Differences between domestic standard JEM and foreign national standards ANSI, GB for medium-voltage switchgear were investigated and summarized in this study. Medium-voltage switchgear for rated voltage above 1kV and up to and including 84kV has been widely applied to electric power company, general industry, and electric railway over 50 years. Main circuit insulation is mainly air insulation type. Recently, SF6 gas, solid, or vacuum insulation types were developed. Interruption media of circuit breaker were changed from air or oil to vacuum. Applications of medium-voltage switchgear were expanded to ships, on/off-shore renewable energy plants. In this paper, recent technology trends were reviewed for corresponding to special specifications and results of performed questioner survey were shown for domestic application of medium-voltage switchgear in Japan.
In this paper, waiting effects of photovoltaic generation (PV) output curtailment by power conditioning system (PCS) on voltage management in a distribution network are quantitatively evaluated, and an appropriate waiting time is determined based on the evaluated results. PV output generation can be categorized to 5 groups based on the weather characteristics, and the waiting effects are evaluated in terms of PV output curtailment amount, cumulative voltage violation amount, maximum voltage violation, and the number of tap operations as the impacts on a distribution system operator and customers. Through a numerical simulation, we found that the waiting time effects in cloudy groups were higher than other groups with keeping the maximum voltage violation lower than the allowable voltage limit. Also, the appropriate waiting time for PCS control was determined based on the relationship between the waiting time and PV output curtailment reduction rate. The results also represent that we do not need to set a longer waiting time from the viewpoints of PV output curtailment reduction rate though the waiting time should be set larger than the time delay of voltage regulators.
When excessive power supply is caused by large amount of renewable energy sources, existing synchronous generators (SG) are supposed to adjust the outputs to balance supply and demand. Transient stability may deteriorate depending on the magnitude of curtailment for SGs. In this paper, transient stability have been evaluated from the viewpoint of critical clearing time (CCT), assuming the case that SG output is curtailed when wind farm (WF) output increases. As the result, it is clarified that SG is difficult to step out and transient stability improves in the case of calculation condition where the difference of acceleration among SGs during fault becomes small. As a method for improving the transient stability utilizing this, we proposed the adjustment method of SG outputs that makes acceleration among SGs during fault coincide.
This paper proposes a novel power smoothing control method for a photovoltaic generation (PV) system using a water electrolyzer (ELY). This method realizes that the long-period components of the PV power fluctuation are supplied to the power grid and the short-period components of the PV power fluctuation are supplied to the ELY. The power supplied to the ELY is converted to hydrogen for fuel cell vehicles. The results of the power smoothing control method are evaluated in terms of the power in the load frequency control (LFC) band and the ability of generating hydrogen. Furthermore, filtering characteristics of the proposed method are clarified. As the result, the proposed method has reduced the power in LFC band by 83.7% compared to the PV power without the power smoothing control. The filter characteristics of the proposed method have shown that fluctuating components with periods less than 1000s, in which the step voltage regulator does not perform, can be sufficiently eliminated from the PV power fluctuation components. When applying the proposed method for a PV system rated at 800kW, the ability of generating hydrogen was about 2.30% of the hydrogen required for one hydrogen station.
The authors analyzed the relation between height of -10°C air layer derived from Meso-Scale Model and 51 examples of damage at low structures by winter lightning at the coastal areas of the Sea of Japan in Tohoku in three winter seasons. We also discussed the number of lightning discharges detected by the Japanese Lightning Detection Network (JLDN) and height of -10°C air layer when those damage occurred. In the results, the characteristics of lightning discharges were often similar to single lightning flash when height of -10°C air layer was less than 2000m. Most damage at low structures by winter lightning occurred when height of -10°C air layer was higher than 2000m. The number of lightning discharges tended to become large when a cold front passed over the coastal area of the Sea of Japan in Tohoku. The damage caused by winter lightning concentrated between the middle of November and early January and did not occur when height of -10°C air layer was less than 1200m.
Accurate evaluation of flashover occurrence at arcing horns is necessary for the analysis of multiple back-flashover phenomena of transmission lines. In this paper, analyzed results of flashover phenomena in parallel gap configuration with residual voltage after flashover at one of the gaps are compared with experiments. The agreement between the theory and experiments is fairly well, which proves the validity of the analysis.
Surge arresters that protect the equipment of the electric power systems against lightning or switching overvoltage may degrade due to the stress of overvoltages and increase leakage currents under normal system voltages. As a preventative maintenance, the increase in the leakage current of surge arresters is normally checked, but this method takes time. As the leakage current increase causes the temperature rise of the surge arresters, the diagnosis with the thermographic camera is available. But this method is not common because there are few studies on the relationship between the temperature rise and the degradation level.
The authors measured the temperature rise of a polymer arrester under normal and aged conditions with a thermographic camera, and compared measured values to simulation results. The simulation showed good agreement with the measurements. The diagnosis by the temperature detection is proposed as one of the useful ways to find the degradation of surge arresters under operation in electric power systems.