Anton Janssen

International Surveys on Circuit-Breaker Reliability Data for Substation and System Studies

Since the 1970s, CIGRE has conducted three worldwide surveys on high-voltage circuit-breaker (CB) reliability. The results of the last inquiry, published last year, are presented and compared with those of the former inquiries. With a focus on the CBs fundamental functions for the system, figures show the growth in reliability during the past decades. The reliability is expressed in failure per 100 CB years (CBY) or per 10 \thinspace000 operating cycles for the relevant failure modes. The overall major failure rate improved largely from the first (1.58 per 100 CBY) to the second (0.67 per 100 CBY) to the third enquiry (0.30 per 100 CBY). The failure rate increases with higher voltage classes; GIS CBs have been shown to be twice as reliable and live tank CBs twice as bad as the average failure rate. Although improved, the mechanical operating mechanism is still the subassembly responsible for most failures; besides, CBs applied for frequent switching purposes show a higher failure rate than average.

Clearing faults near shunt capacitor banks

Fault interrupting tests on high-voltage circuit breakers confirmed the problems of breaking a short-circuit current, which can occur in substations with shunt capacitor banks. Research was conducted on the behavior of different extinguishing media. Tests show that all types of circuit breakers (including the restrike-free breakers) have reignitions after the interruption of faults in the vicinity of a shunt capacitor bank. This is caused by the low rate of rise of the recovery voltage which gives the circuit breaker an opportunity to clear at short arcing times. Reignitions occur because of the low dielectric withstand capability of the small contact gap. Both tests and computer calculations show high overvoltages when the circuit breaker interrupts at one of the superimposed high-frequency current zeros which are created after reignition. Some breakers interrupt at a high-frequency current zero and thus create overvoltages. Other breakers do not interrupt at a high-frequency current zero and wait for a current zero of the power-frequency current, thus prolonging the arcing time. The overvoltages in three-phase circuits are higher than in single-phase circuits. >

Investigations on Requirements for UHV/EHV AC Switchgear by CIGRE Study Committee A3

On request of IEC Technical Subcommittee 17A, CIGRE Study Committee A3 “High Voltage Equipment” investigated the requirements for ultra-high voltage ac switchgear, especially for parameters which cannot be extrapolated linearly from the corresponding equipment characteristics with lower-rated voltage, namely 800 kV. For circuit breakers, such aspects are the first pole-to-clear factor, the amplitude factor, the time to peak to define the transient recovery voltage envelope, the surge impedance of overhead lines, the damping and refraction of traveling waves, long-line faults, transformer-limited faults, initial transient recovery voltage, and the influence of shunt and series compensation. Disconnectors utilized for interrupting the bus transfer currents, bus charging currents and very fast transients, ground switches used for interrupting induced line currents, and high-speed ground or earthing switches have been studied as well. Based on these system and substation studies and simulations, recommendations have been provided to IEC SC 17A.