Regarding specifications and circuit designs of an instrument transformer (Current Transformer: CT, Voltage Transformer: VT) necessary for a protection and control system, this report arranges knowledge such as technical grounds on such designs, because the present state has not been arranged by systematization, though it has been followed from the past to the present.

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We developed highly accurate optical magnetic field sensors. The sensor optical system has a confocal optical system using ball lenses. Bi-substituted rare earth iron garnets were prepared by the LPE method for use as Faraday rotators. We found that a new garnet composition, Bi_1.14Gd_0.57La_0.02Y_1.27Fe_4.53Ga_0.47O_12, showed stable temperature characteristics of the sensitivity; The temperature dependence of the sensor output was less than 2.0% from -20°C to +80°C. We also improved the linearity error in the small magnetic field by using the film thickness dependence of the wall coercive force. The linearity error of the sensor output was within 1.0% for alternating magnetic fields from 3.0 Oe to 420 Oe. The sensors achieved high performance with regard to the linearity and the temperature characteristics.

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We developed highly accurate optical magnetic field sensor probes. New probe-type sensors 5 mm in width were designed, using a confocal optical system with two drum-shaped ball lenses. BiRIG films were prepared by the LPE method. We found a new garnet composition, Bi_0.98Gd_0.92La_0.03Y_1.07Fe_4.72Ga_0.28O₁₂, that showed high temperature stability for sensitivities less than 2.0% from - 20°C to +80°C. The linearity error of the sensor output was obtained within 1.0% for alternating magnetic fields from 5.0 Oe to 450 Oe. We also developed optical current transformers (OCTs), using this sensor and a permalloy magnetic core with a 6 mm air gap. The sensitivity of the new OCTs is about four times higher than that of our previous ones. The OCTs showed high performance in practical use.

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This paper describes the advance of the ground relay of the line protection.
First of all, we examined the ground fault in a resistively grounded power system. The protective relay detects the ground fault by the zero phase current in the system. There are two methods for the current input to the relay. One uses the residual circuit of CT secondary winding, and the other uses the CT tertiary winding. When the ground current is insufficient in the residual circuit, the CT tertiary winding is used. In order to omit the CT tertiary winding and to get more reliability, we reviewed the full-scale of relay and evaluating the error of CT. As result, almost all transmission lines could omit the CT tertiary winding.
Next, we examined the ground fault with high fault resistance in a solidly grounded power system. When that fault occurs in the system, the PCM current differential relay acts by fault current. So, this relay possibly mal-operates when the CT secondary circuit is disconnected. Therefore, it uses a special CT to raise the reliability. Nowadays GIS has become small and the CT installation space has been decreased, it is needed to reduce these CT cores. In order to omit the special CT, we utilized the combination of the power relay and the rate of change of relay current. As a result, there is a prospect that the relay would not need the special CT.
These improvements are effective to reduce cost too.

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A new type of magnetic field sensor based on creation of the Faraday effect by rotation magnetization of YIG single crystals is described. The influences of both the Cotton-Mouton effect and cubic anisotropies on the sensor characteristics are also discussed. The linearity error of the sensor output was very small in external magnetic fields ranging of 0.02-200 Oe. The temperature dependence of the sensor output was obtained with an accuracy of ± 1 % between 20 °C and 120 °C.

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This paper describes the measuring and protection method for the abnormal rise of magnetizing inrush current in a divided type 3 CTs system. By divided type 3 CTs system, it is possible to measure the primary phase current and zero phase current at the same time. In this reason, the divided type 3 CTs system is widely used for the measurement of high voltage distribution line by simply clamping the each phase lines with 3CTs. For the accurate measurement of the phase current and zero phase current, the internal residual current in CT should be small as possible. It is reported that the abnormal rise of the residual current is generated in the practical field use and several ten ampere (converted to the primary current value) is observed in some case. The abnormal rise of the residual current is caused by the primary magnetizing inrush current or by the sum of the influence by electromagnetic field of the nearby conductors. The magnetizing inrush current is caused by the magnetic saturation of the core of CT. It is difficult to eliminate the abnormal residual current by using the bigger size of core. In our test, we used the active elements and independent feed back coils around the right and left core of CT. By using the feed back current from these coils it was observed that the magnetic saturation of the core is improved and the magnetizing inrush current can be controlled.

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Interconections of dispersed generators like co-generation systems to distribution feeders can cause overvoltages to ground on substation bus lines during single-line-to-ground faults. These overvoltages are transient phenomena which exceed 1.73 pu and would occur at the feeder-circuitbreaker tripping. Significant point of causing the phenomena is existance of instantaneous openphases with single-line-to-ground faults. The open-phases are normal operation, because phases of three line currents on the circuit-breaker are different and don't trip exactly at the same time.
The model of a distribution system involving generators is newly made for an analysis of the above problem. This model is used for a static analysis and a transient analysis. The static analysis is to clarify the system condition under which the zero-sequence voltage on substation bus lines reaches to the maximum amount. The transient analysis is to clarify the transient maximum voltage to the ground. The results of a transient digital simulation were corroborated by means of an analog simulator. The overvoltages to ground rarely cause iron-resonances due to the saturation of a GPT (grounding potential transformer). But they can be suppressed by means of a current-limiting resistor in a GPT.

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We developed optical voltage and current transformers (PT and CT) for 72kV three-phase in one enclosure type Gas Insulated Switchgears (GIS). We used Bi₁₂GeO₂₀and flint glass as device material used in the optical PT and CT, respectively. We also designed an optical PT unit with a pin hole type grounded electrode and intermediate electrode in the pin hole and an optical CT located at the ground level with optical transmission by an optical fiber instead of free space light propagation. We assembled the optical PT and CT into GIS and conducted withstand voltage test, ratio error characteristic test, and other-phase influence test, obtaining good assessment results. Finally, we suggested that the developed optical PT and CT have a possibility of practical use.

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