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The complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) expand the effect of commutation failure (CF). The rapid prediction of CF is crucial in ensuring the stable operation of such systems. The simultaneous CF caused by grid fault has received widespread attention in recent years. However, the coupling between the LCC-HVDC with CF and the adjacent normal LCC-HVDC is not considered. When the state of the main circuit suddenly changes and the control system subsequently starts, the LCC-HVDC with CF inevitably produces an effect on the adjacent LCC-HVDC through the coupling of grid voltage. This study describes a new phenomenon, where the former produces a successive CF in the latter under the fault of weak receiving-end grid. The dynamic reactive power characteristics of inverter station and the induced mechanism of successive CF are analyzed. And the dynamic reactive power model of LCC-HVDC is established. A fast prediction method for successive CF is then proposed by calculating the extinction angle of LCC-HVDC with consideration of the effect of adjacent LCC-HVDC with CF. The proposed method can improve the accuracy of CF prediction in multi-infeed LCC-HVDC systems through theoretical derivation and simulation verification.

Limited by the reserve capacity and control speed of synchronous generator (SG), the prediction error of wind power (PEWP) results in severe frequency problems in power systems with high-proportion wind energy. The deloading of doubly-fed induction generators (DFIGs) is an effective way to provide active power support for the grid frequency. However, the existing methods are usually based on a fixed deloading rate, which is difficult to match the uncertainty of PEWP. Accordingly, a new idea of dynamic deloading is proposed considering the influence of safe limit of DFIGs, prediction error of wind power, spinning reserve of SGs, and system operating costs. The positive and negative reserve capacities of DFIG are derived based on coordinating the rotor accelerating control and pitch angle control. The allocation principle of the reserve capacity of DFIGs and SGs for frequency regulation is built. Thus a partitioned scheduling mode of power systems with high-proportion wind power based on the optimal deloading of DFIGs to participate in frequency regulation control is proposed. The simulation shows that the proposed method can simultaneously satisfy the dynamic frequency regulation demand and operating costs of power systems.

With the wide construction of the line-commuted converter-based high-voltage direct current (LCC-HVDC), a large-scale AC/DC hybrid grid pattern has gradually formed. LCC-HVDC is prone to subsequent commutation failure (SCF) during the recovery process of first commutation failure (FCF). This phenomenon poses a great threat to the safe and stable operation of the power grid. LCC-HVDC is equipped with several controllers to prevent SCF. However, these controllers adopt preset fixed parameters and ignore the fault severity and LCC-HVDC self-regulation. The controller effect may not match the dynamic demand of the FCF recovery and even become the driving force of SCF. Accordingly, a new method is proposed on the basis of the quantification of the FCF recovery consequence, and it can avoid SCF by dynamically adjusting the DC current command according to a variable slope of U d - - I d - ord curve. First, the causes and influencing factors of SCF are presented by analyzing the recovery process of CF under the AC grid fault. Second, the correlation factors of the FCF recovery path are analyzed, and the idea of adaptive control against SCF based on the critical slope is proposed. Third, the critical slope of the U d - I d - ord curve is derived with considering the influence of the initial state of the fault and constant extinction angle control. Then, a current-limit control method to prevent SCF based on the adaptive trigger voltage is proposed with considering the influence of the DC current variation on power angle stability. Finally, simulation results verify the effectiveness of the proposed method in preventing SCF.

The doubly fed wind turbine is the main equipment of wind generators. The fault transient characteristics of doubly fed induction generators (DFIG) have elicited the attention of many scholars. However, the short-circuit current (SCC) of power grid-contained DFIG under asymmetrical voltage drop cannot be accurately analyzed. The main difficulty is that the response and coupling of converters under partial voltage drop remain unclear. Thus, this paper presents an imperative study on SCC of DFIG, with particular attention to the transfer of negative-sequence voltage in the windings and rotor-side converter. The electromagnetic process and excitation control are simultaneously deduced in a uniform coordinate space by constructing positive- and negative-sequence vector models. The generating mechanism and analytical expressions of stator SCC are proposed. The positive- and negative-sequence equivalent models of DFIG during fault initial stage and steady-state process are constructed for practical short-circuit calculation. Simulation and physical testing are implemented to verify the theoretical analysis.

The large-scale application of doubly-fed wind turbines has significantly changed the fault transient characteristics of power systems. However, the transient state of a doubly-fed induction generator (DFIG) under large disturbances is difficult to accurately evaluate. The main difficulty lies in the failure to acquire the transient process of the DFIG excitation circuit because of the high orders and the strong coupling of excitation control. This paper presents a detailed transient analysis of a DFIG, with focus on the dynamic behavior and effects of the excitation circuit. The dynamic models of the rotor-side converter and the grid-side converter, which include the excitation regulation and the electromagnetic process, were constructed. The effects of the implementation of controllers on the transient behavior of the DFIG are analyzed by deducing the transitive relation of transient processes. Simplified expressions of DFIG electrical variables are proposed with the excitation regulation considered.

The transient characteristics of power systems with large-scale doubly fed wind turbines undergo profound changes. Doubly fed induction generators (DFIG) under non-deep voltage drop generate short-circuit currents, which involves complicated electromagnetism and control coupling. This phenomenon has only been partially studied in the literature. Thus, this paper presents a detailed analysis of the short-circuit current of DFIG, with particular attention to the influence of converter adjustment. The dynamics of generator and converter control were simultaneously analyzed by vector analysis in uniform coordinate space. The generation mechanism, composition, and analytical expressions of short-circuit current were proposed. DFIG models were constructed to calculate the initial and steady-state short-circuit current. Simulation and experiment were performed to verify the efficacy of the proposed analyses.