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With the increasing installation of the high-voltage direct current device and flexible AC transmission system, the power transfer capability increases significantly. However, the risk of line overloads or even cascading failures caused by DC blocking or line outage also augments, which requires accurate and effective post-contingency corrective control (CC) actions. Traditional sensitivity-based methods are fast, but are less accurate and may suffer from the seesaw problem. Artificial intelligence optimisation-based methods usually exhibit better CC performance but are time-consuming. For fastly performing accurate CC actions, a second-order cone programming (SOCP)-based CC optimisation model for line overloads alleviation in meshed AC/DC power systems is proposed. Initially, to reduce the number of CC actions and optimisation variables (OVs), a heuristic search method is proposed to automatically select the most effective generators/loads for the overloaded lines. Subsequently, thyristor controlled series capacitor (TCSC) compensation levels and multi-terminal direct current (MTDC) power are treated as additional OVs to minimise load shedding/generation rescheduling. Additionally, to boost computational efficiency, SOCP format power flow equations for systems with MTDCs and TCSCs are proposed. Eventually, the effectiveness, accuracy, and superiority in computational efficiency of the proposed method are verified based on the modified IEEE 30-bus test system.

The increasing development of the large-scale southwest hydropower and the AC /DC transmission leads to power angle stability issue. Based on equivalent three machine system model, this paper deduces the function expression of voltage amplitude and position of multi-frequency oscillation center. The function is affected by power potential amplitude ratio, line impedance, frequency of power potential and initial phase of power potential. Thus, the performance of the out-of-step splitting device is influenced by the change of operating mode and migration of oscillation center affect. Therefore, the out-of-step splitting device cannot be well applied in the multi-frequency out-of-step scenario. On this basis, this paper proposes a new out-of-step splitting strategy for oscillation center migration under multi-operation mode based on WAMS information. In detail, the proposed strategy can obtain bus voltage amplitude/phase information provided by WAMS and local Ucosφ criterion to determine position of out-of-step oscillation center and out-of-step period. The simulation results show that the influence of different power system operation modes on the migration characteristics of oscillation center is correctly applied in multi-frequency out-of-step oscillation scenario, and the proposed strategy based on WAMS information has a good adaptability under different operation modes.

The existing out-of-step splitting device may be worse in adaptability under multi-frequency out-step oscillation scenario. Aiming at this problem, this paper presents an out-of-step splitting strategy based on compound information to adapt to the migration of oscillating center. In detail, the proposed strategy can obtain bus voltage amplitude/phase information provided by WAMS and local Ucosφ criterion to determine position of out-of-step oscillation center and out-of-step period. The proposed strategy can greatly simplify the configuration and parameter setting of out-of-step splitting devices. Simulation results show that the correctness of the proposed strategy under multi-frequency out-step oscillation scenario.

With the increase of the scale of AC / DC power grid and the operation mode, the applicability of out-of-step splitting devices based on the two machine model is seriously challenged. The migration law of out-of-step oscillation centers in multi-frequency out-of-step scenario is in urgent need of study. In this paper, based on the equivalent three machine system model, the multi frequency oscillation center voltage and position function are derived, voltage characteristics of multi-frequency oscillation center and the law of oscillation center migration when the system parameters change are analyzed, and research shows that migration scope and direction of oscillation center are jointly determined by the power potential amplitude ratio, the angular velocity and initial phase of power potential, while variation of angular velocity influences the migration speed of the oscillation center. The three machine system example is used to verify the influence of system parameters on the oscillation center migration in multi-frequency out-of-step scenario.

With the aid of power quality monitoring systems (PQMSs), accurate locating of voltage sag sources, which has important significance for guiding maintenance personnel in finding and repairing faults as well as improving power supply reliability, has been becoming a new research hotspot. However, existing methods have unsatisfactory locating accuracy due to the integration of distributed generators (DGs) and fault resistance. In this paper, a novel strategy for accurately locating voltage sag sources in smart distribution networks is proposed. Based on inverse theory, which is well applied in geophysics, the accurate location issue is treated as a two-step optimization model. It aims at making the distribution of voltage phasors and current phasors obtained by theoretical short-circuit calculation match those actually observed as closely as possible. To guarantee the feasibility of the strategy, the effect of inverter-interfaced DGs (IIDGs) which are the main form of DG is considered in the short-circuit calculations. To guarantee the location accuracy of the strategy, fault resistance is treated as an optimization variable in the two-step optimization model to eliminate estimation error of fault resistance. Via two modified IEEE benchmarks with different scales, the validity and the superiorities in applicability and accuracy of the proposed strategy are verified.

