Keisuke Oka

Study of neutral grounding for 22 kV distribution system

To cope with restructuring of electricity market, application of 22 kV distribution system to areas with high load density, is promising. In this paper, we have studied the neutral grounding method considering low voltage rise and induced voltage to communication line in case of ground fault. We have performed EMTP (Electromagnetic Transients Program) simulation on several distribution system models with variation of line length and assembling representing actual distribution systems. We have also compared the simulation result with measurement performed on line imitating actual 22 kV distribution line to confirm the accuracy of simulation result. Consequently, we have clarified the permissible range of neutral grounding resistance.

Analysis of a neutral grounding method for a three-phase four-wire 11.4 kV distribution system

In selection of the neutral grounding method in a three-phase four-wire 11.4 kV distribution system, it is necessary to take into consideration low voltage rise and induced voltage on telecommunication line in case of ground fault. In this paper, we perform EMTP/ATP simulation to select the optimal neutral grounding method and neutral grounding resistance with consideration to above mentioned conditions. The accuracy of the modeling procedure and simulation result is confirmed through verification test performed on test distribution line. As a result, we found (1) that the 11.4 kV distribution system should be associated with a resistance single grounding method; and (2) that the neutral grounding resistance should be above 20 /spl Omega/.

Measurement and analysis of transient overvoltages in full-size 11.4 kV distribution line

To increase the transmission capacity of 6.6 kV distribution lines economically, 11.4 kV distribution system that is constructed of existing 6.6 kV equipment in use is being examined. For the application of the 11.4 kV distribution system to the real field, we are trying to calculate transient overvoltage by EMTP to decide the rational test voltage of 11.4 kV equipment. So, to confirm the calculation precision of EMTP concerned with transient overvoltage in 11.4 kV distribution line, we had made an experiment measuring the characteristics of overvoltages, which occur in 11.4 kV distribution lines. We compared the calculated waveform with the measured waveform, and investigated the difference of the two. We compared the calculated waveform by EMTP with the measured waveform for some test data, and we found that the range of computing errors were from 1.8% to 10.8%.

A calculation of transient overvoltage for rationalization of insulation design of 22 kV cable system

This paper presents a new approach for a reduction of test voltage for power distribution equipment of 22 kV cable system. EMTP calculations are used in this examination for rationalization of insulation design of 22 kV cable system. From the result of parameter analysis by the typical model of 22 kV cable system, the system condition of distribution line that causes the largest transient overvoltage is determined. In the case of switching surge, there is little relationship between neutral resistance and the overvoltage, and then the maximum per unit value of abnormal voltage is approximately 2 pu. On the other side, in the case of fault surge, the overvoltage increases as the neutral resistance increases, and then the maximum per unit value of the abnormal voltage becomes 2.68 pu. According to the above results, the conventional test voltage for 22 kV distribution equipment could be reduced by approximately 20%.

A protection scheme and a method for keeping a voltage balance for an 11.4-kV three-phase four-wire distribution system

An 11.4-kV three-phase four-wire distribution system can economically enhance the supply power by using most of the existing 6.6-kV distribution system facilities. In this paper, the authors made a study on securing safety and electric power quality in the 11.4-kV system by presupposing the use of a single resistance grounded neutral system based on EMTP simulations. For the safety aspect, we conducted a study on applying the major protection methods of the 6.6-kV system to the 11.4-kV system. As a result, the conditions for the application were revealed. For the electric power quality aspect, we propose the load connection phase control method and the Y-/spl Delta/ connection for transformers for motors to improve the voltage unbalance. Through these measures it becomes possible to secure the quality similar to that of the existing 6.6-kV system.