Search Results (564)

The existing basic control system of fractional-order modular multilevel converters (FO-MMCs) is mainly designed for ideal operating conditions and exhibits limited adaptability under grid unbalanced conditions. To address this issue, this paper proposes a fractional-order composite control system for a FO-MMC. The proposed system consists of fractional-order positive-sequence and negative-sequence control loops. The positive-sequence control loop includes a power outer-loop fractional-order proportional-integral (FOPI) controller and a fractional-order decoupled FOPI positive-sequence current inner-loop controller. The negative-sequence control loop adopts a fractional-order decoupled FOPI negative-sequence current inner-loop controller. Combined with three sequence current reference calculation strategies, the proposed system achieves three control objectives: active power oscillation elimination (APOE), reactive power oscillation elimination (RPOE), and balanced positive-sequence current output (BPSC). A FO-MMC simulation model was established based on the MATLAB/Simulink platform, and the proposed control system was verified. The results demonstrate that the proposed control system achieves accurate regulation of positive- and negative-sequence currents. Under different control objectives, the system exhibits excellent dynamic response and steady-state performance, providing a theoretical basis for stable FO-MMC operation under unbalanced grid conditions. Full article

With the integration of renewable energy and power-electronic devices, grid-forming modular multilevel converters (GFM-MMCs) play a critical role in active grid support. An AC grid voltage sag can trigger a large support current, which may cause large voltage fluctuations in submodule capacitors and arm overmodulation, thereby threatening system safety. This paper proposes a multidimensional collaborative method to improve the support capability of grid-forming MMCs under severe grid voltage sags. The multidimensional physical constraints of internal energy fluctuation during fault transients are clarified. The corresponding safe operating boundaries are then established, after which a coordinated optimization strategy is developed. This approach integrates second-harmonic circulating current and zero-sequence voltage injections. Offline optimization utilizes a particle swarm optimization (PSO) algorithm across the full operating range. Expanding the safe P–Q operating region requires no extra hardware costs. A neural network enables a millisecond-level direct mapping control architecture. This architecture addresses the long online computation time of traditional heuristic algorithms by embedding offline optimization data into the network weights. The trained network performs rapid forward computation to generate optimized commands, which is verified by a hardware-in-the-loop (HIL) experiment. The experimental results verify the effectiveness of the proposed method, with clear performance improvements being observed. The strategy suppresses capacitor-voltage peak and prevents overmodulation. This directly improves the MMC support capability during severe faults. Full article

Energy-storage-integrated renewable energy stations contain both grid-following and grid-forming converters, which makes their wideband impedance behaviour difficult to predict. This paper studies a field-oriented harmonic impedance measurement method for such stations. A three-phase cascaded H-bridge converter is used to develop a harmonic impedance measurement device connected to the 35 kV bus. For the measured system, a generalized Norton equivalent model is established for the grid-following part, while a generalized Thevenin equivalent model is derived for the grid-forming energy-storage part. A frequency-domain multi-objective target-oriented decoupled control strategy is then designed for the measurement device. The strategy assigns fundamental power synchronisation, harmonic disturbance injection, and submodule capacitor voltage balancing to different frequency components. This design prevents the fundamental current loop from suppressing the injected harmonic current. Simulation studies are carried out under four conditions, including no harmonic injection and 5th-, 25th-, and 99th-order harmonic injections. The baseline case shows no clear commanded-frequency current component. Under harmonic commands, the device injects the corresponding current components while maintaining stable multilevel bridge-port voltage. The device also demonstrates robust impedance-identification performance under grid background harmonics, measurement noise, and equivalent-impedance variations. The results indicate that the proposed device has application capability for wideband impedance measurement and stability analysis in energy-storage-integrated grid-connected systems. 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

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission systems possess distinctive advantages such as massive transmission capacity and flexible control, making them increasingly popular for offshore wind power integration. However, offshore wind power via a MMC-HVDC transmission (OWPMMC-HVDC) system is highly susceptible to wideband oscillations caused by dynamic control interactions. Although traditional strategies can effectively mitigate oscillations within a specific frequency band, they frequently induce new oscillations in non-target bands—a phenomenon defined herein as the negative effect. To clarify the negative effect mechanism, this study first analyzes the impedance characteristics of the OWPMMC-HVDC system under traditional active and passive mitigation strategies. To reveal the damping drift characteristics across the full-frequency band, a multi-strategy coordinated oscillation mitigation approach based on the damping complementary principle is proposed. Finally, the effectiveness and superiority of the proposed coordinated oscillation mitigation strategy are verified through PSCAD/EMTDC electromagnetic transient simulations based on the 1100 MW/± 400 kV OWPMMC-HVDC system. The analysis results demonstrate that compared with the single-strategy approach, the proposed multi-strategy coordinated control successfully solves the negative effect issue. This comprehensively enhances wideband oscillation damping and the stable operation of the system, thus providing a robust oscillation mitigation strategy for practical engineering applications. 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

The transition toward electric vehicle (EV) architectures exceeding 800 V offers key advantages, including shorter charging times, lower operating currents, and reduced system weight due to smaller conductor cross-sections, all of which enhance overall vehicle performance. However, identifying suitable traction inverter topologies that meet automotive requirements for efficiency, electromagnetic interference (EMI), and reliability remains critical. This study presents a simulation-based converter-level electro-thermal and conducted-EMI benchmark of 2-Level H-Bridge, 3-Level Active Neutral-Point Clamped (ANPC), and 3-Level T-Type inverters under identical output-power operating conditions. The distinguishing feature of this work is the unified evaluation of these topologies under a common external thermal boundary, enabling a consistent comparison of semiconductor losses, junction-temperature behaviour, cooling-burden indicators, conducted-EMI tendencies, and first-order lifetime-oriented thermal indicators. Within this framework, the required effective thermal resistance is used as a cooling-burden indicator, while junction-temperature swing and mean junction temperature are used as relative thermal-stress indicators. Under the considered simplified R–L loading conditions, the results show that multilevel topologies reduce semiconductor losses, peak junction temperature, conducted-EMI excitation, and relative thermal-stress indicators compared with the 2L H-Bridge. These findings are interpreted as comparative topology-level trends under the defined converter-level simulation framework rather than as final vehicle-level EMI compliance or power-module lifetime predictions. 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

