“Cryodielectrics", i.e. electrical insulation technology at cryogenic temperature, has been recognized as one of the fundamental and common technologies for the research and development of superconducting power apparatus. This article describes the importance, present status and future perspectives of cryodielectrics for superconducting power apparatus and systems in the next generation.
Recently customers who need electricity of higher quality have been installing co-generation facilities. They can avoid voltage sags and other distribution system related disturbances by supplying electricity to important load from their generators. For another example, FRIENDS, highly reliable distribution system using semiconductor switches or storage devices based on power electronics technology, is proposed. These examples illustrates that the request for high reliability in distribution system is increasing. In order to realize these systems, fast relaying algorithms are indispensable. The author proposes a new method of detecting magnetizing inrush current using discrete wavelet transform (DWT). DWT provides the function of detecting discontinuity of current waveform. Inrush current occurs when transformer core becomes saturated. The proposed method detects spikes of DWT components derived from the discontinuity of the current waveform at both the beginning and the end of inrush current. Wavelet thresholding, one of the wavelet-based statistical modeling, was applied to detect the DWT component spikes. The proposed method is verified using experimental data using single-phase transformer and the proposed method is proved to be effective.
Renewable energies such as wind power or photovoltaic energy are environmentally focused but the fluctuation of output power of renewable energies may cause excess variation of voltage or frequency of the grid. Increase of the amount of renewable energies would violate the quality of the grid. The micro grid in which dispersed energies compensate the variation from the renewable energies can expand the installation limit of the renewable energies by maintaining the quality of the interconnected grid. In this paper how the gas turbines absorb the power variation from the wind generation and the loads is discussed. In order to control the output power, gas turbines must run at partial load operation, which results in lower efficiency. Considering the efficiency of the micro grid, installed capacity of wind power generation system, and frequency stability for islanded operation, performs the evaluation of the micro grid.
This paper proposes a hybrid method of hierarchical optimization and Parallel Tabu Search (PTS) for distribution system service restoration with distributed generators. The objective is to evaluate the optimal route to recover the service. The improvement of power quality makes the service restoration more important. Distribution system service restoration is one of complicated combinational optimization problems that are expressed as nonlinear mixed integer programming. In this paper, an efficient method is proposed to restore the service in a hierarchical optimization with Parallel Tabu Search. The proposed method is tested in a sample system.
A new risk assessment method for short-term load forecasting is proposed. The proposed method makes use of an Artificial Neural Network (ANN) to forecast one-step ahead daily maximum loads and evaluate uncertainty of in load forecasting. As ANN the model, the Radial Basis Function (RBF) network is employed to forecast loads due to the good performance. Sufficient realistic pseudo-scenarios are required to carry out quantitative risk analysis. The multivariate normal distribution with the correlation between input variables is used to give more realistic results to ANN. In addition, the method of Moment Matching is used to improve the accuracy of the multivariate normal distribution. The Peak Over Threshold (POT) approach is used to evaluate risk that exceeds the upper bounds of generation capacity. The proposed method is successfully applied to real data of daily maximum load forecasting.
Traditionally, thermal current rating of transmission line is considered as a constant. It is defined considering the most severe weather condition of the season. Actual weather condition, however, changes from time to time. Therefore, actual thermal rating should be changed from time to time. It leads to the concept of “dynamic rating". Short term rating of the transmission line may be changed dynamically from time to time according to the power flow and weather condition in the concept of dynamic rating. Detailed calculation method of dynamic rating is discussed in this paper. Furthermore, new subtransmission system operation method based on the dynamic rating of transmission lines is proposed in this study. The effects of the proposed operation on supply reliability and transmission line life are also evaluated.
This paper has investigated induced voltages characteristics from an overhead transmission line to a buried gas pipeline by applying a modeling method of the induced voltages proposed by the authors. EMTP simulation results agree with analytical results obtained from a well-known formula. The induced voltages are significantly dependent on the configuration of an overhead line. A horizontal line induces the largest voltage to the gas pipeline, and an induced voltage by a vertical twin-circuit line is smaller by about 20% than that by a vertical single-circuit line.
ZCT (Zero-phase-sequence Current Transformer) is used widely for the detection of the ground fault current in low or high voltage distribution power line. Especially, the use of pole air switches equipped with ZCT is increasing to prevent the extended accident caused by the ground fault. The accurate detection of low level ground fault is needed from the view of early stage discovery. The internal residual current of ZCT must be small as possible for the detection of low level ground fault. Generally, the magnetic shield around the core of ZCT is used to reduce the internal residual current. Also, the core material and shield effect is related to the reduction of the internal residual current. In this paper, the reducing method of the internal residual current is proposed by using new winding method of the core. In new winding method, the secondary and tertiary windings are provided, and each winding is divided into two or three sections with parallel connection. The leakage magnetic flux caused by the primary current is changed to the cross current in parallel winding. This cross current is used to offset the leakage magnetic flux caused by the primary current. It is observed by the examination that the cross current is effective to reduce the internal residual current without influence to the zero phase current detection.
A new type of high-speed streak camera was developed to observe the process of lightning discharge. The camera consists of a rotating quadrate pyramid mirror and a digital video camera. Using the camera we can observe the lightning discharge in all horizontal direction and continuous observation is available without using high-speed shutter synchronizing with lightning discharge. The visual range, the time resolution and the dead time of the camera were evaluated theoretically and experimentally. From the evaluation we found that the time resolution of less than ten μs is expected at maximum rotating speed of the mirror of 200s-1. The percentage of dead time was evaluated to be less than 50%. By using the camera, we have successfully observed the lightning discharges and from the observation we have evaluated the shape, and the duration of each lightning discharge.
