Archivo para marzo 16th, 2016


Typical compensation system for renewable energy applications based on flywheel energy storage

There are two broad classes of flywheel-energy-storage technologies. One is a technology based on low-speed flywheels (up to 6000 r/min) with steel rotors and conventional bearings. The other one involves modern high-speed flywheel systems (up to 60 000 r/min) that are just becoming commercial and make use of advanced composite wheels that have much higher energy and power density than steel wheels. This technology requires ultralow friction bearing assemblies, such as magnetic bearings, and stimulates a research trend. Most applications of flywheels in the area of renewable energy delivery are based on a typical configuration where an electrical machine (i.e., high-speed synchronous machine or induction machine) drives a flywheel, and its electrical part is connected to the grid via a back-to-back converter, as shown in Figure. Such configuration requires an adequate control strategy to improve power smoothing. The basic operation could be summarized as follows. When there is excess in the generated power with respect to the demanded power, the difference is stored in the flywheel that is driven by the electrical machine operating as a motor. On the other hand, when a perturbation or a fluctuation in delivered power is detected in the loads, the electrical machine is driven by the flywheel and operates as a generator supplying needed extra energy. A typical control algorithm is a direct vector control with rotor-flux orientation and sensorless control using a model-reference-adaptive-system (MRAS) observer.

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006

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Variable-speed wind turbine with a hydrogen storage system and a fuel-cell system that reconverts hydrogen to electrical grid

As the wind penetration increases, the hydrogen options become most economical. Also, sales of hydrogen as a vehicle fuel are more lucrative than reconverting the hydrogen back into electricity. Industry is developing low-maintenance electrolysers to produce hydrogen fuel. Because these electrolysers require a constant minimum load, wind turbines must be integrated with grid or energy systems to provide power in the absence of wind.

Electrical energy could be produced and delivered to the grid from hydrogen by a fuel cell or a hydrogen combustion generator. The fuel cell produces power through a chemical reaction, and energy is released from the hydrogen when it reacts with the oxygen in the air. Also, wind electrolysis promises to establish new synergies in energy networks. It will be possible to gradually supply domestic-natural-gas infrastructures, as reserves diminish, by feeding hydrogen from grid-remote wind farms into natural-gas pipelines. The Figure shows a variable-speed wind turbine with a hydrogen storage system and a fuel cell system to reconvert the hydrogen to the electrical grid…

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Five-level cascaded multilevel converter connected to a multipole low-speed wind-turbine generator

The use of low-speed permanent-magnet generators that have a large number of poles allows obtaining the dc sources from the multiple wounds of this electrical machine, as can be seen in Figure. In this case, the power-electronic building block (PEBB) can be composed of a rectifier, a dc link, and an H-bridge. Another possibility is to replace the rectifier by an additional H-bridge. The continuous reduction of the cost per kilowatt of PEBBs is making the multilevel cascaded topologies to be the most commonly used by the industrial solutions. This as one alternative to multinivel conversors.

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Two HVDC transmission solutions_Classical LCC-based system with STATCOM and VSC-based system

Classical HVDC transmission systems [as shown in Figure (a)] are based on the current source converters with naturally commutated thyristors, which are the so-called linecommutated converters (LCCs). This name originates from the fact that the applied thyristors need an ac voltage source in order to commutate and thus only can transfer power between two active ac networks. They are, therefore, less useful in connection with the wind farms as the offshore ac grid needs to be powered up prior to a possible startup. A further disadvantage of LCC-based HVDC transmission systems is the lack of the possibility to provide an independent control of the active and reactive powers. Furthermore, they produce large amounts of harmonics, which make the use of large filters inevitable. Voltage-source converter (VSC)-based HVDC transmission systems are gaining more and more attention not only for the grid connection of large offshore wind farms. Figure (b) shows the schematic of a VSC-based HVDC transmission system

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Double three-phase VSI

The figure shows the scheme of a full power converter for a wind turbine. The machine-side three-phase converter works as a driver controlling the torque generator, using a vector control strategy. The grid-side three-phase converter permits windenergy transfer into the grid and enables to control the amount of the active and reactive powers delivered to the grid. It also keeps the total-harmonic-distortion (THD) coefficient as low as possible, improving the quality of the energy injected into the public grid. The induction generator of wind turbine is connected to a voltage-source inverter (VSI) used as a rectifier

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Single doubly fed induction machine controlled with slip power dissipation in an internal resistor

