Parallel Rectifiers in Adjustable Speed Drive Systems: Analysis, Modelling and Control

Abstract

Although power electronic converters are widely spread in industrial, commercial, and residential applications, they may be a major harmonics source due to their switching behaviour. Utilising power electronic converters introduces new challenges such as the generation of harmonic distortions, which can negatively impact the grid's power quality and harm other loads connected to the same grid. One of the major consumers in the global electrical market are motor drives systems; improving these systems contributes to a large savings of electrical energy and a healthier grid. Power electronic converters can control the speed of motors, depending on the demand, which increases the efficiency of motor drive systems known as Adjustable-Speed Drive (ASD) or Variable-Speed Drive (VSD). In industrial applications, ASD or VSD generally consists of the AC/DC rectifier and DC/AC inverter stages. A conventional 3-phase Diode rectifier is utilised as the first stage (AC/DC rectifier) due to its reliability and low cost. This rectifier's drawbacks are the limitation of its current conduction period, the generation of high harmonic distortions, and the load's dependence on performance. To overcome these issues, parallel rectifiers are proposed. In this thesis, two solutions have been proposed to reduce the current harmonics in industrial or commercial power networks. The first solution is employing a hybrid active harmonic filter (HAHF) at the point of common coupling (PPC). The aim of the HAHF is to improve the grid’s power quality at a relatively low cost using a new shifted-pulse current modulation method. The HAHF utilises a Particle Swarm Optimisation (PSO) algorithm to optimise the modulation parameters. In other words, the HAHF solves an optimisation problem by reading the harmonics profile at the PCC and generating an optimised opposite harmonic that minimises the overall current harmonics. The second solution is employing a parallel rectifier topology as a first stage for the ASD system to minimise the harmonics generated by motor drive systems. The parallel rectifiers system with a shared DC link employed in this thesis is known as Modular Multi-parallel Rectifiers (MMR). The MMR system consists of a diode rectifier and parallel Silicon Controlled Rectifiers (SCRs); each rectifier is connected to an electronic inductor. The utilisation of parallel SCRs increases the current conduction period while the electronic inductors smooth the current and remove the load performance's dependency. Hence, the power quality of the grid is improved since the generated harmonic distortion is significantly reduced. Although parallel rectifiers' employment increases power quality, reliability, flexibility, power rating, control dynamics, reduced cost, filtering effort, current-voltage ripple, and current stress, it introduces new challenges such as circulating and unbalanced current, which may cause overloading and damage components. Deep insight into the MMR system is needed to comprehend the system’s capability and limitations. In this research, precise mathematical representations of the MMR system with n number of parallel units are proposed. The MMR is analysed for both Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). In addition, the CCM and DCM boundary of the MMR is identified mathematically. The impact of parasitic elements and the firing angle on the system’s performance is investigated. Moreover, the impact of different ASD scenarios on the mode of operation. A proper current sharing controller is essential prior to the implementation of the MMR in industrial applications. In this thesis, a mathematical control-oriented model for the MMR comprising n parallel units is proposed through a generalised small-signal AC model of the parallel configuration. The control-oriented model is used to develop coordinated control for the overall system to ensure that the rectifiers' currents are balanced equally. In addition, this thesis proposes a multivariable Model Predictive Control (MPC) for the MMR. The proposed systems and methods are modelled in MATLAB/Simulink, and a 1 kW experimental setup is constructed to validate the proposed Hybrid Active Harmonic Filter (HAHF), mathematical representations, and cascaded current-sharing controller. The experimental validations demonstrate the effectiveness of the harmonic mitigation method, the accuracy of the mathematical representations, and the stability of the current-sharing controller despite abrupt disturbances.