Fault Blocking Capability Research Articles

Design of Series-Connected Novel Large-Scale Offshore Wind Power All-DC System with Fault Blocking Capability

The utilization of wind power all-DC systems with DC collection and transmission is an effective solution for the extensive development of wind power in deep-sea areas. However, in the event of faults occurring in wind power all-DC systems, the fault propagation speed is extremely rapid, with a wide-ranging impact, and to date, there are no complete DC engineering references available. It is crucial to research the topology and fault isolation methods applicable to large-scale offshore wind power all-DC systems in deep-sea areas. This paper proposes a novel series-connected all-DC system topology and presents corresponding fault isolation methods for internal faults in wind turbine units and faults in high-voltage DC transmission lines. The system simulation model was constructed using PSCAD/EMTDC (v4.6.3), and simulations were conducted for internal faults in the wind turbine units and DC transmission line short-circuit faults. The simulation results demonstrate that the proposed system can isolate various DC faults while maintaining stable operation, thereby validating the effectiveness of the control strategies and fault isolation methods proposed in this paper.

A hybrid mixed‐cells DC‐DC modular multilevel converter with balanced arm energy and DC fault blocking capability for high voltage direct current systems

SummaryDC‐DC converters play a vital role in the integration of renewable energy sources. Modular multilevel converter (MMC) has emerged as a highly promising option for high‐voltage high‐power applications, for DC‐AC and DC‐DC conversion, thanks to its distinctive characteristics. An inherent limitation of using traditional MMC in DC‐DC conversion is the occurrence of arm energy drift, leading to imbalanced voltages across capacitors of submodules (SMs). Many approaches have been presented in the literature to address the energy drift issue during DC‐DC conversion process. In this paper, a novel bidirectional hybrid modular multilevel DC‐DC converter with balanced arm energy has been proposed. The proposed converter consists of one high‐voltage valve and two legs. Each leg is equipped with half‐bridge submodules (HB‐SMs) in the upper arm and full‐bridge submodules (FB‐SMs) in the lower arm. The proposed architecture enables the parallel connection between series‐connected capacitors of the upper arm and series‐connected capacitors of the lower arm during the equalization period. The parallel connectivity enables the energy transfer between the upper and lower arms in the same leg, hence preventing energy drift issues. A detailed illustration of converter operational concept, mathematical analysis, design, and efficiency estimation are presented. The presented simulation and experimental results show the bidirectional power flow ability and the promising behavior of the proposed converter during normal operating conditions with balanced arm energy. Finally, a comparison between the proposed converter and other existing topologies has been made to show the superiority of the proposed converter over the other alternatives. The proposed converter has a lower count of insulated gate bipolar transistors (IGBTs) and eliminates the usage of isolating transformers when compared to the energy equalization modules approach. In addition, unlike the other alternatives, the proposed converter has a DC fault blocking capability.

Analysis and Design of a Novel Hybrid Modular Multilevel Converter With Time-Sharing Alternative Arm Converter

This paper proposes a time-sharing principle-based modular multilevel converter (TS-MMC) for medium- to high-voltage power transmission applications, which is designed with a lightweight running feature. The TS-MMC consists of cascaded switch stacks (CSSs) and alternative arm converters (AACs), integrating the merits of both the MMC and the two-level voltage source converter (VSC). In TS-MMC, the AACs shape the required multilevel arm waveforms, and the CSSs arrange the AACs operating at different intervals for producing the desirable output waveforms. By applying a “time-sharing” principle for the AACs, a dramatic reduction of the sub-module (SM) number is achieved. Additionally, the TS-MMC does not require any arm inductors, whereas only three inductors are equipped in AC side. Moreover, featured by the specific structure, the DC fault blocking capability of the TS-MMC is also enabled. Furthermore, to explore the performance of the TS-MMC, mathematical model and control strategy of the TS-MMC are provided. Finally, the simulation and experimental results are carried out to verify the feasibility of the TS-MMC.

An Isolated Reduced Arm Modular Multilevel Converter with Fault Blocking Capability for MVDC Rail Traction

An Isolated Reduced Arm Modular Multilevel Converter with Fault Blocking Capability for MVDC Rail Traction

New Hybrid Thyristor-Based Multilevel Converter With DC Fault Blocking Capability, for HVDC Applications

New Hybrid Thyristor-Based Multilevel Converter With DC Fault Blocking Capability, for HVDC Applications

A novel triple capacitor multi-level submodule MMC topology with fault current clearing capability

