Karim Karoui

Modeling the primary reserve allocation in preventive and curative security constrained OPF

The paper presents the modelling of the primary reserve allocation that is needed to model accurately post-contingency states in Security Constrained Optimal Power Flow (SCOPF) due to an incident involving a modification of the active power balance. The dynamic behaviour of the network between the pre- and post-contingency steady-states is not taken into account

An optimum power flow tool for optimising the expansion plan of reactive power compensation

This paper describes the methodology to determine the optimum size and location of reactive compensation equipments. The proposed method is implemented with a new interior point optimum power flow program called IPSO (Integrated Power System Optimizer), developed by Tractebel Engineering. The proposed objective functions achieve the trade off between economic objectives (investment cost) and functional objectives (real power losses reduction or security level requirements). The discrete variables are first modelled as continuous quantities then rounded-off sequentially to obtain a applicable operating point. The efficiency of the proposed methodology has been proven on several studies in different Gulf countries (e.g. Oman, Dubai…).

Stochastic optimization for reactive power planning problems

In this paper, a stochastic optimal power flow is developed to solve the reactive power planning (RPP) problem while dealing with uncertainties such as intermittent power generation and loads. The RPP problem is formulated as a mixed integer non-linear problem (MINLP) which is a hard problem to solve. Therefore, Benders decomposition is used in order to get feasible solutions. The uncertainty sources are characterized by probability density functions from which samples are generated. If rare events are considered, conventional Monte Carlo method gives rise to a too large number of samples. The main objective of this paper is to identify the most critical samples leading to possible voltage limits violations due to a lack of reactive power supply and find the required optimal reactive power compensation. An iterative procedure is developed and aims to concentrate the samples in the zones having the highest influence on the RPP. First, both MINLP and Benders formulations are compared on a 9-node system to show the validity of the Benders decomposition. Then the iterative sampling process is assessed on the same test case and allows finding the optimal robust reactive power compensation with a reduced number of samples compared to direct Monte Carlo Simulation (MCS).

Automatic tuning of out-of-step relays

A method and prototype tool for tuning out-of-step relay parameters is presented. A decision making matrix is built from dynamic simulation results. Based on this matrix, an optimization problem is formulated that determines the out-of-step relay parameters that minimize the number of cases in which the relays do not operate satisfactorily. The method has been successfully tested on a simple test case with one relay to tune, and on a four zone test case where several relays are tuned for different operating scenarios.

Efficient samplings in stochastic optimization for reactive power planning problems

Résumé Dans cet article, un optimal power flow stochastique est développé afin de résoudre un problème de planif ication de puissance réactive (RPP) sujet à des productions de puissance intermittentes et des charges stochastiques. Le problème RPP est utilisé pour gérer de manière optimale les écoulements de puissance réactive sur le réseau électrique et consiste en un problème d’optimisation non-linéaire à variables binaires (MINLP) dont la difficulté de résolution est élevée. Une décomposition de Benders est donc utilisée pour obtenir une solution faisable. Les sources d’incertitude sont modélisées par des densités de probabilité (pdf) à partir desquelles des échantillons sont tirés (approche par scénario). L’approche classique de Monte Carlo nécessite un nombre trop élevé d’échantillons si des cas peu probables doivent être identifiés ce qui justifie le développement de techniques d’échantillonnage plus efficace permettant de trouver les échantillons critiques tout en réduisant la variance des résultats. Cet article vise donc à identifier les cas menant à des violations des limites de tension due à une gestion inefficace de la puissance réactive afin de concevoir la compensation réactive nécessaire au bon fonctionnement du réseau. Un processus itératif est développé afin de concentrer les échantillons dans les zones les plus critiques pour le RPP. Cette méthodes est testée sur un réseau à 9 nœuds et permet de trouver la compensation réactive optimale tout en réduisant le nombre d’échantillons par rapport à la méthode de Monte Carlo. Une formulation robuste et une formulation stochastique du RPP sont implémentées. Une extension de cette méthode à de plus grands réseaux nécessitera une adaptation de la stratégie d’échantillonnage stratifié due à l’augmentation du nombre de paramètres incertains.

Dynamic assessment of voltage & reactive power control in Oman Transmission System

The paper presents dynamic assessment of voltage and reactive power control in the Main Interconnected Transmission System (MITS) in Oman. Two case studies are considered; namely the MITS with and without temporary diesel generators. Voltage and reactive power control is achieved by optimization process to maximize reactive power margin of generating units to maintain as much as possible reserves on different generators. The optimal power flow technique is used to determine the optimal settings of the control facilities. These include adjustment of generation reactive power injections, transformer tap-changers and switching on/off of controllable shunt capacitors. The objective is to verify that the MITS is able to achieve an adequate steady-state voltage profile and acceptable dynamic performance. Simulation results are presented to show the dynamic performance of the system when it is subjected to a large disturbance such as outage of a generating unit or outage of a transmission circuit.

Small-disturbance stability of Oman/UAE interconnected system

This paper presents small-disturbance stability studies of the interconnected power systems of the Sultanate of Oman and the State of the United Arab Emirates. The two systems are interconnected through the existing 220 kV double circuit transmission line between Oman and Abu Dhabi leading to a rather longitudinal structure power system. The objective of the paper is to assess the inter-area oscillation damping of this interconnected system during small amplitude disturbances by using eigenvalue analysis. A detailed model of the interconnected system is obtained including representation of generators, prime movers, speed governors, exciters, automatic voltage regulators, transmission lines, transformers, compensators and loads. The system eigenvalues are calculated at various operating scenarios, including peak and minimum loading conditions with zero and maximum power exchanges in both directions and also during N and N-1 conditions. The results have shown that both systems have adequate damping before being interconnected. After interconnection, the eigenvalues profile indicates that the system will remain stable and satisfactorily damped for the considered power exchanges.

Load model development based on monitoring and laboratory staged testing

The paper presents a load model to assess the voltage stability margin of power system with a large share of air conditioning (AC) load. The first part of the paper describes the laboratory tests performed on a set of electrical appliances and the dedicated load monitoring campaign performed in the power system of Dubai (UAE). It illustrates the non linear nature of the load following large disturbances. The second part of the paper presents a general methodological approach to model at system level the distributed undervoltage relays embedded in the electrical AC appliances single and three phase LV connected and MV connected appliances.