Flying Capacitor Multicell Converter Research Articles

Optimized Switching Frequency Voltage Balancing Schemes for Flying Capacitor Based Multilevel Converters

The redundant switching states of flying capacitor-based (FC-based) multilevel converters are used to balance the voltages of the flying capacitors. Attempts to balance capacitor voltages have ignored the switching transitions between converter switching states. In this paper, we propose a generalized voltage balancing scheme with an optimized switching frequency scheme for FC-based multilevel converters, including the classic flying capacitor multicell (FCM) converter, which needs less computation. All the switching transitions between the switching states of adjacent voltage levels are considered by the overall priority index of switching frequency. In addition, an overall priority index is utilized to balance the voltages of flying capacitors. Using these two indexes, three criteria are developed to select the proper switching state of the converter. Using these criteria, the proposed decoupled SVM scheme is implemented intuitively and simply for FC-based multilevel converters. The proposed scheme is evaluated by simulation results of a five-level FCM converter. Moreover, experimental results demonstrate the robustness of the proposed method in the static load and transient conditions.

A Fast-Decoupled Space Vector Modulation Scheme for Flying Capacitor-Based Multilevel Converters

The space vector modulation (SVM) scheme is popularly implemented in multilevel converters. However, in flying capacitor-based (FC-based) multilevel converters, it becomes extremely challenging to implement due to the redundancy in phase voltage levels. The intrinsic property of line voltage redundancy of SVM in addition to the phase voltage redundancy of FC-based converters results in considerably increased computational complexity and storage requirements. In fact, one of the most important concerns about FC-based converters is the voltage control of the FCs. This article proposes a generalized decoupled SVM scheme for the FC-based multilevel converters, including the classic flying capacitor multicell (FCM) converters, which is less computationally complex. For balancing the FC voltages, an overall priority index is employed to determine the switching state with the best balancing characteristics. The proposed decoupled SVM scheme along with the capacitor voltage balancing scheme can be implemented intuitively and simply for all FC-based multilevel converters. The simulation results of a five-level FCM converter show the effectiveness of the proposed method in the regulation of FC voltages. Moreover, the proposed voltage balancing method is robust to static load and transient conditions. Also, experimental results demonstrate the effectiveness of the proposed method.

New Active Capacitor Voltage Balancing Method for Flying Capacitor Multicell Converter Based on Logic-Form-Equations

This paper presents a new active capacitor voltage balancing control method for the flying capacitor multicell (FCM) converters which is fully implemented using logic-form equations. The proposed active capacitor voltage balancing control technique, measures output current and flying capacitor (FC) voltages to generate the switching states in order to produce the required output voltage level and to balance FCs’ voltages at their reference values. Output voltage of the FCM converter controlled with proposed active capacitor voltage balancing method can be modulated with any pulse width modulation (PWM) technique such as the phase-shifted-carrier PWM or level-shifted-carrier PWM. An advantage of the proposed active capacitor voltage balancing control method is its simplicity as it does not require any complex computations and tedious optimization calculations. Simulation results and experimental measurements of a three-cell four-level and four-cell five-level FCM converters are presented to verify the performance of the proposed active capacitor voltage balancing control technique.

Analytical determination of conduction power loss and investigation of switching power loss for modified flying capacitor multicell converters

Considering advantages of modified flying capacitor multicell (MFCM) converter over conventional flying capacitor multicell converter, and noting that conduction loss investigation can be very advantageous in design phase of multilevel converters, this study presents an analytical approach to calculate and investigate conduction losses in MFCM converters. First, rms and average currents of insulated gate bipolar transistors (IGBTs) and anti-parallel diodes are analytically calculated by considering the associated duty cycle of each IGBT/diode in terms of the converter modulation index, load current, and load power factor. Numerical results of derived closed-form equations to calculate rms and average currents of IGBTs/diodes are compared with simulation results and experimental measurements. All simulation, analytic, and experimental results agree well with each other which validate derived closed-form equations. Afterwards, derived equations for rms and average current computations are utilised to calculate the conduction power losses in a 12.4 MVA, 3.3 kV, nine-level (line-to-line) MFCM converter. A 2.5 kV, 1.5 kA IGBT module from ABB is considered as a power switch in the performed case study for MFCM converter. In addition, switching power loss investigation is performed using numeric approach and curve-fitting method for aforementioned MFCM converter. Comparative analysis and evaluation of conduction and switching losses are presented.

