In the future electric power systems, it will be very important to utilize the exsting AC networks more effectively with the help of power electronics technology. It has been made it clear that various apparatus utilizing power electronics technologies such as variable series capactor (VSrC) and high speed phase shifter (HSPS) can improve transient stability and damping of one machine and infinite bus power system. This paper presents a novel control scheme for variable impedance apparatus such as VSrC and HSPS in multi-machine power systems. First, this paper describes a comprehensive approach for control design of VSrC and HSPS. The proposed control scheme is based on an energy function of multi-machine power systems. The controllers are designed so that the time derivative of the energy function is smaller in the negative value than that without controllers. In this sense, the present method assures the improvement of first swing stability and damping. Next, the proposed control scheme is applied to VSrC. Digital simulations and eigenvalue analysis are conducted for a three-machine loop system and five-machine radial system to demonstrate the effectiveness of the proposed method. As a result, it is made clear that the proposed controllers for VSrC can improve both transient stability and steady-state stability of power systems significantly.
A method for on line several-hours-ahead prediction of inflow rate into a dam was developed based on rainfall information. We analyzed actual data of inflow rate and rainfall during past several years at Midorikawa dam. Based on the analysis, we derived the inflow rate model and the stational rainfall model which expressed appropriately the relationship between the inflow rate and the rainfall at the dam area. By using a recursive least-squares estimation of these model parameters, we obtained the on line several-hours-ahead prediction of inflow rate. The results were evaluated based on the actual data in comparison with other three methods. We can conclude that the proposed method gives an accurate prediction of inflow rate by a simple algorithm, adapts to slow changes of characteristics of the inflow rate and the rainfall, because the models are updated on line. The proposed method will be widely accepted in actual fields as an accurate and simple method for predicting inflow rate into dam.
This paper presents an efficient computational algorithm for selecting the optimal generation mix under uncertain circumstances. Subjective, experiential or linguistic uncertainties are selected from among various uncertainties; i.e., we treat a fuzziness in generation expansion planning. The fuzziness can be divided into (1) the fuzziness of decision making and (2) the fuzziness of some planning parameters, such as load growth, fuel price and so on. Both classes of fuzziness are integrated into a fuzzy decision based on fuzzy sets theory, and then the optimal generation mix can be determined by the Fuzzy Dynamic Programming (FDP) technique. The proposed method, which is based on the dynamic programming technique, is extended by using the Bellman-Zadeh maximizing decision. In the method, each generation technology and generation capacity are selected as a stage and state, respectively. The proposed method can easily accommodate not only the fuzziness but also many constraints of generation expansion planning, such as integer solutions of unit capacities, condition of existing units and so on. Furthermore, arbitrary shape of membership function can be used. The effectiveness and feasibility of the proposed method are demonstrated on a typical power system model.
Recently, power demand by air-conditioners is rapidly increasing, and it occupies about 40% of summer peak load. It is guessed that air-conditioners have considerable effects on power system load characteristics. This paper presents a load model for a PWM-inverter air-conditioner. The air-conditioner converts AC into DC, and then inverts DC to three-phase AC. It adjusts thermal output by changing the output frequency of the inverter. The voltage characteritic is basically constant-power. However, if the current reaches a ceiling value, the frequency of the inverter is lowered. The voltage characteristic shifts to constant-current after this. The response to a sudden voltage change consists of a quick electrical oscillation caused by the rectifier, and a slow response due to frequency control of the inverter. The response considerably changes with the operating state. In modeling the air-conditioner, the rectifier is represented in detail, but simple models are used for the inverter and the motor. It is shown that this model accurately simulates the response of the air-conditioner.
As concerns fault condition in dynamic stability study, balanced faults have been mainly utilized. In recent years, however, with the advance of system protective control technologies such as multi-phase reclosing system for faulted transmission line, the needs for stability software which would be able to calculate simultaneous faults without any restrictions on fault types, total numbers or location, has been increasing. This report presents the newly developed multi-faults calculation method based on symmetrical components which is able to calculate simultaneous faults without any restrictions on combination of fault types, total numbers or location. Fault types consist of ballanced/unballanced ground fault, short-circuit fault, line-out of transmission lines, and short-circuit of series condenser. Proposed method also allows for calculation of faults at a arbitrary location on the line without any topological changes of each symmetrical network even if the faults include line-end faults with ensuring outages. An example was shown to clarify the validity of proposed method. Moreover, transient stability limits under the various faults conditions were also discussed.
