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In this article, we introduce a model predictive control with optimal switching sequence (OSS-MPC) for a multilevel static synchronous compensator (STATCOM). The studied STATCOM is based on a hybrid multilevel inverter named HC-FNPC formed by the cascaded connection of H-bridge cells with a full-bridge neutral point clamped cell (FNPC). In this article, the optimal switching sequences are derived for HC-FNPC STATCOM by considering the impact of each switching sequence on the voltage of capacitors. Then, the control objectives of hybrid multilevel STATCOM, i.e., ac current control and capacitors’ voltages balancing, are formulated in terms of OSS-MPC for the first time. In contrast to the conventional MPC, this method provides a fixed-switching frequency. Also, by a combination of optimal switching sequences in MPC, the regulation of capacitors’ voltages with unequal values is realized. The control system's performance is verified on a nine-level (9L) 2.4 Mvar/11 kV HC-FNPC STATCOM in MATLAB/Simulink. Also, the experimental results are provided on a 9L 1.25 kvar/220 V scaled-down hardware prototype. [ABSTRACT FROM AUTHOR]

In the event of a DC side fault in modular multilevel converters (MMCs), the fault current contributions are initially made by submodule (SM) capacitor discharge, which occurs before the fault is detected, followed by the AC side contribution to the DC side fault. While the AC side currents can be regulated using fault blocking SMs, the initial discharge of the SM capacitors results in high DC fault currents, which can take several milliseconds to be brought under control. This paper presents a method to actively control the rate of rise of the DC fault current by regulating the discharge of SM capacitors and accelerating the suppression of fault current oscillations during fault ride-through (FRT) in a full-bridge (FB)-MMC system. In the proposed method, the discharge direction of the FBSM capacitors is reversed following the detection of a DC side fault, which leads to a reversal in the fault current direction and a fast drop-off towards the zero-crossing. Immediately after the zero-crossing of the DC fault current, the DC fault is cleared by adjusting the arm voltage references and operating the MMC as a static synchronous compensator (STATCOM) to provide voltage support to the AC grid. The proposed control scheme provides faster fault current suppression, more effective SM capacitor voltage regulation, low AC side and MMC arm current transient peaks, and an overall superior DC-FRT performance compared to methods in which the conventional fault ride-through operation is initiated immediately upon DC fault detection. [ABSTRACT FROM AUTHOR]

This paper presents a new power electronic topology for a STATCOM allowing the generation of multilevel AC voltages. This topology is characterized by a modular configuration since it uses two well known four-leg two-level voltage source inverters. A Scott transformer with multiple secondary windings connects the two four-leg inverters to the grid. One of the important features of the proposed power topology is the capability to generate AC voltages with high number of levels, in spite of using only two four-leg two-level voltage source inverters. To maximize the number of voltage levels a geometric progression ratio of 3 between the windings of the transformers connected in cascade is proposed. However, to reduce the turns-ratio span, alternatively a fractional geometric ratio of 1.5 is also proposed and analyzed. A control strategy adapted to the proposed power converter topology is also presented. Besides the control of the reactive power the regulation of the four-leg inverter DC voltage will also be considered. To support the theoretical studies and assumptions, simulations and experiments are presented. Experimental results were obtained using a low power laboratory prototype. The results shown give evidence to the characteristics and features of the proposed power converter topology and control system. [ABSTRACT FROM AUTHOR]

This article investigates the operation of cascaded H-bridge (CHB) based static synchronous compensator (STATCOM) after failure in its submodules (SMs). Once a switch fails, the corresponding faulty SM is bypassed. In the post-fault operation, a zero-sequence voltage (ZSV) is injected to the converter phases to generate the maximum possible line-to-line voltage. Using a clamping technique, the fundamental component of injected ZSV (FZSV) is minimized according to the operating point of CHB STATCOM. But, FZSV leads to unequal flow of active power into the converter phases. In the proposed method, negative-sequence current is controlled to manage the active power flow within CHB phases. By doing this, the effect of FZSV on active power of CHB phases is compensated and the voltages of capacitors are kept balanced. This method ensures the continuous operation of CHB STATCOM with the minimum over-design and guarantees the voltage balance of capacitors after a failure. The presented simulation and experimental results validate the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A type of peak-value dc voltage control (denoted as PK control) was proposed in the literature for supporting the use of a smaller dc-side capacitance when the single-phase voltage source converter is operated as a static synchronous compensator. Although it was demonstrated to operate stably under several conditions, it will be revealed in this article how the PK control will suffer from a more severe small-signal stability issue under nonideal grid conditions than the conventional method of controlling the average value of the dc voltage (denoted as Avr control). Especially, it will be shown how the PK control is sensitive to some of the control parameters. To obtain these results, a parameter-oriented stability analysis method is developed in the linear time-periodic framework. Then, it is utilized for parametric stability assessments of the Avr and PK control. Finally, both frequency- and time-domain experimental results verified the effectiveness and accuracy of the applied method in the presented analysis. [ABSTRACT FROM AUTHOR]

