Modular Multilevel Converter HVDC Research Articles

A Novel Integrated Switching Strategy for Reliable Starting of Two Terminal MMC HVDC Transmission System

If the initialization protocol of the Modular Multilevel Converter (MMC) system is not adhered to, the primary winding of the system transformer will experience a surge in inrush current, while the secondary winding on the converter side will exhibit significant starting current draw to charge the capacitors of the MMC-H-bridge. This pronounced initial current surge can trigger the activation of protective measures, leading to the disconnection of the High Voltage Direct Current (HVDC) line and thereby engendering power system reliability and stability concerns. This paper presents a starting methodology for a two-terminal MMC HVDC system. The proposed approach entails employing controlled switching techniques to mitigate inrush currents in the HVDC converter transformer, coupled with the utilization of pre-insertion resistance circuit breakers to diminish starting currents on the converter side. The efficacy of the aforementioned strategy is validated through simulations conducted on a two-terminal MMC HVDC system modeled using PSCAD/EMTDC, with the results confirming its effectiveness.

High‐frequency resonance in HVDC and wind systems: Root causes and solutions

AbstractThis paper presents methods to model and solve high‐frequency resonance problems in HVDC and wind power systems. Control and digital PWM delays are identified as a common root cause for such resonances. A systematic method to include such delays in small‐signal sequence impedance models is presented. The developed models are fully validated and used to characterize the effects of delay on system stability. In addition to full models that can predict the associated impedance and negative damping behaviour accurately, simplified approximate models are also presented to gain insights. The method is applied to type‐III and type‐IV turbines as well as HVDC converters based on modular multilevel converters (MMC). In addition to resonance of MMC and wind turbines with overhead transmission lines, the paper also identifies several other modes of high‐frequency resonance, including resonance between wind turbines and MMC HVDC converters, among different wind turbines, and between MMC HVDC converters and cables. Different methods to damp high‐frequency resonances are also discussed and demonstrated.

SSR Stable Wind Speed Range Quantification for DFIG-Based Wind Power Conversion System Considering Frequency Coupling

The subsynchronous resonance (SSR) characteristics of doubly-fed induction generation-based wind power conversion system (DFIG-WPCS) varies with the wind speed. Obtaining the small signal SSR stable wind speed range is urgently important for wind farm dispatch planning. However, there are few methods to quantify the small signal SSR stable wind speed range of DFIG-WPCS. Moreover, the inherent frequency coupling when DFIG-WPCS is integrated to modular multilevel converter-high voltage direct current (MMC-HVDC) may lead to inaccurate stability assessment results. In this article, the frequency coupling mechanism between WPCS and MMC is revealed by analyzing the law of harmonic interaction in the interconnected system. The wind speed-frequency two-variable equivalent admittance model of DFIG-WPCS is established by taking the frequency coupling account. Relying on the two-variable open-loop transfer function, the multiple phase margin contours plot approach is proposed to quantify the small signal SSR stable wind speed range and damping properties. The effectiveness of two-variable admittance and analysis method are validated against electromagnetic transient (EMT) testbed with MATLAB /SimPowerSystems. The analysis results can provide a reference for the early planning and stable operation of DFIG-WPCS.

Dynamic interaction analysis among different control loops and stability enhancement strategy for MMC-HVDC systems

This paper presents the dynamic interaction analysis method among different control loops of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems, and proposes an improved control strategy with feedback regulation loop and low-pass filter loop for stability enhancement. The small-signal cascade model of MMC with dc voltage control loop, ac current control loop and differential-mode ripple capacitor voltages loop is firstly established, which clearly shows the analytical interactions of inner capacitor voltage response and system input power’s dynamics with simplified closed/open-loop transfer functions. Then the small-signal dynamic analysis of MMC is conducted based on the derived closed/open-loop transfer functions, which reveals the interactions between the system stability and key control loops, as well as the main parameters of the MMC-HVDC system by both the Nyquist criteria and the pole-zero theory. Next, the feedback regulation loop and low-pass filter loop are proposed and introduced in the MMC control system to maximize stability enhancement performance during disturbances, and system dynamic response under the proposed control with different parameters are also explored through theoretical analysis. Finally, several cases on a benchmark two-terminal MMC-HVDC simulation model are carried out in PSCAD/EMTDC to verify the proposed control strategy.

