Modular Multilevel Cascade Converter Research Articles

New Modulation Method to Reduce Cell Capacitor Ripple Current in Modular Multilevel Cascade Converters and its Application to Active Power Filters

New Modulation Method to Reduce Cell Capacitor Ripple Current in Modular Multilevel Cascade Converters and its Application to Active Power Filters

Internal Energy Balance of a Modular Multilevel Cascade Converter Based on Chopper-Cells With Distributed Energy Resources for Grid-Connected Photovoltaic Systems

This article presents a methodology to control the internal energy balance of a modular multilevel cascade converter (MMCC) with distributed energy resources. The converter is characterized by a high voltage multi-terminal DC link connected to a three-phase MMCC with distributed photovoltaic arrays. Each modular arm structure is composed of bidirectional cascaded chopper cells with floating capacitors connected to the photovoltaic array through a DC/DC converter. Based on the power flow equations developed for the individual cells, a control methodology is applied to maintain the internal energy balance of the MMCC under power imbalance between the cells. The operation at different levels of irradiance is discussed to allow the operation with individual cells with zero energy generated from the photovoltaic array. Zero power cells are used to maintain three-phase output voltage and stable MMCC operation. Power balance between the arms is achieved through of the AC circulating currents in the MMCC arms without affecting the DC link and AC output currents. This converter topology and control methodology is suitable for hybrid renewable energy systems such as wind-solar power systems. Comprehensive experimental results are presented to validate operating principles and control methodology.

Reconfigurable Large-Current and High-Voltage Test Bench for HVDC Circuit Breaker Verification

This article presents a novel circuit configuration of a high-voltage direct current circuit breaker (HVdcCB) test bench that is based on a modified H-bridge modular multilevel cascaded converter (MMCC). The modified MMCC is composed of fewer H-bridge cells, and it can be reconfigured during operation to allow the proposed test bench to output large current or high voltage for the current breaking and dielectric withstand tests. Although simultaneous output of large current and high voltage is not possible, the maximum transient interrupt voltage (TIV) withstand test can be performed with reduced ratings. The controllable output allows generation of complex waveforms to simulate a wide range of fault conditions. Furthermore, the modified MMCC has some inherent safety features that can reduce the need for additional protective equipment in case of operational failure of the HVdcCB. In contrast, the conventional charged capacitor and inductor-based designs cannot generate arbitrary waveforms, are only suitable for current breaking tests, and require additional circuits to generate initial conditions for the HVdcCB. AC short-circuit generator-based designs offer one degree of freedom for control of the output waveforms and can sustain the maximum TIV withstand test. However, the ac output is unsuitable for the dielectric withstand test, and additional circuits are required to provide initial conditions for the HVdcCB. The proposed test bench circuit configuration is verified using a downscaled experimental test bench that consists of a total of nine H-bridge cells with an equivalent switching frequency of 92.5 kHz.

Investigative uses of overmodulation techniques in modular multilevel cascaded converter

Sinusoidal pulse width modulation (SPWM) is a method to generate the switching gate pulse of the converter. Overmodulation is a method where the modulation index exceeds the unity value and the system goes into the nonlinear region. To maintain the system in a linear region when operating in the overmodulation region, some techniques are developed. These techniques helped to operate the system in the linear range. Medium and high-power energy conversion systems mostly use a modular multilevel cascaded converter (MMCC), which has been an issue improving significantly in recent years. In this article, MMCC-based overmodulation techniques are compared with conventional PWM and analyzed on DC bus utilization (DBU), and total harmonic distortion (THD). MATLAB/Simulink digital platform used demonstrate overmodulation technique.