With the high penetration of the power electronic loads in the grid, the stability of static synchronous compensator (STATCOM) devices is greatly challenged. However, the conventional control methods for the modular multilevel converter (MMC) based STATCOM only consider the stability with small signal disturbances. This article proposes a novel dual-layer back-stepping control (BSC) for the MMC-based STATCOM. In the first layer, the BSC aims to regulate the sum of the capacitor energy and the reactive output current. In the second layer, the BSC aims to control the circulating current. Therefore, the proposed method possesses a fast dynamic response and accurate tracking with the Lyapunov stability of the MMC-based STATCOM. Compared with the arm-control-based BSC for MMC-based inverters, the proposed method has a simplified structure and a reduced computation burden. Moreover, the proposed method realizes the independent control between the output current and the circulating current. The simulation and experimental results verify the effectiveness of the proposed method. In addition, its robustness toward different circuit parameters and the operation ability under unbalanced grid fault is also verified. [ABSTRACT FROM AUTHOR]

The model predictive control (MPC) methods have been widely applied due to the fast dynamic performance and multiple control targets. However, they show certain limits when applied to the multilevel converters, such as unfixed switching frequency, large computation burden, and complex weighting factors selection. This article proposes a novel modulated MPC (M2PC) method for cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM). Instead of multitarget integration and weighting factor design, the proposed method disintegrates the voltage balancing control process from the cost function. The evaluated control sets for output current control is decided by the search step and the search range in each period. By proper design and selection of the adaptive search step for the output voltage references under α–β coordinate, the search range can be reduced, which greatly reduces the number of evaluated control sets in each control period. Therefore, the dynamic of output characteristics and the low computation burden of the MPC method are guaranteed simultaneously. In addition, with the simple and independent integration of the capacitor voltage balancing control, the proposed method can be easily applied in the CHB converter-based STATCOM application. Furthermore, this article provides a delay compensation for the prediction process, and it provides a fixed switching frequency for each H-bridge cell. Simulation and experimental results verify the effectiveness of the proposed M2PC method. [ABSTRACT FROM AUTHOR]

This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results. [ABSTRACT FROM AUTHOR]

This article develops a static synchronous compensator (STATCOM) based on the hybrid cascaded converter which has the advantage of reduced number of power transistors and floating capacitors compared to the cascaded H-bridge converter. A hybrid modulation scheme is employed in the hybrid cascaded converter to reduce the switching losses while maintaining the output performance, and the corresponding dc capacitor voltage balancing control scheme is proposed. The laboratory test results are provided to verify the feasibility of the developed STATCOM and the effectiveness of the proposed scheme. Furthermore, the proposed scheme is compared with the conventional scheme to reveal the improvement in output quality, power losses, and even fault ride through capability. [ABSTRACT FROM AUTHOR]

This article discusses development and verification test results of the 6.6-kV 200-kVA transformerless static synchronous compensator (STATCOM). This STATCOM is characterized by the use of a modular multilevel single-delta bridge-cell (SDBC) converter and silicon carbide metal-oxide-semiconductor field-effect transistor (MOSFET) modules. The article discusses a control method for the 6.6-kV system with focus on dc-capacitor voltage control. The voltage control presented in this article is different from the ones presented earlier. It consists of intercluster balancing control and intracluster balancing control. The former, also known as cluster balancing control, eliminates the requirement of the separate overall dc-voltage control present in conventional methods, whereas the latter, also known as individual balancing control, is replaced with a new technique based on the control of a dead time of each bridge cell. The article makes a detailed description of the latter. Experimental results obtained from the 6.6-kV 200-kVA verification test equipment validate the effectiveness of the control method. Moreover, successful test results confirm the efficacy of the system for grid-voltage regulation and load compensation. [ABSTRACT FROM AUTHOR]

