Recently, distributed generation using renewable energy sources such as photovoltaic (PV) generation has increased in power systems to solve environmental problems and resource exhaustion problems. However, conventional reliability evaluation methods of power systems cannot easily evaluate power systems with massive penetration of PV. The methods may not accurately evaluate overload exceeding system capacity, expansion of voltage phase difference between adjacent power systems, and voltage violation resulting from a sudden decrease in PV output due to weather conditions. Therefore, this paper proposes a new method to evaluate the supply reliability of power systems by three constraints, namely, capacity constraint, voltage phase difference constraint, and voltage magnitude difference constraint.
Recently, renewable energy sources are rapidly introduced to power grid because of concern for global warming and fossil fuel depletion. However, generating power from variable renewable energy, such as photovoltaics and wind turbine generators, depends on the weather conditions; therefore, balancing demand power and supply power becomes difficult with large penetration of them. One of the proposed solutions is to control power consumption of controllable loads called directive demand response. In this paper, the target device as a controllable load is heat pump water heaters (HPWHs) installed at a hot spring facility. The authors proposed an HPWHs' schedule generation method taken into account not only requirements from aggregators but also hot water demand to prevent from hot water shortage. We make steady-state models of power and generated heat of the HPWH and decide the range of amount of hot water to generate actually. Finally, we make a system to calculate HPWHs’ operation schedules taken into account users’ convenience and aggregators’ requirements by solving a mixed-integer programming problem. We show HPWHs’ schedules calculated by our system and the result of data we operated actually.
This study mainly aimed to compare operation planning schemes for polymer electrolyte fuel cell cogeneration systems (PEFC–CGSs) to analyze derived operation plans from various perspectives. PEFC–CGSs provide hot water by using waste heat produced in electricity generation. Therefore, they have considerable potential for further energy saving and can be efficient systems by generating and storing heat according to household demand. Operation planning scheme based on the prediction of household electricity and hot water demands is a promising approach to achieving further energy saving. However, operation results imply that improvement in accuracy from the viewpoint of prediction error does not always lead to efficient PEFC–CGS operation. In this study, we implemented several operation schemes and analyzed them by simulation from various perspectives, namely, operation cost breakdown, demand types and robustness required for the operation plan, to attempt further operation cost reduction.
This study focuses on the trade of power from wind power plants (WPPs) at an electricity market. To trade power, operators should produce generation schedules for bids and then supply the same amount of energy as the schedules. However, power from WPPs tends to fluctuate, which complicates scheduling. This paper proposes three scheduling methods with an energy storage system (ESS) to solve this problem. The first method considers state-of-charge (SOC) transition to maintain the appropriate SOC and minimize the imbalance between supplied and scheduled energy. The second and third methods consider SOC transition and forecast errors. The second method uses a linear regression model to estimate forecast errors. The third method adopts a bagged trees model, which is a machine learning method, to directly estimate the adjusted forecast data considering errors. Five patterns of the rated power of the ESS are assumed, and these three methods are simulated on each power. In comparison with the basic method, whose schedules are the same as the forecast, the third method can reduce 84% of the imbalance from the schedules when the rated power of the ESS is the minimum. The proposed methods help develop further correct and practical scheduling methods.
In this paper, the authors propose a method of improvement of power quality by static synchronous compensators (STATCOMs) for low voltage system (single-phase STATCOMs) added the function of harmonic suppression. In the proposal method, the STATCOMs control the voltage of the distribution system within the proper range by compensating the reactive power. Additionally, they are operated as the filtering function and suppress harmonics of the voltage of the distribution system. The filtering gain is determined by PI controlling the error between the voltage total harmonic distortion of the point voltage at the installation of STATCOMs and the target value. In order to verify the validity of the proposal method, the numerical calculations are carried out by using an analytical model of distribution system which interconnected photovoltaic generation systems. Simulation results show the effect of improvement of power quality on both sides of voltage deviation and harmonics.
