A tendency to erect ever more wind turbines can be observed in order to reduce the environmental consequences of electric power generation. As a result of this, in the near future, wind turbines may start to influence the behavior of electric power systems by interacting with conventional generation and loads. Therefore, wind turbine models that can be integrated into power system simulation software are needed. In this contribution, a model that can be used to represent all types of variable speed wind turbines in power system dynamics simulations is presented. First, the modeling approach is commented upon and models of the subsystems of which a variable speed wind turbine consists are discussed. Then, some results obtained after incorporation of the model in PSS/E, a widely used power system dynamics simulation software package, are presented and compared with measurements.

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General model for representing variable speed wind turbines in power system dynamics simulations

Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

Grid-connected photovoltaic power systems: Technical and potential problems—A review

Small-signal stability analyzed results of an autonomous hybrid renewable energy power generation/energy storage system connected to isolated loads using time-domain simulations is presented in this paper. The companion paper presents frequency-domain analyzed results of the same hybrid system. The proposed renewable energy power generation subsystems include three wind turbine generators (WTGs), a diesel engine generator, two fuel cells (FCs), and a photovoltaic system (PV) while the energy storage subsystems consist of a battery energy storage system and a flywheel energy storage system. An aqua electrolyzer absorbs a part of generated energy from PV or WTGs to generate available hydrogen for FCs. A time-domain approach based on three mathematical models for three studied cases under various operating points and disturbance conditions is performed. It can be concluded from the simulation results that the proposed hybrid power generation/energy storage system feeding isolated loads can be properly operated to achieve system power-frequency balance condition.

Small-Signal Stability Analysis of an Autonomous Hybrid Renewable Energy Power Generation/Energy Storage System Part I: Time-Domain Simulations

A survey is presented of papers and reports that address various aspects of economic dispatch. The time period considered is 1977-88. Four related areas of economic dispatch are identified and papers published in the general areas of economic dispatch are classified into these. These areas are: optimal power flow, economic dispatch in relation to AGC, dynamic dispatch, and economic dispatch with nonconventional generation sources. >

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A review of recent advances in economic dispatch

In the post-fault dynamic analysis of interconnected power systems, the critical fault clearing time (CCT) is one of the parameters of paramount importance. Critical clearing time is a complex function of pre-fault system conditions (operating point, topology, system parameters), fault structure (type and location) and post-fault conditions that are in part dependent on the protective relaying policy. To define analytically such a relationship would be highly desirable but diversity of variable involved makes this task extremely complicated. Our efforts focus on examination of that complex mapping and investigation of the influence of the various parameters on CCT. The evaluation of CCT involves elaborate computations that often include time-consuming solutions of nonlinear on-fault system equations. Existing conventional pattern recognition techniques are incapable of synthesizing such complex and transparent mappings. Thus, when a human operator tells the machine learning unit (that is the pattern recognizer) that system state belongs to a certain class, say "emergency", the pattern recognizer merely records that classification mindlessly and is not able to look at the pattern with insight and discover what underlies the "emergency" nature of pattern. It is, therefore, highly desirable to have a mechanism which when presented with a sequence of class labeled patterns not only learns an internal structure which allows it to generalize and to classify other and to classify other patterns correctly, but also is able to shed some light on what combination of features give rise to the particular class membership.

Artificial neural-net based dynamic security assessment for electric power systems

A simulation and digital computer modeling effort is described in which a wind turbine- generator system is adapted for stability evaluation using a large scale transient stability computer program. Component models of the MOD-2 wind generator system are described and their digital model equations are provided. A versatile wind velocity model is described, which provides the capability of simulating a wide variety of wind variations, in addition to the usual network disturbances. Computed results obtained from runs of the enhanced stability program are provided that illustrate the wind turbine-generator system dynamic performance for changes in wind velocity.

Stability Simulation Of Wind Turbine Systems

Power system stability analysis is usually performed in a deterministic framework in which the time domain response of the power system is studied for certain specific disturbances to determine the adequacy of the system. However, the occurrence of disturbances and their attendant protective switching sequences are random processes and it would be more meaningful to determine the probability of stability for a power system. An approach for such a determination is presented in this paper. The probability of steady state stability is relatively easier to determine because of the linearization of the system equations. The probability of transient stability, on the other hand, is much more difficult to obtain analytically because of the nonlinear transformations required. However, a Monte Carlo simulation method to determine transient stability probability is shown to be feasible.

A Probabilistic Approach to Power System Stability Analysis

This paper presents an evaluation of the effect on utility operation of photovoltaic (PV) generation that is interconnected to an electric utility grid. Various PV concentrations and performance characteristics are examined and the effect on utility generation control performance is evaluated.

The Effect of Photovoltaic Power Generation on Utility Operation

This paper presents a Monte Carlo simulation of the transient stability of a power system. The simulation time is in the order of years in which the occurrence of disturbances and the subsequent protective action are considered to be stochastic processes. The objective is to obtain a probabilistic measure of transient stability for a power system instead of just its particular response to an individual disturbance. The latter is the usual output of deterministic transient stability programs that are used for present day worst case analysis. A probabilistic analysis is needed if such worst case design criteria is to be replaced by (probabilistic) reliability criteria. This paper describes a computer program that has been developed for such a Monte Carlo simulation and presents some results of its application to representative systems.

Monte Carlo Simulation of Power System Stability

Models for power plants have been developed over several decades for use in long term dynamic simulation of large power systems. These models are derived from, but are significantly simpler than, models used for studying power plants. This is because in system studies, as opposed to power plant studies, only the trajectories of the output variables of many plants are of interest as opposed to the many variables internal to the plant. The first use of these models was in the simulation and study of automatic generation control. This was done by interconnecting the power plant models through a uniform frequency dynamic model of the power system. Later, the addition of the network model and models of other system components provided the capability to simulate all aspects of power system long term dynamics. The power system simulation required in an operator training simulator is very similar to that needed for long term dynamic studies with the added requirement that it must be run in real time. These available power plant models have been used in operator training simulators and the suitability for such application of these models has also been shown. The fast solution requirement of the operator training simulator, however, has often led to the use of simple low order models whose accuracy, though suspect, has never been reported. In this paper, an attempt is made to provide models of varying complexity.

Paul M. Anderson

Papers

Stability Simulation Of Wind Turbine Systems

A Probabilistic Approach to Power System Stability Analysis

The Effect of Photovoltaic Power Generation on Utility Operation

Monte Carlo Simulation of Power System Stability

Power plant models for operator training simulators