Switching faults and safe control of an ARCP multicell flying capacitor inverter

The auxiliary resonant commutated pole (ARCP) multicell converter is a soft-switched variant of the multicell converter. It is shown in this paper that the ARCP technique can be very efficiently used in multicell flying capacitor converters. The main properties of the resulting soft switching multicell converter are very similar to those of the hard-switched version. They are presented and validated by simulations as well as experimental results. In practice, due to the damping, the ARCP multicell converter can suffer from switching faults as in two-level ARCP inverters, but in the case of a multicell converter failures can occur in different cells. So, the main control strategies are evaluated and the switching process is discussed step by step, taking account of the main imperfections of actual control circuits. When they can occur, the switching faults are described and analyzed. Finally, an original quasisoft switching control that should give a high safety of operation is proposed. Experimental results obtained with a 900 V-100 A ARCP multicell inverter leg are given and the performances are compared with those of a hard-switched multicell inverter leg of the same rating.     

A comparison of high-power converter topologies for the implementation of FACTS controllers

This paper compares four power converter topologies for the implementation of flexible AC transmission system (FACTS) controllers: three multilevel topologies (multipoint clamped (MPC), chain, and nested cell) and the well-established multipulse topology. In keeping with the need to implement very-high-power inverters, switching frequency is restricted to line frequency. The study addresses device count, DC filter ratings, restrictions on voltage control, active power transfer through the DC link, and balancing of DC-link voltages. Emphasis is placed on capacitor sizing because of its impact on the cost and size of the FACTS controller. A method for the dimensioning the DC capacitor filter is presented. It is found that the chain converter is attractive for the implementation of a static compensator or a static synchronous series compensator. The MPC converter is attractive for the implementation of a unified power flow controller or an interline power flow controller, but a special arrangement is required to overcome the limitations on voltage control.     

Optimal Modulation of Flying Capacitor and Stacked Multicell Converters Using a State Machine Decoder

Modulation of flying capacitor and stacked multicell converters is complicated by the fact that these converters have redundant states that achieve the same phase leg voltage output. Hence, a modulator must use some secondary criteria such as cell voltage balancing to fully define the converter switched state. Alternatively, the modulator can be adapted to directly specify the cell states, such as has been proposed for the harmonically optimal phase disposition (PD) strategy. However the techniques reported to date can lead to uneven distribution of switching transitions between cells, and the synthesis of narrow switched phase leg pulses. This paper presents an improved strategy that decouples the tasks of voltage level selection and switching event distribution. Conventional PD and centered space vector pulsewidth modulation (CSVPWM) strategies are used to define the target voltage level for the converter, and a finite state machine is then used to distribute the transitions to the converter cells in a cyclical fashion. Experimental results for a four-level flying capacitor inverter are presented, verifying that the natural balancing properties of this converter has been preserved, the cell switching utilization is equal and the expected harmonic gains of PD and CSVPWM compared to phase shifted carrier PWM have been achieved     

一种应用于48V-1V系统的隔离型混合模式降压变换器

提出了一种应用于48 V-1 V系统的隔离型混合模式降压变换器,利用飞电容和变压器实现高转换比应用下的高转换效率。混合变换器结合了开关电容变换器和开关电感变换器,其中飞电容承担了部分电压降,实现了功率开关管电压应力的降低。由于开关节点处的电压摆幅较小,开关损耗随之减小;通过使用更低压的功率开关管,实现功率开关管导通损耗减小。在此基础上,隔离型混合模式降压变换器通过时序控制可以实现软开关,进而实现功率开关管开关损耗减小,使得整体效率提升。在隔离型混合模式降压变换器中,飞电容还具有隔直电容的作用,可以防止变压器偏磁。在典型应用下,即在48 V输入电压、1 V输出电压、500 kHz开关频率下,峰值效率为94.84%。     

Switching frequency imposition and ripple reduction in DTC drivesby using a multilevel converter

