MMC-HVDC Transmission Research Articles

Online DMD-based Kalman filter for current sensor fault-tolerant control of MMC-HVDC transmission systems

With the growing application of modular multilevel converter based high-voltage direct current transmission (MMC-HVDC), the sensor fault-tolerant capability in modular multilevel converters has become critical. This paper aims to boost the sensor fault-tolerant performance of the MMC-HVDC systems using advanced data-driven techniques. A novel online dynamic mode decomposition based Kalman filter (ODMDKF) is designed for fault detection and control reconfiguration. Besides, a modified dual-check fault detection module based on local outlier factor algorithm is proposed to reduce false alarm rate. Simulation studies are conducted on a modified IEEE 9-bus system with MMC-HVDC transmission lines. The results indicate that the proposed sensor fault-tolerant control method can ensure the continuous operation of the MMC-HVDC systems under the condition of three types of current sensor faults and its fault-tolerant capability is better than that of extended Kalman filter-based fault-tolerant control method.

High-Sensitivity Differential Protection for Offshore Wind Farms Collection Line With MMC-HVDC Transmission

High-Sensitivity Differential Protection for Offshore Wind Farms Collection Line With MMC-HVDC Transmission

Coupling characteristics between MMC-HVDC and AC transmission lines on the same tower and their impact on protective relaying

Significant coupling exists between DC and AC lines erected on the same tower or sharing transmission corridors. Electromagnetic and electrostatic coupling between DC and AC lines are investigated based on ± 500 kV MMC-HVDC and 500 kV double-circuit AC lines on the same tower. The interactive coupling characteristics and their impacts on protective relaying are analyzed during normal operation and after DC or AC faults. Voltage coupling ratio and current coupling ratio are defined. The 50 Hz components in MMC-HVDC line may be higher than ripple criterion under the coupling of AC line and can be even more significant when a fault occurs on adjacent AC line. The DC reactor with larger inductance can reduce its impact on MMC-HVDC system. Zero-sequence overcurrent relaying and zero-sequence directional relaying for AC lines may mal-operate when a fault occurs on adjacent MMC-HVDC line. And there is also an interruption risk for AC systems. Improvement schemes are proposed to avoid the mal-operation of protective relaying. Simulation case studies verify the correctness of coupling characteristic analysis and the effectiveness of proposed improvement scheme.

Stability modeling of MMC-HVDC transmission system with differential flat control

AC asymmetry grid conditions put forward higher requirements for the stability control and performance of MMC-HVDC transmission systems. This thesis proposes a small-signal modeling method of an MMC-HVDC transmission system with differential flat control for AC asymmetry grid conditions, incorporating dynamic models of MMC internal characteristics, new control systems, circulation suppressors, and DC lines. To prove the accuracy of the pre-sended small-signal model, a two-terminal MMC-HVDC test system with differential flat control is constructed on the PSCAD/EMTDC platform.

Performance evaluation of MMC-HVDC Transmission System feeding a passive network during disturbances

Performance evaluation of MMC-HVDC Transmission System feeding a passive network during disturbances

Analysis and general calculation of DC fault currents in MMC-MTDC grids

The short-circuit fault of MMC-MTDC has great influence on the safe and stable operation of the MMC-HVDC transmission system. To analyze the changing trend of short-circuit current after fault, this paper studies the short-circuit fault mechanism based on the analytical formula of MMC short-circuit current, and analyzes the influence of capacitance, inductance and resistance on short-circuit current. Subsequently, the discharge characteristics of each converter station in multi-terminal complex power grid under different fault conditions are investigated. Based on the above research, a general calculation method for short-circuit current of multi-terminal complex direct power grid is proposed. Then, a four-terminal power grid and a six-terminal power grid are built based on the PSCAD/ EMTDC electromagnetic transient simulation platform to verify the fault characteristics of MMC and discharge characteristics of multi-terminal network converter stations respectively. Finally, a digital-physical hybrid simulation experiment platform is erected to verify the effectiveness of the analytical calculation method for short-circuit current. The results demonstrate that the theoretical solution method can accurately characterize the fault current variation trend, which has an extremely important guiding significance for the engineering design and control protection of MMCHVDC transmission network.

