Showing papers in "Electrical Engineering in Japan in 1994"

Development of self‐commutated SVC for power system

Abstract: A 50-MVA self-commutated Static Var Compernsator (SVC) using the worlds's largest GTO thyristors rated 6 kV-2500 A has been developed for voltage control of electric power systems. The rated dc voltage is 16.8 kV which is four times higher than the highest one manufactured so far. To achieve the high dc voltage, eight GTO thyristors are connected in series. Voltage unbalance between GTOs during switching transient was investigated. A new overcurrent limiting method is proposed, since conventional fuse protection cannot be applied due to such high voltage. Newly developed technologies for this large and high-voltage self-commutated SVC are described. Although simple star-delta connection transformers are used, harmonics generated from SVC are almost equivalent to the 24-pulse converter by shifting the pulse timing appropriately. The PWM control based on 150 Hz is applied and the SVC is designed to continue operation during system faults. The performance of the controller is tested successfully by a simulator, and typical rest results are introduced.

Compensation characteristics of shunt active and series active power filters

Abstract: Active power filters have been used in practice to suppress the harmonic interference in power systems. To compensate for harmonic currents of loads, active power filters usually are connected to power systems in parallel with the loads. These filters, which are called shunt active filters here, are very effective for loads that can be considered as current sources, such as thyristor rectifiers with large dc reactances. Many papers have covered the shunt active filters applied to these current-source loads, however, none has discussed characteristics of the shunt active filters when they are applied to voltage-source loads. On the other hand, since more and more diode rectifiers with capacitive dc filters have recently been used, harmonics generated by them have become an issue. The diode rectifier with capacitive dc filters behaves as a voltage source rather than a current source. When a shunt active filter is applied to such a diode rectifier, the current injected from the shunt active filter may flow into the diode rectifier. As a result, harmonics of the source current cannot be reduced effectively, and harmonic current flowing into the diode rectifier increases greatly. This paper presents the aforementioned problem of shunt active filters analytically and experimentally. Then a series active filter is proposed to suppress the harmonic current of the diode rectifiers. The features, operating conditions, and considerations of shunt active filters and series active filters are described analytically and demonstrated experimentally. Taking a diode rectifier with capacitive dc filter as a typical voltage-source load, compensation characteristics of shunt active filters and series active filters are discussed by experiment and simulation. The validity of the series active filters is illustrated experimentally.

Investigation of impedance and line parameters of a finite length multiconductor system

Abstract: The earth-return impedance and capacitance formulas of a finite parallel multiconductor with arbitrary positions have been proposed. A comparison with measured results has proved that the accuracy of the impedance formula is satisfactory. By applying the formulas, the characteristics of the finite line parameters such as a propagation constant and a characteristic impedance are investigated in comparison with those given by the conventional assumption of an infinite line. The impedance of the infinite line is greater and the capacitance is smaller than those of the finite line. The difference increases as the line length and source frequency decrease, and the height, radius and separation distance increase. According to the foregoing, the attenuation constant and the characteristic impedance of the infinite line are greater and the propagation velocity is smaller than those of the finite line.

A feasibility study of an 8-MWh flywheel-type power storage system using oxide superconductors

Abstract: When combined with a magnet having a magnetic field gradient (for example, a permanent magnet), a Y-based oxide superconductor is capable of forming a noncontact bearing with a strong levitational force. Since this bearing exhibits low rotational loss, it is very likely to form a highly efficient power storage system in combination with a flywheel. In this paper, an 8-MWh power storage system utilizing a flywheel was designed conceptually to examine its applicability and the possible effects of its introduction. It was found that this system was an effective power storage.

A CO2-recovering nonpolluting high-efficiency gas-turbine power-generation system utilizing saturated steam as its working gas

Abstract: A carbon dioxide-recovering high-efficiency 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 the 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. This makes the system extremely high efficient. By taking saturated steam of 210°C as an example, the characteristics of the system were simulated. The net exergetic efficiency of the system has been estimated to be 48.4 percent 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 percent of large-scale thermal power generation plants using the same natural gas, and is 28.0 percent higher than its efficiency of 37.8 percent, 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.

