Synchrophasor Data Baselining and Mining for Online Monitoring of Dynamic Security Limits

When the system is in normal state, actual SCADA measurements of power transfers across critical interfaces are continuously compared with limits determined offline and stored in look-up tables or nomograms in order to assess whether the network is secure or insecure and inform the dispatcher to take preventive action in the latter case. However, synchrophasors could change this paradigm by enabling new features, the phase-angle differences, which are well-known measures of system stress, with the added potential to increase system visibility. The paper develops a systematic approach to baseline the phase-angles versus actual transfer limits across system interfaces and enable synchrophasor-based situational awareness (SBSA). Statistical methods are first used to determine seasonal exceedance levels of angle shifts that can allow real-time scoring and detection of atypical conditions. Next, key buses suitable for SBSA are identified using correlation and partitioning around medoid (PAM) clustering. It is shown that angle shifts of this subset of 15% of the network backbone buses can be effectively used as features in ensemble decision tree-based forecasting of seasonal security margins across critical interfaces.

I. Introduction

System security is the main asset and responsibility [1] of a system operator. Even when everything is in a normal state throughout the grid, the operator should always be able to determine whether the state is secure or insecure, in order to take preventive security control action in the latter case [2]. In practice, many factors can move the system out of its security boundaries [3]–[6] following a normal-state contingency, such as post-contingency voltage or frequency violations, transient rotor angle instability, undamped oscillations, excessive temperature rise, etc. To ensure convenient online supervision, stability threats are converted into a single stability limit set on the power transfer across key interfaces of the system, which are computed online using DSA packages and offline based on extensive operational planning studies [7]–[11]. Since the introduction of the security control concept in the early seventies and the advent of SCADA shortly afterwards [2], this task has been done by checking in real-time whether 1) the actual power transfer across critical interfaces is below the online or offline pre-determined transfer limits and 2) the voltages of the backbone grid buses and reactive-power reserve remain within prescribed bounds [5], [6]. An automated access to look-up tables containing these threshold values for each likely topology and system status makes it possible to easily compare SCADA measured quantities against the stored limits, compute the security margin and initiate requests for preemptive corrections when the normal state is deemed insecure [3], [4], [7].