Vienna Rectifier Fed Squirrel Cage Induction Generator based Stand-alone Wind Energy Conversion System

ABSTRACT:

 Back to back voltage source converters are normally preferred for interfacing squirrel cage induction generators (SCIG) with loads in stand-alone wind power generation applications as they allow for maximum power point tracking. However, the total converter losses tend to be very high, especially near the rated wind speed. Therefore, to reduce the power converter related losses, a Vienna rectifier is utilized as the machine side converter (MSC) in the present work to interface the SCIG. Owing to the limited reactive power handling capability of the Vienna rectifier a fixed capacitor bank is also required to provide the excitation VAR for the variable speed SCIG. In the present work a new computational method is proposed to calculate the value of this fixed capacitance based on maximizing the yearly energy output from the Wind Energy Conversion System (WECS). A voltage sensor less vector control scheme for the Vienna rectifier fed SCIG with the reference frame oriented along the machine terminal voltage is also proposed which adheres to the operating limits of the Vienna rectifier and gives much better dynamic performance under load and wind speed transients compared to similar Vienna rectifier based VSCF generating systems reported earlier in the literature.

KEYWORDS:

1.      Vienna rectifier

2.      Voltage sensor less

3.      Maximum yearly energy

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 

 

Fig. 1 Equivalent circuit representation of an induction machine with excitation capacitor and Vienna rectifier

 EXPECTED SIMULATION RESULTS:

Fig. 2 Performance of the proposed control scheme for Vienna rectifier assisted SCIG based WECS during load transient

 

Fig. 3 Performance of the proposed control scheme for Vienna rectifier assisted SCIG based WECS during wind speed variation

 CONCLUSION:

The excitation VAR required by the SCIG in a SCIG based stand-alone VSCF WECS can be provided by a combination of a fixed capacitance and a Vienna rectifier. An algorithm is proposed to choose the value of the fixed capacitance based on maximizing the yearly energy. A control scheme is developed to regulate the active power and reactive power adhering to the operating limits of the Vienna rectifier. The proposed control scheme increases the annual energy capture by 8 % to 10 % compared to operating the Vienna rectifier at unity terminal power factor. The performance of the control scheme is found to be much superior (compared to similar control scheme reported in the literature) under load transients, wind speed transients and with nonlinear/unbalanced loads. However, the LUT used in the present scheme to compute the terminal voltage reference is machine parameter dependent which varies with operating condition and ageing effect. The uncertainty in the values of the machine parameters can be mitigated by computing the terminal voltage reference value from the above method using nominal machine parameters and can then be further refined using online search based methods. The proposed converter scheme extracts about 8 % more electrical power at the rated wind speed compared to the B2B scheme and requires less cut-in wind speed compared to the STATCOM assisted SCIG. Further, the Vienna rectifier based WECS is shown to be capable of generating maximum annual energy output at most locations. Hence, this configuration can be regarded as a “standard topology” for small wind energy conversion systems feeding isolated loads.

REFERENCES:

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