Modular Multilevel Converter Based High Voltage Direct Current Research Articles

Parallel multi‐rate simulation scheme for modular multilevel converter‐based high‐voltage direct current with accurate simulation of high‐frequency characteristics and field programmable gate array‐based implementation

AbstractThe real‐time simulation of the modular multilevel converter‐based high‐voltage direct current (MMC‐HVDC) transmission system has become a popular research topic. However, in order to meet the real‐time performance, the real‐time simulation technology will cause additional simulation errors for MMC‐HVDC, especially on its frequency characteristics. Therefore, a parallel multi‐rate simulation scheme for MMC‐HVDC is developed in this work to ensure accurate simulation of high‐frequency characteristics. Firstly, a non‐error method based on converter transformer decoupling is proposed to decouple the converter and alternating current system; direct current transmission line decoupling and arm decoupling methods are used to achieve decoupling among and within converters. A multi‐rate data synchronous mechanism is established by considering the differences among high‐frequency characteristics caused by delayed data interaction. Secondly, the computing architectures of the primary system solver and modular multilevel converter controller are designed based on a field programmable gate array (FPGA). The real‐time simulation platform for a four‐terminal true bipolar MMC‐HVDC is constructed based on the FPGA array. Thirdly, the factors in multi‐rate simulation affecting the simulation accuracy of high‐frequency characteristics are analysed. The simulator is shown to be accurate in steady and dynamic states. The authors also verify its applicability for further research on high‐frequency resonance based on control experiments.

Stable operating limits and improvement methods for hydropower and photovoltaic integration through MMC-HVDC systems

This paper addresses the critical need to determine the stable operating limit of modular multilevel converter-based high voltage direct current (MMC-HVDC) systems, particularly concerning the integration of extensive renewable energy sources. To achieve this, the steady-state mathematical model and state-space model of bundled hydropower and photovoltaic integration through MMC-HVDC systems are established. A novel methodology considering steady-state and small-signal stability constraints is proposed to compute the stable operating region of the system. The quantitative assessment reveals that diminishing AC system short-circuit capacities amplify restrictions from small-signal stability constraints, thereby reducing the system's stable operating region. Eigenvalue and participation factor analyses shed light on the pivotal factors affecting small-signal stability in weak AC systems. To expand the system's stable operating region, a supplementary frequency damping control strategy is proposed. The theoretical analysis and calculation results are validated by building a simulation model for the bundled hydropower and photovoltaic integration through MMC-HVDC systems in PSCAD/EMTDC.

Network based impedance analysis of grid forming based MMC-HVDC with wind farm integration

Grid forming control for modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system is favorable to support the AC systems. This paper presents the hybrid AC-DC impedance model for grid forming based MMC-HVDC with wind farm integration. Comparisons of the hybrid AC-DC impedance with the traditional AC/DC impedance are presented, and the hybrid AC-DC impedance does not contain right half-plane poles caused by DC/AC coupling. Then, the network based impedance analysis is carried out to analyze the stability of grid-forming based MMC-HVDC with wind farm integration. It is found that grid forming based MMC-HVDC may lack damping with two modes of oscillations. Large inertia, DC/AC virtual impedance and wind farm integration can dampen the first mode oscillation, while only AC virtual impedance is effective for the second mode oscillation. The stability comparison of grid forming and grid following based wind farm integration is also presented. Moreover, MMC-HVDC with grid forming control cannot improve the stability of grid following based wind farm integration in weak grid where grid forming based wind farm performs better. Therefore, both wind farm and MMC-HVDC with grid forming control are better in weak grid. Simulation results in PSCAD/EMTDC validate the corresponding analysis.

Optimal Energy Utilization of MMC-HVDC System Integrating Offshore Wind Farms for Onshore Weak Grid Inertia Support

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems have the potential to support grid frequency by utilizing the electrostatic energy stored in sub-module capacitors. However, existing energy control schemes cannot effectively use the MMC capacitor energy to provide sufficient inertia support. Additionally, those control schemes are based on grid following control, suffering from stability issues when the AC grid short circuit ratio (SCR) decreases. Thus, a grid-forming control scheme capable of active energy management for MMC-HVDC systems integrating offshore wind farms (OWFs) is developed, achieving flexible energy regulation and stable operation under weak grid conditions. To improve inertia support performance, an optimal two-stage energy utilization method is further proposed to overcome the drawbacks of existing grid-forming methods. Finally, comparative simulations are performed in a 2-area 4-machine weak grid to verify the effectiveness and advantages of the proposed method.

