Floating Dc Capacitors Research Articles

A Generic Two-Vector Model Predictive Control for Hybrid Multilevel Converters

In this article, a generic two-vector model predictive control (TV-MPC) strategy is proposed for the hybrid multilevel converters (HMCs). The proposed method selects two optimal voltage vectors among all the vector candidates using a geometric positioning approach to reduce the computational burden, which is a common issue in the existing MPC methods for HMC. Then duty cycles of the two selected vectors are optimized to minimize the current tracking error, such that the current tracking performance can be enhanced compared with the conventional MPC. In addition, the voltages of the floating dc capacitors can be balanced by evaluating all switching sequences that belong to the optimal voltage vectors with optimal duty cycles. The concept of the proposed TV-MPC is generic and applicable for any HMCs. A typical HMC based on active neutral-point-clamped topology is adopted as a case study in this work. Comprehensive simulation and experimental studies are performed on an all silicon-carbide HMC prototype to validate the effectiveness of the proposed control scheme.

Two-Phase Current Injection Method for Single Line-to-Ground Fault Arc-Suppression With Revised STATCOM

To suppress the arc current in the single line-to-ground (SLG) fault in neutral isolated distribution network, a two-phase current injection method is proposed for the revised static synchronous compensator (STATCOM), as an extra function during the SLG fault. Therefore, the traditional roles of the dedicated passive coil or power electronic based arc-suppression devices can be taken by this multi-purpose STATCOM. This article first introduces the principles and the models of the arc-suppression current injection during SLG fault. Then, different options of arc-suppression current injection by the revised STATCOM are analyzed in terms of stabilizing the floating dc capacitor voltages of the cascaded H-bridges (CHB) converter. It is discovered that the two-phase current injection is the only viable option to maintain the floating dc capacitors voltages. Then, the arc-suppression current controller and the dc capacitors voltages controller are proposed, and the design process of the control parameters adapting to the varying transitional (grounding) resistances computed online is also introduced. The proposed methods are first validated by simulations. Then, an MVA-rating CHB converter prototype is constructed, and 10 kV feeder SLG fault and arc-suppression experiments are performed to demonstrate the effectiveness of the proposed method.

High-Efficiency Operation of an Open-Ended Winding Induction Motor Using Constant Power Factor Control

A controller is presented for an open-ended winding motor dual-inverter drive (DID), where main and floating inverters are supplied from a dc power source and a floating dc capacitor, respectively. The controller utilizes the efficiency characteristic of induction machines, where high power conversion efficiencies are obtained when operating the machine with a constant fundamental power factor, typically around 0.70–0.75, over a wide load range and under variable frequency. The controller uses the drive's topology to maintain the motor's desired power factor. In essence, the main inverter's output voltage is used to control the floating inverter's dc capacitor voltage to keep the injected fundamental voltages of both inverters at a desired ratio. The floating inverter's voltage is operated with a 90° lead relative to the main inverter and uses a constant maximum amplitude modulation depth to minimize the capacitor's operating voltage. This approach updates the motor's voltage automatically to ensure constant power factor operation and improves the floating capacitor's voltage stability during transient conditions. The inherent voltage-boosting capability of this topology is especially beneficial in extending the constant torque region of the motor and improving performance in the speed range extension region. Simulation and experimental results verify the predicted motor efficiency gains and stability under speed and load transients.

Control of High-Frequency Isolated Modular Converter

Recently, voltage-source converter (VSC)-based high-voltage dc (HVDC) transmission systems have gained more attention. In this paper, a control method for a modular VSC-based HVDC transmission system with high-frequency isolation referred to as high-frequency isolated modular converter is proposed. In the high-frequency isolated modular converter configuration, several floating dc capacitors in all three phases are connected in series, and voltage balancing control of these floating dc capacitors is required. In this paper, an appropriate control structure with the capacitor voltage balancing controller is proposed. The proposed control scheme consists of three layers to control terminal dc bus voltage and balance dc capacitor voltages of each building block. Detailed PSCAD simulation results are presented to evaluate the performance of high-frequency isolated modular converter. Controller hardware-in-the-loop simulation of the high-frequency isolated modular converter is also performed by real-time digital simulator (RTDS), and RTDS results are presented to verify the control structure. Finally, laboratory-scale experimental results are presented to validate the proposed control method.

