Commutation failure analysis in multi-infeed HVDC systems

Commutation failure theory of HVDC system is proposed and its reason is analyzed. Faults at inverter bus in multi-infeed HVDC system can cause local commutation failure and even concurrent commutation failure. Anomalous commutation failure is simulated with six kinds of ac system strength, and it is more and more serious at first as the system strength increases then become less serious as the strength becomes stronger. Coupling impedance is changed to get its relationship with anomalous commutation failure.

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

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.

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.

Commutation failure (CF) is an inherent drawback of line-commutated converter high-voltage DC (LCC-HVDC) system. And CF prevention (CFPREV) control is widely applied in UHVDC lines to mitigate the subsequent CF. However, the formation of a multi-infeed UHVDC system induces power exchange and voltage interaction between AC buses during the transient process, making the behaviors and mitigation of CFs more complex. Consequently, exploring the impact of CFPREV control on adjacent stations and optimizing the CFPREV control to adapt to the interaction characteristics in a multi-infeed system is necessary. Considering the interaction characteristics, the impact of CFPREV control on the commutation process of inverters in a multi-infeed system is analyzed through formula derivation and simulation verification. It is found that although CFPREV control can effectively mitigate the CFs of the inverter near the fault, it will further increase the risk of concurrent CF (CCF) of remote inverters due to the interaction on the AC side. To solve this problem, a coordinated control scheme of CFPREV controls in a multi-infeed UHVDC system is proposed. The output of the CFPREV control of the inverter near the fault can be adjusted adaptively according to the commutation margins of remote inverters. And the probability of CCF caused by CFPREV control is consequently decreased. Case studies are conducted based on Henan provincial multi-infeed UHVDC system in China to verify the analysis result and the effectiveness of the coordinated control scheme of CFPREV controls.

This paper focuses on the investigation of the dynamic characteristics of Ultra High Voltage Direct Current (UHVDC) system Under Hierarchical Infeed Mode (UHVDC-HIM) with Static Synchronous Compensator (STATCOM). The STATCOM at the inverter side of UHVDC-HIM is developed in PSCAD/EMTDC, and their power flow equations are derived. Based on these equations, the control schemes for rectifier and inverter side of UHVDC-HIM and STATCOM are properly designed. Several dynamic characteristics indices such as Commutation Failure Immunity Index (CFII), Commutation Failure Probability Index (CFPI), fault recovery time and Temporary Overvoltage (TOV) are evaluated in detail. The impact of different sizes of STATCOM and Short Circuit Ratio (SCR) of AC sources on dynamic characteristics of UHVDC-HIM system during single-phase and three-phase faults are examined. The analysis shows that higher SCR values and greater capacity of STATCOM can make the UHVDC-HIM system less vulnerable to Commutation Failure (CF), considerably enhance the fault recovery time and effectively reduce the TOV.

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.

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.

A fault division method based on fault current limiting technology was proposed to solve the problem of simultaneous commutation failure (CF) in multi-infeed HVDC transmission system. The relationship formula between the reactance of fault current limiter (FCL) and the voltage drop of non-fault point were deduced. And it's proved that FCLs can compensate the voltage reduction of non-fault point to avoid its CF. The multi-objective optimization configuration was carried out by using the correlation degree method and the non-dominated sorting genetic algorithm-II (NSGA-II). The optimal installation location, number and impedance of FCLs were achieved. Thus the fault division method was realized. Finally, two DC single-feed standard models combined with IEEE 39-node grid were adopted to verify the efficiency and reliability of the fault division method. And it was confirmed that the method can reduce the search space of the configuration, improve the economy and avoid the simultaneous CF of multi-infeed HVDC transmission 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.

With more power electronics deployed in power system, the propagation of cascading failure become more complex and it is necessary to investigate the mechanism of cascading failure in the novel power system. This paper reveals a new cascading failure mode in multi-infeed high voltage direct current (HVDC) systems. Firstly, the mechanism of multi-timescale cascading failure is proposed. Then the simulation results of multi-infeed HVDC systems which is carried out in Power system department-Bonneville power administration (PSD-BPA) indicate that the dominant cascading failure modes in different timescales are different and they are coupling with each other. Finally, several dominant factors of different timescale cascading failure modes are pointed out.

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.

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.

Dynamic reactive power characteristic analysis of inverters under the commutation failure faults in HVDC systems