuit breaker mechanism stores energy, the energy storage motor keeps

After the circuit breaker is closed, the energy storage motor of the operating mechanism starts working. After the spring energy is fully stored, a signal that the spring has stored energy is issued. A pair of normally open auxiliary contacts of the circuit breaker and a pair of normally closed contacts of the travel switch are connected in series in the energy storage circuit. After the circuit breaker is closed, the normally open contacts of the auxiliary switch are connected, the energy storage motor starts working, and after the spring is fully stored with energy, the mechanism rocker arm opens the normally closed contacts of the travel switch, the energy storage circuit is de-energized, and the energy storage motor stops working. The reason why the energy storage motor keeps working is that after the spring is fully stored with energy, the mechanism rocker arm fails to open the normally closed contacts of the travel switch, the energy storage circuit is always energized, and the energy storage motor cannot stop working.

3.4 Increased DC resistance of circuit breaker

Since the contacts of the vacuum interrupter are butt-jointed, if the contact resistance is too large, the contacts will easily heat up when carrying current, which is not conducive to conducting electricity and breaking the circuit, so the contact resistance value must be less than the requirements of the factory manual. The pressure of the contact spring has a great influence on the contact resistance, and it must be measured under the condition of qualified overtravel. The gradual increase in the contact resistance value can also reflect the electrical wear of the contacts, which complement each other. The electrical wear of the contacts and the change in the opening distance of the circuit breaker contacts are the fundamental reasons for the increase in the DC resistance of the circuit breaker.

3.5 The circuit breaker closing bounce time increases

When the vacuum circuit breaker is closed, the contacts always bounce a little, but if it bounces too much, the contacts are prone to burn or weld. The technical standard for the contact bounce time of the vacuum circuit breaker is ≤2ms. As the circuit breaker runs longer, the main reason for the increase in closing bounce time is the decrease in the elastic force of the contact spring and the increase in the wear of the crank arm and the shaft pin gap.

3.6 Discharge from the CT surface of the circuit breaker intermediate box to the bracket

The current transformer is installed in the middle box of the circuit breaker. When the circuit breaker is running, an uneven electric field will be generated on the surface of the current transformer. To avoid this phenomenon, the transformer manufacturer applies a layer of semiconductor glue on the surface of the transformer to make the surface electric field uniform. During the assembly of the circuit breaker, due to space limitations, the semiconductor glue around the transformer fixing bolts is scraped off. During the operation of the circuit breaker, an uneven electric field is generated on the surface of the transformer, causing the transformer surface to discharge to the bracket.

3.7 The arc extinguishing chamber of the circuit breaker cannot be disconnected

Under normal circumstances, whether it is a manual opening operation or a protection action trip, the circuit breaker can effectively disconnect the circuit and cut off the current.

The arc extinguishing principle of vacuum circuit breaker is different from other types of circuit breakers. It refers to a switchgear in which the contacts are closed and opened in a vacuum, that is, a circuit breaker that uses vacuum as an insulating and arc extinguishing medium. When the vacuum degree of the vacuum bubble decreases, there will be a certain ionization phenomenon in the vacuum bubble, and ions will be generated, which will reduce the insulation in the arc extinguishing chamber and cause the circuit breaker to fail to open normally.

4 Treatment methods

4.1 Circuit breaker refuses to close or open

Check the clearance of all connecting parts of the operating mechanism, and replace unqualified parts with new, high-hardness qualified parts.

4.2 Circuit breaker mis-tripping

Check all possible rain leaks and seal them effectively; install a sealing rubber sleeve on the output crank arm connecting rod; turn on the heating and moisture-removing device in the mechanism box.

4.3 After the circuit breaker mechanism stores energy, the energy storage motor keeps

Adjust the installation position of the travel switch so that the normally closed contact of the travel switch can be opened when the rocker arm is in the highest position.

4.4 Increased DC resistance of circuit breaker

Adjust the arc extinguishing chamber contact opening distance and overtravel. The contact resistance can be measured using the DC voltage drop method required by the "Regulations" (the current must be above 100A), otherwise replace the arc extinguishing chamber.

4.5 The circuit breaker closing bounce time increases

(1) Appropriately increase the initial pressure of the contact spring or replace the contact spring.

(2) If the clearance between the crank arm and the axle pin exceeds 0.3 mm, the crank arm and the axle pin can be replaced.

(3) Adjust the transmission mechanism. Take advantage of the fact that the transmission ratio is very small when the mechanism exceeds the dead point of the active arm in the closing position. Adjust the mechanism closer to the dead point to reduce the contact closing bounce.

4.6 Discharge from the CT surface of the circuit breaker intermediate box to the bracket

Apply a layer of semiconductor glue evenly on the surface of the transformer to make the surface electric field uniform.

4.7 The arc extinguishing chamber of the circuit breaker cannot be disconnected

For the treatment of vacuum interrupters that cannot reach the required vacuum degree, if the vacuum degree of the vacuum interrupter has indeed dropped below the required value through testing, the vacuum interrupter should be replaced.

(1) The vacuum interrupter to be replaced must pass the vacuum test.

(2) Remove the original vacuum interrupter and replace it with a new one. Install it vertically. Pay attention to the coaxiality of the moving conductive rod and the interrupter. There should be no torque during operation.

(3) After installing the new vacuum interrupter, the opening distance and overtravel (contact travel) should be measured. If they do not meet the requirements, corresponding adjustments should be made: ① Adjust the bolts of the insulating pull rod to adjust the overtravel; ② Adjust the length of the moving conductive rod to adjust the opening distance of the interrupter.

(4) Use a comprehensive power switch tester to measure mechanical characteristics such as opening and closing speed, three-phase synchronization, and closing bounce. If they are unqualified, adjustments should be made.

5 Conclusion

This article starts with some abnormal conditions that occur during the operation of vacuum circuit breakers, and explains some fault analysis and treatment methods. It is a technical accumulation of vacuum circuit breakers in work. In future work, we need to continue to learn and innovate to ensure that electrical equipment in the power system is always in good operating condition and contribute to safe and high-quality power supply.