Single-Phase AC–AC Double-Star Chopper Cells (DSCC) Converter Without Common DC-Link Capacitor

This paper presents a single-phase ac–ac converter based on a modular multilevel cascade converter (MMCC) with double-star chopper cells (DSCC). The converter topology is composed of three DSCC legs, in which the middle leg is shared, and it does not present a common dc-link capacitor. It is suitable for nonregenerative or regenerative low-power and medium-power ac–ac conversions. A voltage-balancing control is presented for regulating the floating dc capacitor voltages. Furthermore, a grid current and load voltage control is presented to correct the input power factor and to ensure input/output sinusoidal waveforms. A phase-shifted carrier (PSC) pulsewidth modulation (PWM) commands submodules (SMs) using half of PSCs, preserving the characteristics of conventional modulation. Details about a downscaled prototype rated at 3.3 kW and 220 V are also presented. The experimental results allow validating the control strategies and the effectiveness of the proposed system.

スター結線を有するモジュラー・マルチレベル・カスケード変換器（MMCC-xSBC）のパワーフロー解析とコンデンサ電圧制御

The modular multilevel cascade converter (MMCC) family has been attracting attention from power electronics and power system engineers because it has been showing considerable promise as one of the next-generation high-power high-voltage/medium-voltage power converters. This paper provides an intensive discussion on power-flow analysis of three MMCC members with star configuration, focusing on dc-mean-voltage balance/regulation and ac-voltage mitigation control of all the floating dc capacitors. Most existing analysis methods are based on single-phase circuits, which make it difficult to derive capacitor voltage regulation/mitigation methods relying only on circulating currents and zero-sequence voltage. The power-flow analysis and regulation/mitigation methods developed in this paper are characterized by applying the αβ0 coordinate transformation(s). The validity of the power analysis and the regulation/mitigation method is confirmed by simulation using the PSCAD/EMTDC software package.

Modular Multilevel Converter With an Adaptive Observer of Capacitor Voltages

A modular multilevel converter (MMC) is an attractive solution for power conversion without transformers. The MMC consists of cascade connections of floating dc capacitors. In this paper, an adaptive observer design has been proposed to estimate the capacitor voltages from the measurement of arm currents. This work introduces the capacitance value of the cell capacitors as a parameter uncertainty for making the system performance robust with unknown constant parameters. It may be used for predictive control, condition monitoring for capacitors, and diagnosis check for capacitor health. In addition, a pulsewidth modulation (PWM) technique for MMC has been explored. The PWM technique is performed using a carrier-based level-shifted PWM strategy. It does not necessitate the calculation of duty cycles, and can be easily implemented in a DSP. By using the PWM technique, harmonics in the phase voltage is shifted to twice the switching frequency. Theoretical analysis is included in this paper for showing stability and convergence of the proposed observer. Analytical expressions are verified by simulation and experimental results.

A Simple Capacitor Voltage Balancing Method with a Fundamental Sorting Frequency for Modular Multilevel Converters

A Fundamental Frequency Sorting Algorithm (FFSA) is proposed in this paper to balance the voltages of floating dc capacitors for Modular Multilevel Converters (MMCs). The main idea is to change the sequences of the CPS-PWM carriers according to the capacitor voltage increments during the previous fundamental period. Excessive frequent sorting is avoided and many calculating resources are saved for the controller. As a result, more sub-modules can be dealt with. Furthermore, it does not need to measure the arm currents. Therefore, the communication between the controllers can be simplified and the number of current sensors can be reduced. Moreover, the proposed balancing method guarantees that all of the switching frequencies of the sub-modules are equal to each other. This is quite beneficial for the thermal design of the sub-modules and the lifetime of the power switches. Simulation and experimental results acquired from a 9-level prototype verify the viability of the proposed balancing method.

Experiment and Simulation of a Modular Push–Pull PWM Converter for a Battery Energy Storage System

This paper presents a modular push-pull PWM converter (MPC) for a battery energy storage system, which is intended for grid connections to medium- or high-voltage power systems. The converter per phase consists of a center-tapped transformer and two arms based on a cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. This paper discusses the operating performance and control method of the MPC, focusing on voltage balancing of all the floating dc capacitors. Moreover, a comparison is made between the MPC and a modular multilevel converter (MMC), under an assumption that both are used as the same battery energy storage system. The validity of the operating performance and control method is confirmed by both computer simulation using the “PSCAD/EMTDC” software package and experiment using a three-phase, 200-V, 5-kW downscaled model.

