Grid interconnections of distributed generation systems (DG) to the existing power grid are expected to increase in future due to concerns for environmental preservation and effective energy use. In this paper, targets and status of DG introduction are explained first. Then, outline of some projects intended to promote large-scale introduction of DG is described. It is required to consider the technical requirements for grid interconnection of DG to cooperate with existing power grid. History of DG grid interconnection rules is mentioned and recent progress of standards for DG interconnection testing is explained.
Transient stability may be seriously affected when a large amount of distributed generators (DG) stop simultaneously during voltage sag. It is needed to analyze accurately the dynamics of bulk power system with high DG penetration. In this paper, transient stability is studied by analyzing power-angle curves of generator considering load dynamics and model order reduction in lower voltage. According to analysis, decreasing of load internal resistance after voltage sag causes transient instability of generators. The phenomenon is confirmed through simulation using one machine and one load model. This paper also presents that the simulation results might be misled by traditional bulk power system modeling such as using static load model and ignoring impedance in lower voltage. As to the numerical simulation, a large amount of DG-stopping leads to transient step out in bulk power system, and the stability is greatly improved by DG-voltage regulation.
A fundamental study of power system dynamic stability analysis with dynamic load and distributed generation is presented. For this study, we use a single machine and a single load connected to an infinite bus model with voltage dependency parameters of loads. The adequacy of proposed model is demonstrated using power system stability simulations. Using proposed model, we can develop insights into various effects of dynamic loads and distributed generations to the dynamic stability analysis.
SCADA (Supervisory Control And Data Acquisition) systems for demand area power systems (one of the next-generation distribution grids) would be required; 1) to be flexible, 2) to provide various functions on restricted computing resources, and 3) to complete processing within the period required. We have proposed a system design based on mobile agents and Ethernet in order to fulfill the first and second requirements. This paper shows that this system design would satisfy the third requirement. In order to validate the system performance, we constructed an experimental system composed of a distribution line, generators, power devices, computers, mobile agents and Ethernet. The experiment results illustrate that a system based on this design could complete the fault isolation process that has the most stringent execution period.
The connection of a large number of the dispersed generators induces voltage deviations beyond a statutory range in distribution systems. Therefore, a hierarchically cooperative control system has been developed for voltage regulation under the consideration of reactive power application of dispersed generators. In the hierarchically cooperative control, a distribution system is divided into blocks and each block with an on-load tap changing transformer is controlled under a cooperative control algorithm. A distribution system with a tap-changing substation transformer, step voltage regulators, and multiple dispersed generators connected to each feeder has been simulated in an analogue simulator; the hierarchically cooperative control has been demonstrated on the simulator and compared with independent control. The demonstration has experimentally confirmed the successful operation and high voltage regulation ability of the hierarchically cooperative control.
Since photovoltaic generators (PV) have been mainly introduced in residential areas, it is necessary to consider the consequences of setting up large numbers of PVs locally. In such a case, it will be difficult to manage the voltage in distribution lines, because the reverse power supplied from the PV raises the voltage of the power lines. In this study, a large PV system was connected to a distribution system, and for every inverter we used simulation to verify the appraisal of systematic voltage and the electric power generation. As a result, we verified that power factor control is more effective than output control. In addition, we verified that with each user the power generation becomes more unequal. In the future, when a large PV system is connected to a distribution system, the problem of unsatisfactory equalization of demand for home appliance usage may arise. A method for determining the setting value of the inverter with each user was proposed in order to solve this problem, and the effectiveness of the proposed technique was verified by simulation.
