Effects of load variation on a weak grid under unbalanced voltage conditions

This paper presents the basic principle of a laboratory based weak grid emulator in order to describe the effects of load variation at the point of common coupling (PCC). The impact of these effects is significant and for that a voltage regulation feature has been added to maintain the voltage at the load connection. Design and performance are investigated by introducing unbalanced voltage conditions to test the system´s behavior. In case of unbalance, power is injected from the DC link to regulate the voltage at the converter output connected to either passive or an active load. The system has a rating of 1 kW and can handle an output voltage of 50-150% of the nominal voltage peak. A space vector voltage oriented control (VOC) technique is implemented on the converters and the PI controller gains are tuned accordingly. National Instruments (NI) PXI controller is used for the hardware implementation. Voltage unbalance is generated and controlled with the help of dual vector sequence extraction technique.