Approach to Inertial Compensation of HVdc Offshore Wind Farms by MMC With Ultracapacitor Energy Storage Integration

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method.

A Control Strategy for Ground Fault on the AC Side of MMC-HVDC System.

This study explores a control strategy for a flexible Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) system when there is an asymmetrical fault with the AC side system voltage. Aiming at the characteristic that the fault system has a negative sequence current path, this paper proposes a positive/negative sequence controller to ensure the output of original active power, and the validity of the proposed control strategy is verified by using simulation software PSCAD/EMTDC.

FPGA-Based Real-Time Simulation of Dual-Port Submodule MMC–HVDC System

Aiming at the problems of the high switch numbers, complex working mechanisms, and complicated real-time simulation of modular multilevel converters (MMCs) composed of dual-port submodules, in this study, we designed a unified equivalent model of the multiple submodule network by analyzing the combination of parallel submodules in the bridge arm. The proposed model decouples the submodules that do not affect each other in the subnetwork calculation process, thereby reducing the number of prestored parameters in the subnetwork simulation. In the Xilinx Virtex-7 FPGA VC709 (Xilinx Corporation, San Jose, CA, USA) development board, we replaced the inline computation combined with the prestorage of parameters with the proposed equivalent model to optimize the execution unit structure and redesigned the FPGA-Based Real-Time Digital Solver (FRTDS). Taking the P-FBSM-based MMC–HVDC system as the simulation object, we performed a real-time simulation with a step size of 10 μs, which verified the effectiveness of the proposed model and the improvement in the hardware. We compared the results with the offline MATLAB/Simulink simulation results to verify the accuracy of the simulation.

A Generic User-Defined Modeling Method in PSS/E and Its Application in an MMC-HVDC System

This paper studies the user-defined modeling (UDM) method in PSS/E and applies it to a modular multilevel converter high-voltage direct current (MMC-HVDC) system. First, the mechanism of PSS/E UDM is discussed, including the requirements of the model call, storage space allocation, and flows of dynamic simulation. Then, a generic UDM method applicable to various objects is proposed in PSS/E, considering both the internal structures of models and their interfaces with the PSS/E main program. Next, the design of multiple time scales of UDM is presented for hybrid step size simulation. Accordingly, a user-defined MMC-HVDC system model is built in PSS/E considering the converter, DC-line, control system, and system interface. Finally, the model is compared with a PSCAD electromagnetic transient MMC-HVDC model in a modified two-area-four-generator system. The simulation results demonstrate the proposed UDM method.

Fault Diagnosis Method for MMC-HVDC Based on Bi-GRU Neural Network

The Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) system is recognized worldwide as a highly efficient strategy for transporting renewable energy across regions. As most of the MMC-HVDC system electronics are weak against overcurrent, protections of the MMC-HVDC system are the major focus of research. Because of the insufficiencies of the conventioned fault diagnosis method of MMC-HVDC system, such as hand-designed fault thresholds and complex data pre-processing, this paper proposes a new method for fault detection and location based on Bidirectional Gated Recurrent Unit (Bi-GRU). The proposed method has obvious advantages of feature extraction on the bi-directional structure, and it simplifies the pre-processing of fault data. The simplified pre-processing avoids the loss of valid information in the data and helps to extract detailed fault characteristics, thus improving the accuracy of the method. Extensive simulation experiments show that the proposed method meets the speed requirement of MMC-HVDC protections (2 ms) and the accuracy rate reaches 99.9994%. In addition, the method is not affected by noise and has a high potential for practical applications.

