Search Results (185)

Accurate identification of the initial traveling wavefront is essential for two-terminal fault location in modular multilevel converter-based high-voltage direct-current (MMC-HVDC) lines. This study develops a coordinated processing chain that combines a signed-pole modal transformation, offline whale optimization algorithm (WOA) calibration of variational mode decomposition (VMD), wavefront-sensitive intrinsic mode function (IMF) pairing, wavelet synchrosqueezing transform (WSST) diagnostics, and fractional-delay cross-correlation. A common VMD parameter pair is calibrated for both terminals. Candidate IMFs are then screened using wavefront-retention and frequency-consistency constraints. The final time difference is estimated from three fixed correlation windows after band-limited resampling and local parabolic peak refinement. This procedure estimates a sub-sample delay but does not increase the measurement bandwidth. The method is evaluated on a 200 km, 500 kV MMC-HVDC simulation model. The 10 kHz validation matrix contains 36 combinations of four fault types, three fault locations, and three fault resistances. The mean absolute error is 0.374 km, 34 of 36 errors are below 1 km, and the maximum error is 0.920% of the line length. A separate 10/50/100 kHz study evaluates the complete processing chain at all three rates. At the 100 km reference location, the complete method gives a mean absolute error of 0.148 km. The corresponding values are 0.213 km without modal transformation, 11.062 km with fixed VMD parameters, and 0.414 km when CWT replaces WSST. Direct line-mode and WTMM sample-level baselines each give a three-location mean absolute error of 3.433 km. The results support the coordinated design within the tested simulation envelope; noise, synchronization, wave-speed uncertainty, and near-terminal faults require further validation. A six-case robustness matrix with 540 noise realizations gives valid-location rates of 94.4%, 92.8%, and 89.4% at 40, 30, and 20 dB, respectively, with accepted-case median errors of 0.170, 0.181, and 0.270 km. Controlled synchronization, wave-speed, and combined-uncertainty tests further quantify the applicable error envelope. Full article

The growing research interest in diode rectifier unit-based modular multilevel converter (DRU-MMC) configurations has made it imperative to analyze their potential impact on the short-circuit current characteristics of future AC grids. Most existing short-circuit current calculation approaches are oriented toward DC-side fault scenarios, while systematic research on AC-side fault analysis remains incomplete. To address this limitation, this work first illustrates the operating mechanism of the MMC. The short-circuit current at the fault point is decomposed into two independent components based on the superposition theorem. These components are: the current injected by the MMC and the current originating from the AC system. This work further explores the regulatory mechanism by which dq-axis limiting and fault-ride-through current limiting shape the MMC’s output current and derives an analytical equation for its amplitude. Furthermore, the phase correlation between the MMC-injected current and the AC system current under symmetrical fault conditions is clarified, and the computational formula for the aggregate short-circuit current is established. Then, considering the influence of transition resistance, the proposed method is verified to be applicable to both symmetrical metallic and non-metallic faults. Then, the symmetrical component method is used to analyze the sequence component of asymmetric fault short-circuit current, and a negative-sequence suppression (NSS) strategy is introduced. At the same time, considering the influence of transition resistance, the calculation formula of asymmetric metal and non-metal fault short-circuit current is derived. At the final stage of the study, a two-terminal simulation model is constructed in the PSCAD/EMTDC simulation environment. Comparative verification confirms that the results derived from theoretical calculation are in strong agreement with the simulation outcomes. The approach introduced here offers a favorable combination of simplicity and precision, rendering it highly suitable for practical engineering use. It reliably determines the short-circuit current under various fault conditions, thereby supporting fault-current analysis and the coordination of protective relays on the AC side of the receiving-end MMC in a DRU-MMC system. Full article

