Power Grid And Load Identification And Its Application To Voltage Stability Analysis

The past decade has seen major restructuring and deregulation in the electric utility industry; significant changes have been taking place, bringing in the notion of competition and market strategies in the operation of electric power systems, and the need to reduce operating and capital costs. This economic pressure drives the trend to maximize the utilization of high-voltage equipment, operating at conditions closer to the system limits; however, this also put systems at a higher risk for collapse. For safe and efficient operating of large power systems, it is of paramount importance to provide real-time and accurate estimates of the voltage stability margin. Accurate grid models and load models are important in monitoring and protection of wide-area power systems.This dissertation addresses issues in the estimation and application of power grid impedance matrices and dynamic load models based on synchronized phasor measurements, for improving monitoring and better operation of large power systems. The main results and innovations are listed as follows:1) Online impedance estimation of power grids using a recursive least squares estimation algorithm with a forgetting factor.For better understanding overall operation and performace of entire power systems, a multi-bus power grid model is obtained based on Kirchhoff's current and voltage laws. A recursive least squares estimation algorithm with a forgetting factor is presented to identify impedance parameters of the model. This approach is based on synchronized phasor measurements and makes it possible to track changing parameters. The algorithm is tested on two power systems using MATPOWER-generated data, and case studies are conducted for estimation of grid impedances. The results show that the proposed method is effective for online estimation of grid impedances.2) Applications of grid impedance matrices to voltage stability margin analysis and fault detection.Application of power grid impedance matrices to voltage stability margin analysis and fault detection is studied. Based on the Kirchhoff's law, the system admittance matrix can be obtained from the grid model. Then a method to dectect transmission line outage by observing grid impedance changes is proposed. A multi-port method for voltage stability margin prediction is proposed based on the estimated network impedance matrix. A simple5-bus system and the IEEE30-bus power system are used to investigate the proposed methods, and the results showed that the proposed method can be utilized for dectecting transmission line outage and online monitoring of voltage stability with high accuracy.3) Identification of nonlinear dynamic load models of power systems.A second order Taylor series expansion model is presented as the simplification of the popularly used exponential recovery dynamic load model. Then, a frequency weighted least-squares based Hartley modulating functions method is developed to estimate parameters of the load model, which effectively attenuates the effect of noises in the process of parameter identification. However, it is hard to choose the optimal parameters of the Hartley modulating fuction method.To simplify the identification process and to improve the identification accuracy, considering the special characteristic of the exponential recovery dynamic load model, an alternating least squares approach is developed to decompose the nonlinear optimization problem of dynamic load parameter identification to two linear least squares problems, which can be solved in an alternating manner. Then, a DFT-based numerical derivative technique is proposed to attenuate the effect of noises in the process of parameter estimation. Simulation studies are conducted to demonstrate the accuracy of the proposed methods for identifying the dynamic load model in the presence of noticeable additive measurement noise.4) Application of the dynamic load models to stability margin analysis.P-V curves play an important role in understanding voltage instability. Under the same conditions in a power system, load characteristics affect the location of operating point in the P-V curves, which is distinctly important for voltage stability analysis. The influence of dynamic load characteristics on voltage stability is studied. Simulation analysis showed that load characteristics are critical for the stability of the system. The parameters of the dynamic load model are helpful for voltage stability analysis and protection.