Development of fuzzy excitation control system

Abstract: Power system stability is essential to reliable and economical operation of power systems. The quick-response excitation control with PSS is used as a power system stability improvement measure at the generator. However, this method is not always sufficient to apply in a wide operation range of power system configuration, load flow, and so on. This paper includes results of experiments of fuzzy excitation control system on the ac/dc Power System Simulator of CRIEPI. This control system is verified to improve power system stability and has better performance than the conventional excitation control on a one-and two-machine to infinite bus system.

Diamond-like carbon film produced by vacuum arc deposition technique and its surface appearance

Abstract: Diamond-like carbon (DLC) films are deposited in a dc vacuum arc plasma with a graphite cathode. The arc current, the flow rate of hydrogen gas and the pressure are 50 A, 10 ml/min and 0.05 Pa, respectively. The substrate materials are Si (111), soda-glass and Mo. Process parameters are deposition time (3 to 30 min). The films are analyzed by laser Raman spectroscopy and the surface appearance of the films are observed with an optical microscope. The results are as follows: (1) the films deposited under these conditions have diamond-like carbon structure; (2) the films deposited for 3 min on Si or Mo and for 5 min on glass, wrinkle on the substrates; (3) the films deposited for more than 10 min on Si and for more than 15 min on glass, crack and flake off the substrates; and (4) the films deposited for more than 10 min on Mo, plainly stick on the substrate without any wrinkles or flakes. It is found that plain DLC films are able to be produced by the vacuum arc deposition method if the deposition time is selected appropriately, while never by the conventional plasma CVD method.

Degradation diagnosis and estimation of residual life of insulating material using pattern recognition of phase-angle resolved partial-discharge pulse-occurrence distribution

Abstract: A prototype of partial discharge (PD) pulse measurement and degradation diagnosis expert system (PDM/ES-I) is constructed using a personal computer for insulation degradation diagnosis of materials experiencing PDs. For 0.1-mm-thick low-density polyethylene (LDPE) with CIGRE Method II electrode system, temporal change of phase-angle-resolved PD pulse occurrence distribution was measured with PDM/ES-I. The so-called “pattern recognition” of the PD pulse distribution is applied to diagnosis of insulation degradation and assessment of residual life. Experimental results revealed that in spite of large scattering of time to breakdown, i.e., life, from 5 to 43 hours, all the tested samples exhibited similar evolution of the profile of PD pulse occurrence compared with five standard normalized PD patterns which had been registered in advance as five representative degradation stages from the initial degradation category to the final one. The degradation stage determined by the pattern recognition (PR) method proved to vary from the initial degradation category to the final one in a correct turn. Consequently, the PR method permitted an exact determination of the degree of degradation at a given aging time. In other words, the PR method was capable of predicting the residual life of LDPE with a CM-II electrode system.

More intelligent DC railway electrical power systems with traction power control

Abstract: The load of a dc railway has very large peaks while the average power is low. This makes the capacity of wayside power equipment larger. Recently, the ratio of peak power to average power has become even higher because of the introduction of a large number of electric multiple units with a regenerative brake system. This has also made the contact system unreceptiveness of the regenerated energy a significant problem. To solve these problems, researchers are attempting to add intelligence to the power system. This paper describes a novel method of intelligently controlling the traction power of trains. In this system the trains reduce their accelerating power to cut current peaks in wayside substations. With the proposed method the load peaks can be reduced by up to 31 percent, which means that the characteristics of the railway load have been changed drastically. The delays of the trains involved are negligible. Also, the unreceptiveness of the regenerated power can almost be eliminated. In the first part of this paper, the idea of traction power control is described. Then a description is given of how the idea is applied to wayside substation load peak suppression or receptiveness improvement, with some simulation results to prove the method. Finally, the possibility is discussed of further improvement of the system by adding communication capability between trains and wayside equipment.