In islanded hydropower high voltage DC (HVDC) sending systems based on modular multilevel converters, the sending-end system is usually characterized by long electrical distance, weak system strength, and insufficient inertia support, which may easily induce oscillations between the converter station and the grid-side AC system. To address the medium- and high-frequency oscillation problem between the converter station and the AC system in islanded hydropower HVDC sending systems, this paper first establishes an MMC AC-side impedance model suitable for medium- and high-frequency oscillation analysis based on harmonic linearization. A dual-resonance-point model is adopted to equivalently represent the sending-end AC system impedance, and the impedance method is used to analyze the stability of the interconnected system. Then, by comparing the effects of three conventional feedforward additional active filtering methods on MMC impedance improvement, the limitations of conventional methods in oscillation suppression are identified. Furthermore, based on the conventional methods, a multi-link adaptive active damping strategy is proposed from the perspectives of the power loop, voltage feedforward, and phase compensation control links, which consists of power loop filtering, feedforward cascaded notch filtering, and adaptive phase compensation. Finally, an equivalent simulation model of the islanded hydropower HVDC sending system is built in PSCAD, and the effectiveness of the proposed strategy in suppressing oscillations between the converter station and the AC system is verified. Full article

The conventional fault-control strategy for fiber-optic communication failures in modular multilevel converters (MMCs) directly bypasses faulty submodules (SMs). However, once communication is restored, the bypassed SMs cannot be reintegrated, and the converter must shut down when redundancy is exhausted. This paper establishes a fault-propagation model and proposes a long-timescale non-bypass fault-tolerant control strategy that avoids unnecessary SM bypassing and improves system reliability. First, a mathematical fault-propagation model is developed for downlink command loss and uplink status-feedback interruption. The model reveals a positive correlation between modulation-index deviation and current fluctuation during downlink faults and derives the coupling mechanism between switching states and DC-voltage fluctuation during uplink faults. On this basis, a non-bypass fault-tolerant control method is developed in which a faulty SM switches to a local constant-voltage closed-loop mode. This mode preserves the SM’s electrical connection and voltage stability while enabling rapid resynchronization and recommissioning after fiber-optic communication is restored. Hardware-in-the-loop (HIL) results show that the proposed strategy limits current fluctuations to within 0.1% under transient faults and DC-voltage fluctuations to within ±1% under permanent faults while substantially improving system availability compared with the conventional bypass scheme. The proposed method provides a cost-effective and practically implementable approach to maintaining uninterrupted operation of VSC-HVDC MMCs under communication-link failures. Full article

With the rapid development of wind power generation, submarine cables have become a critical transmission channel for delivering offshore wind energy to onshore power stations. Because of the difficulty of submarine-cable maintenance, particular attention should be paid to their overvoltage characteristics during operation. Taking a ±200 kV DC submarine-cable project as the research background, this paper establishes a PSCAD/EMTDC model of a two-terminal MMC-HVDC submarine-cable system and compares the conductor-to-ground overvoltages caused by faults at different electrical locations. Among the investigated AC-side cases, single-line-to-ground faults on the valve sides of the connecting transformer and bridge-arm reactor produce relatively high cable overvoltages, with the bridge-arm-reactor valve-side fault reaching a maximum of 2.48 p.u. (496.48 kV). Among all investigated cases, the grounding fault on the valve side of the DC reactor produces the highest overvoltage, reaching 2.93 p.u. (586.18 kV) at approximately 30 km along the cable. After the initial transient associated with the DC-side grounding faults, the healthy-pole conductor-to-ground voltage remains at approximately 2.0 p.u. The results identify the relatively severe fault locations and cable sections requiring particular monitoring attention and provide a case-specific reference for DC submarine-cable monitoring and subsequent project-specific insulation-coordination studies. Full article