Since a distribution network has many sectionalizing switches, there are huge radial network configuration candidates by states (opened or closed) of sectionalizing switches. Recently, the total number of distributed generation such as photovoltaic generation system and wind turbine generation system connected to the distribution network is drastically increased. The distribution network with the distributed generators must be operated keeping reliability of power supply and power quality. Therefore, the many configurations of the distribution network with the distributed generators must be evaluated multiply from various viewpoints such as distribution loss, total harmonic distortion, voltage imbalance and so on. In this paper, the authors propose a multi evaluation method to evaluate the distribution network configuration candidates satisfied with constraints of voltage and line current limit from three viewpoints ((1) distribution loss, (2) total harmonic distortion and (3) voltage imbalance). After establishing a standard analytical model of three sectionalized and three connected distribution network configuration with distributed generators based on the practical data, the multi evaluation for the established model is carried out by using the proposed method based on EMTP (Electro-Magnetic Transients Programs).
Recently, the total number of distributed generation such as photovoltaic generation system and wind turbine generation system connected to distribution network is drastically increased. Distributed generation utilizing renewable energy can reduce the distribution loss and emission of CO2. However the distribution network with the distributed generators must be operated keeping reliability of power supply and power quality. In this paper, the authors propose a computation method to determine the maximum output of a distributed generator under the operational constrains ((1) voltage limit, (2) line current capacity, and (3) no reverse flow to bank) at arbitrary connection point and hourly period. In the proposed method, three-phase iterative load flow calculation is applied to evaluate the above operational constraints. Three-phase iterative load flow calculation has two simple procedures: (Procedure1) addition of load currents from terminal node of feeder to root one, and (Procedure2) subtraction of voltage drop from root node of feeder to terminal one. In order to check the validity of the proposed method, numerical simulations are accomplished for a distribution system model. Furthermore, characteristics of locational and hourly maximum output of distributed generator connected to distribution feeder are analyzed by several numerical examples.
A Micro-Grid has a small capacity of the system compared with conventional power system. When the Micro-Grid is disconnected from power system and operated under islanding condition, the problems of the power quality such as frequency fluctuation and voltage fluctuation can occur. This paper presents the “local control" method of improvement of the power quality in order to solve the problem. The suggested method is to use an inverter of a battery which is an energy storage system in the Micro-Grid and to improve frequency fluctuation and voltage fluctuation in Micro-Grid islanding operation. The impact of the method is investigated in this paper by analysis using HYPERSIM. Additionally, it is shown that the local control is developed and verified by field test in the Micro-Grid under islanding operation.
As a future power distribution system, Flexible, Reliable and Intelligent ENergy Delivery System (FRIENDS) has been proposed. In FRIENDS, various functions become realizable by installing new facilities called Quality Control Centers (QCCs) between distribution substation and customers. Moreover, in fault case of upstream power system, multiple QCCs become independent of distribution system and an isolated system is constituted by interconnected QCCs. Furthermore, an isolated system operation which uses Distributed Generators (DGs) and Energy Storage Systems (ESSs) which are installed in QCC as the main power supply will be performed. In an isolated system by QCCs, a method for balancing supply and demand based on Distributed Autonomous Operation of QCCs using a frequency deviation is proposed by applying the concept of frequency control which has been performed by the utility. Simulation results to investigate the effectiveness of the proposed method are shown in this paper.
We established a procedure for estimating regional electricity demand and regional potential capacity of distributed generators (DGs) by using a grid square statistics data set. A photovoltaic power system (PV system) for residential use and a co-generation system (CGS) for both residential and commercial use were taken into account. As an example, the result regarding Aichi prefecture was presented in this paper. The statistical data of the number of households by family-type and the number of employees by business category for about 4000 grid-square with 1km × 1km area was used to estimate the floor space or the electricity demand distribution. The rooftop area available for installing PV systems was also estimated with the grid-square statistics data set. Considering the relation between a capacity of existing CGS and a scale-index of building where CGS is installed, the potential capacity of CGS was estimated for three business categories, i.e. hotel, hospital, store. In some regions, the potential capacity of PV systems was estimated to be about 10,000kW/km2, which corresponds to the density of the existing area with intensive installation of PV systems. Finally, we discussed the ratio of regional potential capacity of DGs to regional maximum electricity demand for deducing the appropriate capacity of DGs in the model of future electricity distribution system.
