Figure 18. Response on transformer energization at 70% excitation level in a DFIG based hybrid system.
From the responses above, energization at 100% excitation levels led to the SC field current being above an acceptable technical limit of 2.2pu [35]. This was the case for energization of the all the selected transformer sizes for both SCIG and DFIG based hybrid system. On energization at lower excitation levels (90% for the SCIG and 70% for the DFIG) the field current transient is kept below 2.2 pu for energization of the selected transformer sizes. From the voltage response, the windfarm is assumed to sustain the transients considering typical low voltage ride through capabilities [36]. At both excitation levels, the voltage response from the DFIG system is better, although slower, in comparison to the SCIG based system. The SCIG based system could handle the energization better if the excitation is lowered further while keeping the voltage above 1pu. This is achieved by energization of larger capacitor banks.
3.4. Underground Cable Energization
Although the cable infrastructure can be assumed energized during energization of the windfarm auxiliaries before starting of the WECSs if the energization source is located onshore, a scenario might emerge in which the initial excitation source is located offshore thus the HV submarine cable to the onshore substation requires energization. Response on energization of a 40km cable at 80% and 100% compensation levels is shown in Figs. 19 and 20.