DC Capacitor Voltage Research Articles

A New Variable Frequency Control of 49-Level Cascaded Packed U-Cell Voltage Source Inverter

Requirement of large number of levels with lower number of switching devices has made asymmetrical converters more popular than the symmetrical ones. Asymmetrical cascaded multilevel inverters (ACMLI) can achieve high efficiency by combining switching devices with different voltage ratings and technologies. The proposed ACMLI cascades two or more units of packed U-Cell (PUC) inverters using two or more isolated dc link supplies. In this article, one of the PUC unit is controlled using high switching frequency while the other PUCs are operated in a step mode at low switching frequencies, thus operating them in a variable frequency control mode. The cascading of two 7-level PUC inverters with dc link voltage ratio of 1:7 can produce an output voltage with 49 (7x7) levels. The multilevel output voltage waveform is nearly sinusoidal with very low THD content, and the low switching frequency operation leads to lower power dissipation and greater system efficiency. However, each PUC module requires two dc voltage sources. To address this concern, in this article, each PUC module consists of one dc voltage source and one dc bus capacitor. With the cascaded PUC topology and proposed control algorithm, load current and dc bus capacitor voltage control is achieved simultaneously. The proposed converter and its control technique lead to the breaking of the design tradeoff rule between switching frequency (efficiency) and filter size. This is very useful in various applications such as uninterruptible power supplies and grid-tie inverters. The converter and its control technique are simulated using MATLAB/Simulink software and simulation results for both open loop and closed loop are discussed. Hardware results are obtained by developing a 1-kW experimental prototype. Simulation and experimental results confirm the usefulness and effectiveness of the proposed topology and its control technique.

Parameters Tuning of Photovoltaic Power Generation System with Static Synchronous Compensator Based on Chaos Orthogonal Particle Swarm Optimization

China is a country rich in solar energy resources, with good natural conditions for the utilization of solar energy. At present, the main application mode of photovoltaic power generation system is grid-connected photovoltaic power generation. In order to maintain the voltage stability of the common connection point (PCC), STATCOM is mainly added to the PCC of photovoltaic power station. It can be regarded that the photovoltaic system and STATCOM constitute a whole and are integrated into the grid, and the unified coordination and optimization of the overall control system becomes particularly important. In this paper, the parameter tuning strategy of PI controller for photovoltaic grid-connected system with STATCOM is studied. Taking the optimal overall performance of DC capacitor voltage and PCC point voltage of photovoltaic inverters as the optimization objective, chaotic orthogonal particle swarm optimization algorithm is used to optimize the parameters of PI controller of the system, and simulation is carried out under the condition of voltage variation to verify the effectiveness of the optimization strategy.

An Improved Grid Current and DC Capacitor Voltage Balancing Method for Three-Terminal Hybrid AC/DC Microgrid

In this paper, a three-terminal ac/dc hybrid microgrid with two dc terminals and one ac terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based ac grid interface and two dual active bridge (DAB) converters based dc subgrid interface that connects two isolated dc buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB dc rails according to the system operation requirement. To overcome the imbalanced grid currents and dc rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid ac/dc microgrids show that the generalized effectiveness of the proposed three-phase ac current and dc rail voltage balancing method.

A STATCOM Based on a Three-Phase, Triple Inverter Modular Topology for Multilevel Operation

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.

Multi-objective coordination control of distributed power flow controller

In this paper, distributed power flow controller (DPFC) constraints are analyzed. The energy balance relationship between fundamental wave and third harmonic in series and shunt-side converter is deduced. A proportional integral (PI) controller of the DPFC is constructed. The PI controller uses the voltage amplitude and phase angle injected into the system in the series side, along with the modulation ratio of the three-phase converter on the shunt side as the control variables. A multiobjective coordinated control equation is proposed, which factors the constraints of the energy balance between series and shunt side, device capacity limit, safe operation limit, fundamental component, as well as third harmonic component of the injection voltage at the series side. The equation minimizes the variance between the actual value of the control target and its given value to ensure that the DC capacitor voltage, both in the series and shunt side, is stable at target value. Simulations are conducted to verify correctness and effectiveness of the proposed control method.

A Different Voltage-Source Power Inverter with Carrier Based SPWM for Open-End Connection Loads

This paper presents a new configuration of voltage source inverter with a simplified circuit for generating five-level pulse width modulation (PWM) voltage waveform. Compared with conventional inverter configuration, this circuit is drastically able to simplify the structure and reduce the required number of active switch components. The new inverter circuit is very suitable for the use of open-end connection loads such as open-end winding ac motor drive application. Instead of using separated dc power sources, the new inverter circuit configuration is also possible to utilize only one dc voltage source, so the power supply circuits can be made simpler. Furthermore, to reduce ripples of dc capacitor voltages, the voltage stabilizing circuit of capacitors at the input side was proposed and applied. The stabilizing circuit is capable to work reducing the size of dc capacitors, and maintaining voltage stability of capacitors through charging and discharging operation modes. The working principles of inverter circuit were evaluated and examined by means of computer simulations using PSIM software. In addition, experimental test results of the prototype were also provided. Test results proved that the new five-level PWM inverter operated well generating five-level output waveform with smaller distortion and less voltage ripples of dc capacitors.