A Study of failure Multi-Term Central HVDC Conductors In Wind Power Transmission Systems

Wind power is developing quickly all throughout the planet as a methods for managing the world energy deficiency and related natural issues. Adaptable direct current (DC) transmission network innovation is a viable strategy for huge limit clean energy admittance to control grids, however the DC cut off identification for it is a troublesome issue for wind structure. The multiterminal dc wind farm is a promising geography with a voltage-source inverter (VSI) association at the coastal matrix. Voltage-source converters (VSCs) are robust to ac-side shortcoming conditions. Be that as it may, they are defenseless against dc deficiencies on the dc side of the converter. For the modular multilevel converter HVDC (MMC-HVDC) systems, when a solitary pole to-ground deficiency happens, the flaw current is little, and it is hard to recognize the issue rapidly. Two variable-speed age frameworks are thought of: doubly-fed induction generators (DFIGs) and perpetual magnet simultaneous generators (PMSGs) in light of their ubiquity these days for wind turbines scaling to a few MW frameworks. The impact of the wind power plant control technique in the short out conduct of the HVDC connect has likewise been contemplated. Results show that the elements of the wind power plant commitment to post to-ground shortcomings are somewhat more slow than those of the wind turbines current control circles. Consequently, the wind turbine front end converters can be utilized to diminish the pinnacle and normal estimation of the issue current in such a system.

Voltage Correlation Based Single Pole-to-Ground Fault Detection of MMC-HVDC Transmission Line

Through analysis of various fault voltage characteristics of modular multilevel converter high voltage direct current (MMC-HVDC) transmission system, it shows that the positive and negative voltage relationships of direct current transmission lines with single pole-to-ground faults are obviously different from other faults. Hence, Pearson correlation coefficient is used to describe the degree of difference in this work. In the case of single pole-to-ground fault, correlation coefficient is positive and close to 1, while as for other failures, correlation coefficient is negative and close to −1. This article analyzes the mechanism of correlation coefficient to withstand transition resistance and verifies the effectiveness of the proposed method under high resistance grounding fault. A novel scheme is proposed by combining the quantity sum of voltage variation characteristics, which can effectively identify fault pole. The proposed scheme needs smaller time window and owns high efficiency, as well as great robustness to transition resistance.

Single Pole‐to‐Ground Fault Location Method for MMC‐HVDC System Using Wavelet Decomposition and DBN

Accurate and reliable fault location technology is essential to the stable operation of the modular multilevel converter‐high voltage direct current (MMC‐HVDC) system. Aiming at the difficulty of locating high‐resistance ground faults on MMC‐HVDC transmission lines, this paper proposes a method for fault location of transmission lines based on wavelet transform and deep belief network (DBN). First, the wavelet transform is used to decompose the original single pole ground fault voltage waveform, and then the high‐frequency and low‐frequency components obtained are used as training samples to train different DBN models, the final fault location results are obtained by superimposing the outputs of each model at last. The ±250 kV double‐ended MMC‐HVDC system model is established by using PSCAD/EMTDC, which can simulate the faults of different positions and different transition resistances. In order to verify the fault location performance of the proposed method, it is compared with two machine learning fault location methods. The results show that this method can accurately and reliably locate the single pole‐to‐ground fault of the transmission line with transition resistance of up to 10 000 Ω at low sampling frequency of 20 kHz. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

A Single-End Protection Scheme for Hybrid MMC HVDC Grids Considering the Impacts of the Active Fault Current-Limiting Control

In the hybrid modular multilevel converter (MMC) based high voltage direct current (HVDC) systems, the fault current can be actively suppressed by the converter itself, which endows a smaller requirement for current-limiting reactors (CLR) and a larger time margin for fault detection algorithms, comparing with the half-bridge MMC. But the robustness to fault resistance and noise disturbance of existing boundary protection schemes will be deteriorated with small CLRs. Moreover, the fast response of the fault current-limiting control will change the output DC voltage of hybrid MMC, which affects the fault characteristics and may cause mal-operation of existing protection algorithms. Thus, a single-end protection scheme considering the impacts of the active current-limiting control is proposed for the hybrid MMC based DC grids. The traveling-wave characteristics under different fault stages are analyzed to evaluate the impacts of the fault current-limiting control. In addition, a coordination protection strategy versus different fault conditions is adopted to improve reliability. Various cases in PSCAD/EMTDC are simulated to verify that the proposed method is robust to fault resistance, fault distance, power reversal, AC faults, and immune to noise.

