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# A; How Is Power Converted In A

A; How is power converted in a synchronous generator with respect to induced voltages, and with respect to absorbed torque – which are the mechanism?

The synchronous generator is a AC generator in which the rotation of the shaft is synchronized with the frequency of the supply current this means the rotation period is exactly equal to an integral number of AC cycles.

In the synchronous generator power in converted due to interaction of the magnetic fields of the stator and the rotor. The stator winding consists of 3 phase windings which is supplied with a 3 phase supply from the grid, and the rotor in turn is provided with a DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux creates a rotating magnetic field. The rotor locks in with the rotating magnetic field and rotates along with it, when this happens the motor is said to be in synchronization.

B; How is the magnetization of the rotor accomplished in an asynchronous generator?

The asynchronous generator consists of a winded iron core stator which is placed in a cage of material with high electric conductivity. The magnetization of the rotor in an asynchronous generator is done by letting the grid AC power create magnetic fields that rotates in time with the AC oscillations. Unlike the synchronous generator the asynchronous generator dos not tur synchronous with the stator field, instead the rotor rotates at a slower speed than the stator field.

Therefore, the stator’s magnetic field is changing or rotating relative to the rotor. This induces an opposing current in the generators rotor. The rotating magnetic flux induces currents in the windings of the rotor similar to the currents induced in a transformer’s secondary winding. The currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field.

Due to Lenz’s Law, the direction of the created magnetic field will be such as to oppose the change in current through the rotor windings. The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field

Per Storgärd Wind Turbine Concepts and Application 2015-10-09

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3. Turbine electric system

A; What is the advantages (merits) of an asynchronous generator for a wind power system?

One reason for choosing an asynchronous generator is that it is very reliable due to its simple design, and tends to be comparatively cheap. Asynchronous generators are available in a wide range of electrical sizes from fractional horse power machines to multi-megawatt capacities which makes them ideal for use in both domestic and commercial renewable energy wind power applications. also the asynchronous generator can be connected directly to the utility grid and driven directly by the turbines rotor blades at variable wind speeds.

The generator also has some mechanical properties which are useful for wind turbines. (Generator-slip, and a certain overload capability).

B; Why DFIG and Optislip generators can have a variable speed.

Double fed induction Generator(DFIG); The DFIG is that rotor windings are connected to the grid via slip rings and back-to-back voltage source converter that controls both the rotor and the grid currents. Thus rotor frequency can freely differ from the grid frequency. By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generator’s turning speed. The control principle used is either the two-axis current vector control or direct torque control.

The doubly fed generator rotors are typically wound with 2 to 3 times the number of turns of the stator. This means that the rotor voltages will be higher and currents respectively lower. Thus in the typical ± 30% operational speed range around the synchronous speed, the rated current of the converter is accordingly lower which leads to a lower cost of the converter. The drawback is that controlled operation outside the operational speed range is impossible because of the higher than rated rotor voltage. Thus the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies.

Per Storgärd Wind Turbine Concepts and Application 2015-10-09

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Optislip induction generator(OSIG); The OptiSlip feature allows the generator to have a variable slip (narrow range) and to choose the optimum slip, resulting in smaller fluctuations in the drive train torque and in the power output. The variable slip is a very simple, reliable and cost-effective way to achieve load reductions compared with more complex solutions such as full variable-speed wind turbines using full-scale converters. OSIGs are a variable external rotor with resistance attached to the rotor windings. The slip of the generator is changed by modifying the total rotor resistance by means of a converter, mounted on the rotor shaft. The converter is optically controlled, which means that no slip rings are necessary. The stator of the generator is connected directly to the grid.

C; How can a synchronous generator be used in a variable speed system?

Development of modern wind turbine generators has evolved through the years, in the aspects of output power, dimensions, and the technology used. Conceptual scheme of wind turbines with full scale power converter, with variable speed with a synchronous generator. This type of turbines nowadays may or may not include a gearbox and wide range electrical generators types can be employed, for example, induction, wound-rotor synchronous or multi-pole permanent magnet synchronous. All the power generated by the generator passes through the convertor, thus isolating the dynamic operation of the generator from the grid. Electric frequency of the generator can vary following the change in the wind’s speed, otherwise power frequency injected in the grid remains unchanged, thus allowing variable speed operation of the wind turbine. The power converters can be arranged in different ways. Whereas the generator side converter can be a diode rectifier or a PWM (Pulse Width Modulation) voltage source converter, the grid side converter is typically a PWM source converter. The control strategy of the generator operation and active and reactive power flow to the grid depends on the type of power converter arrangement. The rotor side converter ensures the rotational speed being adjusted within a large range, whereas its grid side converter transfers the active power to the grid and attempts to cancel the reactive power consumption.e…

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