DC Fault Ride-through Capability Research Articles

Multiplexed-Stack Converter With DC-Fault Ride-Through Capability and High Compactness

Multiplexed-Stack Converter With DC-Fault Ride-Through Capability and High Compactness

A hybrid arm‐multiplexing MMC for DC fault ride‐through without blocking

AbstractConsidering both the DC fault ride‐through (FRT) and lightweight requirements of modular multilevel converter (MMC), this paper proposes a hybrid arm‐multiplexing modular multilevel converter (HAM‐MMC). The phase leg of the proposed MMC topology is divided into upper, middle, and lower arms, configured with full‐bridge submodules (FBSMs), half‐bridge submodules (HBSMs), and FBSMs, respectively. The time‐division multiplexing of middle arms between upper and lower arms is achieved by introducing arm selection switches, which considerably enhances the submodule utilization and thus minimizes the submodule number. The structure and zero voltage switching strategy of arm selection switches are presented, and a modified sorting algorithm is developed for capacitor voltage balance. The negative level output capability of FBSMs in upper and lower arms allows the DC voltage to be reduced in response to DC faults. Compared with the conventional hybrid MMC composed of HBSMs and FBSMs in a 1:1 ratio, the HAM‐MMC has the same power quality and DC FRT capability, but uses 25% fewer capacitors for lightweight. Simulation results demonstrate that the proposed HAM‐MMC can smoothly ride through DC voltage dip without interrupting power transmission, and can also rapidly resume normal operation after a zero‐voltage disturbance.

A soft start-up method for DC micro-grid based on improved two-level VSC with DC fault ride-through capability

For the black start-up of DC micro-grids, three-phase charging resistors are required to limit the uncontrollable surge current. The main drawback of this start-up method is the difficulty in determining the appropriate resistance value to achieve a rapid start-up and limit the surge current with the change of grid parameters. To address this problem, this article proposes a soft start-up method for the DC micro-grid based on an improved two-level voltage source converter (VSC). Specifically, an silicon controlled rectifier and anti-parallel diode are added in each up-bridge-arm in the improved VSC. By conducting a dynamic control strategy of the firing angle on the SCRs, the start-up current can always be maintained near a given value to achieve rapid start-up. Moreover, the improved VSC has DC fault ride-through capability. The simulation results based on PSCAD/EMTDC are provided to validate the feasibility of the proposed start-up method.

Research on MMC Improved Sub-Module Topology With DC Fault Ride-Through and Negative Level Output Capability

Modular multilevel converter (MMC) is a modern and preferred topology among the voltage source converters (VSCs) in high voltage direct current (HVdc) transmission. However, the traditional half-bridge sub-module (SM) cannot block the dc fault current. Hence, the novel SM topology with dc fault ride-through capability is introduced. This article proposes an improved diode-clamp SM (IDCSM) structure. Based on the DCSM topology, the two capacitors’ connection in the IDCSM is modified to operate independently and generate output voltage levels of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- u_{c}$ </tex-math></inline-formula> , 0, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$u_{c}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2u_{c}$ </tex-math></inline-formula> based on the additional switching devices. The IDCSM with dc fault ride-through and negative level output capability can increase the number of output levels and significantly reduce hardware cost. In addition, this article adopts an equivalent half-bridge modulation (EHBM) strategy that can modulate all the MMC SM topologies with the comparison algorithm, simplifying the complicated logic calculation process. Then the IDCSM power loss, cost, and dc fault ride-through characteristics are illustrated in detail. Finally, the simulation and experimental results verify the feasibility of the proposed IDCSM topology and EHBM.

A Cost-Effective and DC-Fault Tolerant Alternate Arm Converter With Wide Range Voltage Adaptability

In this article, an enhanced three-arm alternate arm multilevel converter (AAMC), called T-type AAMC (T-AAMC), is proposed for achieving dc fault ride-through capability with reduced semiconductor power devices compared to the hybrid modular multilevel converter (MMC). The upper and lower dc arms of T-AAMC are formed by half-bridge submodules (SMs) and series devices for lower construction costs, while the ac arm uses full-bridge SMs to realize dc fault tolerance. First, the arm phase-shift conducting modulation is applied for energy balance of converter stacks under wide-range operation by adjusting the arm phase-shift angle to match ac grid voltage and power factor. Then, the SM number, power device number, and SM capacitance are evaluated for T-AAMC under optimized modulation design. Analyses illustrate that 43% SMs, 17% power devices, and 44% energy storage requirement are reduced with the proposed topology than that with hybrid MMC. Furthermore, the closed-loop control system is designed to dynamically regulate the energy sharing between dc and ac stacks. Finally, full-scale simulations and down-scaled experiments are performed to verify the feasibility of the proposed converter and the theoretical analysis of its modulation and implementation.

