The impact of induction generator and PWM Inverter with energy storage on weak grids

The impact of wind energy generation connected to a weak grid is studied with SIMULINK as simulation platform. The wind energy conversion system is modelled by a squirrel cage asynchronous induction machine (ASM). The grid is modelled as an infinite bus. In the proposed system the ASM is connected to the grid through a DC-link with incorporated battery storage. An ultracapacitor (UCap) bank in parallel to the battery reduces the current ripple which is originated by the PWM – IGBT bridges and absorbs rapid power peaks. Different degrees of grid strength for 2 disturbing events (A: 3-phase fault and B: Voltage dip) are studied. The simulation model permits a sizing of the storage system according to the desired stabilizing capabilities of the system. From the simulation results can be concluded that under weak grid conditions the DC-link need to have a voltage control to ensure the stability of the system. Special attention must be paid to the power quality at the inverter output of the DC-link if renewable generation exceeds 5 % of the short circuit power at the coupling point. The simulated ASM generator is rated at 75 kW / 400 V. The DC-link operates at 700 V nominal voltage, the battery is rated at 340 kWh (C = 500 Ah), 34 kW at discharge rate C0.1 and 170 kW at discharge rate C0.5 with a UCap of 0.5 F in parallel, rated at 600 A discharge current.