Emerging Technologies. BY JIM ESCH. Producing electricity from renewable energy sources has ... energy sources comprised about 19% of worldwide energy.
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An introduction to the paper by Yaramasu, Wu, Sen, Kouro, and Narimani
High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies BY J IM E SCH Wind turbines when grouped into wind farms can be Producing electricity from renewable energy sources has become vitally important in a world of depleting fossil fuels connected to the electric grid. The turbines can be sited on and increasing environmental anxiety. In 2012, renewable land or offshore. Onshore wind farms are easier to access, energy sources comprised about 19% of worldwide energy feature lower startup and maintenance costs, and are consumption. In particular, wind power has become closer to transmission lines. Offshore wind farms cost legitimately competitive with conventional energy sources. more to install and maintain because stronger foundations Wind power capacity has risen exponentially since 1996, and are required and connections to the grid must be made the industry’s growth rate has risen more than 19%. Of the through submarine cables. Wind farms are connected to the electric power system through 83 nations using commercial wind various series/parallel and ac/dc power, 24 have more than 1 gigawatt configurations, using high voltage (GW) wind power capacity. Wind power capacity has ac and dc transmission systems. Harnessing the power of the Because wind power was intewind is nothing new. Humans have risen exponentially since grated to the grid rather rapidly, used it for shipping, water pumping, 1996, and the industry’s concerns have arisen over the elecand milling, and the conversion of growth rate has risen tric power system’s stability, securiwind kinetic energy to electrical more than 19%. ty, and efficiency. To address those energy started as long ago as 1887. concerns, grid codes have been But it was not until the 1980s that updated and enforced for wind turbine technology reached the point of maturity as a viable utility-scale option. Over the large-scale turbines and wind farms. And high-power past 30 years, we have seen the size of commercial wind wind turbine technology has been upgraded from turbines increase exponentially. In addition, the wind fixed-speed to full-variable speed operation. This paper reviews state-of-the-art progress with energy industry has benefitted from technological improvements in aerodynamic design, mechanical systems, respect to MW wind generator-converter configurations, electric generators, power electronic converters, integra- wind farm configurations and grid code compliance tion to power systems and control theory. Electrical methods. Four categories of generator-converter configengineers should be most intrigued by how electric urations have been identified and analyzed based on generators and power electronic converters can be adapted component count, modularity, reliability, converter/conto wind energy conversion systems (WECS). Various trol complexity, device voltage stress, operation voltage, combinations of generators and converters have led to and achievable power levels. wind turbines of three classes: fixed-speed, semivariable speed, and full-variable speed.
I . OVERVIEW OF HIGH POWER WECS
Digital Object Identifier: 10.1109/JPROC.2015.2418461
Let us first look at high power grid-connected WECS: their major components, operating voltages and grid code requirements. A WECS converts wind kinetic-energy into
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Prolog to the paper by Yaramasu, Wu, Sen, Kouro, and Narimani
electric-energy and consists of three broad component categories: mechanical, electrical, and control systems. Mechanical components relate to the MW as well as the small power scale wind turbines. Mechanical components include rotor blades, tower, nacelle, rotor hub, gearbox, wind speed sensors, pitch drives and yaw drives. The three-bladed design is the most common. The tower, nacelle and rotor hubs give mechanical support to the rotor blades. Wind velocity and direction are measured via sensors, and a yaw drive moves the nacelle toward the wind. Pitch drives are used to change the angle of blades such that turbine output power is limited to the rated capacity during high wind speeds. Turbines typically run at low speed (6 to 20 rpm) and high torque, coupled with a high-speed, low-torque generator shaft via a multi-stage gearbox. Gearboxes can be costly and prone to wearing out. As such, gearless or direct-drive technology eliminates the need for a gearbox, and commercial products are increasingly using them. There are drawbacks, however, such as large diameter and increased weight. Compromise solutions exist, which combine a medium-speed generator and single or two-stage gearbox. Electrical components consist of an electric generator that converts rotational mechanical-energy into electric-energy. Several types exist: squirrel-cage induction generator (SCIG), wound rotor induction generator (WRIG), doubly-fed induction generator (DFIG), permanent magnet synchronous generator (PMSG), and wound rotor synchronous generator (WRSG). Output voltage from the generator and frequency