following Figure (Figure 2 of the full paper) which associates the condition of the arresters (as characterized by the test failure rate) with years of service.
The full paper details the results of the electrical testing, the seal
tests and of the inspection of the internal components of the dismantled arresters. Comparisons are made with the results of similar investiga-
tions conducted in North America about fifteen years ago. The significant findings from the present investigations are summarized in the following Figure (Figure 2 of the full paper) which associates the condition of the arresters (as characterized by the test failure rate) with years of service. The electrical test results showed that after about 10 years of service, there is a marked upturn in the number of arresters with unsatisfactory insulation resistance, and after about 13 years of service, a marked upturn in the number of arresters with reduced power frequency sparkover level. Inspection of the internal components of dismantled arresters confirmed that the likelihood of significant degradation increased markedly with years of service, and was evident in almost 75 percent of arresters with 13 years or more of service. The authors therefore recommend that all gapped silicon carbide arresters with 13 or more years of service be progressively replaced by modem metal oxide arresters. Discussers: J.B. Posey; D. Roby; L. Peirrat, F. Perrot
96 WM 014-1 PWRD, T-PWRD October 1996 Impact of Shunt Capacitor Banks on Substation Surge Environment and Surge Arrester Applications Working Group 3.4.17 of the IEEE Surge Protective Devices
Committee Shunt capacitor banks are used on power transmission systems at all voltage levels, from distribution voltages up-to EHV, with bank sizes ranging from a few Mvar to more than 300 Mvar. The banks are usually installed at substations, wye-connected, with or without grounded neutrals, and connected to the station busbars through circuit breakers or circuit switchers. The intent of this paper is to provide an explanation of some basic overvoltage mechanisms associated with the installation and switching of shunt capacitor banks. It is written primarily for utility personnel to provide assistance in the selection and rating of surge arresters, when used in combination with shunt capacitor banks for overvoltage protection. Overvoltage protection should be considered wherever shunt capacitor banks are installed, regardless of voltage level, size, connection, or switching arrangement. The possibility of overvoltages due to lightning, switching surges, and temporary overvoltages requires a detailed evaluation to determine the duty on any surge arresters close to the capacitor bank. Shunt capacitor banks in shielded stations are exposed to lightning surges resulting from shielding failures or backflashovers on any connected transmission lines. The increase in capacitor bank voltage due to an incomning lightning surge depends on how much charge is absorbed. If the charge results in excessive overvoltages, surge arresters must be installed to discharge some energy and limit the overvoltage to. a safe level. Additional surge arresters, beyond those that already exist at a station may not be necessary at some installations, where existing surge arresters for the protection of other equipment, are rated for lightning surge discharge duty. A detailed study should be carried out to determine if the bank is adequately protected against lightning. Factors such as origin of the incoming surge, magnitude and waveshape, as well as capacitor bank size, configuration, and location should be included in the study. The switching of any shunt capacitor bank produces transient overvoltages. Certain switching operations can present some potentially hazardous overvoltage conditions, not only to the capacitor bank, but to other nearby equipment such as circuit breakers and transformers. Sqwitc-hing suirges- associate-d with the installation of shuint capacitor banks include the following: * Bank energization * Bank deenergization with restrike
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* Energization or deenergization combined with a single lineground fault * Voltage magnification. Transient overvoltages will always occur on "switching in" a capacitor bank, but will only occur on "switching out" a capacitor bank if restrikes occur in the switching device. Arresters installed in a substation to protect transformers and other equipment from overvoltages, can be subjected to severe energy absorption duty during capacitor switching, because of the large energy ('/2CV2) stored in the capacitor bank. The capability of all nearby installed surge arresters to withstand the energies dissipated during capacitor switching is, therefore, an important consideration. In particular, if some existing surge arresters include gapped silicon-carbide units, these units may have to be replaced for one of the following reasons: (1) the higher duty imposed by the addition of the shunt capacitor bank, (2) the sparkover level will cause them to operate on capacitor switching. Due to the frequent switching of shunt capacitor banks, there will be a significant increase in the number and magnitude of transient overvoltages on the power system. Shunt capacitor banks are normnally switched in during peak loading conditions and switched out during light loading conditions or high