Principle Analysis | Deep Analysis of Vacuum Circuit Breakers

1、 Insulation characteristics of vacuum

Vacuum has strong insulation properties. In vacuum circuit breakers, gas is very thin, and the free travel of gas molecules is relatively large, resulting in a low probability of collision with each other. Therefore, collision dissociation is not the main cause of true space gap breakdown, but the metal particles precipitated by the electrodes under the action of a high strength electric field are the main factors causing insulation damage.

The insulation strength in a vacuum gap is not only related to the size of the gap and the uniformity of the electric field, but also greatly influenced by the properties and surface conditions of the electrode material. The vacuum gap has higher insulation characteristics than high pressure air and SF6 gas at small distance gaps (2-3 millimeters), which is why the contact opening distance of vacuum circuit breakers is generally not large.

The impact of electrode materials on breakdown voltage is mainly manifested in the mechanical strength (tensile strength) of the material and the melting point of the metal material. The higher the tensile strength and melting point, the higher the insulation strength of the electrode under vacuum.

The experiment shows that the higher the vacuum degree, the higher the breakdown voltage of the gas gap, but it remains basically unchanged above 10 to 4 torres. Therefore, to maintain the insulation strength of the vacuum interrupter, its vacuum degree should not be less than 10 to 4 torres.

2、 Formation and extinction of electric arcs in vacuum

There is a significant difference between the phenomenon of vacuum arc and gas arc discharge that we have learned before. The dissociation of gas is not the main factor causing the arc, and vacuum arc discharge is formed in the metal vapor evaporated from the contact electrode. At the same time, the characteristics of the arc performance vary depending on the magnitude of the breaking current. We generally classify it into low current vacuum arc and high current vacuum arc.

1. Low current vacuum arc

When the contact is disconnected in vacuum, a cathode spot with a high concentration of current and energy is generated, and a large amount of metal vapor is evaporated from the cathode spot. The density of metal atoms and charged particles in the spot is high, and the arc burns in it. At the same time, the metal vapor and charged particles inside the arc column continue to diffuse outward, and the electrode also continuously evaporates new particles to supplement. When the current passes zero, the energy of the arc decreases, the temperature of the electrode decreases, the evaporation effect decreases, and the particle density inside the arc column decreases. Finally, when the current passes zero, the cathode spot disappears and the arc extinguishes.

Sometimes, evaporation cannot maintain the diffusion speed of the arc column, and the arc suddenly extinguishes, leading to the occurrence of flow interception.

2. High current vacuum arc

When the contact disconnects a large current, the energy of the arc increases, and the anode also generates severe heat, forming a strong concentrated arc column. At the same time, the role of electrodynamics is also evident, therefore, for high current vacuum arcs, the magnetic field distribution between contacts has a decisive impact on the stability and arc extinguishing performance of the arc. If the current is too high and exceeds the limit breaking current, it will cause breaking failure. At this point, the contact heats up severely, and even after the current passes zero, it still evaporates, making it difficult for the medium to recover and unable to disconnect the current.

3、 Structure and working principle of circuit breakers

There are many manufacturers and models of vacuum circuit breakers. According to usage conditions, it is divided into two types: indoor (ZNx - * *) and outdoor (ZWx - * *). It mainly consists of a frame part, an arc extinguishing chamber part (vacuum bubble), and an operating mechanism part.

The circuit breaker body is composed of a conductive circuit, an insulation system, seals, and a shell. The overall structure is a three-phase common box type. The conductive loop is formed by connecting the incoming and outgoing line conductive poles, the incoming and outgoing line insulation supports, conductive clips, and soft connections to the vacuum arc extinguishing chamber.

The mechanism is electric energy storage, electric opening and closing, and also has manual function. The entire structure is composed of closing springs, energy storage systems, overcurrent releases, opening and closing coils, manual opening and closing systems, auxiliary switches, energy storage indicators, and other components.

working principle

When a vacuum circuit breaker utilizes high fidelity air current to flow through zero, the plasma rapidly diffuses and extinguishes the arc, completing the purpose of cutting off the current.

Action principle

Energy storage process: When the energy storage motor 14 is connected to the power supply, the motor drives the eccentric wheel to rotate, and the roller 10 next to the eccentric wheel drives the crank arm 9 and connecting plate 7 to swing, pushing the energy storage pawl 6 to swing, causing the ratchet 11 to rotate. When the pin on the ratchet 11 is against the plate of the energy storage shaft sleeve 32, the two move together, causing the closing spring 21 hanging on the energy storage shaft sleeve 32 to elongate. The energy storage shaft sleeve 32 is fixed by a positioning pin 13 to maintain the energy storage state. At the same time, the crank arm on the energy storage shaft sleeve 32 pushes the travel switch 5 to cut off the power supply of the energy storage motor 14, and the energy storage pawl is lifted to reliably detach from the ratchet wheel.