New Multilevel Converter Based on Cascade Connection of Double Flying Capacitor Multicell Converters and Its Improved Modulation Technique

This paper proposes a new multilevel converter based on the cascade connection of double flying capacitor multicell (DFCM) converters, as multilevel modules, to decrease the voltage diversity of the flying capacitors. Furthermore, a new switching pattern based on the phase-shifted pulse-width modulation technique is proposed to reduce the voltage ripple across the flying capacitors. Moreover, the proposed modulation technique reduces the rms value of the current flowing through flying capacitors. This results in an increase in the life time of flying capacitors and a decrease in the capacitance of the flying capacitors, to keep the same amount of the ripple, meaning a reduction in the physical size of the converter. In addition, this paper presents an analytical approach to calculate the average and rms currents of the insulated gate bipolar transistors (IGBTs)/diodes in the DFCM converter in a closed-form expression. The derived closed-form equations to calculate the average and rms currents of the IGBTs/diodes are utilized to investigate the conduction power losses in a DFCM converter and the proposed multilevel converter. Numerical results of the derived closed-form equations match the simulation results well, which validates the derived equations. Furthermore, simulation results and experimental measurements of the proposed multilevel power converter, configured by cascading two two-cell five-level DFCM converters, are presented to validate the performance of the proposed converter as well as the suggested modulation technique.

Hybrid double flying capacitor multicell converter and its application in grid‐tied renewable energy resources

Multicell multilevel voltage source power converters are the state-of-the-art and key elements for harnessing and integration of medium-voltage high-power renewable energy resources. This study proposes a novel hybrid topology for double flying capacitor multicell (DFCM) converters that is suitable for grid-tied renewable energy systems. The proposed power converter is realised by cascade connection of one DFCM converter and one full-bridge converter (FBC) comprising a floating-capacitor. The floating-capacitor voltage of the FBC is regulated naturally at its requisite voltage level because of the existence of the abundant redundant charging/discharging states provided by the proposed switching and modulation pattern for the suggested hybrid converter. Hence the proposed control technique not only preserves the natural balancing for the FC voltages of the DFCM converter but also provides natural balancing for the floating-capacitor voltage of the FBC. The main objective of the FBC is to generate additional intermediate voltage steps to double the number of DFCM converter's voltage levels for enhancing power quality significantly. The proposed hybrid DFCM converter is simulated to inject/absorb active/reactive-power provided and harvested from renewable energy resources into/from electric power grid. Furthermore, experimental measurements of the proposed hybrid power converter are also presented to validate its viability, merits, effectiveness and the proposed modulation and control strategy.

An EM-based identification algorithm for a class of hybrid systems with application to power electronics

In this paper we present an identification algorithm for a class of continuous-time hybrid systems. In such systems, both continuous-time and discrete-time dynamics are involved. We apply the expectation-maximisation algorithm to obtain the maximum likelihood estimate of the parameters of a discrete-time model expressed in incremental form. The main advantage of this approach is that the continuous-time parameters can directly be recovered. The technique is particularly well suited to fast-sampling rates. As an application, we focus on a standard identification problem in power electronics. In this field, our proposed algorithm is of importance since accurate modelling of power converters is required in high- performance applications and for fault diagnosis. As an illustrative example, and to verify the performance of our proposed algorithm, we apply our results to a flying capacitor multicell converter.

Reduced DC voltage source flying capacitor multicell multilevel inverter: analysis and implementation

Voltage source multicell multilevel converters (VSMMCs) have been extensively exploited in medium-voltage high-power fed industrial applications. One of the typical breeds of the VSMMCs is flying-capacitor multicell converter (FCMC). This study presents an improved configuration for FCMCs, accompanied by its analysis, modelling and implementation. The main advantage of the proposed converter, in comparison with the conventional one, is that the number and voltage rating of the required dc voltage sources are halved in the improved topology which results in reducing the cost, size, installation area and weight of the FCMCs effectively. This amelioration is attained by adding four power switches to the conventional topology of an FCMC without making any amendments to the number and voltage rating of high-frequency power-switches and flying capacitors (FCs). The proposed switching and control methodology for the adopted FCMC is based on the phase-shifted carrier pulse width modulation technique with applying some amendments; therefore the natural balancing phenomenon of the FC voltages, one of the critical advantages of the FCMCs, is preserved in the proposed VSMMC. Experimental measurements taken from the 3-cell-four-level and 4-cell-five-level laboratory prototype systems of the proposed converter are presented in order to validate and corroborate the effectiveness and advantages of the proposed topology, its switching and suggested control strategy. Furthermore, experimental studies are accomplished on an 8-cell-nine-level prototype system of the proposed FCMC to peruse its feasibility and viability for higher number of cells and voltage levels. In addition, comparison of the proposed converter with the other types of multilevel converters is carried out to underline the advantages of the proposed structure profoundly.

