Half-bridge Modular Multilevel Converter Research Articles

Enactment of HBMMC five-level inverter based D-STATCOM using robust controllers

This paper provides the results of a concerted investigation into developing a robust voltage controller for the modular multilevel converter (MMC) used in the D-STATCOM. The simplicity of the half bridge (HB) sub-module, along with the fact that it has minimal conduction losses, led to its adoption in the MMC. For the purpose of controlling the HB MMC switches, an enhanced modulation strategy based on the phase disposition (PD) scheme has been implemented. In the presence of nonlinear load conditions, the converters are experiencing difficulties with the DC voltage balancing. The sliding mode controller (SMC) is utilized so that the DC voltage can be maintained in a balanced state. Performance characteristics including active power, reactive power, dc voltage control and THD are shown and compared. For the purpose of simulating the indicated methodology, MATLAB/Simulink is the tool of choice.

An Active DC Fault Current Limiting Control for Half-Bridge Modular Multilevel Converter Based on Arm Voltage Reconstruction

An Active DC Fault Current Limiting Control for Half-Bridge Modular Multilevel Converter Based on Arm Voltage Reconstruction

Auto tuning of PI Gains using Cuttlefish Optimization for DC Link Voltage Control in a 5-level HB MMC D-STATCOM

The performance of a Half Bridge (HB) Modular Multilevel Converter (MMC) for D-STATCOM application is studied in this article. For the control of MMC switches, the Carrier-Based (CB) Pulse Width Modulation (PWM) scheme has been selected as the appropriate method. There are two types of CB PWM schemes such as phase shift and level shift. Phase Disposition (PD), Phase Opposition Disposition (POD), and Alternate Phase Opposite Disposition (APOD) are the three different types of PWM that are used in level shifts. The PD CB PWM is projected in this work due to its simplicity and accuracy. The conventional PI Controller (PIC) has its disadvantages, including that it is time-consuming and comes with complexity in the estimation of gain values. Hence, the Cuttlefish Optimization Scheme (COS) is proposed for the online tuning of the PI gain values. The verification of performance parameters, including active power, reactive power, DC voltage balance, and Total Harmonic Distortion (THD), is effectively conducted using the COS method and subsequently compared with the conventional PIC. The suggested research is executed using the Matlab/Simulink software package.

A selection principle of submodule switching state vectors for switching frequency reduction in voltage self‐balancing half‐bridge modular multilevel converters

AbstractThe problem of submodule switching frequency reduction in half‐bridge modular multilevel converters (HB‐MMCs) with capacitor voltage self‐balancing control is considered and explored in this paper. A selection principle of submodule switching state vectors is proposed based on the voltage self‐balancing switching state matrix, aiming to lower submodule switching frequency and device losses. The relationship between system stability and submodule switching signals is revealed according to the capacitor voltage self‐balancing characteristics, and the full‐rank constraints on the voltage self‐balancing switching state matrix are proposed. Considering the tradeoff between switching loss and capacitor voltage fluctuation, the evaluation indexes of voltage self‐balancing control effect are determined. The selection principle of submodule switching state vectors and the optimized construction method of switching state matrix are presented. Voltage self‐balancing HB‐MMC models are built in MATLAB/Simulink, and it is verified that the submodule switching state vector selection principle proposed in this study can effectively reduce switching frequency while meeting the evaluation requirements of practical engineering projects, so as to achieve the balance between switching loss and steady‐state capacitor voltage fluctuation.

HEG-T2F: Hybrid Ebola Gannet-Based Type 2 Fuzzy-Based Controller for Modular Multilevel Converter

