PROTECTION AND CONTROL
Performance Assessment of Medium Voltage Circuit Breakers
By Pratap Revuru
Performance assessment of medium voltage circuit breakers identifies status of the breaker and allows the user to prioritize and define a corrective action plan: maintenance, repair, conversion or replacement.
Medium voltage circuit breaker assessment includes:
- Evaluating the circuit breakerÕs overall condition (mostly visual inspection)
- Measuring and comparing actual contact resistance to the manufacturerÕs specifications
- Measuring insulation resistance, per NETA specifications
- Assessing the operating mechanism
Assessment of Overall Condition
This mainly visual assessment is very important and should be performed by an experienced technician, who inspects:
- exterior and interior appearance
- racking mechanism, ground stab, and interlock
- arc chutes and insulation barriers
- main and arcing contacts and bushings
- main and secondary drawout contacts
- auxiliary wiring and plugs
- operating mechanism, motor, lubrication, shock absorber, and arc puffer
The technician rates each of these parameters for further analysis.
Contact Resistance Measurement
Contact resistance measurement is one indicator of contact performance, but cannot accurately assess contact condition. Experience shows that visual inspection can detect worn contacts even when the contact resistance is within the limited range value. Visual inspection is therefore still required and must be correlated with the contact resistance measurement.
In addition, arcing contacts must be visually inspected because contact wear cannot be detected with this measurement. Contact resistance measurement equipment must inject at least 300A to ensure an accurate test result. The measured value in microhms must be compared with the limit specified by the equipment manufacturer (in some cases the upper limits may be up to 20 times the nominal value) and values obtained from the previous inspection (if measurement conditions remain unchanged).
Performance Assessment
The main function of a circuit breaker is to open under a fault condition to break the short circuit. In the case of medium voltage circuit breakers, the current is very high, leaving a short time during which the circuit breaker can open without severely impacting safety and damaging equipment. The time between the impulse to the coil and the actual opening of the circuit breaker indicates how quickly the circuit breaker operates.
For example, if it opens in 450ms instead of the required 65ms, there is a strong probability it will fail at the next fault. However, to accurately assess circuit breaker health, analysis of additional parameters is needed.
Time Travel Analysis
The time travel curve yields a thorough analysis of circuit breaker operation during the opening and closing phases. Curve data is captured from a travel sensor plugged into the main shaft of the operating mechanism (See Figure 1).
Phase 1 is the initial step of the opening/closing operation. The breaker hasnÕt started to open, but the coil is activated and operates the latching mechanism.
Phase 2 is the actual opening/closing. The poles are now separating.
Phase 3 is when the energy involved in the operation creates bouncing. Oscillations are decreased via the shock absorber.
This time travel analysis, combined with the coil current curve and phase contact detects malfunctions such as:
- worn or sticky latching mechanism
- defective coils
- mechanical wear and hardened grease
- defective shock absorbers
- contacts not opening/closing at the same time
Motor Charging Current Curve
The curve in Figure 2 illustrates motor current waveforms of an electrically operated mechanism.
This waveform enables the user to measure the time to charge and detect stiff points during the charging process as well as broken or damaged pinions.
All this information ?including time travel and motor charging current curves ?can be quickly captured by Schneider Canada Services?new Prodiag?software.
Taking just an average of 45 minutes to gather data, Prodiag immediately generates a status report that includes a comparison of measured versus specified values. The manufacturer-specified values vary according to the type of medium voltage circuit breaker being tested.
Breakthrough in Maintenance Technology
This software represents a significant breakthrough in medium voltage circuit breaker maintenance technology, as it provides highly accurate and in-depth results in very little time. To minimize downtime, these tests can be run as part of a regular maintenance schedule that includes assessing medium voltage circuit breaker operation Tests can be performed on an unscheduled, Ôas needed?basis on Ôproblem?medium voltage circuit breakers.
The software is also being adapted to cover a wide range of medium voltage circuit breakers, in order to provide users with information to better manage circuit breakers as they reach the end of their useful life.
Pratap Revuru is with Schneider Canada. For more information visit www.schneider-electric.ca ET
LATEST ISSUE