Characteristics analysis of superconducting power transformer without iron core

Abstract: With the progress of superconducting wires for ac power use, research on superconducting power transformers is increasing. These transformers can be divided into two types: the iron-core type [1, 2]; and the air-core type [3–9]. The latter type has such advantages as absence of iron losses and magnetic saturation, and greater possibility of reduction of size and weight. However, the air-core transformer has a large magnetizing current due to the absence of iron core. Hence, research has been carried out on the possibility of using the air-core transformer also as a shunt reactor in a power transmission system. However, the operating characteristics of the air-core transformer, such as voltage regulation and reactance voltage, are not clear at present. In this paper, the equivalent circuit without losses is proposed first. Since this equivalent circuit is expressed by means of the magnetic coupling factor and self-inductances of windings, the effect of these parameters on the transformer characteristics can easily be investigated. Then, based on this equivalent circuit, the per-unit expressions for the air-core transformer characteristics are derived and the characteristics are analyzed in detail. The validity of the theoretical results are confirmed by experimental results obtained by the use of an experimental superconducting transformer.

A rapid solution for the maximum loading point of a power system

Abstract: This paper presents a new solution method for computing the maximum loading point of a bulk power system under the condition where the loads of the nodes can be parameterized by a scalar which is the loading level of the system. A special loading model, in which the loading level of the system depends on the voltage magnitude of a loading node, is adopted for the purpose of releasing the load-flow computation from the ill-condition near and at the maximum loading point. The loading level of the system is unknown in the aforementioned loading model. The maximum loading point is obtained by adjusting the operating parameter so as to achieve the maximum loading. The operating parameter is adjusted in the converging process of the Newton-Raphson iterative computation. The adjustment is computed based on the least squares estimation using the data set which is obtained from its own iteration process. It is shown in numerical examples that the proposed method is satisfactorily rapid, stable, and accurate.

Method for analyzing eddy currents in moving conductors

Abstract: When magnetic fields in electrical machines with moving conductors such as linear induction motors are analyzed, eddy currents due to the movement of conductors should be taken into account. Although two kinds of coordinate systems (moving and fixed coordinate systems) can be applicable to the analysis of eddy currents in moving conductors, the advantages and disadvantages of both coordinate systems are obscure. In this paper, methods for analyzing magnetic circuits with moving conductors are classified systematically, and the moving and fixed coordinate systems are compared with each other from the standpoints of accuracy, memory requirement, and CPU time. It is shown that the moving coordinate system is superior to the fixed coordinate system for transient analysis.

Reluctance control magnetic suspension system-suspension system with permanent magnet and linear actuator

Abstract: A new type of magnetic suspension system is proposed which uses a permanent magnet. In this system, the attractive force is adjusted by controlling the reluctance of the magnetic circuit. Since no electromagnet is necessary, this system is effective for saving energy and avoiding heat generation. The composition of this paper is as follows. First, the principle of this suspension system is explained and various types of magnetic circuits are introduced for reluctance control. Second, the possibility of one of them is examined from the viewpoint of linear control theory. Third, an experimental device is developed based on the proposed suspension method. The reluctance of this device is controlled by the air gap which is between the magnet and the suspended object, and which is adjusted by driving a permanent magnet by piezoelectric actuator. Experimental results support the theoretical predictions well.

A new voltage control equipment using fuzzy inference

Abstract: Power system control equipment needs higher sensitivity and operational reliability. Advanced voltage control equipment is needed for reducing the frequency of tap changes and improving the characteristics (the relationship between the actual voltage and reference voltage) of the voltage to meet today's power system requirements. However, these objectives are in a trade-off relationship. Studies of voltage control derived from a knowledge base suitable for electric power systems can satisfy these objectives using fuzzy inference. Compared with corresponding conventional equipment, the new equipment improved the deviation of 30 min average voltage of 30 percent. This paper describes the design concept of new voltage control equipment using fuzzy inference. In addition, field test results are described along with rules of fuzzy inference, membership functions, and the deviation of 30 min average voltage through detailed simulation.

Stabilizing control of power system using fuzzy control

Abstract: This paper presents an application of fuzzy control to enhance power system stability. The proposed control consists of the controller for large disturbance (FU 1), the fuzzy controller for small disturbance (FU 2), and the fuzzy judgment mechanism (FU 3). FU 1 is determined based on the fuzzy controller [FU 1(F)] is determined according to the control rules and its input signals, i.e., speed deviation and acceleration at every sampling time of the machine. FU 2 consists of two controllers, namely, FU 2-ω and FU 2-P; FU 2-ω has the same mechanism as FU 1, while the output signal of FU 2-P is determined according to the rules together with the change of error of electrical power and terminal voltage. To obtain the optimal desired control signal during both the large and the small disturbances, the operations of FU 1 and FU 2 are judged by FU 3, where the magnitude of speed deviation is chosen as its input signal. The determined control signal is fed to AVR of the machine. The implementation of the proposed control is simple due to the small amount of calculations and required data. The effectiveness of the proposed control is demonstrated by the one-machine infinite-bus system model and very good system performance is obtained throughout all the simulations.