Wind turbine generation systems (WTs) normally do not have mechanisms to control their output, so that the growth in WT penetration has brought difficulty in the frequency control operation of power grids. This paper presents a frequency adjustment mechanism for the WTs consisting of the following controllers: an active power controller, a governor controller, a pseudo synchronization controller and a pitch angle controller. The governor and the synchronization controllers calculate a correction value to make variability of the WT output in response to its share of power generation. Besides, the pitch angle controller is customized from a viewpoint of relaxing restraint in the frequency adjustment mechanism. By these controllers, the proposed mechanism supplies the frequency adjustability to the power grid in consideration of the actual WT output while effectively suppressing the rapid change of rotor angular speed. Through numerical simulations and discussions on their results, validity of the authors’ proposal is verified.
Recent power system operations are faced with the difficulties due to large penetration levels of intermitted renewable energy sources (RESs), for example, photovoltaic power (PV) generations and wind-turbine (WT) generations. Operations of a large number of uncontrollable RES also cause an unforeseen issue for the reliability in power system operation. To tackle the issue, this paper proposes a novel real-time generation schedule (GS) integrated with RES’ power curtailment. The proposed real-time GS become to effectively compute the outputs of the associated generating units so as to meet the required electricity load. The GS computation is frequently updated every 5-min in real-time operation. The proposed approach is applied for real-time dynamic economic load dispatch for a real-time GS that completely treats the limited controllable energy resources under the uncertainties. Furthermore, real-time PV forecasting with their covariance matrices of the prediction errors is effectively utilized in the approach. In case of a unforeseen situation when the estimated electricity load cannot match the controllable unit’s capability, the approach can be detect a minimum amount of supply-demand mismatch in advance, and can manage it reliably for the considerable time horizon.
In this paper, photovoltaics (PV) outputs are not only limited constantly but also controlled by bidirectional communications to compensate short period power fluctuations. In this case, if qualities of communication systems are not high, communication delays may not be ignored. The effect of communication delays is evaluated. If communication delays are not very small, a system reaches a stability limit, and a PV output may oscillate continuously. Then, a compensation method of them is proposed. The compensation method uses a disturbance observer. In this method, it is a merit that models of communication delays are not necessary and only a PV output model is necessary. It is found that the proposed method can compensate communication delays to some extent by simulating step responses. Furthermore, compensation of short period power fluctuations in a micro grid is considered by using the proposed method. Short period power fluctuation may be reduced by the compensation method of communication delays.
This paper describes a new three-phase rectifier circuit using the partial switching strategy. A passive method to be composed of a diode rectifier circuit and an active method with the high-frequency switching were used for the circuit to convert from A.C. into D.C. mainly. However, the passive method and the active method had a problem of low power factor and low efficiency, each other. In contrast, the partial switching strategy realizes high efficiency and high power factor and low harmonics to a moderate degree. Therefore the single-phase partial switching voltage doubler rectifier circuit was applied to air conditioner. By the superior performance, most air conditioner adopts the partial switching strategy. In this paper, circuit configuration and a principle of operation and the experimental result of the three-phase partial switching rectifier circuit are reported.
This paper presents a peer-to-peer (P2P) energy auction mechanism based on transaction zoning in a distribution system with a number of distributed generators. A transaction zone can be configured by the power flow monitoring method using the bids and offers from auction participants. In this study, the power flow monitoring method was applied to check how much power flow passes through branches and divide the whole network into sub-networks based on branched with small power flows. Energy trading in the transaction zone configured in this manner has no significant effect on other zones in terms of power flow because only little power flow passes through the branch, which is connected in adjacent to the transaction zone. Therefore, supply and demand can be balanced in the transaction zone and energy trading can be established without violating network constraints. To verify the effectiveness of the transaction zoning, the proposed P2P trading environment was simulated with an IEEE 37-node test feeder. Simulation result shows that P2P energy transaction using transaction zone does not violate network constraints.
