Wind power has shown marked increase both in the world and in Japan. However, since wind power generation has distinct characteristics—e.g., intermittency of output power—from conventional power source, its large-scale integration provokes concerns on its adverse effects on an electricity grid. In Japan, governmental committees have discussed the effects and recommended the researches on the following topics: output fluctuation characteristics of wind power, stabilization of output fluctuation of wind power with energy storage technologies, wind power prediction, etc. The article summarizes the results of the researches as well as the researches in Japan on control methods of wind power and an electricity grid frequently examined in the papers of IEEJ Transactions on Power and Energy.
The rapid increase of wind power plants and enlargement of capacity makes the lightning problem of the wind power plants including blades serious. This report summarizes the result of the comprehensive surveillance study carried out for these several years. The contents lightning outages, the lightning phenomena and the result of the electric discharge experiment in connection with the measure against lightning of a windmill blades. Furthermore, the state of the present condition of the protective measures in the blades of windmill, mechanical parts, connecting power lines and communication lines was also considered. Especially grounding methods for wind power plants are considered.
Shimane Nuclear Power Station Unit 1 (Shimane-1) of Chugoku Electric known as the first Japanese-made nuclear power plant scrammed automatically before dawn 12th September 1985 due to a power transmission grid trip result from thunderstroke. Until then, Shimane-1 was believed to be able to continue operation regardless of such power transmission grid trouble because the turbine bypass system had sufficient capacity to process total rated steam flow generated in the reactor. This paper describes the concerted efforts to achieving islanding operation of Shimane-1 by plant fabricator Hitachi, CRIEPI and Chugoku Electric with this incident as a start.
This paper proposes a new control method for reducing fluctuation of power system frequency through smoothing active power output of wind farm. The proposal is based on the modulation of rotaional kinetic energy of variable speed wind power generators through power converters between permanent magnet synchronous generators (PMSG) and transmission lines. In this paper, the proposed control is called Fluctuation Absorption by Flywheel Characteristics control (FAFC). The FAFC can be easily implemented by adding wind farm output signal to Maximum Power Point Tracking control signal through a feedback control loop. In order to verify the effectiveness of the FAFC control, a simulation study was carried out. In the study, it was assumed that the wind farm consisting of PMSG type wind power generator and induction machine type wind power generaotors is connected with a power sysem. The results of the study show that the FAFC control is a useful method for reducing the impacts of wind farm output fluctuation on system frequency without additional devices such as secondary battery.
Wind generators are rapidly gaining acceptance as some of the best alternative energy sources. However, wind speed is not constant and the power output of wind generators is proportional to the cube of the wind speed. In order to control the power output for wind power generators, it requires method of wind speed estimation. This paper proposes 3 hours ahead power output forecasting of wind generators based on wind speed forecasting by using Recurrent Neural Network (RNN). The validity of the proposed RNN is confirmed by comparing the forecasting abilities of feed-forward neural network (FFNN) and RNN in simulation results of 1∼3 hour ahead forecasting. The proposed method is not require complicated calculation.
Most of the methods forecasting the wind speed proposed so far are not universal and available only for specific area. In the present study, forecasting method of a time-series wind speed 24-hours ahead at any location in Japan was proposed, which was based on classified forecast models by wind conditions (called FCCM; Forecasting Method using Classified forecast Model). The model was structured by the radial basis function network (RBFN) and the performance was compared to the conventional feed forward neural network (FFNN). The procedure of the FCCM was as follows; (1) All the points where wind speed had been measured by the Japan Meteorological Agency were divided into the groups depending on their wind conditions (mean wind speed and standard deviation of wind speed). (2) RBFN was learned and the network model was optimized to forecast wind speed in each group. (3) If the wind speed was not measured in the forecast point, it was estimated by using measured data around there. (4) Then, the time-series wind speed of the forecast point was obtained by the network model for the group. The wind speeds at 6 points in Japan where the wind condition were different were forecasted by FCCM. As a result, the mean absolute error was about from 1.0 to 1.5m/s. Although forecast error of RBFN and that of FFNN were similar, RBFN was able to learn fast than FFNN.