Wind turbine generation systems are built at locations where few tall structures are found nearby so as to obtain good wind conditions, and thus, they are often struck by lightning. To promote wind power generation, lightning-protection methodologies for such wind turbine generation systems have to be established. This paper presents the result of an experimental study of lightning overvoltages in wind turbine generation systems using a reduced-size wind turbine model. Overvoltages observed at wavefronts of lightning surges are focused on in this study. In the experiments, lightning strokes to one of the blades and to the nacelle were considered, and voltages and currents at various positions of the wind turbine model were measured. The following points have been deduced from the results: (i) The voltage rise due to the tower footing resistance can cause a significant voltage difference between the tower foot and an incoming conductor led from a distant point. Also, a voltage difference between the bottom of down conductors installed inside the tower and an incoming conductor can be of significance. (ii) The lightning current flowing through the tower body induces voltages in main and control circuits which form loops, and the induced voltages can cause overvoltages and malfunctions. (iii) Traveling-wave phenomena in a wind turbine generation system for a lightning strike to the tip of a blade and to the nacelle have been made clear from the measured waveforms. This information can be used for developing an EMTP simulation model of wind turbine generation systems.
Clarification of metallic particle motion is important to attain high reliability of gas insulated switchgear (GIS). Existing calculation methods have been developed only for AC voltage, and have been not applicable for DC voltage. Because partial discharges at particles in gas gap have a great influence on particle motion under DC voltage and these existing calculation methods have not considered the partial discharges. Partial discharges of particle were investigated in order to adopt them into the calculation model. The calculation model considers the following phenomena: (1) moment of the particle around the center of gravity, (2) electric charge and discharge at both pointed tips of the particle, (3) mirror image of the particle near the wall. The model has been applied to DC electric field conditions, and was able to correctly calculate the crossing motion or firefly phenomenon of the metallic particle.
This paper provides the theoretical calculation results of thermodynamic and transport properties of CO2, CO2-O2 mixture, CO2-H2 mixture under thermal equilibrium condition at temperatures of 300-30,000K and at pressures of 0.1-10MPa. The gas CO2 is one of the candidates for the environmentally-benign arc-quenching medium in a circuit breaker. Furthermore, effect of addtional gases O2 and H2 on the thermodynamic and transport properties of CO2 was also invesitigated in this paper. The hydrogen atom included CO2 is similar to the polymer ablated vapor in a switching devices. First, equilibrium compositions of CO2, CO2-O2 mixture, CO2-H2 mixture were calculated through Gibbs' free energy minimization method. Secondly, thermodynamic properties were computed using the calculated composition. Finally, transport properties were calculated by the first order approximation of Chapman-Enskog method using the collision integrals between species. Inclusion of H2 increases the electrical conductivity of CO2 in range 3,000-6,000K because CHO molecules produced in this temperature range emit more electron due to the lower ionization potential of CHO. It also increases the thermal conductivity of CO2 especially due to dissociation reactions of H2 around 3,900K and ionization of H around 15,000K. These properties data provided here can be used for CO2 thermal plasma simulation.
Since SF6 gas was identified as a greenhouse gas at COP3 in 1997, alternative insulation gases to SF6 have been investigated. The CO2 gas insulation is one of the candidates, which is a natural gas and has lower global warming potential (GWP). However, CO2 gas has a lower withstand voltage level than SF6; therefore, it is important to rationalize the equipment insulation level and reduce the test voltage of electric power equipment as low as possible. The actual lightning surge waveform (called non-standard lightning impulse waveform) in actual substations is different from the standard lightning impulse waveform (1.2/50μs). The actual lightning surge waveform generally rises steeply, however, the decay of the overvoltage is large, and the insulation requirements are not as severe as those of the standard lightning impulse waveform. This paper describes the insulation characteristics of CO2 gas for single-frequency oscillatory waveforms with various frequencies from 1.3 to 4.0MHz and damping ratios. From experimental investigation, similarly to SF6, it might be possible to reduce the test voltage of CO2 gas insulation system by evaluating actual lightning surge waveform in terms of the equivalent standard lightning impulse waveform.
Probability distribution of surge discharging current of arresters provided a basis for conventional theory of grounding systems. In order to rationalize the grounding systems, it is necessary to grasp the statistical data of lightning surges on distribution lines caused by direct lightning strokes and indirect lightning strokes. Lightning phenomena on TEPCO's distribution lines had been continuously observed for the rationalization of lightning protection design of distribution lines. The observation had been carried out with still cameras and monitoring sensors of lightning surges. This makes it possible to find out new interesting facts that can be useful for rationalization of lightning protection design of distribution lines. Cumulative frequency distribution of conventional data is close to that of ZnO discharging current in case of direct strokes and indirect strokes through TEPCO's observation. Moreover, to verify the cumulative current distribution in concrete poles, the authors have compared the cumulative distribution of current through ground lines with that of current through ground lines and concrete poles. The result shows that the distribution of current through ground lines and concrete poles is larger than that of current through only ground lines for high currents exceeding 1kA. This fact suggests that lightning surge current flows not only in ground lines but also in concrete poles.
Surge arresters have contributed to supply of electric power by suppressing lightning surge on transmission lines in case of lightning phenomena. Surge arresters using zinc oxide (ZnO) elements are suitable for insulation coordination, and are enable to reduce LIWV (Lightning Impulse Withstand Voltage) and construction cost of power systems. This paper describes applications and results of surge arresters to verify effectiveness of surge protection for 66-500kV power systems in Japan.
The authors have developed a method of analysis of induced voltages where the Agrawal's model is adopted as a field-to-line coupling model. In this letter, the sparkover rate of a power distribution line associated with indirect hits is investigated based on numerical calculations taking account of statistics on return-stroke current. It turns out that the sparkover rate associated with a subsequent stroke is higher than that associated with a first stroke when the line is equipped with surge arresters.