Anomalous Commutation Failure Phenomenon in Multi-Infeed HVDC Systems

Summary form only given. The commutation failure vulnerability of multi-infeed HVDC converters to ac side faults is analyzed. Faults in multi-infeed systems can cause commutation failure events in the nearby local converter or concurrently in both converters. The immunity of the system to both these events is investigated and suitably parameterized. “Commutation Failure Immunity Index” (CFII) is calculated for multi-infeed systems with different ratings. Anomalous failure in valve current commutation due to voltage distortion has been investigated.

A commutation failure risk analysis method considering the interaction of inverter stations

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

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.

This paper presents a wavelet based commutation failure (CF) detection technique in HVDC system, and selects the suitable mother wavelet. The different properties of wavelets are analyzed in detail. It is shown that the orthogonality, regularity, moments, and symmetry of a wavelet function may affect the CF detection to some extent, and the critical factor is the moments of the mother wavelet. Discrete Wavelet Transform (DWT) has executed respectively on normal sinusoidal voltage signal and alternative voltage record with CF in Tianguang HVDC project. The transform result shows that DWT by Daubechies wavelet function with high vanishing moments is a cogent tool for CF detection in HVDC system.

Introduction: The critical extinction angle of the HVDC converter is closely related to commutation failure and varies with the operating conditions. Existing research ignores the dynamic variation characteristics of the critical extinction angle, which will weaken the effect of the commutation failure suppression strategy. Considering the dynamic variation characteristics of the critical extinction angle, a commutation failure suppression strategy by dynamically compensating the extinction angle is proposed to improve the effect of commutation failure suppression. Methods: Firstly, this paper analyzes the influencing factors of the critical extinction angle, and establishes a calculation model of the critical extinction angle. Then, based on the proposed model, the influence of the critical extinction angle variation on the commutation failure process is analyzed. Considering the dynamic variation characteristics of the critical extinction angle during AC short-circuit fault, a commutation failure suppression strategy based on extinction angle compensation is proposed. Results: The proposed strategy can improve the suppression effect of commutation failure under different operating conditions, decrease the drop of AC voltage, and increase the recovery speed of DC power. Discussion: The proposed control strategy takes into account the dynamic variation of the critical extinction angle, enhances the suppression effect on commutation failure in HVDC systems, and improves the grid stability in modern power systems. Conclusion: Commutation failure is affected by the dynamic variation characteristics of the critical extinction angle. The proposed strategy considers the characteristics that can accelerate the fault recovery and improve the commutation failure resistance ability.

Control strategy improvement is an effective method for commutation failure mitigation in line-commutated-converter-based high voltage direct current (LCC-HVDC). However, for the multi-infeed HVDC system with multiple dc links located in close mutual proximity, existing strategies based on adjustment of dc current or the firing angle may lead to negative influence even commutation failure in adjacent HVDC. Therefore, in order to achieve coordinated recovery of multi-infeed HVDC systems, novel control strategy for improvement of the voltage dependent dc current order limiter (VDCOL) is put forward after analyzing the reactive power interaction between dc systems. Then crucial parameters in the proposed strategy are determined based on evaluation of influence on stability of the multi-infeed HVDC system caused by each dc system and reactive power-voltage sensitivity between ac busbar of inverters. Simulation results in PSCAD/EMTDC verify effectiveness and advantage of the method on commutation failure mitigation as well as coordinated recovery performance.

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.

The behaviors of transient phenomena in HVDC systems, including DC line faults, commutation failures caused by AC faults and missing firing are difficult to be identified automatically by the control systems, while the effective protection control for commutation failures depends on the rapid and correct identification of such faults. This paper proposes a method to identify different causes leading to commutation failures based on the wavelet transform. By using the technique of wavelet multi-resolution analysis (MRA), the transient signals generated by the faults are decomposed into different resolution levels. The features of each fault are extracted. Two auxiliary parameters are defined as the criteria for the identification, based on which four thresholds are set to distinguish different faults. Simulation results indicate that the proposed approach could make a definite identification of commutation failures from DC line faults and normal operations. Furthermore, the discriminations between AC short circuit faults and missing firing faults, which cause commutation failures, also could be obtained.

Compared to traditional multi-infeed HVdc (MIDC) systems with inverters coupling only in the ac-side, more complex inter-inverter interactions exist in emerging hierarchical-infeed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -UHVdc (HIdc) systems with inverters coupling in both the ac- and dc-sides. However, the effectiveness and accuracy of the earlier interinverter interactions evaluation approach in HIdc systems are limited since it is established by the empirical reasoning without rigorous theoretical basis. Thus, in this article, the hierarchical-infeed voltage interaction factor (HVIF) index is first used to quantify the complex inter-inverter interactions in HIdc systems. The salient feature of the HVIF is that both the voltage magnitude and angle couplings at various inverter buses are considered in its definition. Hence, the definition of the HVIF enables the proposed analytical evaluation approach to be more effective than the earlier approach. Moreover, with various dc control modes comprehensively considered, the inverter power-voltage dependence terms in the quasi-steady-state power-flow model of HIdc systems are derived based on the rigorous theoretical analysis. This lends the fundamental mathematical basis to subsequently develop the analytical calculation method of the HVIF, which makes the proposed approach more accurate than the earlier approach. Finally, both the simulated and experimental results sufficiently validate the proposed approach.

Commutation failure is one of the common faults in LCC-HVDC transmission systems, threatening safety and stability of power grid. To focus on the key issues of commutation failure and reduce commutation failure frequency, a review of commutation failure in LCC-HVDC transmission systems is given in this paper. Firstly, the mechanism of commutation failure is analyzed from both device and system aspects. Since commutation failures are mostly caused by AC system faults on the inverter side, commutation failures are classified into three categories: first commutation failure, continuous commutation failure during the fault period and commutation failure after the fault. Then, the influencing factors and prevention methods of commutation failure in each category are summarized. Finally, the problems to be solved in the field of commutation failure are pointed out.

Commutation failure is one of the most frequent failures in line-commutated converter based high-voltage direct-current (LCC-HVdc) systems which may lead to the outage of HVdc links, and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, a commutation failure prevention module (CFPM) is developed, which inserts the optimal value of resistance proportional to the reduced extinction angle in series with the fault current path using a simple switching method. When a fault occurs at the inverter ac side, a self turn- off switch starts switching with a pre-specified frequency and controlled modulation index, which limits the fault current to the desired value and inhibits commutation failure. The salient feature of the proposed CFPM is that it is fully controllable. In addition, despite the available controller modification methods which are not effective for low-impedance faults, the proposed CFPM can prevent commutation failure regardless of the fault severity. The practical performance and feasibility of the developed CFPM is validated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively eliminate the commutation failure or repetitive commutation failures under different fault types.

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.

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.

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.