The philosophy behind maintaining circuit breakers focuses on preventive maintenance to maximize the longevity of the electrical apparatus. The transmission division of Hydro-Québec TransÉnergie (Québec, Canada) divides activities in this field into three categories: corrective, conditional and systematic. Moreover, maintenance is an integrated part of a continuous improvement process of practices, methods and tools done in collaboration with the manufacturers of circuit breakers and test instruments.

MAINTENANCE DESCRIPTIONS

Corrective and conditional maintenance are related to tasks performed to repair an apparatus following a fault or with a defect. In the first case, the primary function of the apparatus cannot be maintained; while in the other case, it is still possible to operate the equipment. Whether it is corrective or conditional maintenance, the impact of these interventions is significant as they concern the safety of personnel and the operation of the power network.

Systematic maintenance is based on a specific predefined time frame or number of breaker operations, depending on the first occurrence. The content and frequency of these inspections are continuously adapted to the technical aspects, such as the reliability of the technology and the position of the equipment in the substation (lines, power plants, capacitor banks and inductors), as well as problems previously experienced with the same type of equipment.

The preventive inspection of high-voltage circuit breakers includes traditional tests such as alternating-current insulation, static contact-resistance measurement and timing tests with contact travel curves. In addition, one of the interrupting chambers must be inspected, based on the results of the previously mentioned tests. Because of the specific procedures regarding handling and recovery of SF₆ gas, these interventions can become quite complex and demanding. Thus, the idea is to limit the number of intrusive interventions and, in turn, minimize their cost.

DYNAMIC CONTACT-RESISTANCE MEASUREMENT

The original approach for dynamic contact resistance was to measure the resistance during a slow open operation of the breaker. Tests were done at low speed, so the partial contact separation was not present, making it easy to measure the resistance with a direct current of 100 A. However, during a field test, a minor incident occurred on the spring operating mechanism due to the slow operation of the breaker. Moreover, extremely high resistance values were measured due to the accretion of SF₆ byproducts on the contacts.

Although this method was interesting because of the simplicity of its implementation, the measurement at 100 A could not be universally applied to all types of breakers.

A complementary study was launched to validate the measurement at nominal speed and high current. On one hand, tests on the electrical network confirmed that a continuous current source of 2800 A could burn the SF₆ powders, verifying the actual condition of the breaker contacts. On the other hand, validation tests at nominal opening speed were performed on various types of SF₆ breakers. A direct current of 700 A was determined to be sufficient to avoid the partial contact separation and produce a clean dynamic-resistance curve.

The dynamic-resistance measurement allows a precise evaluation of the wear of both the arcing and main contacts without having to open the breaker. The static-resistance measurement gives information on the status of the permanent contacts, but the wear mainly occurs on the arcing contacts, which are subjected to the heat and energy produced by arcing during each breaker operation. This measurement is applicable to high-voltage circuit breakers (69 kV to 735 kV) insulated with SF₆ and fitted with two sets of parallel contacts (main and arcing). For dead tank breakers, this method is only applicable if necessary precautions are taken to avoid the magnetization of the current transformers. Figure 1 shows a typical dynamic contact-resistance curve.

ARCING CONTACT MISALIGNMENT

A series of tests allowed Hydro-Québec TransÉnergie to establish threshold levels that are easily interpretable by the users of the equipment. For example, the utility was able to detect an anomaly on a 120-kV circuit breaker on a capacitor bank. The results of the dynamic contact-resistance measurement are presented in Fig. 2. An internal inspection of this breaker confirmed the diagnosis determined during the test. In fact, one of the arcing contact fingers on the moving side was misaligned, while the fixed side clearly showed abnormal wear due to excessive arcing (Fig. 3).

TUNGSTEN TIP OF ARCING CONTACT UNSCREWED

In another case, a 120-kV circuit breaker on a capacitor bank cumulated more than 4000 operations. The measurement indicated a partial separation of the arcing contact prior to the other phase (Fig. 4). An internal inspection showed that the tungsten arcing tip had begun to unscrew as illustrated in Fig. 5.

In the two previous cases, an internal diagnosis of the interrupting chambers allowed a necessary and well-timed intervention, which avoided an eventual breaker failure.