In the high voltage direct current system based on modular multilevel converter (MMC-HVDC), the half-bridge submodule (HBSM) has become the preferred submodule for significant projects due to its convenient control and low cost. However, the DC short-circuit protection performance of MMC-HVDC will seriously restrict its development in areas such as overhead lines, high voltage, and large capacity. This article proposes a triple capacitor multi-level submodule (TCMSM) topology of modular multilevel converter with DC fault-blocking capability. It utilizes at least one-third of the module capacitance in the bridge arm to reverse to suppress DC fault current and turn off the diode. At the same time, the unidirectional conductivity of the diode can also prevent arc reignition at the fault point. The article also conducted in-depth research on the DC short-circuit fault elimination mechanism of this topology, and compared its performance with other topologies. Through analysis, it can be concluded that TCMSM reduces the initial investment and operating costs by reducing the number of devices inside the module while possessing the fault current blocking ability. Finally, MATLAB/Simulink was used to simulate and verify the proposed new topology, and its performance was analyzed.

A new hybrid multilevel thyristor-based DC-DC converter

The rapid growth in HVDC grids is becoming inevitable for long-distance power transmission. Therefore, the idea of interconnection between the point-to-point links becomes essential. However, these point-to-point connections face several challenges such as the requirement of DC fault blocking capability, interfacing of different grounding schemes, offering multi-vendor interoperability, and difficulty to achieve high DC voltage stepping. DC-DC converters are considered the optimum solution to tackle these challenges in DC grids interconnection. In this paper, a new hybrid modular DC-DC converter is proposed that achieves a low number of semiconductors, low losses, and cost in comparison to other DC-DC converters due to the utilization of thyristors. The new DC-DC converter consists of two hybrid MMC bridges connected through an isolating transformer. Each MMC bridge is comprised of half bridge submodules and bidirectional thyristors. Detailed mathematical analysis, design, and control are illustrated. A comparison is carried out between different topologies in terms of semiconductor count, power loss, and cost. Also, both simulation model and experimental test rig are built to validate the proposed hybrid modular DC-DC converter under different scenarios. Finally, another variant of the hybrid-thyristor based converter (version two) is proposed for multiport DC-Hub application to achieve DC fault blocking without turning off all connected bridges.

A Novel Hybrid Multilevel DC–DC Converter Employing Trapezoidal Modulation

This paper presents a new hybrid multilevel converter for medium voltage dc-dc interconnection. The structure of the topology is a hybrid of two level converter and the modular multilevel converter (MMC). The proposed converter provides three-phase trapezoidal output voltages with a controlled slope while using only a single MMC leg. Therefore, using fewer SMs and sensors, and with its feature of low energy storage requirement (ESR), the overall size of the converter reduces. The front-to-front configuration of the proposed converter provides the desired voltage matching, grounding flexibility and dc fault blocking capability. Moreover, the phase leg switches and the bidirectional switches (BSs) of the converter are operated in the soft switching mode reducing the converter losses. A detailed mathematical analysis is presented to substantiate the low ESR and losses of the proposed converter. The performance evaluation of the proposed dc-dc converter is carried out in PSCAD/EMTDC and analyzed in detail. The operation of the converter is also verified on a developed scaled-down laboratory prototype. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>

An Adaptable Fault Current Derivative Calculation Method for Derivative Relays in HVDC Grids

This paper presents an adaptable method for fault current derivative calculation in high voltage direct current (HVDC) grids composed of modular multilevel converters (MMCs). The proposed method can be used for current derivative calculation under different fault scenarios including pole-to-pole, pole-to-ground, and pole-to-metallic. The proposed method is adaptable as it can provide accurate fault current derivatives in various grid topologies such as symmetric monopole, asymmetric monopole with ground or metallic return, and bipole with ground or metallic return, as well as grids with different types of converters with and without fault-blocking capability including full-bridge MMCs (FB-MMCs), half-bridge MMCs (HB-MMCs), or a mix of HB- and FB-MMCs. The paper also demonstrates how the proposed current derivative calculation method can be used to form a derivative relay, which is fast, selective, computationally efficient, and insensitive to fault resistance. Furthermore, using the proposed current derivative calculation method, all relay settings are analytically calculated instead of being obtained through time-consuming simulation studies. Simulation results for various fault scenarios, grid topologies, and converter configurations show that the calculation method is accurate and the presented relaying algorithm can detect various faults within 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s, even when the fault resistance is as high as 500 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Upomega$</tex-math></inline-formula> .