Characteristic Investigation and Overvoltage Suppression of MMC-HVDC Integrated Offshore Wind Farms Under Onshore Valve-Side SPG Fault

The valve-side ground fault in the converter station of modular multilevel converter based high voltage direct current (MMC-HVDC) is a serious threat to the security of power systems. Hence, this paper analyzes the most common single-phase-to-ground (SPG) fault characteristics of valve-side faults in onshore converter stations of MMC-HVDC integrated offshore wind farms. The propagation and impact mechanism of onshore SPG fault to offshore HVDC transmission lines, offshore converter station, and wind farms are deduced based on the MMC control principle. Furthermore, an overvoltage suppression strategy based on zero-sequence voltage modulation is proposed. The effect mechanism of this strategy on the transient stability of the system is analyzed. The key parameter of the proposed strategy is also designed considering both the practical engineering requirements and the impact of the system operation. Finally, the theoretical analysis of fault characteristics and the proposed overvoltage suppression strategy are verified by the PSCAD/EMTDC simulation platform. Simulation results not only show good agreement with the theoretical analysis of the fault characteristics, but also prove the effectiveness of the proposed overvoltage suppression strategy.

Dual Grid-Forming Control With Energy Regulation Capability of MMC-HVDC System Integrating Offshore Wind Farms and Weak Grids

To integrate weak onshore grids, some grid forming (GFM) control schemes taking advantage of DC voltage synchronization have been proposed for modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems. In these control schemes, the sub-module (SM) capacitor energy is only used for grid synchronization. Since the SM capacitor is an energy storage device, it has the potential to actively regulate the stored energy. To fully exploit the energy management capability of MMC, a dual grid forming control scheme with energy regulation capability is presented in this paper. The proposed control scheme enables grid forming for both onshore and offshore MMC stations. Meanwhile, the capacitor energy of MMCs can be flexibly regulated to provide frequency support for the onshore grid. To verify the effectiveness of the proposed control scheme, case studies are first carried out in a point-to-point offshore wind farms (OWFs) integrated HVDC system. In addition, the frequency support performance is evaluated in a 3-area 4-machine grid integrated with two OWFs-HVDC systems. Finally, a single-terminal three-phase MMC prototype is developed to test the voltage decoupling and energy management capabilities of the proposed control strategy.

Small-Signal Stability Analysis and MOSMA-Based Optimization Control Strategy of OWF with MMC-HVDC Grid Connection.

The recent oscillation events in offshore wind farms (OWFs) connected via a modular multilevel-converter-based HVDC (MMC-HVDC) system are developing towards a wider frequency band, which causes complex a small-signal interaction phenomenon and difficulties in the stability analysis and control. In this paper, the wideband dynamic interaction mechanism is investigated based on the impedance analysis method and an improved control strategy using an optimization algorithm is proposed to improve the small-signal stability and reduce the oscillation risks. First, the detailed impedance models of the grid-connected system are established considering the distribution characteristics of the submarine cable, control delay and frequency coupling effect. Then, combined with the active damping control method, the wideband resonance mechanism is analyzed, and the stability constraints of controller parameters are obtained using the impedance stability criterion. Finally, an improved multi-objective slime mold algorithm (MOSMA)-based coordinated optimization control strategy is proposed to enhance the adaptability of the controller parameters and the wideband damping ability of a grid-connected system, which can improve the wideband stability of the system. The simulation and experimental results verify the proposed control strategy.