Faults in Modular Multilevel Cascade Converters—Part I: Reliability, Failure Mechanisms, and Fault Impact Analysis

Modular Multilevel Cascade Converters (MMCCs) are considered a promising power electronics topology in industry. Their scalability allows to reach (ultra/very) high voltage levels with low harmonic content and high efficiency and makes MMCCs an ideal solution for high-power applications; such as electrical drives, solid-state transformers and high-voltage direct-current (HVDC) transmission systems. However, the high levels of thermal, electrical and mechanical stress on the power electronics devices and the large number of components (e.g. capacitors or semiconductors) make MMCCs prone to faults reducing its reliability. In this first part of the paper, a comprehensive overview of reliability on MMCCs, failure mechanisms and fault impact analysis in MMCCs, including failure rates and fault modes is presented. Also a set of tables which collect all information to easily detect and identify faults in MMCCs is presented.

Faults in Modular Multilevel Cascade Converters–Part II: Fault Tolerance, Fault Detection and Diagnosis, and System Reconfiguration

Modular Multilevel Cascade Converters (MMCCs) are considered a promising power electronics topology in industry. Their scalability allows to reach (ultra/very) high voltage levels with low harmonic content and high efficiency and makes MMCCs an ideal solution for high-power applications; such as electrical drives, solid-state transformers and high-voltage direct-current (HVDC) transmission systems. However, the high levels of thermal, electrical and mechanical stress on the power electronics devices and the large number of components (e.g. capacitors or semiconductors) make MMCCs prone to faults. Fault detection and diagnosis (FDD) in combination with fault isolation and system reconfiguration techniques, based on cell redundancy, can increase the reliability, availability and safety of MMCCs, which is crucial for their utilization in critical energy applications. This second part of the paper comprehensively surveys: (i) fault tolerance and fault detection & diagnosis (FDD; e.g. expert system, model- or hardware and data-based FDD methods) and (ii) system reconfiguration strategies (e.g. cold- or hot-redundant) for MMCCs. Finally, the state-of-the-art, challenges and future research trends and opportunities towards reliable MMCC-based systems are revealed.

Inter-cluster Voltage Balancing Control of Modular Multilevel Cascaded Converter Under Unbalanced Grid Voltage

This paper presents a novel inter-cluster direct current (DC) capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) under unbalanced grid voltage. The negative-sequence component of grid voltage at the point of common connection (PCC) causes unbalanced active power flow in the phase limbs of converter. This leads to the imbalance of DC voltages of the submodule capacitors across the MMCC phases, and consequently, the malfunction of converter. The proposed solution is to inject both negative-sequence current (NSC) and zero-sequence voltage (ZSV) into the phase limbs of MMCC. A quantification factor Q <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</inf> is used to achieve the sharing of intercluster active power between the NSC and ZSV injection methods. Accurate determination of the quantification factor has been presented. In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced, it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion. The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STAT-COM with 3-level bridge cells as sub-modules. The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.

Design and Analysis of a Novel HVDC Circuit Breaker Test Bench Based on an H-Bridge Cell MMCC

This paper proposes a novel circuit configuration for a high-voltage direct-current circuit breaker (HVDCCB) test bench that differs significantly from conventional test benches. The proposed test bench consists of a modular multilevel cascaded converter (MMCC) that is based on H-bridge cells, an output inductor, and an auxiliary capacitor bank. The proposed test bench is capable of generating controllable output currents up to several kiloamperes and output voltages up to several hundred kilovolts because each MMCC cell is operated by phase-shifted pulse-width-modulated (PSPWM) signals. Consequently, the proposed test bench can simulate a wide range of fault conditions within hardware limitations to test different HVDCCB types with various current and voltage ratings. The flexibility of the proposed test bench is complemented by a longer service lifetime with inherent circuit protection in the case of operational failure of the HVDCCB. The concept of the proposed test bench is verified experimentally on a downscaled test bench that consists of nine H-bridge cells and operates at an equivalent switching frequency of 92.5 kHz.