This paper proposes a dc-link voltage shaping method for a cascaded H-bridge (CHB) static synchronous compensator (STATCOM). A recently proposed low capacitance-STATCOM system, which intentionally operates with large voltage ripples on the dc-link capacitors, offers the advantage of superior harmonic performance. However, the capacitor voltage ripples decrease when operating at below-rated reactive current, which removes the aforementioned harmonic performance benefit of this system. In addition, such harmonic performance benefit is only achieved in capacitive operation mode. In order to expand the operating region where advantageous voltage ripples are present, a new voltage shaping method is proposed. The proposed method uses a CHB multilevel converter coupled with simple dc–dc converters to shape voltage ripples on the dc-link capacitor regardless of its operating point. This is achieved by controlling the average current of the dc–dc converter using model predictive control. The concept is best applied to an enhanced STATCOM, or E-STATCOM, application in which the STATCOM is integrated with either a renewable energy source or energy storage system. The operational principle of the proposed approach is validated via simulation and experimental results on a five-level single-phase CHB STATCOM. [ABSTRACT FROM AUTHOR]

Delta-configured Cascaded H-Bridge (DCHB) topology is a suitable alternative for Static Synchronous Compensator (STATCOM) applications. However, under unbalanced voltage and/or current conditions, zero-sequence current needs to be injected to guarantee dc-link capacitor voltage balancing. The control loop of this zero-sequence current is analyzed in this paper, in order to determine the requirements that the implemented controller must fulfill. Considering these requirements, appropriate transient response and stability margin indicators are defined to quantify and evaluate the performance of different controllers that could be employed —Proportional-Resonant controller (PR), PR controller with delay compensation (PR d), and Vector Proportional-Integral controller (VPI) are proposed and analyzed for this application in this paper. Based on the defined indicators, the VPI is the most appropriate among the studied control techniques. Experimental results validate the analytical model of the controllers and their performance. • Identification of the particularities of the i 0 control loop in a DCHB STATCOM. • Definition of appropriate indicators to quantify the performance of the controllers. • Comparison of the PR, PR d and VPI controllers. • VPI preferable option to the PR and PR d in the scenario under study. • Experimental validation in a 100 kVA set-up. [ABSTRACT FROM AUTHOR]

In this paper, a static synchronous compensator (STATCOM) is proposed in which a new modular structure with conventional three-phase voltage source inverters is used to multilevel operation. The inverters are connected to a transformer in an open-end winding configuration and with three independent DC links. In this structure, the inverters will be connected in a cascade configuration. Besides the multilevel operation, the system is also characterized by low required blocking voltages for switch ratings, since the proposed STATCOM can apply the double of the DC capacitor voltage to the transformer windings. The development of the control system for the proposed topology is also presented. The operation of the proposed topology and the performance of the developed control system will be tested through several simulation and experimental tests. [ABSTRACT FROM AUTHOR]

Fault-current handling capability of the modular multilevel converters (MMCs) under dc-cable short-circuit conditions is a major concern for the MMC applications on the high-voltage direct-current (HVDC) transmission systems, where the MMCs based on half-bridge submodules (SMs) cannot block the fault currents to protect the converter devices. In this paper, a comprehensive review for the fault-ride-through capability of the HVDC transmission systems based on the MMCs adopting different SM schemes is presented, where the MMCs can block the fault currents and compensate the reactive currents to the electric grid during the dc faults. An analysis of the dc short-circuit faults in the MMC is introduced and then the operation principle of different SM circuits building the MMC for blocking the fault currents is highlighted. The fault-tolerant operation of these MMC schemes as static synchronous compensator to enhance the ac grid stability during the dc faults is also investigated. A comparison in terms of investment cost, loss, volume, and controllability for various MMC topologies is performed. Comprehensive simulation results for the most promising topologies of the MMC with a capability of fault-ride through under dc-fault conditions are finally presented. [ABSTRACT FROM AUTHOR]