A new strategy for direct torque control with imposed switching frequency (DiCoIF) is proposed. This strategy was specially designed to operate with a multicell (flying capacitors) inverter with any number of levels, which means it can also be used for standard two-level inverters. This approach combines the well known advantages of the multicell. inverter with those of a direct torque controlled (DTC) based strategy. It is shown that the multicell topology presents enough degrees of freedom to control both torque and flux with very low ripple and high dynamics on one hand, and to impose the switching frequency and the capacitors voltage balance on the other hand. Experimental and simulation results, obtained with a standard two-level inverter and with a four-level multicell inverter, are presented and discussed. Finally, a comparative analysis either with the classical DTC and field oriented techniques is carried out     

Natural Current Balancing of Multicell Current Source Converters

This paper explores the issue of balancing the intermediate inductor currents of a multilevel current source inverter (CSI). The paper begins by identifying that single-phase multicell CSIs and flying capacitor voltage source inverters (FCVSIs) form a topological dual pair. This allows the established understanding of the natural balance process of the FCVSI intermediate capacitor voltages to suggest that the intermediate inductor currents of the CSI should also settle to their target balance values when phase-shifted carrier (PSC) pulse width modulation (PWM) is used. This concept is confirmed by full switched simulations. Next, an analytic model is developed to explore the dynamic response and robustness of the balancing process, using a double Fourier series representation of the converter switching signals to linearise a nonlinear transient circuit model. The model is verified against simulations, and predicts that the resistance of the intermediate inductors can perturb the steady state balancing point. The model further predicts that the resistance of the inductance must be small compared to the load resistance to minimize this effect. Results obtained on a five-level experimental CSI precisely match the theoretical model and hence support these conclusions.     

Analysis and Design of a Novel Three-Phase AC–DC Buck-Boost Converter

This paper proposes a novel three-phase ac-dc buck-boost converter. The proposed converter uses four active switches, which are driven by only one control signal. This converter is operated in discontinuous conduction mode (DCM) by using the pulsewidth modulation (PWM) technique, and the control scheme very easily and simply achieves purely sinusoidal input current, high power factor, low total harmonic distortion of the input current and step-up/down output voltage. Also, the proposed converter provides a constant average current to the output capacitor and load in each switching period. Thus, the ripple component of sixth times line frequency will not appear in the output voltage. Therefore, a smaller output capacitor can be used in the proposed converter. Moreover, the steady-state analysis of voltage gain and boundary operating condition are presented. Also, the selections of inductor, output capacitor and input filter are depicted. Finally, a prototype circuit with simple control logic is implemented to illustrate the theoretical analysis.     

A ZVS imbricated cell multilevel inverter with auxiliary resonant commutated poles

The imbricated-cell multilevel converter is well suited to high power applications. It allows the series connection of n switches with natural voltage sharing between these switches enabled through the connection of n-1 flying capacitors. This paper deals with the application of soft-switching on this topology; to date, only the hard-switching mode has been studied. The use of soft switching enables an increase of the switching frequency (resulting in the size reduction of the flying capacitors) without a decrease of the converter efficiency. Of the soft switching methods considered, the Auxiliary Resonant Commutated Pole (ARCP) technique was chosen due to the relative ease in which it can be incorporated into the converter topology. Furthermore, this technique offers numerous advantages: loss reduction, no added stress to the switches and compatibility with PWM control. The main properties of the ARCP multicell converter are the same as the hard-switched topology: an increase of the apparent output switching frequency and natural self-balancing of the flying-capacitor voltages. This paper presents the results of both simulations performed and measurements taken from an experimental set-up in order to study the viable system functioning. The introduction of soft-switching strongly complicates the theoretical study of the balancing mechanisms, however. As a result, the authors depend on simulations to validate the natural balancing effect during soft switching. Lastly, a general method of loss measurement is presented. Results show that the converter losses are reduced by at least 30%.     

Multilevel inverters: a survey of topologies, controls, and applications

Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.     

