This paper shows the dynamic behavior of the HVDC system which uses forced commutation converter when an AC fault occurs in it. The model HVDC system is unidirectional transmission into a load system without any AC voltage source. Computer simulation program using transient analysis has been developed to perform many runs changing several parameters. First, the results of computer simulation when load impedance changes in the receiving system are shown and they indicate the superiority of voltage-source converter in dynamic behavior. Then, the results of computer simulation when an AC fault occurs in the receiving system are shown. Simulated AC faults are 1 Line-to-Ground (1 LG), 2 Line-Short-Circuit (2 LS) and 3 Line-to-Ground (3 LG). The results of computer simulation indicate that the voltage-source converter must be turned off quickly to protect the switching devices while the current-source converter has no severe problem in the AC fault condition. The overcurrent and overvoltage for the voltage-source converter are shown with the parameters of AC reactance and turn-off delay.

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This paper discusses newly developed control strategies of a self-commutated converter applied to HVDC system. An upper and lower power-limited DC voltage control establishes high performance in a two-terminal HVDC system. A voltage margin method to reverse DC transmission power is also introduced. Even when the communication system is shut down, if required, DC transmission power can be changed with our proposed controls applied to the terminal control. Furthermore, the DC voltage control scheme with two-stage voltage control characteristic is presented for multi terminal HVDC system use. When one terminal is collapsed, the multi terminal HVDC system can be operated because of the proposed control characteristics.
Simulator studies were carried out to verify the proposed control performance in the two-terminal HVDC system and successful test results such as power reversal and a terminal start-up during other terminal operation are shown.

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The introduction of an HVDC system into the trunk power supply system requires to transmit as much DC power as possible for improving the whole AC/DC system stability. Continued operation scheme of HVDC system which makes it possible to operate even under very low commutation bus voltage is adopted to cope with such system requirement. Further stability improvement can be achieved by adding damping control which modulates converter active and/or reactive power.
To take full advantage of these control effects, it is essential that HVDC system can maintain stable operation during the AC system faults and reclosing. Particular attention should be given to a repetitive commutation failure following the unbalanced AC transmission line faults. This paper first discusses the cause of the repetitive commutation failure when supplied with unbalanced AC system voltage based on an experiment using the CRIEPI's AC-DC Power System Simulator. Then a stabilization control scheme with an on-line extinction angle estimation is proposed and the effect is verified by digital simulation.

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In the case of DC insulator strings, a special phenomenon, so called single unit flashover, has been recognized to occur under some special contaminated and wetted conditions not only in laboratories but also in actual lines.
Higher magnitudes of audible noise (AN), radio interference (RI), and television interference (TVI) from this single unit flashover may give a serious environmental impact to the neighbourhood of the transmission line. Some investigations have ever been made on this problem simulating such phenomenon under normal operating voltage conditions.
Here we explain the investigation results of single unit flashovers under cold-wet-switch-on conditions. Such a condition occurs when a de-energized transmission line is re-energized under a wet condition.
Higher occurrence probability of single unit flashovers was obtained under cold-wet-switch-on conditions compared with the case of normal operating voltage conditions.
Some candidates for countermeasures to suppress the single unit flashover have also been evaluated.

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In the DC transmission line, electrical performances of insulators are very important subjects. It is necessary to clarify the withstand voltage characteristics of insulator strings under contamination and snow-covering conditions, to establish the insulation design criteria.
For this purpose, DC withstand voltage tests of contaminated insulators were carried out using a DC power source which is supplied by 500kV, 10MVA prototype transformer in the UHV Fog Laboratory of CRIEPI. DC withstand voltage tests of double tension insulator strings covered with snow were also carried out using a thyristor controlled power supply at the Yonezawa testing station.
This paper describes the test results obtained so far on the electrical performance of insulator strings, which is requisite for the design of external insulation of DC transmission lines.

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Since the electrical environmental factors such as electric field, ion current and ion density under HVDC transmission lines, are regarded as important factors relating to biological effects, measurements have been conducted in many countries.
One of important concerns in Japan is the electric shocks (similar to carpet shocks) which are caused by charged voltage due to ion current flows into the human bodies wearing shoes with highly resistive sole under HVDC transmission lines. Consideration is given to the prevention of such shocks in designing the height of the HVDC transmission lines.
In order to clarify the characteristics of ion flow electrification, long-term-measurements have been carried out under HVDC test lines since 1971 in CRIEPI.
Charged voltage is a key factor in determining the height of line conductors, because the reduction in charged voltage influences directly the cost of DC transmission lines.
Therefore, a series of tests and analysis were carried out in CRIEPI as a part of the research project for evaluation of the electric environment and corona effects under HVDC bipolar double circuit transmission lines.
This paper describes the influence of conductor height, pole spacing and pole arrangement on the charged voltage and proposes an empirical equation to estimate the charged voltage on the basis of long-term-measurements.

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This paper deals with static var compensators using self-commutated inverters (which is socalled SVG) compared with conventional SVC and Rortary Condenser.
Principle how to improve power system stability, is explained by using simple system model and checked by dynamic digital simulation.
In the latter part of this paper, system outline, basic specifications and control diagram of the SVG rated 154KV, ±80MVA, which has been installed at INUYAMA Switching Station are briefly presented.
Field tests at INUYAMA verify that power swing is effectively damped and transmission capacity is increased.

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A new control system for a self-commutated coverter compensator was studied which applied real and imaginary component of instantaneous power as control signals. The adequacy of the system was verified by digital simulation of EMTP under various conditions of system disturbances. Sampling periods of input signals for digitalizing the control system were also investigated both by digital simulations and Bode diagram. Both results were well agreed with each other, and a prospect of digitalizing the system was obtained.

