Abstract
Nowadays, the electric connection circuits of power plants (based on fossil fuels as well as renewable sources) entail generator circuit-breakers (GCBs) at the generator terminals, since the presence of that electric equipment offers many advantages related to the sustainability of a power plant. 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 highly asymmetrical short-circuit currents, 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. For shedding light on the proper moment when the generator circuit-breaker must operate, using the space phasor of the short-circuit currents, the time expression to the first zero passing of the short-circuit current is determined. Here, the manner is investigated in which various factors influence the delay of the zero passing of the short-circuit current. It is shown that the delay time is influenced by the synchronous machine parameters and by the load conditions which precede the short-circuit. Numerical simulations were conducted of the asymmetrical currents in the case of the sudden three-phase short circuit at the terminals of synchronous generators. Further in this study it is emphasized that 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 article points out that just after the short-circuit current interruption by the generator the circuit-breaker (when the GCB has been subjected at the metal contact terminals to the high temperature of a plasma arc, up to 50,000 K) between its opened contacts, there arises the transient recovery voltage (TRV) which constitutes the most important dielectric stress after the electric arc extinction. 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.
Highlights
The concepts of science, including concepts related to sustainability such as exergy and embodied energy, were developed to describe aspects of the universe
Since the magnitude and shape of the Transient Recovery Voltage (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
Since the synchronous generator stator can flow via high asymmetrical short-circuit currents, which will not pass through zero many cycles after the fault appearance, in this paper, for shedding light on the proper moment when the generator circuit-breaker must operate, using the space phasor of the short-circuit currents, the time expression to the first zero passing of the three-phase short-circuit current fed by the generator has been determined
Summary
The concepts of science, including concepts related to sustainability such as exergy and embodied energy, were developed to describe aspects of the universe. A representation of phase currents in the case of sudden short-circuit at the terminals of synchronous generator emphasizes that, regardless of the switching conditions, within each time cycle there are two zero passings of the short-circuit current [1,2]. Further we account for the hard situation, when α0 = −θ, corresponding to the maximum value of the aperiodic component of the short-circuit current in the phase A (see Equation (5)) This situation is related to the longest asymmetrical short-circuit current, with the maximum delay of the first zero passing. Taking into consideration that the delay time is influenced by the synchronous machine parameters and the load conditions which precede the short-circuit, several numerical simulations are presented of the asymmetrical currents performed for the case of the sudden three-phase short-circuit at the terminals of synchronous generators from Turceni-Romania electrical power plant. The corresponding increase of electric energy represents a useful component, meaning that exergy is not destroyed during the action of the voltage regulator
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