Inter-cluster voltage balancing control of a delta connected modular multilevel cascaded converter under unbalanced grid voltage

A new inter-cluster DC capacitor voltage balancing scheme for a delta connected modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) is presented. A detailed power flow analysis of applying negative sequence current (NSC) and zero-sequence current (ZSC) injection methods in addressing the issue of inter-cluster DC voltage imbalance under unbalance grid voltage is carried out. A control scheme is proposed which integrates both inter-cluster methods using a quantification factor QF. This is used to achieve the sharing of the inter-cluster active power between the NSC and ZSC injection methods. An accurate method of determining the quantification factor is also presented. The proposed method offers better sub-module DC capacitor voltage balancing and prevents converter overcurrent. The influence of unbalanced grid voltage on the delta connected MMCC-based STATCOM rating using this integrated cluster balancing technique is investigated. The control scheme is verified with a 5 kV 1.2MVA MMCC-STATCOM using 3-level bridge sub-modules, and the results show the advantages of the proposed method over other inter-cluster methods.

Online harmonic elimination pulse width modulation technique for modular multilevel cascade converter

This paper presents a novel online technique, which simultaneously exploits the global search ability of differential evolution (DE) and rapid convergence of Newton Raphson (NR) methods (named as DE-NR) to solve intricate simultaneous transcendental trigonometric set of harmonic elimination pulse width modulation equations for modular multilevel cascade converters based power systems. Major contribution of this paper is a harmonically efficient online algorithm with rapid convergence. Switching angles for an extensive range of modulation index (M,0.86⩽M≤9.97) are computed online to demonstrate the harmonically efficient working of proposed method. Total harmonic distortion values of the filtered line-to-line output voltages during online operation are restricted to 0.35% of the fundamental component, which depict successful removal of unwanted harmonics and are significantly better than the values allowed by IEEE standard 519-2014. Comparison between DE-NR, differential evolution and Newton Raphson methods demonstrates the rapid convergence behavior of DE-NR as it requires only (on average) 4 iterations compared to 490 and 182 iterations respectively. It has also shown superior harmonic control than the recently devised online Middle-Level Selective Harmonic Elimination Pulse-Amplitude Modulation method. Successful simulation and experimental validations of DE-NR method have been done by developing three-phase modular multilevel cascade converter in MATLAB-Simulink and single-phase eleven-level modular multilevel cascade converter hardware prototype.

Single-Delta Bridge-Cell Converter With Single Single-Phase Medium-Frequency Transformer Capable of Active Power Control

A modular multilevel cascade converter based on single-delta bridge cells, hereinafter referred to as the single-delta bridge-cell (SDBC) converter, is a prospective power converter for a static synchronous compensator intended for voltage regulation and stability improvement of electric power systems. However, the SDBC converter cannot exchange nonzero active power with the three-phase ac mains unless energy storage elements such as batteries are connected to the dc side of each bridge cell. This article presents a dc/1φ/3φ power converter that combines the SDBC converter with a single-phase converter via a single-phase medium-frequency transformer with an operating frequency of 150 Hz. It can control the active power of 37% of the rated reactive power between the dc source and three-phase ac mains, thus enhancing voltage regulation and stability improvement. The operating principles and control method are first discussed, followed by a description of experiments using a downscaled 110-V, 5-kVA model.

Analysis and Control of a Modular Multilevel Cascaded Converter-Based Unified Power Flow Controller

This article presents a novel configuration for a unified power flow controller (UPFC) comprising a modular multilevel cascaded converter (MMCC) with a full-bridge inverter. The MMCC has one end of each phase-leg shunt-connected to the transmission line. The other end connects in parallel to the primary terminals of a series line transformer, and the ac output terminals of a full-bridge dc–ac inverter. The submodules in the MMCC are full-bridge flying capacitor converters. This UPFC is compared to another type of MMCC-UPFC which uses double-star configuration, and submodules of half-bridge chopper type; this is referred to as the Double Star Chopper Cells UPFC (DSCC-UPFC). The comparison is in terms of footprint, cost, and performance. The new topology is lighter, more efficient, and cheaper than the DSCC. Its operating principle and control scheme, which combines the regulation of voltage and of transmission line power flow, are presented. Simulation studies for the proposed MMCC-UPFC realizing power flow control in a dual voltage sourced power network are presented and show good performance under varying operation conditions. The article also evaluates the power control margins of this device.