Closing operation process: When the mechanism receives the closing signal (the switch is in the disconnected and stored energy state), the iron core of the closing electromagnet 15 is sucked downwards, and the positioning component 13 is pulled to rotate counterclockwise to release the energy storage maintenance. The closing spring 21 drives the energy storage shaft sleeve 32 to rotate counterclockwise, and its cam presses the transmission shaft sleeve 30 to drive the connecting plate 29 and rocker arm 27 to move, causing the rocker arm 27 to buckle onto the half shaft 25, causing the mechanism to be in a closed state. At this point, the interlocking device 28 locks the positioning component, preventing the positioning bull from rotating counterclockwise, achieving the purpose of mechanism linkage and ensuring that the mechanism cannot be closed in the closed position.

Opening operation process: After the circuit breaker is closed, the opening electromagnet receives a signal, the iron core pulls in, and the top rod in the opening release 19 moves upwards, causing the release shaft 16 to rotate, driving the top rod 18 to move upwards, pushing the bending plate 26 and driving the half shaft 25 to rotate counterclockwise.

Half axis 25 and rocker arm 27 are released, and under the action of the opening spring, the circuit breaker completes the opening operation.

4、 Debugging of circuit breakers

The measurement of the opening distance and overtravel of a circuit breaker can be based on Figure 3. The difference between the X value measured in the open and close state is the opening distance of the circuit breaker, and the difference between the Y value is the overtravel of the circuit breaker. The adjustment method is to lengthen or shorten the insulated operating rod 3 or the connecting rod between the mechanism and the spindle.

Adjustment of opening and closing mechanism

1. The connection amount between rocker arm 27 and half shaft 25 is 1.5-2.5mm, which can be achieved by adjusting screw 24.

2. When the transmission shaft sleeve 30 rotates at its maximum angle, there should be a gap of 1.5-2mm between the rocker arm 27 and the half shaft to ensure that when the transmission shaft sleeve falls back to the closed position, the rocker arm 27 can automatically buckle onto the half shaft 25, which can be adjusted by the screw 31.

3. The conversion of auxiliary switch 2 should be accurate and reliable, which can be achieved by adjusting the position of the crank arm 3 and the length of the lever 4 of auxiliary switch 2.

4. During the energy storage process, when the pawl reaches the highest point of the last tooth, it should be ensured that the crank arm on the energy storage shaft sleeve 32 can reliably switch the contacts of the travel switch, cut off the motor power supply, and achieve this by adjusting the up, down, front, and back positions of the travel switch 5.

5. Adjust the pre tension length of the opening and closing spring to ensure reliable opening and closing of the circuit breaker, and ensure that the opening and closing speed reaches the specified value.

5、 Control circuit of circuit breaker

In the 35KV standardized substation of China's rural power grid, the principle of separating the control busbar and the closing busbar is adopted.

Connect a pair of normally open contacts of the energy storage travel switch of the circuit breaker in series into the control circuit between the auxiliary normally closed contact of the circuit breaker and the closing coil. In this way, the closing operation cannot be carried out without energy storage in the circuit breaker. It prevents closing without energy storage in the circuit breaker, maintaining the closing circuit, and burning the closing coil.

Meanwhile, during the wiring process, it is important to ensure that the polarity between the closing busbar and the control busbar in the energy storage travel switch contacts is consistent, in order to prevent the arc in the closing circuit from breaking through the travel switch during energy storage, causing the control fuse to fuse or the control air switch to trip.

This should be particularly noted in integrated automation substations.

6、 Operation maintenance and repair test

Vacuum circuit breakers have short arcing time, high insulation strength, high electrical life, small contact distance and stroke, and low operating energy, therefore, their mechanical life is also high. In daily operation, the maintenance workload is very small, mainly checking the wear of the moving parts of the mechanism, whether the fasteners are loose, removing dust from the insulation surface, and injecting some lubricating grease into the moving parts.

In the preventive test of spring inspection, the DC resistance test of the switch should be compared with historical data, and problems should be promptly handled and replaced. The power frequency withstand voltage test of the fracture is an effective method to check whether the vacuum bubble is leaking. (Indoor vacuum circuit breakers can refer to the color of the flashing light inside the vacuum bubble when the load is disconnected to preliminarily determine the vacuum degree of the vacuum bubble. When the color is dark red, it indicates a decrease in vacuum degree, and when the color is light blue, it indicates a good vacuum degree.) When verifying the protection and installation of the circuit breaker, a low voltage on-off test is conducted to verify whether the switch operates reliably when the voltage drops during a busbar fault state.

Analysis of the Development and Performance of Vacuum Circuit Breakers

1、 Dedicated vacuum circuit breaker

Faced with extremely different breaking tasks, new specialized circuit breakers have emerged. If the super large capacity vacuum circuit breaker (with a short-circuit breaking current of 63-80kA or above) is used for generator protection circuit breaker, the standard vacuum circuit breaker (with a short-circuit breaking current of 25-50kA), the economical vacuum circuit breaker (with a short-circuit breaking current of 16-25kA), the frequent vacuum circuit breaker (with an operating frequency of 50000 to 60000 times), and the ultra frequent and complex vacuum circuit breaker (with an operating frequency of 100000 to 150000 times). For example, Siemens' 3AH series circuit breakers are divided into five models based on their usage. The 3AH1 and 3AH3 models are standard with 10000 operations, the 3AHZ model is frequent with 60000 operations, the 3AH4 model is overclocking with 120000 operations, and the 3AH5 model is economical with low prices.