Control of DC-Fault-Resilient Voltage Source Converter Based HVDC Transmission System under DC Fault Operating Condition

Voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission systems have attractive advantages compared to classical thyristor-based HVDC transmission systems. However, VSC-based HVDC transmission systems are vulnerable to dc side fault, and expensive dc circuit breakers are required to protect them against dc fault. This paper proposes a control method of a dc fault-resilient VSC which can be protected against dc fault without using expensive dc circuit breakers. In the VSC configuration, several H-bridge modules are connected in cascade, so the voltage balancing control of several floating dc capacitors is required. In this paper, an appropriate control structure with the capacitor voltage balancing controller is proposed. The appropriate control algorithm for dc fault operation and recovery after dc fault is also proposed. PSCAD simulation results are presented to validate the proposed control structure under normal and dc fault operating conditions. Real-time-digital-simulator results are also presented to verify the control structure.

Control and Verification of a Modular Push-Pull PWM Converter for a Battery Energy Storage System

This paper presents a modular push-pull PWM converter (MPC) for a battery energy storage system, which is intended for grid connections to medium-voltage or high-voltage power systems. The converter per phase consists of a center-tapped transformer and two arms consisting of a cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. This paper discusses the operating performance and control method of the MPC, focusing on voltage balancing of all the floating capacitors. Moreover, a comparison is made between the MPC and a modular multilevel cascade converter (MMCC-DSCC), both of which are used as the same battery energy storage system. The validity of the operating performance and control method is confirmed by both computer simulation using the “PSCAD/EMTDC” software package and experiment using a three-phase, 200-V, and 5-kW downscaled model.

Coupling Effect Reduction of a Voltage-Balancing Controller in Single-Phase Cascaded Multilevel Converters

This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system.

Control and Analysis of the Modular Multilevel Cascade Converter Based on Double-Star Chopper-Cells (MMCC-DSCC)

This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment.

二重スター・チョッパセル方式のモジュラー・マルチレベル・カスケード変換器（MMCC-DSCC）の理論解析と制御

This paper presents a modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC), which is intended for grid connection to medium-voltage power systems without the use of line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. It is necessary to achieve voltage-balancing control of all the dc capacitors. However, the voltage control achieved by averaging control and individual-balancing control imposes certain limitations on the operating conditions. This paper proposes arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is shown by performing a computer simulation and experiment.

Control and Experiment of Pulsewidth-Modulated Modular Multilevel Converters

A modular multilevel converter (MMC) is one of the next-generation multilevel converters intended for high- or medium-voltage power conversion without transformers. The MMC is based on cascade connection of multiple bidirectional chopper-cells per leg, thus requiring voltage-balancing control of the multiple floating DC capacitors. However, no paper has made an explicit discussion on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of pulsewidth-modulated modular multilevel converters (PWM- MMCs) with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the PWM-MMCs to achieve voltage balancing without any external circuit. The viability of the PWM-MMCs, as well as the effectiveness of the voltage-balancing control, is confirmed by simulation and experiment.

Voltage Monitoring on Capacitor of Modular Multilevel Converter

Voltage Monitoring on Capacitor of Modular Multilevel ConverterA modular multilevel converter is an attractive solution for power conversion without transformers. As modular multilevel converter consists of cascade connections and floating dc capacitors, it requires continuous voltage monitoring. This paper represents voltage measurement circuit of a DC-storage capacitor including power supply with results of experiments.

Control and Performance of a Transformerless Cascade PWM STATCOM With Star Configuration

This paper presents a three-phase transformerless cascade pulsewidth-modulation (PWM) static synchronous compensator (STATCOM) intended for installation on industrial and utility power distribution systems. It proposes a control algorithm that devotes itself not only to meeting the demand of reactive power but also to voltage balancing of multiple galvanically isolated and floating dc capacitors. The control algorithm based on a phase-shifted carrier modulation strategy is prominent in having no restriction on the cascade number. Experimental waveforms verify that a 200-V 10-kVA cascade PWM STATCOM with star configuration has the capability of inductive to capacitive (or capacitive to inductive) operation at the rated reactive power of 10 kVA within 20 ms while keeping the nine dc mean voltages controlled and balanced even during the transient state.