4. Micro-grid model

A typical MG model shown in Figure 6 is used for simulation studies; the SimPowerSystems® package of the MATLAB/Simulink software is employed to carry out the simulations. This model system is composed of a 132-kV-bulk power system (represented as an infinite AC bus)

Figure 6. Single-line diagram of the test power system with an AC micro-grid including wind generation and the TCCC/ VRFB system.

Figure 7. Capability curves of the wind farm: (a) voltage curve and (b) frequency curve.

connected to a 13.2 kV MG through a 160 km transmission line. The MG is composed of a 10 MVA steam turbine, a constant load, a 12 MW wind farm composed of eight double-fed induction generator (DFIG) wind turbines, and a 1.5 MW TCCC/VRFB unit. The energy and power ratings of the TCCC/VRFB unit are determined according to Refs. [35, 36] and proved through the simulations presented in Section V. The mathematical models of the TCCC and the steam turbine are detailed in the Appendix, and the wind farm model is developed in Refs. [37, 38].

The wind farm protection system is represented in Figure 7, which illustrates the voltage and frequency capability curves in Figure 7(a) and (b), respectively. The protection system disconnects the wind farm when the terminal voltage is less than 0.2 pu during 0.5 s. On the other hand, frequency oscillations can also trip the wind farm. If the frequency of the MG is in the 49–48 or 51–52 Hz range during 15 s at least, the wind farm is switched off. If the frequency is lower than 48 Hz or higher than 52 Hz, the wind farm is instantaneously disconnected. The wind farm operates normally when the frequency of the MG is in the range of 49–51 Hz.