will change as the wind speed changes. A generator can be coupled directly to the grid or interfaced through a power electronic converter. Many power converter topologies are possible. When power electronic converters are combined with electric generators, different WECS configurations can be devised. Harmonic filters are used to solve switching harmonics problems when using power converters. They reduce harmonic distortion of generator currents and voltages. Grid-side harmonic filters connect the power converter output to a three-phase grid through a step-up transformer, electric switch gear, and a circuit breaker. Several slave control systems and a master control system are used to maintain performance for the WECS. Variables monitored include wind speed velocity, wind direction, generator voltages and currents, grid voltages and currents, etc. Adjustments of system operating states or variables can then be made, such as passive stall, active stall, pitch control. Control systems are implemented via computer, microcontroller, digital signal processor or field programmable gate arrays. Grid codes ensure the grid stability and consumer power quality, and their primary elements include active power control, reactive power control, grid power quality, flickers, harmonic oscillations, fault ride-through operation and system protection. Fault ride-through is a particular concern for wind turbine and power converter makers. Another
important requirement is reactive power control, which compensates the transmission equipment (cables and transformers) while maintaining voltage stability. Using different designs and combinations of generators and power electronic converters, many kinds of WECS commercial configurations are possible, including these types: 1) a fixed-speed SCIG-based WECS without power converter interface; 2) a semi variable-speed WECS using WRIG and partial rated (10%) power converter (often known as Optislip control); 3) a semi variable-speed WECS using DFIG with partial rated (30%) power converter; 4) a full-variable speed WECS with SCIG, PMSG or WRSG and full-scale (100%) power converter; 5) a direct grid-connected WRSG with speed/torque converter. Each type is described and compared in this paper, using the categories: generator, power converters employed, capacity of power converter, achievable speed-range, soft-starter requirement, gearbox and external reactive compensation, maximum power point tracking ability, aerodynamic control, compliance with fault ride-through requirement, technology status, and market penetration. It is found that Types 3 and 4 are best suited for MW-level application, and this paper examines them in detail.
II. POWER CONVERTERS FOR MW-WECS Power electronics technology is rather closely coupled with grid-connected wind turbines, and state-of-the-art full-scale converters perform energy conversion and grid integration at the turbine and wind farm levels. This paper provides an overview of power converters, their technical requirements and a classification of power converters for MW-WECS. A power converter must enable variable-speed operation and eliminate the need for soft-starter and reactive power compensation. This objective is achieved by converting the variable voltage/frequency of the wind generator to fixed voltage/frequency, which is accomplished by a variety of power conversion stages. Converter topologies can be broadly classified as direct and indirect. The most important requirements for MW-WECS power converters include: • initial cost, as a fraction of overall wind turbine cost; • high reliability, modular structure, and low maintenance cost; • higher efficiency, typically more than 98%; • power quality with output voltage close enough to sinusoidal waveform and lower total harmonic distortion of the generator and grid currents; • compliance with grid-codes; • high power density while maintaining a small footprint and low weight; • minimized cable size and cost. Vol. 103, No. 5, May 2015 | Proceedings of the IEEE
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Prolog to the paper by Yaramasu, Wu, Sen, Kouro, and Narimani
There are four groups of generator-converter configurations classified in the paper: Back-to-Back (BTB) Connected Converters, Passive Generator-Side (PGS) Converters, Converters for Multiphase Generators, and Converters without Intermediate DC-Link. Features and drawbacks for each type are discussed in detail. Back-To-Back Connected Power Converters. A back-to-back (BTB) converter has identical converters on the generator and grid sides, linked via a dc-link and performing a conversion of variable voltage/frequency output of the generator to dc, then dc to ac, with fixed voltage/frequency for the grid connection. Power flow is bidirectional. There are two types of BTB converters: low voltage (G 1 kV) and medium voltage (1–35 kV). For low voltage, four kinds of configurations are discussed: full-scale BTB two-level voltage source converters (2L-VSCs), Partial-Scale BTB 2L-VSCs, Parallel BTB 2L-VSCs with Common DC-Link, and Parallel BTB 2L-VSCs with Individual DC-Links. For medium voltage converters, four types of configurations are discussed: series connected switches, BTB Neutral-Point Clamped Converters, Other Voltage Source Multilevel