Generalized Impedance Analysis and New Sight at Damping Controls for Wind Farm Connected MMC–HVdc

The small-signal stability of wind farm (WF) connected modular multilevel converter (MMC)-based high voltage direct current (MMC-HVdc) systems has been widely studied by impedance analyses. However, it is generally difficult to calculate the poles of WF-connected MMC-HVdc due to complex internal dynamics. Thus, the existing impedance analysis methods usually assume that the studied subsystems are stable. In some special cases, the MMC-HVdc itself may be unstable but the WF-connected MMC-HVdc may be still stable. If the poles of the subsystem are not calculated, these phenomena cannot be well revealed by impedance analysis. In order to address these special issues, this article divides the WF-connected MMC-HVdc into three subsystems and proposed a 3-order minor loop-gain matrix to investigate the unified stability of both ac and dc systems. Based on the proposed model, the impact of ac/dc current-based damping controls in WF side MMC on the system stability are investigated. As a result, the proposed model makes up for the deficiencies in ac/dc impedance analysis method and provides a new sight at evaluating the impact of active damping controls on system stability. The simulation results in PSCAD/EMTDC validate the proposed models and stability analyses.

Disturbance Interaction Analysis and Suppression Strategy of MMC-HVDC Systems Considering Sub-Module Capacitor Voltage Ripples

This paper presents the interaction analysis of external disturbances and internal dynamics of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems considering sub-module (SM) capacitor voltage ripples, and proposes an enhanced control strategy for disturbance suppression. The disturbance equation indicating the interaction of the external MMC output currents and internal SM capacitor voltage ripples are firstly established. Then based on the disturbance equation, the dynamic analytical expressions of the differential-mode capacitor ripple voltages and the converter currents are derived in the d-q reference frame, which explains that the resultant current term i0 can be regarded as an equivalent disturbance that will lead to transients in the capacitor voltages during external perturbations. Next, the disturbance observer is introduced to track the resultant disturbance current term and an enhanced control for the MMC based on disturbance suppression strategy is proposed to improve the system dynamic response during external transients. Furthermore, system stability characteristics under the enhanced control with different parameters are explored through theoretical analysis. Finally, a benchmark single-terminal MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the proposed control.

Imbalance recovery performance analysis and improvement for MMC‐HVDC systems considering DC current dynamics

This study presents the imbalance recovery performance analysis of modular multilevel converter high voltage direct current (MMC-HVDC) systems considering dc current dynamics, and proposes an enhanced control to improve the system's internal stability response. The numerical expressions of the imbalance recovery speed and overshoot amplitude response of dc current components in upper and lower arms of the MMC phase unit are firstly derived analytically, which reveal that the system can be prone to poor imbalance recovery performance after sudden disturbance impacts with no auxiliary control. According to the analysis results, an improved control based on the adaptive virtual resistance method is proposed for the MMC-HVDC systems to reduce the recovery time regarding different system operating states. The key control parameter design of the adaptive virtual resistance is also simplified since a wider adjustment range of the parameters is allowed in the proposed method. In addition, the dc current components rebalance controller consisting of a low-pass compensator is proposed to further reduce the overshoot amplitude of system internal response. With the implementation of the proposed control schemes, improved stability recovery performance of MMC-HVDC systems can be achieved. Simulation results verify the effectiveness of the proposed control.

