Typical PMSG based wind turbine consists of an electrical generator, machine-side converter (MSC), grid-side converter (GSC), dc-link capacitor, and a passive filter to connect the GSC to the grid. Under unbalanced grid voltage, the GSC can be used to control the active and reactive powers to implement fault ride through (FRT) algorithm. In such a case, the role of MSC reduces to regulating the dc-link voltage to a constant command value by controlling the stator currents of the PMSG. However, grid voltage unbalance can generate second-order harmonic ripples in the dc-link voltage, which can create a sinusoidal oscillation of frequency 2w in the torque of PMSG, where w is the grid frequency in rad/s. This paper proposes the use of notch filter to prevent the propagation of the dc-link voltage oscillation to the torque of PMSG. Moreover, disturbance observer-based control (DOBC) approach is used as a basis to design robust controllers for both GSC and MSC. The key feature of the proposed DOBC technique is its ability to achieve asymptotic regulation and accurate tracking of sinusoidal references of known frequencies to cope with voltage unbalance. Simulation tests have been conducted to verify the effectiveness of the proposed controllers.