Embedded Modular Multilevel Converter-Based Multi-Terminal Direct Current (MMC-MTDC) systems have become an important solution for enhancing transmission capacity and operational flexibility in urban hybrid AC/DC power grids. However, the coexistence of Grid-Following (GFL) and Grid-Forming (GFM) MMC stations introduces complex dynamic interactions through both AC and DC networks. Existing small-signal stability studies often neglect MMC internal dynamics, such as submodule capacitor voltage fluctuations and circulating current-related states, or simplify the AC network as an ideal voltage source, which may lead to inaccurate stability assessment and limited control parameter optimization performance. To address these issues, this paper proposes a coordinated small-signal stability enhancement strategy for an embedded heterogeneous MMC-MTDC system considering AC/DC bilateral coupling. First, a system-level full-order small-signal state-space model is established by incorporating the internal dynamics of both GFL-MMC and GFM-MMC stations, non-ideal AC networks, and DC transmission links. Then, eigenvalue analysis and participation factor-based sensitivity evaluation are performed to identify weakly damped oscillation modes and screen the key variables and control parameters associated with dominant oscillations. Furthermore, a quadratic performance index is constructed by weighting the sensitivities of key control parameters, and particle swarm optimization is employed to obtain coordinated optimized parameters for heterogeneous MMC stations. Comparative case studies and PSCAD/EMTDC time-domain simulations verify the effectiveness of the proposed strategy under different scenarios. The quantitative active-power indices show that, compared with the unoptimized parameters, Strategy 2 reduces the settling time by 49.1% in the power step response, suppresses the power step overshoot from 6.4% to 0%, shortens the settling time by 53.8% under grid-strength variation, and reduces the active-power peak and settling time by 28.0% and 74.8%, respectively, under the fault ride-through scenario. Full article

Press-pack insulated-gate bipolar transistor (IGBT) devices are key components of modular multilevel converter (MMC)-based HVDC systems, yet the parallel connection of tens of chips inside one sealed housing makes their internal aging difficult to monitor—existing methods cannot determine which chip or thermal interface has degraded without invasive sensing. This paper proposes a two-layer aging condition monitoring framework based solely on non-invasive external case temperature measurements, comprising an online detection layer and a detection layer. For the diagnosis layer, a coupled thermal network state-space model of the press-pack module is established; its observability matrix is shown to be of full rank, proving that aging at any internal location is detectable from external case temperatures, and a steady-state sensitivity analysis reduces the required measurement to the module-center case-temperature pair and yields a failure-signature rule that discriminates five aging modes. A 3D electrothermal finite-element model, validated against fiber Bragg grating (FBG) measurements on an MMC sub-module with deviations below 1.2 °C at all load levels, is then used to build an aging fingerprint database of 270 scenarios that maps a measured case-temperature distribution to the aging location, the affected component, and the severity of any cooling-system degradation. For the detection layer, a deep neural network (DNN) trained only on healthy-state data provides online early warnings through its prediction residual. Experiments on a 2.2 kV/2100 A MMC sub-module platform and a solid-state DC–DC transformer platform confirm that cooling-system degradation is reliably detected and that the detection layer transfers across converter types. Even in the least sensitive aging case, the predicted case-temperature signature (0.036 °C) exceeds the 0.01 °C resolution of standard thermocouple instrumentation, which indicates that genuine device aging would be resolvable as well. The framework provides a non-invasive online health-monitoring solution that can also locate the degraded element in high-power press-pack devices. Full article

To meet the demand for large-capacity and long-distance transmission of deep-sea offshore wind power and coordinated export of multiple energy sources in coastal clean-energy bases, this paper investigates the topology configuration and operating characteristics of a multi-energy DC collection system for offshore wind power integration. Based on the characteristics of offshore wind power, nuclear power, onshore photovoltaic generation and pumped storage, a source-type–converter-topology–control-function matching relationship is established. A ±800 kV true-bipolar DC large-bus system is then constructed, in which the four sources are configured as a lightweight offshore wind export branch, a stable power-export branch, a fast controllable renewable-energy branch and a system regulation resource, respectively. A polarity-interface conversion link is introduced to match the local offshore wind export structure with the main true-bipolar DC system, and the power-balance relationship among sending-end injection, receiving-end absorption and DC-bus voltage is formulated. PSCAD/EMTDC simulations are performed under steady-state operation, wind-speed step disturbance, a sending-end AC three-phase metallic grounding fault and a submarine-cable pole-to-ground fault. The results confirm that the proposed differentiated topology can support multi-energy collection, true-bipolar voltage coordination and continuous stable operation under typical disturbances. Full article