Converters, and BTB Current Source Converters. BTB converters are compared with respect to power/voltage rating, semi-conductor/passive component count, voltage stress of switches, reliability, power quality, converter and control complexity, grid code compliance, technology status and market penetration. Passive Generator-Side Converters. Because WECS power flow is unidirectional (generator to grid), less-expensive and more reliable passive (diode-bridge) converters can be used on the generator side compared to pulse width modulated (PWM) converters, and passive converters can be utilized in PMSG/WRSG type wind turbines. Passive converters are not usable with induction generators. Four types of configurations are described in the paper: Diode Rectifier + 2L-VSC, Diode Rectifier + 2L-Boost Converter + 2L-VSC, Diode Rectifier + 3L-Boost Converter + 3L-VSC, and Diode Rectifier + Buck Converter + CSC. Converters for Multi-Phase Generators. The wind energy industry has used multi-phase generators and distributed converters as another approach to increase power-handling capacity. Generators with two sets of three-phase windings are called six-phase generators. Generators with more than six-phases are known as open-winding generators. Both types are described and compared in the paper. Six-phase generators have been adapted for use in Type 4 wind turbines, capable of two-times power handling capacity. Two types of converters for six-phase generators are compared: distributed converters and cascaded converters. Similarly, in wind generators with multiple windings (open winding generators), distributed converters and cascaded converters are discussed. Power Converters Without DC-Link. Matrix converters (MC) enable direct ac-ac conversion with no intermediate dc-link. This results in more silicon-based conversion at a 738
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low cost with small footprint. MCs can produce wide ranges for output voltage/frequency. They can convert variable voltage/frequency to fixed voltage/frequency for connecting the wind turbine to the grid. By eliminating the dc-link capacitors, reliability will increase. This is an important requirement for offshore wind turbines. A proper control system is needed to maintain correct operation of matrix converters. Voltage source converters and matrix converters are summarized and compared in the paper. These include LV matrix converters and MV matrix converters.
I II . WIND FARM CONFIGURATIONS Early wind farms were land-based and relatively easy to build and maintain. Offshore wind farms are desirable because more power can be produced, thanks to stronger and steadier winds. They also reduce impact on the landscape. By 2020, offshore wind power capacity could rise to as much as 40 GW. By optimizing the interconnection of wind turbines, a wind farm’s costs can be lessened, its efficiency heightened, and its reliability improved. Of the many proposed configurations, only a few have seen their way to practical implementation. This paper studies the more pragmatic and promising configurations, which include Parallel AC Configuration + HVAC Transmission, Parallel AC Configuration + HVDC Transmission, Parallel DC Configuration + HVDC Transmission, and Series DC Configuration + HVDC Transmission. Each features the series or parallel connection of WT output terminals, the coupling of WT output ac or dc terminals, and the connection of the wind farm to the grid by ac or dc transmission lines. They differ in the implementation of those features. The paper weighs the pros and cons of each type.
I V. FAUL T RIDE-THROUGH COMPLIANCE IN MW-WECS One major concern for makers of wind turbines and power converters, in particular Type 3 and 4 turbines (the types most focused on here) is fault-ride-through (FRT). A wind turbine system must provide reactive current during grid faults, and Type 4 turbines completely decouple the generator from the utility grid, so transmission faults are rendered invisible to the wind generator, drive train and gearbox. With an electrical control system, active power output can be decreased to zero, but the mechanical system response cannot keep pace with the electrical response, resulting in injections of active power to the power converter. Due to surplus energy, the net dc-bus voltage would increase exponentially with respect to the grid fault duration and destroy the complete power conversion system. Type 3 turbines are more complicated than Type 4 turbines, and because the DFIG stator terminal is directly connected to the three-phase grid,
Prolog to the paper by Yaramasu, Wu, Sen, Kouro, and Narimani
the DFIG experiences uncontrolled dynamics during grid faults, and FRT transients cause sever torque transients that threaten the drive-train and gearbox. Methodologies for solving the FRT problem in Type 3 and 4 turbines are examined in this paper. Types of solutions include regulating the active power generation using a pitch control system; storing surplus energy in the dc-link, external energy storage systems, and turbine-generator rotor inertia; dissipating surplus energy in the dc-link chopper, ac crowbar, and braking resistor; using compensation devices; and using control systems at wind farm, wind turbine and power converter levels.
