Circuit Breaker Motor Monitoring

Breaker Motor Monitoring

While much attention is given to monitoring a circuit breaker’s timing and integrity of SF6, a better understanding of how the breaker’s charging motor is performing, in conjunction with the type of stored energy system being utilized, can provide critical information as to the condition of the stored energy system. Through comprehensive monitoring of the breaker’s charging motor, many defects which could result in motor failure or breaker mis-operation can be detected in their incipient stages, allowing for better maintenance practices and a reduction in the cost associated with equipment failure.

For motor monitoring to be effective, it is first crucial to have a good understanding of how a motor for a specific type of stored energy system should perform.

Mechanism Types

Spring Mechanism

For a spring type mechanism, a motor run should always be expected during the closing operation of the breaker. This should also be the only time the motor runs during normal operations. This is due to the design of the stored energy system which charges the closing springs upon the complete close of the breaker. This allows the breaker to have enough stored energy to perform a trip operation, as the trip springs are charged by the energy exerted during the close operation, and a subsequent close operation. Should this motor run not occur during the breaker close operation, the breaker would not have the stored energy to perform a close operation after the next trip operation occurred, resulting in a major breaker fail. For spring type mechanisms, the Dynamic Ratings Breaker Performance Monitor (BPM), is programmed to provide an alarm should this close motor run fail to occur. It also provides alarms concerning changes in motor current as this can be indicative of changes in spring tension or performance of the motor itself. A motor runtime is provided for both total motor runtime, which is necessary for maintenance purposes as well as run time per start, which provides an indication to the health of the motor as well as the limit switches. Also, any motor run which occurs other than the required close operation run would be reported as an excessive motor run as these should never occur with spring type mechanisms.

Spring Mechanism
Hydraulic Mechanism

Hydraulic Mechanism

Hydraulic mechanisms rely on hydraulic or a combination of hydraulic spring energy to operate. A different logic is required when monitoring the motor of this type of breaker as there are several key differences in how these motors function to keep the breaker in a ready state. The first key difference is that a motor run should be expected on both the close and open operation of the breaker. The BPM is programmed to provide an alarm should either of these motor starts not occurring as this could result in the breaker failing to operate. Also, another major difference in the normal operation of this type of stored energy system is that motor runs are to be expected when the breaker is in a static state. These non-operational motor runs occur to maintain the required pressures within the stored energy system which can be affected considerably due to changes in ambient temperature. The BPM provides a daily set point for the allowable number of nonoperational which may occur before an alarm is raised. The purpose of this alarm is to notate the difference between a motor which is running nominally or one which is running excessively due to a leak occurring within the breakers stored energy system. Alarms are also provided based on the motor’s runtime and current. However, these must be programmed to allow for the variation of current associated with changes in viscosity of the hydraulic fluid as it correlates to ambient temperature.

Pneumatic Mechanism

The BPM truly offers several unique features as it pertains to monitoring of pneumatic mechanisms. This type of mechanism works in a similar manner to hydraulic mechanism but relies on pneumatic pressure as the primary stored energy source. The BPM is still programmed to record both operational and nonoperational starts and a daily set point is programmed to detect the occurrence of a leak within the pneumatic system. However, due to the use of atmospheric air being stored in tanks via a compressor, moisture can become problematic when maintaining this type of system. Due to this moisture build up, it is required that regular maintenance be performed to manually drain air storage tanks. If this maintenance isn’t carried out in a timely manner, excessive moisture can build up in the breaker’s air lines. In cold weather environments this can result in moisture freezing in the small lines between the breaker’s limit switches and main pneumatic storage system. When this occurs, it can result in excessive compressor runtimes which can cause thermal failure of the compressor motor. The BPM has introduced custom logic to better manage this type of failure. This is accomplished through monitoring of moisture levels in the breaker’s airlines, providing better guidance as to when tanks need to be drained due to build up of free water from condensation. Transducers to monitor actual pressure at the mechanism are utilized to indicate if limit switches are functioning properly. This allows an alarm to be issued should a motor run occur when the air system is above the cut on pressure of if a motor continues to run after cutoff pressure has been reached. Finally, the BPM also allows for monitoring of the compressor temperature, providing an alarm if the compressor temperature exceeds a set percentage of the OEM’s recommended max temperature rating. There is also an optional feature that allows the BPM to shut off a compressor run when this alarm point is exceeded. This would allow a maintenance of the compressor to be performed before failure occurred, allowing for a reduction of the repair cost and downtime associated with these events.

Pneumatic Mechanism

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Additional Resources

WEBCAST

SF6 Gas Monitoring and Diagnostics

WHITE PAPER

Improving Reliability of Circuit Breakers Through Directed Maintenance

ARTICLE

The Ins and Outs of Circuit Breaker Monitoring