Power systems are extremely non-linear systems which require substantial modeling and control efforts to run continuously. The movement of the power system in parameter and state space is often not well understood, thus making it difficult or impossible to determine whether the system is nearing instability. This dissertation demonstrates several ways in which the power system stability boundary can be calculated. The power system movements evaluated here address the effects of inter-area oscillations on the system which occur in the seconds to minutes time period.
The first uses gain scheduling techniques through creation of a set of linear parameter varying (LPV) systems for many operating points of the non-linear system. In the case presented, load and line reactance are used as parameters. The scheduling variables are the power flows in tie lines of the system due to the useful information they provide about the power system state in addition to being available for measurement. A linear controller is developed for the LPV model using H₂/H∞ with pole placement objectives. When the control is applied to the non-linear system, the proposed algorithm predicts the response of the non-linear system to the control by determining if the current system state is located within the domain of attraction of the equilibrium. If the stability domain contains a convex combination of the two points, the control will aid the system in moving towards the equilibrium.
The second contribution of this thesis is through the development and implementation of a pseudo non-linear evaluation of a power system as it moves through state space. A system linearization occurs first to compute a multi-objective state space controller. For each contingency definition, many variations of the power system example are created and assigned to the particular contingency class. The powerflow variations and contingency controls are combined to run sets of time series analysis in which the Lyapunov function is tracked over three time steps. This data is utilized for a classification analysis which identifies and classifies the data by the contingency type. The goal is that whenever a new event occurs on the system, real time data can be fed into the trained tree to provide a control for application to increase system damping. / Ph. D. / The goal of the utility, reliability coordinators, academics, and regulators is to keep the lights on. The contributions presented in this dissertation aims to provide a methodology and algorithm with which that goal can be met. Although the power system requires the balancing of many different components, it can be boiled down to ensuring equilibrium between load served and generation provided. Because the utility goal is to keep the lights on – and thus not change the load of the customers by turning their lights off, the utility only has control over the generation side of this equation. A see-saw can be used to imagine this balance, but it will also require a feedback loop to ensure that generation will increase or decrease as the load changes.
Another way to visualize the power system is to imagine a marble at the bottom of a bowl. If the marble is perturbed too much, it will fly out of the bowl and become what is called unstable. However, if the marble is nudged lightly, it will return back to its resting place at the bottom of the ball – which could be considered a stable equilibrium. A possible control for this type of system would tilt the bowl with a feedback signal based on the location or speed of the marble as it moved around the inside of the bowl. By providing a feedback control, the strength with which the marble can be hit can increase beyond if the bowl were to remain stagnant. However, if the marble is hit very hard, it will not matter if there is feedback control, the marble will veer out of the bowl into instability. This example serves as an analogy for the power system where the current operating point of the power system is the marble and the stable areas of operation are represented by the bowl. The feedback control for the work explored here utilizes information about the generator states to feedback to HVDC lines to strengthen the system. The power system modeling and control design involved in this dissertation aims to determine how much the power system can be perturbed before reaching the edge of the “bowl.”
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/78322 |
| Date | 07 July 2017 |
| Creators | Vance, Katelynn Atkins |
| Contributors | Electrical and Computer Engineering, Thorp, James S., Marathe, Madhav Vishnu, Wicks, Alfred L., Centeno, Virgilio A., Phadke, Arun G. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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