Flying Capacitors Reduction in an Improved Double Flying Capacitor Multicell Converter Controlled by a Modified Modulation Method

This paper proposes an improved configuration of double flying capacitor multicell (DFCM) converter. The main advantages of the proposed converter, compared to the conventional DFCM converter, are the doubling of the number of output voltage levels and improvement of the output voltage frequency spectrum. This progress is achieved by adding only two low-power switches and one dc voltage source, whose voltage rating is a small fraction of the main dc-link voltage rating, to the conventional configuration of the DFCM converter. However, the number and voltage rating of high-frequency switches and capacitors and the number of high-frequency switchings during a full cycle are kept constant. The doubling of the number of output voltage levels in the proposed converter makes it possible to decrease the number of cells, the number of flying capacitors, as well as their voltage rating and the amount of stored energy in flying capacitors. Moreover, a modulation method based on phase-shifted carrier pulsewidth modulation is proposed to control the new converter. Simulation and measured experimental results are presented to illustrate the performance of the proposed configuration and its control strategy.

New mixed stacked multicell converter with interesting advantages

Multilevel converters are very interesting alternatives for medium and high-power applications. This study presents a new configuration of multicell converters. The main advantages of the proposed converter, in comparison with conventional flying capacitor multicell (FCM) and stacked multicell (SM) converters, are doubling the number of output voltage levels and the root mean square (RMS), improving the output voltage frequency spectrum and lowering the number and rating of components and decreasing the converter losses. Furthermore, the stored energy and rating of flying capacitors will be decreased to half in this topology in comparison with FCM and SM converters. These improvements are achieved by adding only four low-frequency switches to the conventional SM converters configuration. In this topology, the number of high-frequency switches is decreased while the voltage ratings of them during a full cycle are kept constant. Also, a modified phase shifted pulse-width-modulation-based control method has been applied to the proposed topology, which can decrease the harmonic distortions and THD. The circuit is simulated using PSCAD/EMTDC software and simulation results are presented to validate the effectiveness and advantages of the proposed configuration. In addition, the experimental results of proposed configuration prototype have been presented to examine its practicability.

New method for estimating flying capacitor voltages in stacked multicell and flying capacitor multicell converters

Multicell converters are an interesting alternative for medium voltage and high power applications, because of the increased number of output voltage levels and apparent frequency. The two most significant types of multicell converter are the flying capacitor multicell (FCM) converter and its derivative, stacked multicell (SM) converter. Balancing flying capacitor voltages is an important constraint to the proper performance of FCM and SM converters. Thus, observation of the flying capacitor voltages used in active control is valuable, but using voltage sensors for observation increases cost and size of the converter. This paper deals with a new strategy to estimate the flying capacitor voltages of both FCM and SM converters. The proposed strategy is based on a discrete time model of the converter and uses only a load current sensor. The circuit was simulated using PSCAD/EMTDC software and simulation results were presented to validate the effectiveness of the proposed estimation strategy in observing the flying capacitor voltages. Simplicity is the most significant advantage of the proposed strategy, its performance being based on simple equations.

Double Flying Capacitor Multicell Converter Based on Modified Phase-Shifted Pulsewidth Modulation

Multilevel converters are very interesting alternatives for medium and high-power applications. The main reason is the increase in the number of output voltage levels and its apparent frequency. This paper presents a new configuration of flying capacitor multicell (FCM) converter. The main advantages of the proposed converter, in comparison with FCM and stacked multicell converters, are doubling the rms and the number of output voltage levels, improving the output voltage frequency spectrum, and canceling the midpoint of dc source. This progress is achieved by adding only two low-frequency switches to the conventional configuration of FCM converter while the number of high-frequency switches and capacitors, voltage ratings of capacitors and switches, and the number of high-frequency switchings during a full cycle are kept constant. This converter is controlled by a modified phase-shifted pulsewidth modulation, therefore, the self-balancing property of the flying capacitor converter is maintained in the proposed converter. The circuit is simulated using power systems computer-aided design/electromagnetic transients in DC systems (EMTDC) software and simulation results are presented to validate the effectiveness and advantages of the proposed configuration as well as its control strategy. Additionally, measurements taken from an experimental setup are presented in order to study the practical configuration.