2、 Low overvoltage type vacuum circuit breaker

As is well known, vacuum circuit breakers can cause voltage cutoff due to current cutoff, especially when interrupting small inductive negative interrupts such as electric motors. In general, overvoltage absorption devices such as Sic, RC circuit, ZnO lightning arrester, etc. are equipped to limit overvoltage in vacuum circuit breakers, which makes the circuit breaker structure large and complex, and some limit overvoltage not ideal.

Several Japanese companies have taken a different path and developed low overvoltage vacuum circuit breakers. It does not require the addition of overvoltage absorption devices and uses newly developed contact materials to limit overvoltage to one tenth of the conventional value. Low overvoltage contact material: Toshiba is AgWC, Hitachi is Co Ag Se, and Mitsubishi is Cu Cr Bi - α， Fuji is a CuCr+high vapor material. These companies generally achieve 20kA at 7.2kV, with only Toshiba achieving 40kA at 7.2kV.

3、 Multifunctional vacuum circuit breaker

As is well known, vacuum circuit breakers have so far completed the closing and breaking tasks in two I positions (i.e., closing and opening). Now multifunctional vacuum circuit breakers have emerged, giving them multiple functions, such as closing opening isolation grounding, etc. Siemens, Alstom, and Hitachi all have such products. Siemens' latest NXACT modular vacuum circuit breaker has multiple functions: integrating making, breaking, isolating, grounding, and interlocking. The vacuum circuit breaker equipped by Alstom company with VISAX switchgear is in three I positions (closing opening isolation). The 24kV vacuum circuit breaker developed by Hitachi in collaboration with Tokyo Electric Power Company has four I positions (closing opening isolation grounding).

In order to make the product multifunctional, there are two methods from the perspective of existing products: firstly, the phase column of the vacuum circuit breaker moves or rotates after opening, forming isolation and grounding; The other is the rotation of the contacts in the vacuum arc extinguishing chamber to complete isolation and grounding. Siemens NXACT products complete isolation and grounding by moving the phase column after disconnection, while Alstom completes the isolation task by rotating the phase column after disconnection, and Hitachi completes the isolation and grounding task by rotating the contact in the arc extinguishing chamber.

4、 Synchronous circuit breaker

Synchronous circuit breakers are also known as phase selective vacuum circuit breakers or controlled vacuum circuit breakers. The basic principle is to make the vacuum circuit breaker close or open at the most favorable moment of voltage or current.

Compared with ordinary vacuum circuit breakers, synchronous circuit breakers have the following advantages: 1. reducing transient overvoltage loads in the power grid; 2. Improved the quality of power supply in the power grid; 3. Improved the electrical life and performance of the circuit breaker; 4. Simplified power grid design, thereby reducing the overall system cost.

ABB has developed synchronous vacuum circuit breakers using digital electronic devices and magnetic operating mechanisms, which is a good start.

5、 Intelligent vacuum circuit breaker

The intelligence of vacuum circuit breakers is based on modern sensing technology and digital control technology. Foreign manufacturing companies have made their products intelligent, which is not only necessary for distribution automation, but also for the control and protection of circuit breakers themselves. For example, Alstom's DCX programmable digital control device, ABB's REF542 control and protection device, and Siemens' second-generation digital protection device.

From the above, it can be seen that vacuum circuit breakers have developed rapidly. Although there are many reasons for this, there are two basic ones: firstly, the progress of vacuum arc extinguishing chamber technology; The second is the advancement of operating mechanism technology. The vacuum arc extinguishing chamber is the heart of a vacuum circuit breaker. The progress of vacuum arc extinguishing chambers is reflected in the transformation of contact material from CuBi to CuCr, which improves the breaking ability and reduces the cutoff value. At the same time, the magnetic field shifts from transverse to longitudinal magnetic fields, improving the breaking ability and reducing contact burning loss. In terms of technology, the adoption of a one-time sealing process greatly improves the performance and reliability of the arc extinguishing chamber.

The operating mechanism is called the central nervous system of vacuum circuit breaker. Originally using electromagnetic mechanisms, spring mechanisms have emerged, and the latest is the emergence of permanent magnet mechanisms. The spring mechanism has a complex structure with a large number of parts (up to 200), high machining accuracy requirements, and the output characteristics of the spring mechanism do not match the load characteristics of the vacuum circuit breaker. Therefore, it is necessary to design it reasonably on the cam contour curve and connecting rod structure. The mechanical structure of permanent magnet mechanisms is particularly simple, with fewer components than any other mechanism, and the number of moving parts can be reduced to one, resulting in particularly high mechanical reliability. Moreover, the output performance of permanent magnet mechanisms is well matched with the load characteristics of vacuum circuit breakers. The permanent magnet mechanism uses permanent magnet locks, capacitors (or DC screen power supply) for energy storage, and is electronically controlled. Permanent magnet mechanisms are particularly suitable for frequent operations, such as up to 60000 to 150000 times

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