voltage. Overvoltage protection should be considered for the following conditions that may be associated with the installation of a shunt capacitor bank: * On the capacitor primary and backup switchgear to limit transient recovery voltages (TRV) when shunt capacitors are being switched out * At the end of transformer-terminated lines to limit phase-phase overvoltages resulting from capacitor switching or line switching in the presence of shunt capacitor banks * On transformers when energized in the presence of shunt capacitor banks * To control overvoltages due to resonance when switching shunt capacitor banks (a) in series or parallel with transformers, (b) in the presence of other capacitor banks. * Due to voltage magnification on an inductively coupled lower voltage system, which has shunt capacitors, when a capacitor is switched on the higher voltage system * On the neutrals of ungrounded shunt capacitor banks In addition to the contents of the paper, additional information can be found in application standards and other papers cited in a reference list of 30 entries. Discusser: J.E. Harder
Switchgear 96 WM 33&-4 PWRD, T-PWRD October 1996 Comparison of Synthetic Test Circuits for Ultra-High-Voltage Circuit Breakers B.L. Sheng (High Power Laboratory, KEMA, Arnhem, The Netherlands), L. van der Sluis (Senior Member, IEEE; Power Systems Laboratory, Delft University of Technology, Delft, The
Netherlands)
In order to meet the requirements from both designers and users, several synthetic test circuits have been developed to verify the breaking performance of ultra-high-voltage (765 kV) circuit breakers. Although some of them have already been used in the laboratory for lower voltage rating a detailed understanding of the validity and the applicability of these test circuits has yet not been published. Three synthetic test circuits dimensioned for a 765 kV, 63 kA, 50 Hz circuit breaker are compared. They are the Hitachi four-parameter synthetic test circuit, the EPIC synthetic test circuit and the series voltage injection synthetic test circuit. It was found the voltage injection synthetic test circuit has the lowest capacitive energy requirement, the Hitachi four-parameter synthetic test circuit gives the best four-parameter prospective TRV waveform and Power Engineering Review, October 1996 ~~~~~~~~~~~~~~~~~~~~~~~~~~IEEE
the EPIC synthetic test circuit has the best equivalence during the thermal interaction interval of the test breaker. Both the Hitachi circuit and the EPIC circuit are based on the parallel current injection method in order to keep the circuit parameters as close as possible to that of the direct test circuit. Both circuits have a modest capacitive energy requirement and match the TRV reference line of IEC-56 well. In spite of the complicated circuit lay-out, these circuits can be used to verify the interrupting performance of ultrahigh-voltage circuit breakers. The series voltage synthetic test circuit overstresses TB significantly during the thermal interaction interval. The voltage injection synthetic test circuit has a limit to perform the short line fault test duties because of the limited current source driving voltage. The performed computations reveal that the EPIC synthetic test circuit is better than the Hitachi four-parameter synthetic test circuit with respect to the equivalence in the thermal interaction interval and on the ability to adjust the dU/dt. A higher current injection frequency, an improved current waveform near zero and, the same surge impedance as the direct test circuit are the advantages of the EPIC synthetic test circuit.
96 WM 340-0 PWRD, T-PWRD October 1996 Vibration Analysis for Diagnostic Testing of Circuit Breakers M. Runde (Norwegian Electric Power Research Institute (EFI), Trondheim, Norway), G.E. Ottesen (SINTEF DELAB, Trondheim, Norway), B. Skyberg (Norwegian Grid Company, Oslo, Norway), M. Ohlen (Programma Electric AB, Taby,
Sweden) Presently, there is a clear tendency among electric utilities to shift from periodic to condition-based maintenance practices. Consequently, the demand for reliable and simple methods that make it possible to assess the need for invasive inspections and overhauls are increasing. Among the novel approaches that have been proposed, is to apply vibration analysis for diagnostic testing of high-voltage circuit-breakers. The mechanical vibrations from closing and opening operations are recorded by using accelerometers and a data acquisition system. These vibration "signatures" or "fingerprints" are compared with a reference, which can be an earlier recording from the same breaker or the signature from another of the same type. The basic idea is that mechanical malfunctions, excessive contact wear, misadjustments and other irregularities and faults can be detected as changes in the recorded vibration patters. The feasibility of vibration analysis for diagnostic testing of highvoltage circuit-breakers has been evaluated through a field test program comprising 31 breakers (93 identical single-phase units). The breakers were assumed to be in good condition with no known irregularities at the time of testing. Thus this is a true "blind test" carried out under realistic circumstances on circuit-breakers in normal service. Several serious faults, including an incipient rupture of the contact plug shaft, an incorrectly assembled crank, and major lubrication problems were disclosed.
