• Energy Research

Dynamic simulation is vitally important in power system analysis, but traditional approaches based on numerical integration over small time steps are time-consuming. Also, the Newton-Raphson method suffers from difficulty in convergence when solving nonlinear algebraic equations. This paper proposes a novel dynamic simulation approach based on holomorphic embedding. By obtaining a high-order approximation of system dynamics, it achieves a much larger time step and thus enhances the computational efficiency significantly. In addition, the new approach avoids non-convergence issues in solving algebraic equations, which improves robustness. The approach includes flexible modeling of synchronous generators and controllers, and we propose a method for modeling generator coordinate transformations. The approach is tested on the IEEE 39-bus, 10-generator system and a Polish 2383-bus, 327-generator system. The results demonstrate promising computational efficiency and satisfactory numerical robustness for the analysis of large-scale power systems.

Abstract In this paper, a battery energy storage system (BESS) based control method is proposed to improve the damping ratio of a target oscillation mode to a desired level by charging or discharging the installed BESS using local measurements. The expected damping improvement by BESS is derived analytically for both a single-machine-infinite-bus system and a multi-machine system. This BESS-based approach is tested on a four-generator, two-area power system. Effects of the power converter limit, response time delay, power system stabilizers and battery state-of-charge on the control performance are also investigated. Simulation results validate the effectiveness of the proposed approach.

In this study, controller parameters of static var compensators (SVCs) at planned locations are optimized to mitigate fault-induced delayed voltage recovery issues and improve angular stability of a multi-machine power system. The problem is formulated as a nonlinear optimization problem involving constraints on post-fault trajectories of voltages and frequencies. This paper proposes a mesh adaptive direct search based algorithm interfaced with a power system simulator for optimization of SVC controller parameters. The proposed method is tested on the NPCC 140-bus system to optimize controller parameters for three SVCs. Simulations on critical contingencies verify that post-fault transient voltages and generator speeds can both quickly recover and transient stability of the system is improved.

In this paper, in order to enhance the numerical stability of the unscented Kalman filter (UKF) used for power system dynamic state estimation, a new UKF with guaranteed positive semidifinite estimation error covariance (UKF-GPS) is proposed and compared with five existing approaches, including UKF-schol, UKF-$��$, UKF-modified, UKF-$��Q$, and the square-root unscented Kalman filter (SR-UKF). These methods and the extended Kalman filter (EKF) are tested by performing dynamic state estimation on WSCC 3-machine 9-bus system and NPCC 48-machine 140-bus system. For WSCC system, all methods obtain good estimates. However, for NPCC system, both EKF and the classic UKF fail. It is found that UKF-schol, UKF-$��$, and UKF-$��Q$ do not work well in some estimations while UKF-GPS works well in most cases. UKF-modified and SR-UKF can always work well, indicating their better scalability mainly due to the enhanced numerical stability. accepted by IEEE Transactions on Smart Grid

The recently proposed non-iterative load flow method, called the holomorphic embedding method, may encounter the precision issue, i.e. nontrivial round-off errors caused by the limit of digits used in computation when calculating the power-voltage (P-V) curve for a heavily loaded power system. This letter proposes a multi-stage scheme to solve such a precision issue and calculate an accurate P-V curve. The scheme is verified on the New Eng-land 39-bus power system and benchmarked with the result from the traditional continuation power flow method. This manuscript was submitted to IEEE Power Engineering Letters, which contains 2 pages and 4 figures. Minor modifications suggested from the first round review have been addressed and the manuscript has been submitted for the second round review

This paper proposes a measurement-based voltage stability monitoring method for a load area fed by N tie lines. Compared to a traditional Thevenin equivalent based method, the new method adopts an N $+1$ buses equivalent system so as to model and monitor individual tie lines. For each tie line, the method solves the power transfer limit against voltage instability analytically as a function of all parameters of that equivalent, which are online identified from real-time synchronized measurements on boundary buses of the load area. Thus, this new method can directly calculate the real-time power transfer limit on each tie line. The method is first compared with a Thevenin equivalent based method using a 4-bus test system and then demonstrated by case studies on the Northeast Power Coordinating Council (NPCC) 48-machine, 140-bus power system.

This paper develops an analytical method for assessing the safety margins of a generation rejection scheme (GRS) reliably. It also presents a practical framework for implementing the proposed method integrated with energy management system and synchrophasor data in power grid operations. By employing a concept of virtual load connected to the critical generation bus of the single machine equivalent of the real-time operations case, we calculate, similar to transfer analysis, the allowable power to the virtual load in MW after tripping the pre-planned number of generation units and thus determine the required rejected power for the GRS initiating scenario. This virtual loading can be interpreted as the safety margin of the designed GRS to ensure its stabilizing operation. This research further develops a computationally efficient technique for refining the safety margin potentially with the measured synchrophasor data to improve the robustness of the GRS in practice. Understanding the safety margin is envisioned to help investigate and identify other practical options than tripping generators for protecting the system integrity. Accuracy and efficacy are demonstrated for real Korea power system cases.

Voltage collapse is a critical problem that impacts power system operational security. Timely and accurate assessment of voltage security is necessary to detect post-contingency voltage problems in order to prevent a large scale blackout. This paper presents an online voltage security assessment scheme using synchronized phasor measurements and periodically updated decision trees (DTs). The DTs are first trained offline using detailed voltage security analysis conducted using the past representative and forecasted 24-h ahead operating conditions. The DTs are also updated every hour by including newly predicted system conditions for robustness improvement. The associated synchronized critical attributes are obtained in real time from phasor measurement units (PMUs) and compared with the offline thresholds determined by the DTs to assess security. This approach is tested on the American Electric Power (AEP) system and properly trained DTs perform well in assessing voltage security. Several new ideas to improve DT performance are also introduced.