Power‐system stabilizing control by SMES using fuzzy techniques and neural networks

Abstract: It has been clarified that a superconducting magnetic energy storage (SMES) is very effective for power system stabilization. The control methods proposed for power system stabilization by SMES include the pole assignment, the optimum control, and so on, each of which, however, has its drawbacks. The application of fuzzy control is considered to overcome these drawbacks. This paper considers the power system stabilization by fuzzy control of the active and reactive power of SMES. First, the adequate fuzzy control rules of an SMES for the model power system is derived. Then, to alleviate the dependence of the fuzzy control on the operating condition and the fault, a method is proposed which adjusts the fuzzy parameter according to the operating condition and the fault using a neural network. The validity of the proposed method is examined by computer simulations.

Long‐term load forecasting using improved recurrent neural network

Abstract: In general, electric power companies must prepare power supply capability for maximum electric load demand because it is very difficult at present to store electric power. It takes several years and requires a great amount of money to construct power generation and transmission facilities. Therefore, it is necessary to forecast long-term load demand exactly in order to plan or operate power systems efficiently. Several methods have been investigated so far for the long-term load forecasting. However, because the electric loads consist of many complex factors, good forecasting has been very difficult. This paper proposes a long-term load forecasting method using a recurrent neural network (RNN). This is a mutually connected network that has the ability of learning patterns and past records. In general, when interpolation is used for unlearned data sets, the neural network provides reasonably good outputs. However, when extrapolation is used, such as in long-term load forecasting, some kind of tunings have been necessary to obtain good results. Therefore, to solve the problem, a method is proposed in which growth rates are used as input and output data. Using the proposed method, successful results have been obtained and comparisons have been made with the conventional methods.

Evaluation of photovoltaic modules considering spectral solar radiation

Abstract: For the optimum design of photovoltaic power systems, it is very important to estimate their annual maximum amount of output. In general, the annual maximum amount of output has been calculated by using photovoltaic module efficiency under irradiance of 1 kW/m2 at AM 1.5 with solar cell temperature of 25°C. Since the spectral distribution of solar radiation changes with weather, season and time, the efficiency of solar cells is not always constant. This paper describes a method evaluating the maximum amount of output in photovoltaic modules, ‘the spectral method,’ by using the measured annual spectral solar radiation and the spectral response of various photovoltaic modules. The calculated values using the spectral method agreed well with the measured values for amorphous silicon photovoltaic modules from March 1991 to February 1992. The spectral method proved most effective for optimum design of amorphous silicon photovoltaic power systems.

An efficient method of modifying Z‐matrix elements in short‐circuit capacity calculations

Abstract: Short-circuit study is one of the most fundamental planning analysis procedures for electric power utilities. Measures for increasing transfer capabilities such as installation of additional facilities were originally implemented for the purpose of improving system stability. However, the reinforcement of transmission networks is commonly the major factor in increasing short-circuit currents. Therefore, short-circuit studies should be given greater attention in the future, especially with regard to coordination of power system availability and reliability. To effectively solve these problems, it is necessary to develop an advanced algorithm to easily compute short-circuit currents in one round for a large number of alternative network configurations corresponding to operational changes in bus connections at main substations. This paper proposes a new calculation method to efficiently modify the Z-bus matrix elements for short-circuit studies of large power networks, and also demonstrates its practicality by applying the proposed method to the calculation of short-circuit currents in two model networks.

