This paper summarizes the power system issues that are of concern when the introduction of renewable energy progresses, and explains the technology and trends of smart inverters, virtual synchronous generators, and grid forming inverters that are expected to be countermeasures.

The service restoration within a short period, performed when the permanent fault occurs in a distribution system, is essential for high supply reliability. The service restoration can be achieved by searching the fault location and controlling switches to isolate the fault area. However, the service restoration faces difficulties in the uncertainty of the fault location and the multiple crews' scheduling of the switching operations. It is important to search for the unknown fault location efficiently and optimize the crews' scheduling for their service restoration procedures. This paper proposes an optimization method of the service restoration procedures performed by crews considering the search of the unknown fault location by crews to handle the difficulties. The proposed method combines an efficient fault location search with scheduling for multiple crews. We formulate the scheduling problem as a mixed-integer linear programming problem which allows us to schedule the procedures efficiently using optimization solvers. The proposed method is verified by the computational experiments using the large-scale distribution system model with many manual and remote switches.

Photovoltaics (PV) output prediction, which is indispensable for power system operation, can affects demand and supply adjustment adversely when large prediction error occurs. Thus, the reduction of large error as well as average error is required in PV power prediction. In 2019, the operation of the Meso-scale Ensemble Prediction System (MEPS) of numerical weather prediction started from the Japan Meteorological Agency, and the amount of forecasting information would be potentially useful for the improvement of PV power prediction. However, very few studies on inputting multiple meteorological elements of the MEPS have been reported. In this paper, we newly develop the prediction model for an area day-ahead PV power output composed of Just-In-Time Modeling (JIT Modeling) with multiple elements of the MEPS. The developed method achieves precise forecasts with low computational load by both selecting meteorological elements valid for improving prediction accuracy and adequately devising the structure of JIT Modeling. Some numerical examples demonstrating the effectiveness of the developed method are also presented. In particular, the proposed method reduces large error significantly.

If an inverter-connected power supply is introduced into the power system with the expansion of the introduction of renewable energy, there is a concern that the inertia of the system will decrease, so research on inverters with inertial functions is underway. At the same time, there is concern about the influence of a decrease in short-circuit current. The authors focused on the influence on protection relays for transmission lines, clarified the influence of the fault current that occurs when the inverter power supply with inertia function continues to operate while suppressing the overcurrent in the event of a fault, comparing it with the synchronous generator, and examined the necessity of countermeasures. This paper reports the results.

This paper proposes a new method of Distributed Energy Resource Management System (DERMS) Volt-Var droop curve control for electric power distribution systems using reactive power demand response, which we call VarDR. The DERMS aggregates the droop-curve-designated reactive power from a cluster of dozens of air-conditioners in neighboring buildings near the location of a problem in the distribution system. Computer simulations using a VarDR DERMS were carried out using a model scenario with large PVs, extremely fast-charging EV stands, and battery energy storage systems (BESSs). The VarDR DERMS droop curve control showed stable recovery from transient over/under voltages without significantly degrading the air-conditioning performance.

In March 2021, the Shin-Shinano Substation of TEPCO Power Grid, Inc. was connected to the Hida Convertor Station of Chubu Electric Power Grid Co., Inc. by HVDC transmission system with an interconnection capacity of 900MW in order to increase the interconnection capacity between the eastern and the western areas in Japan. The 200kV HVDC overhead transmission lines have a pair of energized conductors and a return conductor. It has been investigated that TACSR (410mm² [AC410], 610mm² [AC610], 810mm² [AC810]) would be used as the return conductor. However, as the return conductor plays the role of a ground wire, the melting and breaking of TACSR strands due to lightning strike was concerned. Therefore, in this paper, numerical evaluations of melting and breaking characteristics of the return conductor strands were carried out supposing the return conductor was struck by DC arc discharge simulating lightning whose maximum current was 400kA. As a result, the number of breaking strands of AC610 was more than that of AC410 and AC810 because the diameter of strands of AC610 was smaller than that of AC410 and AC810. The remaining rate of cross-section area of strands of AC410 decreased more than AC610 and AC810 as DC arc current increased.