昨年12月に沖縄電力の給電指令所を見学する機会があった。沖縄では家庭用太陽光発電が急速に普及し，電力システムの運用がとても大変になってきていると聞いていた。実際に過去3年ほどの夏場の日負荷カーブ（需要から太陽光発電を差し引いた電力）を目にして，その形状の異様さに

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The stable electric power supply requires to maintain the system frequency within a specified band by matching the supply and demand constantly with the help of Economic load Dispatching Control (EDC) or Load Frequency Control (LFC). On the other hand, recently the penetration of renewable energy sources (RESs) such as photovoltaic generations (PVs) and wind turbines (WTs) are rapidly increasing due to the introduction of feed-in tariff (FIT) and growing environmental awareness. However, the output power generated by these RESs depends on the climatic conditions, and as such the output may not always be available when required. As a result, it would be more difficult to ensure the stable power supply by balancing the supply and demand in view of the uncertain outputs of RESs at all the time. In order to grasp the influences and to verify the effectiveness of countermeasures for various issues such as the frequency deviation caused by a large penetration of RESs, the analysis and simulation technologies for the electric power supply and demand control are becoming more important. Hence in this paper, the recent trends of these technologies are reviewed.

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Recently, introduction of the distributed power generation using renewable energy sources like photovoltaic (PV) and wind turbine (WT) generation is expanded. However, the output fluctuation and the unpredictability may have an impact on system frequency. In this paper, the methods to suppress the frequency fluctuation by the control of existing generators and PV are proposed. The probability of occurrence of the ramp rate violation and the available generating capacity shortage, and fuel cost are also checked. First, the method using pumped-storage generators is simulated. The operation time of pumped-storage generators is changed to prevent the surplus power. Second, the methods using thermal generators are simulated. Reserve margin is increased to prevent the deficit power, and 10-min spinning reserve (SR₁₀) is increased to prevent the short-term power fluctuation. Finally, the method using PV is simulated. Suppression control of PV is simulated to prevent the surplus power.

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There is a growing interest in a time-domain simulation of active power balancing and frequency control to examine the frequency stability in long-term time scale (up to several hours or a day). In order to perform the accurate simulation, proper models of thermal power plants that play principal roles in the balancing and frequency control are essential.  In this paper, new models of conventional steam power plants and gas turbine combined cycle power plants suitable for balancing and frequency control analysis are presented. In the developed models, thermal power plant dynamics and plant control systems are simplified without affecting the accuracy. This simplification decreases the number of parameters and reduces the computational burden. The developed models were validated through the comparison between the model response and the measured response of representative thermal power plants.

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We have developed a unit commitment (UC) model which can consider the confidence intervals of photovoltaics (PV) forecast data. We also analyzed supply-demand balance for the Tokyo Electric Power Company area with highly penetrated variable renewable energy sources in 2030. For 2030, 53GW of installed PV capacity in Japan (17.5GW in Tokyo area) was assumed. The case study was carried out to investigate the effect of the UC considering forecast error on day ahead supply-demand balance. As a result, the number of operating thermal power plants increased and the rate of operation of coal-fired power plants decreased to reserve the Load-Frequency Control capacity whose amount increased due to highly penetrated PV. Pumped storage plants stored water in the daytime, when the power of PV is peak, not in midnight to increase the amount of electric load and the operation rate of thermal power plants. Therefore broaden confidence intervals or increasing the amount of forecast errors considered in UC raised the expected operational cost.

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Sudden and large changes in renewable energy generators called ramp events are one of the major issues to be addressed for the stable operation of electric power system. By classifying the change of spatial average irradiance in a day, which is the primary factor of aggregated power output of photovoltaic power generation system (PV system), this study statistically evaluated the frequency of ramp events in a year. As a result for the spatial average irradiance in the Chubu area in Japan, the ramp event occurs in 42 days (12%) in a year. The duration of ramp event varies mainly between 30-120min. Then, this study statistically evaluated the short-cycle fluctuation characteristics of spatial average irradiance during ramp event. As a result, in the case of ramp down of relatively large width, the short-cycle fluctuation before and during the ramp event is as large as that in quasi-fine day, while it after the ramp event is small enough as in cloudy day. The results can be useful as approximate evaluation regarding generation flexibility required in electric power system with high penetration PV systems.

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Large error of a day-ahead forecast of aggregated power output of photovoltaic power systems (PVS) can threaten power supply-demand balance in electric power system. When the forecast error is large, the power supply flexibility such as load-frequency control (LFC) resources would be exhausted to maintain supply-demand balance. Such a situation should be taken into account in an impact assessment of a day-ahead forecast error by using UC-EDC simulation model. Based on the availability of several minutes interval data of electricity demand and spatial average irradiance, this study proposes a UC-EDC simulation model including constraints on actually required LFC resources. As a result, because power supply resources can be used as much as possible for supply-demand balancing, electricity supply shortage and surplus are smaller than those in a conventional simulation model including constraints on fixed amount of LFC resources regardless of the real fluctuation levels.

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The growth of renewable energy sources such as photovoltaic and wind power generation requires more reserve capacity to maintain supply-demand balance. This paper proposes a novel battery utilization method for assist of Load Frequency Control (LFC) by compensating the lack of generation rate for LFC served by thermal power plants. In the proposed method, the battery output is driven by excess component of LFC signal over the generation rate. In order to check the performance of proposed method, a comparison study has been done with a conventional method. When the generation rate is high, frequency fluctuation is not suppressed in proposed method since LFC units respond to LFC signal quickly and the battery hardly supplies reserve capacity. On the other hand, when the generation rate is low, frequency fluctuation is suppressed in proposed method. This is because LFC signal exceeds the generation rate and the battery provides with reserve capacity which is not served by thermal power plants. As a conclusion, the proposed method is adaptable for the total generation rate of LFC units and useful for suppressing the deterioration of frequency stability when the total generation rate of power system is relatively reduced by large penetration of renewable energy sources.

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