Single Pole-to-Ground Fault Analysis of MMC-HVDC Transmission Lines Based on Capacitive Fuzzy Identification Algorithm

The probability of a single pole-to-ground fault in high voltage direct current (HVDC) transmission lines is relatively high. For the modular multilevel converter HVDC (MMC-HVDC) systems, when a single pole-to-ground fault occurs, the fault current is small, and it is difficult to identify the fault quickly. Through a detailed analysis of the characteristics of the single pole-to-ground fault of the MMC-HVDC transmission line, it is found that the single pole-to-ground fault has obvious capacitance-related characteristics, and the transient process after the single pole-to-ground fault is the discharge process of the distributed capacitance of the line. However, other faults do not have such obvious capacitance-related characteristics. Based on such feature, this paper proposes a novel capacitive fuzzy identification method to identify the single pole-to-ground fault. This algorithm can effectively identify both the fault of single pole-to-ground and the fault pole, which can contribute to the large database of the future smart grid.

Combined Line Fault Location Method for MMC–HVDC Transmission Systems

A combined algorithm based on traveling wave (TW) theory and time-frequency characteristics is proposed to estimate the DC line fault location in modular multilevel converter-high voltage direct current (MMC-HVDC) systems. The relationship between instantaneous frequency and TW propagation velocity is analysed, and the local voltage signal is utilized to extract fault features. A TW distortion-based scheme is improved to obtain the approximate fault location and accurately detect TW arrival times. The arrival times of the initial TW from the fault point and opposite terminal are determined using intrinsic mode functions under variational mode decomposition. Given the attenuation property of the TW-based method, the Teager energy operator is used to overcome the weakness in fault feature extraction. The instantaneous frequency of the TW front is obtained via ensemble empirical mode decomposition and Hilbert-Huang transform. The fault location is predicted with the detected TW arrival times and corresponding TW propagation velocity. The effectiveness and superiority of this method are verified through simulations in PSCAD/EMTDC and computation in MATLAB.

Design Procedure of MMC-HVDC System: Comprehensive Consideration of Internal and External Dynamics

This paper proposes a systematic design procedure with comprehensive consideration of the internal and external dynamics in modular multilevel converter high voltage direct current (MMC-HVDC) transmission system. Previous studies on MMC parameter selection separately deal with each specific component such as energy storage capacity for voltage ripple of sub-module (SM) capacitor, arm inductance for second harmonic circulating current reduction, maximum allowable modulation index for MMC operating condition, which considered only a single purpose. However, the parameters respond dynamically to their characteristics and interact directly with the MMC performance, power system conditions, and specific requirements. In this study, we investigate the mutual relationships between the parameters and their performance. Then, we determine the parameter values based on a proposed systematic design procedure with the desired objectives and restricted conditions, which could be cumbersome and time-consuming to approach proper and acceptable parameter values. Therefore, this study could provide engineering evaluations and insights to help MMC-HVDC system engineers and project developers in intuitive approaches regarding the design aspects of the technology requirement challenges. The efficacy and accuracy of the analysis and design method for the MMC-HVDC system parameters were validated by PSCAD/EMTDC time-domain simulation and real-time digital simulation with hardware-in-loop system.

An Improved DC Fault Protection Algorithm for MMC HVDC Grids Based on Modal-Domain Analysis

To detect the dc faults for the modular multilevel converter (MMC)-based dc grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis, and sensitivity to fault resistance and noise disturbance. To overcome these shortcomings, this article proposes an improved dc fault protection algorithm using the modal-domain approach for the MMC-based overhead dc grids, which decouples the interaction between positive and negative poles and mitigates the strong frequency dependence of the characteristic impedance in phase-domain. The dc fault equivalent circuits are established in modal-domain and the fault characteristics during the initial stage are analyzed. Based on the modal-domain analysis, the line-mode reactor voltage which combines the fault characteristics of negative and positive reactor voltages is used to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles is used to select the faulted pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, ac faults, and dc circuit breaker (DCCB) operation, which are validated and evaluated by the simulations in PSCAD/EMTDC.