This paper proposes a new control method for maintaining a charging level for a battery energy storage system (BESS) coupled with a wind farm (WF) to stabilize WF output. Since output of wind power varies with wind speed, operation of a power system will be influenced if a large number of WFs are interconnected to the power system. The authors have made a research project to conduct demonstration tests of a BESS coupled with a WF to smooth out short-term fluctuations in WF output. In operating the BESS, it is essential to maintain a charging level of a battery within a proper range. This paper thus proposes a new control method called “State-Of-Charge Feedback Control (SOC-FB control)” to stabilize the charging level. In this paper, characteristics of the SOC-FB control are discussed through theoretical analyses with its transfer functions as well as through simulation studies. Analyses on the transfer functions show that the SOC-FB control is effective to stabilize the charging level of the battery. Sensitivity of the control parameters of the SOC-FB control is examined from the viewpoints of the performance of smoothing the output fluctuations and stabilizing the charging level. The performance of the BESS is also examined through simulations using actual WF output data. The results show that charging level is kept within its proper range without deteriorating the smoothing performance by adopting the SOC-FB control, while the charging level goes beyond its proper range without the SOC-FB control. These results clarified the effectiveness of the SOC-FB control to maintain the charging level of the battery. Results of the sensitivity of the control parameters are considered useful to select proper parameters for the control.
In this paper, we propose a simple wind power prediction system using a self-tuning fuzzy reasoning and an error persistent model. The self-tuning fuzzy of which fuzzy rules are automatically adjusted so that the tuning error between a measured and a predicted wind power decreases, and converges to a small constant value by using predicted wind speed and direction data of the mesoscale spectral model (MSM)-GPV providing from the Meteorological Agency. After tuning process is completed, adjusted fuzzy rules imply wind power prediction rules which is considered geographical features, distance between the nearest wind forecast point and the site of the wind power generator system. Moreover, to decrease a prediction error, we apply an error persistent model to prediction results under the fuzzy reasoning. Some simulation results demonstrate the usefulness of the proposed simple wind power prediction system.
Performance of wind turbines is much influenced by hazards associated with lightning hits. Damages of wind turbines by lightning hits in Japan show seasonal and regional variations, which may reflect characteristics of lightning hits on tall structures dependent on the season and the region. It was revealed that serious lightning faults of high voltage power transmission lines, experienced so far in winter along the coastal area of the Sea of Japan, were mostly due to upward lightning initiated from transmission towers. Wind turbines in the same region, reportedly suffer from a high lightning fault rate, must also generate frequent upward lightning, which is likely the cause of the frequent lightning damages. In this paper, it is proposed to apply the method to predict lightning performance of transmission lines, in winter in Hokuriku area, to evaluate frequency of upward lightning from tall structures in cold regions of Japan in winter. Moreover, the method to evaluate the risk of lightning hits on high structures in Japan through the year, in combination with the LLS data, is proposed.
A wind turbine generator system is composed of blades, a nacelle, a tower etc. In recent year, to make the blades and the nacelle lighter, the blades are often made from GFRP (Glass Fiber Reinforced Plastics) and the nacelle is composed of steel grids and GFRP covering it. When the lightning strikes the wind turbine, the current flows into the ground through the down conductor in the blades, the steel grids of nacelle, and the tower. The current flowing near the nacelle produces comparatively large magnetic fields inside the nacelle. Therefore, the communication and control systems are broken down or malfunction. In this paper, the transient magnetic fields in a nacelle due to lightning strokes at the tip of the blade and the rear portion of the nacelle are studied analytically and experimentally. In the analytical studies, the FDTD (Finite Difference Time Domain) method which is one of numerical electromagnetic field analyses is used. The reduced-size model of a wind turbine is used for experimental studies. As the results of the researches, it has been found that the magnetic fields at the front of the nacelle are larger than those at the rear of the nacelle regardless of the injected points of the lightning current. The lightning current following through the down conductor connected to the nacelle through the blades has a strong influence on the magnetic fields in the nacelle. We have proposed that the front of the nacelle should be covered with the conductor plate or mesh to decrease the large magnetic fields.
