Analysis of spatial characteristics of HVDC commutation failure in complex power grid

Since a large-scale wind farm is connected, it has an impact on the hybrid High Voltage Alternating Current/Direct Current (HV AC/DC) power grid with the characteristics of 'strong DC and weak AC'. When the connected capacity of the wind farm drops instantaneously, it may cause the DC system commutation failure. In this paper, firstly, the impact of wind power grid connection on the converter bus voltage is analyzed. Then, the relationship between the commutation bus voltage variation and the critical commutation voltage reduction is established, the correlation factor of the DC commutation failure caused by the wind power grid connection is proposed, and the criterion for determining the DC commutation failure caused by the wind power grid connection is defined. As a result, the maximum variation of wind power access capacity could be obtained.

Hybrid cascaded multi-terminal DC transmission provides a more flexible transmission method, improves the voltage stability of the inverter AC system, and reduces the probability of commutation failure. The hybrid LCC-VSC at the receiving end of the DC system is a new type of topology.In order to study the influence of coupling characteristics between VSC and LCC on commutation failure in AC power grid at receiving end,firstly,a hybrid LCC-VSC simulation model is built on PSCAD to analyze the operation characteristics of the system during the fault and verify the reactive power support ability of VSC to LCC, and the improvement effect of VSC on LCC-HVDC under different reactive power control modes is compared and analyzed. In order to study the influence of AC/DC coupling on commutation failure of LCC, commutation failure immune factor (CFII) was introduced to study the influence of AC system strength on commutation failure of LCC. Finally, considering that the influence of AC/DC coupling on commutation failure is closely related to the degree of coupling, the electrical distance between LCC and VSC is further studied. The results show that the conclusion can provide theoretical guidance for the design and analysis of related projects.

Chinese energy resources and load are in reverse distribution, and it is necessary to vigorously develop UHV AC and DC technologies suitable for long-distance and large-power transmission. The pattern of UHV AC-DC hybrid power grid has been initially formed at present. However, the faults of UHV DC with large transmission power and complex fault forms have great impact on the power grid in both active and reactive power. In this paper, based on the analysis of the current situation and characteristics of UHV AC/DC power grids in China, the impact of UHV DC restart and commutation failure on the stability of the power grid is analyzed. The different types and triggering factors of DC commutation failures are analyzed, and the effects of single or multiple DC commutation failure types on the transient angle stability, the voltage stability and the transient overvoltage are simulated. The UHV DC restart fault time sequence is studied, and the effects of different restart modes on the system transient angle stability and transient overvoltage are demonstrated through simulation. The research results have laid the foundation for grasping the UHV DC fault morphology mechanism and the influence law on the stability of the power grid, and provided support for the safe and stable verification of the UHV AC/DC power grid.

The mathematical relationship between reactive power consumed by DC system and AC voltage on inverter bus is derived under different control modes, respectively.Based on these relationships, a new definition of multi-infeed short circuit ratio (MISCR) is formulated from the power-flow model of PQ decoupled method.The specific influence of different control modes on voltage stability is analyzed.Considering the relationship between MISCR and voltage stability of AC/DC hybrid system, the characteristics of values of new MISCR are derived.Case study of an actual regional power grid validates these characteristics.

With the increasing integration of HVDC projects, multi-infeed HVDC systems are becoming more prevalent. The strong coupling between DC links can lead to successive commutation failures across multiple lines, known as cascading commutation failures. These failures cause significant transient power imbalances in both sending and receiving systems and may trigger voltage instability or even system-wide cascading faults due to insufficient reactive power support. To address this issue, this paper proposes a discrimination method for cascading commutation failures in AC/DC hybrid grids with multi-infeed HVDC systems. Based on a quasi-steady-state model of the DC system, the method analytically computes the voltage sag at converter buses, helping to reveal the mechanism of post-fault cascading failures in hybrid AC/DC systems. It enables rapid prediction of whether successive commutation failures will occur across multiple DC lines—without relying on specific grid operating conditions—and provides a theoretical foundation for subsequent control strategies. Finally, the method in this paper is verified by simulation in an actual system to prove its effectiveness.

