Research and Simulation Analysis of Systematic Voltage Fluctuation in DC Commutation Failure

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.

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.

Set up a model of typical minimum ac/dc system. From the electrical distance and the phase difference of steady state between fault point and converter bus to research the spatial characteristics of HVDC commutation failure. Simulation results show that under the condition of invariable system support strength, the electrical distance between fault point and the converter bus is the main factor affecting the spatial characteristics of HVDC commutation failure. The closer the electrical distance is, the more likely to cause a dc commutation failure. The influence of power flow to HVDC commutation failure is mainly embodied in the phase difference of steady state between the fault point and the converter bus. The lager the phase of the fault point is, relative to the phase of the dc converter bus, the easier the HVDC occurs commutation failure.

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.

In recent years, the violation and fluctuation of system voltage has occurred with greater frequency with the integration of high-penetration distributed photovoltaic generation. In this paper, the voltage violation and fluctuation in a high-penetration distributed photovoltaic integrated system is analyzed, and then a corresponding suppression strategy is proposed. Firstly, based on solar cell and photovoltaic control system models, the influence factors of photovoltaic output are analyzed. Secondly, the voltage violation and fluctuation caused by photovoltaic integration is analyzed, and the quadratic parabola relationship between bus voltage fluctuation and photovoltaic power variation is constructed. Next, according to the virtual synchronous generator characteristic of distributed photovoltaics, a double-hierarchical suppression strategy is proposed to make full use of reactive power regulation capability, which can maintain the symmetry of power supply while meeting standard requirements. The proposed strategy can conveniently realize quick response and support the photovoltaic extensive access. Moreover, with the employment of the proposal, the system voltage violation and fluctuation can be suppressed effectively. Finally, considering the photovoltaic access location, capacity, and partial shading, the effectiveness of the proposed strategy is verified in IEEE 33-bus distribution system with field measured data. After distributed photovoltaic accesses the system, more than 60% of buses appear to have undergone bus voltage violation. With the proposed method, more than 20% of the voltage deviation and more than 6% of the voltage fluctuation are effectively suppressed so that the system voltage can be kept below 1.07 p.u. and the voltage fluctuation can be kept within 4%, meeting the requirements of power quality standards.

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.

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.

Aeroelastic aspects of Wind Turbine (WT) have severe effects on power and voltage fluctuations of wind power systems. To mitigate these fluctuations, Energy Storage Systems (ESSs) can be utilized as a suitable option. In short term applications, Flywheel Energy Storage System (FESS) is considered as a proper choice among ESSs because of its fast response. In this paper, a wind farm comprising four fixed-speed WTs is simulated, and FESS is proposed to reduce the power and voltage fluctuations caused by aerodynamic aspects of WT (i.e., yaw error, horizontal, and vertical wind shear). To comprehensively model a WT, FAST and AeroDyn packages have been used in Simulink platform. The simulation results demonstrate the effectiveness of FESS in reducing voltage and power fluctuations. Moreover, the capability of FESS in reducing voltage fluctuations is compared with STATCOM.

Coordinated power control of electrochemical energy storage for mitigating subsequent commutation failures of HVDC

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

With the enhanced interaction between the AC and DC power system, the post-contingency cascaded DC commutation failure (CF) has been a potential security risk. This paper proposes the steady-state security region (SSR) formulation with the post-contingency cascaded CF considered. The voltage reduction at the post-contingency quasi-steady-state is selected to identify the cascaded CF. The decoupled linearized power flow (DLPF) is introduced so that the voltage issues can make contributions to the SSR depiction. The proposed SSR provides the scope of the dispatch decisions where the cascaded CF can be inhibited. Case studies between different types of SSR are performed in the IEEE modified 9- and 30-bus test systems to validate the effectiveness of the proposed SSR.