Modular multilevel converter (MMC) plays a significant part in high voltage power electronic industries due to its wide range of benefits such as modularity and reliability. The circulating component exists between three-phase half-bridge MMC causing certain issues in the design of MMC such as system loss, voltage instability, current disturbance, etc. To handle such problematical factors, a novel circulating current control mechanism named ‘hybrid ebola gannet based generalized type 2 fuzzy controller (HEG-GT2FC)’ is proposed in this paper. Here, the main controlling parameters of MMC namely harmonic distortion, current ripple, and circulating current settling time are adjusted or minimized using the generalized type 2 fuzzy controller. The hybrid ebola gannet algorithm solves the high voltage conversion problem by determining the optimal solution. The proposed HEG-GT2FC mechanism is evaluated using the MATLAB/Simulink tool. The performance of the proposed HEG-GT2FC mechanism is investigated in terms of measures such as current ripple, total harmonic distortion, and circulating current settling time. The simulation results illustrate that the proposed HEG-GT2FC mechanism achieves a very low percentage of current ripple (0.09%), total harmonic distortion (4.98%), and circulating current settling time (0.8 sec) than other compared state of art techniques.

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> .

Exploration of the HBSM MMC Five-Level Inverter for D-STATCOM Application

This paper presents the concert exploration of a Modular Multilevel Converter (MMC) and its application for D-STATCOM. The Half Bridge (HB) sub-module is adopted in the MMC due to its simplicity and low conduction losses. An advanced modulation technique of the Phase Disposition (PD) scheme is applied for the control of the HB MMC switches. The converters are facing DC voltage balance problems under non-linear load conditions. The Proportional and Integral (PI) controller is adopted for the balance of DC voltage. Performance parameters such as apparent power, active power, reactive power, power factor, DC voltage balance, and Total Harmonics Distortion (THD) are presented and compared with the ones of the traditional two-level converter. The proposed scheme is simulated in MATLAB/Simulink.

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.

Impedance Phase Based Active Injection Protection for Radial VSC-HVDC Grid

Effective DC fault identification is very important for a half-bridge modular multi-level converter (HB-MMC) based high voltage direct current (HVDC) grid. The performance of the traditional protection principles lacks reliability and sensitivity, and there is a further limitation under the current limiting strategies. To address the problem, a scheme coordinating the control and protection is proposed, which actively creates the known feature for fault identification and limits the fault current at the same time. In this paper, the characteristics of the input impedance phase under different faults are analyzed in various frequency bands. Based on the analysis of the input impedance phase, the converter switches the control mode and generates a characteristic signal at a particular frequency after fault detection. Then, the Prony algorithm is employed to acquire the phase information at the injection frequency. Finally, the faulty zone can be identified by the value of the input impedance phase. This active injection protection scheme is applicable to identify different faults, verified in PSCAD/EMTDC, and tested by hardware-in-the-loop simulation in RTDS.

초고속차단기와 바이패스 사이리스터를 이용한 MMC의 DC-Link 고장전류 차단 방법

The modular multi-level converter (MMC) has recently gotten great attention due to the installation of large renewable generation farms and high voltage direct current transmission (HVDC). In the same manner as the conventional inverters, MMC operates based on multiple insulated-gate bipolar transistors (IGBTs). Each IGBT is generally combined with an anti-parallel diode in the half-bridge MMC, the most efficient MMC topology. Due to the diode path, the DC fault current, caused by the AC phase voltage differences when its DC-Link bus shorts, flows the MMC freely. This paper proposes a DC-Link fault current interrupting method based on Ultra Fast Disconnectors (UFDs) and bypass thyristor. The proposed method is verified based on electromagnetic transient (EMT) level simulations, including several case studies with various short circuit ratios (SCRs) and reluctance per resistance (X/R) ratios. As a result, the proposed method successfully blocks the DC-Link fault current within at most one cycle. The case studies are conducted using a power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDCTM).

Low-Cost and Compact Asymmetrical Unidirectional-Current Modular Multilevel Converters

