10-05-2014, 12:03 PM
Modeling and Real-Time Simulation of Non-Grid-Connected Wind Energy Conversion System
Modeling and Real-Time Simulation.docx (Size: 1.89 MB / Downloads: 12)
Abstract
Method for wind turbine controller design and test in laboratory still remains a problem. This paper presents a real time simulator of non-grid-connected wind energy conversion system suitable for controller design and test. The simulation model, which is composed of a fixed-pitch wind turbine, a permanent magnet synchronous generator with PWM voltage source vector control and DC-DC converter with peak current control, is especially suitable for electrochemical industries. High energy efficiency can be achieved by adjusting the rotor speed according to the wind speed. Simulation results show the validity of system modeling and control strategy.
INTRODUCTION
The terminal loads of non-grid-connected wind energyconversion system are no longer the conventional power grid,the characteristic of it lies in that power can be applied to singleunit immediately, which makes it quite suitable for high energyconsuming electrochemical industries such as electrolyticaluminum and seawater desalination. Cancelation of auxiliaryappliances that needed for grid connection introducessimplicity of configuration to the plants which helps to reducethe manufacturing cost of the system and the price of windpower energy can also be cut down.
With rapid advances in computer technologies, it isincreasingly advantageous to make a more efficient approach tosystem prototyping using Hardware-in-the-Loop (HIL) digitalsimulation, where one or more devices (wind turbinecontrollers) are tested while connected to a real-time dynamicequivalent of plant. If the system is correctly modeled, the windgenerator controllers under test will behave as if they wereconnected to the real system. The tested controllers cantherefore be tested over a wide range of parameters without riskto the main system.
Wind power generation
Windpower technology dates back many centuries. There are historical claims that wind machines which harness the power of the wind date back beyond the time of the ancient Egyptians. Hero of Alexandria used a simple windmill to power an organ whilst the Babylonian emperor, Hammurabi, used windmills for an ambitious irrigation project as early as the 17th century BC. The Persians built windmills in the 7th century AD for milling and irrigation and rustic mills similar to these early vertical axis designs can still be found in the region today. In Europe the first windmills were seen much later, probably having been introduced by the English on their return from the crusades in the middle east or possibly transferred to Southern Europe by the Muslims after their conquest of the Iberian Peninsula.
It was in Europe that much of the subsequent technical development took place. By the late part of the 13th century the typical ‘European windmill’ had been developed and this became the norm until further developments were introduced during the 18th century. At the end of the 19th century there were more than 30,000 windmills in Europe, used primarily for the milling of grain and water pumping.
MAKING POWER FROM WIND
The blades on a wind turbine are similar to the propeller blades on an airplane. The rotor blades generate lift from the passing wind, causing them to rotate the hub of the turbine. The rotating action of the hub then turns a generator, which creates electricity. A gearbox is generally necessary to optimize the power output from the machine. That power is then either fed into the electric grid or stored in batteries for use on-site. While wind speed is important, so is the size of the rotor. On a turbine, the power available to the blades is proportional to the square of the diameter of the rotor.
Principles of wind energy conversion
There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either lift or drag force (or through a combination of the two). The difference between drag and lift is illustrated by the difference between using a spinnaker sail, which fills like a parachute and pulls a sailing boat with the wind, and a Bermuda rig, the familiar triangular sail which deflects with wind and allows a sailing boat to travel across the wind or slightly into the wind.
Drag forces provide the most obvious means of propulsion, these being the forces felt by a person (or object) exposed to the wind. Lift forces are the most efficient means of propulsion but being more subtle than drag forces are not so well understood.
Grid connected or battery charging
Depending on the circumstances, the distribution of electricity from a wind machine can be carried out in one of various ways. Commonly, larger machines are connected to a grid distribution network. This can be the main national network, in which case electricity can be sold to the electricity utility (providing an agreement can be made between the producer and the grid) when an excess is produced and purchased when the wind is low. Using the national grid helps provide flexibility to the system and does away with the need for a back-up system when windspeeds are low.
