Magne Runde

Continuous monitoring of circuit breakers using vibration analysis

This paper reports the results from three years of continuous condition monitoring of high-voltage circuit breakers (CBs) using vibration analysis. More than 1000 vibration patterns from the operating mechanism of three different spring-operated SF/sub 6/ breakers in normal service are analyzed. The patterns are recorded during each opening/closing operation and compared with references to identify timing and frequency deviations. Two of the three investigated CBs were operating well, while the third suffered from several severe problems. A developing failure that caused this breaker to not open on command was identified in advance. The reproducibility of the vibration patterns is high for a faultless CB and this makes it possible to separate faulty conditions from natural variations. The vibration analysis method is, however, dependent on a well-conditioned vibration pattern without noise and with distinct events. If the instrumentation is made sufficiently robust to withstand both the electrical disturbances in a substation environment and the mechanical shocks during CB operation, vibration analysis seems to be a promising technique that can be used on a continuous basis to detect malfunctions in the operating mechanism.

Application of high-T/sub c/ superconductors in aluminum electrolysis plants

The electric power system in an aluminum electrolysis plant has several features that may advocate use of superconducting technology: high power, low voltage, system compactness, direct current, and substantial conductor cost and losses per meter. Two case studies where the cost of conventional busbar conductors is compared with the predicted cost of a future nitrogen-cooled high-amperage superconductor based on Ag/BSCCO tapes are carried out. To be an attractive alternative the investment and capitalized operational costs of a superconducting line should not exceed USD 70 per meter per kiloampere rating. Simplified estimates of cost and properties of a superconducting cable show that these requirements are very hard to satisfy, even if long lengths of Ag/BSCCO tapes with operating current densities as high as 50 kA/cm/sup 2/ become available.<<ETX>>

Commercial Induction Heaters With High-Temperature Superconductor Coils

The induction heater for aluminum, copper and brass extrusion billets is an application where high-temperature superconductors (HTSs) provide clear benefits compared to conventional technology. With an energy efficiency of just some 50% for conventional technology and limitations in processing speed, the potential for improvements is evident. A novel heater design based on superconductors, and a fast product development have led to an industrial breakthrough. With one unit in operation since August 2008 and four more sold, the superconducting induction heater has become the first true commercial HTS product. Radical efficiency increases, improvements in process speed, quality and cost have been demonstrated in a rough industrial environment. In this paper we explain how to fully utilize the superconductor in this application by altering the established concepts of induction heating. The electromagnetic realization as well as cryogenic integration based on off-the-shelf cryo-coolers are described. Furthermore, operational experience from heating up 10,000 tons of aluminum (350,000 billets) in the first commercial installation is presented.

Thirty-Six Years of Service Experience with a National Population of Gas-Insulated Substations

This paper shows the result of a failure survey including the entire population of gas-insulated substations in the Norwegian power transmission system. The survey comprises the complete service experience up to now-nearly 12000 circuit-breaker years. Information about 180 failures has been recorded during 36 years: 77 internal flashovers/arcing faults have occurred, four of which burned through the encapsulation and another six caused the pressure-relief devices to operate. No severe injuries have been recorded. The overall failure rates and their causes are comparable to the findings of other reliability surveys, although the occurrence of severe arcing faults appears to be clearly higher than noticed elsewhere. The 300-kV and 420-kV gas-insulated substations put into service around 1980 have a significant and negative influence on the overall failure rate, whereas a large number of 145-kV substations installed since the early 1990s have proven very reliable. Even though a large portion of the population is above or approaching the designed service life of 30 years, no significant aging problems could be revealed with a basis in the failure statistics.

Design, building and testing of a 10 kW superconducting induction heater

Conventional 50/60 Hz induction heaters for aluminum billets have very large losses. By replacing the copper windings with windings of high-temperature superconducting (HTS) tapes, there is a substantial potential for efficiency improvements, especially if low AC loss HTS tapes become available. To examine the feasibility of using HTS in induction heaters, a first, small-scale working model has been designed and built. The induction coil is made of 24 double pancake coils of Bi-2223/Ag tapes. In the initial test, a workpiece of aluminum situated in the warm bore of the coil was heated up to 300/spl deg/C.