A Flexible DSP-FPGA Based Platform for Experiments with Modular Multilevel Cascade Converters

&#x0D; The growing use of Modular Multilevel Cascade Converter (MMCC) topologies in applications such as High Voltage Direct Current Transmission, Static Var Compensation, Alternating Current (AC) Drives and renewable energy systems, among others, turns their study relevant. However, as the number of Power Submodules in the MMCCs increases, the applied control strategies, data communication and digital hardware complexity increases as well. This paper proposes a flexible and scalable platform, that is adequate for implementing different MMCC topologies (single star, single delta or double star), communication schemes (star or ring) and control strategies (centralized or distributed). The Processing Board explores the availability of Field Programmable Gate Arrays, Digital Signal Processors and ARM microcontrollers. The Power and Measurement Submodules processing capability is provided by Complex Programmable Logic Devices and ARM microcontrollers. Description of the tasks implemented in the processing devices and experiments are presented for a test case, in which a double star topology with star communication and centralized processing strategy is tested using fixed switching frequency and equally displaced carriers.&#x0D;

A delta-connected MMCC-based active power conditioner for unbalanced load compensation and harmonic elimination

Modular multilevel cascaded converters (MMCC) are seen in medium and high voltage applications and have gained attention from industry. This paper presents new control techniques to improve accuracy and control response speed when applied to a delta-connected MMCC- based Active Power Conditioner used for negative sequence and harmonic current elimination. A novel reference current generation scheme combining a notch filter with Decoupled Double Synchronous Reference Frame (DDSRF) is proposed which can extract the negative sequence and a selection of dominant harmonic currents accurately with the minimum time delay. Fast and accurate tracking the extracted reference current is obtained by using a model-based predictive controller with a derivative reference current term. The control strategy is applied to an experimental delta-connected MMCC-APC and the resultant waveforms are shown and analysed, confirming that the grid current quality after compensation has improved significantly.

Development of an SDBC-MMCC-Based DSTATCOM for Real-Time Single-Phase Load Compensation in Three-Phase Power Distribution Systems

This paper proposes a newly developed single-delta bridge-cell, modular multilevel cascade converter (SDBC-MMCC)-based distribution-level static synchronous compensator (DSTATCOM) for single-phase load compensation in three-phase, three-wire electric power distribution systems. Each main circuit arm of the DSTATCOM uses a modular multilevel cascade converter based on full-H-bridge (FHB) cells. The three main DSTATCOM arms are delta-connected to allow phase-independent operations for phase balancing and unity power factor correction of the single-phase load in three-phase, three-wire electric power distribution systems. By using the symmetrical components method, a feedforward compensation algorithm was employed for the DSTATCOM. A simulation of the DSTATCOM was performed for functioning verification. Finally, a hardware test system was built by using a multi-DSP-based control system. The test results verified the effectiveness of the proposed SDBC-MMCC-based DSTATCOM in single-phase load compensation.

Carrier-based sinusoidal pulse-width modulation techniques for flying capacitor modular multi-level cascaded converter

Carrier-based sinusoidal pulse-width modulation (PWM) techniques, such as phase disposoed PWM(PD-PWM) and phase shifted PWM (PS-PWM), are widely applied to control the modular multilevel cascaded converters (MMCC) having full H-bridge as sub-modules. This paper evaluates these PWM techniques when controlling a variant of the H-bridge MMCC, i.e. the MMCC five-level flying capacitor converter as sub-modules. This MMCC poses an extra challenge to PWM schemes; namely maintaining two inner floating capacitor voltage balancing. Two novel PWM techniques known as the swapped carrier PWM techniques are introduced for the control of this converter. The paper compares them with the two conventional ones using a performance metrics composed of voltage waveform performance, capability in natural flying capacitor voltage balancing, converter power loss, and switch utilisation. The results show that the proposed new PWM schemes outperform both conventional methods in both switching and conduction power losses and achieve similar performance like the PS-PWM under the three other metrics.