This paper presents an estimation method of operating lifetime for electric power components, such as electric poles and pole transformers, utilizing massive records on their inspection and maintenance. The proposed method analyzes initial information, which is easily available, and real historical records of the inspection and measures for the inspection (maintenance) storing the database in an electric power company (e.g. 1.4 million and 53.4 thousand cases in the concrete electric poles). Subsequently, lifetime estimation models on each component are constructed using relationship between the initial conditions and the taking measures in the maintenance record. This method is aiming to apply on expansion planning phase of the power grid, and therefore the initial conditions are only used as its input in estimation process. Through numerical simulations, it was confirmed that accuracy of the lifetime estimation exceeds 70% (approximately 7 years). In addition, factors, which have an influence on the operating lifetime of the target component in process of lifetime estimation were clarified.
Electric power companies are storing massive records such as results of inspection and maintenance through their daily operations. Although the massive records have been expecting to utilize for efficiency improvement of the power grid operations and planning, applications of the massive records have been limited in a small portion until now. The authors analyse 3.0 million sets of inspection scores and 0.9 million cases of their measures (need follow-up observation, or need repair or replacement) that have been actually stored in an electric power company. Moreover, based on the analysis results, a decision support model is constructed for judging maintenance necessity (need repair or replacement) in response to the inspection scores. A decision tree analysis, which represents its decisions and decision-making process visually and explicitly, is applied in the process. Usefulness of the authors’ proposal is verified through numerical simulations and discussions on their results.
Microgrids are small-scale power grids that combine a renewable energy source with an energy storage system (ESS) to power loads. They are operated in connection with commercial power grids and operate independently in mountainous areas where commercial power grids are not provided. Microgrids are mainly classified into AC and DC types. Independently operating microgrids typically use diesel generators to supply power to customers. However, the use of distributed generators, such as solar power, wind turbines, and ESSs, is increasing worldwide as conventional fossil-fueled power systems face the problem of gradual depletion of resources. DC microgrids are being actively studied because they require only voltage control, and most distributed generators produce DC power. In particular, the voltage of the DC bus must be maintained using ESSs to maintain the voltage of the microgrids. In this study, we propose two stable power supplies, namely, MAIN ESS for voltage control in DC microgrids and SUB ESS for power supply of loads, and a stable operation by switching a voltage-controlled ESS unit at the MAIN ESS dropout. We demonstrate that DC microgrids can be modeled and operated stably throughout PSCAD/EMTDC.
In this paper the role of wind energy conversion systems (WECS) was investigated, In particular, variable speed wind turbines (VSWT) based on double-fed induction-generator (DFIG) in control and the optimization frequency with different wind penetration in the isolated system including Traditional Thermal and Non-thermal units have been investigated too. DFIG is capable of providing power at different mechanical speeds and reducing the Instantaneous speed, thus the release of stored mechanical energy; it is able to support traditional units of frequency tuning system. By achieving this through setting the desired speed, controlling DFIG at different levels of wind penetration is possible. This technique utilizes the particle swarm optimization (PSO) algorithm. The simulation results have been compared to the integral of squared error (ISE). The optimal penetration of WECSs by considering changing parameters in the microgrid (MG) frequency is investigated. The presence of wind turbines has been shown to improve the oscillation frequency and In particular the penetration rate of the wind turbine in the MG. The best frequency control will be achieved. To achieve the computing purposes, use of intelligent algorithms is much better than the methods trial and error methods.
The aim of this paper is to analyze the reliability of Variable Frequency Drives (VFD) when used to control condensate pump motor in a nuclear power plant. IEC 60812 system reliability analysis procedure for Failure Mode and Effects Analysis (FMEA) was used to determine the potential failure modes for VFD and its effects on the operation of the condensate pump motor. In addition, Electrical Transient Analysis Program (ETAP) was used to simulate the performance and evaluate system reliability indices of the VFD for APR1400 auxiliary power system. The objective for adopting these two approaches is to use the FMEA for comprehensive identification and evaluation of unwanted effects of VFD malfunction on the condensate pump operation; and the ETAP was used to quantify the impact of VFD operation on system reliability indices. From the FMEA model, the Insulated Gate Bipolar Transistor (IGBT) and the motor stator winding were identified as the components with high susceptibility to failure. The overall reliability assessment showed that VFD failure effects are generally incipient. Average system availability increased marginally when VFD was used instead of the existing throttle valve for condensate flow control.
