Abstract:The increasing integration of renewable energy sources has caused spatially and temporally uneven inertia distribution in power systems. This paper proposes a regional inertia estimation and virtual inertia configuration method using dynamic mode decomposition (DMD). A regional equivalent inertia model is established based on inertia estimation theory for renewable-penetrated power systems. The power system is partitioned via spectral clustering, and frequency measurement nodes are selected according to Pearson correlation coefficients of regional nodes. Regional inertia is then estimated through the DMD method. Critical regional inertia requirements are calculated based on frequency security constraints to configure virtual inertia for renewable energy and energy storage systems. An improved IEEE 10-machine 39-node model with wind-storage systems is implemented in PSCAD for validation. Simulation results demonstrate that the proposed method achieves regional inertia estimation with errors below 5%. The online virtual inertia configuration strategy for wind turbines and energy storage systems effectively prevents frequency violations and enhances frequency security and stability.

Abstract:The evaluation and improvement methods for commutation failure resistance ability in multi-infeed direct current (MIDC) system are of great significance for testing commutation failure characteristics, addressing assessment challenges, and applying commutation failure defense technology in power system planning and operation. Therefore, a systematic and scientific summary and generalization are conducted in this paper. Firstly, the fundamental definitions of MIDC system and commutation failure are presented, analyzes the impacts of various fault types on commutation failure, identifies key influencing factors, and summarizes existing commutation failure criteria. Secondly, the existing evaluation methods of commutation failure resistance ability are also reviewed. Then, the current improvement methods of commutation failure resistance ability in MIDC system are summarized from three aspects: reactive power compensation optimization, control and protection optimization, and converter topology improvement. Finally, the key issues that need to be focused on in the future mainly include: adopting a normalized effect evaluation method, conducting repeatable calibration on a simulation platform with standard significance, forming a systematic and comprehensive improvement method for the commutation failure resistance ability in MIDC system, and establishing a multi-level coordinated enhancement strategy encompassing converter station level, converter level and system level.

Abstract:Linearized modeling of large-scale renewable energy stations with a voltage-source converter-based high-voltage direct current (VSC-HVDC) system faces significant challenges. These arise from the diverse dynamic characteristics of generating units and the high system order. As a result, conventional modal analysis methods are often ineffective. To address this issue, a reduced-order analysis method is proposed for the stability evaluation and oscillation source identification of such systems. Renewable energy plants are equivalenced as multiple identical units. The equivalent systems are then reduced in order, considering the dynamic interactions with the external system. System stability is assessed using modal analysis. Oscillation sources are identified based on unstable modes. Dominant components are determined through participation factor analysis. The proposed method is validated through simulation studies. It is demonstrated that the reduced-order approach significantly lowers system order, reduces computational complexity, and effectively evaluates instability risks. Moreover, system stability is enhanced by tuning parameters of the dominant components.

Abstract:Fault detection and protection of modular multilevel converter high voltage direct current (MMC-HVDC) power grids constitute a critical technology within the domain of electric power systems. When the DC power grid fails, equipment damage is caused and grid stability is jeopardized by the rapid escalation of fault currents. Consequently, extremely high requirements are placed on the rapidity and reliability of the fault protection system. By comparing the changes in the voltage value of the current limiting inductor on the line before and after the fault, a method for identifying line faults using the voltage ratio of the current limiting inductor is proposed. The proposed method is validated using a four-terminal MMC-HVDC power grid model constructed on the PSCAD/EMTDC simulation platform. The validation encompasses fault initiation, type identification, and pole selection. Through accurate fault identification, the appropriate DC circuit breaker (DCCB) is activated to isolate the fault effectively. Additionally, feasibility analysis is conducted on system performance metrics along with four critical factors: fault transition resistance values, post-fault noise interference, fault location variations, and communication error impacts. The final research outcomes demonstrate that the proposed fault protection scheme can accurately detect and rapidly isolate faults, thereby safeguarding the stable operation of the power grid.

