Ignacio Egido

Maximum Frequency Deviation Calculation in Small Isolated Power Systems

Large frequency deviations due to a number of disturbances are frequent in small isolated power systems. The maximum frequency deviation in the system is limited to prevent other generator tripping. It is important to have an accurate model to calculate it, both for system planning and operation. A new simplified model to calculate the maximum frequency deviation when either a generator or load-related disturbance occurs in these systems is presented. This model takes into account the response of governor-prime mover even when different technologies are present in the power system. Model parameters can be easily obtained from either more complex models or from test records. Simulation results for an actual power system aimed at checking the model accuracy are presented. High accuracy is obtained while computation time is reduced due to the simplicity of the model.

A Method for the Design of UFLS Schemes of Small Isolated Power Systems

This paper presents a systematic method for the design of robust and efficient underfrequency load-shedding (UFLS) schemes. UFLS schemes play an important role in protecting the system integrity. The systematic method consists first in selecting representative operating and contingency (OC) scenarios by means of a clustering algorithm and subsequently, in tuning UFLS scheme parameters by dint of a simulated annealing optimization algorithm. The approach is applied to a small isolated Spanish power system. The systematic method leads to a robust and efficient UFLS scheme. The resulting design is also compared to a design based on OC scenarios determined by the common practice of OC scenario selection. The possibility of rearranging UFLS stages and the influence of minimum allowable frequency constraints is analyzed as well. Finally, an analysis of the impact of increasing converter-connected generation (CCG) is presented.

Modeling of thermal generating units for automatic generation control purposes

A simple discrete time model of a thermal unit has been formally developed for designing automatic generation control (AGC) controllers. This model has been developed using data obtained from specific tests and historical records. This model consists of a nonlinear block followed by a linear one. The nonlinear block consists of a dead band and a load change rate limiter, while the linear block consists of a second-order linear model and an offset. Although most of these elements have already been included in unit models for AGC presented in the literature, a certain mix up exists about which of them are necessary. This is clarified in this paper. It has been found that the unit response is mainly determined by the rate limiter, while the other model components are used for a better fitting to the real response. An identification procedure is proposed to estimate the values of the models parameters.

Representative Operating and Contingency Scenarios for the Design of UFLS Schemes

This paper studies an approach to identify representative operating and contingency (OC) scenarios for the design of underfrequency load-shedding (UFLS) schemes. In small isolated power systems, contingency scenarios are outages of generating units. Usually, only N-1 outages are considered. In this paper, simultaneous outages of several units are also taken into account. Data mining techniques such as K-Means and Fuzzy C-Means algorithms are used to group scenarios in terms of system frequency and to identify representative OC scenarios. The approach has been applied to the design of UFLS schemes of two of the Spanish isolated power systems. The results have also been compared to the common practice of scenario selection. Clustering techniques yielded to satisfactory results, i.e., representative OC scenarios can be identified. Furthermore, these representative OC scenarios cover a wider range of possible system responses than the scenarios selected following the common practice.

Principles of a Centralized UFLS Scheme for Small Isolated Power Systems

This paper presents the principles of a centralized underfrequency load-shedding (UFLS) scheme based on a frequency stability boundary curve suitable for small isolated power systems. The centralized UFLS scheme is illustrated in case of two small isolated Spanish power systems.

The Spanish AGC System: Description and Analysis

This paper describes and analyzes the Spanish AGC system. The differences with respect to a standard hierarchical structure are explained. A simple model useful for simulation is proposed. Using this model, the performance of the system and the influence of several parameters in system response is evaluated. The paper also suggests some changes in the configuration and parameter settings of the system to improve its performance.

Performance Analysis of UFLS Schemes of Small Isolated Power Systems

This paper presents a method for analysis of the performance of underfrequency load-shedding (UFLS) schemes. UFLS schemes play an important role in protecting the system integrity. The proposed method makes use of a Monte Carlo (MC) approach to evaluate the impact of step size variations and non-responding turbine-governor systems on the performance of UFLS schemes. The approach is applied to two isolated Spanish power systems of different size. Step size variations and non-responding turbine-governor systems are modeled by different probability density functions and their impact on the UFLS schemes of the two power systems is analyzed and compared.

Sizing and Controller Setting of Ultracapacitors for Frequency Stability Enhancement of Small Isolated Power Systems

This paper studies the impact of the size and controller settings of an ultracapacitor (UC) on frequency stability. Underfrequency load-shedding (UFLS) schemes play an important role in protecting the system against frequency instability and their performance is used as a measure for the enhancement of frequency stability thanks to the UC. To this end, a simplified but still accurate model of the UC is developed. In addition, a meaningful procedure is proposed to size the UCs power and energy storage capacities in function of the system parameters and operation constraints in order to improve frequency stability. Finally, the impact of the UC on the UFLS scheme is studied for many generation dispatch scenarios and for all possible single generating unit outages of a Spanish isolated power system. An analysis with respect to the UCs control parameters and its size in terms of power and energy storage capacities is carried out, too. Appropriate control parameter settings improve the UCs impact on frequency stability.

Local Hosting Capacity Increase by Means of Wind Farm Voltage Control Provision

This paper identifies when the transmission network local hosting capacity for a wind harvesting network may be limited because of steady-state bus voltage limits. In addition, the paper addresses how with the wind farm voltage control provision, such constraints may be overcome and the local hosting capacity can be increased. To answer these questions, actual Spanish system data is used on different network models of increasing complexity. Firstly, a simplified model of both transmission network and harvesting network is discussed to show that generally, only buses with low short-circuit power and low or high reactance-resistance ratio may limit local hosting capacity significantly. Secondly, in order to assess how modeling simplifications affect the results, the full model of an actual Spanish harvesting network is considered: the real reactive capability of the harvesting network at the transmission network connection node is computed and the local hosting capacity recalculated. Finally, in the last step, the results of the aforementioned simplified models are validated using the complete model of the Spanish transmission network. In addition, a complementary area hosting capacity analysis is included in order to show the importance of steady-state bus voltage constraints when large amounts of power need to be transported over long distances.

Frequency stability boundary of small isolated power systems

This paper explores new frequency protection schemes based on the analysis of the ω-dω/dt (frequency — rate of change of frequency) phase plane and on the definition of frequency stability boundary in particular. Due to their nature, these frequency protection schemes belong to advanced UFLS schemes and they could be applied in small isolated power systems. The definition of the frequency stability boundary is inspired by the definition of the region of angular stability. The derivation of the frequency stability boundary is based upon a simplified power-system model which allows summing power outputs of n generators without creating an (n+1)th-order system. It can be shown that the frequency stability boundary is a separatrix, describing a region of frequency stability in the sense of Lyapunov stability theory. This region helps determining if and how much load must be shed to guarantee frequency stability for a particular disturbance. The region of frequency stability is described by an ellipsis in the phase plane.