One-factor mean-reversion equation (OFMRE) having three parameters of reversion speed, mean-reversion level, and volatility is a mathematical model to simulate a characteristic of electricity price which tends to revert to the mean-reversion level with a random walk. An applicability of OFMRE to 1 year, 3 months, and 2 months-periods of electricity price at three foreign deregulated day-ahead spot markets (Nord Pool, National Electricity Market of Australia, and PJM day-ahead market) at year 2000-2001 was studied. 1 year-period of electricity price at all the markets could not be applied to OFMRE, but several 3 months-period or 2 months-period of the electricity price at Nord Pool and PJM day-ahead market could be applied, which was related with price spike appearances and/or normal distribution of the electricity price.
This paper presents development of analysis function for low voltage (LV) distribution system with distribution generators. The conventional interconnection evaluation tool had three phases unbalanced load flow calculation which made primary distribution system applicable to analysis. On the other hand, in LV distribution system, the authors are anxious about generating of the voltage rise by the photovoltaic power generation. In this paper, pole transformer model, LV distribution line model, and photovoltaic power generation model have been developed as models for LV distribution systems to analyze LV system integrated with primary distribution system.
Recently voltage-source or current-source inverter based various FACTS devices have been used for flexible power flow control, secure loading, damping of power system oscillation and even for the stabilization of wind energy generation. In this paper, we propose the static synchronous compensator (STATCOM) based on voltage source converter (VSC) PWM technique to stabilize grid connected wind generator system. A simple control strategy of STATCOM is adopted where only measurement of rms voltage at the wind generator terminal is needed. Fuzzy logic controller rather than conventional PI controller is proposed as the control methodology of STATCOM. Multi-mass shaft model of wind turbine generator system (WTGS) is also considered as shaft modeling has a big influence on the transient performance of WTGS. Transient performance of STATCOM connected WTGS is compared also with that of pitch controlled WTGS. Both symmetrical and unsymmetrical faults are analyzed. Moreover, the steady state performance of STATCOM connected WTGS is analyzed. It is reported that STATCOM can reduce the voltage fluctuation significantly. Finally STATCOM is applied to a wind park model with multiple wind generators. Comprehensive results are presented to assess the performance of STATCOM connected WTGS, where the simulations have been done by PSCAD/EMTDC.
Methods for estimating available capacity and voltage changes during the discharge of a large-capacity lithium-ion battery were developed and their effectiveness was demonstrated. The cell was developed for such uses as a backup in power supply systems etc. Lithium-ion cells with storage capacity ratings of 10 and 40Ah were exposed to high-temperature acceleration tests. We observed a correlation between a cell's available capacity and voltage drops that we believe is effective for estimating the available capacity of the cell. We were able to predict the voltage change in a varying load-current discharge with a high degree of accuracy. This is because for a lithium-ion cell, the voltage change profile in a stepwise-varying constant-current load can be expressed as a conjunction of partial discharge voltage curves of first, second and other sequent discharge current every time discharge duration of each constant current is terminated. We believe the methods are practical as a monitoring function in power systems equipped with a lithium-ion battery.
In distribution system, Distributed Generation (DG) is expected to improve the system reliability as its backup generation. However, DG contribution in fault current may cause the loss of the existing protection coordination, e.g. recloser-fuse coordination and breaker-breaker coordination. This problem can drastically deteriorate the system reliability, and it is more serious and complicated when there are several DG sources in the system. Hence, the above conflict in reliability aspect unavoidably needs a detailed investigation before the installation or enhancement of DG is done. The model of composite DG fault current is proposed to find the threshold beyond which existing protection coordination is lost. Cases of protection miscoordination are described, together with their consequences. Since a distribution system may be tied with another system, the issues of tie line and on-site DG are integrated into this study. Reliability indices are evaluated and compared in the distribution reliability test system RBTS Bus 2.