Abstract:With the continuous expansion of clean energy integration, hybrid energy storage systems (HESS), which are capable of smoothing power output, have garnered increasing attention. The dual active bridge converter, known for achieving electrical isolation and soft-switching capability, is commonly employed in HESS. To broaden the voltage gain range and reduce the current ripple on the energy storage device side, this paper proposes a novel HESS topology based on a current-fed bidirectional resonant converter. Firstly, the converter topology and its equivalent circuit are presented. Its switching modes and operational principles are analyzed, leading to the derivation of an equivalent circuit model, a voltage gain expression, and the characteristics of the low-voltage-side current ripple. Subsequently, a decoupling control strategy is proposed. This strategy independently controls the power transfer of the super capacitor and the battery by adjusting the duty cycle of the corresponding full bridge on each energy storage side. The power share of each port is dynamically regulated according to the power allocation command. Simulation results verify that the proposed system achieves zero voltage switching (ZVS) for all switches and exhibits fast dynamic response along with good stability during load transitions and power command changes.

Abstract:Model predictive control (MPC) is applied in modular multilevel converter (MMC) control due to its advantages of fast response and simple modeling. However, previous research has not addressed high-frequency resonance suppression. Firstly, a mathematical model for MMC predictive control is established. By combining the dq impedance modeling method in traditional control, the Z-transform is used to achieve positive and negative sequence impedance modeling of the MPC. Secondly, the discrete transfer function of the converter station body and conduct stability analysis are derived. Under the premise of a stable ontology, the impedance method is used to reveal the mechanism of the station high-frequency resonance in model predictive control. Then, in response to the high-frequency resonance problem caused by delay, a multistep predictive control method is used to suppress it. Based on the impedance model, the optimization effect on the local high-frequency region is analyzed. Finally, the theoretical analysis shows that the converter station can compensate for the increased delay by increasing the number of predicted steps. And the effectiveness and correctness of multi-step predictive control in suppressing high-frequency resonance are proved through electromagnetic transient simulations.

Abstract:The modular multilevel converter (MMC) is widely used in flexible direct current projects. The fault clearance speed of hybrid MMCs is closely related to the proportion of full bridge sub-modules (FBSMs). To address the issue of sub-module overvoltage caused by unbalanced energy absorption between half bridge sub-modules (HBSMs) and FBSMs during the clearance of a bipolar short-circuit fault, this paper designs an energy-assisted isolation branch suitable for hybrid MMCs with a low FBSM ratio. By bypassing the HBSMs before blocking, the energy released by the submodules is reduced, thereby lessening the transient energy absorption burden on the FBSMs and suppressing FBSM overvoltage. To shorten the fault clearance time, this paper proposes a dual-branch coordinated operation strategy. A transient energy absorption branch is connected in parallel at the port side to assist in absorbing the fault's transient energy and accelerate fault clearance. A two-terminal hybrid MMC-based flexible direct current transmission system model is built on the PSCAD/EMTDC platform to verify the effectiveness of the proposed scheme. Simulation results show that the proposed dual-branch coordinated operation strategy can effectively suppress sub-module overvoltage, shorten the fault clearance time, and reduce the operational cost associated with rapid self-clearing of faults in hybrid MMCs.

Abstract:In the new power system, a large number of new energy sources and power electronic equipment are connected to the AC power grid. When a fault occurs in the busbar area, affected by the control strategy, the amplitude and angle of the fault current are controlled, and the harmonic content is high. As a result, the operating performance of the ratio differential protection declines. The basic principle of the traditional ratio differential algorithm is introduced, and the problems of the traditional ratio differential algorithm in the new power system are analyzed. An improved algorithm for ratio differential protection for busbar protection is proposed, which is not affected by the angle difference and harmonics of the fault currents. The current phasors of each bay with different angles are mapped to the same coordinate system. Then, the differential current and restraint current are calculated. The operating performance of the improved ratio differential algorithm is analyzed under internal and external faults of the busbar area and current transformer (CT) saturation during external faults. An improved logic for the ratio differential protection is proposed. The real time digital simulation (RTDS) test results compare the operating performance of the traditional ratio differential protection and the improved ratio differential protection. It is proved that the improved ratio differential protection enhances the operating sensitivity without reducing the reliability of protection.