Control of MMC-HVDC transmission system: a review on internal and external converter control under grid strength

Control of MMC-HVDC transmission system: a review on internal and external converter control under grid strength

The improved 2DoFs-PLL for MMC-HVDC transmission system

The performance of a modular multilevel converter (MMC) is highly related to the three-phase phase-locked loop (PLL). The presence of the DC component, the harmonic component, and the negative sequence component results in the poor dynamic and steady-state performance of the PLL, such as fundamental frequency oscillations in the phase estimated by the PLL. In order to suppress the influence of parameters variation and disturbance of the three-phase grid voltage, an improved PLL with a moving average filter with two degrees of freedom PID (2DoFs-PID) is proposed. Moreover, to improve the phase margin and gain margin, an extra compensator with a pole and a zero is introduced into the 2DoFs-PLL. The improved 2DoFs-PLL can track the frequency and phase with superior accuracy, stability, and rapidity. Furthermore, the methodology for the parameters design of the 2DoFs PID controller is introduced, based on the index of integrated time and absolute error. A simulated model and experimental platform are established to verify the performance of the improved 2DoFs-PLL. The results show that the improved 2DoFs-PLL can meet the needs of both steady-state performance and dynamic performance by employing the 2DoFs-PID controllers.

A simplified model design of MMC-HVDC transmission system for steady state and transient stability analyses

A simplified model of a modular multilevel converter (MMC) implemented on a high voltage direct current (HVDC) transmission system is presented in this study for the analyses of steady-state and transient stabilities. The simplified model design’s goal is to reduce the complicated modeling of the MMC circuit as well as the simulation time of the analysis. The validity of the model design has been examined using the PSCAD/EMTDC simulation program, where the simulation outcomes of the detailed model and the simplified model are contrasted. It has been established that the simplified model is accurate enough and that simulation time can be substantially decreased. It is concluded that the suggested model design is very significant as a simplified model for analyzing the steady state and transient stabilities of MMC-HVDC transmission system.

Stability Enhancement of Modular Multilevel Converter - High Voltage Direct Current Systems Interacted with Weak Power Grid Based on Current-mode Virtual Inertia Control

The extensive growth of large-scale flexible high voltage direct current (HVDC) transmission systems compromises the strength of the alternating current (AC) power grid. Sub-synchronous oscillations will likely arise within such flexible HVDC systems, which would significantly threaten the grand electric power system’s stable operation. A current-type enhanced virtual inertia control is proposed to suppress the sub-synchronous oscillations induced by a weak AC power grid in the modular multilevel converter (MMC)-based HVDC systems. Firstly, a small signal model of MMC is established to study the stability of the system under a weak power grid. Focally, a current-mode virtual inertial control is proposed to suppress the AC power grid’s sub-synchronous oscillation by directly taking the grid current as the virtual inertia control variable. Finally, the proposed virtual inertia control approach is verified using both offline and real-time simulation based on the Chongqing-Hubei flexible MMC-HVDC transmission system. The simulation results demonstrate that the proposed current-mode virtual inertia control strategy can efficiently suppress sub-synchronous oscillations in the MMC-HVDC system interacting with a weak AC grid, which enhances the stability and reliability of the power system.

Comprehensive Analysis of PV and Wind Energy Integration into MMC-HVDC Transmission Network

Renewable energy will play a vital role in greenhouse gas emissions reduction. However, renewable energy is located far away from the load center. Modular multilevel converter-(MMC) based VSC-HVDC systems became competitive for remotely located renewable energy grid integration. Unlike the average model for MMC and renewable energy side converter, this paper presents a detailed model-based control and analysis of the MMC-HVDC system for solar and wind energy integration. Furthermore, it optimally tracks PV energy employing the modified incremental conductance method and wind energy using field-oriented control. Instead of decoupled control, a feedforward controller is utilized to establish a standalone AC voltage for renewable energy grid integration. This work considers a doubly fed induction generator (DFIG), permanent magnet synchronous generator (PMSG), and squirrel cage induction generator (SCIG) for wind energy integration. The results from MATLAB/SIMULINK platform agree with the controller hardware in the loop results from RTDS-dSPACE platform. The results confirmed the optimum solar and wind energy tracking during wind speed, irradiance, and temperature variations. However, it improved the fault ride-through capability during balanced and unbalanced low voltage disturbances at the point of common coupling (PCC) of AC grid.