V. TRENDS IN HIGH-POWER WECS Wind turbines are tending to get larger. At present, the largest turbine is 8 MW, but 10–15 MW turbines have been announced already. Major trends and challenges are summarized here. With respect to mechanical technologies, gear-box and drive-train technology must be developed to bring in lighter weight mechanical components. Direct-drive technology will reduce maintenance requirements in offshore wind turbines. Multiple drive-train technology will help reduce mechanical torque on each generator shaft. Wind turbine blade technology has become more aerodynamic, and the three-blade design is the most common standard. Challenges include lightning protection, noise reduction, optimum shape, increasing power-to-area ratio, and ease of manufacture. Newer blade sandwich technology can lower manufacturing and transportation costs. Wind turbine foundation design poses several challenges for offshore use. Gravity and monopile foundation are suitable for shallow waters with depths less than 20 m. Floating foundations, as have been used in offshore oil rigs, could be promising for deep water applications. With respect to electrical technologies, high power density electrical components are needed. It is expected that Type 4 turbines will overtake Type 3 as the dominant turbine type, and turbine manufacturers will seek alternative generator configurations with high power density. One attempt is to use a high temperature super conducting synchronous generator in place of PMSG/WRSG. Semiconductor switching devices are evolving as well. Emerging switching devices and advanced modulation and control schemes can improve power conversion efficiency. Asymmetric IGCTs are good for multilevel voltage source converters, and symmetric IGCTs are used in current source converters. Wind energy uses insulated gate bipolar transistors (IGBTs) in both LV and MV power converters. Next-generation power semiconductor devices are based on wide band-gap materials with significant performance improvements over silicon-based devices. The increasing power level in wind turbines drives power electronics technology towards the medium-voltage operation. Expect MV power converters to be dominant for the next
generation multimegawatt turbines. Research into new MV power converters and advanced control schemes will play an important role. Reliability of power converters will improve and fault tolerance will shorten turbine down times and yield high annual energy production. More efficient designs will decrease the size of power converters. With respect to integration to power systems, it is expected that WECS should behave like conventional power generation units by exhibiting black start capability, being able to restore normal operation from shutdown mode without help from the external power network. We can expect the grid code requirements in terms of frequency support, active and reactive power control, short-circuit power level, voltage variations, flicker, harmonics and stability to become more vigorous in future years. Ride-through requirements will evolve and affect high-power wind turbine design. More research is needed in studying asymmetrical grid fault conditions. Current source converters and partial scale voltage source converters need more attention as well. As offshore wind farms become more common, the cost, size, reliability and efficiency of interconnection approaches will be a crucial factor. It is likely that HVDC transmission systems will take over from current HVAC transmission. With respect to trends in control theory, control schemes enforce the WECS to achieve desired operation, increase wind energy conversion efficiency, reduce cost of energy, increase life time of turbine components, decrease structural loading, reduce turbine down times, and provide superior dynamic and steady-state performance. Mechanical and electrical power conversion units are both important, but this paper concentrates on electrical. Generator side converters are usually controlled by vector control or field oriented control. The reference control variables, such as generator speed, torque or power, are usually provided by a maximum power point tracking (MPPT) algorithm. Its methods include optimal tip-speed-ratio control, power signal feedback control, hill climbing searching control, optimal torque control and optimal power control. Power converter switching is adjusted to keep variables maintained at their reference value. Grid side converters in Type 3 and 4 WECS are controlled by decoupled voltage oriented control (VOC) or direct power control (DPC). We can also expect to see developments in the areas of finite control-set model predictive control. Aerodynamic techniques have also been proposed to control output power. In sum, we can expect enormous growth in wind energy systems worldwide. Technology development will make these systems more efficient, and switching to wind power will get us closer to the goals of energy independence and reduced pollution in the environment. This review paper on WECS technology serves as a comprehensive background reference material for practicing engineers, researchers and graduate students working in the field of renewable energy, particularly wind energy. h Vol. 103, No. 5, May 2015 | Proceedings of the IEEE
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