Simulation based methods to determine linear equivalent models of power system devices
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Date
2019
Authors
Rupasinghe, Akbo
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Abstract
Small signal stability analysis (linear analysis) of power systems is a very useful component in any power system study. Greater insight into the system can be obtained by it. It also opens the power systems and its controllers to a large body of knowledge on linear systems and controls.
It has become the trend of the power system device manufacturers to make available an as-built detailed model of their product. Such a model is usually in the electromagnetic transient (EMT) simulation domain. The model is made available as a device that can be connected to a larger power system on a simulation case running on an EMT software. Due to proprietary constraints, the model may be black-boxed and only a limited number of outputs and inputs may be available to the user. In instances it is not black boxed, it is usually a device with complex controllers, making it a tedious task to determine its mathematical model. In both cases, the complete mathematical model is not available to the user.
Since it is not possible to linearize the system without the knowledge of all the devices in the power system, small signal stability analysis is often skipped in a power system study. This research focuses on using available input and output signals of black boxed EMT simulation models of power system devices, and utilize other available knowledge of the system to determine model data of their linear equivalent models, in such a way, complete small signal stability studies can be carried out at any operating point, and connected to any network.
Power system devices can be divided into two main subsystems. (1) The current injection device, which is operating point dependent. (2) The auxiliary controller, which is operating point independent within a given operating mode. Dynamic data of the current injection device subsystem (i.e. synchronous machine, induction machine) can be considered as a-priori knowledge of the system. A Prony Analysis method, augmented by eigenstructure assignment has been proposed to incorporate the a-priori knowledge of the system to the system identification process. Using a result derived from Mason's Gain Rule, the linearized model of the power system device is divided into the known subsystem of the operating-point-dependent current injection device; and the operating-point-independent unknown subsystem of the auxiliary controller. The modes identified from the proposed system identification procedure at multiple operating points are then used to solve a system of polynomial equations, for the unknown auxiliary controller transfer functions.
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Keywords
power system linear simulation, small signal stability analysis, system identification