Accurate locating and capacity assessment of capacitor switching can be used to verify whether the instruction of capacitor switching from distribution automation (DA) master station is correctly carried out. In this paper, a novel locating strategy, which can obtain the accurate locations as well as capacities of simultaneously switched capacitors at different places in distribution network, is proposed. Firstly, the steady state and perturbation transient information collected by power quality monitoring system (PQMS) is synthetically analyzed to judge whether the disturbance is caused by capacitor switching. And, the indices of regional net increments of reactive and active power, which can distinguish the mutual influence between switched capacitors in different regions, are proposed to determine the candidate area in DA master station. Further, based on inversion match of power flow, an optimization model is developed to realize precise locating and capacity assessment. Eventually, via the IEEE 33-node benchmark, the effectiveness and superiorities of the proposed strategy are verified.

In this paper, a multi-objective economy/security unit commitment approach is proposed, in which security region is considered as constraint and one of the objectives. Augment ε-constraint method is used to minimize the total operational costs and maximize security region. Meanwhile, Pareto optimal solutions are found. Finally, fuzzy decision making process is employed to select the best compromise non-dominated solution. The effectiveness of the proposed unit commitment approach is verified on a small scale power system over 24-h period.

Isolated grid (IG) usually has small inertia constant, and the primary frequency regulation ability of IG is limited. In addition, high penetration level of photovoltaic power plants (PV-PPS)integrated to IG deteriorate system inertial and primary frequency regulation ability. To ensure the stability of frequency, UC model considering dynamic frequency limit (DFL) is proposed in this paper, and PV-PPS are deloaded to participate in frequency regulation. Based on frequency response model with PV-PPS participating in frequency regulation, the expression of required minimum frequency regulation capacity of PV-PPS is deduced, which guides PV-PPS retain sufficient but not excessive capacity. Then, a UC model with DFL and frequency regulation capacity of PV-PPS constraints is proposed. And the problem is decomposed into a master problem for UC with DFL and a subproblem for PV-PPS' frequency regulation capacity assessment. Because the master problem is a complex nonlinear mixed integer optimization problem, in order to avoid complexity solving procedure, unit regulation power is used to present frequency regulation ability and generate optimization cut. The optimization cut and benders cut are used for eliminating frequency limit/regulation capacity violations in iteration method. Finally, an IG test system is used to verify the effectiveness and advantages of the proposed scheme, comparing with other two schemes: conventional UC and UC considering DFL without PV-PPS participating in frequency regulation.

Harmonic resonance, which is a main concern in power quality, may cause damage of equipments in power system, and consequently affect stable operation of power grid. In addition, with the continued growth of distributed generators and controllable FACTS devices,for example SVCs, in power grid, harmonic resonance becomes more complicated as frequent changes of their equivalent harmonic impedance parameters. Thus, the prevention and control of harmonic resonance is worthy of study, in particular, the security region of harmonic resonance in parameter field. In this paper, based on eigenvalue estimation, a novel method to establish parameters' security region of harmonic resonance, is proposed. As harmonic resonance can summarized as the phenomenon of singular matrix, the Gerschgorin theorem and matrix similar transformation are applied to formally state sufficient conditions for preventing harmonic resonance. Further, an optimal planning model with linear constrains is established and solved, to obtain the parameters' security region of harmonic resonance. Based on a three-bus test system, results of case study verify the effectiveness and superiority of the proposed method.

Harmonic resonance may cause abnormal operation and even damage of power facilities, further threatening normal and safe operation of power systems. For renewable energy generations, controlled loads and parallel reactive power compensating equipment, their operating statuses can vary frequently. Therefore, the parameters of equivalent fundamental and harmonic admittance/impedance of these components exist in uncertainty, which will change the elements and eigenvalues of harmonic network admittance matrix. Consequently, harmonic resonance in power grid is becoming increasingly more complex. Hence, intense research about prevention and suppression of harmonic resonance, particularly the parameter feasible domain (PFD) which can keep away from harmonic resonance, are needed. For rapid online evaluation of PFD, a novel method without time-consuming pointwise precise eigenvalue computations is proposed. By analyzing the singularity of harmonic network admittance matrix, the explicit sufficient condition that the matrix elements should meet to prevent harmonic resonance is derived by the extended Gersgorin theorem. Further, via the non-uniqueness of similar transformation matrix (STM), a strategy to determine the appropriate STM is proposed to minimize the conservation of the obtained PFD. Eventually, the availability and advantages in computation efficiency and conservation of the method, are demonstrated through four different scale benchmarks.