The failure of high-power DC commutation leads to large fluctuations in the voltage of the transmitting end grid, which brings great harm to the safe and stable operation of the transmitting end grid. In this paper, the detailed analysis of PMU data is carried out for the voltage fluctuation events of the transmitting end caused by the frequent DC commutation failure in recent years. By constructing a simulation model in BPA and RTDS, the fault back-tracking of the voltage fluctuation of the power supply with the commutation failure is carried out. The simulation results show that the main cause of the fluctuation is caused by the timing coordination and the delay of the rectification side AC filter. This is of great significance for reducing voltage fluctuations in the case of subsequent DC system commutation failure, and it also has great significance for maintaining safe and stable operation of the power grid.

HVDC commutation failure and recovery, the reactive power exchanged between HVDC and AC systems will change largely, showing the external characteristic of ‘large capacity reactive impact load’, so it is quite necessary to carry out studies on this problem. In this study, first, the HVDC sending‐side and receiving‐side reactive power equations in steady state and transient process are given. Second, the HVDC sending‐side and receiving‐side reactive power characteristics during commutation failure and recovery are analysed. Finally, the impact of the HVDC control system parameters is studied. All the contents are verified by simulation of the HVDC equivalent case system.

The multi-infeed HVDC systems are closely coupled between the inverter stations, which is easy to cause concurrent commutation failures (CFs). In this paper, the electrical distance is adjusted by the installation of series reactors (SRs) to reduce the multi-infeed interaction factor (MIIF). At the same time, reactive power (VAR) compensators are added at some buses with SRs installed to control the voltage drop and power flow of the system. Finally, a mathematical model for the SRs and VAR compensators installation is established, and the particle swarm optimization (PSO) algorithm is used to optimize the installation. The simulation results of the power grid show that the optimized installation can inhibit the problem of concurrent CFs.

In this paper, a novel analytical model is proposed to predict the average transmission-capacity loss and load loss during a cascading failure as a function of time and their steady state values. Cascading failures in the power grid are described using a Markov-chain approach, in which the state transition probabilities depend on the number and capacities of the failed lines. The transition matrix is characterized parametrically using Monte Carlo simulations of cascading failures in the power grid. The severity of cascading failure is estimated using two metrics: the expected number of transmission-line failures and the amount of load shedding/load loss (inferred from the average transmission capacity loss) in the steady state. These two metrics provide critical information regarding the severity of a cascading failure in a power grid (in terms of both the distribution of blackout sizes and the amounts of load shedding). One of the benefits of this model is that it enables the understanding of the effect of initial failures and of the operating parameters of the power grid on cascading failures.

This paper analyzes the essence of commutation failure of HVDC transmission systems, and points out that the load model has an important influence on commutation failure. The Commutation Failure Immunity Index (CFII) is used to quantitatively calculate the influence of the load type and percentage of the induction motor load on the commutation failure. It is proposed that the induction motor load can provide the rotary inertia to the system at the instant of fault, suppress the converter bus voltage drop and reduce the probability of commutation failure. The above discussion is verified by electromagnetic transient simulation.

Accurate detection and effective control strategy of commutation failure (CF) of high voltage direct current (HVDC) are of great significance for keeping the safe and stable operations of the hybrid power grid. At first, a novel detection method for consecutive CF is proposed. Concretely, the 2nd and higher orders’ derivative values of direct current are summarized as the core to judge CF by analyzing the physical characteristics of the direct current waveform of the converter station in CF. Then, the Daubechies wavelet coefficient that can represent the 2nd and higher order derivative values of direct current is derived. Once the wavelet coefficients of the sampling points are detected to exceed the threshold, the occurrence of CF is confirmed. Furthermore, by instantly increasing advanced firing angle β in the inverter side, an additional emergency control strategy to prevent subsequent CF is proposed. Eventually, with simulations of the benchmark model, the effectiveness and superiorities of the proposed detection method and additional control strategy in accuracy and rapidity are verified.

