Figure 12.20 compares the MG frequency behavior during the reconnection of loads and non-controllable MS when considering both non-controlled charging and V2G service. At t = 5 s the MGCC enables the reconnection of the first load block causing a frequency deviation of 1.5 Hz, which is then corrected by the secondary control by increasing the controllable generation output. The load reconnection is interleaved with the reconnection of the non-controllable MS at t = 30 s, followed by the reconnection of the second block of loads at t = 50 s. After the restoration
Fig. 12.22 Active power injected by the MG main storage unit
Fig. 12.23 SSMT active power response to the MGCC centralized secondary frequency control
of load there is still sufficient reserve capacity and the MGCC enables the increase of the EV charging power, which is divided in three steps: 20 % of their nominal power at t = 70 s, 40 % at t = 80 s and 60 % at t = 90 s.
As it can be observed in Figs. 12.20 and 12.21, the active participation of the EV in the frequency regulation reduces the frequency deviation caused by the reconnection of loads and the non-controllable MS. When the frequency deviation falls outside of the P-f droop frequency dead-band (from 49.9 to 50.1 Hz), the EV inject power during the reconnection of loads and increase their consumption during the reconnection of the non-controllable MS. The participation of EV in frequency regulation becomes more significant for large frequency disturbances, according to their droop characteristics. Following the MG frequency stabilization (t = 65 s) and since the MG had enough reserve capacity, the P-f droop parameter Prated is increased gradually, in order to allow the EV to charge their batteries.
Figure 12.22 shows the MG main storage power output for both case studies. As demonstrated, when considering the V2G operation mode there is a reduction of the active power required from the MG main storage unit.
The active power response of the SSMT to the centralized secondary load frequency control is shown in Fig. 12.23. The SSMT change their power output considering the frequency set-points sent by the MGCC. During the restoration procedure the SSMT coupling inverters are controlled as VSI with a MMO strategy. The DC coupled storage provides immediate power response to the MGCC frequency control signals, compensating the SSMT slower power response.