Figure 6 . Response of both SCIG and DFIG based WECSs on the described scenario.
It can be seen that at excess generation conditions and maximum dump load absorption capacity, the system would collapse on overfrequency for the SCIG based system and overvoltage for the DFIG based system. The DFIG based system tried to regulate its power output through the pitch control system but it was not enough to avoid the overfrequency which happened at a later stage as compared to the SCIG based system. The simulation was discontinued at the 80th second due to the collapse thus other events are not shown. An additional simple pitch regulation scheme dependent on the dump load power is proposed. It is implemented using a PI controller obtaining its input from dump load power and if it reaches 80% of its capacity, the pitch angle is increased to de-load the WECS. Fig. 7 shows the improved network response.