Identifying Long Term Voltage Stability Caused by Distribution Systems vs Transmission Systems

Monitoring the long term voltage stability of the power grid is necessary to ensure its secure operation. This paper presents a new phasor based methodology that distinguishes between long term voltage stability caused by distribution systems versus transmission systems. From a conceptual understanding of a simplified system, a Transmission-Distribution Distinguishing Index (TDDI) is proposed to distinguish between the two scenarios. A methodology to calculate the TDDI for multi-bus systems using quasi-steady state phasor measurements is described and validating results are presented for the IEEE 9 Bus system with a load replaced by various distribution feeders. The results verify that the TDDI can indeed be used to distinguish between transmission limited and distribution limited systems. This information can be utilized by the operator to effectively choose controls in distribution and transmission systems to improve the system margin.