Abstract:Modular multilevel converter based multi-terminal direct current (MMC-MTDC) systems rely on energy-dissipating devices to handle surplus power caused by AC-side faults at the receiving-end, which suffers from poor economic efficiency and significant energy waste. To fully exploit the inherent surplus power absorption capability of MMC-MTDC systems and reduce dependence on energy-dissipating devices. A master-slave energy coordination strategy is proposed for interactive power absorption among multiple converter stations. Firstly, an MMC-MTDC control model is established, and the feasibility of surplus power absorption through energy-based control is analyzed. Subsequently, a three-dimensional energy model of the MMC is introduced to achieve decoupled energy control for each pole of the converter stations. Based on a simplified MMC-MTDC system model, active energy control schemes are designed for different types of converter stations. Furthermore, inspired by the master-slave control concept, a timing-based energy coordination logic is developed to address various AC-side fault scenarios at different receiving-end stations and two categories of surplus power levels, thereby enabling coordinated utilization of available energy margins across multiple converter stations. Finally, a MMC-MTDC system is implemented in PSCAD/EMTDC for simulation validation. Results demonstrate that the proposed strategy effectively coordinates multiple converter stations energy control without requiring energy-dissipating devices. The strategy can adapt to diverse surplus power conditions and successfully achieve fault ride-through.

Abstract:This paper addresses issues of large power fluctuations and low voltage levels in 100% renewable energy power systems without synchronous power support. The control mechanism of energy storage converters is analyzed, and a reactive power optimization model is established, considering energy storage control and DC transmission characteristics. A reactive power optimization method based on energy storage converter control is proposed. This method utilizes energy storage converter control to optimize the system's reactive power, enhancing the voltage regulation capability while participating in active power balance and ensuring economical operation. Additionally, a mathematical model based on DC channel operational characteristics is established to meet the demands for DC transmission in renewable energy systems. The optimization results can be applied to DC transmission curve planning. Finally, a case study is conducted on a county-level power grid in Northwest China without synchronous power support. The active and reactive power decision variables at each node in the grid are solved using the Yalmip platform in MATLAB, verifying the effectiveness of the proposed method.

Abstract:To address the challenges of high volatility and stochasticity in photovoltaic (PV) power series, a multi-modal adaptive PV power optimization forecasting model based on Q-Learning is proposed. The original PV power series are first decomposed into different submodalities using the variational mode decomposition algorithm optimized by the whale optimization algorithm. An integrated feature selection model is then employed to identify the most sensitive meteorological features for each submodal series. Four basic forecasting models: back propagation neural network, bidirectional long short-term memory, gated recurrent unit and temporal convolutional network, are constructed to predict the power sub-series. Given that different models exhibit varying forecasting abilities for sub-series with different frequency characteristics, Q-Learning is utilized to adaptively select the optimal combination of forecasting models for each modality. The final forecasting result is obtained by superimposing and reconstructing the forecasts of the different submodalities. The proposed model is validated using a high-resolution PV meteorological power dataset. The results demonstrate that the proposed multi-modal adaptive photovoltaic power optimization and combination forecasting based on Q-Learning achieved a 16.18% reduction in mean absolute error and a 17.00% reduction in mean squared error compared to the single model.

Abstract:Synchronous condenser is an important equipment to provide reactive power compensation and voltage support in the ultra-high voltage direct current (UHVDC) transmission system. The safety and reliability of its operation are of great significance to UHVDC projects. A fault diagnosis method based on stator branch circulating current to address the challenges of diagnosing minor stator inter-turn short-circuits (SISC) and locating fault slots is proposed in this paper. Firstly, from the perspective of a single coil in the stator branch, the influence of different positions of SISC faults on the armature magnetomotive force and stator branch circulating current is analyzed. Secondly, a field-circuit coupling model for SISC in synchronous condenser is established to simulate the fault characteristics at different positions. Finally, the variation law of the stator branch circulating current is summarized at different positions. The results of simulation and experiment show that when the number of turns of SISC stays the same, the closer the short circuit position is to the axis of the branch winding, the larger the amplitude of the stator circulating current, the further the short circuit position is from the winding axis, the smaller the amplitude of the stator branch circulating current.

