DFIG Turbine With PV Hybrid System
DFIG wind turbines are largely deployed due to their
variable speed feature and hence influencing system dynamics.
In recent years, wind energy has become one of the most important and promising sources of renewable energy, which demands additional transmission capacity and better means of maintaining system reliability. The evolution of technology related to wind systems industry leaded to the development of a generation of variable speed wind turbines that present many advantages compared to the fixed speed wind turbines. These wind energy conversion systems are connected to the grid through Voltage Source Converters (VSC) to make variable speed operation possible. The studied system here is a variable speed wind generation system based on Doubly Fed Induction Generator (DFIG). The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-back converter which is dimensioned to stand only a fraction of the generator rated power. To harness the wind power efficiently the most reliable system in the present era is grid connected doubly fed induction generator. The DFIG brings the advantage of utilizing the turns ratio of the machine, so the converter does not need to be rated for the machine’s full rated power. The rotor side converter (RSC) usually provides active and reactive power control of the machine while the grid-side converter (GSC) keeps the voltage of the DC-link constant. The additional freedom of reactive power generation by the GSC is usually not used due to the fact that it is more preferable to do so using the RSC. However, within the available current capacity the GSC can be controlled to participate in reactive power generation in steady state as well as during low voltage periods. The GSC can supply the required reactive current very quickly while the RSC passes the current through the machine resulting in a delay. Both converters can be temporarily overloaded, so the DFIG is able to provide a considerable contribution to grid voltage support during short circuit periods. This report deals with the introduction of DFIG, AC/DC/AC converter control and finally the SIMULINK/MATLAB simulation for isolated Induction generator as well as for grid connected Doubly Fed Induction Generator and corresponding results and waveforms are displayed.
With increased penetration of wind power into electrical grids, DFIG wind turbines are largely deployed due to their variable speed feature and hence influencing system dynamics. This has created an interest in developing suitable models for DFIG to be integrated into power system studies. The continuous trend of having high penetration of wind power, in recent years, has made it necessary to introduce new practices. For example, grid codes are being revised to ensure that wind turbines would contribute to the control of voltage and frequency and also to stay connected to the host network following a disturbance. In response to the new grid code requirements, several DFIG models have been suggested recently, including the full-model which is a 5th order model. These models use quadrature and direct components of rotor voltage in an appropriate reference frame to provide fast regulation of voltage. The 3rd order model of DFIG which uses a rotor current, not a rotor voltage as control parameter can also be applied to provide very fast regulation of instantaneous currents with the penalty of losing accuracy. Apart from that, the 3rd order model can be achieved by neglecting the rate of change of stator flux linkage (transient stability model), given rotor voltage as control parameter. Additionally, in order to model back-to back PWM converters, in the simplest scenario, it is assumed that the converters are ideal and the DC-link voltage between the converters is constant. Consequently, depending on the converter control, a controllable voltage (current) source can be implemented to represent the operation of the rotor-side of the converter in the model. However, in reality DC-link voltage does not keep constant but starts increasing during fault condition. Therefore, based on the above assumption it would not be possible to determine whether or not the DFIG will actually trip following a fault. In a more detailed approach, actual converter representation with PWM-averaged model has been proposed, where the switch network is replaced by average circuit model, on which all the switching elements are separated from the remainder of network and incorporated into a switch network, containing all the switching elements. However, the proposed model neglects high frequency effects of the PWM firing scheme and therefore it is not possible to accurately determine DC-link voltage in the event of fault. A switch-by-switch representation of the back-to-back PWM converters with their associated modulators for both rotor- and stator-side. Converters has also been proposed. Tolerance-band (hysteresis) control has been deployed.
However, hysteresis controller has two main disadvantages: firstly, the switching frequency does not remain constant but varies along the AC current waveform and secondly due to the roughness and randomness of the operation, protection of the converter is difficult. The latter will be of more significance when assessing performance of the system under fault condition. In order to resolve the identified problems, a switch-by-switch model of voltage-fed, current controlled PWM converters, where triangular carrier-based Sinusoidal PWM (SPWM) is applied to maintain the switching frequency constant. In order to achieve constant switching frequency, calculation of the required rotor voltage that must be supplied to the generator is adopted. Various methods such as hysteresis controller, stationary PI controller and synchronous PI controller have been adopted in order to control current-regulated induction machine. Among which, synchronous PI controller has been acknowledged as being superior. Power quality is actually an important aspect in integrating wind power plants to grids. This is even more relevant since grids are now dealing with a continuous increase of non-linear loads such as switching power supplies and large AC drives directly connected to the network. By now only very few researchers have addressed the issue of making use of the built-in converters to compensate harmonics from non-linear loads and enhance grid power quality. In, the current of a non-linear load connected to the network is measured, and the rotor-side converter is used to cancel the harmonics injected in the grid. Compensating harmonic currents are injected in the generator by the rotor-side converter as well as extra reactive power to support the grid. It is not clear what are the long term consequences of using the DFIG for harmonic and reactive power compensation. some researchers believe that the DFIG should be used only for the purpose for which it has been installed, i.e., supplying active power only .
Doubly Fed Induction Generator
Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generator.Where Vr is the rotor voltage and Vgc is grid side voltage. The AC/DC/AC converter is basically a PWM converter which uses sinusoidal PWM technique to reduce the harmonics present in the wind turbine driven DFIG system. Here Crotor is rotor side converter and Cgrid is grid side converter. To control the speed of wind turbine gear boxes or electronic control can be used.