Frequency stability is becoming an increasingly critical issue in asynchronously interconnected (ASI) power systems, where DC interconnections inherently decouple frequency dynamics between asynchronous AC grids and conventional synchronous generators are progressively displaced by renewable energy resources with limited frequency-support capability. To address this issue, this paper proposes a primary frequency control strategy for a hybrid High Voltage Direct Current (HVDC) system composed of a line-commutated converter (LCC) rectifier and a modular multilevel converter (MMC) inverter. In the proposed scheme, the MMC adopts a dual-port grid-forming (GFM) control mode, thereby establishing voltage-source characteristics at both its AC and DC ports, while the LCC rectifier regulates the DC current. Through the interaction between the DC-voltage-forming MMC and the DC-current-regulating LCC, bilateral primary frequency support can be achieved for the two asynchronous AC systems without requiring frequency-signal communication between the converters. Furthermore, a frequency-response model is developed to characterize the frequency-coupling dynamics of the ASI system and to quantify the frequency-support capability enabled by the proposed control. Based on this model, the parameterization of the proposed frequency control is further discussed. Finally, real-time digital simulation studies are carried out on an RTDS platform to validate the effectiveness of the proposed control strategy and the accuracy of the developed analytical model. Full article

Against the backdrop of the transition toward power systems with high shares of renewable energy and power electronics and the rapid growth of urban load, large receiving-end urban grids are fed by multiple line-commutated-converter HVDC (LCC-HVDC) links, so that their maximum receiving capability is frequently limited by the static-voltage-stability margin. To assess the receiving capability of such large urban grids, this paper proposes a method for evaluating the maximum receiving capability of a receiving-end urban grid that incorporates a Modular-Multilevel-Converter-based multi-terminal embedded DC (MMC-MTEDC) system. First, a quasi-steady-state model of the receiving-end urban grid with LCC infeed and an embedded MMC-MTEDC system, in which the DC-network equations characterize the mutual coupling among the AC active-power injections of the receiving-end converter stations, is established. Second, an augmented extended Jacobian that incorporates the MMC control equations and the DC power-flow equations is constructed; its minimum singular value is adopted as the static-voltage-stability index, and the corresponding sensitivities are derived to reveal the mechanism by which the receiving capability is formed. On this basis, a unit-commitment optimization model that centers on the stability-margin constraint and accounts for the converter-capability curve, the bus-voltage limits, and the line-loading limits is built; the model is solved iteratively by a column-and-constraint-generation (CCG) method, and the feasibility of the unit commitment is used to estimate the maximum receiving capability. A modified IEEE 39-bus system is used as a case study, which quantitatively verifies the effectiveness of the MMC-MTEDC in enhancing the receiving capability of the receiving-end urban grid. Full article

China’s offshore wind power is rapidly developing towards the direction of “deep-water and far-shore, large-scale, and clustered”. Existing offshore wind power transmission schemes based on the MMC are technologically mature but highly expensive. Although transmission schemes based on the DRU possess economic advantages, they lack AC voltage support and reverse power flow capability. To combine the control performance of the MMC and the economic advantages of the DRU, this paper proposes a heterogeneous DC transmission system for offshore wind power based on the parallel operation of MMC-HVDC and DRU-HVDC, which can realize the clustered transmission of deep-water and far-shore wind power. First, the configuration scheme of the system is introduced, and the basic control strategy is proposed. Secondly, the small-signal model of the system is established, and the small-signal stability analysis is conducted. Then, the control strategies for the system under near-zero power conditions and AC/DC faults are proposed, respectively. Finally, the effectiveness of the proposed topology and control strategies is verified through PSCAD electromagnetic transient simulations. Full article