Abstract:In response to the challenges of low-carbon operation in distribution networks and to fully exploit the flexible regulation potential of distributed resources, a bi-level peer-to-peer (P2P) trading model for virtual power plants (VPPs) based on integrated electricity-carbon marginal pricing is developed. In the upper level, a carbon-aware optimal power flow model based on carbon emission flow (CEF) technology is established by the distribution system operator (DSO). An integrated electricity-carbon marginal price is calculated, which is used by the DSO to coordinate low-carbon scheduling of VPPs. In the lower level, a multi-VPP coalition is formed to aggregate electric vehicles (EVs) at scale. A flexible EV scheduling mechanism guided by carbon signals is introduced. An asymmetric Nash bargaining model based on contribution degrees is constructed, where VPPs balance individual and coalition interests under price signals to determine optimal production and trading strategies. The model is solved by the adaptive-scaling alternating direction method of multipliers (AS-ADMM) to address convergence issues caused by variable coupling. Finally, simulation verification is carried out on a modified IEEE 33-bus distribution system. Case study results show that the proposed trading model reduces VPP operating costs and lowers carbon emissions of the distribution network by improving distributed energy utilization and optimizing load distribution.

Abstract:During the annual maintenance of ultra-high voltage (UHV) lines, there are arc erosion marks on the wire at the grounding clamp. In severe cases, it can lead to strand breakage, wire breakage, and even grounding clamp shedding, which is easy to cause safety accidents. However, the arc ablation characteristics of the wire at the grounding clamp and the process of strand breakage during the maintenance of UHV lines remain unclear. Therefore, based on the induced current calculation result from an actual case of arc ablation on an UHV blackout line, this paper uses the current action integral equivalent method to build a research platform for the arc ablation characteristics of the wire at the grounding clamp. The results show that the discharge arc is accompanied by a large number of heat, light and shock wave effects. The arc generates severe arc ablation along the discharge channel between the lower end of the wire and the clamp, and there are multiple arcing processes. By increasing the contact area between the wire and the grounding clamp, the local arc energy density is reduced, and the ablation degree is obviously weakened. By reducing the gap distance between the wire and the grounding clamp, the arc discharge ablation and impact are more concentrated and severe, and the degree of wire ablation damage is more significant. Under the cumulative effect of arc ablation, the test wire exhibits broken strands, and the fracture is up to 5 mm at the severe melting point, which aggravates the ablation of the adjacent strands. When the arc current action integral is the same, the cumulative effect of wire arc ablation energy under small amplitude, low frequency and long time action current is more obvious, and the damage to wire ablation is stronger. The research results provide a reference for ensuring the personal safety of maintenance staff and prolonging the operating life of transmission equipment.

Abstract:In power transformers, insulating oil deteriorates continuously due to aging, overheating, discharge, and other factors, accompanied by the production of characteristic gases such as H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, and varying degrees of diffusion within the oil. However, due to differences in gas structure and insulation systems, the diffusion characteristics of characteristic gases in gas to liquid (GTL) insulating oil are not yet clear, and the interactions among multiple gas molecules remain unclear. In order to elucidate this diffusion mechanism, this study employs molecular dynamics methods to investigate the diffusion behavior of mixed gases in stationary GTL insulating oil at the microscopic level. By comparing the diffusion coefficients, trajectories, free volume fractions, and interaction energies of single-component, binary, and multicomponent gas systems, the influence of mixed gas addition on diffusion is analyzed. The results indicate that for single-component diffusion systems, the diffusion coefficients of gases in GTL insulating oil exhibit the order: H₂ > hydrocarbon gases, and the diffusion coefficients of hydrocarbon gases are inversely proportional to molecular mass, with diffusion of different gases conforming to the "vacancy jump diffusion theory". For binary diffusion systems, the diffusion of gas molecules in mixed gas systems exhibits a synergistic effect, manifested by repulsive interactions between different gas molecules. Moreover, the addition of mixed gases reduces the interaction energy of CH₄ with GTL insulating oil by 9.21 kJ/mol and that of H₂ by 3.76 kJ/mol, respectively; the free volume fractions of H₂ and CH₄ increase by 27.5% and 113.7%, respectively, expanding gas movement space, weakening GTL's binding effect on gases, and increasing gas diffusion coefficients. Clarifying the diffusion characteristics of gases in GTL insulating oil will effectively serve the fault diagnosis of power transformers.