CB Control Schematics
This article is a technical piece that delves into the schematics for the close and charging motor circuits on a medium voltage circuit breaker. Control circuits are pivotal to the proper operation of MV switchgear. The integrity of these control circuits is crucial for the switchgear’s appropriate functioning. Detailed records of commissioning and maintenance activities will be vital for troubleshooting purposes.
Providing a close-up and detailed explanation of the motor control circuits in a power circuit breaker (pictured: Siemens’ medium voltage vacuum circuit breaker). To effectively troubleshoot and comprehend the sequence of operations, one must also have a solid understanding of interpreting the control circuits.
All control circuits are fundamentally based on the CB coils that initiate closure or tripping actions. Switchgear must incorporate interlocking systems to prevent accidental closures, which could jeopardize the safety of both individuals and equipment.
Unlike low-voltage breakers, medium-voltage breakers do not integrate protection relays, and these relays are not powered by the primary circuit’s current. Safety relays can be procured externally. Consequently, medium voltage circuit breakers rely on control power to precisely and consistently trip or open the breaker in case of malfunctions.
Given this, the control power source must exhibit high reliability, as the protective functionality of a circuit breaker hinges on the availability of control power.
In electric generation stations owned by utilities, the most dependable source is a DC source from a system battery. Even if AC power is lost in the power station, the battery voltage will be sustained, allowing breakers to carry out their circuit protection functions.
However, the medium voltage breaker is closed or opened through mechanical springs housed within the breaker. Additionally, a manual trip and close button is situated on the breaker’s front.
The operational mechanism employs a stored energy mechanism. The closing spring can be charged either manually or electrically. Operating levers transmit force from the operational mechanisms to the pole assemblies.
To close the breaker, one can unlatch the closing spring either manually using the local “ON” pushbutton or electronically via remote control. As the breaker closes, the closing spring charges up the contact or opening pressure springs. The discharged closing spring will be recharged either automatically or manually through the mechanism’s motor.
Subsequently, the springs are programmed with the operating sequence OPEN – CLOSE – OPEN. A position switch can be utilized to verify the charging state of the spring.
This post was written by Justin Tidd, Director at Becker/SMC. For nearly a half a century, Becker Mining has been at the forefront of safety, producing the best Vacuum Circuit Breakers in the industry. Becker/SMC is the industry’s leader in increasingly more sophisticated electrical control systems. Most of the major innovations, design features and specialized electrical components have been developed by Becker/SMC