A superconducting multistranded cable is used to realize high current capacity for AC use. The critical current value of the cable is reported to be less than the simple summation of individual critical current value of each strand. The causes for such a degradation of the critical current value have not been revealed. This paper investigates the current distribution in multistrands before and after their quenching by using 7-strand superconducting cable and 7×7 cable. The following experimental results are derived: (1) The quenching is initiated at one strand in the cable, (2) The current in the quenched strand is transferred into the other strands, (3) An avalanche of quenching is induced among the strands, (4) The central strand is quenched finally among the strands. The critical current values of the 7- and 7×7- stranded cables are also measured. These values are in good agreement with the predicted values based on the mutual inductance among the strands. It is concluded that the unbalance of the current distribution in the superconducting multistrands can be one of the promising causes for the degradation of the critical current value.
The flashover characteristics of rod to plane air gaps up to gap length of 30m under positive switching impulse voltages had been predeterminded by use of our combined engineering model of leader propagation published in a previous paper. However, flashover voltages of long air gaps published previously appear to have remakable variations depending on each of discharge conditions such as electrode arrangement, waveshape of switching surge voltage and humidity in air. Important flashover characteristics used in insulating designs under various discharge conditions were determinded empirically from experimental results. In this paper, a simple method for the calculation of flashover characteristic under each discharge condition has been proposed as an expert system with a microcomputer.
Results of power generation experiments with an improved heat exchanger system in the FUJI-1 facility were described. One of the main purposes was to study the effect of working gas temperature on generator performance. The results with argon working gas showed that the gas temperature of 1, 850K is enough to eliminate the effect of inlet relaxation under the present experimental conditions and that gas temperature does not affect much the output performance so long as the inlet relaxation is not significant. For the case of helium working gas, the voltage drop due to the inlet relaxation is remarkably decreased and improvement in both output power and enthalpy extraction can be observed by the increase of gas temperature.
A carbon dioxide-recovering high-efficient gas turbine power generation system is proposed in which carbon dioxide (CO2) generated is recovered by adopting the oxygen (O2) combustion method and no thermal nitrogen oxide is generated. In the system, saturated steam produced by utilizing waste heat is adopted as working fluid of the gas turbine. Thus, the compressing process of the working fluid gas, which is the most energy-consuming process in generating power by using a gas turbine, is not needed, and this makes the system to be extremely high-efficient. By taking saturated steam of 210 Centigrade as an example, the characteristics of the system was simulated. The net exergetic efficiency of the system has been estimated to be 48.4% by considering both the exergy of the saturated steam and the electric power required not only to generate high-pressure oxygen but also to liquefy the recovered CO2. The value is higher than the exergetic efficiency, 37.8%, of large scale thermal power generation plants using the same natural gas, and is 28.0% higher than its efficiency of 37.8%, the one estimated if the CO2 generated is removed and recovered from the stack gas by using alkanolamine-based solvent and the recovered CO2 is liquefied.
Double-circuit faults of transmission lines of often occur in winter in Japan. One of the causes for such faults in winter has been thought the reduction of the flashover voltage under the influence of space charge with the presence of ice crystals or water in the thundercloud. In this paper, we describe the result of the flashover experiment which was performed with a rod-to-rod gap or a suspension insulator in thunderclouds at the top of a mountain of about 1, 000m in altitude in winter. Summary of the results is as follows. (1) The change of the lightning impulse flashover voltage of rod-to-rod gap in the thundercloud with rain is less than 3% in comparison with the flashover voltage in rain. (2) The lightning impulse flashover voltage of a suspension insulator in the thundercloud with rain in the negative electric field is 7% less in comparison with the flashover voltage in rain, and 13% less in comparison with that under fine weather. (3) The lightning impulse flashover voltage of a suspension insulator in the thundercloud with snow in the negative electric field is 9% less in comparison with the flashover voltage in snow, and 22% less in comparison with that under fine weather.