The Enhanced Modular Multilevel Converter With DC Fault Blocking Capability

The large energy-storage requirement and lack of dc-fault tolerant capability are the major drawbacks of modular multilevel converter (MMC). To address these issues, alternate hybrid topologies have been developed, but these are all non-modular and hence MMC remains the most preferred topology for high-power applications. In this paper, a new submodule (SM) structure is proposed that consists of five switches and a capacitor. Based on this SM, a novel Enhanced Modular Multilevel Converter (EMMC) is developed, which is fully modular, scalable and requires fewer switches and capacitors. It also possesses the features of reduced energy-storage requirement and dc-fault tolerant capability. The structures and operating principles of proposed SM and EMMC topology are presented. The proposed topology is compared with state-of-the-art topologies in terms of the switch counts, SM capacitors and isolated power-supplies requirements to highlight the main features of EMMC. The proposed EMMC is an effective and more efficient converter solution compared to the major existing high-power hybrid converters. Moreover, it has fully modular structure. A 21-level, grid-connected EMMC simulation model is developed to validate its working principles and dc-fault tolerant capability. The working principles and dc-fault tolerant capability are further validated using a scaled-down, 5-level EMMC experimental prototype.

An Isolated Single Input-Multiple Output DC–DC Modular Multilevel Converter for Fast Electric Vehicle Charging

This article proposes an isolated single input-multiple output (SIMO) dc–dc modular multilevel converter (MMC) for fast charging of multiple electric vehicles (EVs). The proposed topology provides galvanic isolation and facilitates a high-voltage conversion ratio from medium voltage direct current (MVDC) to LVDC (EV battery voltage levels) via intermediate medium-frequency transformer. The proposed topology, as compared to conventional MMC, has reduced number of arms, semiconductor devices and has inherent dc fault blocking capability. Operation of the converter with medium frequency reduces the size of passive components present in it. The output voltage waveshape of the converter results in nearly sinusoidal wave being impressed across transformer's primary with very less <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> . Analysis of the converter in steady state and during fault conditions have been included in this article. The proposed converter is validated in simulation model and scale down laboratory prototype.

Novel fault isolation scheme under protection failure in MVDC distribution networks without DC circuit breakers

In medium voltage direct current (MVDC) distribution networks without DC circuit breakers, converters with fault-blocking capability are employed to de-energize the network, which facilitates the disconnection of DC switches. The fault current is cleared rapidly under DC faults, posing difficulties to the reliable fault isolation due to the rapid disappearing fault features. To tackle this problem, a novel isolation scheme based on active control of converters is proposed, where trapezoidal wave current is injected from the converter to the fault point. The faulty line could be located by counting the number of high-level pulses of trapezoidal wave current, and then isolated by the corresponding DC switches during the low-level pulses of trapezoidal wave current. Finally, the healthy parts of the network could be restored to normal operation. Compared with other fault location & isolation methods, the proposed scheme based on the active control of converters could cooperate with DC switches without communication. In addition, the innovative utilization of the number of injected pulses rather than the amplitude or frequency can provide a backup function if the DC switch fails to operate. The proposed scheme further improves the reliability of the MVDC network and is easy to apply, the effectiveness of the proposed scheme is verified in PSCAD/EMTDC.

Modular Multilevel Matrix Converter as Solid State Transformer for Medium and High Voltage AC Substations

The use of power converters as solid state transformers is an attractive solution to modernize the power network, but this solution has not been fully addressed for MV and HV substations. This paper presents a customized and simple control for the Modular Multilevel Matrix Converter (M3C), specially conceived for its operation on synchronous ports, which is the case of AC substations. The control allows bidirectional power transfer, generation/absorption of reactive power and provisions of ancillary services. The converter is compared to the back-to-back Modular Multilevel Converter (B2B-MMC) where the key performance indicators to carry out the comparison are power efficiency, number of semiconductor devices, passive components required, footprint, voltage cell balance, fault blocking capability and stress of components. The simulation results show the features, performance and attractiveness of the M3C topology in a 33/11 kV, 16 MW substation under different operating conditions, including grid faults and dynamic operation. The M3C presents similar efficiency and performance than the B2B-MMC, but it uses fewer semiconductor devices, passive components and total cell capacitor energy than the B2B-MMC, reducing cost and footprint. The experimental results show the performance of the M3C under less ideal conditions including a substation transformer saturation and power step response.

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.

A Novel Medium-Voltage Hybrid-Cascaded DC–DC Converter With DC Fault-Blocking Capability and Reduced Switch Capacity

In this article, a medium-voltage nonisolated dc–dc converter with dc fault-blocking capability is proposed. The converter is designed to connect two high-voltage dc networks with varying voltage levels. The proposed converter is composed of a string of cascaded modules and two sets of series-connected switches. The cascaded modules have all the merits of modular design, and the series-connected switches avoid dynamic balancing issues due to the zero-voltage switching feature. The single-string structure of the proposed converter helps reduce the number of switches, power losses, and volume. The power efficiency of the converter is evaluated to be as high as 99.3%. The feasibility of the proposed converter is verified via 32-kV 22.5-MW simulations and 240-V 1.45-kW experiments.