Micro-grids distribute electricity to smaller areas, typically a village or town. When wind is used for supplying electricity to such a grid, a diesel generator set is often used as a backup for the periods when windspeeds are low. Alternatively, electricity storage can be used but this is an expensive option. Hybrid systems use a combination of two or more energy sources to provide electricity in all weather conditions. The capital cost for such a system is high but subsequent running costs will be low compared with a pure diesel system.
In areas where households are widely dispersed or where grid costs are prohibitively expensive, battery charging is an option. For people in rural areas a few tens of watts of power are sufficient for providing lighting and a source of power for a radio or television. Batteries can be returned to the charging station occasionally for recharging. This reduces the inconvenience of an intermittent supply due to fluctuating windspeeds. 12 and 24 volt direct current wind generators are commercially available which are suitable for battery charging applications. Smaller turbines (50 -150 watt) are available for individual household connection.
SYNCHRONOUS MACHINE
A synchronous machine is an ac rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature. The magnetic field created by the armature currents rotates at the same speed as that created by the field current on the rotor, which is rotating at the synchronous speed, and a steady torque results. Synchronous machines are commonly used as generators especially for large power systems, such as turbine generators and hydroelectric generators in the grid power supply.
Because the rotor speed is proportional to the frequency of excitation, synchronous motors can be used in situations where constant speed drive is required. Since the reactive power generated by a synchronous machine can be adjusted by controlling the magnitude of the rotor field current, unloaded synchronous machines are also often installed in power systems solely for power factor correction or for control of reactive kVA flow.
Such machines, known as synchronous condensers, may be more economical in the large sizes than static capacitors. With power electronic variable voltage variable frequency (VVVF) power supplies, synchronous motors, especially those with permanent magnet rotors, are widely used for variable speed drives. If the stator excitation of a permanent magnet motor is controlled by its rotor position such that the stator field is always 90o (electrical) ahead of the rotor, the motor performance can be very close to the conventional brushed dc motors, which is very much favored for variable speed drives.
Angle in Electrical and Mechanical Units
Consider a synchronous machine with two magnetic poles. The idealized radial distribution of the air gap flux density is sinusoidal along the air gap. When the rotor rotates for one revolution, the induced emf, which is also sinusoidal, varies for one cycle as illustrated by the waveforms in the diagram below. If we measure the rotor position by physical or mechanical degrees or radians and the phase angles of the flux density and emf by electrical degrees or radians, in this case, it is ready to see that the angle measured in mechanical degrees or radians is equal to that measured in electrical degrees or radians, i.e. q = qmwhere q is the angle in electrical degrees or radians and qm the mechanical angle.
HIL REAL-TIME SIMULATOR OF THE SYSTEM
Nowadays, real-time simulation plays an important part in rapid control prototyping, system integration and hardware-inthe- loop testing. Electrical systems have higher bandwidth than mechanical systems and therefore command smaller simulation time steps, the problem gets most when simulating powerconverters like PWM motor inverters and DC-DC converters because of their high switching frequencies with regards to the simulation sampling frequency. RT-LAB electrical drive simulator proves to be a powerful tool in real-time simulation because of its state-of-the-art hardware and software technique.
The PWM signals generated by the real controllers are sampled with a high frequency counter card based on FPGA running at much faster rate than the simulation process, whose frequency is as high as 100MHz. Along with it, there is a special “time stamp” technique. When some transition occurs on the gate, the counter card stop counting and therefore “stamps” the time of the transition with 10ns resolution. The count is then transferred to the Time stamped bridge as a normalized ratio on the Pentium side of the simulator where the wind generator is simulated. Besides that, ARTIMES solver provides superior simulation efficiency than other solvers. A complex simulation model can be divided into several parts and distributed to different CPU cores, which can shorten the simulation time step greatly