The power fluctuations of the wind power generation due to the wind speed variation cause a considerable frequency deviation to the power system in Tohoku and Hokkaido, etc. The frequency deviation has hardly an allowance. It is necessary to smooth the power fluctuations to interconnect a new wind power generation to the power system in those areas. Thus, the power storage system such as the battery system must be installed together with wind power generation. It needs the inverter system because there is DC power supply in the battery system. However, the system is expensive. A recent wind power generation has inverter system. The smoothing performance for wind power fluctuations can be given to the wind power generation by using the inverter system. The paper proposes a new wind power generation for which the smoothing performance is provided by such a scheme. The smoothing performance is examined in using simulation software PSCAD/EMTDC. The simulation result shows that a new wind power generation has an excellent smoothing performance for wind power fluctuations.
Wind power provokes increasing concerns on its impacts on a power system as many wind power generators have been integrated to a power system. One of the concerns is a challenge to keep supply-and-demand balance in a power system. Battery energy storage systems (BESS) installed at a wind farm are considered as a promising option for the challenge. Two types of operation strategies were proposed for its operation: fluctuation mitigation and scheduled output types. On the other hand, in order to constitute an economical BESS, selecting battery size as small as possible is vital. However, for the scheduled output operation, no study has been published both on the required battery size and on a methodology to examine the size. This paper proposes a methodology to evaluate the required kWh capacity of battery for the scheduled output operation using accumulated prediction error of a wind farm. The methodology evaluates a standard deviation of accumulated prediction error with autocovariance functions of prediction error. Based upon the statistical characteristics obtained for actual prediction error of wind power, the paper presents results of a sample study to show the effectiveness of the methodology.
Battery is installed for with wind power generator to level the output power fluctuations, since output power fluctuations of wind power generator are large. However, if large battery is installed in wind turbine generator, the capital cost for wind power system will increase. Hence, the smallest size of battery should be preferable to save the capital cost. In this paper, we propose a methodology for controlling combined system output power and storage energy capacity of battery system. The system consists of wind turbine generator and battery energy storage system. The generated power fluctuation in low and high frequency range are smoothed by pitch angle control and battery charge or discharge. This coordinated control reduces the rated battery capacity and windmill blade stress. In our proposed method, we apply H∞ control theory to achieve good response and robustness. The effectiveness of the proposed control system is simulated.
The use of renewable energy sources such as the sun and wind is increasing steadily. High penetration of wind power may affect electric power system frequency, because the fluctuations in wind power generation are much larger than that of conventional power plants. To evaluate wind power fluctuation and its effect on the power systems, it is important to acquire actual fluctuation data of wind farm power. In this paper, the authors report the results of an analysis of wind power fluctuation and smoothing effect from data measured at existing wind power sites in the Hokuriku region. A novel index is proposed to evaluate the smoothing effect, and impacts of high wind penetration are estimated using this index. This paper also proposes a control strategy of battery storage system for stabilizing wind farm output.
In the coast of the Japan Sea which has many good places for wind turbine generator system, winter lightning occurs frequently. Many results of investigation have been reported not only damages of wind turbine generator system itself but also failures of distribution line arresters caused by part of the lightning current which flows into distribution lines when the customer's structure is struck by lightning. The lightning back flow current and ground potential rise are also important factors for a lightning protection design of distribution line arresters, which are connected to the wind turbine generator system struck by lightning. This paper describes simulation results of the energy absorption to estimate the lightning damages in the arresters considering the back flow current and the ground potential rise using the EMTP.
