分散型電源大量導入時における動的負荷を考慮した過渡安定度

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems.

With large wind energy integration into power systems, wind farms begin to influence power systems in a much more significant manner. As wind energy systems utilize different generator technologies from the one utilized in the conventional power plants, the steady-state, transient and small-signal dynamics, as well as, power system stability will thus be significantly affected. The impact of wind energy systems on the power system dynamics and stability is thus of practical importance. As there is a significant increase in installation of wind turbines equipped with doubly-fed induction generator (DFIG) in recent years, a dynamic model of the DFIG wind turbine is firstly developed in this thesis. The model is validated against field measurement data, and the validation gives confidence about the accuracy and applicability of the developed model. DFIG wind farms consist of tens to hundreds of identical DFIG wind turbines increasing the complexity of the wind farm model and simulation time. A novel aggregation technique is developed in this thesis that incorporates a multiplication factor, namely mechanical torque compensation factor (MTCF), to the mechanical torque of the full aggregated wind farm model. The MTCF is initially constructed to approximate a Gaussian function by using fuzzy logic method. By optimizing the MTCF on a trial and error basis, less than 10 percent discrepancy is then achieved between the proposed aggregated model and the complete model. The proposed aggregation technique is then applied to a 120 MVA offshore wind farm comprising of 72 DFIG wind turbines and shows higher accuracy in approximating the wind farm dynamics as it appears at the point of common coupling (PCC) as compared to the full aggregated model. The proposed aggregated model computes faster than the complete wind farm model by 90.3 percent during normal operations and 87 percent during grid disturbances. To overcome the adverse effects due to the fluctuating nature of wind, two wind power smoothing methods are proposed using a fuzzy logic pitch angle controller for a smooth performance with a minimum drop in output power. One method performs partial smoothing with only 4.74 percent drop in the output power while the other method offers complete smoothing with a 8.28 percent drop in output power. The impact of wind energy integration on power system transient stability (PSTS) is studied quantitatively with the transient energy margin (TEM), which is calculated through the evaluation of the transient energy function (TEF). This study is carried out in two ways in the thesis. One is to analyse the impact of transient fault on the DFIG wind turbine as compared to SGs for different factors, like the fault clearing time, the grid coupling, the inertia constant and the voltage sag. The study reveals that transient stability of the DFIG wind turbine is hardly affected by the grid coupling, the inertia constant and the generator terminal voltage sag variations indicating its consistent transient performance within a wide range of these factors. The fault clearing time should be almost 11 percent faster for the system with the DFIG than the synchronous generator (SG). The other is to investigate the impact of the DFIG wind farm on the PSTS with the variation of different factors, which are the voltage sag, the fault clearing time, the load and the wind power penetration level. The study reveals that power systems integrated with DFIG wind farms are sensitive to transient events with high voltage sag, high fault clearing time, low load operation and high wind power penetration level. Machines at different locations individually possess distinct fault response and suffer from a large-scale power imbalance. As a result, reliable operation of DFIG wind farm integrated power systems demands upgraded equipment, such as advanced switchgear, fast breakers/isolators, efficient power reserve systems and advanced reactive power compensating device, etc.

The load stability in bulk power system may be seriously affected when a large amount of distributed generators (DG) stop simultaneously due to voltage sag. It is important to understand the mechanism of load instability, which cannot be analyzed by well-known P-V curve only. For analyzing load stability without detailed simulations, this paper presents torque-speed characteristic curves of induction motor considering power system condition and constant impedance loads. As the results of examining the curves in various conditions (model order reduction methods, load dropping out), this paper also presents some cautions for bulk power system modeling with DG. A bulk power system model whose lower voltage subsystems are reduced by an accurate model order reduction method is introduced in order to check out the problems by DG-stopping. As to the numerical simulation, a great number of DG-stopping leads to voltage instability in bulk power system, and the instability is greatly improved by DG-voltage regulation.

This paper presents the influence of distributed generation (DG) location and capacity for mitigating voltage sag caused by faults. The voltages sags are caused by the faults were simulated on the IEEE 13 bus system for single line to ground close to 650 and three-line phase faults at bus 646 using Alternative Transient Program (ATP) software. The voltages are monitored at every buss system. The DG is installed for one DG, two DG and three DG as the alternative. The DG locations are installed based on trial and error. DG total capacity is 2/3 load total installed in the system. As the alternative DGs installed viz. for one DG at Bus 611 with the capacity of 2.5 MVA; Two DGs at Bus 611 and Bus 680 with their respective capacities of 1.25 MVA; Three DGs are located at Bus 611, Bus 680 and Bus 633 with their respective capacities of 1 MVA. The results show that DG location placement in many locations gives the better results for mitigating voltage sags.

