Power factor improvement by active distribution networks during voltage emergency situations

As more Distributed Generation Units (DGUs) are integrated into medium-voltage grids, the interaction between transmission and distribution networks plays a higher role in power system security. The contribution of the so-called Active Distribution Networks (ADNs) to system stability is expected to become more and more significant. This paper focuses on long-term voltage stability issues and shows that, contrary to what might be expected, a mere reactive power injection by DGUs has detrimental effects on system stability and might bring along counterproductive effects that precipitate voltage instability and collapse. Hence, an alternative control scheme for power factor improvement by ADNs during voltage emergency situations is proposed. The power factor is measured at the point of common coupling between transmission and distribution networks. The controller enhances system stability and eliminates the identified counterproductive effects. The conclusions are derived from time-domain simulations of a very simple, but concept-friendly, 5-bus system.