An Enhanced AC Fault Ride through Scheme for Offshore Wind-Based MMC-HVDC System

This study presents an improved, communication-free Fault Ride-Through (FRT) strategy for type-3 and type-4 wind turbine integrated modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems in offshore wind power plants (OWPPs). The research aims to enhance the reliability and resilience of OWPPs by ensuring their connection with AC grids remains intact during and after faults. Simulation results conducted on a 580 kV, 850 MW MMC-HVDC system using PSCAD/EMTDC software v.4.6.2 demonstrate quick post-fault recovery operation and the ability to effectively manage DC link and capacitor voltages within safe limits. Furthermore, the circulating current (CC) and capacitor voltage ripple (CVR) remain within acceptable limits, ensuring safe and reliable operation. The study’s major conclusion is that the proposed FRT strategy effectively mitigates the adverse effects of short circuit faults, such as a rapid rise in DC-link voltage, on the performance of the MMC-HVDC system. By promptly suppressing DC-link overvoltage, the proposed FRT scheme prevents compromising the safe operation of various power electronics equipment. These findings highlight the significance of FRT capability in OWPPs and emphasize the practical applicability of the proposed strategy in enhancing the reliability of offshore wind power generation.

Multi-Stage Sequential Network Energy Control for offshore AC asymmetric fault ride-through of MMC-HVDC system integrated offshore wind farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) is the preferred solution to the problem of long-distance transmission from large-scale offshore wind farms (OWFs). When an offshore AC asymmetric fault occurs, OWFs and MMC-HVDC will face the challenge of fault ride-through with power electronic converters on both sides of the fault location. In this paper, a Multi-stage Sequential Network Energy Control (MSNEC) is proposed for offshore AC asymmetric fault ride-through (FRT). Firstly, the constraint relationship among the sequence networks under AC asymmetric faults is revealed. Secondly, the energy control potential of WFMMC is fully explored from the perspective of energy transformation and transfer. Energy control reduces the fault phase current amplitude by putting in submodules at the fault phase. Additional sub-modules are put in the non-faulted phase to realize series voltage dividing effect to actively prevent non-faulted phase fault overvoltage. The MSNEC considers the FRT requirements for multiple stages of fault occurrence, fault steady state, and fault recovery. It combines sequential network control, energy control, and voltage support of the converter station. It realizes AC asymmetric FRT from four aspects: power equivalence, negative sequence suppression, energy control, and voltage stabilization. Finally, a simulation model of OWFs connected to the onshore grid via MMC-HVDC is established in PSCAD/EMTDC. Simulation results verify MSNEC not only improves the AC asymmetric FRT capability of the OWFs and WFMMC, but also promotes the efficient consumption of renewable energy and improves the economics of the OWFs grid-connected system.

Active Energy Control for Enhancing AC Fault Ride-Through Capability of MMC-HVDC Connected With Offshore Wind Farms

Modular Multilevel Converter-based High Voltage Direct Current (MMC-HVDC) connected with offshore wind farms could suffer serious challenge: surplus power of MMC-HVDC caused by AC fault in onshore power grid. This paper proposes an active energy control (AEC) scheme for converter stations utilizing MMC sub-module capacitance. It can be used to actively absorb the surplus power of the MMC-HVDC in the stage of grid-side AC fault and enhance the AC fault ride-through (ACFRT) ability. First, the energy decoupling principle of MMC is analyzed, and a steady-state control scheme is proposed for MMC-HVDC with energy decoupling capability. Based on this strategy, an AEC scheme between onshore and offshore converter stations is proposed for ACFRT. It is divided into the following four stages: passive energy recovery, active energy recovery, active energy maintenance, and active energy release. Then, a set of coordinated control schemes for AEC and active power reduction of offshore wind farms are designed. This is used as an example to illustrate that the proposed AEC can effectively enhance the fault ride-through ability of the existing strategies, such as active power reduction of offshore wind farms. Finally, the correctness and effectiveness of the proposed control schemes are verified via the PSCAD/EMTDC platform.

Semi‐hot redundant control method for modular multilevel converter‐based high‐voltage direct current systems under submodules fault

AbstractA modular multilevel converter‐based high voltage direct current (MMC‐HVDC) system involves a large number of cascading submodules (SMs). The failure of SMs may critically degrade the system performance. Therefore, several redundant SMs can be incorporated and reasonably controlled to ensure the system reliable and continuous operation even if certain SMs fail. This paper proposes a semi‐hot redundant method for MMC‐HVDC systems. In the proposed strategy, when the system operates normally, the redundant SMs are controlled in a lower voltage range. These SMs do not require frequent charging and discharging, which decreases the switching loss. When ordinary SMs fail, the faulty SMs are bypassed and the corresponding number of redundant SMs can be promptly recharged to the rated voltage to replace the faulty SMs. The semi‐hot reserved SMs can shorten the transient process of faulty SMs replacement, and in this process, the absence of faulty SMs does not considerably influence the system. Thus, the original control and modulation strategies of the system do not need to be modified when several SMs fail. The performance of the proposed method is compared with those of the existing methods, and its validity is demonstrated through a small‐scale experimental prototype.

