MMC-based DC Grid Research Articles

Resonance stability analysis of MMC-based DC grid with coupled multiphase DC lines

The resonance stability of modular multilevel converter (MMC) based high-voltage direct current systems has been extensively researched using the impedance-based stability theory. However, when there are coupled multiphase DC lines (CMDCLs), the DC side stability issues would become more complicated and harder to be analyzed by many well-established stability criteria. To address this challenge, this paper proposes a resonance stability analysis method for MMC-based DC grid with CMDCLs, based on the nodal admittance matrix analysis. First, the DC side impedance models of MMCs with typical control modes are established using the harmonic state-space theory. The parameter matrices of CMDCLs are also introduced and their elements are transformed into the s-domain. Second, the nodal admittance matrix of the DC grid when there are CMDCLs is derived in detail. The resonance mode of the DC grid can then be obtained by solving zeros of the determinant of the s-domain nodal admittance matrix. Finally, the resonance stability characteristics of a bipolar four-terminal MMC-based DC grid are investigated. Simulations in PSCAD/EMTDC verify the effectiveness of the analysis results.

A Fast Protection Relay for MMC-Based DC Grids Robust to Cyber-Physical Anomalies

Half-bridge modular multilevel converters (HBMMCs) are widely applied in modern DC grids, where converter substations have a cyber-physical structure similar to that of a typical intelligent substation. One major issue for HBMMCs is the lack of sufficient fault interruption capability. Hence, a reliable protection relay is necessary. However, conventional relays need a very robust cyber-physical system, and unintended events are not rare in intelligent substations. In this paper, several types of cyber-physical anomalies and their impacts on protection relays are first analyzed. Then, a novel protection relay is proposed based on the difference between the theoretical and the calculated modal measurements. To eliminate inter-substation asynchronization, the Bhattacharyya distance is introduced to quantify the above graphic feature in a sliding time window with high robustness to noise contamination; also, historical data is employed to identify and erase upcoming abnormal samples in real time. Case studies validate the robustness against external faults, power order change, and cyber-physical anomalies. Additionally, the proposed protection relay is highly sensitive to internal faults. Discussions are also conducted to investigate the quantitative capability against abnormal samples, to determine the scope of applications for different DC lines and to compare the proposed protection relay with several state-of-the-art methods.

A Fault Clearance and Restoration Approach for MMC-Based MTDC Grid

With the growth in continuous energy demand, high-voltage Multi-Terminal DC (MTDC) systems are technically and economically feasible to transmit bulk power and integrate additional energy sources. However, the high vulnerability of the MTDC systems to DC faults, especially pole-to-pole (P2P) faults, is technically challenging. The development of DC fault ride-through techniques such as DC circuit breakers is still challenging due to their high cost and complex operation. This paper presents the DC fault clearance and isolation method for an MMC-based MTDC grid without adopting the high-cost DC circuit breakers. Besides, a restoration sequence is proposed to re-energize the DC grid upon clearing the fault. An MMC-based four-terminal DC grid is implemented in a Control-Hardware-in-Loop (CHIL) environment based on Xilinx Virtex-7 FPGAs and Real-Time Digital Simulator (RTDS). The RTDS results show that the MTDC system satisfactorily rides through DC faults and can safely recover after DC faults.

Fault Current and Voltage Estimation Method in Symmetrical Monopolar MMC-based DC Grids

Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current (MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.

An Improved DC Line Fault Detection Scheme Using Zone Partition for MTDC Wind Power Integration Systems

The MMC based DC grids are an effective solution to integrate bulk wind power. Under DC faults, the wind power is continuously fed into DC grids, resulting in large fault currents. To guarantee the uninterrupted and safe operation of healthy parts, DC faults should be detected and isolated selectively. Most existing DC fault detection schemes rely on large current-limiting reactors (CLR) to guarantee high selectivity. The reliability of them will be deteriorated under weak boundary conditions. Besides, some schemes fail to identify close-in faults. Though fault detection schemes independent of CLRs are proposed, they cannot work well for remote faults. Hence, to protect the entire transmission line with smaller CLRs, an improved DC fault detection scheme using zone partition is proposed. Firstly, according to different fault distances, internal faults are partitioned into four zones along the transmission line. The fault characteristics in different zones are analyzed. Then, the polarities and arrival times of traveling-waves are used to design the criteria dedicated to different zones. The proposed method is endurable to fault resistance and noise disturbance. Simulation results show that the MTDC wind power integration systems can operate safely during DC fault isolation.

Reclosing Strategy of a Hybrid DC Circuit Breaker on Overhead Lines in Half-Bridge MMC-Based DC Grids Considering Fault Type Discrimination

Flexible DC grids with dedicated metallic return (DMR) encounter two major challenges when reclosing hybrid DC circuit breakers (DCCBs): (1) identifying permanent and nonpermanent faults to determine whether DCCBs should be reclosed; and (2) identifying pole-to-ground (P-GND) and pole-to-DMR (P-DMR) faults to determine whether all DC lines should be isolated when a permanent fault occurs. Under a permanent P-DMR fault, all DC transmission lines should be isolated instead of maintaining half of the power transmission. In this study, the equivalent circuits and analytical expressions of the fault currents under the P-GND and P-DMR faults are deduced. On this basis, two different criteria are proposed to discriminate these two faults. Permanent and nonpermanent faults are recognized according to the characteristics of the residual voltage of the DC transmission line. Finally, a comprehensive reclosing strategy considering fault type discrimination for a hybrid DCCB is developed. A simulation model of a four-end flexible HVDC grid with DMR is constructed in PSCAD/EMTDC. The validity of the proposed reclosing strategy is verified through different simulation cases.

