A power component detection (PCD) method and an improved current order limiter control (improved COL) are proposed to mitigate the commutation failure in a line-commutated converter-based high-voltage direct current (LCC-HVDC). Integrating the instantaneous voltage and instantaneous current characteristics under single-phase and three-phase-to-ground faults, a fault detection method based on the power component is proposed, and the corresponding setting principle is presented. Then, an improved dc current order limiter is introduced. To evaluate the effectiveness of the proposed methods, a dual-infeed HVDC system is developed in PSCAD/EMTDC, and the presented methods are also implemented. The transient performances of dual-infeed HVDC, under single-phase and three-phase-to-ground faults at the inverter ac busbar of one LCC-HVDC link, are investigated. Simulation results show that the PCD method, with the voltage and current components as an additional judgment criterion, is more sensitive to detect the single-phase and three phase faults, and even more reliable than the detect method taking only voltage or current components for faults recognition. It can also be concluded that the improved COL that is integrated with the advancing firing angle control has the ability to make the LCC-HVDC less susceptible to commutation failure. Finally, the improvement of commutation process by improved COL is further demonstrated.

A small signal model of a multi-infeed high-voltage direct current (HVdc) transmission system containing a line commutated converter (LCC) and a voltage source converter (VSC) is developed. This model represents the LCC and VSC converters as operational impedances as seen from the converter ac busbar. This permits the converters to be included in the effective short-circuit ratio (ESCR) calculations. The resulting ESCR is referred to in this paper as the “Impedance based Effective Short Circuit Ratio” (IESCR). It is shown that the maximum power transfer limit (referred to as the maximum available power or MAP) of the converters is better predicted by this index compared to the conventional ESCR which ignores the operational impedances of the converters. The question also arises as to how close to this theoretical maximum power transfer limit can the HVdc system operate. Using the small-signal model, it is shown that with commonly used control strategies, the predicted MAP can only be achieved by reducing the controller gains. The results are validated using detailed electromagnetic transients simulation of the multi-infeed VSC-LCC system.

This paper develops a comprehensive dynamic phasor based small-signal model for the MMC system under unbalanced grid conditions, which takes into consideration of the dynamics of the MMC system under balanced grid condition as well as the other emerging internal harmonics produced by unbalanced grid components and more complex controllers. The small-signal model is validated by comparing the time-domain responses from a detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC. Based on the eigen-analysis and PSCAD-based time-domain simulation studies, this paper reveals the poorly damped modes that cause unstable oscillations in MMC system under unbalanced grid conditions. From the results, more poorly damped modes involved in the internal harmonics are emerging under unbalanced grid conditions compared with the modes under balanced grid conditions, and can highly impact the system dynamic behaviors by the selection of the control parameters. It is also shown that, under unbalanced grid conditions, the feasible region of the control parameters of the vector current control (VCC) gets narrowed indicating the reduction of the stability margin, smaller (larger) value of proportional (integral) gain of the circulating current suppressing controller (CCSC) can enhance the system stability, and larger values of proportional and integral gains of the negative-sequence current suppressing control (NCSC) can result in the harmonic instability of the MMC system. Besides, the parameters of multiple complex-coefficient filters (MCCF) for positive- and negative-sequence components extraction also have big impact on the small-signal stability of the MMC system, and are required to be properly selected.

This study investigates the impact of phase‐locked loop (PLL) and DC‐side voltage controller on small‐signal dynamics of the high‐voltage direct‐current (HVDC) inverter station system which utilises line commutated converter (LCC) technology. The studies are conducted based on eigenanalysis of a linearised model of a study system and verified by time‐domain simulation studies in the PSCAD platform. The studies show that under weak grid conditions, e.g. short‐circuit ratio (SCR) = 1, parameters of PLL and DC‐side voltage controller of the LCC‐HVDC station can highly impact the damping of the oscillatory modes and even cause instability. The studies also show that the voltage control parameters can be selected to prevent PLL‐induced instability. The studies also (i) show that maximum available power and critical SCR are highly affected by PLL and DC‐side voltage control and (ii) propose a procedure to systematically determine these indices.

The voltage source converter (VSC) system has stability issue under weak AC grid conditions. In this paper, the multi-input multi-output (MIMO) transfer function model of VSC system in frequency-domain is derived. Then, the individual channel analysis and design (ICAD) approach is employed to develop the equivalent single-input single-output (SISO) feedback control model for stability analysis. On that basis, the stability of VSC system and the coupling interaction intensity among the multiple control loops, including the active power control (APC), reactive power control (RPC) and phase-locked-loop (PLL) are investigated. The results show that larger PLL bandwidth increases the coupling degree of active power control (APC) and reactive power control (RPC) closed loops, resulting in the instability of the system under weak AC grid conditions. Finally, a derivative feedback compensation(DFC) approach is proposed to enhance the stability margin for VSC system connected to weak AC grid. The results based on MATLAB-based theoretical analysis and electromagnetic transient (EMT) simulations show that the proposed DFC approach can effectively suppress the instability of VSC system due to the close coupling of APC and RPC closed loops caused by the large PLL bandwidth, thus effectively improve the stability margin of the system.

With the increasing penetration of renewable energy, the inertia of AC system gradually decreases. When modular multilevel converter (MMC) based high voltage DC transmission (HVDC) system is connected to a low inertia AC system, the low-frequency oscillation may occur due to the interaction between MMC and synchronous generator (SG). To investigate this issue, a detailed small-signal model considering the dynamics of MMC and SG is developed and the corresponding eigenvalue analysis is conducted. Based on the developed model, the impact of the inertia on the low-frequency oscillation of MMC-HVDC system is studied, and the dominant low-frequency mode is investigated by participation factor and control parameter sensitivity analysis. From the results, it is found out that there exists close control interaction between SG and MMC under low inertia condition. To damp the low-frequency oscillation of MMC-HVDC system under low inertia condition, a supplementary damping control (SDC) approach is proposed, the parameters of the SDC approach are tuned by pole-assignment approach. Finally, the damping effect of the SDC approach is validated by theoretical analysis and simulation study in PSCAD.