Practical design of superconducting generator—electrical characteristics equations

Abstract: A superconducting generator (SCG) consists of a nonmagnetic cryogenic rotor, which contains a super-conducting field winding cooled by liquid helium, and an airgap armature winding, which makes it possible to obtain a high magnetic field in the airgap. Since the SCG construction differs from that of the conventional generator, practical application requires that its electrical and mechanical design methods be established. This paper describes electrical characteristics equations of the SCG, electromagnetic shield characteristics of double dampers and proof test results of a 50-MVA experimental SCG. Equations for the generator parameters (voltages, output capacity, reactances, time constants, flux densities), which are useful for the practical design of the SCG, are derived from two-dimensional electromagnetic analysis considering winding thickness and the edge effect. From comparison with measured and calculated values of no-load, sudden 1–3 phase short-circuit and slip extended tests, these equations were confirmed as valid.

Fuzzy inference application to planning of distribution network switching

Abstract: Optimal switching pattern planning is the main subject of distribution network reconfiguration to recover from a blackout or to avoid a blackout due to maintenance and construction. This is a combinatorial optimization problem, and an enumerative check of every pattern is necessary to obtain the optimal solution. In practice, successive heuristic search methods are adopted to obtain the quasi-optimal solution within a limited computation time. However, a very long computing time is still required when the problem size becomes slightly larger. The step-by-step nature of the methods results in many iterations, although it ensures satisfaction of the constraints. Reduction of the iterations is considered to be necessary to enhance processing speed. In this paper, “supply adequacy” is introduced as a new criteria index to guess the solution, which is obtained by fuzzy inference for each possible pair of source and load zone. The reconfiguration pattern is obtained at once or in a few processing steps by assigning to each zone the source with the highest adequacy. Numerical examples show that the proposed method can obtain the solution faster than the ordinary method, more remarkably with the increase of problem size.

High‐precision voltage‐measuring method using fast sampled data for electric power systems

Abstract: Control and protection equipment in power systems requires higher sensitivity and operational reliability to meet today's changing power system requirements. The voltage-measuring deviation requirement for advanced voltage and var control equipment is less than 0.1 percent under conditions of harmonic distortion in the voltage waveform and power-system frequency variation. Studies on digital signal processing suitable for electric power systems showed that these requirements are satisfied using fast sampling and very fast 32-bit floating point operations by a Digital Signal Processor (DSP). This paper describes the design philosophy of a high-precision power system voltage-measuring method using fast sampled data. In addition, total voltage-measuring deviation characteristics under a combination of the techniques are described along with digital filter characteristics, frequency-measuring deviation characteristics, frequency variation versus gain-compensation characteristics of a digital filter, and peak value operating principles.

Stability analysis of a controlled-current source-fed induction motor vector control system without a speed sensor

Abstract: One of the representative vector-controlled induction motor drive systems without a speed sensor is composed of a rotor flux estimator, a PI torque current controller and a conventional vector-controlled current source. However, the stability analysis of this system has not been reported. In this paper, the flux estimator is interpreted as a flux observer. Then a linear model of the system is proposed by deriving the equations of the flux estimator in a synchronously rotating reference frame. By computing the trajectories of the poles and zeros and the transient responses, the following results are obtained. (1) If the rotor flux is estimated using only the stator equations (voltage model), the system becomes unstable when the actuator stator resistance is smaller than the value used by the controller. (2) By choosing a small time constant of the first-order lag element in the flux estimator (reciprocal number of observer gain), the poles and zeros on the imaginary axis move to the stable region. However, another pair of poles has reverse performance. (3) From the viewpoint of stability, the gain of the PI torque current controller should be chosen as large as possible and the stator and rotor resistances in the controller should be set to values smaller than their actual values.

Quenching current level coordination in superconducting power transmission systems

Abstract: In the present power transmission system, electric power devices are equipped under the concept of insulation coordination and are protected from lightning overvoltage and switching surge by the arrester. However, the future superconducting power transmission system is characterized by the heavy current transmission capability, the lower transmission loss and lightning-surge-free circumstances. Thus, in the superconducting power transmission system, superconducting devices should be protected rather from quenching by the overcurrent of a short-circuit fault than from the breakdown by over-voltage. In this paper, a new concept of the coordination for the superconducting transmission system is proposed as a “quenching current level coordination” instead of the insulation coordination. Furthermore, it is pointed out that the quenching current level coordination has the duality relationship with the present insulation coordination. A superconducting fault current limiter should play an important part of the quenching current level coordination to prevent the other superconducting devices from quenching.