Internal Energy Based Grid-Forming Control for MMC-HVDC Systems With Wind Farm Integration

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.

Fault Ride-through Hybrid Controller for MMC-HVDC Transmission System via Switching Control Units Based on Bang-bang Funnel Controller

This paper proposes a fault ride-through hybrid controller (FRTHC) for modular multi-level converter based high-voltage direct current (MMC-HVDC) transmission systems. The FRTHC comprises four loops of cascading switching control units (SCUs). Each SCU switches between a bang-bang funnel controller (BBFC) and proportional-integral (PI) control loop according to a state-dependent switching law. The BBFC can utilize the full control capability of each control loop using three-value control signals with the maximum available magnitude. A state-dependent switching law is designed for each SCU to guarantee its structural stability. Simulation studies are conducted to verify the superior fault ride-through capability of the MMC-HVDC transmission system controlled by FRTHC, in comparison to that controlled by a vector controller (VC) and a VC with DC voltage droop control (VDRC).

A multi-level coordinated DC overcurrent suppression scheme for symmetrical bipolar MMC-HVDC integrated offshore wind farms

This paper proposes a multi-level coordinated control (MLCC) scheme for symmetrical bipolar modular multilevel converter based high voltage direct current (MMC-HVDC) integrated offshore wind farms. MLCC makes full use of the energy control capability of MMCs to effectively suppress DC overcurrent triggered by submarine cable breakage. This MLCC scheme consists of four control levels. The first level is for the offshore fault pole MMC. Once a DC submarine cable is broken, the fault pole MMC of the offshore converter station passively absorbs power from wind farms using the storage capacity of internal sub-module capacitors. This buys time for the rest of the control to start. The second level is for the onshore non-faulty pole MMC, which can actively raise the DC voltage to suppress DC overcurrent. The third level is for offshore non-fault pole MMC, which balances the power delivered to the DC system in real-time based on the power received from wind farms. The fourth level is for offshore wind farms, which reduce power to restore the non-fault pole submarine cable transmission power and current to the allowed state. The correctness and effectiveness of the proposed MLCC in different scenarios are verified using the PSCAD/EMTDC.

A practical impedance modeling method of MMC-HVDC transmission system for medium- and high-frequency resonance analysis

The modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission system can arouse resonance that threatens the safety and stability of the power grid. In this paper, the impedance model and resonance mechanism of an MMC-HVDC transmission system are investigated at medium and high frequencies. First, an MMC-HVDC transmission system equivalent circuit model is proposed based on the partitioning impedance modeling method. Second, the concept of normalized sensitivity index is proposed to quantify the influential effects of crucial factors on the MMC equivalent impedance, including the control gains, control link delay, and circuit parameters; subsequently, the influencing factors are graded, and reduced-order impedance models are proposed for different situations. Then, the resonances in the MMC AC- and DC-side systems are analyzed considering both active and passive resonance, i.e., with or without a harmonic source, to systematically reveal the resonance mechanism of the MMC-HVDC system. Finally, a two-terminal MMC-HVDC simulation model is built to verify the proposed model and resonance analysis.

Analysis and research on the influence of multiple factors on the steady operation area of large-capacity MMC-HVDC transmission system

Aiming at the MMC-HVDC (modular multilevel converter based high voltage direct current), this paper studies the influence of multiple factors such as the AC system and converter characteristic limits on the steady-state operating range of the MMC-HVDC transmission system. First, a mathematical model based on the steady-state operation of the MMC-HVDC transmission system is established, and the AC-side power quality requirements and stability limiting factors of the MMC-HVDC system are analyzed and summarized, as well as the constraints of multiple limiting factors on the converter side. Based on this, the power operation range of MMC-HVDC transmission project under different conditions is obtained. Finally, combined with the actual project, the effects of modulation ratio, equivalent reactance between the AC outlets of the converter and AC system equivalent impedance on the steady-state power operating range are analyzed and studied separately.