We have developed a full-scale prototype of transformerless static synchronous compensator (STATCOM), rated at 6.6 kV and 100 kVA, based on a hybrid cascade multilevel converter using SiC devices. The topology employs multivoltage converter cells, Si and SiC semiconductor devices, and hybrid modulations in the converters. One phase of the STATCOM has two Si insulated-gate bipolar transistor converter cells and two SiC-JFET converter cells connected in series. The SiC cells are operated with high-frequency pulse width modulation. The Si cells carry higher dc voltage, resulting in smaller converter volume by reducing the number of cascaded cells. To minimize the loss, the Si cells are operated with one-pulse. The balance control of dc voltage of the hybrid cells is also discussed. We tested the developed STATCOM in a full-scale 6.6-kV distribution grid. The experimental results show a stable rated operation and fault-ride-through performances. Moreover, we analyzed its loss and volume and discussed them based on the developed STATCOM. The estimated loss was approximately 0.3% at rated power, which is comparable to an all-SiC configuration. It also has an advantage in its small volume compared with other configurations. These analyses revealed that the hybrid configuration is a suitable topology for low-loss and small-size distribution STATCOM. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive control scheme for a delta-connected cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of zero-sequence circulating current regulation of delta-connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive dc cell capacitor voltage fluctuation. The validity of the proposed method is demonstrated by mathematical proof. Finally, the full-scaled simulation results and the down-scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions. [ABSTRACT FROM AUTHOR]

A new static synchronous compensator (STATCOM) based on the diode-clamped modular multilevel converter (DCM2 C) is proposed in this paper. In this converter topology, the capacitor voltage is clamped by using a low power rating diode in each submodule. The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore, the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of modular multilevel converter, the DCM2C-STATCOM has the capability of Var compensation and negative-sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this paper. Experimental results obtained from a laboratory prototype validate that the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensations are effective. [ABSTRACT FROM AUTHOR]

As more distributed renewable energy sources are installed in the utility grid, the static synchronous compensators are applied to manage the power factor and the grid voltage in the medium-voltage level of the power system. The cascaded multilevel converter with single-star topology is well applied to this application. The dc-capacitor voltage balancing control is a fundamental issue, and it can be accomplished by negative-sequence current and/or zero sequence voltage injection. Unfortunately, the zero sequence voltage injection increases the risk of over-modulation and the negative-sequence current injection results in high peak current. This paper provides the voltage balancing strategy to manage the peak current or the modulation index. Besides, all the asymmetrical grid voltages are considered in the proposed method to satisfy the grid fault operation. Laboratory test results verify that the proposed method limits the peak current or manages the modulation index during the grid fault operations. [ABSTRACT FROM PUBLISHER]

De-icing of overhead transmission lines is necessary to prevent damage to the transmission system and outages. In this paper, a modular multilevel dc De-Icer (M2D2) is introduced, based on the structure of the modular multilevel converter with full-bridge submodules. The M2D2 exhibits a lot of salient features: high modularity, scalability, compactness, high efficiency, wide dc voltage range, and no harmonic filter requirements, etc. Moreover, during good weather, the M2D2 can be configured as a static synchronous compensator, providing reactive power support to the grid and alleviating power quality problems. Operating principle of the M2D2 is explained, harmonic features under phase-shifted carrier modulation are analyzed, and selections of the phase-shifted angle are derived to achieve best harmonic performance. Control schemes for the M2D2 are also developed, including control of de-icing current, capacitor voltages, and reactive power. Finally, experimental results, utilizing a downscaled prototype, confirm the validity of this topology and effectiveness of the proposed control strategies. [ABSTRACT FROM PUBLISHER]

This paper presents a simple controller integrating a new reactive current reference algorithm for enhancing the transient performance of static synchronous compensator (STATCOM). A multilevel cascaded inverter with separated dc capacitors which is driven by carrier-based pulse width modulation is used to implement the STATCOM. The voltage across each dc-link capacitor is regulated by the rotated switching swapping scheme. In this paper, the STATCOM is controlled to provide both reactive power (VAR) compensation and grid power factor correction at the point of common coupling with a dynamically varying reactive load system. The proposed algorithm enhances the transient performance of the closed-loop system with only proportional controller and minimizes the STATCOM reactive current ripples. STATCOM based on a five-level cascaded inverter is presented in this paper and the performance of the proposed controller is investigated through various simulation studies using MATLAB/Simulink software for both steady state and transient conditions. A laboratory prototype is also developed to verify the simulation results where a good match between simulation and experimental results is achieved. [ABSTRACT FROM AUTHOR]

