Mirsad Kapetanović

Computer simulation of high-voltage SF 6 circuit breakers: approach to modeling and application results

Nowadays, computer simulation of processes during current interruption is widely used in design optimization of HV circuit breakers. A simplified physical enthalpy flow arc models provide possibility to get relatively simple, and easy to use, simulation software for HV SF6 circuit breakers. Such software has been developed and successfully used in development of various SF6 circuit breakers, based on classical puffer and puffer assisted self-compensated principle. General outline of the computer program, as well as some simulation results and their experimental verification obtained on a real 245 kV SF6 circuit breaker, are presented. The interrupter is based on puffer assisted selfcompensated principle where arc energy is used to partly compensate compression forces during high current interruptions. It additionally uses a double speed contact stroke created by a simple mechanism placed below the moving contact.

Linking a physical arc model with a black box arc model and verification

The arc behavior in the current zero region is critical in the case of very steep rising TRV, such as after clearing a short-line fault. Therefore, intensive and abundant short-line fault tests (L90) of a 245 kV SF6 circuit breaker were performed at the KEMA High Power Laboratory. For the purpose of a comparative analysis three different sets of data were obtained during the tests: 1) High-resolution measurements of near current-zero arc current and voltage were carried out. The current zero measurement system (CZM) works as a standalone system in addition to the standard laboratory data acquisition system. The arc conductance shortly before current zero and the arc voltage extinction peak give a clear indication of the interrupting capability of the breaker under test. 2) From the measured traces of every individual test, arc parameters (3 time constants and 3 cooling-power constants) were extracted for the composite black box arc model, which has been developed by KEMA High Power Laboratory and is based on more than 1000 high-resolution measurements during tests of commercial high-voltage circuit breakers. Its aim is to simulate interruption phenomenon in SF6 gas, evaluate performance of HV SF6 circuit breakers in testing and enable the prediction of the performance under conditions other than those tested. 3) After each test, using specially developed computer software, based on a simplified physical enthalpy flow arc model, the values of the arcing contact distance, gas mass flow through the nozzle throat and pressure inside the compression cylinder were calculated. The values of these characteristic quantities at the current zero are relevant indicators for successful interruption. In the comparative analysis, mathematical relations and statistical correlations between the evaluated parameters of the composite black box arc model and the characteristic output quantities are established and discussed. The link has been verified by MatLAB simulation of every individual test. This approach enables acceptable prediction of interruption success in a similar circuit and with a similar interrupter without SLF tests and CZM.

Nozzle ablation model: Calculation of nozzle ablation intensity and its influence on state of SF 6 gas in thermal chamber

Energy released by electric arc during short circuit switching is mostly absorbed by the surrounding cold SF6 gas. However, a considerable part of this energy is also transferred and absorbed by other elements of the circuit breaker interrupter which are located near the electric arc. The most important parts are the transfer of energy to the arcing contacts and to the nozzles, absorption of the energy by these elements and the resulting effects. The absorption of the energy causes heating, melting and finally the vaporization of structural material and it is the main cause of wearing of arcing contacts and nozzles, where the latter is commonly referred to as the nozzle ablation. The nozzle ablation causes an increase in the nozzle throat diameter which generally has a negative effect on the circuit breakers breaking performance. The other significant effect is the mixing of SF6 gas and the nozzle vaporized material in the nozzle space and in the surrounding chambers. It is obvious that the ablation process has a considerable influence on the state of SF6 gas in the contact gap but also in the adjacent interrupting chambers, in particular on the state of gas in the thermal chamber in case of self-blast interrupting units. In this paper, a method of calculation of intensity of nozzle ablation is presented as well as a variety of calculation results. The calculated nozzle ablation intensity is verified by comparing the calculated results of the nozzle diameter increase and mass losses, with experimentally obtained data. In addition to the nozzle ablation intensity, the influence of the ablated nozzle material on the state of SF6 gas in the thermal chamber is also analyzed and discussed. The model is incorporated into a computer application for high voltage circuit breaker interruption simulation.

Design Improvement of a 245-kV

This paper presents the results of current zero measurements during short-line fault interruption tests performed on three variants of an SF6 circuit breaker (CB) (245 kV, 40 kA) with a new mechanism for increasing the contact motion speed, shortly named double-speed mechanism, in order to distinguish between double-motion systems where both contacts are moving. The application of a double-speed mechanism provides the necessary increase of contact separation speed, without a significant increase of opening energy. Besides that, it does not requires any fixed mechanical connection between the stationary and moving contacts through the nozzle. This feature has a positive impact on the CB reliability and creates the possibility of easier assembly and dismantling of the interrupter from its insulator. High-resolution measurements of near current-zero arc current and voltage were carried out during these tests. Different levels of information on the “quality of interruption,” obtained from current zero measurements are presented. Direct observation of arc current and arc voltage data are analyzed. The arc conductivity very shortly (500 and 200 ns) before current zero, as an indicator of the performance of the breaker under test is discussed. All information obtained during current zero measurement is in correlation with the direct results of testing and with design improvements in successive variants of the CB.

