Nonisolated Three-Phase Current DC-Link Buck–Boost EV Charger With Virtual Output Midpoint Grounding and Ground Current Control

Common-mode (CM) leakage current is an important symptom in cable insulation monitoring. However, traditional current sensors suffer from low accuracy when extracting small-amplitude leakage current from large-amplitude load current. This article proposes an open-type high-precision CM leakage current sensor model to achieve online monitoring of CM leakage current in cables. Through symmetrically arranging four tunnel magnetoresistance (TMR) chips outside the shielding ring and averaging their output results, the load current component can be eliminated to the lowest level and the CM leakage current component can be retained. This open-type structure resolves the installation challenges associated with closed-type shielding ring in long cables, and four symmetrically arranged TMR Chips can further reduce the error introduced by the open structure, which enables the extraction of 20 mA CM leakage current with relative error less than 5%, even in the presence of 200 A load current.

This paper presents the touch current (TC) suppression for the semiconductor-based galvanic isolation (SGI) solution. The solution achieves galvanic isolation by utilizing semiconductor switches to deliver differential-mode (DM) power during their ON states, while sustaining common-mode (CM) isolation voltage and blocking CM leakage current during their OFF states. In this solution, TC is resulted as the switch output capacitance is charged by the CM voltage at every switching event, and the TC rises as the switching frequency increases, which limits the system power density. This paper reviews multiple TC-suppression approaches for the SGI to meet the safety standard IEC60950. These approaches include preventing the TC generation, increasing CM impedance, bypassing the TC, and partially reduce the TC. Among them all, it is practical to reduce the TC via partial compensation for the switch output charge, using distributed auxiliary current-source circuits. To verify this approach, a simplified off-line power supply with a SGI-based dc/dc stage is built and tested. The results show the TC can be reduced effectively. It also allows the converter to operate at higher switching frequency to achieve higher power density, while meeting the IEC60950 TC requirement.

Abstract: Owing to low cost and system compactness, transformerless inverters became popular in single-phase grid connected photovoltaic (PV) systems. Improvement of conversion efficiency, reduction in distortion of the grid current and minimizing common mode leakage current are the major concerns in these inverters. The purpose of this paper is to review on various transformerless solar PV inverters. In this paper, the single-phase inverters are classified into two major categories: 1) Nonpatent Survey and 2) Patent Survey. These two categories are further divided in various subcategories. Each subcategories are elaborated and their merits and demerits are discussed. This paper provides in depth analysis of several single phase solar PV inverters. Thus, it can be taken as reference for researchers. Index Terms: Transformerless Solar PV inverter, Conversion efficiency, inverter topology, Common Mode (CM) leakage current, Common Mode voltage Cite this Article: Pankaj Kumar Singh, Swapnil Kushwaha, Kishan Bhusan Sahay. A Review of Single-Phase Grid Connected Transformerless Solar PV Inverter Topology. Journal of Electronic Design Technology. 2019; 10(1): 1–7p.

Power generation systems (PGSs) based on renewable energy sources are finding ever-widening applications, and many researchers work on this problem. Many papers address the problem of transformerless PGSs, but few of them aimed at conducting research on structures with multilevel converter topologies as part of a PGS. In this paper a grid-tied transformerless PV-generation system based on a multilevel converter is discussed. There are common-mode leakage currents (CMLCs), which act as a parasitic factor. It is also known that common-mode voltage is the main cause of the common-mode leakage current in grid-tied PV-generation systems. This paper considers the space vector pulse-width modulation (PWM) technique, which is used to suppress or reduce common-mode leakage current. The proposed PWM technique with the reduction of common-mode leakage current for a generation system based on the multilevel converter controlled with a PWM technique was verified experimentally. The experimental results accurately confirmed the mathematical model and the compensation achieved without errors. In the experiment, there was an approximately six-fold decrease in the common-mode leakage current (10.3 mA in rejection mode and 61 mA in non-rejection current). This can lead to the elimination of CMLC in a multilevel semiconductor converter only by changing the modulation mode. This suggests the possibility of using these devices as part of transformerless generation systems. Suppression of CMLC can only be carried out by changing the PWM algorithm. Both considered topologies can implement this mode of operation. The proposed converter has a higher efficiency up to a frequency multiplicity of 2000.