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In the single-phase AC railroad electrification, the voltage unbalance and voltage fluctuation of three-phase electric power system have to be reduced. For suppression, three-phase power is converted to two-phase power by a three phase-to-two phase transformer, and the electric power is received from a powerful or high-tension source.
However, in the case of the newly AC electrified railroad projects, it is likely that a strong power supply may not be available locally. Further, recently, re-generative braking has come to be applied to electric rolling-stocks. The result is poor when a three phase-to-two phase transformer is used to reduce the unbalance power. So, in some cases reactive power compensation may be found more profitable than a three phase-to-two phase transformer.
This paper describes a theoretical study and results of tests on a mini-model which utilizes reactive power compensation on primary-side of the modified wood-bridge connected transformer. On AC electrified railroad, in which two single-phase loads change independently of each other, reactive power compensation capacity differs for each of the three phases. Therefore, reactive power capacity has to be controlled separately for each phase. Authors examined a method of detecting two single-phase loads to control voltage variation and found the test results about the mini-model satisfactory.

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Recent steady increase of electric power demand causes power sources to be larger and farer from cities based upon demands consentration in megalopolis, and wide area power interchanges leads to make a power system larger and more complicated. The network is susceptable to poor damping power swing oscillations of relatively low frequency which influence to the whole system.
This report describes the optimization method of damping controller's constants on SVC which is added to SVC's voltage regulator, which is from a standpoint that the SVC is powerful equipment for not only restraint of node voltage fluctuation but also stabilization of power oscillation as multi-function controller. This method has major features as follows.
(1) It is able to make the stability of all over the large scale power system improved without appearing the local unstable phenomenon on each controllers.
(2) Combination with S-method, which was developed by CRIEPI for the eigen value analysis in the large scale power system, makes the optimal design of damping controller easy and efficient.
Remarkable effectiveness for stability enhancement is comfirmed by appling optimal designed SVC to the large scale longitudinal power system to compare with the results of only voltage regulation control.

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This paper introduces a coordination technique for a multi-objective generation scheduling of thermal generating system. Today's quality requirement for electrical power makes power system operators to minimize generation cost, while maintaining environmental condition and transmission security as desired. The priority of these objectives may change according to load pickup rate, weather, and other various conditions. However, these objectives are usually in a trade-off relation-ship and often non-commensurable. Therefore, it is difficult to handle by conventional approaches which optimize a single objective function. In this approach we measure the adaptability of performance indices, including economy, NOx emission, and transmission security, by membership functions in fuzzy set theory. Maximization of the fuzzy decision-making function, formed by combining the adaptability indices, yields a coordinated solution of the multi-objective optimization problem. The definition of a membership function, reflecting an operator's vague intention, influences the solution of the multi-objective problem, thus enables to incorporate system operator's varying strategy, realizing a flexible operation according to system operating conditions.

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This paper presents a mixed integer programming model for scheduling the periodical maintenance of thermal and nuclear generating facilities in an electric power system. The periodical maintenance has an impact on system reliability and economics because of planned outages. The system reliability should be the first consideration over the system economics. Based on the idea, first, a maintenance scheduling problem is formulated so as to maximize the minimum reserve capacity under various constraints. This max-min problem can be solved by a mixed integer programming technique. Next, the first object function is modified in order to improve the system reliability by aiming at leveling reserve capacities over a given period as well as possible. Economic facts are handled as simultaneous constraints of allowable maintenance number of thermal generating units which have similar fuel cost curves. The proposed method is tested in a middle-scale power system model. The computational results show that the reliable and economic maintenance schedule is obtained in a practical computer time and the proposed method is a feasible and useful tool.

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The lightning surge response of 15m height cylindrical conductor which stood vertically on the ground is measured by the direct method. Fast optical measuring systems which have the rise time of 5ns are used in the measurements. The wave shapes of the current and the voltage at the top, at 6m below the top and at the foot of the conductor are measured. All current and voltage are measured simultaneously with the tower top current which enables to obtain the time relationship between them.
An equivalent distributed constant line model of a vertical conductor is developed, which produces the surge response wave shapes close to the actual measurements. One section of distributed constant line whose surge impedance is given by the function of conductor radius and height, is found to express the surge response of a vertical conductor. The surge propagation velocity along the line is given by the velocity of light. The proposed model is found to be applicable for the case of short rising time of the imposed current. But the deviation of the response of this model from the actual one increases with the rising time. The correction factor of the proposed model is derived and expressed by the function of the rising time.

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Dissipation of phosphoric acid from fuel cells operating for 2 years is found to cause an increase of gas permeability between the fuel electrode and air electrode and a rapid decrease of the cell voltage. After phosphoric acid is replenished in the cells, gas permeability decreases and the cell voltage is recovered.
Characteristics of cells before acid replenishment are as follows.
(1) Low open circuit voltage.
(2) High temperature rise due to reactive gas supply for the open circuit condition. The temperature rise is in proportion to the conductance of gas permeability between the fuel electrode and air electrode.
(3) Temperature rise depends mostly on the flow rate of fuel gas. Heat of direct combustion can be separated from the heat of the electro-chemical reaction by the measured temperature rise. The ratio of direct combusion to the electro-chemical reaction decreases by increasing fuel utilization.
(4) Air utilization, fuel utilization or pressure difference dependency of the cell voltage have hardly any change due to acid replenishment.

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