Ecological and economical stand point of view becomes the most significant issues in electric power technology these days. There are many research works and reports on the development of ecological and economical power technology including new machines and also new asset management methods. In this report, several research results are explained mainly based on the research works presented in 2004 CIGRE Paris meeting.
SF6 gas has been used as an excellent insulating and arc-quenching gas for electric power apparatus such as SF6 gas insulated switchgear (GIS). However, since SF6 gas was specified to be a greenhouse gas because of its high global warming potential (GWP), environmental-friendly gases or gas mixtures alternative to SF6 gas are strongly required to be developed. Especially, the development of insulation gases without SF6 gas is an important task for the future electric power system. We focused our research on CO2 gas and CO2-based gas mixtures such as CO2/O2, CO2/N2 with small GWP as SF6 substitutes, and investigated partial discharge (PD) and breakdown (BD) characteristics under lightning impulse and ac voltage applications. In this paper, PD inception electric field (PDIE), BD electric field (BDE) and the impulse ratio were obtained in CO2, N2, SF6 gases and CO2-based gas mixtures, and discharge mechanisms of CO2 gas were discussed. Experimental results revealed that impulse ratio in CO2 gas was higher than those in SF6 and N2 gases, and dielectric-coating effect in CO2 gas was higher than that in SF6 gas. These results were attributed to the lower generation probability of initial electrons in CO2 gas.
With introducing the competitive electrical power market, large-scale customers can select electric power suppliers. Customers need to consider not only the economical efficiency but also reliability, to decide the amount of electrical power to purchase. This paper develops an economic electric power purchase strategy for customers focusing on reliability. A bilateral contract model expressing electric power suppliers as generators with a forced outage rate is proposed and introduced to assess potential outage risks of the bilateral contract between an electric power supplier and a customer. The outage-related cost, consisting of potential outage risks and the estimated outage cost, is also proposed and introduced as an index of the reliability on the customer side, and an optimal reliability level of a customer is obtained by using the index. Several numerical examples demonstrate the availability of the proposed electric power purchase decision method and reliability improvement strategies for customers are discussed.
A severe fault may cause loss of synchronism of a small number of generators. And it may make many generators out of service resulting in wide area blackout. When two groups of generators lose synchronism, the magnitudes of voltage on some lines becomes zero. Therefore an approach that lines with zero voltage are opened has been employed actually in order to prevent loss of synchronism from extension. This paper presents a new method using voltage and current measured on a line during disturbance to detect the line with zero voltage at the instant when loss of synchronism takes place. A method to detect loss of synchronism using voltage and current measured on a line was already proposed. This paper presents results of the tests performed by using power system simulator which is composed of a DC-motor, a synchronous generator, artificial transmission line, circuit breaker equipments and so on. The validity of proposed methods was verified by finding loss of synchronism correctly.
The solid oxide fuel cell (SOFC) has a problem in durability of the ceramics used as its cell materials, because its operating temperature is very high and cell temperature fluctuation induces thermal stress to the ceramics. The cell temperature distribution in the SOFC, therefore, should be kept as constant as possible during the variable load operation through the control of the average current density in the cell. Considering this fact, the authors investigate the relation between the average current density and the temperature distribution in the co-flow and counter-flow type planar SOFC single cells by numerical simulations. In the calculations, the fuel utilization is kept constant and the air utilization is selected as the controllable variable. It is made clear that the change of the temperature distribution can be suppressed to sufficiently low level and the variable load operation can be realized for both types of cells by properly regulating the air utilization.
Generally the prediction value of the amount of annual power generation of the actually installed photovoltaic generation system is calculated as follows: Multiply the rated capacity (kW) and the monthly fixed coefficient (Summer: 0.8, Winter: 0.9, Spring/Autumn: 0.85, etc.) by the monthly solar isolation on the receiving surface. Calculate the annual total of these values, and acquire the annual amount of power generation regarding the losses of the system. However, these temperature coefficients are rough values, so the prediction value of the amount of annual power generation cannot be so accurate. Here, we propose the method to calculate “Monthly temperature coefficient" for each photovoltaic generation system as accurately as possible using the simulation calculation. It is shown that thus calculated “Monthly temperature coefficient" is quite corresponding to “Monthly temperature coefficient" calculated using the actually measured values of the 40kW class photovoltaic generation system. We examined the influence on “Monthly temperature coefficient" caused by differences of regions, photovoltaic module characteristic values, and photovoltaic module frames using the simulation calculation.
We already proposed the calculation method of “Monthly temperature coefficient” with a new concept to calculate the amount of annual power generation of the photovoltaic generation system simply and accurately. This report is related to the method for evaluating the amount of power generation using this “Monthly temperature coefficient”. An example of the calculation is shown, in which the rate of the loss caused by the above-mentioned temperature rise is calculated by applying this “Monthly temperature coefficient” to the 40kW class photovoltaic generation systems at two places in Japan. It found out that this value (“system performance ratio with temperature compensation”) is approximately the same value all through the year if monthly “system performance ratio” regarding the loss of the temperature rise is calculated. Next, we confirm accuracy and reliability of this calculation result using the actually measured data.
SF6 gas has excellent dielectric strength and interruption performance. For these reasons, it has been widely used for gas insulated switchgear (GIS). However, use of SF6 gas has become regulated under agreements set at the 1997 COP3. Presently, development of a gas circuit breaker (GCB) using CO2 gas and development of a high voltage vacuum circuit breaker (VCB) are being pursued. GIS consists of disconnectors (DS), earthing switches (ES) and buses in addition to GCB. Since the interruption performance is not an important requirement for DS, ES and BUS, use of a gas with high dielectric strength is better than use of a gas with good interruption performance. Air and N2 are not greenhouse gases, and their dielectric strengths are higher than those of other SF6 alternative gases, but only about one-third of the dielectric strength of SF6 gas. This paper deals with a suitable insulation gas which has no greenhouse effect as an SF6 alternative gas. The N2/O2 mixed gas was investigated by changing the ratio of O2. Moreover, the effect of an insulation coating was investigated and compared with the dielectric strength of SF6/N2 mixed gas. The dielectric strength of air under the coating condition was equal to that of 10%SF6/N2 mixed gas.
In this letter, we have demonstrated a simple method to evaluate the resistivity and relative permittivity of a lossy medium. The method comprises two parts: the first one is to measure the frequency characteristic of impedance between two parallel electrodes having a lossy medium inside, and the second one is to calculate the resistivity and relative permittivity of the medium, in terms of the resistance and capacitance between the electrodes, from the measured frequency characteristic of impedance.
This paper deals with an analytical study about a decentralized autonomous control strategy of a super-distributed energy system where small-scale dispersed generators such as micro gas turbines and fuel cells have a dominant. Using the dynamics of the Hopfield neural network model, it is proven that the proposed decentralized autonomous control strategy provides a stable ground state of the energy function of the Hopfield neural network, which state exactly corresponds to a state where local supply-and-demand balance is maintained.