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

To address the current issues of single-objective control and the inability to achieve coordinated operation between the Modular Multilevel Converter (MMC) and the thyristor rectifier in parallel-type ice melting systems, this paper proposes a master-slave coordinated control method for such systems. The MMC operates in constant PQ mode, while the thyristor rectifier operates in constant-voltage mode. The thyristor rectifier provides voltage support to the MMC, and an adaptive U-I droop control is introduced in the control loop to cope with voltage drop. The MMC adjusts its active power output according to the operating condition of the thyristor rectifier, enabling coordinated ice melting operation, while simultaneously performing reactive power compensation and active filtering for the system. This paper elucidates the operating principle of the ice melting system, analyzes its harmonic characteristics, designs a control strategy, and constructs a PSCAD 5.0 simulation model to validate the proposed control strategy. The results demonstrate that the proposed strategy can coordinate the active power allocation between the MMC and the thyristor rectifier, laying the foundation for the safe and stable operation of the ice melting system while suppressing the harmonic currents and reactive power introduced into the power grid by the thyristor ice melting system. Full article

This paper systematically analyses the fault characteristics and investigates fault ride-through (FRT) control strategies for a low-frequency (LF) transmission system in offshore wind power based on Modular Multilevel Matrix Converter (M3C). The study addresses transient issues of power imbalance, submodule capacitor overvoltage, and bridge-arm overcurrent arising from three-phase ground faults on both the industrial-frequency (IF) and LF sides. The underlying mechanisms of power surplus and submodule capacitor overvoltage, induced by decoupling control and current-limiting protection during IF-side faults, are examined in detail, along with the transient characteristics of bridge-arm currents under voltage sags on the LF side. Two innovative control strategies are proposed to enhance system resilience: (1) For IF-side faults, a controllable energy dissipation device on the LF side achieves precise dissipation of surplus power via real-time monitoring of the average submodule capacitor voltage. (2) For LF-side faults, the FRT strategy based on dynamic adjustment of the LF modulation voltage rapidly reduces the reference to 0.1 p.u. and restores it linearly at a predefined rate, thereby enabling fault information transmission and wind turbine derating. The effectiveness and feasibility of the proposed scheme are verified through simulations on a 1000 MW system model. 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

Low-frequency alternating current (LFAC) transmission has attracted increasing attention for medium- and long-distance offshore wind power transmission, as this application scenario is typically characterized by long transmission distance and large installed capacity. Converting offshore low-frequency alternating current into onshore power-frequency alternating current requires a dedicated frequency conversion device. Compared with power–electronic converter-based schemes represented by the modular multilevel matrix converter (M3C), grid connection via a motor–generator pair (M-G) enables the renewable energy port to retain intrinsic synchronous-machine characteristics, including inertial support, voltage support, and fault isolation. This paper elaborates the operating principles and mathematical models of the two types of frequency conversion solution for LFAC transmission systems, and systematically analyzes the frequency support, voltage support, and fault-isolation capabilities of the M-G scheme. Simulation results demonstrate that under a sudden increase in onshore active power load, the M-G system can provide strong frequency support by releasing rotor kinetic energy, and a larger inertia time constant mitigates the frequency drop more effectively. Under a sudden increase in onshore reactive power load, the M-G scheme offers a greater reactive power margin benefiting from its strong short-term overcurrent capability. Moreover, increasing the excitation gain on the motor side and installing shunt reactors at both ends of the submarine cable can effectively improve the voltage profile along the cable. Full article

Maintaining a constant frequency is vital for grid stability and reliability, especially during dynamic changes in load and generation, which are caused by the increasing incorporation of renewable intermittent energy sources such as solar and wind power. These energy sources decrease the system’s inertia, which compromises the primary frequency regulation, a process historically sustained by the speed regulators of conventional synchronous generators. In this study, to mitigate this issue, we investigate a battery energy storage system (BESS) operating with virtual synchronous machine (VSM) control to provide ancillary services of primary frequency control. A multilevel cascade H-bridge static converter with eleven levels is controlled to emulate the dynamic behavior of a conventional synchronous machine, allowing primary frequency control support. The case studies are evaluated using Matlab/Simulink R2024a software and tested under contingency scenarios involving load rejection and step-load insertion within an isolated power grid comprising other synchronous machines, alongside an analysis of the BESS-controlled power dispatch. Our simulation results demonstrate that the energy storage system, operating under a virtual synchronous machine (VSM) strategy, effectively emulates the dynamic behavior of a conventional synchronous generator, enabling controlled active and reactive power dispatch. Furthermore, the proposed control strategy provides virtual inertia support, mitigating the Rate of Change of Frequency (RoCoF) following disturbances, improves the damping of frequency oscillations, and ensures a smoother frequency recovery after load variations. These findings indicate that the proposed BESS can provide effective primary frequency control support in power systems characterized by a high penetration of converter-interfaced renewable energy sources. Nonetheless, further investigations into the influence of VSM parameters on the system’s dynamic response are needed to further optimize the performance of the proposed solution. Full article