Grid filter design for a multi-megawatt medium-voltage voltage source inverter

This paper describes the design procedure and performance of an LCL grid filter for a medium-voltage neutral point clamped (NPC) converter to be adopted for a multimegawatt wind turbine. The unique filter design challenges in this application are driven by a combination of the medium voltage converter, a limited allowable switching frequency, component physical size and weight concerns, and the stringent limits for allowable injected current harmonics. Traditional design procedures of grid filters for lower power and higher switching frequency converters are not valid for a multi-megawatt filter connecting a medium-voltage converter switching at low frequency to the electric grid. This paper demonstrates a frequency domain model based approach to determine the optimum filter parameters that provide the necessary performance under all operating conditions given the necessary design constraints. To achieve this goal, new concepts such as virtual harmonic content and virtual filter losses are introduced. Moreover, a new passive damping technique that provides the necessary damping with low losses and very little degradation of the high-frequency attenuation is proposed.Postprint (author’s final draft

Power quality improvement utilizing photovoltaic generation connected to a weak grid

Microgrid research and development in the past decades have been one of the most popular topics. Similarly, the photovoltaic generation has been surging among renewable generation in the past few years, thanks to the availability, affordability, technology maturity of the PV panels and the PV inverter in the general market. Unfortunately, quite often, the PV installations are connected to weak grids and may have been considered as the culprit of poor power quality affecting other loads in particular sensitive loads connected to the same point of common coupling (PCC). This paper is intended to demystify the renewable generation, and turns the negative perception into positive revelation of the superiority of PV generation to the power quality improvement in a microgrid system. The main objective of this work is to develop a control method for the PV inverter so that the power quality at the PCC will be improved under various disturbances. The method is to control the reactive current based on utilizing the grid current to counteract the negative impact of the disturbances. The proposed control method is verified in PSIM platform. Promising results have been obtaine

Bidirectional Multilevel Converter for Grid-Tie Renewable Energy and Storage with Reduced Leakage Current

This thesis discusses a transformerless multilevel converter (MLC) applied to a domestic level renewable energy system consisting of PV panels and EV batteries in their 2nd life applications. MLCs enable the use of conventional switching devices due to reduced voltage stress. Being able to produce a multilevel output voltage waveform, MLCs require less filtering and therefore may produce better quality waveform when compared to a standard 2-level voltage source converter (VSC). In this study, various modulation techniques for MLCs are implemented and the performance of the converter analysed regarding regulations and standards. The system is designed to have two-stage power conversion, including a DC-DC boost converter for adjusting each stage battery voltage, and maximum power point operation of the PV panels in each module. This provides a stable input voltage for the DC-AC converter stage. The cascaded H-bridge converter (CHB) is selected for the DC-AC conversion due to its isolated DC source requirement. This topology enables the separation of the total DC link voltage into different modules, increasing the accessibility of EV batteries in their 2nd life application. The base system is designed to be coupled without a transformer to the single-phase UK utility grid. A systematic approach is adapted for examining the MLC system. The design procedure starts with system parameter definition and component selection. This is then validated using simulation analysis and hardware implementation to demonstrate the practicability of the system for the planned application. The control algorithm is implemented in a National Instruments (NI) CompactRIO FPGA that can transform graphical programming into VHDL code. To accelerate the implementation and optimisation process, a co-simulation environment is used between NI LabVIEW and NI Multisim software. This ensures the optimisation of control code before compilation and enables testing without having analogue circuitry. Converters without galvanic isolation may exhibit ground leakage currents when coupled with grounded PV panels. This thesis analyses the common-mode and differential-mode voltages that CHB modules generate, and their effect on ground leakage current. The mathematical analysis suggests that leakage current may be supressed solely on changing the modulation method in a CHB converter. A novel leakage reduction pulse width modulation (LRPWM) technique is proposed, which successfully diminishes the ground leakage current to within the limit allowed by VDE-0126-1-1 (withdrawn, accessed in 2018) or IEC 62109-2 standard. The experimental results show that LRPWM has superior performance when compared to conventional MLC modulation technique

Advanced Inverter control for mixed source microgrids

This thesis focuses on investigating virtual oscillator control (VOC) and applying it to mixed source microgrids to address several stability issues. A detailed comparison between VOC and droop control in a three-phase system is presented in terms of transient responses of a single inverter under small load disturbances and the synchronization speed in multiple paralleled inverters under various inverter terminal voltage amplitude and frequency regulation settings. In the single-inverter microgrid, it is demonstrated in both simulation and experiment that the two control models produce similar transient responses in the output voltage and current amplitudes. However, VOC has a faster instantaneous frequency transient response whilst still maintaining the terminal voltage amplitude transient response of the droop controller. In microgrids with multiple inverters, the synchronization speed of the VOC is faster than that of the droop control when the terminal voltage’s frequency regulation range is allowed to be wide. The conclusion is verified with different types of loads. A virtual inertia design method for the VOC inverter with a mixed source microgrid is presented to improve the frequency stability issues of the system. The per unit inertia constant of a VOC inverter is derived when coupled with a synchronous generator in an islanded microgrid. The control parameters of the virtual inertia are designed via small-signal analysis. A dual second order generalized integrator - frequency locked loop (DSOGI-FLL) is adopted for digital implementation of proposed virtual inertia based VOC. With the use of virtual inertia block, the frequency nadir is improved by 22% and rate of change of frequency is improved by 29% compared with the unmodified VOC inverter during the transient period induced by load disturbances. Simulation and experimental results verify the enhanced transient response of system frequency. A voltage and current dual-loop control structure is added to the VOC inverter to solve the voltage drop issues at the inverter terminals caused by the inverter dead-time effects, non-ideal semiconductor and LCL filter. A complete small-signal model for a multiple-inverters microgrid with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations. Finally, a method to limit current in the VOC inverter under large disturbances in a mixed source microgrid is proposed. During a large load change in the islanded microgrid, the inverter based sources may get temporarily overloaded until other generations with sufficient power margin take the remaining load burden. The original VOC inverter lacks the ability to constrain the current within limits during the transient period. The dual-loop structure proposed in this thesis can limit the transient current with the use of virtual impedance. Such virtual impedance is presented by the desired maximum current magnitude and virtual voltage drop. Compared with a recently proposed fault ride through VOC inverter, the proposed virtual impedance based current limitation method can effectively constrain the inverter current within the pre-set value under large disturbances, which augments the range of application of VOC and enhances its robustness