This paper presents the topology and modulation technique of a three-phase three-level modified Z-source neutral-point-clamped (MZS-NPC) inverter, which combines a modified Z-source impedance network and a three-phase three-level NPC inverter. The boost factor of the proposed MZS-NPC inverter is twice as high as the three existing representative topologies combing an impedance network with a three-level NPC inverter. A modulation scheme for the proposed topology, based on a maximum boost control method, is designed to achieve the maximum voltage gain with simple implementation and to balance the dc-link neutral-point voltage. A closed-loop control of the ac load voltage in the fuel-cell or photovoltaic applications based on the proposed inverter is realized, in order to supply a desired voltage to the critical load in islanding mode of a microgrid. The boosting ability and operation validity of the proposed topology and modulation technique are demonstrated with simulation and experimental results.

Topology and Modulation Scheme for Three-Phase Three-Level Modified Z-Source Neutral-Point-Clamped Inverter

This paper does some theoretical analysis and simulation studies on the most common topologies of Z-source inverter. As we all know, the traditional Z-source inverter has some problems, such as the voltage of the capacitor which belongs to this topology is higher and there is a startup shock, which caused researchers proposed a variety of improved Z-source topologies to solve these problems. Some of these topologies can increase boost capacity, and some can reduce the capacitor voltage. However, these topologies can improve only part of the problems of the traditional Z-source inverter, which leads to that the users are hard to choose which topologies. Faced with this situation, eleven Z-source topologies are analyzed in terms of boost capacity, inductance start-up current, capacitance stress and economy in this paper. Finally, two kinds of promising topologies were gotten.

A comparative study of Z-source inverter and enhanced topologies

The maximum boost control (MBC) technique of voltage boosting offers the highest output voltage gain among other boost control techniques of Z -source inverter (ZSI). But, this technique suffers from the major drawback of sixth fundamental frequency (6thF1) ripple components of the impedance network inductor current. Due to this, the size/cost of the impedance network increases, which limits the practical applications of MBC. This article proposes an improved version of MBC via appropriate modification in the conventional space vector pulse width modulation of three-level ZSI (3L-ZSI). The improved MBC eliminates the problem of the 6thF1 impedance network inductor current ripple of 3L-ZSI without affecting its input/output gain and switching stress across power switches as compared to the existing MBC. Also, two new improved reduced common-mode voltage (RCMV) switching patterns have been proposed. As compared to the existing ones, the proposed RCMV switching patterns of 3L-ZSI offers reduced size of the impedance network while limiting the common-mode voltage magnitude to one-sixth of the available dc-link voltage. The above-mentioned findings have been successfully validated using theoretical analysis, simulation, and experimental results.

Improved Maximum Boost Control and Reduced Common-Mode Voltage Switching Patterns of Three-Level Z- Source Inverter

An interesting feature of a Z-source inverter is to provide buck–boost ability in a single stage. However, this concept is most popular in the conventional inverters. In a few multilevel inverters (MLIs), Z-source is utilized to have higher gain along with retaining the advantages of an MLI such as better electromagnetic interference compatibility, low harmonic distortion, etc. However, the potential of the Z-source concept is not fully exploited in MLIs. To widen the feasibility of the Z-source concept in the MLI, a modular MLI is considered. In this article, a generalized switched inductor cell is selected as a Z-source network for integration, however, other Z-source networks such as quasi-boost network, quasi-switched boost network, two winding coupled inductor network, three winding coupled inductor network, etc., may be used in place of a switched inductor cell. The quantitative and qualitative analysis is done for the proposed converter in both the continuous current mode and discontinuous current mode. The analysis shows that a higher voltage gain can be achieved in the discontinuous current mode as compared to the continuous current mode. To control the proposed converter, two new modulation techniques are proposed, i.e., full shoot through and upper shoot through/lower shoot through. Finally, the proposed converter is validated experimentally in both the modes and for different modulation techniques.

A Generalized Switched Inductor Cell Modular Multilevel Inverter

The Photo-voltaic (PV) tied Z-source Neutral-point clamped multilevel inverter (Z-NPC-MLI) is used in solar grid connected applications due to its single stage conversion and better performance. Though the Z source inverters adaptation is accepted in grid connected technology, the need for suitable controller and PWM scheme are necessary to meet out the performance such as shoot through switching, neutral point balancing, and harmonic reduction. The space vector pulse width modulation (SVPWM) strategy is a prominent modulation technique for Z-source NPC-MLIs due to the flexibility to select the appropriate voltage vector. Previous publications have shown the control of a Z-source MLI using the SVPWM with and without modification of shoot through switching. However, the current controller (CC) based SVPWM is not matured, which is the most essential consideration for the grid connected inverter to provide neutral point balancing, shoot through control for low harmonic distortion and a high quality current. With all these aims, this paper presents a PV tied Z-NPC-MLI grid connected system with a unique hysteresis current control SVPWM (HSVM) strategy with neutral point (NP) balancing control and direct current control in the inverter input side. Also, the proposed HSVM is assuring the grid connection with high quality voltage and current waveforms. This CC based SVPWM for Z-NPC MLI has been validated through simulation and FPGA based experimental investigations. The results are confirmed the feasibility and reliability of the proposed HSVM for the PV tie grid connected Z- Source NPC-MLI.

