Technologies for estimation and forecasting of photovoltaic generation output supporting stable operation of electric power system

Abstract

In recent years, Japan has rapidly extended its use of electricity obtained via photovoltaic (PV) generation. When the proportion of PV in the generation mix was small, its unpredictable output fluctuations had little effect on the supply and demand operation. However, as the role of PV in grid supply grows, predicting these fluctuations becomes increasingly necessary to ensure stable and economical operation of the electric power system. In this study, we discuss the current approaches of estimating and forecasting the PV output. PrefaceElectricity supply networks must balance the supply capacity with the fluctuating demand. Capacity is adjusted by bringing power stations on-and off-line and changing the power generated. However, the price of electricity from different units varies according to the generation method used and, therefore, it is necessary to seek the most economical combination at any demand level [1].Recent years have seen an expansion in the role played by renewable energy in Japan as the country attempts to build a low-carbon society. These renewable energy sources are clean and sustainable and do not emit greenhouse gases, but their output fluctuates in response to the weather conditions, posing challenges to the stability of the supply system. Photovoltaic (PV) generation is an example of the most widely deployed renewable source, so the transmission departments of power companies are investigating and deploying various measures to support demand/supply balancing in a system with a large PV component [2].In this study, we discuss the challenges that this poses and the approaches currently being pursued. However, the generation units that can be directly observed by telemetry from the central load dispatching center of a power company are limited to those interconnected with power lines of 60 kV or greater, which excludes most of the grid-connected PV. New techniques are therefore needed to monitor (estimate) the power generated by PV units. Necessity of Estimating and Forecasting PV outputIn the case of PV, abrupt fluctuations in output can be caused by short-term changes in weather conditions. These are too brief to be predicted from the daily weather forecast alone. Load balancing requires thermal power stations, hydroelectric generation, and pumped storage to be ramped up or run down to compensate for these abrupt fluctuations in the PV output.With the high penetration of PV, the grid operators may no longer be able to ensure constant power supply because thermal and hydroelectric generation constitutes a smaller proportion of the generation mix.In particular, mid-merit-order thermal stations, which play a key role in supply balancing, require several hours to bring the power units off or on line, making it necessary to evolve precise shortterm forecasting of PV output and ensure low power generation costs. PV Output Estimation TechnologyThe output of a PV unit can be calculated by multiplying the solar radiation intensity at the installation location by the conversion efficiency of the P...

Research on Frequency Control of Islanded Microgrid with Multiple Distributed Power Sources

“…(6) P BAT_MIN is the charging power limit that PCS can withstand, and P BAT_MAX is the discharge power limit that PCS can withstand. (7) Turn off the diesel gensets and reduce the PCS discharge power appropriately. End the control of the cycle and wait for the next cycle of data to make a judgment.…”

“…End the control of the cycle and wait for the next cycle of data to make a judgment. (9) If there is unloaded load in the system, go to step (9), otherwise jump to step (7). (10) Connect the load to the system one by one according to the order of priority of the load.…”

“…(6) Estimate the stability of the system. If the Equations (7) and (8) are satisfied, the frequency of the system is still in the offset state and the situation is deteriorated.…”

“…It indicates that the frequency of the system is still in the offset state and the situation is deteriorating. At this point, it should be transferred to step (7), and report "system frequency CL area fault" to the upper monitoring system, try to maintain the stable state of the system, and wait for the command of the upper monitoring system. End the cycle operation and wait for the next cycle data to make a decision.…”

“…The isolated microgrid, which is not connected to the external large power grid, can provide high-quality power supply to areas where it cannot be covered by the large power grid, such as the marginal mountains and islands [2][3][4][5]. However, the unpredictability of load and random output of existing distributed power sources, such as wind power generation and photovoltaic power generation, will have a great impact on the power quality of microgrid [6][7][8][9]. If the proportion of distributed power in the large power grid is too high, there will also have a certain disturbance to the large power grid, affecting the safe operation of the large power grid [10,11].…”