Control and Optimal Operation of Grid-Connected Wind Power Plants

Abstract

This thesis focuses on design of control and optimal operation of wind power plants (WPPs) especially in the case of grid integration. For grid integration, a WPP needs to control its power for reliable and efficient operation. For these operations, each wind turbine (WT) should be controlled to produce power as commanded. Current technology of WT enables it and used for more advanced WPP active power control (APC) by manipulating each WT. From this APC, WPPs can help improve grid reliability and also they could protect themselves by limiting the power from the WPPs. Thus, the APC of a WPP has been attracting much interest from the researchers who studying about operation of WPPs. For this APC, this thesis handles some issues which includes a maximum power point tracking (MPPT) in terms of a WPP, WPP inertial response, coordinated control of WT and Energy storage system (ESS). To withstand the grid fault event, Moreover, the advanced coordinated (low voltage ride through) LVRT control is also developed which is very important issue for grid integration of wind power. Lastly, the dynamic economic dispatch for wind power plant control is presented. Therefore, the issues about control and optimal operation of grid connected WPPs were handled. Firstly, the mathematical model of permanent magnet synchronous generator (PMSG) model is described for wind power modeling. Wind power system includes wind turbine mechanical part, generator part and power conditioning system (PCS) for power transforming. By accounting whole wind power system (WPS), the effective efficiency of WPS can be evaluated and also the MPPT control scheme can be described. The wake interaction model is also described for WPP MPPT control. For obtaining WPP MPPT, the upstream WT should reduce its power production as de-loaded operation. By reducing its power production, the down stream wind turbine could produce more power from increased wind speed because upstream WT extracts less power from the given wind speed. Secondly, the predictive control algorithm considering wake model is introduced for WPP MPPT. By using predictive control algorithm, the more fast converging phenomenon and more optimal power production can be achieved compared other data driven method for WPP MPPT. Since data driven method depends mainly on output power from WPP, the method reduce its dependency on parameter accuracy. Two case which has different number of WTs in a WPP (3 and 16 WTs), has been investigated for the performance of the method. Third, wind power plant active power control is developed for grid support. Recently, the APC from WPP is requested from grid operator as the penetration level of WP is increased. At least, the same requirement of the APC from the conventional generators should be satisfied from WP since it replace significant portion of generation capacity in power systems. Since active power control is a global problem which affects power system frequency as a result, the total power from WPP is key issue for this problem. From the consideration about the power coefficient factor, $C_p$ of individual WT, and consideration of wake effect among WPP, the APC during frequency event such as a sudden large generator failure or increase of load. From these advanced APC of WPP, the grid frequency stability can be improved. For reliability of WP, reducing WP fluctuation is a important topic and many researches have been done for this. Recently, energy storage system (ESS) is used for supporting renewable energy and reducing the WP fluctuation is one of many roles that the ESS is used for. However, since a ESS is very expensive device until now even though it will be reduced afterward, it is not a efficient to give a whole burden of reducing WP fluctuation to the ESS. Thus, WT and ESS coordinated control is better than the previous method that ESS handles whole roles about smoothing the WP fluctuation. This approach can reduce the overall cost of the system and also enhance the overall system reliability. For reliability of both WPS and grid, low voltage ride through (LVRT) capability is important requirement when there is grid fault. And for this purpose, both coordinated control of converters are proposed. From the proposed method, rotor side converter reduces its power from wind turbine to regulate the dc link voltage by storing kinetic energy to the rotor. Moreover,the grid side converter also reduce its power to enhance the dc link voltage regulation performance. Lastly, the dynamic economic dispatch is developed considering stochastic operation cost. Wind energy has no cost for fuel. However, it has intermittent nature which incurs negative effect to power system. To generate WP economically, the probabilistic data (i.e. Weibull PDF) should be accounted for reliable and economic operation of WPP.