Flux Rate Controlled Harmonic Filtering of Integrated Magnetic-Based Isolated-Inverter Topologies and Dynamically Variable Inductors

Abstract

An intermediate filtering stage is mandatory to deliver smooth sinusoidal output voltage across the load. Several multilevel and modular multilevel-based inverter topologies are reported in the literature to reduce the filtering requirements. The medium to high-voltage and high-power applications requires isolated systems to maintain safety and minimize transmission of electromagnetic interference. A load-side power electronics stage causes EMI/EMC transmission to the output side. It also increases dependency on the rating of the power semiconductor devices for voltage and power scaling purposes. In most cases, a separate line frequency transformer is placed between the load and the inverter circuit. This work presents a novel DC-to-AC converter topology based on the magnetic flux rate switching principle. The proposed converter can operate in a single or dual output mode, selected using the low-voltage, high-frequency, and bi-directional control switches. The proposed inverter provides inherent isolation to the load side ports and eliminates dependency on the power semiconductor device ratings. A hardware prototype of the proposed isolated converter is built using a commercially available three-legged EE-type ferrite core. The performance of the proposed converter is verified by correlating the simulated and experimental results. A flux-rate controlled harmonic filtering of an integrated magnetic-based isolated single-phase inverter topology is presented in this work. The proposed system comprises a high-voltage, high-power main inverter, a low-voltage, low-power control inverter, and a three-limbed magnetic circuit. The main inverter performs a square wave mode of operation throughout the complete operating range at the fundamental operating frequency to set up the primary flux in the magnetic circuit. This port delivers most of the active power demand, contributing minimal switching losses to the system. The fundamental subtracted pulse width modulation (FSPWM) drives the control inverter to perform the flux-rate control operation in the magnetic circuit. This port delivers the harmonic power demand of the system during Case I operation and extends the modulation range by contributing active power during Case II operation. The main and control inverters are integrated with the magnetic circuit, which performs the flux rate control operation to deliver a smooth sinusoidal waveform to the output port. The load is connected directly to a galvanically isolated port of the magnetic circuit. In the case of modified integrated magnetic circuit topology, the load is connected through a proper arrangement to reduce leakage flux and improve the coupling. As mentioned, the arrangements free the output port from power electronics components and filtering requirements. These attributes of the proposed scheme enable its usage in medium- to high-voltage applications and make it less dependent on the semiconductor device ratings. The proposed inverter scheme uses the widely used bond-graph modeling technique to observe the flux, current, and voltage waveforms at different ports. The operation of the proposed inverter scheme is validated using analytical calculations and simulated results, and finally, experimentation is performed using a lab-built prototype. This study compares the proposed scheme with the conventional isolated single-phase inverter system regarding power loss, area product, and the number of turns. An integrated magnetic-based isolated three-phase inverter scheme is proposed in this chapter. Here, the harmonic components are filtered using the flux-rate control technique. The system incorporated an integrated magnetic interface and the main and control inverter. The high-power main inverter performed a square wave mode of operation at the fundamental frequency throughout the operating range, causing lower switching losses. The control inverter performed PWM (pulse width modulation) operation and set up the necessary flux rate in the magnetic interface, operating at low power. Based on the control inverter's harmonic and active power sharing, the operation is divided into two cases, Case I and Case II. During Case I, the control inverter handles the harmonic power demand of the system, whereas Case II deals with the harmonic and active power, causing the voltage boost operation. The integrated magnetic circuit comprised E-I-E lamination types and contained different windings to interface the main inverter, control inverter, and loads. It causes inherent isolation to the output port and frees the load side from power electronics components and filter interface. The bond graph modeling technique formulated the dynamic model of the system to conduct simulation studies for understanding the performance. A lab-built prototype is used to validate the working of the proposed scheme. The experimental studies comprised the inverter's working for different modulation, dynamic operations, and an induction machine's open loop v/f operation. The study included the proposed inverter's power loss and size comparison with the conventional 2-level and 3-level NPC (neutral point clamp) inverters. The variable inductors are widely used in LED driving circuits, auto-tuning tank circuits, and various other applications. The existing saturable core-based variable inductors require an external biasing circuit to vary the inductance value. The biasing circuits use external power sources. Several modified air gap-based and composite material-based variable inductor techniques have been reported in the literature. This work proposes a distinctive approach to sweep the inductance value smoothly using a flux rate-controlled variable inductor. The proposed variable inductor comprises a commercially available three-limb core, two windings, and a control switch, which gives a low-cost solution. The control switch is realized using a bidirectional switch or a variable resistance. The duty ratio or the variable resistance value controls the magnetic circuit's flux rate, causing the inductance value's swing. The proposed adjustable inductor is modeled using the widely used bond graph technique to conduct the simulation study. The working of the variable inductor is validated through several applications. The experimentation for the DC load impedance variation application is carried out using a lab-built prototype considering both flux rate control methods. The inductance values obtained from the experiments closely follow the simulation study's calculated values. The AC load variation experimentation is performed at 500 Hz to observe the change in phase angle due to the inductance variation. In the following applications, the variable inductor is incorporated in a conventional and synchronous buck converter to enhance the positive inductor current region. The working of the proposed concept is validated using simulation and experimental results. A traditional PI (Proportional Integral) controller performs the close loop control of the SIC-based synchronous Buck and Boost converters to validate the system stability during inductance variation.