Analyzing the Interruption Processes in the Generator Circuit Breaker through Sustainability Concepts

Abstract

The concepts of science, including concepts related to sustainability including exergy and embodied energy, were developed to describe our knowledge about aspects of the universe. A convincing example of the usefulness of embodied energy and exergy for analyzing systems which transform energy is the generator circuit-breaker (GCB) disconnection process. Nowadays, the electric connection circuits of power plants (based on fossil fuels as well as renewable sources) entail GCBs at the generator terminals, since the presence of that electric equipment offers many advantages related to the sustainability of a power plant. A classic circuit-breaker is an automatically electrical switch designed to protect against inherent operation faults, such as overload or short-circuit. A generator circuit-breaker is located between the generator and the main step-up transformer, this location influencing the operating conditions since GCBs are significantly more difficult to apply to some operating regimes than classical network circuit-breakers. Consequently, the electrical and mechanical performance required of a GCB exceeds the requirements of a standard distribution circuit-breaker. Generally, a circuit-breaker must detect a fault condition, and once a fault is detected, electric contacts within the circuit-breaker must open to interrupt the circuit. In an alternating current (a.c.) circuit the interruption of a short-circuit is performed by the circuit-breaker at the natural passing through zero of the short-circuit current. During the current interruption, an electric arc is generated between the opened contacts of the circuit-breaker. This arc must be cooled and extinguished in a controlled way. Since the synchronous generator stator can flow via high asymmetrical short-circuit currents, which will not pass through zero (at least on one phase) many time periods after the fault appearance, the phenomena which occur in the case of short-circuit currents interruption determine the main stresses of the generator circuit-breaker; the current interruption requirements of a GCB are significantly higher than for the distribution network circuit breakers. Although the phenomena produced in the electric arc at the terminals of the circuit-breaker are complicated and not completely explained, the concept of exergy is useful in understanding the physical phenomena. The electric arc study can prove that the limits between the microscopic and macroscopic phenomena are fragile and certain phenomena could be studied in related frames of work. The electric arc that occurs during the interruption processes in a circuit-breaker can be studied as a very high temperature continuous plasma discharge, and thermodynamic parameters must be taken into consideration; alternatively it could be seen as an electric conductor by a resistance depending on the current intensity (under a constant low voltage) and studied within the Faraday's macroscopic theory. Electric arc interruption is of great importance, because an uncontrolled electrical arc in the apparatus could become destructive since, once initiated, an arc will draw more and more current from a fixed voltage supply until the apparatus is destroyed. However, the appearance of an electric arc at the terminals of the circuit-breaker should not be necessarily seen as a damaging phenomenon since if the electric arc would not appear the network embedded magnetic energy would be converted to electric energy, leading further to high over-voltages. Consequently, during the conversion process of the system magnetic energy in the arc thermal energy, the exergy is not destroyed, and it must be taken into consideration as embodied energy, used further on in the interrupting process. Just after the short-circuit current interruption by the generator circuit-breaker (when the GCB has been subjected to a 50,000 degree plasma arc), between its opened contacts arises the transient recovery voltage (TRV) which constitutes the most important dielectric stress after the electric arc extinction. If the rising rate of TRV exceeds the rising rate of dielectric strength across the open gap within the extinction chamber of the GCB, the electric arc will rekindle (re-strike) and this time the electric arc exergy will be entirely used in a mechanical destructive process determined by the electrodynamics forces. Since the magnitude and shape of the TRV occurring across the generator circuit-breaker are critical parameters in the recovering gap after the current zero, in this paper, we model, for the case of the faults fed by the main step-up transformer, the equivalent configurations, with operational impedances, for the TRV calculation, taking into account the main transformer parameters, on the basis of the symmetrical components method. This study focuses on this fault location because the transformer-fed-fault currents can be very high since the full energy of the power system feeds the faults.