The Virtual Inertia Control of MMC-HVDC Considering the Effect of Active Power Loss of DC Line

Modular multi-level converter based high-voltage Direct-Current (MMC-HVDC) grid integration is an effective measure to achieve offshore wind power grid integration of large-scale and long-distance. As the offshore wind farms get farther and farther offshore, the active power loss of their submarine DC cables has become a non-negligible part of offshore wind power's participation in frequency regulation. In this paper, on the basis of simulating the inertia of synchronous generator with DC capacitor energy, a virtual inertial control strategy considering the influence of active power loss of DC cable is derived. And, the influence on the frequency stability of the system after considering the active power loss of the DC cable is also analyzed at the same time. Finally, a model of offshore wind power grid-connected through MMC-HVDC is built on MATLAB/Simulink. The simulation results show that the virtual inertia control strategy considering the active power loss of the DC line can effectively improve the frequency stability of the power system.

Study on Calculation Method for Steady-State Short-Circuit Current of MMC During a DC Pole-to-Pole Fault

The modular multilevel converter based high-voltage direct current (MMC-HVDC) has drawn great attention in the field of HVDC transmission. Research on short-circuit fault current calculation method is of great significance for equipment protection and key MMC components selection. In common cases, IGBT will be blocked within a short period of time after the occurrence of a bipolar fault, and the short-circuit current will be suppressed. However, there are also cases where the fault progresses to stable stage. Therefore, research on the steady-state short-circuit current calculation method is of high importance for protection design and failure analysis. Taking AC-side reactance and DC-side short-circuit impedance into consideration, a steady-state short-circuit current calculation method based on solving the circuit initial value is proposed. Current calculation is discussed in three situations divided based on different fault parameters after defining the commutation overlap angle are respectively analyzed. The short-circuit current can be solved by formulating the equations based on the conduction state of the diode in the converter, and then solving the electrical quantities through the current boundary conditions of each state. Finally, a fault circuit model is built in PSCAD/EMTDC for simulation, and the accuracy of the calculation method is verified under different fault parameters.

Analysis for magnetic field disturbance of modular multilevel converter based high voltage direct current (MMC‐HVDC) converter valve

The standardised design and operational reliability enhancement of the flexible DC converter valve are impending with the construction of high shares of renewables and power electronics power system. A power frequency magnetic field calculation model able to obtain the electromagnetic disturbance at the submodule (SM) drive board during normal operation of a flexible DC converter valve is presented. Hundreds of SMs with complex structures in a converter valve tower are simplified by the equivalent to the line current to fulfil calculation accuracy requirements. In addition, a random function is employed to simulate the non-sequential switching process of the SMs. By comparing the calculated results with the measured data obtained from the SM equivalent operation experiment of 2200 V/2150 A, the validity of the calculation model is confirmed. According to the final calculations, the influence of non-sequential switching cannot be ignored when recognising the time-domain distribution features of the power frequency magnetic field of the drive board. However, engineering design can be guided only by the power frequency magnetic field intensity of the drive board calculated based on SM’s full removal in the valve tower.

Impact of control system on the DC impedance characteristics of low-medium frequency bands and stability of modular multilevel converter based HVDC system

The resonance in modular multilevel converter based high voltage direct current (MMC-HVDC) system poses a potential threat to the safe and stable operation of the power grid. The mechanism of the resonance phenomenon is complicated, which is closely related to the impedance characteristics of MMC system. By establishing the DC impedance model of MMC-HVDC system at both ends, the influence of parameters of control system in converter stations on the DC impedance characteristics of MMC system was analyzed, revealing the main influencing frequency bands and effect of each control parameter. The DC impedance characteristics of low and medium frequency bands could be shaped in a targeted manner by adjusting the control parameters, the stability margin of the system will be improved as well. The correctness of the results was verified by time domain simulation. The analysis results of this article can provide reference for the selection of the parameters of control system and the improvement of stability margin of MMC-HVDC system.