Hybrid Asymmetric Cascaded Multilevel Inverters Based on Three- and Nine-Level H-Bridges

This paper proposes two hybrid asymmetric cascaded multilevel inverters for driving medium-voltage open-end winding motors. Each phase of the proposed systems consist of a three-level (3L) H-bridge cell and a nine-level (9L) H-bridge cell connected in series. An unequal dc-voltage ratio of 1:8 between cells is used, providing 19 phase voltage levels. The topologies of the 9L H-bridge cell are based on modular multilevel cascade converter (MMCC) with double-star chopper cells, while the 3L H-bridge cell is fed by a floating capacitor. Compared to conventional MMCC for open-end stator winding motor drives, the proposed inverters can produce more phase voltage levels. In addition, they require a smaller number of power semiconductor switches, drivers and capacitors. Simulations and experimental results are presented in order to demonstrate the performance and feasibility of the proposed topologies.

A DC-Link Capacitor Voltage Ripple Reduction Method for a Modular Multilevel Cascade Converter With Single Delta Bridge Cells

This article proposes a capacitor voltage ripple reduction method used for a modular multilevel cascade converter (MMCC) with single delta bridge cells (SDBC), by applying a third harmonic zero-sequence current. A practical case study on an 80 MVar/33 kV MMCC-SDBC-based STATCOM is used to demonstrate the method. The impact of the third harmonic zero-sequence current level of the capacitor ripple reduction and the electrothermal stresses on insulated gate bipolar transistor modules are analyzed. An optimal parameter of the current level is obtained by compromising the above-mentioned performance factors. The obtained result shows that the required capacitance, as well as the capacitor bank volume, is reduced by 20% without increasing the total power semiconductor losses by using the proposed method.

Two-leg Modular Multilevel Cascade Converter as Active Power Filter - Control Improvement

This paper analyses the use of a simplified ac-side current control algorithm strategy, independent of the number of submodules, applied to a Modular Multilevel Cascade Converter acting as an Active Power Filter (MMCC-APF) with a reduced hardware structure. Here, Model Predictive Control (MPC) with a reduced number of states is used for both ac-side current control and submodules' capacitor voltage balancing algorithm. Furthermore, analytical criteria are developed, to define the minimum number of states required for current control. This approach improves the control strategy, thus reducing computational effort for MMCC in APF applications. For the calculation of APF compensating currents, two main strategies are addressed, in order to assess ac-side currents control limitations: (i) total and (ii) specific frequencies harmonic compensation. The system analyzed in this paper consists of a nine-level MMCC connected to a medium voltage distribution grid, compensating the harmonics drained by a diode rectifier. All the analyses are conducted in the environment of circuit simulation PSIM.

Asymmetrical Reactive Power Capability of Modular Multilevel Cascade Converter Based STATCOMs for Offshore Wind Farm

Modular multilevel cascade converters (MMCCs) are becoming attractive solutions as high-voltage Static Synchronous Compensators (STATCOMs) for power plants in renewable energy generation, in order to satisfy the strict grid codes under both normal and grid fault conditions. This paper investigates the performances of four potentially used configurations of the MMCC family for the STATCOM in large-scale offshore wind power plants, with special focus on asymmetrical low-voltage ride through capability under grid faults. The specifications and the sizing of components of each type of practical 80-MVar/33-kV-scaled MMCC-STATCOM are carefully designed and compared. The total cost and volume are compared based on the total power semiconductor chip area and the total energy stored in the passive components. Asymmetrical reactive power delivering operation of the MMCC family considering the dc-link capacitor voltage-balancing method is solved mathematically in order to quantitatively understand the performance limitations and behaviors. The electrothermal stress of the power modules used in each type of the MMCC for a practical 80-MVar/33-kV-scaled STATCOM is analyzed. The asymmetrical reactive power capability of the MMCC solutions is compared under different scenarios of grid faults, while considering the device temperature limits as well as voltage saturation. It is found that the MMCC configuration with double-star bridge cells becomes the most attractive circuit configuration for the STATCOM application based on the obtained results.