The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR).

This letter proposes an analytical expression on transient overvoltage peak value caused by DC blocking (DCB) and commutation failure (CF). The proposed method can obtain the transient overvoltage risk without modeling the complex AC/DC hybrid system. The method only requires the equivalent parameters of the AC system and the operation parameters of the DC system to effectively calculate the overvoltage without using the dynamic simulation software. The simulation results verify the effectiveness and correctness of the proposed method.

After a tripping of a transmission line, if the fault is transient, fault arc may be extinguished in three different kind of periods: initial, middle and ending period. Existing adaptive auto-reclosing (AAR) schemes cannot cover all kind of situations, which seriously affects application of those AAR schemes. an adaptive comprehensive auto-reclosing (ACAR) scheme for shunt reactor-compensated transmission lines is proposed in this paper. Based on a novel partial tripping method (PTM), two criteria are included in the proposed ACAR scheme. The first criterion is based on fault phase reactive power low-frequency component (RPLC), which can deal with faults extinguished in initial and middle periods; and the other criterion is based on high-frequency component distortion (HCD), which can deal with faults extinguished in middle and ending periods. The theoretical analysis and simulation experiments based on PSCAD/EMTDC indicate that, for different fault categories, locations and transition resistances, the proposed ACAR can distinguish transient faults from permanent ones and determine the fault extinction time rapidly and accurately.

With the increasing penetration of wind power in the power systems, the uncertainties in wind power significantly challenge the reliable and economic operation of power systems. Recently, the worst scenario-based robust optimization approaches have been employed to manage the uncertainties in the unit commitment problem. To further improve the robustness and economic efficiency of power system operation, this paper proposes a flexible robust risk-constrained unit commitment formulation, in which flexible reserve capacities of conventional generators and energy storage are allocated to cope with the uncertainty of wind power. The proposed model optimizes the unit commitment and dispatch solutions for the base case while guaranteeing that the flexible reserve capacity can be adaptively adjusted after wind generation realization. In contrast to the predefined uncertainty set in the conventional robust unit commitment, the proposed model constructs an adjustable and flexible uncertainty set via balancing the operational costs and the operational risk. The model establishes worst-case constraints to optimally allocate the flexible reserve capacity. The proposed model can be equivalently transformed into a single-level optimization problem using the strong duality theory. Numerical case studies on a modified standard test system demonstrate the effectiveness and the efficiency of the proposed model.

As the only power output channel of wind farms, wind‐powered outgoing lines carry the risk of reclosing to faults as an applied single‐phase reclosing scheme recloses the outgoing line after a fixed time without distinguishing transient and permanent faults. Although various transient‐fault identification (TFI) methods were proposed for conventional power transmission lines with shunt reactors, they can be hardly utilised on wind‐powered outgoing lines with shunt reactors due to the influence of outgoing line complex working conditions and wind farms inconsistent output power. In this study, reactive power was calculated using the instantaneous power algorithm to obtain the equivalent reactance of the outgoing line faulty phase shunt reactor. Then, a TFI method for the outgoing lines with shunt reactors based on online parameter determination was proposed, a relevant adaptive single‐phase auto‐reclosing (ASPAR) scheme was established. Finally, the validity and feasibility of the proposed TFI method and ASPAR scheme were verified by conducting simulation experiments on the software PSCAD/EMTDC. The experiments confirmed that the proposed ASPAR scheme can achieve immunity to changes in fault location and transition resistance, and are therefore suitable for inconsistent output power of wind farms and complex working conditions of outgoing lines.

In this study, the influences of the signal delay duration between rectifier and inverter in ultra-high voltage direct current (U-HVDC) transmission systems have been analyzed. The simulation results with different signal delay time under the same fault condition show that the signal delay time has a significant influence on the control system response during the fault and recovery periods. In the worst condition, it may lead to a commutation failure in the inverter. To eliminate the influence of the signal delay time on the DC control system, a fast calculation method for computing the voltage and current has been proposed by establishing the equivalent capacitance and inductance of the DC line. A voltage dependent current order limiter is added in the rectifier control system, which can calculate the DC current order value. Based on this concept, this paper proposes an improved control strategy to eliminate the influence of signal delay. The analysis and simulation results show that the calculation method can effectively improve the response speed and shorten the fault recovery time of the HVDC systems.