This paper describes a study of grounding system of wind turbine generators in a windfarm. A horizontal grounding conductor is often used to obtain a low steady-state grounding resistance. The grounding resistance of the grounding conductor shows transient characteristics. Effective length of the grounding conductor is an important parameter to estimate peak value of lightning overvoltages on the grounding system. This paper proposes approximate formulas to investigate an influence of such parameters as soil resistivity and boundary condition of the grounding system on the effective length.
Electric power from renewable energy sources may cause unstable situation to power grid, because of its electric power fluctuation. In order to smooth the power fluctuation from the renewable energies, there are some researches to simulate the watt-hour capacity of the power storage systems based on the survey data of renewable energy outputs. However, the results only show in limited situation. In a micro-grid, its power load receives both fluctuation of supplies from the renewable energies and demands. It is important to compute the theoretical storage capacity consider both the renewable energies and demands. The aim is to determine the numerical values of the power storage capacity when examining the micro-grid power system; and to explain analytically how capacities computed. In this paper, a mathematical model of power storage capacity has been modeled using the efficiency of power storage, the maximum and average output of micro-grid, and the range of smoothing.
This paper describes the results of the experiments of wind turbine blades with rocket triggered lightning. A number of wind power stations have been projected and planted. Lightning damage to wind turbines has been an increasing problem recently. So development on protection of wind power plants from lightning is necessary to be fully run for the future. In the experiments, the 1.8m long blade was struck by the lightning discharge triggered by rocket. For the blade kept dry inside, the very strong discharge of positive peak current 28kV, total charge 520 Coulombs, was triggered, but the breakdown did not occur through the blade into inside. Another blade polluted by salty wet inside was struck by the lightning discharge of negative peak current of 4kA with 0.5 Coulombs. The lightning was small, nevertheless, the blade was broken at the upper edge and the blade was disconnected by crack. For the protection of blade, the blade surface was covered with stainless steel plate. The blade itself was safe when the big positive lightning discharged, while most part of stainless steel cover was burned out. Supplement breakdown tests of wind turbine blade were carried out with lightning impulse voltage in laboratory. As a result, it became clear that the blade kept dry inside was an effective lightning protection of wind turbine blades.
This paper aims to clarify the battery capacity required by a residential area with densely grid-connected photovoltaic (PV) systems. This paper proposes a planning method of tomorrow's grid-connection power from/to the external electric power system by using demand power forecasting and insolation forecasting for PV power predictions, and defines a operation method of the electricity storage device to control the grid-connection power as planned. A residential area consisting of 389 houses consuming 2390 MWh/year of electricity with 2390kW PV systems is simulated based on measured data and actual forecasts. The simulation results show that 8.3MWh of battery capacity is required in the conditions of half-hour planning and 1% or less of planning error ratio and PV output limiting loss ratio. The results also show that existing technologies of forecasting reduce required battery capacity to 49%, and increase the allowable installing PV amount to 210%.
In this paper, a coherency-based dynamic reduction method for power system stability studies is proposed. The proposed method is based on the principle that both short circuit current at each retained node and operating condition at boundary nodes after reduction is matched to those of original network. Two types of any radial and loop configured external network can be reduced, and each line constant and nodal power condition for those reduced networks can be automatically determined from the principle point of view stated above. The one of notable features of the proposed method is that generator internal voltage angle after the reduction can be specified as mean value of those angles in original network for radial reduction, and another one is that no negative branch impedance solution can be given for looped reduction while negative impedance is often resulted in conventional method. The proposed method is applied to both the IEEJ West-30 standard model and realistic bulk power system model, and its sufficient accuracy is verified by comparing transient stability swing to conventional method.
The aim of this letter is to propose a novel construction method of equivalent systems for bulk power system. Here, a bulk power system is rewritten into a one-machine one-load equivalent system model without approximation by the power flow calculation at each generator-bus or each load-bus.