Voltage-source active power filter based on multilevel converter and ultracapacitor DC link

A new topology for active power filters (APF) using an 81-level converter is analyzed. Each phase of the converter is composed of four three-state converters, all of them connected to the same capacitor dc link voltage and their output connected in series through output transformers. The main advantages of this kind of converter are the negligible harmonic distortion obtained and the very low switching frequency operation. The single-phase equivalent circuit is analyzed and their governing equations derived. The dc link voltage control, based on manipulating the converter's voltage phase, is analyzed together with the circuit's characteristics that determine the capability to draw or deliver active and reactive current. Simulation results for this application are compared with conventional pulsewidth-modulated (PWM) converters, showing that this filter can compensate load current harmonics, keeping better-quality sinusoidal currents from the source. The simulated configuration uses a 1-F ultracapacitor in the dc link, making it possible to store energy and deliver it during short voltage dips. This is achieved by applying a modulation control to maintain a stable ac voltage during dc voltage drops. A prototype of the filter was implemented and tested, and the obtained current waveforms showed to be as good as expected.     

Extended operation of flying capacitor multilevel inverters

Recent research in flying capacitor multilevel inverters (FCMIs) has shown that the number of voltage levels can be extended by changing the ratio of the capacitor voltages. For the three-cell FCMI, four levels of operation are expected if the traditional ratio of the capacitor voltages is 1:2:3. However, by altering the ratio, the inverter can operate as a five-, six-, seven-, or eight-level inverter. According to previous research, the eight-level case is referred to as maximally distended (or full binary combination schema) since it utilizes all possible transistor switching states. However, this case does not have enough per-phase redundancy to ensure capacitor voltage balancing under all modes of operation. In this paper, redundancy involving all phases is used along with per-phase redundancy to improve capacitor voltage balancing. It is shown that the four- and five-level cases are suitable for motor drive operation and can maintain capacitor voltage balance under a wide range of power factors and modulation indices. The six-, seven-, and eight-level cases are suitable for reactive power transfer in applications such as static var compensation. Simulation and laboratory measurements verify the proposed joint-phase redundancy control.     

A generalized Undeland snubber for flying capacitor multilevel inverter and converter

This paper proposes a snubber circuit for a flying capacitor multilevel inverter and converter. It also explains the concept of constructing a snubber circuit for a multilevel inverter and converter. The proposed snubber circuit makes use of an Undeland snubber as a basic snubber unit, and thus can be regarded as a generalized Undeland snubber for a flying capacitor multilevel inverter and converter. It has such an advantage of Undeland snubber used in the two-level inverter. Compared with a conventional RLD/RCD snubber for multilevel inverter and converter, the proposed snubber keeps such good features as fewer number of components, reduction of voltage stress of main switching devices due to low overvoltage, and improved efficiency of system due to low snubber loss. In this paper, the proposed snubber is applied to a three-level flying capacitor inverter, and its features are in detail demonstrated by computer simulation and experimental result.     

Multicell converters: active control and observation of flying-capacitor voltages

The multicell converters introduced more than ten years ago make it possible to distribute the voltage constraints among series-connected switches and to improve the output waveforms (increased number of levels and apparent frequency). The balance of the constraints requires an appropriate distribution of the flying voltages. This paper presents some solutions for the active control of the voltages across the flying capacitors in the presence of rapid variation of the input voltage. The latter part of this paper is dedicated to the observation of these voltages using an original modeling of the converter.     

Research of the equivalent relationship between the space vector and the triangular carrier-based PWM modulation strategies in the flying capacitor multilevel inverters

ABSTRACT

The equivalent relationship between the carrier-based modulation (CBPWM) and the space vector modulation (SVPWM) based on the flying capacitor multilevel inverters has not been analysed systematically. Due to the redundancies of the flying capacitor multilevel inverters, this paper firstly focuses on the switching sequence and the working ratio of each switching state and then proposes a novel method of decomposing modulation wave. This method can distribute the ratio of redundancies arbitrarily to satisfy the requirements of the output. Taking three- and five-level inverters as examples, by injecting a zero-sequence voltage into the CBPWM modulation wave and then decomposing the new modulation wave, we can get the same sequence of SVPWM modulation. This method takes the advantages of using simple CBPWM to realise the complex output of SVPWM, such as double- and quadruple-frequency output. At last, we deduce the equivalent relationship between the SVPWM and CBPWM in n-level with an arbitrary number of segments. Experimental results verify the correctness of the equivalent relationship.     