A Critical Review of Modular Multilevel Converter Configurations and Submodule Topologies from DC Fault Blocking and Ride-Through Capabilities Viewpoints for HVDC Applications

Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are unable to prevent the AC side contribution to DC side fault currents, thus necessitating circuit breakers (CBs) for protection. A solution to this problem is using submodules (SMs) that are capable of blocking the flow of current from the AC grid to feed the DC side fault. The full-bridge submodule (FBSM) is one type of fault blocking SM where the presence of two extra switches ensures that in the event of a DC fault, the reverse voltage from the FBSM capacitor is placed in the path of the AC side current feeding the DC side fault through the antiparallel diodes. However, the additional semiconductor switches in the FBSMs increase the converter cost, complexity, and losses. Several SM configurations have been proposed in recent years that provide DC fault blocking capability with lower losses and device counts than those of FBSMs. Besides, many of the proposed hybrid converter configurations that combine different topologies to optimize converter performance are also capable of providing DC fault blocking. Furthermore, certain SM topologies are capable of riding through DC faults by remaining deblocked and operating in static synchronous compensator (STATCOM) mode to provide reactive power support to the AC grid. In this paper, noteworthy SM and MMC configurations capable of DC fault blocking and ride-through are reviewed and compared in terms of component requirements, semiconductor losses, and DC fault handing capability. The review also includes a discussion on control strategies for MMC arm/leg energy balancing during STATCOM operation.

Dc-MMC for the Interconnection of HVDC Grids With Different Line Topologies

DC-DC converters are needed for the future development of high voltage direct current (HVDC) grids, as they allow to interconnect lines with different voltages and topologies. The dc-dc converters can increase the grid controllability adding power flow control, voltage regulation and/or fault blocking capability. The dc modular multilevel converter (dc-MMC) is a non-isolated solution proposed to interconnect HVDC systems with the same line topology. This paper proposes a new dc-MMC with a control strategy, which allows the converter to interconnect different line topologies (e.g., rigid bipole connected to a symmetric monopole). The paper presents the different line topologies in HVDC installations. Then, a mathematical model with a variable transformation is proposed for the new dc-dc converter. A control structure is proposed and implemented in Matlab/Simulink using an average arm model and simplified dc grids. The results validate the control in normal operation, fault blocking capability and post-fault scenario (degraded mode).

Reverse Blocking Devices Based Three-Level MMC Sub-Module Topology With DC Side Fault Blocking Capability

Conventional half bridge modular multilevel converter (MMC) system will suffer great over-current when there is a dc side fault. Therefore, topology study in sub-modules with dc side fault blocking capability have become one of the hotspots in MMC field. However, fault blocking topologies using conventional IGBT devices must employ more semiconductors than half bridge topology. The extra semiconductor devices employed in the topology will lead to the increase of cost and conduction loss. To reduce the conduction loss of the fault blocking topology, a novel sub-module topology is proposed in this paper. The proposed topology employs reverse blocking devices to form the low conduction loss path. Conduction loss analysis is conducted in this paper to show the priority of the proposed topology. Simulation and experiment results verified the dc fault blocking capability of the proposed topology.

Overmodulation Operation of Hybrid Modular Multilevel Converter With Reduced Energy Storage Requirement

The hybrid modular multilevel converter (MMC) has the advantages of dc fault blocking and overmodulation capability as the promising candidate in future multiport HVdc systems. Especially, the overmodulation ability could increase the modulation index higher than 1 without voltage distortions on the ac side. However, the hybrid MMC under the overmodulation operations could suffer from the voltage deviations between the half-bridge submodule (HBSM) and full-bridge submodule (FBSM) capacitors. In this article, the structure of the self-balancing branches (SBBs) is introduced into the hybrid MMC to eliminate the aforementioned voltage deviations. As a result, the energy storage requirement (ESR) of the hybrid MMC under the overmodulation operations could be greatly reduced compared with the conventional hybrid MMC without SBBs. Besides, the voltage balancing strategy is improved to avoid the impulse current in the conduction path of the SBB, by which the current stress of the power devices could be decreased significantly. The simulations and experiments are carried out to verify the effectiveness of the proposed hybrid MMC. It shows that the ESR of the hybrid MMC could be reduced by 30% with decreased power losses under the overmodulation index 1.5 compared with the conventional hybrid MMC.

An Improved Submodule Topology of MMC With Fault Blocking Capability Based On Reverse-Blocking Insulated Gate Bipolar Transistor

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology.