This paper explains and discusses the current situation and future trend of the intelligent system applications in the field of electric power systems. Papers from various fields are surveyed and classified which include (1) novel optimization approach, (2) consideration of multi-objective and/or realistic constraints, and (3) human-friendly representation of the knowledge and the expertise. The authors sincerely hope that this small article helps the interested readers studying the related topics.
The theories of operation of existing relays are roughly divided into two types: one is the current differential types based on Kirchhoff's first law and the other is impedance types based on second law. We can apply the Kirchhoff's laws to strictly formulate fault phenomena, so the circuit equations are represented non linear simultaneous equations with variables fault point k and fault resistance Rf. This method has next two defect. 1) heavy computational burden for the iterative calculation on N-R method, 2) relay operator can not easily understand principle of numerical matrix operation. The new protection relay principles we proposed this paper focuses on the fact that the reactance component on fault point is almost zero. Two reactance Xf(S), Xf(R) on branch both ends are calculated by operation of solving linear equations. If signs of Xf(S) and Xf(R) are not same, it can be judged that the fault point exist in the branch. This reactance Xf corresponds to difference of branch reactance between actual fault point and imaginaly fault point. And so relay engineer can to understand fault location by concept of “distance". The simulation results using this new method indicates the highly precise estimation of fault locations compared with the inspected fault locations on operating transmission lines.
This paper describes the development of the new islanding preventive device using interharmonic current injection method for 22kV or 33kV electric power system interconnection and the demonstration test results at Akagi Test Center of CRIEPI. The islanding preventive device using interharmonic current injection method is limited for 6.6kV electric power system interconnection use, because the impedance of a 22kV or 33kV electric power system is too small for the device to detect the islanding phenomena of the 22kV or 33kV electric power system. We have developed the new islanding phenomena detection methods. These methods are composed of the noise elimination technique using digital filter and the small-sized technique using more than one interharmonic.
Effects of the fluctuation inherent in wind speed are studied by a probabilistic method. The random variation in wind speed makes random behavior in output power and internal voltage of a wind power generator. In case of fault occurrence at the instant of high internal voltage the resultant short circuit current will be big, and vice versa. The dc component is also affected. According to the study, 2.4% and 1.3% increase of short circuit current in ac and dc components are observed respectively in a large variation case. This implies that the wind speed variation should be considered for accurate short circuit study.
Wind power, a renewable and environmentally friendly energy source, is becoming increasingly important and is now considered to be a viable source of alternative energy. In recent years, many large-scale wind farms have been incorporated into power grids. However, one of the biggest problems of wind power is that output cannot be fully controlled due to the output fluctuations of the wind caused by wind gusts. With large-scale integration of wind power into a power grid, the variation of wind power may cause some problems, e.g., voltage instability and frequency control problems, which should be resolved adequately to avoid negative effects on the existing power grid. In this work, the improving-effects such as power leveling and voltage fluctuation control by one of the energy storage systems—EDLC (Electric Double Layer Capacitor) on the properties of wind power generation systems is investigated, and frequency characteristics and transient stability in these cases are verified. The results illustrate the fact that a properly controlled EDLC may bring about good features to wind power generation systems.
The largest output fluctuation is an index used to quantify disturbance against a power grid caused by wind power plants and photovoltaic power generation systems connected to it. To develop its estimation method, we investigate the relationship between the largest output fluctuation and the standard deviation of a newly proposed stochastic variable derived from the output variation of photovoltaic power generation systems. Output fluctuation coefficients are defined and estimated using the measured data of photovoltaic power generation systems located at 52 places. An approximation formula is presented to predict correlation factors between the stochastic variables corresponding to photovoltaic power generation systems using distances between their locations. Finally it is shown that the largest output fluctuation of many photovoltaic power generation systems dispersed in a wide area is predictable by using the output fluctuation coefficients and the formula to approximate the correlation factors.
From the viewpoint of reducing the cost of electric transport, a pole transformer has been operated, for a trial, under a short-time overloaded condition. It is not clear how long it takes a pole transformer to deteriorate under an overloaded condition, therefore, a pole transformer lifetime evaluation method is necessary. To realize this method, we developed a transient temperature calculation method from the load pattern and temperature of the outer section of the temperature, and evaluated its accuracy by comparing measurement results and calculations. We developed an estimation method of the degree of polymerization of insulating paper at varying temperature from the measurement result at various temperatures and times in the pole transformer atmosphere.
Transmission line must be immediately inspected when it was struck by lightning. It is necessary to find and change the damaged part promptly in case the damage is serious. Such case, however, is actually hardly seen due to the fact that the lightning protection system is well designed recently. Therefore, it is important to compare the data of past lightnings and of the transmission line, and make criteria for deciding more precisely in which case the actual inspection should be carried out. The criteria would be as follows: (1) Insulator voltage exceeds critical voltage. (2) Arc current and arc duration exceed characteristic of insulator destruction. (3) Accident phase exceeds 3 line ground fault. (4) Tower foot resistance is up to 10ohm or more. (5) Lightning current is positive characteristic. By using this criteria, the number of lightning fault inspection was reduced by 34%.
This paper describes the high voltage etching fuse for protecting semiconductors. At first the performance required to the fuse was investigated. Then test results of trial products of 7.2kV rating etching fuses were shown. At present the specified national standard for high voltage fuse for semiconductor protection has not been established. Therefore, firstly we confirmed the general characteristics of high voltage fuses in accordance with JEM standard for the low-voltage fuses for the protection of semiconductor device. Next we investgated the breaking performance on the basis of the following 3 items, since the fuse applied to inverters must have special performances in addition to general characteristics. (a) Possibility of endurance against maximum voltage occurring after current interruption, (b) Possibility of breaking current having the high di/dt value due to being discharged from smoothing capacitor in inverter circuit, and (c) Mechanical strength against fatigue by thermal expansion and shrinking due to repetition of high current flow. It was found that an etching fuse have higher current breaking performance and mechanical strength. Current breaking test results exhibited that a 7.2kV, 100A rating fuse had small operating I2t value, that is excellent current breaking performance, and cleared above three points.
Lightning surges propagate into a house mainly from four directions, power distribution lines, communication lines, antenna cables and grounding lines. However lightning performance of indoor wires has not been cleared yet because of the complexity of the mechanism. Moreover, lightning protection devices such as SPDs are available, but they still are not popular because the effects of them have not been cleared. In this study, experiments were conducted with a full-scale system. In order to clarify the effects of the SPD against lightning surges from several directions, applied current for each case kept a certain level. Propagations of surge currents and over-voltages were observed in various conditions. The results confirmed effectiveness of the SPD in the switchboard against the surges from the various directions.