Series-Connected-Based Offshore Wind Farms With Full-Bridge Modular Multilevel Converter as Grid- and Generator-side Converters

Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm.

Analysis of Maximum Load Unbalancing Limits of Cascaded H-Bridge Rectifier Under Unity Power Factor

In a cascaded H-bridge rectifier (CHBR), the dc capacitor voltages need to be controlled to balance by the voltage balance controller when the loads are unbalanced. However, the voltage balance controller will fail to balance the dc capacitor voltages when the load unbalancing range exceeds a certain limit under unity power factor; that limit is constrained by the AC voltage and transmission power of each H-bridge. First, the relationships in CHBR were analyzed. Then, the expression between AC voltage and the power of each H-bridge was derived. Then the load unbalancing limits to maintain the dc capacitor voltage balance of each H-bridge were further deduced. In addition, the influence of ripple power and total load power on the load unbalancing limits was investigated. Finally, an experimental prototype of a single-phase three-cell cascaded H-bridge rectifier was built and the correctness of the proposed load unbalancing limits was verified by the experiment. The proposed load unbalancing limits are universal, which provides theoretical support for the stable operation and power transmission of CHBR.

Packed E-Cell (PEC) Converter Topology Operation and Experimental Validation

This paper proposes a novel single-dc-source multilevel inverter called Packed E-Cell (PEC) topology to achieve nine levels with noticeably reduced components count, while dc capacitors are actively balanced. The nine-level PEC (PEC9) is composed of seven active switches and two dc capacitors that are shunted by a four-quadrant switch to from the E-cell, and it makes use of a single dc link. With the proper design of the corresponding PEC9 switching states, the dc capacitors are balanced using the redundant charging/discharging states. Since the shunted capacitors are horizontally extended, both capacitors are simultaneously charged or discharged with the redundant states, so only the auxiliary dc-link voltage needs to be sensed and regulated to half of the input dc source voltage, and consequently, dc capacitors' voltages are inherently balanced to one quarter of the dc bus voltage. To this end, an active capacitor voltage balancing integrated to the level-shifted half-parabola carrier PWM technique has been designed based on the redundant charging/discharging states to regulate the dc capacitors voltages of PEC9. Furthermore, using the E-cell not only reduces components count but also the proposed topology permits multi ac terminal operation. Thus, five-level inverter operation can be achieved during the four-quadrant switch fault, which confers to the structure high reliability. The theoretical analysis as well as the experimental results are presented and discussed, showing the basic operation, multi-functionality, as well as the superior performance of the proposed novel PEC9 inverter topology.

Analysis and Optimization of Multi-DC Current Flow Controllers With Different Configuration Positions in M2TDC Grid

In multi-terminal HVDC (MTDC) grids, the voltages and currents at the converter DC terminals can be accurately regulated by the control of converter stations. For the DC line currents in meshed MTDC (M 2 TDC) grids, they are naturally distributed based on the line resistance and the voltage difference at DC ports, which greatly affects the controllability and optimal distribution of the DC power/current flow. In this paper, regarding different configurations of positioning (COP) DC CFCs in M 2 TDC grids, the dynamic behaviors and optimized configuration scheme are investigated. Firstly, topological structure of a generalized M 2 TDC is presented. Two different COP schemes for double DC CFCs in a four-terminal M 2 TDC grid are proposed according to control objectives of the DC current flows. Secondly, in order to achieve the coordinated control of DC line currents and capacitor voltage of the DC CFC, a generalized voltage-droop control strategy is applied. It can be applicable to multiple DC CFCs under different COP without the need of communication. Apart from that, considering the impact of DC CFCs under specific COP on the control characteristics of DC current flows and control margins, an auxiliary control strategy, which is capable of mitigating the confliction of multiple DC currents, is proposed. It can effectively improve the control performance of the DC CFC. Based on the time-domain simulation PSCAD/EMTDC, the M 2 TDC grids with different COP of DC CFCs are established. The merits and drawbacks of different COP, in terms of the complexity and implementation of control systems, dynamic behaviors in mitigating transient voltage spikes and current surges, control margins and robustness of the controller design, are discussed. In addition, the effectiveness of the proposed generalized voltage-droop control and auxiliary control strategies is verified with an optimized COP scheme of multiple DC CFCs proposed.

