Dan Floricau

Evaluation of three-phase transformerless photovoltaic inverter topologies

This paper analyzes and compares three transformerless photovoltaic inverter topologies for three-phase grid connection with the main focus on the safety issues that result from the lack of galvanic isolation. A common-mode model, valid at frequencies lower than 50 kHz, is adopted to study the leakage current paths. The model is validated by both simulation and experimental results. These will be used to compare the selected topologies, and to explain the influence of system unbalance and the neutral conductor inductance on the leakage current. It will be demonstrated that the later has a crucial influence. Finally, a comparison of the selected topologies is carried out, based on the adopted modulation, connection of the neutral and its inductance, effects of unbalance conditions, component ratings, output voltage levels, and filter size.

New Multilevel Converters With Coupled Inductors: Properties and Control

In the paper, a new five-level (5L) converter is presented. It is called 5L-active-neutral-point-clamped with coupled-inductor (5L-ANPC-CI) and consists of a three-level bidirectional (3L-B) commutation cell connected to two parallel basic commutation cells. The proposed topology can be seen as two cascaded current stages controlled at different frequencies. The high-current stage is specific to the 3L-B cell controlled at low-frequency, while the low-current stage corresponds to the parallel cells with coupled-inductor controlled at high-frequency (HF). Due to this advantage, the new converter appears as a useful solution to increase the output current, while the switched current through the HF power devices is reduced. The operation principle of the new structure is verified by simulations both for single- and for three-phase topologies. Experimental results for single-phase 5L-ANPC-CI converter on a low-voltage test bench are also provided to validate the proper operation of this concept.

A new stacked NPC converter: 3L-topology and control

In this paper a new three-level (3L) voltage source converter is presented. This new topology called Stacked Neutral Point Clamped (3L-SNPC) increases the apparent switching frequency compared with other 3L structures (3L-Stacked Cells, 3L-Neutral Point Clamped and 3L-Active NPC). This advantage leads to a better balancing of total losses in power switches under all specified operating conditions (low or high modulation index). The topology has more degrees of freedom and allows a parallel conversion. The load current passes simultaneously or alternatively through two parallel paths, depending on PWM strategy. In the paper three PWM strategies are proposed and their switching states and commutations are analyzed. The total losses in power switches are calculated and compared to the most popular 3L structures. Experimental and simulation results are illustrated in the paper to confirm the operation of the proposed topology.

New Active Stacked NPC Multilevel Converter: Operation and Features

In this paper, the operation and the features of a new three-level converter are presented. The proposed topology was named three-level active-stacked neutral point clamped (3L-ASNPC). It is a derivative of the 3L-SNPC structure, having two additional active switches connected antiparallel with the clamp diodes. The main advantage of 3L-ASNPC converter is the reduction of the average switching frequency for all power devices. In the same time, the apparent switching frequency of the output voltage is doubled. Experimental and simulation results are shown in order to validate the proposed structure and the analysis of the switching states.

New Multilevel Converters Based on Stacked Commutation Cells With Shared Power Devices

In this paper, a new three-level (3L) converter is presented. It is called 3L Vienna neutral-point-clamped (3L-VNPC) converter and consists of two stacked high-frequency (HF) commutation cells working with a shared power device. The HF cells are connected to a single-phase switched bridge controlled at low frequency. The 3L-VNPC topology is extended to 3L active VNPC (3L-AVNPC) concept, which is generalized for five levels (5L) and N odd voltage levels (N L). The proposed multilevel converters can be seen as two cascaded voltage stages. The high-voltage stage is specific to the single-phase switched bridge, while the low-voltage (LV) stage corresponds to the stacked HF cells with shared power devices. The new concept AVNPC is a useful and suitable solution for medium-voltage applications, where small modulation index (M) is as crucial as large M. The analyses of the new converters are verified by simulations both for 3L and for 5L topologies, and their features are generalized for NL. Experimental results for single-phase 3L-AVNPC converter on an LV test bench are also provided to validate the proper operation of the new concept.

A comparison of efficiency for three-level NPC and Active NPC voltage source converters

The 3L-NPC (three-level Neutral-Point-Clamped) is the most popular multilevel converter used in high-power medium-voltage applications. An important disadvantage of this structure is the unequal distribution of losses among the switches. The performances of 3L-NPC structure were improved by developing the 3L-ANPC (Active-NPC) converter which have more degrees of freedom. In this paper the switching states are studied for different PWM strategies. A numeric calculus method for the total losses in switches was also presented in order to compare the efficiency for the 3L-NPC and 3L-ANPC converters. The PSIM simulation results are shown in order to validate the analysis of the switching states.

New multilevel Flying-Capacitor inverters with coupled-inductors

In the paper a new five-level (5L) inverter based on unidirectional Flying-Capacitor switching cells is presented. It is named 5L-Flying-Capacitor with Coupled-Inductor (5L-FC-CI) and consists in parallel connection of two unidirectional (P and N) three-level (3L) Flying-Capacitor switching cells with a coupled-inductor. The PWM output voltage is vastly improved and the switch control dead times is eliminated, particularly when compared with the 3L inverter obtained with bidirectional Flying-Capacitor cells. The phase-shifted carrier (PSC) modulation is used to control the new 5L topology. Thus, the proposed structure ensures an apparent switching frequency in each winding of coupled-inductors twice of switching frequency. The analysis of the new inverter is verified by numerical simulations both for single- and for three-phase topologies and their features are generalized for N odd voltage-levels (NL).

New three level topology with shared components: Properties, losses, and control strategy

The multilevel topologies allow the power transfer for applications in the range of medium voltage drive with switching frequency of about kHz. In the three phases applications, some of these topologies have many switches. In order to reduce this number of switches, it is possible to share some of them between several phases and thus reduce the total number of switches. Based on a known topology of three phases ASNPC [1−2], this paper proposes to share some of these switches in order to optimize the silicon used but retaining all the properties of the initial topology. This new topology has interesting properties regarding the redundancy of the combination for many levels. These redundancies are then used to reduce overall system losses.

Availability modelling and evaluation of the longitudinal coupler used in power systems

Generalised stochastic Petri nets (GSPN) are the most common state-space model types used in dependability modelling. The great complexity of GSPN models, even for simple repairable systems, makes it necessary to use a less complex model. Combining the GSPN properties and high level Petri nets facilities, a structural simplified model logical explicit stochastic Petri nets (LESPN), having the same modelling power as GSPN, is built. Primitive architectural modules are used in repairable power systems dependability modelling. The paper presents a comparative analysis of different LESPN models associated to the longitudinal coupler used in power systems.

New unidirectional five-level VIENNA rectifier for high-current applications

In this paper a new AC/DC unidirectional PWM Vienna switching cell with five voltage levels is proposed. It is called 5L-Parallel-Unidirectional-Vienna (5L-PUV) cell and can be connected between an AC current source - on the input side, and a DC voltage source - on the output side. The novelty of 5L-PUV topology is based on two cascaded stages that work at different currents and different switching-frequency. The first stage contains two diodes that operate at high-current and low-frequency. The second stage works at low-current and high-frequency and contains two groups of stacked switching cells parallel connected by means of coupled-inductors (CIs). An interesting property of the 5L-PBV switching cell is the use of two CIs which work alternatively on a half-cycle. Thus, the currents in inductor windings of CIs are naturally set to zero on each cycle and any difference between these currents is cancelled periodically. Due to the parallel connection of high-frequency switching cells, the 5L-PUV concept appears as a useful solution for high-current applications. The operation principle of the 5L-PUV cell is verified by digital simulations both for single- and for three-phase unity power factor unidirectional PWM rectifiers.