Recent Advances and Experience with Power System Oscillation Monitoring and Control using Wide-area Phasor Measurements

Power oscillations are a growing concern among power system operators worldwide. Traditionally, the main countermeasure against dangerous power oscillations has been the installation of power system stabilizers (PSS). Essentially, the potential for inter-area power oscillations depends on the strength of the tie lines between different areas and the load on the ties. From a European perspective, with the anticipated integration of remote renewable energy sources such as offshore wind power from the Northsea region and solar power from southern Europe or Africa, we can expect the average transmission distances to grow and consequently also tie line flows. Unless tie lines are also reinforced we expect more oscillation events in the European grid in the future. From an operational point of view, it is of high priority to be able to estimate the damping of oscillatory modes reliably in real-time in order to take appropriate and timely measures in case damping becomes poor. Recent developments in wide-area phasor monitoring have resulted in a new power oscillation monitoring algorithm that uses multiple measurements from different locations in the grid. An equivalent system model of the power grid is estimated in real-time and based on this model, the damping and frequency as well the activity of oscillatory modes can be determined from ambient process variations. As basis for this, a wide-area measurement system (WAMS) can provide time synchronized signals from phasor measurement units (PMUs) that can measure voltage, current and frequency with adequate accuracy and resolution in time. This paper shows results from pilot operation of the new application at swissgrid, including recordings from an actual and representative event in the continental ENTSO-E interconnected power system. This example demonstrates the performance of the new application as well as provides information about the oscillatory modes present in the continental ENTSO-E system today. 1.0 INTRODUCTION Wide-area monitoring systems (WAMS) offer the possibility of supervision of power system dynamic phenomena. They are based on phasor measurement units which are most often synchronized using a global positioning system (GPS) clock to provide time-synchronized voltage and current phasor, as well as frequency measurements with synchronization accuracy better than microseconds [1-4]. This paper describes a new damping estimation application that employs WAMS data to estimate the damping of oscillatory modes from ambient data before a disturbance [5-7]. This can give an indication of the damping of transient oscillations that will follow a disturbance, once it occurs. The application is based on a system identification procedure that is carried out in real-time using a sliding measurement window. If the transfer between two grid areas is limited by inadequate damping, the damping monitor can assist in maximizing the power transfer capability by providing a faster and more accurate view of the current situation than one obtained via off-line dynamic simulations. This is possible since the PMU measurements are not subject to modeling errors that are potentially present in the off-line simulation models. Such modeling errors could lead to definition of overly conservative or even inadequate transfer limits. The new damping monitoring application also identifies the parts of the power system that participate in a detected oscillation through modal activity information that can be used to determine the root cause of a poorly damped oscillatory mode, and serve as starting point for the definition of improvement measures. Furthermore, any fast power electronic devices able to directly or indirectly modulate electric power flows (such as FACTS, HVDC or the excitation systems of synchronous generators), can be used as actuators to improve poor damping, provided they are equipped with appropriate controller extensions [8-10].