Dispersed generation systems, such as micro gas-turbines and fuel cells, have been installed on some of commercial facilities. Smaller dispersed generators like solar photovoltaics have been also located on the several of individual homes. The trends in the introduction of the these generation systems seem to continue in the future and to cause the power system to have the enormous number of the dispersed generation systems. The present report discusses the near-future power distribution systems.
Generally, home PV systems use single-inverter systems as their power conditioners. However, single-inverter systems suffer limitations in terms of efficiency for the following two reasons: (1) PV arrays can not be operated simultaneously at maximum power points if all the PV arrays are not identical in installations such as azimuth, tilt, surroundings, and so on. (2) It is impossible to avoid the operation mode at which the load factor is low and consequently, the conversion efficiency of the inverter is low. In this paper, we proposed a novel PV system by using multiple inverters. Compared to the conventional system, our simulation results show that the proposed system can improve the annual efficiency by nearly 2%.
We have developed a digital real time simulator of Power Electronics Controllers, so called FACTS (Flexible AC Transmission Systems) Controllers and/or Custom Power by using MATLABTM/SIMULINKTM and dSPACETM System. This paper describes the modeling and the calculation accuracy of a UPFC (Unified Power Flow Controller) model. Hence the developed simulator operates at a large time step, in order to improve simulation accuracy, a correction processing of the switching delay is implemented into the UPFC model. Calculation accuracy of the real time UPFC model is the same level as EMTDCTM results. We confirm stable operation of the developed UPFC model with connecting a commercial real time digital simulator (RTDSTM).
This paper investigates the function of DASM (Doubly-fed ASynchronous Machine) with emphasis placed on its ability to the stabilization of the power system including wind generators. P (active power) and Q (reactive power) compensation from DASM can be regulated independently through secondary-excitation controlling. Simulation results by PSCAD show that DASM can restore the wind-generator system to a normal operating condition rapidly even following severe transmission-line failures. Comparison studies have also been performed between wind turbine pitch control and proposed method.
Recently, diagnosis of gas insulated switchgears (GIS) is important because number of aged GIS is increasing and maintenance method of GIS is changing from TBM (Time Based Maintenance) to CBM (Condition Based Maintenance). In this paper, the mechanism of electromagnetic wave propagation in a GIS tank is investigated for understanding of fundamental characteristics for partial discharge detection using UHF method. In this paper, the output characteristics of UHF sensor in non-reflection coaxial waveguide are obtained under the various parameters of installation and shape of UHF sensor. The mechanism of electromagnetic wave propagation is investigated with comparison of the experimental result and EMTP (Electro-Magnetic Transients Program) analysis of equivalent circuit. In consequence, it is found that the mechanism of electromagnetic wave propagation can be treated as an equivalent electric circuit in the frequency region below cutoff frequency of TE11 mode, and another mechanism of electromagnetic wave with higher mode dominantly propagates.
In these days, integrated circuits have been increasingly used for Low Voltage (hereinafter LV) distribution equipments for the purpose of making more multi-functional and downsizing. However insulation of the circuits is easily broken by lightning surge from the power line, so the number of troubles is also getting increased. On the other hand, there is no authorized standard value of the lightning withstand performance for LV distribution equipments. Here, we implemented Lightning Impulse Withstand Voltage (hereinafter LIWV) test and Lightning Impulse Withstand Current (hereinafter LIWC) test in order to verify the value. As a result, we made it clear that if LV distribution equipments’ integrated circuits are protected by surge absorber, the lightning withstand performance is determined not only by LIWV performance but also by the performance of surge absorber, i.e. LIWC test should be implemented for the performance evaluation.
This paper discusses the synchronization phenomena of wind turbines in a wind farm. The phenomena denote the situation that a blade revolution of a wind turbine is pulled into phase with that of the other turbines in a wind farm. If blades of many turbines simultaneously pass in front of their towers, output power fluctuations of the turbines due to tower shadow effects also become synchronized. This causes voltage fluctuation to increase and may give rise to voltage flicker problems. The purpose of this paper is to examine the phenomena theoretically. Obtaining a differential equation that describes the phenomena with output power, voltage and slip fluctuations of a wind turbine, we examine dependence of the phenomena on the related parameters. We also prove the validity of the equation through simulation analyses.