96 WM 341-8 PWRD, T-PWRD October 1996 Hydrodynamic Model for Electrical Arc Modeling P. Chevrier (Centre de Recherches A2, Schneider Electric, Grenoble Cedex, France), M. Barrault (Centre de Recherches A2, Schneider Electric), C. Fievet (Centre de Recherches A2, Schneider Electric) In our thought process, the first aim of electrical arc modeling consists in gathering better understanding of switching arc phenomena, IEEE Power Engineering Review, October
1996
for instance, in determining the main physical phenomena during the interaction between the arc and the flow. The second step consists in building a software sufficiently sophisticated to be an useful tool for circuit breakers design. Various models combine Bernoulli flow and expansion by introducing variable boundaries between the cold flow and the hot gas. In our hydrodynamic model, the electrical arc and the ambient gas are considered as the same fluid. The electrical arc is defined as the conducting part of the hot fluid. For medium voltage applications and simulations in SF6 gas, we have already successfully compared numerical results obtained with different software. Our software has been developed, step by step, each step (cold compressible flow modeling, radiation modeling, real gas modeling, ...) being validated by comparisons between calculations and measurements. Industrial applications have already been computed. These applications concern high voltage (HV) circuit breakers, medium voltage (MV) circuit breakers and low voltage (LV) circuit breakers. The main difficulties that we have met are the following: * Firstly: we had to account for discontinuities such as pressure shocks or boundaries between the arc and the flow, these discontinuities move, and so, the numerical scheme must be well adapted to discontinuity modeling. * Secondly: the model has to account for all the dominant phenomena with the same degree of approximation, the level which has been chosen here is the energy level. Thus, the model does not account for every physical details but is a macroscopic model which we require to be energetically consistent with the physical phenomena. The following hypothesis were retained: * Local thermodynamical equilibrium (LTE) is assumed with a single temperature for all species. The electrical arc is taken as the volume where the gas is electrically conducting. * Dissociation and ionization are taken into account through the state laws. * Energy exchanges between the flow (or the arc) and the walls (or the electrodes) are taken into account with the help of an energy balance at the boundary. * Laplace (or Lorentz) forces, which are dominant for low voltage circuit breakers, are calculated using the Biot and Savart law. We have solved the compressible Navier Stokes equations in two dimensions (plane or axisymmetrical); special source terms are used to take into account radiation and Joule heating. The paper presents our approach to modelize the flow, the real gas, the electrical arc and the interactions between the electrical arc and the electrodes or the walls. In a first step, cold flows have been simulated (for temperature less than 1000 K). A brief analysis of flows in a circuit breaker shows that these flows are compressible and transient. Indeed, the pressure rise (or the energy rise) due to the presence of the electrical arc leads to observable shock waves and contact discontinuities. The switching arc phenomenon is, in all the applications we are interested in, a transient phenomenon. The simulations had to be done on complex geometries with, possibly, moving boundaries; these complex geometries result from the circuit-breaker's geometries whilst the moving boundaries are necessary to account for the motion of a piston or of electrical contacts. To solve numerically this system of equations, because of its excellent accuracy in presence of transient shocks, a finite volume Van Leer scheme is used. We have shown that this scheme could be used on unstructured meshes (with quadrangles, triangles or both) and with moving boundaries. In a second step, to account for dissociation and ionization of the gas (which could be SF6 or air) when the temperature is increasing (from 300 K to 30,000 K), we have used the values of transport coefficients and state variables for SF6 and N2 mixtures tabulated by Gleizes. The electrical arc is defined as the gas area which is electrically conducting. The energy source arises from Joule heating. This energy is then transferred by radiation, thermal conduction and convection. To determine the current density in the electrical arc, we solve the Maxwell equations under the assumption of quasi-steady state. The radiation model is built on the assumption of an optically thin plasma. 53~~5