A common-mode (CM) voltage reduction modulation for a three-level full bridge phase-shift DC-DC converter is proposed in this paper. With conventional modulation, the three-level full bridge phase-shift DC-DC converter generates considerable CM voltage. The CM voltage is undesired because it causes CM current noise and deteriorate EMI performance. To solve this problem, methods including bulky EMI filter or additional cancellation circuits are usually used. However, it increases system cost, and reduces efficiency. The proposed modulation method can reduce the CM voltage close to zero and reduce the CM current substantially. The method does not require any additional circuit. The size of EMI filter could also be reduced due to the reduced CM voltage and current. Compared with conventional methods, the proposed CM voltage reduction method has the benefits of effective, low cost and high efficiency. The principle of the proposed modulation method is presented in this paper. The correctness and effectiveness of the method is verified by simulation and experimental results.

The reduction of common mode leakage current in centralized inverter for grid-connected PV (photovoltaic) system has been well studied. However, the research in cascaded multilevel inverter PV system is not far enough. This paper focuses on analyzing the generation mechanism of common mode leakage current and building equivalent circuit model of cascaded PV system. By clarifying the reason why the conventional cascaded H4 (full-bridge inverter) PV system fails to reduce the common mode leakage current, a new cascaded iH6 inverter PV system is proposed. The related theoretical analysis and simulation results are given, and it is verified that the common mode leakage current in three phase grid-connected system with proposed converters can be greatly reduced compared with the conventional cascaded H4 system.

For flyback converters with long cables attached, radiated common mode (CM) current is the dominant radiation source. Radiated CM current is caused by the CM voltage driving the CM noise propagation loop including the converter and the antenna. This paper proposes a radiated CM current model to quantify the relationship between the CM voltages, impedance and currents. It is discovered that the significant impact from the transformer on radiated CM current is primarily from the CM voltage transformation. To characterize the CM voltage transformation, a transformer CM model extraction method is proposed. Based on the model, for radiated CM current reduction, this paper reviews transformer structures and proposes an interwinding capacitive coupling minimized coaxial shielding technique. And this paper also studies on the EMI filter design for radiated CM current reduction. The proposed modeling and reduction techniques are verified through experiments performed on a flyback converter with cables attached.

The development of the world demands on the progress of electricity operation and quality enhancement. Microgrid (MG) is a solution that manages the additional need of electricity. The MG generates electricity mostly from renewable energy sources, e.g. solar, wind and hydro. Most renewable energy sources require energy storage system as they are highly unpredictable in nature. An inverter is needed to convert that stored DC power to AC. The presence of a capacitor within the grid provides a galvanic connection that varies the common mode (CM) voltage of the inverter. Thus, the leakage current as well as the total harmonic distortion (THD) of the grid voltage and current increases. The increase of the leakage current hampers the safe operation of the MG. Now-a-days transformerless inverter is gaining more attention because of high efficiency, low cost, less weight and the capability of eliminating the high frequency component from the inverter CM voltage. In addition, a controller inclusion with a transformerless inverter can maintain a near-seamless performance. This paper proposes a blended technique of an improved H5 topology with an Integral Linear Quadratic Gaussian (ILQG) controller for grid-connected MG operations, that reduces the inverter leakage current by keeping the CM voltage constant. Thus it has the capability to reduce the THD of voltage and current. A comparative analysis has been carried out between this technique and existing topologies with different modulation techniques that helps to find out the most optimum inverter package to eliminate the leakage current and reduce the THD of grid voltage and grid current to almost zero.

This paper presents a PWM strategy for the reduction of common-mode (CM) voltage generated by variable-speed motor drives composed of two-level, three-phase active-front-end rectifier and three-phase inverter. This method theoretically provides complete elimination of the CM voltage by synchronizing all inverter commutations with rectifier commutations, so that the resulting CM voltage does not vary. The degrees of freedom of this strategy are studied and an experimental implementation is provided on a 15 kW prototype to validate the method effectiveness. Taking into account dead-time compensation, measurements in time and frequency domains show that the CM voltage is strongly reduced and that more than 15 dB reduction is achieved in a wide frequency range.

In this paper, a new method to mitigate the common-mode (CM) voltage in neutral point clamped (NPC), three-level converters is proposed by analyzing the voltage vectors and related CM voltages. Currently, only 19 of 27 available voltage vectors are used in order to reduce the amplitude of CM voltage to be one sixth of the dc-link voltage. This is achieved by adding a constraint to the universal, multilevel PWM algorithm proposed by an earlier paper. The constraint will determine the value of the zero-sequence component adding to the fundamental component, thus selecting of the required voltage vector. Simulation and experimental results validate the proposed method, and show that the CM voltage is effectively mitigated and the maximum output voltage is not affected.

