Tadashi Koshizuka

DC current interruption in HVDC SF/sub 6/ gas MRTB by means of self-excited oscillation superimposition

The Kii-Channel HVDC Link under construction in Japan is equipped with metallic return transfer breakers (MRTBs) in one converter station. A new MRTB for an interrupting current of 3500 A DC was developed. To interrupt a DC current, a method that produces a current zero point by superimposing a self-excited oscillatory current on a DC arc current was employed. For this purpose, a LC circuit was coupled in parallel to a SF/sub 6/ gas circuit breaker. Developing a SF/sub 6/ gas circuit breaker that has a large arc voltage drop gradient against currents permitted a large oscillatory current to be generated. A modified Mayr-type dynamic arc equation was newly presented. The DC interruption limits calculated using this equation agreed with the measured values.

Application of controlled switching to 500-kV shunt reactor current interruption

To suppress reignition overvoltages caused when a 500-kV shunt reactor current is interrupted by a 550-kV one-break circuit breaker (CB), a study was carried out on controlled switching. Using a full-scale test circuit, reactor current interruption tests were carried out to obtain the relation between opening phase angle and generation of reignition. The results showed that even with the dispersion of CB operations taken into account, there were contact separation points free from high reignition overvoltages. It was also proved that no voltage escalations were caused by reignition and high-frequency arc extinction, and that overvoltages due to current chopping were at a safe level in terms of equipment insulation.

RDDS (rate of decrease of dielectric strength) measurement for gas circuit breaker

RDDS (rate of decrease of dielectric strength) is one of the important characteristics of the circuit breaker for applying controlled closing technology. This characteristic is supposed to be affected by contacts roughness and consumption of the circuit breaker caused by rated current or short circuit current interruptions. Also the number of interruptions of the circuit breaker influences the RDDS characteristic. In this paper, the influence of the contact wearing of the circuit breaker by current interruptions to the RDDS was investigated. In a high-power laboratory, the RDDS of a high-voltage one-break circuit breaker was measured at three conditions as follows: before current interruptions; after rated current interruptions; and after short circuit current interruptions. In the field, the RDDS characteristics using another one-break circuit breaker were measured. The results showed that current interruptions have little influence on the RDDS. It is possible to apply controlled closing to circuit breakers after current interruptions. The RDDS calculated by electric field analysis was in good agreement with measurements.

Controlled switching for energizing 3-phase transformers in isolated neutral system

When an unloaded transformer is being energized, a large inrush current may flow depending upon the residual flux in the core of the transformer and the closing phase of the circuit breaker. A large inrush current may cause voltage fluctuations in the transmission system. One method of suppressing the inrush current is to perform controlled switching of the circuit breaker. In this paper, a new method of controlled switching of a 3-phase transformer in an isolated neutral system was examined. A method employing 3-phase circuit breakers with one operating mechanism and also single phase circuit breakers was used to devise a method of control switching for suppressing inrush current when the transformer is energized. In experiment using a 3 kV transformer, actual inrush currents were greatly suppressed.

TRV under transformer limited fault condition and frequency-dependent transformer model

The TRV amplitude factors of 4kVA and 300kVA transformers under transformer limited fault conditions were measured. The measured factors were 1.4 and were lower than the IEC standard. The frequency response of the transformer impedance was investigated using a frequency response analysis (FRA). FRA measurement graphs showed that the short-circuit inductance value of test transformers gradually decreases along with frequency. From this result, a frequency dependent transformer model was constructed. Simulated frequency response of transformer impedance was in good agreement with measured values. Simulated amplitude factor using frequency-dependent transformer model was 1.5 and is good agreement with measured values.

Ferroresonance by open-phase on transformer with delta winding and grounded neutral

It was clarified that the ferroresonance overvoltage phenomenon is caused by an open phase in a power transformer with delta windings and a directly grounded neutral. An analysis of this phenomenon was performed using a 1050 kV transformer, and also a model test using three 3.3 kV transformers. The cause of magnetic saturation of the iron core, which is the cause of ferroresonance, was the appearance of a voltage in the transformer corresponding to the phase that was not connected to the power source, via the delta windings. Previously, the above ferroresonance phenomenon in a transformer was not very well known.

Steep fronts at transient recovery voltages appearing with the interruption of inrush currents of transformers

High lightning overvoltages do not appear in underground substations connected to transmission cables. Consequently, it is very important to thoroughly investigate switching overvoltages and to achieve rational insulation coordination for apparatus installed in such underground substations. This paper discusses the occurrence of steep fronts at transient recovery voltages (TRV) appearing at circuit breakers when the inrush currents of transformers are interrupted. Caused by a steep front at the TRV, reignitions occur at circuit breakers, resulting in the generation of high overvoltages with high frequencies. The overvoltages are among the highest switching overvoltages appearing at the terminal of a transformer. The authors clarified the mechanism of the generation of steep fronts at TRV by means of EMTP analysis, as well as by carrying out tests in a high-power laboratory.

Analytic method using laplace transform for a modified TRV of a circuit breaker

When circuit parameters are known, it is easy to calculate the modified transient recovery voltage (TRV) with resister breaking or MOSA operating by numerical simulations. But, when the TRV is only known, it is very difficult to calculate the modified TRV by numerical simulations. This paper shows the theoretical analytic method of the modified TRV at such a circuit impedance modification as breaking with parallel resister or MOSA operating. A TRV can be calculated by injecting a current from circuit breaker terminals to back impedance. TRV and injected current wave shapes can be expressed with a group of ramp waveforms in Laplace domain. By using our analytic method, the back impedance can be easily derived in the Laplace domain from the TRV and injected current waveforms. As a result, it is shown that the modified TRV at circuit impedance modification can be calculated. Moreover, the same method can be used to calculate the TRV that is reduced by asymmetrical current breaking.