Hans-Joachim Knaak

Fixed-reference-frame-control: A novel robust control concept for grid side inverters in HVDC connected weak offshore grids

Offshore wind energy plants built at a long distance from public ac-grid are connected by HVDC-transmission. Hence an offshore ac-island-grid results, in which the HVDC station generates the grid while the inverters of the wind power plants synchronize themselves on that gird. As the power of the HVDC-station is not significant higher than that of the sum of all wind power plants, the grid is relative weak. This could lead to stability problems of wind power plants grid side converters.

DFIG wind turbines operating in a fixed reference frame

HVDC connected offshore wind farms represent a weak grid that causes stability issues in voltage oriented controlled converters. In addition the conventional line side converter control is not applicable for island grids without an existing AC-voltage source. A Fixed Reference Frame (FixReF) control proved already beneficial in full scale converter wind turbines. In this paper, the usage of FixReF in wind energy plants with doubly fed induction generators is investigated.

Voltage and current behavior in a FixReF controlled offshore wind farm using a HVDC transmission system based on uncontrolled diode rectifier units

Future HVDC systems may connect offshore wind farms via uncontrolled diode rectifiers instead of VSC-HVDC converters. Due to this wind turbine inverters must own a grid forming capability. This paper describes a control strategy that requires only minor changes in the conventional wind turbine inverter control software to form the AC voltage in the offshore island grid connected via diode rectifier HVDC link.