Two-terminal fault combination location method on MMC-HVDC transmission lines based on ensemble empirical mode decomposition

In the HVDC transmission system, the main factors that affect the accuracy of traditional two-terminal fault traveling wave distance measurement are the selection of appropriate wave velocity and whether the wave head can be identified. The line dispersion and transition resistance of the DC transmission will make it more difficult to determine the time when the wavehead reaches the two ends of the two sides, and the boundary elements of both terminal of the line will cause distortion of the wave head. Based on fault traveling waves characteristics of the time–frequency domain, we propose a method of Two-Terminal Fault Combination Location (TTFCL) on High Voltage Direct Current based on MMC (MMC-HVDC) transmission line. The method combines the natural frequency fault location method and the traveling wave fault location method. The first step of the walking wave natural frequency method is used for rough ranging, so that the approximate time range of the initial traveling wave of fault to reach the busbar can be obtained, and then can be calculated the rough position range of the fault point. Then the second step of accurate ranging is carried out, using the Ensemble Empirical Mode Decomposition (EEMD) algorithm to decompose the processed signal, extract the traveling wave component of the high-frequency, identify the traveling wave of initial head recognition and calibration, obtain the time parameters required for fault ranging, and calculate the more accurate fault point location by the two-ended fault ranging formula. In PSCAD/EMTDC, we built a simulation model of MMC-HVDC system line fault, and the simulation experiments are compared with different fault types and locations, and the results show that the combined fault ranging method eliminate the wave speed influence on accurate positioning and effectively identifies the wave head, which further improves the ranging accuracy.

Research on oscillation characteristics of wind farm sending system based on participation factor

The MMC-HVDC transmission system of wind farms has a very broad application prospect, but there is gradually growing major concern that the system is prone to broadband oscillation. And the mechanism of oscillation also remains to be clarified. In this article, based on the basic principle of eigenvalue analysis, the theoretical calculation equation of the quantitative evaluation index of the participation factor is deduced. The small-signal model of the MMC-HVDC transmission system for a wind farm is established. Combining with the case of 200 wind turbines connected to the grid, the eigenvalue analysis method is used to obtain the dominant oscillation mode of the system. The participation factors of 11 oscillation modes of the system are calculated to further analyze the relationship between the oscillation modes and the state variables in the MMC-HVDC transmission system of the wind farms. And the correlation between the participation factors of each oscillation mode, wind farm, and the MMC system is investigated, which laid a foundation for the formulation of broadband oscillation suppression strategies.

Redundancy configuration strategy and condition maintenance arrangement of converter valves for MMC-HVDC transmission systems considering both reliability and economy

The converter valve is a key equipment in the flexible DC transmission system, and in order to coordinate its reliability and economy, the redundant configuration strategy and condition maintenance arrangement of the converter valve are studied. Firstly, the reliability model of the converter valve bridge arm is established based on the half-bridge sub-module topology and k/n(G) model, and the Markov method is used to model the fault state transfer process of the converter valve bridge arm sub-module; secondly, for the condition maintenance problem, the impact of condition evaluation and the service age rollback effect brought by the maintenance are quantified, and the sub-module state difference caused by the maintenance is also considered, and the redundant configuration and condition maintenance Finally, the cycle time and average cost of condition maintenance under different redundancy degrees are derived by means of calculation examples. The results show that increasing the redundancy configuration can effectively extend the maintenance cycle and reduce the average cost. The strategy can provide a reference for the redundancy configuration and condition maintenance of the converter valve.

Short-Time Wavelet Entropy Integrating Improved LSTM for Fault Diagnosis of Modular Multilevel Converter

The modular multilevel converter (MMC) is the main part of MMC-based high-voltage direct current (HVDC) system. The MMC bridge arm inductance fault and the submodule IGBT fault have the greatest influence on the transmission quality of transmission systems. Therefore, this article proposes a novel fault diagnosis method based on short-time wavelet entropy integrating the long short-term memory network (LSTM) and the support vector machine (SVM). The proposed short-time wavelet entropy calculation method is used to extract the fault information. First, the optimal short-term wavelet packet calculation period is determined. Moreover, the improved LSTM topology can process the wavelet entropy fault information in the time dimension. Then, the output of the LSTM is set as the input of the SVM to obtain the fault diagnosis result based on the adaptive classification. Finally, through the MMC fault diagnosis experiment of the double-ended MMC-HVDC transmission system, the effectiveness of the proposed method is verified. Compared with the traditional fault diagnosis method, the proposed method has better robustness, adaptability, and accuracy, which can greatly reduce the number of electrical signal samples and realize the fault diagnosis of multiple fault types by collecting a single signal.