A Novel Submodule Voltage Balancing Scheme for Modular Multilevel Cascade Converter—Double-Star Chopper-Cells (MMCC-DSCC) Based STATCOM

A novel capacitor voltage balancing scheme for modular multilevel cascade converter-double-star chopper-cells (MMCC-DSCC)-based STATCOM is proposed in this paper. Compared with the conventional software voltage balancing methods, the proposed scheme has three main advantages. First, only six voltage sensors are needed, and other voltage sensors have been removed. Then, the control framework can be simple without sort and select process, or some balancing control loops. Additional delay process caused by the sampling and control signals transmission is also mitigated. Finally, the faster capacitor balancing speed can be obtained. In addition, compared with some other hardware voltage balancing schemes, there are superior characteristics in terms of the reduced number of switches and only the popular power modules in the additional units are applied. For these additional modules, the parameters design guide of the proposed scheme has been discussed. Verified results from the 13-level simulated system and 9-level experimental platform for the MMCC-DSCC based STATCOM show the effectiveness of the proposed scheme. By the proposed scheme, both DC capacitor voltage balancing control and reactive power compensation are simultaneously realized for the MMCC-DSCC-based STATCOM even in balanced and unbalanced load.

Control of a Type-IV Wind Turbine With the Capability of Robust Grid-Synchronization and Inertial Response for Weak Grid Stable Operation

With the increasing penetration of wind power, the effective inertia of the power system reduces. Besides, a series of interactive instability issues including sub-synchronous oscillations and harmonic oscillations were reported due to the weak grid effects. Broadly speaking, those issues are closely related to the PLL and its tuning. Therefore, recent literature make efforts to avoid using the PLL for converter control, e.g., the virtual synchronous generator (VSG) control. However, the VSG control usually employs multiple loops, where a fast switching frequency of the converter is required to decouple each loop's dynamic. This prerequisite is usually not met for wind power converters with high capacity and low switching frequency. To address these issues but also inherit the merits of a VSG on the grid-integration, this paper employs a new concept of PLL-less control and applies it to the Type-IV wind turbine, in which the grid-synchronization is realized by the dynamics of dc capacitor voltage. The virtual capacitor control is designed and added to the machine-side converter to eventually deliver adequate inertia to the grid. To justify the effectiveness of the proposed control, both the soft start-up validation and the thorough analysis of the overall small-signal stability are presented. Several concerns of vital importance regarding the virtual capacitor design and stabilization control are discussed, where the mechanism of stability is revealed through the complex-power coefficient-based analysis. On this basis, a stabilization control method is proposed, which can enlarge the stable range of virtual capacitor coefficient and enhance the inertial response effect. Finally, the performance of the proposed method on the inertial response and the weak grid operation is evaluated by time domain simulations in PSCAD/EMTDC, which is proven effective overall.

A Highly Efficient Single-Phase Three-Level Neutral Point Clamped (NPC) Converter Based on Predictive Control with Reduced Number of Commutations

This paper proposes a highly efficient single-phase three-level neutral point clamped (NPC) converter operated by a model predictive control (MPC) method with reduced commutations of switches. The proposed method only allows switching states with none or a single commutation at the next step as candidates for future switching states for the MPC method. Because the proposed method preselects switching states with reduced commutations when selecting an optimal state at a future step, the proposed method can reduce the number of switchings and the corresponding switching losses. Although the proposed method slightly increases the peak-to-peak variations of the two dc capacitor voltages, the developed method does not deteriorate the input current quality and input power factor despite the reduced number of switching numbers and losses. Thus, the proposed method can reduce the number of switching losses and lead to high efficiency, in comparison with the conventional MPC method.

A New Diode-Clamped Multilevel Inverter With Balance Voltages of DC Capacitors

This paper proposes a new diode-clamped multilevel inverter that can effectively balance the voltages of dc capacitors. For the proposed inverter, the inductors are inserted between the diodes and the main switches. Also, the voltage balance of the dc capacitors can be guaranteed by controlling the opening time of the corresponding main switches in particular modes. Thus, the parallel circuits on the dc side in the conventional inverters are canceled. Consequently, the proposed topology requires fewer switches, diodes, and capacitors, although it adds the compensation currents to the corresponding main switches. Both the simulation and experimental results demonstrate the suitability and applicability of the proposed topology.

A Fault-Tolerant Control Strategy for the Delta-Connected Cascaded Converter

This paper proposes an open-delta operation as the fault-tolerant control strategy for the delta-connected cascaded converter. Based on the detailed power analysis, the fault-tolerant control enables continued operation and maintains dc capacitor voltages balancing during the occurrence of bridge cells fail. This enhances the system's reliability and availability. The simulation and laboratory test-bench, combining a seven-level delta-connected cascaded converter apply in a STATCOM operation and a PV system is tested to verify the proposed control technique.

