It is still a significant challenge to accurately locate faults of grounding electrode lines in high voltage direct current (HVDC) transmission systems, especially high resistance faults and remote faults. In this paper, a novel fault location approach based on characteristic-harmonic measured impedance (CHMI) is proposed. This method works well under high resistance faults and remote faults. Considering that there are abundant and steady characteristic harmonics in electrical signals of grounding electrode lines, the CHMI is defined first according to the equivalent circuit of the grounding electrode system. Then, the CHMIs of the two grounding electrode lines under faults are analytically derived. On the basis of that, the fault branch is identified by the amplitude ratio of the two CHMIs. Furthermore, since both of the CHMIs of the two lines are relevant to fault distance and fault resistance, the precise fault distance is obtained by solving two fault location functions simultaneously and eliminating fault resistance. The proposed algorithm is finally validated through numerous simulations covering various fault scenarios carried out in PSCAD/EMTDC.

This letter presents a new phenomenon that the integration of voltage source converter based high voltage direct current (VSC-HVDC) can increase the risk of commutation failure (CF) of line-commutated converter based HVDC (LCC-HVDC) caused by sending end AC faults, and reveals the underlying mechanism. It is found that VSC-HVDC accelerates the recovery of the active power of LCC-HVDC after fault clearance in a hybrid multi-infeed HVDC. This leads to a phase shift of the commutation voltage and high DC current in the LCC inverter, which is the inherent reason for the increased risk of CF under severe faults. And the DC impulse current is the direct reason for the increased risk of CF under moderate faults.

This letter proposes a novel fault diagnosis method to detect the fault of high voltage capacitor (HVC) for DC filter (DCF) in high voltage direct current (HVDC) systems. The proposed diagnosis principle is developed through identifying the change of HVC capacitance, which is calculated in real-time using the measured voltage and current of DCF. Compared with the traditional methods, the proposed approach is superior in detecting successive faults and symmetrical faults since any HVC fault will change the capacitance parameter of HVC essentially. Simulation and experimental results verify the performance and superiority of the presented method.

The hybrid cascaded (HC) LCC-MMC HVDC scheme, wherein the LCC serves as the rectifier, and a single LCC is connected in series with multiple MMCs in parallel as the inverter, amalgamates the merits of both LCC and MMC technologies. However, the utilization of thyristors in the LCC inverter still introduces a potential risk of commutation failure (CF). To mitigate this concern and bolster the stability of the hybrid system, an analysis of the LCC CF and its recovery process is undertaken, along with a mathematical investigation of the LCC's reactive power (RP) characteristics in the inverter. Subsequently, a mitigation strategy rooted in RP feedback control (RPFC) for the subsequent CF (SCF) within the HC-HVDC system is proposed. The transient performance of the system under AC fault conditions is evaluated with the proposed control approach in comparison to conventional methods. The outcomes indicate that this proposed approach effectively harnesses the RP support capabilities of MMCs, thereby endowing the LCC with enhanced resistance to potential SCF and improved post-fault recovery performance for the overall system.

The hybrid high-voltage direct current (HVDC) transmission system with cascaded MMC converters has become a promising alternative for possessing the technical merits of both line-commuted converter (LCC) and voltage source converter (VSC), resulting in favorable characteristics and potential control of good prospect. This paper pays heightened attention to the feasible power and DC voltage control modes of a hybrid HVDC system; characteristics of master–slave control show higher flexibility than the LCC-VSC HVDC system, which demonstrates that the exceptional potential can serve to stability support the AC power grids. To optimize the control effect, besides damping level to attenuate power oscillations, the robustness suitable for various faults is also considered to obtain a multi-objective control problem. A detailed solution is proceeding using the TLS-ESPRIT identification algorithm and H₂/H_∞ hybrid robust control theory. This motivates multi-terminal controllers in the LCC rectifier and MMC inverters, which immensely improve the stability of both sending and receiving girds at the same time. According to the parameters of the actual hybrid HVDC project, the simulation model is established in PSCAD v4.6.2 software, and proposed control methods have been verified to satisfy damping objectives and perform well in multiple operating scenarios.

As a typical single-phase electric power system, high-speed railway system gradually absorbs the more single-phase electric traction vehicles (trains) with its rapid development. Traction power load (TPL) is the electrical part of the train. The incompatible interactions between traction power network (TPN) and TPL generally cause the serious wideband oscillations, fortunately, which can be readily analyzed with the identified wideband impedances. As for impedance identification, the multi-frequency harmonic generator is of great significance to rapidly excite the desired responses for impedance calculation, however which is vacant for the single-phase TPL, and needs a further study. Aiming at it, this paper proposes a multi-frequency harmonic generator based on the transformer-less cascaded H-bridge (CHB) converters with floating DC capacitors. In detail, its corresponding multi-frequency harmonic control loop is derived by the internal model principle (IMP), and the inserted notch filter and repetitive controller can strengthen the multi-frequency harmonic output performance. Finally, results of simulation and corresponding down-scale physical experiment can well verify that the proposed harmonic generator can output multiple harmonics simultaneously with high purity and well-behaved tracking performance.