An improved energy management for MVDC distribution system based on exponential droop control

The energy management optimization and DC voltage stability are the main challenges when loads surging and renewable energy sources (RESs) fluctuations occur in the medium-voltage DC distribution system (MVDC-DS). To solve these issues, this paper proposes a dual-time scale energy management method based on improved droop control for the MVDC-DS. Firstly, a ±10 kV three-terminal hybrid modular multilevel converter (MMC)-based ring MVDC-DS with DC fault ride-through capability is constructed, which including electric vehicle charging stations, energy storage systems, multiple loads and RESs. Secondly, the droop control adopted by the hybrid MMC is improved by exp-function to achieve higher power quality. On this basis, a dual-time scale energy management method is proposed to minimize the electricity purchasing cost. The reference power of each controllable unit is optimized in the long time scale, and the parameters of improved droop control are optimized in the short time scale. Finally, a case study is conducted on the ±10 kV three-terminal hybrid MMC-based ring MVDC-DS, and the results indicate that the proposed method can improve the economy and power quality of system, and help to realize the normal system operation under different conditions.

A Line-Frequency Commutated HV VSC Embedded Modular Multilevel Converter With DC Fault Blocking Capability

Modular multilevel converter (MMC) has become one of the most favored topologies in medium-voltage and high-voltage (HV) applications. However, it still suffers from some drawbacks, including a lot of sizeable capacitors, high conduction losses, and no dc-fault ride-through capability. To circumvent these issues, this article presents a modified MMC, namely hybrid-leg MMC (HL-MMC). HL-MMC integrates a pair of three-phase line-frequency commutated HV voltage source converters (VSC) to replace up to 40% MMC submodules (SMs), leading to lower cost and conduction losses. Due to the ripple energy cancellation in the three-phase HV VSCs, the total energy storage requirement in HL-MMC could be also reduced, leading to up to 30% energy storage capacitance reduction compared with MMC. In addition, by combing the dc hybrid circuit breaker, the HV VSCs in the proposed HL-MMC also help provides the dc fault blocking capability, enabling a dc-fault tolerant HL-MMC without using the full-bridge (FB) MMC SMs. Therefore, a significant device and loss saving can be attained compared with the traditional MMC with FB SMs. The detailed converter operational analysis, control method, high-voltage direct current simulation model, and scale-down prototype are provided to validate the effectiveness of proposed topology.

Control Design and Fault Handling Performance of MMC for MMC-Based DC Distribution System

Modular multilevel converters (MMCs) have emerged as a viable choice in future DC grid architectures due to their scalability to meet voltage level requirements. However, MMC-based DC distribution systems are at risk of short-term outages during the faults in either the DC or AC networks feeding the MMC, so it remains a challenge to accomplish AC and DC fault ride-through (FRT) capability in such applications. To ensure stable operations of the DC terminals, FRT strategies are required for the faults on both the AC and DC sides of the converter. This paper proposes a FRT strategy for the AC and DC side of the converter to ensure stable and economically viable operation of the DC distribution network. The asymmetrical faults in the upstream AC grids are managed by using the integrated energy of the MMC. Whereas, the DC FRT capability of the MMC is accomplished by changing the redundant submodules of the MMC to full-bridge submodules (FBSMs), thus allowing a DC FRT to be achieved by using DC circuit breakers that are low cost and reduced in size. Applying the proposed DC FRT strategy, which makes possible the use of low-cost and reduced in size DC circuit breakers in DC distribution, results in a reduction in the overall initial investments.

A DC Solid-State Transformer With DC Fault Ride-Through Capability

This article investigates a dc solid-state transformer (DCSST) with dc fault ride-through (FRT) capability for medium-voltage dc (MVDC) systems. The FRT requirement for DCSST in dc grid is investigated. Short-circuit fault clearance on the medium-voltage (MV) side is realized by introducing full-bridge converters with cascaded dc converter modules on the MV side of the DCSST. The FRT strategy under typical voltage sags is analyzed. Factors affecting the fault clearing and system recovery are considered, including the number of full-bridge converters, the values of dc inductance, and the dc-link bus voltage. The concept is verified with both simulations and experiments.