Although the modular multilevel converters (MMCs) that contain submodules (SMs) with negative voltage capability have various advantages, they usually demand many more semiconductors than the conventional half-bridge SM-based MMC (HB-MMC). This study proposes a unidirectional-current clamp-double submodule (UC-CDSM) by combining two unidirectional-current full-bridge SMs (UC-FBSMs) using a shared switching device. The sharing design enables the UC-CDSM-based MMC (UC-CD-MMC) to have 25% fewer switching devices compared with the UC-FBSM-based MMC. The quantity of switching devices is rather similar to that in a conventional HB-MMC while the UC-CD-MMC still retains the advantages, such as low capacitor usage, dc fault clearing capability, and wide-range dc voltage adjustability. Moreover, a unidirectional-current hybrid MMC composed of UC-CDSMs and UC-FBSMs (UC-HYB-MMC) is presented to further enlarge the adjustable range of dc voltage. Detailed comparisons indicate that the UC-CD- and UC-HYB-MMCs can reduce the valve costs by 32% and 25%, respectively, and volumes by 39% and 34%, respectively, compared with HB-MMCs. Simulation and experimental results verify the steady-state and dc-fault clearance of the proposed topologies, and that the capacitor voltages in the UC-FBSMs and UC-CDSMs are well maintained and balanced in the UC-HYB-MMC.

Continuous Operation of Wind Power Plants Under Pole-to-Ground Fault in an HVDC System Consisting of Half-Bridge MMCs and Disconnecting Switches

This article focuses on a multicircuit HVDC system which can localize the dc line fault without dc circuit breakers. The conventional fault clearing method causes the shutdown and the restart of the wind power plants, which results in the suspension of the power supply. The suspension may affect the balance of supply and demand, and may cause frequency drop in the onshore ac power system. To solve the problem, this article proposes a system switching method to enable the continuous operation of the wind power plants under the dc line fault. The proposed method utilizes the half-bridge modular multilevel converter to clear the fault while keeping the wind power plants operating. As a result, the proposed method realizes the continuous operation of wind power plants and shorten the transmission outage to 0.2 s. Thus, it greatly increases the supply reliability. Experimental results demonstrate the validity of the proposed method.

Harmonic Current Emission Estimation of Modular Multilevel Converters in MV Charging Stations

This paper proposes a novel framework for modular multilevel converter (MMC) current harmonic emission estimation from medium voltage (MV) charging stations. The framework is enlightened by wave decomposition, exploiting an analytical derivation of the emission, without a time-domain model. Accordingly, voltage harmonic distribution in grids can be estimated with a frequency-domain model, using the estimated voltage-dependent current emission in the iterations. Compared to conventional time-domain simulation, such method not only enables analytical harmonic estimation integrated with a large load-flow model, but also benefits from the potential of distributed computation. A 10.5 kV grid-tied single-phase half-bridge MMC simulation case on PLECS has been implemented and investigated for comparison. Corresponding results revealed that the estimated harmonic series kept the absolute average difference to the PLECS model lower than 0.02% and a correlation ratio higher than 99.990% with the simulated ones. The effectiveness of the proposed estimation framework is successfully verified.

A Pilot Protection Scheme of DC Lines for MMC-HVDC Grid Using Random Matrix

The over-current capacity of half-bridge modular multi-level converter (MMC) is quite weak, which requests protections to detect faults accurately and reliably in several milliseconds after DC faults. The sensitivity and reliability of the existing schemes are vulnerable to high resistance and data errors. To improve the insufficiencies, this paper proposes a pilot protection scheme by using the random matrix for DC lines in the symmetrical bipolar MMC high-voltage direct current (HVDC) grid. Firstly, the 1-mode voltage time-domain characteristics of the line end, DC bus, and adjacent line end are analyzed by the inverse Laplace transform to find indicators of fault direction. To combine the actual model with the data-driven method, the methods to construct the data expansion matrix and to calculate additional noise are proposed. Then, the mean spectral radiuses of two random matrices are used to detect fault directions, and a novel pilot protection criterion is proposed. The protection scheme only needs to transmit logic signals, decreasing the communication burden. It performs well in high-resistance faults, abnormal data errors, measurement errors, parameters errors, and different topology conditions. Numerous simulations in PSCAD/EMTDC confirm the effectiveness and reliability of the proposed protection scheme.

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.

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.