{\rm SF}_{6}

The interruption of capacitive currents is a very common switching case, unlike the breaking of short-circuit currents. The current involved is quite small and therefore easy to interrupt. However, a recovery voltage higher than 2 per unit appears across the contacts half a cycle after current zero. This circumstance inevitably increases the risk of restrikes, which may have rather undesirable effects, such as voltage escalation. Combined electric field and flow field simulations are widely used as a tool for evaluation of the dielectric performance of high voltage SF6 circuit breakers in these conditions. The computational fluid dynamics (CFD) simulation starts from the completely closed contact positions and, therefore, it takes CPU time of several hours running a workstation. In this paper, in order to shorten the calculation time to several minutes, a new numerical method for calculating the coupled fields has been developed. The new method uses simulation software for HV SF6 circuit breakers based on an integral-physical enthalpy flow arc model to obtain boundary conditions for CFD simulation. This way, a higher precision CFD simulation combined with electric field calculations can start at any given moment during recovery voltage after the current reaches zero and cover only a short interval of time proximate to an increased risk of restrikes. The new approach additionally provides a full correlation between all design parameters of the circuit breaker (including its operating mechanism) and estimated breakdown voltage during the interruption of capacitive currents.

Circuit Breaker With Double-Speed Mechanism Through Current Zero Analysis

A considerable part of the energy released by electric arc during breaking of a short-circuit current is being absorbed in the moving and stationary arcing contacts. The rest of the energy is being released as thermal stresses of the nozzle and of other parts of the arcing chamber, as well as heating, dissociation and ionization of the arc extinction medium, and in losses to the nearby environment. The absorption of the energy causes heating, melting and finally the vaporization of contact material and it is the main cause of contact erosion. It is obvious that the process of arcing contact erosion has a considerable influence on the state of SF6 gas in the contact gap, but also in the adjacent interrupting chambers. A contact erosion model is incorporated into a computer program for high voltage circuit breaker interruption simulation. The calculated contact erosion intensity is verified by comparing the calculated change in the contact shape and mass losses with experimentally obtained data. The influence of vaporized contact material on the state of SF6 gas in the interrupter chambers and the dielectric performance of the contact gap is also analyzed and discussed.

New approach to breakdown voltage estimation after interruption of capacitive currents

High voltage SF6 circuit breaker simulations based on an integral-physical enthalpy flow arc model are very powerful tool in designing and optimization of high voltage SF6 circuit breakers. This is especially important in case when full correlation between the electric arc, interrupter unit and operating mechanism parameters is needed. This paper presents computer simulations of an improved operating mechanism proven on a 123/145kV 31.5/40 kA GIS circuit breaker and verified using experimentally obtained data. The optimum value of the opening energy for rated short-circuit current interruption of the analyzed circuit breaker is estimated by using an improved operating mechanism model. In addition, several analyses have been made in order to investigate how some interrupter parameters affect the contact travel and breaking performance.

Influence of contact erosion on the state of SF6 gas in interrupter chambers of HV SF6 circuit breakers

Partial discharges, as their name states, only partially bridge a small portion of electrical insulation in the form of a tiny electrical arcs, which burn inside the defects that could appear in insulation system. Because of the fact that extruded cable system insulation is very sensitive on partial discharge activities detection vise, partial discharge measurements could be used as a powerful diagnostic tool in evaluating the actual condition of cable system through measuring procedures during after laying tests. If such procedures would be included in standards, they would provide an effective way to identify and detect the defects that might appear during the cable system installation and to forestall their appearance during exploitation, ultimately reducing the probability of failure. Very first aim of this paper is to address some shortcomings of current IEC standards related to analyses of cable systems with polymer insulation (IEC 60840 and IEC 62067). In order to justify these statements, a review of a recent alignment between IEC 60840 and IEC 62067, simulation support, using the contemporary software tool (COMSOL Mph), backed up with experimental results for two artificially induced defects in cable accessories, are provided in this paper.

Simulations of an improved operating mechanism for high voltage SF6 GIS circuit breaker

High reliability of HV SF6 switchgear makes an internal arc fault an extremely rare event. However, its occurrence cannot be completely avoided, and therefore must be considered in the design process. Internal arc testing in SF6 is not recommended due to its harmful environmental impact, but if necessary, tests should be performed only inside special containers, that will prevent the release of SF6 into the atmosphere. Having in mind that tests in SF6 and air are not yet fully comparable, accurate modeling of pressure rise due to internal arc faults is still the main means to evaluate required design parameters of SF6 switchgear in respect of safety from internal arc faults. A simulation tool, which calculates the pressure rise due to an internal arc inside a metal-enclosed SF6 compartment, was developed and used in the design of a new HV GIS. The calculation procedure and obtained results were described and discussed. Validation of the tool was performed using experimental data from SF6 internal arc tests, dating back several decades ago, when internal arc tests in SF6 were not questionable as today.

Partial discharge measurements and IEC standards: Justification of the use for their inclusion in afterlaying test for extruded cable systems

In this paper, the impact of several 145 kV 40 kA GIS (Gas Insulated Switchgear) circuit breaker design parameters, such as length of the main nozzle throat, contact travel and average opening speed, on the estimated breakdown voltage of the contact gap of an interrupter during capacitive current breaking is analyzed. A new approach for breakdown voltage estimation, which provides full correlation between all circuit breaker design parameters and dielectric characteristics of the contact gap and the criterion for breakdown occurrence, was used for this purpose. The estimated capacitive current breaking capability of the circuit breaker was verified by type testing in KEMA High Power Laboratory, the Netherlands.