A Differential Pilot Protection Scheme for MMC-Based DC Grid Resilient to Communication Failure

Primary single-end line protection strategies of modular multilevel converter (MMC)-HVdc systems are difficult to make a tradeoff between fast detection speed and high reliability. To improve reliability, the pilot protection schemes based on communication and data exchange can be adopted. However, the communication-based schemes suffer from potential communication failure problems, such as data error, data loss, and time synchronization error. To avoid blocking of protection devices during communication failures, a resilience-oriented differential pilot protection method is proposed in this article. To address the problem of synchronization error, a startup element based on the multiresolution morphological gradient (MMG) of traveling wave (TW) is proposed. For the problems of data error and data loss in communication, the sampled data are preprocessed by morphological filtering (MF). Also, the correlation of TWs is used to identify the internal and external faults; the ratio of the morphological gradient of pole voltages is adopted to discriminate the faulty poles. A four-terminal MMC-based dc grid model is built in PSCAD/EMTDC interfaced with the optical fiber-based communication system built in MATLAB/Simulink. The simulation results show that the protection scheme can effectively identify the faults against serious communication problems of 1‰ bit error rate and 5% data loss.

Accurate Impedance Modeling and Control Strategy for Improving the Stability of DC System in Multiterminal MMC-Based DC Grid

This article proposes an accurate dc-side impedance modeling method of modular multilevel converter (MMC) and a control strategy for improving the stability of the dc system in MMC-based dc grid. First, the impedance modeling method based on harmonic linearization is adopted to establish the dc-side small-signal impedance model of MMC, which considers the internal multiharmonic coupling characteristics and the complete control system. Second, an equivalent model is established to analyze the frequency impedance characteristics of the dc ports in MMC-based dc grid, and the stability of dc ports is further analyzed based on the impedance-based stability criterion. Then, considering both the dynamic characteristics and stability of the dc system, a specific design method of controller parameter optimization and additional virtual damping controller is proposed. Based on MATLAB/Simulink, a detailed time-domain simulation model of MMC-based dc grid is established. The simulation results prove the accuracy of the proposed impedance modeling method and the validity of the proposed control strategy.

Fault current limiter for the MMC‐based multi‐terminal DC grids

In the modular multilevel converter (MMC)-based multi-terminal high-voltage direct current (HVDC) system, the fault current rises rapidly due to sub-module capacitors discharging when a DC-side short-circuit fault occurs, which makes the MMC easily blocked for self-protection. To avoid the blocking of the converter station and lower the DC fault current, this study proposes a novel fault current limiter (FCL) for MMC-based multi-terminal DC grids. First, the fault current limiting circuit, which consists of resistance, capacitance and power electronic components is designed. Then, the operation principle of the FCL is analysed in detail. Afterwards, the startup criterion is constructed, and the selection method of the key parameters in the fault current limiting circuit is also introduced in order to prevent the converter station from blocking in the case of DC short-circuit fault. Finally, a simulation model was built using PSCAD/EMTDC software, and extensive simulation results verified the effectiveness of the proposed FCL.

DC Grid Protection Method Based on Phase Planes of Single-End Common-and Differential-Mode Components

This study develops a high-speed protection method for a modular multilevel converter (MMC)-based DC grids. The method utilizes the single-end derivative of common-mode (CM) and differential-mode (DM) currents to construct a protection phase-plane that divides different DC fault types into different regions. When the protection starts, the first sampling point that enters the fault regions of the protection phase-plane has the largest value. When five sampled data satisfy the corresponding fault conditions, the protection signal will be sent. This scheme ensures the rapidity of protection. CM and DM circuits of MMC, DC transmission line, and fault transition resistance under different fault types are derived following the principle of modulus decomposition. Finally, the synthesized equivalent circuits are obtained. On this basis, the expressions of CM and DM currents are derived to analyze the characteristics of fault currents on the protection phase-plane and provide the rules for distinguishing different fault types. A four-terminal MMC-based DC grid simulation model is established in PSCAD/EMTDC. The rapidness and reliability of the proposed protection method are then verified using different simulation cases. The proposed method is also validated on a scaled-down hardware setup in the laboratory.