ESS is widely used in FR power system in Korea. In this paper, we propose a method of starting the supply generator by energizing the transmission line to the ESS instead of the self-starting generator when some area of the system is completely out of order. Simulation should be carried out to find out whether there is any problem in replacing the self-starting generator with the ESS through the calculation of the power flow and the transient stability. We used EPRI ‘s CBEST model for simulation, and utilized actual system data of KEPCO.
Renewable energy such as wind generation is a countermeasure against global warming and fossil-energy resource depletion. However, wind-farm (WF) output fluctuates because of unstable wind conditions and the occurrence of long-term fluctuations, which are called ramp events. WF power generation operator needs to observe the grid code (GC) by controlling wind generators themselves or adjusting a WF output with the energy-storage system (ESS). Accordingly, scheduling the operation of WF with ESS and WF output forecast is necessary. Scheduled operation has two parts: scheduling of generation plans (SGP) and operating WF (OWF) outputs with the charge/discharge of ESS. This paper proposes the scheduling method, which can control ramp events and satisfy GC formulation such as mixed integer liner programming in the SGP part, and the balancing method, which can reduce the imbalance energy due to control delay of ESS in the OWF part. We also evaluate these methods by annual numerical simulation. We further confirm the validity and effectiveness of the proposed method, as well as approximate its battery capacity requirement.
Recently, Smart grid is one the important concepts that improve the energy future. This paper proposes an economic optimization technique for battery management to reduce the operating cost of a grid-connected microgrid. The proposed microgrid includes photovoltaic, wind power, and battery storage system. The proposed objective concerns with determination of the optimal hourly management for the operating power system to meet the required profile of load demand. In addition to, the excess/lack amount of electricity can be sold to/purchased from the utility grid. Interior Search Algorithm (ISA), an efficient heuristic algorithm, is applied to solve the problem with an objective of total operational cost minimization. The optimization technique is applied to a typical microgrid system with different loading conditions as a case study to test its effectiveness. The proposed platform can be considered as a significant part of comprehensive energy management system (EMS). The simulation results indicate high potential savings for the total operating cost.
Codebook design is a key point in beam-forming which is necessary for millimetre Wave (mmWave) communication to compensate its huge free space path loss. In beam-forming codebook, the transmitter and the receiver co-operate to select the optimum pre-designed code vectors to satisfy robust, efficient, and reliable transmission link between two pairs. In our study, we propose a new codebook structure based on unique phase states. Based on the Array Factor (AF) and Signal-to-Noise Ratio (SNR), the proposed codebook is compared and evaluated with the uniform weighting codebook, circular array codebook, and IEEE 802.15.3c standard codebook. The results show that the proposed codebook achieves comparable SNR and array directivity with the IEEE 802.15.3c, but higher peak and wide dynamic range of SNR and better array directivity compared with the uniform weighting and circular antenna codebooks. Thus, the proposed codebook is expected to improve signal quality and increase user throughput significantly.
In recent years, plant factories have been drawing attention for their capability to solve global food crisis. However, the energy cost of plant factories is high due to the power consumption of air conditioning and cultivation systems necessary for realizing semi-automatic production. As this high cost hinders their propagation,, plant factories must minimize the energy cost of growing plants. We propose an operation planning method of cultivation systems for minimizing energy cost minimization while producing plants with the same amount and quality. The effect of electricity charge and weather factors on electric power consumption under various real-world constraints are used to decide the appropriate operation plan of cultivation systems. Simulation results show that the operation plan of cultivation systems properly reflects the effect of electricity charge and weather factors on electric power consumption to reduce energy cost.