The impact of numerous distributed generators (DGs) coupled with the implementation of virtual inertia on the transient stability of power systems has been studied extensively. Time-domain simulation is the most accurate and reliable approach to evaluate the dynamic behavior of power systems. However, the computational efficiency is restricted by their multi-time-scale property due to the combination of various DGs and synchronous generators. This paper presents a novel projective integration method (PIM) for the efficient transient stability simulation of power systems with high DG penetration. One procedure of the proposed PIM is decomposed into two stages, which adopt mixed explicit-implicit integration methods to achieve both efficiency and numerical stability. Moreover, the stability of the PIM is not affected by its parameter, which is related to the step size. Based on this property, an adaptive parameter scheme is developed based on error estimation to fit the time constants of the system dynamics and further increase the simulation speed. The presented approach is several times faster than the conventional integration methods with a similar level of accuracy. The proposed method is demonstrated using test systems with DGs and virtual synchronous generators, and the performance is verified against MATLAB/Simulink and DIgSILENT PowerFactory.

This paper presents a method for optimum allocation of a number of Distributed generation (DG) units in distribution system (DS). The main aim of the present work is to draw attention to some effects of DG's installation on DS. These effects are studied in both normal steady state operation (line power losses) and transient periods (voltage sag losses). As a matter of fact addition of DG to DS decreases grid losses, however it is highly probable that this addition will increase the losses of voltage sags resulting from short circuit (S.C.) on any bus of DS. This makes it important to include the voltage sag losses in optimum allocation problem. The paper applies the conventional particle swarm optimization (PSO) technique with two modifications. The proposed method is applied to the IEEE 14-bus system. Results ascertained contradictive effects of DG installation between line and voltage sag losses which assured the importance of voltage sag consideration in the problem of DG allocation.

In the current power and energy scenario, distributed generation (DG) has generated a lot of interest across the globe due to the growing concerns about gradual depletion of fossil fuels, steep load growth, environmental pollution and global warming caused by greenhouse gas emissions. Renewable DGs such as wind generators and solar photovoltaic are well-recognized now-a-days as sources of clean energy. Voltage dips have always been a serious problem in electricity networks accounting for the disruption, poor power quality and affecting the cost and productivity of power utilities and the consumers. Therefore, with more DG penetration into the network, utilizing the DGs for improving power quality through voltage dip mitigation has become an important area of research in itself. In this context, this paper presents a novel technique for voltage dip mitigation with Distributed Generation (DG) using a simple feed forward Artificial Neural Network (ANN) to mitigate the effects of the power quality problems arising out of voltage dips in a distribution network. The scheme is simulated in DIgSILENT Power Factory 14.0 software and the tests are carried out on IEEE 9-bus test system. A three-phase short circuit fault on a line is simulated as the disturbance causing the voltage dip in the system. The model is trained, tested and validated in Matlab using mean square error and regression analysis.

Integrating a large amount of hybrid Distributed Generation units has a substantial effect on the transient stability of IEEE 39-Bus System Base Model. With the increasing number of Distributed Generation (DG) sources connected to the power grid, the impact on the power system's stability is definitely higher compared to the previous decades. From the previous studies, small amount of DG connected to the grid has a negligible impact but a large amount of different DG connected to the same reference grid has a positive and negative impact on the transient stability when a fault occurred on a system. Using IEEE 39-Bus System Base Model with a Hybrid DG such as solar, wind, synchronous generators and energy storage system, the following important results were determined: The overall Transient Stability improves due to the increases in the number of stable conditions in comparison to the IEEE 39 Bus Model System without DG - Base Case; and, the removal of ESS in the Hybrid DG connection decreases the overall system stability. The integration of Hybrid DGs to the system resulted to the decrease in the reduction of Electrical Power Output of the 10 Generators during fault application.

This paper presents impacts of distributed generation (DG) on voltage sag assessment in distribution systems. Voltage sag assessment is analyzed by using the method of fault positions in order to determine area of vulnerability (AOV), voltage sag frequency and voltage sags index such as System Average RMS Variation Frequency Index (SARFI). Impacts of DG's locations, sizes, operation modes and transformer connections are investigated. Simulation results of various case studies on Roy Billinton Test System (RBTS bus 2) are summarized. Results show that the system with DG installed has better voltage sag performance than the system without DG installation.