This paper presents a transformerless static synchronous compensator (STATCOM) system based on multilevel H-bridge converter with star configuration. This proposed control methods devote themselves not only to the current loop control but also to the dc capacitor voltage control. With regards to the current loop control, a nonlinear controller based on the passivity-based control (PBC) theory is used in this cascaded structure STATCOM for the first time. As to the dc capacitor voltage control, overall voltage control is realized by adopting a proportional resonant controller. Clustered balancing control is obtained by using an active disturbances rejection controller. Individual balancing control is achieved by shifting the modulation wave vertically which can be easily implemented in a field-programmable gate array. Two actual H-bridge cascaded STATCOMs rated at 10 kV 2 MVA are constructed and a series of verification tests are executed. The experimental results prove that H-bridge cascaded STATCOM with the proposed control methods has excellent dynamic performance and strong robustness. The dc capacitor voltage can be maintained at the given value effectively. [ABSTRACT FROM AUTHOR]

Grid-connected power electronics converters/inverters usually have certain amount of dc component at their ac terminal. They are likely to inject unwanted direct current into the power grid, unless a line-frequency transformer is employed. This study investigates this dc injection problem for static synchronous compensator (STATCOM) and connects the dc injection with current transformer (CT), a current sensing element widely used in high-voltage converters. By introducing CT's model into the STATCOM's model, this study found that using CT for current feedback control can cause large dc injection, if no extra means is taken. Expressions of the dc injection for different controllers were derived. Then, a dc injection elimination method was proposed. This method eliminates the dc component of the output current by building an indirect dc feedback loop; therefore, it can prevent injecting the direct current even when it cannot be sensed. Experimental results taken on a 25-level cascaded multilevel STATCOM prototype verifies the analysis and the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a space vector pulsewidth modulation (PWM) (SVPWM) algorithm is proposed, which is in \alpha^\prime\beta^\prime frame with dc-link capacitor voltage equalization for diode-clamped multilevel converters (DCMCs). The \alpha^\prime\beta^\prime frame is a coordinate system similar to the \alpha\beta frame. In this frame, some original complex calculations are substituted by integer additions, integer subtractions, truncations, etc. It brings the time and area efficiency to fixed-point digital realization, particularly for the application in a field-programmable gate array. Meanwhile, a minimum energy property of multiple dc-link capacitors is applied as the basic principle for voltage equalization based on a capacitor current prediction algorithm. By evaluating the redundant vectors in each pulse dwelling period, the balancing algorithm chooses an optimal vector, generates the optimal PWM signals, and sustains the voltage stability. After that, an arbitrary multilevel SVPWM intellectual property core is designed and analyzed in the \alpha^\prime\beta^\prime frame. At the end of this paper, a five-level DCMC-based static synchronous compensator is built and tested. The experimental results verify the balancing algorithm and the system steady-state and dynamic performances. [ABSTRACT FROM PUBLISHER]

Wide-scale penetration of distributed-generation (DG) units into a weakly meshed distribution system is one of the major concerns for power system engineers in recent years. As grid standards limit voltage control operation of small DG units, a lack of sufficient reactive power support brings the problem of slow voltage recovery and this leads to the usage of expensive flexible ac transmission system (FACTS) devices, such as a static synchronous compensator (STATCOM). In this paper, a systematic methodology has been developed for fast voltage recovery at the DG bus. This methodology reduces the number as well as the rating of STATCOMs through the tuning of control parameters. The STATCOM rating at selected locations has been tested and verified to be adequate at various dispatch levels of DG units. Additional benefits of STATCOM tuning on the damping of weak modes have also been studied. An IEEE industrial test system has been used for validating the methodology, providing grid compatible voltage recovery and satisfactory damping performance. [ABSTRACT FROM PUBLISHER]