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A Hysteresis Space Vector PWM for PV Tied Z-Source NPC-MLI With DC-Link Neutral Point Balancing

A new three-level neutral-point-clamped quasi-Z-source inverter topology is proposed. The topology can effectively reduce the Z-source network capacitor voltage stress and inhibit the start inrush current, thus low-voltage capacitors can be selected to reduce the volume and cost of the inverter. In addition, the changed structure will not affect the boost ability and all existing modulation methods can be used directly. The proposed topology is validated by simulations.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/el.2016.3930

Three-level neutral-point-clamped quasi-Z-source inverter with reduced Z-source capacitor voltage

The invention relates to a virtual synchronous generator control method based on self-adapting moment inertia. The output of a grid-connected inverter in distributed generation is connected with an AC bus of a filter capacitor Cf which is in parallel connection with the grid-connected inverter through a filter inductor Lf; sampling is performed to obtain the output power, the voltage and the current of an output interface of an inverter; the output power, the voltage and the current are transmitted to a VSG (virtual synchronous generator) algorithm; the calculated terminal voltage of a VSG stator is sent to be controlled by a voltage external loop; the voltage external loop adopts a PI controller to stably load the voltage; the current of the filter capacitor is sent to be controlled by a voltage internal loop; the voltage internal loop adopts a PI controller to increase the response speed; a control signal is outputted by a voltage-current dual-closed loop to the grid-connected inverter; the moment inertia in the virtual synchronous generator in the control method can be changed in a self-adapted manner according to the change capacity of the system frequency. Through the adoption of the virtual synchronous generator control method disclosed by the invention, the transient state process of the system is reduced; a great deal of harmonic waves in the system is offset; the stability of frequency and the stability of voltage can be improved; the adaptability and the power supply quality of a micro-electrical network are improved; the problem that in a conventional method, the output frequency is reduced, is solved.

Virtual synchronous generator control method based on self-adapting moment inertia

The invention relates to a three-dimensional fault line selection method based on random resonance and a transient current signal. The method comprises the following steps of: (1) after a power distribution network is faulted, recording the zero-sequence current iZn(t) and the fault phase current iXn(t) of each branch line fault, and computing transient zero-sequence current izn(t) and transient fault phase current ixn(t); (2) performing variable-scale bistable treatment on the transient zero-sequence current izn(t) and transient fault phase current ixn(t); (3) computing a cross correlation coefficient matrix M and an integrated correlation coefficient Mn among the transient characteristic zero-sequence current iczn(t) of the branch lines and selecting the line ln with the minimum Mn; (4) performing seven-layer db10 wavelet packet decomposition on the transient characteristic phase current icxn(t) of each branch line and computing simplified energy E*sn and simplified gravity center frequency f*gn; and (5) forming a three-dimensional coordinate system by using the simplified gravity center frequency f*gn as a horizontal ordinate, the simplified energy E*sn as a vertical coordinate, and a polarity parameter value as a longitudinal coordinate. Compared with a method in the prior art, the method is high in reliability and is not liable to suffer from external interference.

Three-dimensional fault line selection method based on random resonance and transient current signal

The invention relates to a fault line detection method for a DG-containing power distribution network based on a transient zero sequence current. The method comprises the following steps: (1) calculating an optimized bistable system potential function and the computational step of a fourth-order Runge-Kutta algorithm; (2) after the failure of the DG-containing power distribution network, obtaining the transient zero sequence current of each branch line in the 1.25 power frequency cycle after the failure; (3) introducing the transient zero sequence current of each branch line into the optimized bistable system potential function and solving, and obtaining the characteristic current of each branch line; (4) calculating the characteristic angle theta of each branch line according to the characteristic current, n being 1, 2, ..., s, s being the total number of the branch lines; (5) making i = 1; (6) judging whether theta is greater than 90 DEG, if so, determining the i-th branch line to be the fault line, otherwise, executing the step (7); and (7) judging whether i equals s, if so, determining that a bus is faulty, otherwise, making i = i + 1, and returning to the step (6). Compared with the prior art, the method is higher in correct rate of fault line detection.

Fault line detection method for DG-containing power distribution network based on transient zero sequence current

The invention relates to a grid-side converter control method of a dual-feed motor under voltage unbalance of a power grid. The method include steps of building positive and negative order models according to current dual-loop control of an inner loop of a grid-side converter (GSC) under the voltage unbalance of the power grid, and calculating the grid-side current reference value under different control goals; based on a Hamilton with dissipation (PCHD) model with controlled port, applying internet and damping distribution passivity control (IDA-PBC) method, designing a GSC passivity controller; specific to parameter perturbation, disturbance and other uncertain factors of the current loop in the controller, adding a state observer based on an inner model control (IMC) on the basis of the passivity control; optimizing the observer parameter by a pole assignment method, realizing the current compensation control so that the current steady-state wave motion is small. Compared with the prior art, the grid-side converter control method has the advantages of advanced theory, fast dynamic response speed, strong robustness, and so on.

Zhang Yu

Papers

Three-level neutral-point-clamped quasi-Z-source inverter with reduced Z-source capacitor voltage

Virtual synchronous generator control method based on self-adapting moment inertia

Three-dimensional fault line selection method based on random resonance and transient current signal

Fault line detection method for DG-containing power distribution network based on transient zero sequence current

Grid-side converter control method of dual-feed motor under voltage unbalance of power grid