Power system faults can cause voltage sags that, if they are less than voltage sensitivity threshold of equipment, can lead to interruption of supply and lead to incurring of financial losses. The impact of distributed generation (DG) on these financial losses is investigated in this work. Using the method of fault positions, a stochastic approach to determine voltage sag performance, profiles of magnitudes of remaining voltages at a monitoring point for faults occurring along lines in the network is developed. It follows that an expected number of critical voltage sags at a monitoring point is calculated and the expected cost of these sags is derived for various voltage sensitivity threshold limits. An illustrative study is carried out comparing the expected costs of voltage sags for a network without DG with a DG case, for various mixes of customers. It is shown that in the presence of DG, the expected costs of voltage sags are lesser for all voltage sensitivity criteria assumed and for all customer mixes. The study demonstrates that the impact of incorporating DG sources results in a reduction in the expected cost of voltage sags.

The use of Distributed Generators (DGs) in distribution system has increased at fast pace in recent times. These DGs can work both as grid connected or standalone mode. Thus when it comes to DG placement, getting the optimal location for its placement and the optimal sizing becomes very important. In this paper, finding the optimal location and optimal size of DGs is dealt keeping active power loss as the objective, considering different load models. A novel method called Backtracking Search Optimization Algorithm is used to find the most significant positions for DG placement. DGs supplying both active and reactive power have been studied. Case studies have been done about the effect of 1, 2 or 3 DGs on the IEEE 69 bus test system for DGs operating at 0.85 pf. on different load models.

In this paper controller design for voltage sag ride-through in hybrid fuel cell/battery energy storage distributed power generation system has been presented. As the amount of fuel cell power generation and other distributed generation (DG) with power electronic in the grid grows, it becomes unacceptable to disconnect generating units every time a disturbance occurs, as was common practice in the past. Keeping the VSC on line during unbalanced voltage sags becomes thus a very critical issue. Hence, modeling, controller design, and simulation study of a hybrid distributed generation system are investigated. Based on the classification of unbalanced faults that can occur in the grid, resulting in voltage sags at the bus where the hybrid power system is connected, the maximum current that the converter valves must be bale to withstand is calculated. Simulation results are given to show the overall system performance including active power control and voltage sag ride-through capability of the hybrid distributed generation system.

A stochastic performance analysis is a useful tool for an unbalance distribution feeder having voltage deepened load characteristics under deregulated environment. As the planning, smooth operation, and distributed generation (DG) integration aspects of an unbalance distribution feeder affected by the DG penetration level and may have a favorable or adverse impact on feeder performance. Therefore a cautious analysis to assess the impact on feeder performance with a reasonable penetration level of DG is required. The proposed strategy has been implemented on a modified IEEE 37 feeder having stochastic static ZIP load models. A constant power (PQ) model of distributed generation integrated with above feeder to simulate penetration of DGs with three different scenarios for 24 hours. These scenarios have different penetration level and DG locations with diverse DG capacity frameworks. The outcome has demonstrated the adequacy of the DG penetration levels under time varying scenarios.

The advent of distributed generation (DG) units has posed many problems to the planning and operation of distribution networks (DS). The selection of the “best” site for the DG installation is a major problem in this context. Different strategies and methods have been developed in the recent years which deal with this problem. However, Complexity of solutions and inadequate manipulation have hindered their utilization by DS planners. This paper presents a robust and practical tool which helps the distribution network planner in getting the best possible location of DG unit taking into account both reliability of supply and voltage sag problems. The method suggested uses knowledge-base expert system (ES) to help in selecting the appropriate locations. The method suggested is based on economic and technical considerations. It investigates with economic loss due to both voltage sag and forced outage of DS components. It concerns with the effect of location of the added DG on the overall network performance during both steady-state when a forced outage occurring on one or more component of the DS and transients after sudden short-circuit. Using ES avoids mathematical complications of solving complex analytical model. By judicious rule-base ES, selection of optimum locations becomes clear and simple. The suggested method is explained and implemented through practical distribution network and its effectiveness is tested and discussed.

In order to solve the low-voltage problem of three-phase four-wire system in the low-voltage distribution network by using distributed generation (DG), this paper establishes the mathematical model of the DG configuration, and presents the DG placing method and configuration principle, considering the influence of unbalanced load, reactive power compensation and the existing DG of the network. For the phase voltage deviation and the phase power flow calculation, the mathematical model and the calculation method are given in this paper. According to the voltage deviation and the power flow under the maximum load condition, the placing site of DG is determined. Based on the expectation of the node voltage deviation percentage wanted to improved, the maximum current and maximum capacity of DG are deduced. In order to ensure the normal operation of the low-voltage distribution network, the voltage distributions are checked under the minimum load after conFigure the DG, and the minimum injection node current and minimum capacity of DG are determined in the case of minimum load. Taking the practical 0. 4kV three-phase four-wire network as an example, the DGs are configured to solve the low voltage problem. By the analysis and comparison of voltage distribution, the proposed placing DG method can be applied to different load cases, and it is an effective method to solve the low-voltage problem.