Virtual-Flux-Based Voltage-Sensor-Less Power Control for Unbalanced Grid Conditions

This paper presents a virtual flux-based method for voltage-sensor-less power control of voltage source converters under unbalanced grid voltage conditions. The voltage-sensor-less grid synchronization is achieved by a method for virtual flux estimation with inherent sequence separation in the stationary reference frame. The estimated positive and negative sequence (PNS) virtual flux components are used as basis for calculating current references corresponding to the following objectives for control of active and reactive powers under unbalanced conditions: 1) balanced positive sequence currents, 2) elimination of double-frequency active power oscillations, and 3) elimination of double-frequency reactive power oscillations. For simple implementation and flexible operation, the derived current references are synthesized into one generalized equation where the control objectives can be selected by real coefficients. Since the converter has a limited current capability, a simple, generalized, method for current limitation is also presented with the purpose of maintaining the intended power flow characteristics during unbalanced grid faults. The proposed strategies for virtual flux-based voltage-sensor-less operation have been investigated by simulations and laboratory experiments, verifying the expected performance of active and reactive power control with different objectives.