Facing the demand of long-distance, high-voltage and high-power transmission, the research on MMC-UHVDC (Modular Multilevel Converter based Ultra High Voltage Direct Current) has become a hot issue. This paper focuses on the ±800 kV/8 GW UHVDC transmission system to conduct simulation modelling and insulation coordination studies. First, broadband models of the main circuit and primary equipment are established to simulate and analyse the distribution characteristics of both switching and lightning transient overvoltages under typical faults. Second, based on the overvoltage severity at critical nodes, two surge arrester configuration schemes with distinct internal valve protection topologies are proposed. Finally, an Improved Fuzzy Analytic Hierarchy Process (FAHP) is introduced to perform a quantitative techno-economic evaluation of the comparative schemes. The results demonstrate that the optimised configuration successfully suppresses extreme overvoltages at vulnerable sub-module nodes, maintaining adequate insulation margins. These research findings provide a highly reliable mathematical framework and engineering reference for the safe design of UHVDC systems. Full article

The series-connected chain MMC with DC-blocking capacitor (C-SCMMC) is an emerging topology for HVDC tapping applications with high voltage and relatively low power capacity. However, the DC-blocking capacitor can be bulky and costly, which deteriorates its economy and flexibility. This paper investigates the feasibility of DC-blocking capacitor reduction, with special emphasis on the characteristics, control, and parameter design. The principle of C-SCMMC considering the DC-blocking capacitor dynamics is firstly modeled and analyzed, and the ripples and harmonics, as well as their influences on the external performance of the converter, are analyzed; then, improved control strategies, namely, the grid-tied harmonics suppression and current control, are proposed considering the enlarged DC-blocking capacitor voltage ripple; after that, the influence of reduced DC-blocking capacitor on the operation range and parameter design are analyzed, and the economic advantage is demonstrated via parameter design and comparison based on a typical bench mark. Analysis shows that the DC-blocking capacitor voltage ripple is coupled with other parameters, such as the arm output voltage and SM capacitance, and the advised range is from 0.1 to 0.3 p.u. at unity power factor to reduce the overall cost. In a typical design, the C-SCMMC can reduce the number of SMs by 2/3 and the capacitor energy storage capacity by 15% compared to the conventional MMC. Finally, simulation results obtained in MATLAB/Simulink 2024b are provided to verify the feasibility of the proposed converter and the correctness of the parameter design. Full article

Multi-terminal voltage-source-converter-based HVDC (VSC-MTDC) systems are increasingly used to integrate renewable energy and interconnect asynchronous AC grids, but conventional fixed-coefficient droop control cannot simultaneously limit DC-voltage deviations, reduce operating losses, and preserve converter power margins during operating-point switching. This paper hypothesizes that a rule-based fuzzy adjustment of the droop slope can provide smooth multi-objective coordination without inter-station communication. A dual Mamdani fuzzy controller is developed: one controller adjusts the weighting between loss-oriented and power-margin-oriented droop coefficients according to converter power margin, while the other introduces a voltage-deviation correction according to DC-bus voltage. The controller is implemented and verified in a five-terminal MMC-based VSC-MTDC model built in PSCAD/EMTDC, where simulation data are generated under heavy-load, light-load, and power-reference switching scenarios using specified line and converter parameters. Compared with conventional droop control, the proposed strategy improves power-margin utilization, reduces operating-point discontinuities, and raises the minimum DC voltage from 370.2 kV to 381.4 kV in the severe switching case. The results confirm that fuzzy-slope droop control can achieve smoother operating-point switching and better coordinated optimization among voltage stability, operating loss, and converter reserve margin. Full article

In the context of the new power system, modular multilevel converter high-voltage direct current (MMC-HVDC) has become a key technical solution for the large-scale grid integration of wind power. However, when a fault occurs in the AC grid at the system receiving end, the high-voltage direct current (HVDC) system faces challenges such as wind power redundancy, DC overvoltage, and equipment overcurrent. To address this, this paper proposes an energy storage-coordinated fault ride-through (FRT) control strategy suitable for different fault scenarios. The strategy optimizes the allocation of energy storage capacity according to the state of charge (SOC) of the energy storage units (ESUs), preventing individual ESUs from prematurely shutting down and reducing energy dissipation. Finally, a comparison with a conventional DC dissipation resistor scheme on the PSCAD/EMTDC platform demonstrates that the proposed strategy provides smoother power regulation characteristics and smaller DC voltage fluctuations, thereby enhancing the economic efficiency and reliability of system operation. Full article