Transient recovery voltage analysis for various current breaking mathematical models: shunt reactor and capacitor bank de-energization study

ARCHIVES OF ELECTRICAL ENGINEERING VOL. 64(3), pp. 441-458 (2015) DOI 10.2478/aee-2015-0034 Transient recovery voltage analysis for various current breaking mathematical models: shunt reactor and capacitor bank de-energization study PIOTR ORAMUS, TOMASZ CHMIELEWSKI, TOMASZ KUCZEK, WOJCIECH PIASECKI, MARCIN SZEWCZYK ABB Corporate Research Center Starowiślna 13A, 31-038 Kraków, Poland e-mail: (Received: 29.09.2014, revised: 11.02.2015) Abstract: Electric arc is a complex phenomenon occurring during the current interruption process in the power system. Therefore performing digital simulations is often necessary to analyse transient conditions in power system during switching operations. This paper deals with the electric arc modelling and its implementation in simulation software for transient analyses during switching conditions in power system. Cassie, Cassie-Mayr as well as Schwarz-Avdonin equations describing the behaviour of the electric arc during the current interruption process have been implemented in EMTP-ATP simulation software and presented in this paper. The models developed have been used for transient simulations to analyse impact of the particular model and its parameters on Transient Recovery Voltage in different switching scenarios: during shunt reactor switching-off as well as during capacitor bank current switching-off. The selected simulation cases represent typical practical scenarios for inductive and capacitive currents breaking, respectively. Key words: arc modelling, overvoltages studies, EMTP-ATP program, transients analysis 1. Introduction Physical phenomena occurring upon current interruption are very complex. For this reason, it is still a challenge to use appropriate arc models for circuit breaker design and insulation coordination studies. The used model should represent non-linear behaviour of the circuit breaker electric arc as well as the interaction between the switching process and the system components. Moreover, since very small time constants are involved, a correct numerical treatment of the arc-circuit problem is an important aspect as well. Various interrupting media with different dielectric strength characteristics are applied in HV circuit breakers depending on particular design, application, price, etc. The dielectric strength determines the maximum electric stress that the dielectric can withstand without breakdown. It is a multi-variable function of the switching process and design of the particular switchgear component. Brought to you by | Biblioteka Glówna Zachodniopomorskiego Uniwersytetu Technologicznego w Szczecinie Authenticated Download Date | 2/5/16 12:41 PM 442 P. Oramus, T. Chmielewski, T. Kuczek, W. Piasecki, M. Szewczyk Arch. Elect. Eng. The general models of the electric arc, such as Mayr/Cassie, are often used in a modified form to reflect the arc voltage condition obtained in the measurement (e.g. the modified Mayr eq. [1]). The switching process can be measured in synthetic circuit, which allows to reproduce the realistic Transient Recovery Voltage (TRV), arc voltage and post-arc current conditions. The approach proposed by Mayr [2] is well suited for vicinity of current zero crossing. Not only the parameters, but the model itself can be modified according to the measured curves of the arc voltage and post-arc current. The example of the modified Mayr and Cassie equations is the Schwarz-Avdonin model [3]. The application of Mayr and Cassie models is recommended by IEC standards for insulation coordination studies to represent the physics of the circuit-breaker [4]. Cassie and Mayr models describe the evolution of the conductance (or resistance) of the electric arc during current interruption process, in accordance with the behaviour of the voltage and current appearing at the circuit breaker terminals. In insulation coordination analyses related to switching overvoltages, the arc equations are implemented in numerical software, such as EMTP-ATP [5]. This article contains the comparison of various electric arc models behaviour in selected simulation cases to investigate the impact of the electric arc model on the transient analysis results for the interruption process of the inductive and capacitive current breaking. The paper is based on this recommendation which is specifically applicable for simulations. Hence, the measurements of switching transients are not object of this analysis. Parameters of the models analysed in this paper are used here based on the research reported in e.g. [1, 3]. 2. Inductive and capacitive current switching The switching capabilities of high-voltage circuit breakers are defined by international standards that also describe the normative laboratory tests (using standard circuits) for inductive and capacitive current breaking [6, 7]. Along with capability of specific current breaking, the overvoltages generated during these operations are studied. The transient system response may be different for various configurations and topologies of the de-energized circuit. These aspects can be analysed through insulation coordination process, with the possible use of various simulation environments. The guidelines regarding modelling aspects as well as definition of the events that should be verified, are well established and described in [4, 8, 9]. In the routine switching study both fault clearing and load rejection scenarios should be considered. In this paper, switching of capacitive and inductive currents is analysed. These cases were chosen to analysis due to especially difficult conditions of current interruption process (the dielectric withstand of the contact gap can be exceeded just after arc quenching process, what can cause to generate re-ignitions, an in consequence an unsuccessful current breaking operation). According to [8] and [9], switching of capacitive currents involves energization or deenergization of unloaded cables, overhead lines and capacitor banks. For inductive currents, switching operations are typically related to motor starting as well as energization and deenergization of transformers and shunt reactors at no-load state or during inrush current flow. Brought to you by | Biblioteka Glówna Zachodniopomorskiego Uniwersytetu Technologicznego w Szczecinie Authenticated Download Date | 2/5/16 12:41 PM Vol. 64 (2015) TRV analysis for various current breaking mathematical models 443 Large amount of the energy accumulated in the system during the breaking process causes specifically demanding TRV conditions for the breaker due to possible re-ignition and voltage escalation. Moreover, breaking of inductive currents can lead to high frequency overvoltages if the current is chopped. At low currents (a few Amps) the arc becomes unstable, which may result in chopping before its natural zero crossing. This phenomenon may generate high transient overvoltage peak values due to the oscillations of the energy trapped in the oscillatory circuit, which is for (...truncated)