The WaveDragon is a 7 megawatt wave energy converter, that is currently developed for clean offshore energy production. The system will be floating several kilometres off the Pembrokeshire coast, Wales, UK, and transfers energy using a submarine power cable. Key interest is the control of the power take-off system at disturbed voltages and under weak-grid condition due to long submarine cable and remote, rural location. A multi-reference frame controller is implemented for harmonics compensation. A parameter estimator is implemented to estimate the grid impedance. Simulation and measurements results from a laboratory test setup that illustrate properties of developed method are shown.