Load Shedding Based on Synchrophasor Measurements from PMUs

 

Synchrophasor measurements by Phasor Measurement Units (PMUs), have enabled new applications in power systems. Focus here is on the emergency actions and load shedding in particular. This research aims at developing a new method for load shedding, based on combined frequency and voltage stability. An additional outcome of this work consists in the formulation of measurement requirements for this application. In fact, although a new IEEE standard on synchrophasor was released in 2011, this still does not cover the measurement requirements for specific applications.

  Copyright: RWTH Aachen Schematic diagram about the proposed load shedding method and its operation

With the development of the deregulated electricity markets and the growth of electricity consumption, power systems often operate close to the stability limits. As a consequence, large disturbances are more likely to initiate cascading effects, resulting in the serious imbalance of active and reactive power simultaneously, eventually leading to combined frequency and voltage instability. In this background, load shedding is regarded as the last resort to protect the stability and security.

PMUs can provide synchronized measurements of voltage and current magnitude and phase angle, frequency and rate of change of frequency. By means of the high accuracy measurements at a high sampling rate, these measurements are critical to wide-area monitoring and control systems (WAMCS). In particular, the information of phase angle greatly improves and accelerates the centralized applications based on Jacobian matrix, which may lead to a breakthrough in future power system operation and management.

The proposed load shedding method consists of simultaneous active and reactive power load shedding, to address

the voltage and frequency stability issues respectively. In comparison with the existing methods, this new approach shows, within the limits of the case study adopted up to now, better performance in terms of addressing voltage and frequency instability, in terms of loadability in the recovered steady state, and also in terms of transient behavior during the load shedding. In practical applications, the execution of the load shedding will need the assistance of reactive power compensation.

The tests were performed in simulation with the Real Time Digital Simulator (RTDS) and the offline analysis was performed in MATLAB. IEEE standard 39-Bus test case with multi-generator and dynamic loads, is modeled in RTDS to demonstrate the entire dynamic process. Meanwhile, the system states are monitored by PMU components of RTDS, which provide the synchrophasor measurements.

In future work, the proposed algorithm will be tested with

different test scenarios, and even in different systems for further verification. A critical step is to find out the proposed load shedding coordination with reactive power compensation in practice. Moreover, this also supports the implementation of the method in online application in the near future, hence strengthening its applicability in real power systems.