Abstracts:The letter proposes a smooth Rate Limiter (RL) model for power system stability analysis and control. The proposed model enables the effects of derivative bounds to be incorporated into system eigenvalue analysis, while replicating the behavior of conventional non-smooth RLs with high fidelity. In addition, it can be duly modified to enhance the system's dynamic control performance. The behavior of the proposed model is demonstrated through illustrative examples as well as through a simulation of the New York/New England 16-machine 68-bus system.

Abstracts:Transmission expansion planning is often approached via iterative partitioning, which involves a master problem and a subproblem. The master problem provides investment proposals, while the subproblem computes the resulting operation cost and the sensitivities of the investment proposals with respect to operation cost. These sensitivities are critical for the master problem to progressively refine its investment proposals. We propose an analytical procedure based on the Lagrangian function of the subproblem to compute these sensitivities, efficiently and accurately. The outcomes of the proposed analytical procedure are contrasted with those obtained from the common but cumbersome practice of using auxiliary constraints to fix investment variables to master problem investment proposals, and with those obtained by numerical perturbation.

Abstracts:This letter investigates the impact of typical reactive power control (RPC) on the low-frequency oscillation of virtual synchronous generators, where a unified P/ω admittance is proposed to indicate the coupling effect resulting from RPC, with clear physical meanings. It is found that the mechanisms of different RPC for affecting oscillation damping are distinct. Specifically, the proportional RPC weakens system damping by reducing synchronization reactance, while inertial and PI-type RPC directly introduce negative conductance. A criterion is then proposed to identify the negative damping caused by inertial and PI-type RPC. Finally, the theoretical analysis is validated by simulations.

Abstracts:This letter demonstrates how synthetic inertia can be obtained with the control of flexible discrete devices to keep the power balance of power systems, even if the system does not include any synchronous generator or conventional grid-forming converter. The letter also discusses solutions to cycling issues, which can arise due to the interaction of uncoordinated discrete inertia controllers. The effectiveness, dynamic performance, and challenges of the proposed approach are validated through simulations using modified versions of the WSCC 9-bus test system and of the all-island Irish transmission system.

Abstracts:The accuracy and reliability of the Q/V sensitivity for voltage security assessment is influenced by the outliers present in the calculation results. An unsupervised machine learning approach, empirical- cumulative- distribution- based outlier detection (ECOD), is introduced in this letter to detect and eliminate outliers to address this issue. A comparison of the results with those of the proposed approaches on the standard test power system CSEE-VS demonstrate that, compared with advanced outlier detection algorithms, ECOD can eliminate outliers from the Q/V sensitivities with higher accuracy and less computation time and realize online voltage security assessment with superior accuracy and reliability.

Abstracts:In this letter, a systematic singular perturbation approach is presented to define the criteria for maintaining interarea oscillations in a reduced multi-area frequency model. This gives also an indication on the corresponding minimum value of time constants to be kept for frequency stability analysis.

Abstracts:Lyapunov's direct method plays an important role as a very commonly used analytical tool in studies on transient synchronization stability (TSS) of grid-following converters. To bridge the gap that energy-type Lyapunov function (LF) is only effective in the positive damping area and thus too conservative, a novel LF whose domain of definition includes part of negative damping area is proposed for TSS analysis of grid-following converters. Its corresponding TSS criterion and estimated region of attraction (ROA) exhibit significantly reduced yet still guaranteed conservativeness, endowing them with more practical value and higher efficiency in analytical TSS assessment works.

Abstracts:Post-disturbance frequency nadir (FN) is an important index of system frequency security. The FN constraint should be integrated to the security-constrained unit commitment (SCUC) to avoid under-frequency load shedding. Due to its nonlinear and nonconvex feature, the SCUC problem with original FN constraint cannot be directly solved. This letter proposes a two-stage linearization method to transform the FN into a linear function of generator status variables, enabling good interpretability of FN. Case studies of large-scale systems indicate that this method exhibits high accuracy and low computation workload. Thus, it provides the SCUC with a fast tool.

Abstracts:An accurate aggregated flexibility cost of distribution network is essential for the coordination of transmission system operator (TSO) and distribution system operator (DSO). However, existing methods face accuracy and computational efficiency challenges in capturing the nonlinear characteristic of the aggregated flexibility cost through multi-segment linear approximation. Therefore, this letter proposes a cost surrogate model to replace the original total cost of generators, controlled loads, and other operational cost in the distribution grid, using polynomial chaos expansion (PCE). This cost surrogate model is represented as a nonlinear analytical function between the total cost and the exchange power at the interface of TSO-DSO. The accurate cost surrogate model can avoid considering the detailed model of the whole distribution network. The simulation results demonstrate the effectiveness of the proposed model.