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.

A New Control Method of Suppressing DC Capacitor Voltage Ripples Caused by Third-Order Harmonic Compensation in Three-Phase Active Power Filters

This paper proposes a new control method suitable for active power filters to reduce the dc capacitor voltage ripples associated with the third-order harmonic current compensation. The proposed method superimposes a negative-sequence fundamental current on the compensating current to cancel out the active power ripple caused by the third-order harmonic current. As a result, the proposed method has the capability to eliminate the dc capacitor voltage ripple at double the source frequency. Experimental results obtained by a 10-kW three-phase diode rectifier verify the validity of the proposed method. The proposed method exhibits a good suppression performance of the dc capacitor voltage ripple at double the source frequency.

Single phase nine level inverter using single DC source supported by capacitor voltage balancing algorithm

This study introduces a new single phase nine-level inverter using two capacitors and a single DC source. Voltage imbalances in the capacitors are eliminated using two control algorithms namely charging algorithm and discharging algorithm. The charging algorithm is used to change the inverter switching states when the measured voltage across the capacitors is less than the prescribed value. Alternatively, if the voltage across the capacitors is greater than the set value, the discharging algorithm determines the switching pattern of the inverter switches. The proposed voltage balancing algorithm is very simple to implement and makes the proposed inverter attractive for industrial applications. An extensive comparison of the proposed inverter is made against other topologies proposed in the literature in terms of the components used. The proposed inverter is tested in both standalone and grid connected modes. The proposed inverter is simulated and implemented as a prototype in the laboratory. Experimental results obtained from the prototype confirm the high-quality transient performance of the proposed inverter in terms of its dc capacitor voltage balancing capability.

Interval Type-2 Fuzzy Logic Controller-Based Multi-level Shunt Active Power Line Conditioner for Harmonic Mitigation

The use of variable speed drives in industries introduces harmonics that should be attenuated so as to improve the power quality. To overcome the shortcomings of the traditional passive filters in electric power system, shunt active power line conditioners (APLC) are employed. The regulation of DC capacitor voltage plays a vital role in deciding the performance of shunt APLC. Several intelligent controllers have been developed for the voltage control; however, due to high uncertainties associated with the electric power system, an interval type-2 fuzzy logic controller (IT2 FLC) is proposed. This work compares IT2 FLC, type-1 fuzzy-tuned proportional integral (PI) controller, and their performances are assessed with reference to conventional PI controller using MATLAB. Thus, the paper deals with the investigation of three-level Multi-Level Inverter which is used as shunt APLC in high-power applications using a fuzzy logic scheme.

Improved Modulation Mechanism of Parallel-Operated T-Type Three-Level PWM Rectifiers for Neutral-Point Potential Balancing and Circulating Current Suppression

In high-power applications, parallel-operated T-type three-level pulse width modulation (PWM) rectifiers (T3LPRs) are widely employed to improve the power capacity and system reliability. For parallel-operated T3LPRs, there are two important issues need to be properly addressed: 1) the neutral-point potential (NPP) balancing, and 2) the zero sequence circulating current (ZSCC) between the common ac and dc bus. In this paper, the fluctuation of NPP in a T3LPR and the generative mechanism of ZSCC in parallel-operated T3LPRs are first investigated. The zero sequence voltage (ZSV) difference between the parallel-operated T3LPRs is identified as the main excitation source of the ZSCC problem. Meanwhile, the disconnection of the neutral point and the control effect difference of NPP balancing in each T3LPR also affect the ZSCCs within the parallel-operated T3LPRs. To adjust the ZSV of T3LPRs, the dwell time of the positive small voltage vector in each switching period of the converter, which is usually adjusted to balance the NPP, is also required to be controlled to suppress ZSCC. Therefore, in this paper, to avoid the possible conflict, an improved modulation mechanism is designed to guarantee simultaneous ZSCC suppression and NPP balancing. The neutral points of each T3LPR are connected. Then, by adjusting the dwell time of the positive small voltage vector, the charging time of two dc capacitors is adjusted to balance the NPP, and the ZSV differences between different T3LPRs are adjusted to suppress ZSCCs. To avoid the control effect difference of independent NPP balancing in different T3LPRs, all T3LPRs share the same value marking the difference between two dc capacitor voltages. Thus, a communication with a greatly reduced baud rate requirement is needed for NPP balancing to transfer the difference between the two dc capacitor voltages. Moreover, no extra circuit is needed for ZSCC suppression. These approaches make the proposed improved space vector PWM strategy much more practical and easy to employ. The proposed method is verified with experiments on two 3-kW T3LPR prototypes.