In recent years, the penetration level of renewable-based distributed energy resource (DER) units has increased significantly. Consequently, standards have been developed and deployed demanding small DER units to operate in constant power factor mode and large DER units in voltage control mode. This results in exposing small DER units to the problem of slow voltage recovery for contingencies like faults. Hence, this paper proposes a methodology of static and dynamic reactive power compensation to avoid tripping of small DER units due to slow voltage recovery. A new sensitivity index has been developed for the placement of STATic synchronous COMpensator (STATCOM) to ensure fast voltage recovery at all the buses of interest. The case studies involving two IEEE test systems with varying size and load compositions validate the proposed methodology and index. [ABSTRACT FROM PUBLISHER]

This paper presents a transformerless static synchronous compensator (STATCOM) system based on hybrid multilevel H-bridge converter with delta configuration. This hybrid multilevel STATCOM is characterized by per-phase series connection of a high-voltage H-bridge converter operating at fundamental frequency and a low-voltage H-bridge converter operating at 5 kHz without any other circuit for dc voltage control. A new control strategy is proposed in this paper with focus on dc voltage regulation. Clustered balancing control is realized by injecting a zero-sequence current to the delta-loop, while individual voltage control is achieved by adjusting the fundamental content of ac quasi-square-waveform voltage of high-voltage converter. A downscaled experimental prototype rated at 100 V and 3 kVA is constructed in author's laboratory. Experimental results show that the hybrid multilevel STATCOM performs satisfactory not only improving efficiency and waveform quality, but also compensating reactive power and negative-sequence current while maintaining dc voltage at the given value. [ABSTRACT FROM AUTHOR]

This paper presents a novel dc capacitor voltage balance control method for cascade multilevel static synchronous compensator (STATCOM) and a general analytical method for balance control strategy. Considering that the imbalance of dc capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chain's output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it. The effectiveness of proposed control scheme has been verified by experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype. [ABSTRACT FROM AUTHOR]

Industrial plants are faced with stringent requirements by the utility to maintain a near unity power factor. These plants have traditionally utilized switched capacitor banks as a cost-effective means for power factor correction (PFC). However, if there is a significant level of harmonics present in the current that is disturbing the neighboring loads, then a separate unit for harmonic filtering is required as well. By the time a realistic system is configured, significant cost and complexity are encountered. The concept of a dynamic capacitor (D-CAP) was previously introduced as a means by which to achieve dynamic VAR injection as well as active harmonic filtering in one single integrated unit. The topology consists of taking an existing asset like a PFC capacitor, adding a pair of ac switches and small LC components per phase, and thereby realizing an asset with augmented functionalities. Although the power stage is simple, the underlying control is very challenging. This paper discusses the intricacies of controlling such an architecture and provides keen insights into how one would go about designing a controller. [ABSTRACT FROM AUTHOR]

This paper presents a new feedback control strategy for balancing individual dc capacitor voltages in a three-phase cascade multilevel inverter-based static synchronous compensator. The design of the control strategy is based on the detailed small-signal model. The key part of the proposed controller is a compensator to cancel the variation parts in the model. The controller can balance individual dc capacitor voltages when H-bridges run with different switching patterns and have parameter variations. It has two advantages: 1) the controller can work well in all operation modes (the capacitive mode, the inductive mode, and the standby mode) and 2) the impact of the individual dc voltage controller on the voltage quality is small. Simulation results and experimental results verify the performance of the controller. [ABSTRACT FROM AUTHOR]

In this paper, a novel controller with fixed modulation index (MI) and variable dc capacitor voltage reference to minimize voltage and current harmonics is presented for a distribution static synchronous compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a dc capacitor and is used to provide reactive power compensation and regulate ac system bus voltage with minimum harmonics. A systematic design procedure based on pole-zero cancellation, root locus method, and pole assignment method has been developed to determine proper parameters for the current regulator, the dc voltage controller, and the ac voltage controller of the STATCOM. With the proposed STATCOM controller, harmonic distortions in the inverter output current and voltage can be reduced since the MI is held constant at unity in steady state. In addition, a fast adjustment in the STATCOM output reactive power is achieved to regulate the ac bus voltage through the adjustment of the dc voltage reference during the transient period. Simulation and experimental results for the steady- state operating condition and transient operating conditions for the system subjected to a reactive current reference step change, a three-phase line to neutral fault, and a step load change are presented to demonstrate the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions. [ABSTRACT FROM AUTHOR]