Pulse switching characteristics of MAGTs for pulsed power applications

Abstract: For pulsed power systems such as lasers and accelerators, semiconductor switches with their longer service life have actively been developed as replacements for thyratrons. The MOS-driven thyristors are suitable for pulsed power applications because they have high-power handling and fast turn-on capabilities. The MOS-assisted gate-triggered thyristor (MAGT), designed especially for pulsed power, is a promising candidate in this field. This paper presents the results of an investigation into the performance of MAGTs. Using a pulse-forming network (PEN), the pulse-switching characteristics and the dynamic resistance characteristics during the current flow are investigated. A maximum current density of 21.8 kA/cm2 and di/dt of 106 kA/μs/cm2 with 1550-V anode voltage on a single-shot basis were obtained. Furthermore, a life test with 109 shots at a high repetition rate showed no degradation in the observed characteristics. Based on these experimental results, a carrier flow model of MAGT during turn-on process is proposed and the turn-on mechanism is considered.

Lightweight type linear induction motor and its characteristics

Abstract: At the Institute for Posts and Telecommunications Policy, a postal transportation system has been studied. This system would make use of linear induction motor (LIM)-driven vehicles for transporting mail between post offices in the Tokyo Metropolitan area. The system would connect the main post offices in the Tokyo area with tunnels circularly constructed underground and would transport mail with linear induction motor-driven vehicles. In this study, it is found that if vehicle weight is reduced, climbing capability could be considerably improved. Accordingly, the potential for reducing the weight of vehicle-mounted LIMs has been explored, and a prototype lightweight LIM has been manufactured. This paper reports on the lightweight LIM and its characteristics. 8 refs.

Characteristics of discharge induced by laser-generated plasmas in atmospheric air

Abstract: The possibility of laser-induced lightning has been investigated for the future application in the protection of electric power systems from lightning strokes. This technique induces lightning strokes artificially along a laser-produced breakdown plasma channel and conducts them to a harmless place on the ground. To clarify the plasma formation mechanism and the discharge (sparkover) induction process, sparkover induction experiments are performed for two types of long-gap electrode configurations, plate-to-rod and plate-to-plate, with a 50-J TEA CO2 laser. In the present paper, the sparkover voltage characteristics and interferometric measurement on laser-produced plasma mainly are described, together with sparkover induction for a series of metal beads arranged equidistantly which simulate a plasma channel.

Neural‐network steering control of an automated guided vehicle

Abstract: With the progress in factory automation in industry has come the demand for higher speed drive and performance for an AGV (automated guided vehicle). A new steering control of the AGV by fuzzy control has been proposed to replace the PI control. However, much time is required to investigate the regulations and to adjust the scaling factors for excellent performance in the fuzzy control. In this paper, a new steering control for an AGV based on the neural network using the backpropagation method is proposed. The good steering control results by the fuzzy control are adopted for the teaching signal of the neural network. First, the effect of the number of learning and the learning errors on the steering control results are discussed by computer simulation using the AGV model. Further, the ability of generalization in the turning radius and the traveling speed also are investigated. It becomes clear that the AGV can travel along a designated route provided the neural network learns both the right and left turning at the maximum traveling speed and the minimum turning radius. Then it is proved by an experiment using the AGV constructed for the test that the proposed steering control method is very affective.

Active‐passive power circuit

Abstract: The recent development of a semiconductor device technology realizes small-size, lightweight, high performance power switching devices. However, reactive elements such as an inductor or a PWM inverter achieves high-performance current control, which is applied to the active filter, and so on [2, 4]. If a compact and high efficient reactive component is developed, the power converter technology may drastically change the application field. In this paper, the concept of variable active-passive reactance (VAPAR) which consists of an inverter and passive elements is proposed. The VAPAR behaves like a two-terminal reactive element and the produced active reactance component works as a passive reactive component, including in transient state. The principle of VAPAR and two possible control schemes to realize VAPAR are realized. Through simulations and experiments, it is verified that VAPAR shows reactive characteristics not only in steady state, but also in transient state, as proposed, and basic characteristics of both control schemes are clarified.