6. Conclusions

and frequency oscillation in isolated systems [2–4]. Bearing these aspects in mind, the dynamic performance of the MG is evaluated during the isolated operation. The operation conditions are as follows: the steam turbine is generating 4 MW (minimum operating power), the average wind farm active power is 3 MW, and the constant load is 7 MW. Therefore, the wind

Figure 10(a) shows the frequency of the MG by considering two cases: with and without energy storage. When the TCCC/VRFB unit is not employed, the steam turbine presents problems in establishing the nominal value of the frequency as a result of random variations in wind power. When the TCCC/VRFB is used, the frequency deviations are less than 0.01 Hz

Figure 10(b) shows the power output of the steam turbine and the wind farm. When the TCCC/VRFB unit is not employed, the steam turbine operates below the minimum operating power (4 MW) during several intervals. This situation is avoided when the TCCC/VRFB performs the load leveling of the wind power generation (Figure 10(c)), causing a reduction of the mechanical stress of the steam turbine. Figure 10(d) demonstrates that the steam turbine

These simulations show that the TCCC/VRFB unit and the new control system enhance the dynamic response of MGs which incorporate wind generation, by performing the load leveling of wind turbines and carrying out the frequency regulation of the MG. It is important to notice that the DFIG wind farm and the TCCC/VRFB unit complement each other; the reactive power fluctuations generated by the TCCC are compensated by the wind farm, whereas the fluctua-

Figure 10. Dynamic response of the MG in Case 3: (a) frequency deviation, (b) steam turbine/wind farm power, (c) TCCC/

and the wind farm can control effectively the voltage of buses 2 and 3, respectively.

tions of the wind power generation are smoothed by the TCCC/VRFB unit.

penetration is approximately 43%.

96 Redox - Principles and Advanced Applications

VRFB active power, and (d) bus voltage.

approximately, enhancing the power quality supply of the MG.

This work proposes a new control strategy for the TCCC/VRFB unit for enhancing the transient response of a MG with high wind power penetration. The control strategy has been implemented with a multilevel control topology. The high-level control system, which is the core of the contribution, sets the power that exchanges the TCCC/VRFB with the MG. The aims of this power flow are (1) to smooth the active power generated by a DFIG wind farm and (2) to provide the generation reserve required for PFC and SFC. The model aspects of the VRFB and the TCCC unit are also presented.

From the results obtained, it can be concluded that the developed control algorithms work satisfactorily. The TCCC/VRFB unit, with the operation of the load-leveling controller, effectively compensates the active power fluctuations from a wind farm. The complete system (wind farm plus TCCC/VRFB) generates a smoother power response than that of the system without the TCCC/VRFB. Moreover, the complete control system, now with the operation of the PFC and SFC controllers, also contributes to the recovery of the frequency when severe disturbances occur in the MG. Therefore, the incorporation of the TCCC/VRFB compensator and its new control system improves the integration of wind turbines into AC MGs.