The fast-switching speed and high-switching frequency of wide-bandgap (WBG) devices causes increased-level of motor common-mode (CM) overvoltages and bearing currents. The current generated by the CM overvoltage on the motor bearing and motor windings can degrade the motor bearing and reduce motor lifetime. These CM overvoltages and bearing currents are influenced by how the system is grounded. In this paper, two types of grounding methods including the dc-link capacitor midpoint grounding and the heatsink grounding have been tested and analyzed. The difference of the two grounding types is mainly that the CM path of the inverter-motor drive system is different. It has been found that the motor terminal CM voltage is distorted and the motor stator winding neutral point CM overvoltage is reduced when using the heatsink grounding type. However, the heatsink grounding type cannot completely eliminate the motor terminal CM voltage and the motor neutral point voltage. To further reduce these CM voltages, a CM filter is adopted, and its mitigation effect is compared for the two grounding types. The analysis and experimental results show that, after adding the CM filter, the motor terminal and neutral CM voltage is much reduced for the two grounding types. The above observations have been analyzed and tested with a 7.5kW motor, driven by a gallium-nitride (GaN) three-phase inverter.

Power inverters produce common mode voltage (CMV) and common mode current (CMC) which cause high-frequency electromagnetic interference (EMI) noise, leakage currents in electrical drives application and grid-connected systems, which consequently drops the efficiency of the system considerably. This CMV can be mitigated by designing suitable EMI filters and/or investigating the effects of different modulation strategies. In this paper, the effect of various modulation techniques over CMV and CMC are investigated for two-level and three-level inverters. It is observed that the modified third harmonic injection method reduced the CMV and CMC in the system by 60%. This modified pulse width modulation (PWM) technique is employed along with EMI chokes which results in reduced distortion of the system.

In response to the problem of large common-mode (CM) voltage magnitude and high-frequency variation under traditional modulation strategies for three-level three-phase four-leg (3L3P4L) inverters, this paper proposes a modulation strategy based on carrier combination and modulation wave splitting. Firstly, this paper establishes a CM model for the 3L3P4L inverter and derives mathematical expressions of the system’s CM voltage in terms of switch states and system parameters. Building upon this foundation, this paper explains the reasons for high-frequency variations in CM voltage under traditional schemes and provides a detailed introduction to the optimization process and principles of the proposed strategy. Finally, a simulation model is developed by using Matlab/Simulink software. Through comparison with traditional approaches, the effectiveness of the proposed strategy is validated.

For many years, numerous high-power applications have been found to cause mechanical damage to the rotating shaft. This research demonstrates that common mode voltage (CMV), which causes common mode leakage currents to flow into the motor bearings and spinning shaft, is one of the main contributing factors to mechanical shaft failures. By applying a carrier based Sinusoidal pulse width modulation (PWM) approach in conjunction with a proportional integral or PI controller technique in the feedback, the study aims to reduce leakage currents by mitigating the common mode voltages induced by the inverter output asymmetry. To accomplish the stated goal, an asymmetrical inverter architecture with a 21-level is employed. The literature emphasized the trade-off between total harmonic distortion (THD) and CMV, which is attempted to be kept well below IEEE's allowable bounds. The system stability is verified and proved by plotting the bode diagram.

Variable speed drive (VSD) systems including diode rectifiers and two-level voltage source inverters (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{2 L}$</tex>-VSI), connected to the induction motor through long cables, are common in modern industrial applications. However, the inverter's switching behaviors generate unwanted side effects such as common-mode (CM) voltage due to fast switching transients, while the long cable may cause over-voltage at the motor terminal. CM voltage generates CM current through parasitic coupling paths in the VSD system, causing electromagnetic interference (EMI). Overvoltage increases stator winding stress, leading to premature motor failures such as winding-to-ground and turn-to-turn insulation faults. These stator insulation faults create impedance imbalance, and inturn affects the propagation of the CM current. This paper evaluates the stator winding-to-ground insulation fault by analyzing CM current at different measurement points using a laboratory VSD reference system. A frequency-domain CM simulation model is proposed to precisely validate this fault and predict the CM current up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 0 ~ M H z}$</tex> at the motor side.