We have developed cubicle-type gas-insulated switchgears (C-GISs) applying compressed air insulation technology. To apply the compressed-air insulation to the C-GISs, we identify whether breakdown in compressed air depends on the maximum field strength on high-voltage conductors, and specify the relationship between the breakdown field strength vs. non-uniformity of the electric field distribution of gaps. In this work we use three types of sample gaps to simulate the inside structure of an actual C-GIS. Field utilization factor, U is used to express the non-uniformity of electric field distribution of the sample gaps. As a result, for all types of sample gap, the breakdown field strength at the point where the maximum field is generated on the high-voltage electrode increases as U decreases. There is only small difference in the breakdown field strength among sample gaps that have the same value of U. From these results, we judge that insulation design based on both the field strength on high-voltage conductor and U is possible. Furthermore, we provide an example of actual insulation design applied to a developed C-GIS.
The behavior of arc spot and arc column on long gap DC free arcs between a pair of parallel rod conductors were analyzed quantitatively from two orthogonal views by two high-speed video cameras. In addition, the partial shorting phenomenon, which occurred between the arc column and the rod conductor, was studied experimentally. The pair of iron or aluminum parallel rod conductors was arranged horizontally with the gap of 1.6m and the movement sections for arc spots on the conductors were set to be 1m. Objective arcs were ignited with the current in the range of 0.1 to 8kA and the duration time of 0.3s. When the arc spots moved to the load side after the arc ignition, the middle of arc column hardly moved with the arc spots, that is, it always remained behind. Therefore the partial shorting, which brought about the jumps of the arc spot to the power source side, frequently occurred between the arc column and the rod conductor as the arc current increased. To study the occurrence conditions of above partial shorting, the relationship between the path length of disappeared arc column and the straight distance was investigated. As the result, it was clarified that this relationship was almost in a direct promotion. Accordingly it was suggested that this relationship could be used as a certain index about the occurrence conditions of partial shorting.
This paper presents a new concept of parallel redundant STATCOM system. This system consists of a number of medium capacity STATCOM units connected in parallel, which can achieve a high operational reliability and functional flexibility. The proposed STATCOM system has such redundant operation characteristics that the remaining STATCOM units can maintain their operation even though some of the STATCOM units are out of service. And also, it has flexible convertibility so that it can be converted to a BTB or a UPFC system easily, according to the diversified change of needs in power systems. In order to realize this concept, the authors developed several important key technologies for the STATCOM, such as the novel PWM scheme that enables effective cancellation of lower order harmonics, GCT inverter technologies with small loss consumption, and the coordination control scheme with capacitor banks to ensure effective dynamic performance with minimum loss consumption. The proposed STATCOM system was put into practical applications, exhibiting excellent performance characteristics at each site.
GIS is compacted by progress of arrester and reduction of impulse test voltage for specifications recently. Since field strength of inner surface for GIS tank is more strengthened than ever, it is more necessary that included particle in GIS is taken into consideration. When GCB and DS are opened, residual dc voltage occurs at conductor between GCB and DS. It is well known that movement of particle under ac voltage is different from that of dc voltage. Therefore it is very important that particle movement under dc voltage is taken into consideration, even if GIS is for ac. From the above, this paper deals with firefly phenomenon of particle under dc voltage in SF6 gas. It investigates that prevention of fire fly phenomenon by electric field strength of conductor. And the minimum electric field strength to continue firefly phenomenon of particle is made clear. Then the breakdown voltage with firefly particle is compared with that of particle; with a minute gap, fixed to conductor.
4 units differential equations were introduced to propose individual loading guide instead of established loading guide in the previous paper. This paper showed how to determine the coefficients of the equations above mentioned by using the manufacturers examination reports of each transformer and further, to regulate the coefficients by the field measurement data. But, for on-load tap-changing transformers (especially distribution transformers), tap position is continuously changing during load cycle, therefore primary resistances and stray load loss shall also change accompanied with this phenomenon. Therefore tap changing facility should be added to the equations to make individual loading guide more reliable and accurate than the before. To realize mathematical method, impedance test examination in each manufacturer examination reports are also used to assume primary leakage inductances, second leakage inductances and the mutual inductances. In this paper, the method of how to detect the position of tap changer and the comparison between the calculation results of established loading guide and proposing individual loading guide shall be showed, thereafter consider the overall results.
The authors have previously proposed a simulation model of pole-mounted distribution transformers for electromagnetic transient calculations and validated it by comparisons between calculated and laboratory-test results. However, we recently found that the previously-proposed model gives inaccurate results for a specific type of transformers. In this paper, we make clear that the error comes from the fact that the skin effect of the secondary windings is neglected, and an improved model taking the skin effect into account is proposed. The improved model is validated by comparing simulation and laboratory-test results of surge transfer from the primary to the secondary winding and also from the secondary to the primary winding. Furthermore, the model is used in a practical lightning-surge simulation, and the result is shown to be accurate in comparison with corresponding field-test result.