Research on Capacitor-Switching Semi-Full-Bridge Submodule of Modular Multilevel Converter Using Si-IGBT and SiC-MOSFET

This article presents a capacitor-switching semi-full-bridge (CS-SFB) submodule (SM) using Si-IGBT and SiC-MOSFET for a modular multilevel converter (MMC). Compared with the conventional hybrid half-/full-bridge MMC, the CS-SFB MMC has the same dc fault ride-through capability and lower power losses. The conduction loss reduces due to the current sharing phenomenon between Si-IGBTs and SiC-MOSFETs. The switching loss reduces in virtue of the fast recovery characteristic of SiC-diode. The derivation process and working principle of the proposed SM are first introduced. Then, the power loss models are established in the switch, SM, and MMC system levels, respectively. Afterward, the piecewise calculation and simulation results reveal the low-power-loss characteristics of the proposed SM and MMC. Finally, these features are verified by experimental results acquired from a downscaled MMC prototype with Si-IGBTs and SiC-MOSFETs.

The Alternate Arm Converter “Extended-Overlap” Mode: AC Faults

This article presents ac fault ride-through strategies for the “extended-overlap” operating mode of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. The AAC offers several benefits over the half-bridge modular multilevel converter, such as requiring fewer submodules with a smaller capacitance and providing dc fault ride-through capability. Novel symmetrical and asymmetrical ac fault ride-through strategies are described and these strategies are experimentally validated by using a small scale prototype.

Reliability Modeling and Analysis of Hybrid MMCs Under Different Redundancy Schemes

The hybrid modular multilevel converter (HMMC), consisting of half-bridge and full-bridge submodules (SMs), has become a competitive topology due to the DC fault ride-through capability (FRTC). Redundancy schemes need to be employed to improve the reliability, including fixed-level and voltage-sharing active schemes, as well as passive scheme. However, different schemes affect reliability differently, which entails specific models. The contributions of this paper involve that analytical reliability models of HMMCs are proposed with considerations of voltage stress in active scheme, as well as imperfect bypass switch in passive scheme, and the comparison of different schemes is conducted to help the redundancy design. The reliability of HMMCs is divided into basic and extra components by the requirement of FRTC, which are modeled through the Markov chain and iteration method. A case study is conducted to validate the feasibility and accuracy of the proposed models, and the reliability analysis is presented, together with comparisons of different redundancy schemes. The results illustrate that reliability improvements of both fixed-level mode and passive scheme are prone to suffer saturation effect with the increase of redundant half-bridge SMs, which may even cause reliability decline in voltage-sharing mode.

A Hybrid-Arm Modular Multilevel Converters Topology With DC Low Voltage Operation and Fault Ride-Through Capability for Unidirectional HVDC Bulk Power Transmission

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs.

A Practical DC Fault Ride-Through Method for MMC Based MVDC Distribution Systems

Modular multilevel converter (MMC) based medium voltage DC (MVDC) distribution system has become a promising option in the future smart grid architecture due to many distinguishing features. However, MMC-MVDC distribution system is prone to face the short-term outage problem during bipolar DC faults, so how to implement DC fault ride-through (FRT) capability for MMC-MVDC is still a challenge. This paper intends to change the redundant half-bridge submodules (HBSMs) of the HB-MMC into the redundant full-bridge submodules (FBSMs), thus, a practical DC FRT method is proposed by using the combination of the hybrid MMC and the hybrid DC circuit breakers (CBs) for the MMC-MVDC system. Then, the DC fault current characteristics of the MMC with the proposed FRT method are discussed. Lastly, data analysis and simulations of a three-terminal MMC-MVDC system have been performed in PSCAD/EMTDC to validate the effectiveness of the proposed DC FRT method. The proposed DC FRT method is not only easy to implement, but also reduces the requirements and the investment of the DC CB.

Comparative Analysis of an MV Neutral Point Clamped AC-CHB Converter With DC Fault Ride-Through Capability

The ac-side cascaded H-bridge converter with a two-level (2L) main bridge has previously been proposed as a fault-tolerant converter for high-voltage dc. This paper explores the benefits of replacing the 2L bridge with a neutral point clamped (NPC) three-level bridge for medium voltage dc applications and defines the optimum operating conditions for this case. By modifying the topology to include an NPC main bridge, the peak stack voltage during normal operation is decreased considerably, which results in a 58% reduction in the required number of submodules (SM), thereby significantly increasing efficiency. However, this sacrifices the fault ride-through capability as the stacks are no longer able to support the ac voltage and thus two new SM topologies are proposed. The proposed topologies function as single full-bridges with two capacitors in parallel during normal operation. Under fault conditions, the SMs divide into two series-connected full-bridges to enable dc fault ride-through. Reverse-blocking integrated gate-commutated thyristors or ultra-fast mechanical switches are used to bypass the insulated gate bipolar transistors that are unused in normal operation and therefore the topology maintains a high efficiency. Simulation results are shown, and the proposed topologies are compared with the more conventional designs in terms of efficiency, energy storage requirement, and device count.