A number of turbines, ranging from 600 kW to 2.75 MW, have the variable-speed conditions are achieved dissipating the energy within a resistor placed in the rotor, as shown in Figure. Using that technology, the efficiency of the system decreases when the slip increases, and the speed control is limited to a narrow margin. This scheme includes the power converter and the resistors in the rotor. Trigger signals to the power switches are accomplished by optical coupling

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Single doubly fed induction machine with two fully controlled ac–dc power converters

Variable-Speed Concept Utilizing Doubly Fed Induction Generator (DFIG):In a variable-speed turbine with DFIG, the converter feeds the rotor winding, while the stator winding is connected directly to the grid. This converter, thus decoupling mechanical and electrical frequencies and making variable-speed operation possible, can vary the electrical rotor frequency. This turbine cannot operate in the full range from zero to the rated speed, but the speed range is quite sufficient. This limited speed range is caused by the fact that a converter that is considerably smaller than the rated power of the machine is used. In principle, one can say that the ratio between the size of the converter and the wind-turbine rating is half of the rotor-speed span. In addition to the fact that the converter is smaller, the losses are also lower. The control possibilities of the reactive power are similar to the full power-converter system. For instance, the Spanish company Gamesa supplies this kind of variable-speed wind turbines to the market. The forced switched power-converter scheme is shown in Figure. The converter includes two three-phase ac–dc converters linked by a dc capacitor battery. This scheme allows, on one hand, a vector control of the active and reactive powers of the machine, and on the other hand, a decrease by a high percentage of the harmonic content injected into the grid by the power converter.

Source:
Juan Manuel Carrasco, Leopoldo García Franquelo, Jan T. Bialasiewicz, Eduardo Galván, Ramón C. Portillo Guisado, Ángeles Martín Prats, José Ignacio León and Narciso Moreno-Alfonso “Power-Electronic Systems for the Grid integration of Renewable Energy Sources: A Survey”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006


Other example of microgrid con cell fuel wind turbine PV microturbine battery bank and loads

This microgrid have different elements: wind turbine, photovoltaics, fuel cell, battery bank, microturbine and interconection with main grifd. The level power is little but it is a interesting microgrid for study. It is a typical AC microgrid with load distribuited in many locations into microgrid. Main grind is a sub-transmission network in 20 kV.

Image Source:
Aris L. Dimeas, Nikos D. Natziargyriou “Operation of Multiagent System for Microgrid Control” IEEE Transactions on Power Systems, Vol. 20, No. 3, August 2005.


Actions sequence for the market operation in the time domain and Powerflows and bids in the microgrid

The overall procedure is the following:

1. The Market Operator (MO) announces the prices for selling (SP) or buying (BP) energy to the Microgrid. Normally it is SP>BP.
2. The local loads announce their demands for the next 15 minutes and an initial price (DP) for the kWh. It is DP>BPand DP<SP.
3. The production units accept or decline the load offer according to an Auction Price (AP).
4. The negotiation continues for a specific time (5 min).
5. After the end of the negotiation time, all the units have adjusted their set points. If there is no production unit within the Microgrid to satisfy the load demand, the power is bought from the grid. In addition, the grid can be considered as a load too, so the production or storage units can sell energy to the grid.

Source:
Aris L. Dimeas, Nikos D. Natziargyriou “Operation of Multiagent System for Microgrid Control” IEEE Transactions on Power Systems, Vol. 20, No. 3, August 2005.


Control levels of the microgrid environment

The DNO’s responsible for the technical operation in a medium and low voltage area, in which more than one Microgrids may exist. In addition, one or more MO’s are responsible for the Market Operation of this area. These two entities do not belong to the Microgrid, but they are the delegates of the grid. The DNO
refers to the operational functions of the system and the MO to the Market functions. It should be noted that, despite the autonomous operation of the Microgrid, it should ideally appear as a controlled, intelligent unit in coordination with the DNO.

The MGCC is the main responsible for the optimization of the Microgrid operation, or alternatively, it simply coordinates the local controllers, which assume the main responsibility for this optimization.

The LC’s control the Distributed Energy Resources (DER), production and storage units, and some of the local loads. Depending on themode of operation, they have certain level of intelligence, in order to take decisions locally. Of course, in any type of operation there are certain decisions that can be taken only locally.

Source:
Aris L. Dimeas, Nikos D. Natziargyriou “Operation of Multiagent System for Microgrid Control” IEEE Transactions on Power Systems, Vol. 20, No. 3, August 2005.