Joao Paulo Abreu Vieira

Long-Term Voltage Stability Analysis of Variable Speed Wind Generators

This paper presents the impacts caused by the integration of variable speed wind turbines on long-term voltage stability. The technologies used are fully rated converter (FRC) and doubly fed induction generator (DFIG) with two control strategies: grid-side converter (GSC) at unity power factor, which is usually adopted, and GSC controlling reactive power. Also, this paper considers wind turbines capability curves and its variable limits, since they are subject to several limitations that changes with the operating point and wind speed. This study also considers the dynamic models of over excitation limiter (OEL) and on-load tap changers (OLTC) combined with static and dynamic loads using time domain simulations. Different penetration levels of wind generation are analyzed. The results show that long-term voltage stability can be improved when GSC of DFIG is controlling reactive power. Moreover, the capability curve plays an important role in this analysis since reactive power is a key requirement to maintain voltage stability.

Reactive power control of direct drive synchronous wind generators to enhance the Low Voltage Ride-Through capability

This paper explores the performance of alternative voltage control strategy applied to direct drive synchronous wind generators, more specifically with permanent magnetic (PMSG). The reactive power control of the grid-side converter is investigated for voltage control purposes. In Brazil, the Grid National Operator (ONS) requires that wind turbines stay connected to the grid during voltage dips, but does not stipulate yet the need of reactive power injection during faults in the electric grid. It just specifies the Low Voltage Ride-Through (LVRT) capability curve for voltage dips that the wind generators should follow to avoid the trip of the under-voltage relay. Criteria of the synchronous wind generators protection are evaluated starting from short-circuit simulations in a test grid with adoption of the Brazilian grid code, without reactive power injection, being compared with those of other countries that adopt reactive power injection curves.

Impact of different DFIG wind turbines control modes on long-term voltage stability

This paper presents a comprehensive study showing the impacts of different doubly fed induction generator (DFIG) wind turbines control modes on long-term voltage stability. The study considers fixed voltage control and power factor control modes. The analyses also consider the dynamic models of Over Excitation Limiter (OEL) and On Load Tap Changers (OLTC) combined with static and dynamic loads using time domain simulations. A hypothetical network is used for the scenario of 20% load increase. The impact of each control strategy is studied and the resulting change in long-term system stability is quantified, as well as the interactions between OLTC and OEL equipments. The results show that the manner in which DFIGs are operated will have a significant impact on system stability.

Design of optimal PI controllers for doubly fed induction generators in wind turbines using genetic algorithm

This paper presents a design procedure based on evolutionary computation, more specifically on genetic algorithm (GA) to obtain optimal PI controllers to the static converter connected to the rotor of doubly fed induction generators (DFIGpsilas), in variable speed wind generation systems connected to the electrical grid. The converter of the DFIG has a protection system that monitors continuously the machine operation, emitting a blocking command when the limit of the rotor current is violated, due to transient disturbances occurring in the electrical grid. That implies in deactivating the DFIG control loops which affect negatively the system global controllability. This control action of the DFIG converter is accomplished by PI controllers, which gain adjustments are not a trivial task, due to the nonlinearities and the high complexity of the system. In this way an appropriate fitness function is derived to express the time domain evaluation of the DFIG with the objective to assure the DFIG continuous operation even under a fault condition and improve at the same time its transient behavior as compared with the formal methodology to design PI controllers using poles placement. The results obtained, confirm the efficiency of the proposed control design.

New Fuzzy Control Strategies Applied to the DFIG Converter in Wind Generation Systems

This paper presents new fuzzy control strategies which may be applied to the converter connected to the rotor of doubly fed induction generators (DFIG) composing a variable speed wind generation system connected to a real electrical grid. The fuzzy control proposed strategies are of the type supervised “look-up-table”. The performance of these intelligent controllers are compare with the fixed parameters PI controllers for fault ocurrence in the power system. The fuzzy controllers are supposed to better the transient performance of the electrical power system as compared with the conventional PI controllers. The obtained results from simulation studies have of the fuzzy controllers. To formulate the simulation studies an equivalent mathematical model of a significant number of wind turbines was implemented using the MATLABTM software packcage.

Security-Constrained Optimal Dispatch of Combined Natural Gas and Electricity Networks Using Genetic Algorithms

This paper proposes a method based on genetic algorithm (GA) for the security-constrained 11 optimal dispatch of integrated natural gas and electricity networks, considering operating scenarios 12 in both energy systems. The mathematical formulation of the optimization problem consists of a 13 multi-objective function which aims to minimize both cost of thermal generation (diesel and natural 14 gas) as well as the production and transportation of natural gas. The joint gas-electricity system is 15 modeled by two separate groups of nonlinear equation, which are solved by the combination of 16 Newtons method with the GA. The applicability of the proposed method is tested in the Belgian gas 17 network integrated with the IEEE 14-bus test system and a 15-node natural gas network integrated 18 with the IEEE 118-bus test system. The results demonstrate that the proposed method provides 19 efficient and secure solutions for different operating scenarios in both energy systems. 20 21

Hybrid Method for Protective Devices Placement, Sizing and Coordination in Electric Distribution Systems

This work presents a hybrid method to solve the protective devices allocation problem in electric distribution systems. Current methods only consider optimization results, without taking into account standards established by electric energy companies. These standards reflect distribution systems particularities observed by these companies. Firstly, recloser placementis determined through an optimization process, aiming to reduce reliability indices. Fuse links are placed according to heuristic rules established by electric companies. To validate the proposed methodology, a comparison with three other references from the literature is carried out, showing good results and proving the methods effectiveness.

Comparing NSGA-II and SPEA2 metaheuristics in solving the problem of optimal capacitor banks placement and sizing in distribution grids considering harmonic distortion restrictions

This paper focus on the optimal capacitor banks placement and sizing problem in radial distribution systems considering the presence of non-linear loads and their effects on the voltage harmonic distortion. A comparison between two multiobjective optimization methods used to solve this problem is accomplished, namely, NSGA II and SPEA 2. The performed analysis has shown that both algorithms are appropriate to solve the problem, particularly by the multiple solutions that form the Pareto front from which can be extracted convenient solutions. It is also emphasized the differences between the two methods and their consequences in determining the results. The relevance of this work is founded on new challenges that are related to resonance problems involved in the placement of capacitor banks in distribution systems.

Using genetic algorithm to obtain optimal controllers for the DFIG converters to enhance power system operational security

It is proposed in this paper a methodology to obtain optimal controllers gains for the rotor-side converter of doubly fed induction generators (DFIGs) using a genetic algorithm approach. The main objective is to enhance the operational security and robustness of the power system, by a more effective contribution of the DFIG controllers to the system controllability. To reach this goal, the crow-bar protection scheme is activated during the fault period when severe voltage sags occur in order to maintain the rotor-side converter connected to the DFIG. Immediately after the fault is cleared the crow-bar protection scheme is deactivated and simultaneously the rotor-side converter optimal controllers are turned on which permits the improvement of the converter ride-through capability and also contribute to enhance the overall power system stability margin. To validate the optimal proposed methodology several simulation results were obtained using a real electrical network which are presented in the paper.