Enrique A. Zamora-Cárdenas

Generalized State Estimation of Flexible AC Power Systems Considering Wind Generators and Primary Frequency Control

Abstract This paper proposes a practical approach to incorporate the mathematical models of both fixed-speed and variable-speed wind turbine generators, automatic load frequency controls as well as voltage magnitude and frequency dependent loads into a weighted least squares-based state estimation algorithm suitable for the analysis of flexible alternating current (AC) transmission systems. As opposed to conventional static state estimators, where the inclusion of these electric components has been neglected so far, the proposed approach permits the determination of the steady state operation of a power system in the event of a supply-demand unbalance by estimating the magnitude of the frequency deviation from its nominal value. The state estimation is based on measurements related to those that should be obtained by a supervisory control and data acquisition (SCADA) system and phasor measurement units. For the purpose of this paper, the set of values associated with SCADA measurements (nodal power injections, power flows, and voltage magnitudes) and phasor measurement unit (PMU) measurements (voltage and current phasors) are generated from a power flow analysis of the network under study. Lastly, numerical simulations are reported to demonstrate the effectiveness of the proposed approach.

Practical implementation of a VSC-HVDC in WLS-based state estimation

This paper proposes a practical approach to incorporate a High Voltage Direct-current (HVDC) link in a state estimation algorithm based on the technique Weighted Least Squares (WLS). The power measurements of the HVDC are directly appended to traditional measurements to estimate the equilibrium point of the power system. The proposed approach simultaneously upgrades the estimated values of the state variables of the HVDC controllers and the state variables of the rest of the electric network, to obtain a unified solution in a single-frame of reference. Results are presented to demonstrate the effectiveness of the proposed approach to assess the estimation of the system state and to set the parameters of the HVDC controllers for given control specifications.

Computational development of a practical educational tool for state estimation of power systems using the MATLAB optimization toolbox

State estimation is one of the most important processes to perform a reliable monitoring and control of the steady-state operating condition of modern electric power systems; thus, it is currently a fundamental part in the development of research to enhance the monitoring and security of the smart grids operation. This important topic is taught in advanced courses of operation and control of power systems, for graduate and undergraduate power engineering students. However, the most used software packages for simulation and analysis of power systems by researchers, students, and educators have put little attention on the state estimation module. Due to this fact, this paper proposes an approach to develop the computational implementation of a practical educational tool for state estimation of electric power systems using the MATLAB optimization toolbox. In this proposal, the formulation of the state estimation problem consists of developing a general digital code to implement an objective function based on the weighted least squares method. While the lsqnonlin function of the MATLAB optimization toolbox solves the formulated state estimation problem. Simplifying both research and educational processes, this tool helps graduate and undergraduate students to improve learning, understanding, and the times of implementation and development of research in state estimation. Simulations of an equivalent model of the Mexican interconnected power system consisting of 190 buses and 46 machines are used to test and validate the proposal performance.

Directional Derivative-Based Transient Stability-Constrained Optimal Power Flow

This paper proposes a novel active power redispatch sequential approach to solve the transient stability-constrained optimal power flow (TSC-OPF) problem for the preventive control of transient stability. Based on an independent transient stability assessment of the power system for a given contingency scenario, two proposed power redispatch constraints are formulated and embedded into a conventional OPF formulation. Since these two new constraints only depend on steady-state variables, the dimension of the resulting TSC-OPF model is similar to that of a conventional OPF model and can be solved by standard optimization methods to perform a nonheuristic active power redispatch. The stabilization process is performed by sequentially solving the optimization and the transient stability problems until a suitable generation dispatch that provides a transiently stable equilibrium point is assessed. The validity and the effectiveness of the proposed method are numerically demonstrated in the WSCC three-machine, nine-bus system and an equivalent model of the Mexican power system.

A novel sequential transient stability constrained optimal power flow

A different approach for formulating the sequential TSC-OPF problem is proposed in this paper, where only two power re-dispatch constraints are added to a conventional OPF model to formulate the proposed TSC-OPF model. These two constraints are derived from a proposed best direction vector that provides the power re-dispatch direction in which the rotor angular deviations at the time to instability decrease at the maximum rate of change. In this way, the resulting TSC-OPF model forces the magnitude and direction of the power re-dispatch in order to steer the power system operation state to a transiently stable equilibrium point. The effectiveness of the proposed method is numerically demonstrated in the WSCC 3-machine, 9-bus system.

Output filter control for matrix converter in synchronous applications

In this paper, the design of an output filter for the matrix converter when it operates as series and shunt power compensator is presented. For each of the synchronous grid-tie applications mentioned, the matrix converter is modeled as a voltage and current source correspondingly. The developed filter is comprised only by passive storage elements, in order to improve the efficiency of the overall compensation system and its design is based on a linear control law. Since the proposed filter topology does not include passive damping elements, the control strategy utilized ensures the stability and an adequate transient response of the compensator. The operating limits of the compensator are found analytically, which indicate the maximum power that can be injected to the main grid under the operative conditions considered. Finally, detailed simulations were carried out in order to demonstrate the effectiveness of the proposed filter design and the results obtained are used to verify the proper operation of the matrix converter in power compensation applications.

Non-heuristic selection criterion for transient stability-constrained optimal power flow analysis

A novel approach based on both single machine equivalent (SIME) and trajectory sensitivity methods is proposed to formulate a transient stability-constrained optimal power flow (TSC-OPF) in the Euclidian space, where only one single stability constraint is necessary in the optimization problem to represent all dynamic and transient stability constraints of the multi-machine system, resulting in a tractable approach to the preventive control of transient stability in realistic power systems. A unified framework of time domain analysis is proposed, where the transient stability, trajectory sensitivity and SIME analyses are all combined to assess the systems stability and to compute the sensitivity coefficients of the proposed transient stability constraint. Based on these sensitivity coefficients, a non-heuristic selection criterion is proposed to perform the preventive control by rescheduling only a selected number of generators, which is the commonly accepted practice followed by the systems operators. The validity and the effectiveness of the proposed method is numerically demonstrated in the Mexican 46-machine, 190-bus system.