This paper discusses how to integrate variable power source such as wind power and photovoltaic generation into a power grid. The intermittent renewable generation is expected to penetrate for less carbon intensive power supply system, but it causes voltage control problem in the distribution system, and supply-demand imbalance problem in a whole power system. Cooperative control of customers' energy storage equipment such as water heater with storage tank for reducing inverse power flow from the roof-top PV system, the operation technique using a battery system and the solar radiation forecast for stabilizing output of variable generation, smart charging of plug-in hybrid electric vehicles for load frequency control (LFC), and other methods to integrate variable power source with improving social benefits are surveyed.
Recently, the introduction of wind power generation is increasing rapidly. The ratio of wind power generation to the capacity of a total generation is getting higher and higher. When the phase-to-phase fault occurs in the power system, the frequency of power system is lower due to disconnecting of the wind power generation with doubly fed induction generator (DFIG). Therefore, the power system might become unstable. This paper describes the LVRT (low voltage ride through) performance improvement scheme of the wind power generation with DFIG. The wind power generation is disconnected from the grid in case of the power system fault. It is independently in operation from the grid by controlling of the inverter equipped in the generation. After clearance of the power system fault, the wind power generation is immediately re-connected to the grid. As a result, instability in the power system disappears. The performance of LVRT is confirmed by using simulation software PSCAD/EMTDC. The simulation result shows an excellent result to the three-phase short-circuit fault of the voltage dip 100%.
Recently, the number of system interconnection of the renewable energy sources (RES) such as the photovoltaic generation (PV) and wind power generation is increasing drastically, and there is in danger of changing the voltages in a distribution system by the precipitous output variation of RESs. In this study, the authors propose one voltage control method of the distribution system by the power factor control of plural PV systems in consideration of cooperation with the load ratio control transformer (LRT) of laggard control response installed beforehand in the distribution system. In the proposed method, the slow voltage variation is controlled by LRT, and the steep voltage variation uncontrollable by LRT is controlled by plural PV systems, as a result, all the node voltages are controllable within the proper limits. In order to verify the validity of the proposed method, the numerical calculations are carried out by using an analytical model of distribution system which interconnected PV systems.
Large capacity photovoltaic (PV) systems will be installed in future power systems. According to Japan's energy outlook, the target capacity of PV systems for 2030 is about 50GW. When the large volume PV systems are installed in a power system, the reverse power flow from PV systems may cause the surplus electric power. To utilize the surplus electric power without the reduction of PV power generation, operation of customer equipment such as heat pump water heater (HPWH) according to PV power generation is expected to be one of the solutions. In this paper, cooperative control method of customer equipment with the PV power generation is proposed, and the effectiveness of the proposed control is evaluated by simulation analyses.
It is generally believed that a large amount of battery system will be needed to store surplus electric energy due to high penetration of renewable energy (RE) such as photovoltaic generation (PV). Since main objective of high penetration of REs is to reduce amount of CO2 emission, reducing kWh output of thermal generation that does emit large amount of CO2 in power system should be considered sufficiently. However, thermal generation takes a important role in load frequency control (LFC) of power system. Therefore, if LFC could be done with battery and hydro generation, kWh output of thermal generation would be reduced significantly. This paper presents a method for LFC using battery in power system with highly penetrated PVs. Assessment of the effect of the proposed method would be made considering mutual smoothing effect of highly penetrated PVs.
In order to integrate a substantial amount of fluctuating solar energy into the power grid, accurate irradiance prediction systems are required to estimate the power output of solar power plants. This study presents a simple statistical approach to predict irradiance on the basis of regional numerical weather prediction data that is forecasted up to three days by the Japan Meteorological Agency. Annual irradiance forecasts of individual sites in the Hokuriku region demonstrate a root mean square error of 0.43-0.45MJ/m2 for the intra-day forecast, increasing to about 0.51MJ/m2 for the two-days-ahead forecast. Furthermore, we calculated error indexes under different conditions to estimate the magnitude of the influence of different assumptions on the results.
The capacity of Distributed Generators (DGs) connected to grid by inverters are growing year and year. The inverters are generally controlled by PLL (Phase Locked Loop) in order to synchronize with power system frequency. Power systems will become unstable, if the capacity of inverter type DGs become larger and larger, because inverter frequency is controlled just to follow the frequency decided by other synchronous generators. There is the idea that inverters are controlled to behave like a synchronous generator. This concept is called Virtual Synchronous Generator (VSG). In this paper, a control scheme of VSG is presented, and the design method of required energy storage and the ability of grid stabilizing control by VSG is investigated by computer simulations.
A microgrid (MG) is one of the measures for enhancing the high penetration of renewable energy (RE)-based distributed generators (DGs). For constructing a MG economically, the capacity optimization of controllable DGs against RE-based DGs is essential. By using a numerical simulation model developed based on the demonstrative studies on a MG using PAFC and NaS battery as controllable DGs and photovoltaic power generation system (PVS) as a RE-based DG, this study discusses the influence of forecast accuracy of PVS output on the capacity optimization and daily operation evaluated with the cost. The main results are as follows. The required capacity of NaS battery must be increased by 10-40% against the ideal situation without the forecast error of PVS power output. The influence of forecast error on the received grid electricity would not be so significant on annual basis because the positive and negative forecast error varies with days. The annual total cost of facility and operation increases by 2-7% due to the forecast error applied in this study. The impact of forecast error on the facility optimization and operation optimization is almost the same each other at a few percentages, implying that the forecast accuracy should be improved in terms of both the number of times with large forecast error and the average error.
It is of prime importance to solve the voltage maintenance problem caused by the introduction of a large number of distributed generators. The authors have proposed “voltage profile control method” using reactive power control of distributed generators and developed new systems which can give economical incentives to DG owners who cooperate the voltage profile management in the previous works. However, it is difficult to apply the proposed economical systems to real-time operation because they are based on the optimization technology and the specific amount of incentive is informed after the control action has finished. Therefore, in this paper, we develop a new method that can determine the amount of incentives in real-time and encourage the costumers to cooperate voltage profile control method. The proposed method is tested in one feeder distribution network and its effectiveness is shown.
This paper presents the experimental results of a grid connected inverter. The control of the inverter is based on the virtual synchronous generator model of algebraic type. In case of using the virtual synchronous generator model of algebraic type, there is no evidence for the satisfactory operation with unbalanced load and nonlinear load, because the generator model are constructed under the assumption that the load is linear and balanced. By choosing the feedback control loop gain appropriately, the satisfactory operation is achieved even if the unbalanced and nonlinear load is connected. Experimental results show that the power controls, active and reactive, are achieved preferably in grid connecting mode. Smooth transitions are also achieved from connecting mode to the island mode. The inverter can operate satisfactorily when the unbalanced load is connected. Harmonic analysis is done when a three-phase full-wave rectifier is connected as a nonlinear load. The harmonic current is supplied by the inverter and its output LC filter. It is shown that the harmonic current is supplied mainly by the inverter and that the voltage waveform is practically acceptable.
The large scale photovoltaic (PV) generation station is expected to spread in the future. However, output power of renewable energy sources such as PV is affected by weather conditions and their output tends to be unstable. As a result, the penetration of PV power station makes it difficult to maintain frequency of power system in allowable range. The authors have developed a suppression system to stabilize output power fluctuation of a large PV generation station. To reduce short term fluctuation, storage batteries applying SCiBTM are used. In this paper, verification test results are explained and simulation results to improve control performance are also shown.