Control and stability of power inverters feeding renewable power to weak AC grids with no or low mechanical inertia

The number of power inverter installations feeding renewable energy to AC grids is worldwide increasing. The type of inverter used depends on the power level and on the AC system conditions at the terminal connection point. Line-commutated inverters need sufficient short circuit power. Self-commutated inverters can even operate without an existing grid. In both the cases the mechanical inertia decreases in proportion to the installation of static generation. The main question to be answered is therefore: can inverters be equipped with electrical inertia so that the existing load-frequency controls will be maintained? This work combines an analytical approach using MATLAB with transient simulations using PSCAD/EMTDC. The results demonstrate that stability is achievable with no or low mechanical inertia. The results are relevant for distributed generation as well as for remote off-shore wind farms supplying bulk power via HVDC transmission systems into coastal areas.