Introduction: Existing simulation models of thyristors ignore the dynamic turn-off characteristics, resulting in inaccurate simulation of commutation failure in HVDC converters. To accurately characterize the turn-off characteristics of thyristors and simulate commutation failure characteristics of converters, custom digital models of thyristors and converters have been established in this study. Methods: The influence of current zero-crossing rate (di/dt|t=t1), forward current peak (IF), and junction temperature (Tj) on the turn-off characteristics of the thyristor was analyzed. A characterization method for the influence of di/dt|t=t1, IF, and Tj on the turn-off characteristics has been proposed, and a custom digital simulation model of thyristors considering turn-off characteristics has been established. An electromagnetic transient model of the converter that can accurately reflect the characteristics of commutation failure has also been constructed. Results: Compared to the experimental results, the simulation error of the peak reverse recovery current of the proposed thyristor model has been found to be less than 5%, and the simulation error of the turn-off time has been found to be less than 4%. Discussion: The proposed digital model of converters has addressed the problem of inaccurate simulation of commutation failure found in traditional models. This model has significantly enhanced simulation accuracy, providing reliable support for the research on commutation failure in HVDC systems and being of great significance for ensuring system stability. Conclusion: The turn-off process of thyristors and influencing factors have been analyzed in this study, the digital simulation models for thyristors and converters have been established, and the validity and accuracy of the digital models have been verified through simulation.

In smart power grid, power grid and communication network are connected and mutually dependent. The failure in power grid might cause failures in communication network, and vice versa. A tiny failure in either of them could trigger cascade of failures within the entire system. In this paper, we focus on understanding the structure of smart power grid and studying the underlying network model, their interactions, relationships and how cascading failures occur in the system. We propose a practical model for smart power grid as interdependent complex network. The interdependency between two networks is `oneto-multiple': each node in the communication network has only one support link from the power grid, while each node in power grid is connected to multiple communication nodes. We study the effect of cascading failures using percolation theory, and present detailed mathematical analysis of failure propagation in the system. We analyze the robustness of our model caused by random attacks or failures by calculating the size of functioning parts in both networks. Using simulations, we prove that there exists a threshold for the proportion of faulty nodes, beyond which the system collapses. Also we determine the critical values for different system parameters. To the best of our knowledge, this is the first work that models smart grid as interdependent complex networks and studies its fault tolerance.

With the large-scale deployment of renewable energy, transmission systems for new energy sources are increasingly exposed to transient overvoltage issues induced by DC blockages and commutation failures. To address the challenges of an imprecise response to multiple fault scenarios and the inefficiency of independent device actions in existing dynamic reactive power control schemes, this paper proposes a coordinated optimization control strategy integrating multiple dynamic reactive power sources tailored to different fault characteristics. An equivalent model of the renewable energy DC transmission system is established to elucidate the underlying mechanisms of transient overvoltage formation under various fault conditions. By employing trajectory sensitivity analysis and parameter perturbation methods, the influence patterns of control parameters on transient overvoltage behaviors across different fault scenarios are quantitatively assessed, thereby overcoming the limitations of traditional empirical parameter tuning approaches. Subsequently, a multi-source coordinated optimization model is developed with the objective of minimizing transient overvoltages under simultaneous dual-fault conditions. A multi-objective particle swarm optimization algorithm, incorporating comprehensive trajectory sensitivity and dynamically adaptive inertia weights, is introduced, alongside Pareto front theory, to achieve rapid and balanced optimization among competing objectives. Simulation results validate that the proposed strategy significantly enhances transient overvoltage suppression across diverse fault conditions. The findings provide robust theoretical foundations and practical guidance for the refined parameter tuning and high-efficiency coordinated control of dynamic reactive power sources in renewable energy transmission systems.

Switching on no-load transformer in AC system will cause serious inrush current. When the inrush current travels to the AC bus of inverter in DC system, it will lead to the variation of fundamental voltage and distortion, which inevitably affect the commutation process of the inverter. At present, the research mainly focuses on the variation of fundamental voltage, while the commutation failure caused by voltage distortion is rare. To ensure the safety and stability of the power grid operation, this paper elaborates the mechanism of commutation failure created by voltage distortion based on harmonic voltage-time area method, and also proposes an evaluation method which can quickly identify the commutation failure caused by inrush current. Finally, the PSCAD/EMTDC simulation results are provided to illustrate the effectiveness of the proposed method.