When connecting single-phase loads such as railways to the three-phase grid this might lead to severe imbalance conditions. Imbalance is measured in terms of negative-phase sequence voltage and current. These quantities are regulated in Grid Codes, stipulating how much negative-phase sequence can be tolerated at the point of common connection with the grid.
The relationship between the railway load and the fault level of the feeding grid is what determines the imbalance caused by the rail load. The larger the load and the lower the fault level, the worse the unbalance becomes, and thereby, the larger the negative-phase sequence components.
Besides violating grid codes, imbalance between phases induces extra losses, as well as causes wear to rotating machines connected to the same grid. Rotating machinery, as a rule, has its own specifications as to the amount of phase imbalance can be tolerated for any amount of time. Dealing properly with imbalance is therefore essential in more than one way. The remedy is FACTS, where SVC as well as SVC Light are capable of restoring the feeding grid to three-phase balance, thereby fulfilling the Grid Code, as well as alleviating the other shortcomings due to imbalance between phases caused by railway loads.