In recent years, repeated voltage fluctuations frequently occur in the actual operation of large-scale wind power integration system, which has the characteristics of large voltage fluctuation range and second fluctuation period. This kind of abnormal voltage fluctuation is significantly different from the voltage fluctuation caused by the randomness of wind energy. Owing to the voltage fluctuations, the wind turbines can be disconnected from the grid by continuous low voltage ride through (LVRT) failures, which seriously threatens the safe and stable operation of the power grid. Such a kind of operating accident involves the coupling between wind turbine fault ride through strategy and the characteristics of weak power grids, which is difficult to be explained directly by traditional power system voltage stability analysis methods. In this paper, an analysis method for integration system active power and voltage curve, i.e. PV curve, is proposed to accurately describe the system operating point moving trajectory during the repeated voltage fluctuation, considering the LVRT characteristics of wind turbines. Furthermore, the optimized control strategy for wind turbine LVRT is proposed to suppress the repeated voltage fluctuation. The simulation analysis based on the actual power system data shows that increasing the wind turbine LVRT exiting threshold and adopting the strategy of maintaining active power during LVRT can effectively suppress the repeated voltage fluctuation.

HVDC transmission is an important approach to delivery large-scale wind power for a long distance. When the commutation failure occurs, the voltage of sending end will fluctuate dramatically, which threatens the stable operation of wind farm. Meanwhile, different characteristics of wind turbines will further affect the transient voltage of sending end, so the impact of wind farm on the transient voltage must be considered, especially for the high proportion of wind power generation. Firstly, the sending end transient voltage characteristics are analyzed and the mechanism of transient overvoltage caused by commutation failure is revealed. Then, doubly-fed wind power generation systems with different control modes are modeled, and the dynamic reactive power response capabilities of them are studied, so as to analyze the influence of the doubly-fed induction generator (DFIG) control modes on the transient overvoltage. Finally, a simulation example is established which verifies the correctness of the theoretical analysis. The results show that when DFIG adopts the constant voltage control strategy with the quickly response speed of the converters, it has stronger reactive power output capability, and can suppress the transient overvoltage, which is beneficial to the safe and stable operation of the power grid.

Ultra-high Voltage Direct Current (UHVDC) serves as an effective means to facilitate the integration of renewable energy. Commutation failure, an inevitable characteristic of UHVDC, leads to fault features of non-step changes in voltage amplitude at the receiving end of the grid, posing a threat to the safe and stable operation of wind turbines connected to the UHVDC receiving end on a large scale. To analyze the transient active characteristics of wind turbines at the receiving end of the grid under commutation failure, this paper divides the transient response under grid faults into three states of the direct-drive wind turbine system. Taking into account the fluctuation characteristics under grid faults and the control parameters of the wind turbines, a transient active power response model for direct-drive wind turbines is established. The accuracy and effectiveness of the mathematical model are verified through a combination with simulation models. Furthermore, based on the established transient active power mathematical model of direct-drive wind turbines, the influence of system control parameters on transient active characteristics is analyzed.

In practical engineering, large-scale wind power integration typically requires long-distance transmission lines to deliver power to load centers. The resulting weak sending-end systems lack support from synchronous power sources. Under extreme weather conditions, the rapid increase in active power output caused by high wind power generation may lead to voltage instability. In existing projects, a phenomenon of repeated voltage fluctuations has been observed under fault-free system conditions. This phenomenon is induced by the coupling of the characteristics of weak sending-end systems and low-voltage ride-through (LVRT) discrimination mechanisms, posing a serious threat to the safe and stable operation of power grids. However, most existing studies focus on the analysis of voltage instability mechanisms and the optimization of control strategies for single devices, with insufficient consideration given to voltage fluctuation suppression methods under the coordinated operation of wind power and energy storage systems. Based on the actual scenario of energy storage configuration in wind farms, this paper improves the traditional LVRT discrimination mechanism and develops a coordinated voltage ride-through control strategy for permanent magnet synchronous generator (PMSG) wind turbines and energy storage batteries. It can effectively cope with unconventional operating conditions, such as repeated voltage ride-through and deep voltage ride-through that may occur under extreme meteorological conditions, and improve the safe and stable operation capability of wind farms. Using a hardware-in-the-loop (HIL) test platform, the coordinated voltage ride-through control strategy is verified. The test results indicate that it effectively enhances the wind–storage system’s voltage ride-through reliability and suppresses repeated voltage fluctuations.