A secure system integrated with DC-side energy storage for renewable generation applications

Massive energy storage capability is tending to be included into bulk power systems especially in renewable generation applications, in order to balance active power and maintain system security. This paper proposes a secure system configuration integrated with the battery energy storage system (BESS) in the dc side to minimize output power fluctuation, gain high operation efficiency, and facilitate fault ride through, which is suitable for unidirectional renewable power generation systems (power transfer from renewable sources to the grid). The system utilizes robust diode units (DUs) to protect receiving-end devices against dc faults. Also, the BESS and half-bridge modular multilevel converter (MMC) at the receiving end can operate safely and flexibly to achieve stable and high-quality power transfer, in both source power intermittency and dc-link fault cases. Depending on BESS sizing, the source system power fluctuation can be reduced (absorbed by the receiving-end BESS) when a receiving-end grid fault occurs. Topological configuration and control design of the proposed system are presented. Simulation results show the effectiveness of the proposed system in both dc and ac fault cases, with power fluctuation elimination functionality highlighted. The receiving-end operation losses are investigated, showing a high-efficiency system. In addition, key system implementation considerations regarding the proposed system are elaborated.

Adaptive sequential reclosing strategy for hybrid HVDC circuit breakers in MMC‐based DC grids

DC circuit breaker (DCCB) is one of the most promising solutions for handling DC fault in half-bridge modular multilevel converter (MMC)-based DC grid. Generally after fault isolation, DCCBs are required to have the ability to quickly reclose so as to restore power transmission. However, the traditional simultaneous reclosing scheme may make the whole system suffer from secondary strikes such as overvoltage and overcurrent in the event of permanent faults. To solve this problem, many adaptive reclosing schemes have been proposed. Among them, the sequential reclosing scheme can achieve rapid recovery without laying any burden on the sampling and protection system. Unfortunately, secondary strikes under a reclosing failure still exist though they can be suppressed. Due to this, this paper illustrates that the overcurrent protection-based fault identification method is not able to identify the fault in time, and thus the secondary strikes are generated during the second tripping. Optimal configuration of DCCBs' parameters can reduce these adverse impacts but cannot avoid them. Based on this, an adaptive reclosing scheme is proposed. Permanent and temporary faults are recognised according to the voltage characteristics at the beginning of the fault line as soon as the arresters are conducted. Extensive simulations on a four-terminal DC grid in PSCAD/EMTDC show that the proposed method can eliminate the potential adverse impacts and is robust to fault resistance.

A global fault current limiting strategy for the MMC-HVDC grid with a reduced DC reactor

The large current limiting reactor (CLR) and DC circuit breaker (DCCB) are required in the half-bridge modular multilevel converter (HB-MMC) project to isolate the DC fault. To limit the fault current and reduce the CLR, a global virtual impedance-based fault current limiting (VIFCL) control strategy is proposed in this paper. This strategy includes two parts: the first part is a simplified adaptive fault current limiting control for limiting the DC fault current by reducing the arm modulating voltage of the MMC adaptively. The second part is a virtual impedance control for limiting the AC currents and the arm currents of the MMCs by mapping the virtual impedance to the reactive power control. The effectiveness of the proposed VIFCL control strategy is verified in the PSCAD/EMTDC using a four-terminal HVDC grid. The proposed global VIFCL control can limit the interrupted currents of the DCCBs and reduce the CLR. The proposed strategy is beneficial for the MMCs to ride through the DC faults safely without a significant influence on the power recovery of the HVDC system.

Active injection protection scheme for flexible HVDC grids based on amplitude of input impedance

High-voltage direct current (HVDC) grids require fast and reliable protection of the DC lines. The performance of traditional protection schemes is easily impaired by the limitations of the boundary condition and nonlinearity from the control of converters. One of the key technologies for flexible HVDC grids is the half-bridge modular multilevel converter (HB-MMC). Considering the high controllability of HB-MMC, this study proposes an active injection protection scheme to improve the reliability and sensitivity of the HVDC grid protection. The HB-MMC is used to inject a sinusoidal characteristic signal, at the specified frequency, into the DC lines. Then, the voltage and current at the specified frequency are extracted using the Prony algorithm to calculate the input impedance, which is used for the identification of internal and external faults. The active injection protection scheme was simulated for various cases in the simulation software Power Systems Computer Aided Design. The simulation results indicate that the proposed protection scheme is highly reliable and can overcome transition resistance.