Online Voltage Stability Monitoring and Coordinated Secondary Voltage Control
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- Institutt for elkraftteknikk 
The development towards increasing use of renewable energy sources and smart grid applications represent a paradigm shift in power system operation. Due to the large share of variable generation and more dynamic load patterns, operation conditions in the future grid will change faster than today’s situation. This will render the grid less predictable and consequently affect system operation, and voltage stability will undoubtedly remain one of the major concerns. In this scenario, it is necessary to have new real-time monitoring tools to timely identify operational limits in order to devise preventive and corrective schemes to maintain security of operation. Aiming at developing new online monitoring tools for power systems, the PhD project has produced two methods to assess voltage stability in real time. Both are based on estimation of the Thevenin impedance and consequently the maximum power transfer. The first one estimates the Thevenin impedance by only local phasor measurements; meanwhile, the second approach obtains the Thevenin impedance by combination of system topology and measurements from phasor measurement units (PMUs). Although the second method takes the system topology into its calculation, it requires only information of the studied area, which is quite small since it is limited by boundary nodes. Therefore, both proposed methods are suitable for online implementation. Through simulations of the dynamic model of the Norwegian transmission system, the two algorithms result in comparable estimations of the Thevenin impedance, and both successfully detect the margin to voltage instability. In addition, with the topology-based method, it is able to estimate the post-contingency Thevenin impedance in real time. Thanks to this capability, it is possible to issue early warning of impact of critical disturbances on voltage stability, which is vital for the secure and reliable operation of power systems. Apart from the estimation of the Thevenin impedance, the PhD work also introduces a new indicator called S-Z sensitivity indicator (S-ZI) for online voltage stability monitoring. The algorithm for the new indicator is simple and requires only PMU measurements of voltage and current of the considered load. Since the S-ZI is computed directly from the local phasor measurements, the method works robustly; it does not face any problems with divergence as often experienced in other approaches. The proposed indicator also shows a good performance in detecting the margin to the voltage stability limit, with both simulation and real PMU data. In addition, the S-ZI also functions as a calibrating tool to verify accuracy of the online estimated Thevenin impedance. Based on this new indicator and the proposed algorithm for estimation of the Thevenin impedance, a prototype of online voltage stability monitoring has been built and tested with live stream of PMU data obtained from the Norwegian transmission system. The results from the tests at two locations in the 420kV and 130kV networks have shown that the proposed methods have performed very well with real measurements in power systems. Additionally, a scheme for coordinated secondary voltage control for systems with multiple VAr reserves has been introduced in the PhD project. The proposed scheme is inspired by the concept of multi-agent system. Like an agent, the local controller takes on the assigned tasks itself and contacts its neighbors for support when needed. The control structure incorporates not only controllable VAr sources but also mechanically switched capacitor banks and reactors, resulting in increased online reactive power reserves for critical contingencies. The approach is suitable for areas with high penetration of FACTS devices, distributed generation connected to power systems through voltage source converters (VSCs), or VSC-HVDC systems. Moreover, the proposed scheme is also simple and flexible in terms of coordination, implementation and expansion. Through simulations, the control scheme has shown a good performance in coordinating reactive power sources to obtain a flat voltage profile and sufficient reactive power reserves, not only in normal operation but also under disturbance conditions.