Performance Analysis and Digital Implementation of 2-level and 3-level (NPC) Converter based Shunt Active Power Filter for Different Control Schemes

Abstract

Extensive use of power electronics based converters in industrial as well as domestic appli-cations has resulted into degradation of power quality. This is because, these power electronics based converters serve as non-linear loads thereby introducing current harmonics into the system. From the viewpoint of a utility supply system, presence of current harmonics in the system cause operational and life expectancy problems for other equipments connected to the same system but owned or operated by some other party. Therefore, it becomes essential to ask consumers to rectify harmonics injected into mains from their side by using devices such as filters. Previously, Passive Power Filters (PPF) were a popular solution for mitigation of current harmonics but these suffered from disadvantages such as series or parallel resonance with the grid impedance, bulky size and incapability to compensate for different harmonic orders present in the system. As a result, Active Power Filters (APF) were proposed by researchers as a solution to above-mentioned disadvantages of PPF. Active Power Filters can compensate current and voltage harmonics, compensate reactive power, improve power factor, and improve voltage balance in three phase systems. Because of handling these many tasks, Active Power Filters are often called as Active Power Line Conditioners. Under this dissertation work, the current harmonics present in the system are extracted by Instantaneous Reactive Power Theory. SAPF operates as a controlled current source that injects harmonic components generated by the load but phase shifted by 180o. As a result, harmonic components present in load current get cancelled, and the source current becomes sinusoidal and in-phase with the respective phase-to-neutral voltage. The power electronics converter used in SAPF is either a 2-level conventional inverter or a multi-level inverter depending upon the voltage and power rating of the application. For medium voltage high power applications, working with multi-level inverters is beneficial. The advantageous and topologies of multi-level inverters are explained in detail in the proposed work. This project involves study of two-level and three-level converters based SAPF for different control schemes. The control scheme employed for two-level SAPF is hysteresis current control. Simulation results under steady-state operation and under dynamic load variation are presented for the same. Two different control techniques are employed for three-level neutral point clamped (NPC) converter based SAPF. The first technique is a novel multiband hysteresis modulation control scheme. Detailed explanation of the control scheme along with simulation results under steady-state as well as under sudden load change are presented. The second technique employed is multi-carrier PWM control scheme. There are different multi-carrier PWM control schemes mentioned in literature such as- phase shifted multi-carrier PWM modulation (PS-PWM), in-phase disposition multi-carrier PWM modulation (PD-PWM), alternative phase opposite disposition multi- carrier PWM modulation (APOD-PWM), phase opposite disposition multi-carrier PWM modulation (POD-PWM). Explanation of these control schemes is given in the report. In this project work, simulation results for three-level NPC based SAPF using PD-PWM scheme and APOD-PWM scheme are presented. Results of hardware implementation of two-level SAPF using hysteresis current control scheme are presented. the micro-controller used for digitally implementing IRPT algorithm and hysteresis current control logic is STM32F407VGT6 ARM Cortex-M4. The STM32F4 ARM Cortex-M4 processor is the latest generation of ARM processors for embedded systems. It was developed to provide a low-cost platform that meets the needs of micro-controller (MCU) implementation, with a reduced pin count and low-power consumption, while delivering outstanding computational performance and an advanced response to interrupts.