Detecting
mechanical problems using vibration measurements
To correctly fulfill its task of opening and closing under various voltage and current conditions, a circuit breaker must operate very quickly. Indeed, the operation times of a high-voltage circuit breaker are on the order of a few hundredths of a second (i.e. 3 to 6 cycles). The various components of the operating mechanism are subjected to high speeds and accelerations. At the end of the travel, high stresses are also placed on all components because the contacts must stop suddenly with as little rebound as possible. The entire process generates high accelerations and decelerations that put the various operating mechanism components under very difficult stresses. Just as the human ear can detect a motor that is running badly, or a dog's hearing can discern odd noises in an even wider frequency range, the vibro-acoustic diagnostic method is a very efficient way to track down mechanical problems in circuit breakers. Carefully placed accelerometers, combined with a recording system capable of sampling the signals at frequencies above 150 kHz, can supply precious data on the condition of the operating mechanism. The vibrations generated during the circuit breaker's operation are transformed into an acceleration signal with a rich content of frequency and amplitude variations that follow the various mechanical events in an operation. Then it is only a question of comparing the measured data with a reference signal, which is a normal vibration signature for the same type of mechanism in good condition. Dedicated software has been developed to perform this analysis by determining the amplitude (ΔA) and time (Δt) differences, which are then compared against tolerance, or alert, values.
Figure 1 shows an example of vibration signals measured on a defective spring-type mechanism. In this example, one can clearly see an event that is out of line with the reference signal, which allows us to suspect a mechanical problem. Indeed, after disassembly of the mechanism, a severely deformed gear was found. Experimentation of this method at Hydro-Québec has shown that other mechanical problems can be detected in high-voltage circuit breakers:
- early stages of a command rod unscrewing or shearing off;
- low oil level in the closing damper of a spring-type mechanism;
- early stages of an auxiliary contact unscrewing.
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