A DC Line Protection Scheme for MMC-Based DC Grids Based on AC/DC Transient Information

With the rapid development of renewable energy, multi-terminal flexible DC grids composed of modular multilevel converters have been widely used in power systems. To ensure the safe and stable operation of a DC grid, this paper proposes a DC line protection scheme for MMC-based DC grids using AC/DC transient information. In the proposed scheme, the difference between the transient voltage root mean square ratios (DTVR) on both sides of the current-limiting inductor of DC lines is used to identify the DC line fault, and the instantaneous zero sequence voltage (IZSV) of the AC side of the converter station is used to discriminate the faulted poles. According to the theoretical analysis result, a fault criterion setting method suitable for faulted pole discrimination is proposed. The proposed scheme is based on local information and tested in a PSCAD/EMTDC model of a four-terminal MMC-based DC grid. The simulation results show that the scheme can fast and accurately identify faults.

ANN‐based robust DC fault protection algorithm for MMC high‐voltage direct current grids

Fast and reliable protection is a significant technical challenge in the modular multilevel converter (MMC)-based DC grids. The existing fault detection methods suffer from the difficulty in setting protective thresholds, incomplete function, insensitivity to high-resistance faults and vulnerable to noise. This study proposes an artificial neural network (ANN)-based method to enable DC bus protection and DC line protection for DC grids. The transient characteristics of DC voltages are analysed during DC faults. On the basis of the analysis, the discrete wavelet transform is used as an extractor of distinctive features at the input of the ANN. Both frequency-domain and time-domain components are selected as input vectors. A large number of offline data considering the impact of noise is employed to train the ANN. The outputs of the ANN are used to trigger the DC line and DC bus protections and select the faulted poles. The proposed method is tested in a four-terminal MMC-based DC grid under PSCAD/EMTDC. The simulation results verify the effectiveness of the proposed method in fault identification and the selection of the faulty pole. The intelligent algorithm-based protection scheme has good performance concerning selectivity, reliability, robustness to noise and fast action.

Real-Time Simulation of Hybrid Modular Multilevel Converters Using Shifted Phasor Models

The real-time simulation of modular multilevel converter (MMC) is essential to the evaluation and validation of its control and protection systems. Moreover, the dynamics at both sub-module level and system level are expected from the real-time simulations of MMCs. To achieve this objective, this paper proposes the shifted phasor modeling (SPM) of the MMC by representing each sub-module with a Thevenin equivalent circuit that is derived using shifted phasors with improved accuracy. The efficiency of the SPM is guaranteed by modeling each arm as a switch-dependent Thevenin equivalent circuit. The computational burden remains almost unchanged even when the number of sub-modules increases considerably. The proposed model is materialized using field programmable gate array. And, thus the real-time simulation of MMC-based DC grids can be realized to capture the dynamics at the system level as well as the sub-module level. The effectiveness of this paper has been validated in terms of both accuracy and efficiency on a two-terminal MMC-based low-voltage direct current system.

DC Fault Detection and Identification of a Four-Terminal MMC Based DC Grid Using ANN Method

DC Fault Detection and Identification of a Four-Terminal MMC Based DC Grid Using ANN Method

A Transient Voltage-Based DC Fault Line Protection Scheme for MMC-Based DC Grid Embedding DC Breakers

Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination, and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC-based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analyzed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme.

A Traveling-Wave-Based Fault Location Scheme for MMC-Based Multi-Terminal DC Grids

This paper presents a novel fault location scheme of DC line in modular multilevel converter (MMC)-based multi-terminal DC (MTDC) grids. Considering the low-inertia characteristics and the meshed topology, the scheme, based on traveling-wave principle, is divided into three steps, namely, faulty pole identification, faulty segment determination and fault-distance calculation. With accurate amplitude, polarities and arrival times of the first arrival current traveling waves (FACTWs) collected from time-synchronized measurements taken just at the converter stations, the proposed scheme can correctly determine the faulty pole, the faulty segment and the precise fault location. The continuous wavelet transform (CWT) is deployed to extract the required features of the input signals at the DC lines. Since the scheme merely needs the features of FACTWs, the practical difficulties of detecting subsequent traveling waves are avoided. A four-terminal MMC-based high voltage direct current (HVDC) grid was built in PSCAD/EMTDC software to evaluate the performance of the fault-location scheme. Simulation results for different cases demonstrate that the proposed fault-location scheme has high accuracy, good adaptability and reliability. Furthermore, the algorithm can be used for a MMC-MTDC grid with any number of meshes.

Simulation Framework for DC Grid Control and ACDC Interaction Studies Based on Modular Multilevel Converters

Modular multilevel converter (MMC) topology is discussed intensively as a crucial part of future dc grids, especially to integrate offshore wind power. While generalized voltage-source converter models are usually utilized for multiterminal investigations, conclusions on transient dc grid behavior and on ac–dc interactions during distorted ac grid conditions are hard to draw or are limited in their significance. In this paper, a MMC-based dc grid framework with a tripartite focus on an internal converter as well as dc and ac grid control is presented. Besides, an evaluation of the implemented MMC model that incorporates the available arm sum voltage, a decoupled ac and dc current system control concept that is suitable to handle unbalanced voltage conditions is derived for HVDC applications. Combined with wind farm arrays, this collocation allows a meaningful examination of hybrid ACDC structures and the implemented dc grid control methodologies. To provide proof of the universal applicability of this framework, simulations are carried out on a five-terminal system.