Operating Principle of DFIG
The stator is directly connected to the AC mains, whilst the wound rotor is fed from the Power Electronics Converter via slip rings to allow DIFG to operate at a variety of speeds in response to changing wind speed. Indeed, the basic concept is to interpose a frequency converter between the variable frequency induction generator and fixed frequency grid. The DC capacitor linking stator- and rotor-side converters allows the storage of power from induction generator for further generation. To achieve full control of grid current, the DC-link voltage must be boosted to a level higher than the amplitude of grid line-to- line voltage. The slip power can flow in both directions, i.e. to the rotor from the supply and from supply to the rotor and hence the speed of the machine can be controlled from either rotor- or stator-side converter in both super and sub-synchronous speed ranges. As a result, the machine can be controlled as a generator or a motor in both super and sub-synchronous operating modes realizing four operating modes. Below the synchronous speed in the motoring mode and above the synchronous speed in the generating mode, rotor-side converter operates as a rectifier and stator-side converter as an inverter, where slip power is returned to the stator. Below the synchronous speed in the generating mode and above the synchronous speed in the motoring mode, rotor-side converter operates as an inverter and stator- side converter as a rectifier, where slip power is supplied to the rotor. At the synchronous speed, slip power is taken from supply to excite the rotor windings and in this case machine behaves as a synchronous machine.
Generally the absolute value of slip is much lower than 1 and, consequently, Pr is only a fraction of Ps. Since Tm is positive for power generation and since ωs is positive and constant for a constant frequency grid voltage, the sign of Pr is a function of the slip sign. Pr is positive for negative slip (speed greater than synchronous speed) and it is negative for positive slip (speed lower than synchronous speed). For supersynchronous speed operation, Pr is transmitted to DC bus capacitor and tends to rise the DC voltage. For sub-synchronous speed operation, Pr is taken out of DC bus capacitor and tends to decrease the DC voltage. Cgrid is used to generate or absorb the power Pgc in order to keep the DC voltage constant. In steady-state for a lossless AC/DC/AC converter Pgc is equal to Pr and the speed of the wind turbine is determined by the power Pr absorbed or generated by Crotor. The phase-sequence of the AC voltage generated by Crotor is positive for sub-synchronous speed and negative for super synchronous speed. The frequency of this voltage is equal to the product of the grid frequency and the absolute value of the slip. Crotor and Cgrid have the capability for generating or absorbing reactive power and could be used to control the reactive power or the voltage at the grid terminals.
This consists of the blades of the turbine, along with the hub; upon which the blades are mounted. The performance of a wind turbine is greatly affected by blade geometry, and in many designs, this component is also the most expensive part of the turbine unit.
Connecting the rotor to the generator is the drive train. In larger wind turbine systems, the drive train includes gearing to increase the speed of rotation from the rotor into the generator. Small turbines do not have this feature; the drive train for these systems is simply a connecting shaft.
The generator converts the mechanical rotation of the drive train into electricity. Small turbine generators are commonly of the 3- phase, permanent magnet type; however other generator types have been used.
To protect the system, in addition to converting the output of the generator to domestic voltages, a power electronic interface converter is necessary.
There are two ways in which we can divide the complete control strategies of the machine, one is the scalar control and the other one is the vector control. The limited use of scalar control makes way for the vector control. Although it is easy in executing the scalar control strategies, but the inherent coupling effects present give very slow response. This problem is overcome by the vector control. An Induction machine can be executed like a dc machines with the help of vector control strategy. Vector control is employed for achieving a decoupled control for both active and reactive powers. The base on which the vector control theory is based is d-q axis theory.
D-q axis transformation (reference frame theory)
Direct-quadrature zero conversion is a mathematical conversion employed to make easy the analysis of a three phase circuits, where three AC quantities are converted to two DC quantities. Various mathematical calculations are performed on the imaginary DC quantities and the AC quantities are again recovered by performing an inverse transformation on the DC quantities. It is very similar to Park’s transformation, and it solves the problem of AC parameters that are varying with time. Employed to simplify the analysis of three phase circuits, where three AC quantities are converted to two DC quantities. Owing a smooth air-gap in the induction machine, the self-inductances of both the stator and rotor coil is constant, but the mutual inductances vary with the rotor movement with respect to that of the stator. Therefore the analysis of the induction machine in real time becomes very complex because of varying mutual inductances, as the voltage is nonlinear. Change of variables are therefore employed for the stator and rotor parameter to remove the effect of varying mutual inductances. This conversion leads to imaginary magnetically decoupled two phase machine. The orthogonally placed balanced windings are called d and q windings that can be considered as stationary or rotating relative to that of the stator. In the stationary reference frame, the ds and sq. axes are fixed on the stator, with either ds or qs axis coinciding the phase axis of the stator. In the rotating frame, the rotating d-q axes may be either fixed on the rotor or made to move with synchronous speed.
This paper describes the model of variable speed wind turbines with 3-phase induction motor using back-to back PWM voltages source converters and the corresponding control schemes. The variable speed wind turbine is capable of controlling the output active and reactive power independently. To control the active and reactive power we use STATCOM using PWM switching. The proposed wind farm controller using STATCOM is simulated. Hence STATCOM was used to inject reactive power to maintain voltage level within limits and also eliminates power fluctuations and this confirms the excellent performance of the proposed system for power quality improvement.