Travelling wave fault location approach for MMC-HVDC transmission line based on frequency modification algorithm

Travelling wave (TW) fault location principle is widely utilized on actual power transmission line, which shows better fault location performance than conventional algorithms. But, considering the TW fault location principle is generally implemented using TW front arrival times and transmission velocities, the fault location accuracy is high influenced by frequency distortion. Meanwhile, modular multilevel converter (MMC) based high-voltage direct-current (HVDC) transmission line has more complex boundary characteristic required to be researched. Therefore, in order to acquire better fault location performance, a novel TW fault location approach for MMC-HVDC transmission line is proposed based on frequency modification algorithm. Firstly, the frequency modification algorithm of TW front is studied. Secondly, the detailed calculation principle of system impedance and transmission line’s wave impedance is proposed. Thirdly, the novel TW fault location principle based on modified frequency spectrum is described. Eventually, via PSCAD/EMTDC simulation software, a typical ±400 kV simulation system is constructed to validate the performance of proposed TW fault location principle under the influences of fault type, fault location and grounding resistor issues.

Analysis of AC line single-phase reclosing strategy which connecting with MMC-HVDC

The reclosing strategy of AC line which connecting with MMC-HVDC transmission is an issue deserves intensive research. The electromagnetic transient characteristic during AC line reclosing is studied in this paper, with Chongqing-Hubei back to back MMC-HVDC project as an example. The influence of fault ride through control logic of MMC-HVDC line disconnection is analyzed. The analysis results show that after the line disconnection, the positive sequence controller and the negative sequence controller have an interaction effect, which causes the control system to be unstable. The overvoltage the overcurrent may occur when the fault is recovered. Therefore, when single line with MMC-HVDC operates and the break fault occurs, the power transmission of the straight line should be minimized during the fault time, or the reclosing function of the AC line should not be enabled.

Fault location method for MMC-HVDC transmission line based on adaptive optimal feature reconstruction

Applying the regression mechanism of intelligent algorithm to transmission line fault location is a new research direction in recent years.The location accuracy of such methods largely depends on the feature selection of the training samples of the algorithm model.This paper proposes a feature extraction method that combines regression algorithm, waveform decomposition and optimization algorithm, which can adaptively extract optimal features according to training samples, thereby ensuring the location accuracy of the algorithm model to the greatest extent. Through the test in a MMC-HVDC transmission line simulation model, the optimal feature reconstruction method proposed in this paper can greatly improve the fault location accuracy of transmission line, and has good location performance in case of high resistance grounding fault.

Stability Modeling of a Three-Terminal MMC-HVDC Transmission System

Multi-terminal modular multilevel converter (MMC) power transmission is the current development trend in DC transmission. However, owing to its fast control and dynamic characteristics, it may interact with the other inductive devices or the grid and cause stability issues. This paper describes the use of the impedance analysis method in the frequency domain to analyze system stability. First, the multi-harmonic linearization method is used to model the impedance of a three-terminal MMC system. To make the model more accurate, the modeling considers the effect of frequency coupling caused by an asymmetric controller and the AC sampling site position. This paper then proposes an equivalent method of the grid impedance for the three-terminal system that can accurately judge the stability of the grid-connected system when the AC side is coupled. This work provides a theoretical basis for the prediction and suppression of the oscillation of MMC grid-connected system with coupling on the AC side. Finally, a three-terminal MMC model is built in Matlab/Simulink and the Nyquist stability criterion is used to verify the correctness of the analysis.

Single-ended protection method of MMC-HVDC transmission line based on random matrix theory

The physical electrical boundary at both ends of the MMC-HVDC transmission line is composed of smoothing reactors and has a high-frequency blocking effect. This behavior causes the time-domain characteristics of the initial part of the polar line current waveform to be significantly different under the internal and external MMC-HVDC line faults, and can lead to two different fault modes. However, the operation of the DC line traveling wave protection with du/dt as the core can be easily affected in the presence of high resistance (HR). Furthermore, it is difficult to set the criterion setting value. To solve the aforementioned problems, this paper proposes a line single-ended protection method based on the random matrix theory (RMT). First, the difference between the internal and external line current fault modes is analyzed. Second, the physical characteristics of the two modal differences in the RMT are analyzed. Third, the single Ring Law is used to achieve visualization, and mutation energy is used as the starting criterion. The fixed values of the spectral radius and Euclidean distance of the spectral radius are utilized as the identification criteria for the internal and external line faults. Furthermore, the set value of the spectral radius is used as the fault selection criterion. Last, a 77-level bipolar MMC-HVDC system grounded through the grounding electrode lead is built in RTDS to verify the adaptability of the proposed method to HR and noise. The results show that the method can quickly and reliably detect DC line faults and fault poles under different fault locations and different transition resistances, and the RMT protection can be further improved with the increasing number of fault samples.