Spring-spring mechanism is the most commonly used circuit breaker operating mechanism today. It offers many advantages. Manufacturers use this as the simplest and most economical mechanism in circuit breakers. It also provides high reliability. This mechanism is much more compact than hydraulic or pneumatic mechanisms. Previously, engineers widely used spring-spring mechanisms for 11 kV indoor circuit breakers and 33 kV indoor and outdoor circuit breakers. Today, they use spring-spring mechanisms for circuit breakers up to 400 kV and above.
Basic Concept and Theory of Spring-Spring Mechanism
There are two springs. One spring is the closing spring, which is larger in size. The other is the tripping spring. The tripping spring, also called the open spring, is smaller than the closing spring. Generally we have discussed here about motor operated spring – spring mechanism of a circuit breaker. When we switch on the supply of the motor, it charges the closing spring. The motor does not have any gear or lever link with the tripping spring. That means that motor cannot charge the tripping spring, only it can charge the closing spring. Suppose the circuit breaker is open. The closing spring is already charged after the motor supply turns on.
Closing Operation of a Circuit Breaker
When you apply a closing command to the circuit breaker, the plunger of the closing coil acts and displaces the latch holding the charged closing spring. As a result, rapidly the closing spring will be relaxed, means it will release its stored potential energy. That potential energy was stored in the spring during the charging operation of the spring.
The spring transmits its movement through a cam and roller assembly during discharge. As a result, the cam will rotate and the roller will push a lever. The lever is hinged at the center. When the cam and roller assembly pushes one end, the other end moves and pushes the moving contact to touch the fixed contact. This action closes the circuit breaker.
Charging Opening Spring during Discharging Closing Spring
When you apply the closing command to the spring mechanism, the closing spring starts discharging. The released potential energy is not fully utilized to close the circuit breaker. Part of this energy charges the tripping spring. During the closing operation, the tripping spring deforms and stores energy. That means the tripping (opening) spring becomes charged during the discharging of the closing spring. This prepares the circuit breaker for the opening operation immediately after it has been closed.
Opening Operation of a Circuit Breaker
Now, if we apply the opening pulse either from the control panel or locally from the circuit breaker mechanism box, the trip coil gets energized and hence actuated. The plunger of the tripping coil strikes the latch holding the tripping spring in its charged position. As a result, the latch is displaced, and the tripping spring is suddenly discharged. This rotates the lever in the opposite direction. This causes the moving contact, to move backward to open the circuit breaker.
Advantages of Spring – Spring Mechanisms
This is the basic theory of the spring-spring mechanism of a circuit breaker. There are several advantages of using the spring-spring mechanism in circuit breakers.
Fast Operation
Other mechanisms, like hydraulic or pneumatic mechanisms, have become almost obsolete nowadays. The primary advantage of the spring-spring mechanism is its very rapid operation. Springs enable quick opening and closing of contacts. During charging, the spring stores potential energy, which it releases rapidly during discharging.
Reliable Operation
This stored mechanical energy allows consistent operations without relying on an external power source. When the motor rotates, it provides potential energy to store in the spring. The mechanism uses this potential energy during the closing and opening operations of the circuit breaker. Even if no power supply is available, the breaker can still perform at least one open-close-open cycle.
If the circuit breaker is already open, it can perform one close followed by one open operation. This feature provides a major advantage. The circuit breaker can operate without external power because the spring stores potential energy in advance. During operation, it does not need an external power source except to actuate the closing or opening coils. A DC voltage with battery backup is enough to actuate the closing and tripping coils. This ensures consistent operation. Even in the event of a total incoming power failure, the spring-spring mechanism can perform an open-close-open operations.
Compact Design
The design is very compact because it does not require a compressor, cylinder, or other complex piping networks found in hydraulic and pneumatic systems. Due to the absence of reservoir containers and compressors, the spring-spring mechanism is quite compact and small in size. Its simplicity also makes it more robust than hydraulic and pneumatic breaker operating mechanisms. Consequently, the durability of this mechanism is higher than that of other mechanisms, and its maintenance requirements are minimal.
Drawbacks of Spring – Spring Mechanisms
The spring-spring mechanism offers a fast response. It acts quickly, which is a significant advantage. However, the spring-spring mechanism also has several drawbacks. Despite its simple design, it requires many small parts, such as springs, latches, linkage gears, and other components, all of which must be properly aligned and placed. If any element is misplaced or misaligned, it affects the performance of the entire mechanism.
Another disadvantage is the potential for wear or corrosion in the assembled mechanical parts of the spring-spring mechanism. For long, consistent performance, there should be no wear or corrosion of the components. The materials selected for the gears, latches, levers, and springs must be of very high quality, which increases the cost of the mechanism.
If the spring constant of the closing and tripping springs reduces due to continuous operation, the entire system can malfunction. Reduced spring constant means less stored potential energy, which can lead to degraded performance of the mechanism. To maintain a constant spring force over a long lifespan, designers must choose the spring material precisely. This selection is a challenging task.
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