Enhanced Modular Multilevel Converter for HVdc Applications: Assessments of Dynamic and Transient Responses to AC and DC Faults

This article describes the operating principles and theoretical relationships that underpin the modeling and control system of the enhanced modular multilevel converter (EMMC). A full-scale model of a point-to-point high-voltage direct current (HVdc) link that employs EMMCs is used to examine its performance during normal operation in all four quadrants and resiliency to symmetrical and asymmetrical ac and dc faults. Results of exhaustive simulation studies reveal that the improved ac and dc power qualities, which are achieved by incorporation of a few full-bridge (FB) cells into the arms of the conventional half-bridge modular multilevel converter (HB-MMC) with medium-voltage cells to create the EMMC, do not affect its ac and dc fault ride-through capability nor its dynamics during normal operation as active and reactive power set points being varied. In addition, a variant of the EMMC is proposed, in which the number of FB cells to be added into the arms of HB-MMC could be increased to offer bespoke features beyond that explicitly defined in the original vision of the EMMC, such as reduced dc voltage operation during the pole-to-ground dc fault, and potential extension of fault clearance times in multiterminal HVdc grids. Moreover, the validity of the new variant has been confirmed using results obtained from high-fidelity HVdc link models developed in the EMPT-RV platform, in which the EMMCs are replaced by the proposed variant.

Cost and reliability optimization of modular multilevel converter with hybrid submodule for offshore DC wind turbine

The hybrid modular multilevel converter (HMMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore wind energy conversion system, especially all-DC output wind turbines. The topology of HMMC needs to be optimized for lower cost and higher reliability with active redundancy. This paper presents an optimization method for HMMC topology with cost and reliability objectives. The reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs, which is different from the former model. Firstly, reliability and cost models with power loss, considering active redundancy configuration, are derived. Then, cost and reliability, with different numbers of original and redundant SMs, are quantified by taking a case study of 10-MW and 10-kV HMMC for offshore DC wind turbines. Finally, a multi-objective optimization approach that regards maximum reliability and minimum cost is proposed, and the optimal solution from Pareto-optimal set is selected according to fuzzy membership function, which determines the optimal topology for HMMC. The results show that reliability can be improve with redundant full-bridge SMs (FBSMs) but weakened with excessive half-bridge SMs (HBSMs). Furthermore, the incremental cost with a redundant FBSM is approximately 3 times that of HBSM. The optimal topology, 10 original SMs per arm (5 HBSMs and 5 FBSMs) with 1 redundant HBSM and 4 FBSMs, is validated by comparing with the results of former reliability model and single-objective optimization.

Review on topology‐based dc short‐circuit fault ride‐through strategies for MMC‐based HVDC system

Modular multilevel converter (MMC) based high-voltage direct-current (HVDC) transmission system is a promising solution to the bulk power transmission over a long distance, especially for renewable energy integration. However, conventional half-bridge submodules MMC topology is susceptible to dc faults. In this study, dc short-circuit fault analysis and a review on recent advances in clearing dc fault current utilising different topologies are presented. Furthermore, the MMC can also work as a static synchronous compensator, producing reactive power to support the grid and improving the stability of the system when dc fault occurs. Finally, the comparison for different topologies in terms of device cost, estimated power losses, dc fault ride-through capability, and reactive power compensation capability is given.

A new precharging strategy for hybrid MMC based on HBSM and FBSM

Precharging of the modular multilevel converter (MMC) is an important basis for ensuring the normal operation of MMC-HVDC system. The hybrid MMC based on full bridge sub-module (FBSM) and half bridge sub-module (HBSM) has reliable performance in DC fault ride-through capability and is receiving more and more attention. Due to the different charging characteristics of FBSM and HBSM, the sub-modules (SMs) capacitor voltages may be different at the end of the uncontrolled precharging process. This paper introduces conventional uncontrolled precharging strategy of the hybrid MMC and points out the problems in the process. Then a three-stage precharging strategy is proposed in this paper. This strategy can eliminate the capacitor voltages imbalance of different SMs and solve the problem of unsuccessful self-taking energy of HBSM during precharging process. Finally, simulation and analysis of the uncontrolled precharging process based on the proposed strategy are carried out.

The Analysis of DC Fault during the Hybrid MMC Start-up Process

The hybrid Modular multilevel converter(MMC), which includes both full-bridge sub-modules (FBSM) and half-bridge sub-module (HBSM), has DC fault ride-through capability and is receiving more and more attention.Current researches are focused on the DC fault in the normal operation of the system.However, there is a lack of analysis of the DC fault during start-up process.This paper introduces the characteristics of the pole to pole fault during the start-up process and analyzes the effect of the pole to pole fault on the sub-module charging.Simulation in PSCAD/EMTDC is implemented to verify the correctness of analysis in this paper.Based on that, new suggestions for DC fault protection are proposed.