Efficiency analysis of a high-temperature superconducting induction heater

In an induction heater for heating of aluminum billets, the main reason for using high-temperature superconducting (HTS) tapes instead of conventional copper conductors is to reduce the large power losses in the induction coil. In this work we present how to calculate the different loss contributions of an induction heater based on HTS tapes. Calculations of these losses are used in the design of an HTS induction heater with a rated power of about 10 kW operating at liquid nitrogen temperature. The calculations predict an optimal current that yields the highest efficiency of the induction heater.

Comparative Study of Arc-Quenching Capabilities of Different Ablation Materials

Gases released from polymers placed near a burning electric arc are known to influence the behavior of the arc. In switching equipment, such ablation materials can be used to improve the interrupting capability. Current interruption experiments have been carried out with the purpose of comparing the arc-quenching capabilities of four common polymers. The test setup has static electrodes and two ablation polymer plates placed in parallel on both sides of the arc. Each ablation material is tested according to an “up-and-down” procedure that determines the current magnitude giving 50% probability for successful interruption. Current is supplied from a capacitor unit, and a sinusoidal waveform is created by means of a damped RLC circuit. The electrodes and the ablation materials are replaced after each interruption test. Polypropylene shows the best arc-quenching performance among the tested materials and interrupts about 2.7 times as high current as polyetetrafluoroethylene (PTFE), which is used as a reference material due to its low gassing. Polycarbonate and poly(methyl methacrylate) also greatly improve the current interrupting properties, interrupting a 2.2-2.3 times higher current compared to PTFE. As also pointed out by others, the arc-quenching capabilities of the polymers seem to increase with an increasing content of hydrogen in the ablation polymer.

Condition assessment of contacts in gas-insulated substations

A new method for assessing the condition of electric contacts and joints inside gas-insulated substations (GISs) that overcomes the limitations of conventional diagnostic methods is proposed. The contact resistance in degraded or overstressed electrical contacts varies slightly when currents of different amplitudes are passed. The diagnostic method is based on detecting this nonlinear behavior. For testing of the contacts in large GIS, excitation currents up to several thousand amperes are required. By using a capacitor-based current supply, such currents can be achieved with a portable source. The testing can be carried out without dismantling the GIS. No earlier measurements or manufacturer specifications are required. In full-scale testing under realistic operating conditions on 145-420 kV GIS using prototype instrumentation a very good sensitivity for detecting contact degradation in an early stage was obtained.

Failure Frequencies for High-Voltage Circuit Breakers, Disconnectors, Earthing Switches, Instrument Transformers, and Gas-Insulated Switchgear

Failure frequencies for circuit breakers (CBs) (only single pressure SF6), disconnectors and earthing switches, instrument transformers, and gas-insulated switchgear (GIS) have been determined through a comprehensive worldwide utility survey. For CBs and GIS, where comparable results from earlier surveys exist, significant reductions in failure frequencies are observed.

A Review of Results From Thermal Cycling Tests of Hydrogenerator Stator Windings

Thermal cycling of generators, occurring during intermittent operation of power stations, increases the thermomechanical stress on the stator groundwall insulation and can, therefore, contribute to failure of the generator. Thermal cycling stresses can, to some extent, be replicated and accelerated in laboratory tests. Such tests are performed to ascertain whether a new stator winding insulation system can handle intermittent operation, to evaluate the relative difference between different types of insulations or to investigate degradation processes and failure mechanisms. Results from thermal cycling tests of hydrogenerator stator windings are reviewed. Only stator windings of air-cooled hydrogenerators with thermoset groundwall insulation of mica-epoxy or mica-polyester are considered. The published literature of laboratory experiments performed in order to study effects of thermal cycling on groundwall insulation through different types of diagnostic measurements is described and discussed. Partial discharge (PD) and tanδ measurements gave, in some cases, an indication of a trend in the condition during thermal cycling, while in other cases it was difficult to interpret the test results. Many agree that measurement of breakdown voltage of stator windings can give an indication of the aging and remaining lifetime of the insulation. The ability of other measurements such as tan δ, PD, and voltage endurance to assess the remaining lifetime of the stator winding is less convincing since the results vary greatly.