Modified Phase-Shifted PWM Control for Flying Capacitor Multilevel Converters

The issue of voltage imbalance remains a challenge for the flying capacitor multilevel converter. The phase-shifted pulsewidth modulation (PS-PWM) method has a certain degree of self-balancing properties. However, the method alone is not sufficient to maintain balanced capacitor voltages in practical applications. The paper proposes a closed-loop modified PS-PWM control method by incorporating a novel balancing algorithm. The algorithm takes advantage of switching redundancies to adjust the switching times of selected switching states and thus maintaining the capacitor voltages balanced without adversely affecting the system's performance. Key techniques of the proposed control method, including selection of switching states, calculation of adjusting times for the selected states, and determination of new switching instants of the modified PS-PWM are described and analyzed. Simulation and experimental results are presented to confirm the feasibility of the proposed method     

New zero voltage switching DC converter with flying capacitors

A new soft switching converter is presented for medium power applications. Two full-bridge converters are connected in series at high voltage side in order to limit the voltage stress of power switches at V_in/2. Therefore, power metal–oxide–semiconductor field-effect transistors (MOSFETs) with 600 V voltage rating can be adopted for 1200 V input voltage applications. In order to balance two input split capacitor voltages in every switching cycle, two flying capacitors are connected on the AC side of two full-bridge converters. Phase-shift pulse-width modulation (PS-PWM) is adopted to regulate the output voltage. Based on the resonant behaviour by the output capacitance of MOSFETs and the resonant inductance, active MOSFETs can be turned on under zero voltage switching (ZVS) during the transition interval. Thus, the switching losses of power MOSFETs are reduced. Two full-bridge converters are used in the proposed circuit to share load current and reduce the current stress of passive and active components. The circuit analysis and design example of the prototype circuit are provided in detail and the performance of the proposed converter is verified by the experiments.     

Digital control and simulation for power electronic apparatus in dual voltage railway locomotive

An object-oriented simulation-in-the-loop C-written tool for power electronic apparatus in a dual (AC and DC) voltage prototype railway locomotive is proposed. By considering also the hardware elements of the control board, the simulator structure ensures the straightforward portability of the control codes from simulation to implementation, thus allowing to check the control algorithms and codes via realistic numerical tests. Each component of the locomotive is modeled as an object, by using state space dynamic models for the dynamic subsystems. The two line-side converter configurations, the field oriented controlled induction motors with Kalman filter as flux estimator and the space-vector modulated voltage source inverters are considered as examples. A new control strategy for the line-side converter ensuring voltage regulation and harmonic active filtering is also proposed and simulations show the effectiveness of the controller. The digital implementation of the control algorithms on an electronic control unit equipped with a DSP and a microcontroller is detailed, showing the time relationship between the control board tasks and the converters switching functions.     

Implementation of a Three-Phase Capacitor-Clamped Active Power Filter Under Unbalanced Condition

A capacitor-clamped voltage-source inverter for active power filter operation under balanced and unbalanced conditions is proposed to suppress current harmonics and compensate the reactive power generated from the nonlinear loads. The adopted voltage-source inverter is based on a three-level capacitor-clamped topology to reduce the voltage stress of power semiconductors. Two control loops are used in the control scheme to achieve harmonic and reactive currents compensation and to regulate the inverter dc side voltage. In the adopted inverter, the neutral point voltage is compensated by a voltage compensator to obtain the balanced capacitor voltages on the dc side. In order to control the flying capacitor voltages, two redundant states in each inverter leg can be selected to compensate the flying capacitor to obtain a better voltage waveform with low harmonic contents on the ac terminals. The balanced and sinusoidal line currents are drawn from the ac source under the balanced and unbalanced conditions. The feasibility of the proposed scheme is confirmed through experimental results     

Capacitor voltage balancing in full binary combination schema flying capacitor multilevel inverters

Recently, the full binary combination schema (FBCS) method has been introduced to control the flying capacitor multilevel inverter. This method has the primary advantage that the number of voltage levels can be increased for a given number of semiconductor devices when compared to the conventional control methods. However, due to the difficulty of balancing the capacitors, the new schema requires fixed floating sources to provide the DC voltages. This paper reveals an approach of balancing the capacitors, thus expanding the application fields of FBCS inverters to the family of the flying capacitor multilevel inverters under the condition of choosing a suitable modulation index. Simulation results demonstrate the proposed voltage balancing control.