Showing papers by "Brendan McGrath published in 2013"
TL;DR: In this article, a linearized dynamic model of a DAB that accurately identifies its transient response to both a reference voltage change and an output load-current change is presented, and a feed-forward compensation strategy is presented to improve the DAB's transient response.
Abstract: An essential requirement for a high-performance dual active bridge (DAB) dc-dc converter is to rapidly and accurately maintain its DC output voltage under all operating conditions This paper uses a novel harmonic modeling strategy to create a linearized dynamic model of a DAB that accurately identifies its transient response to both a reference voltage change and an output load-current change Using this model, a feedforward compensation strategy is presented that significantly improves the DAB's transient response to an output load change The transient performance is then further enhanced by analytically compensating for the nonlinear dead-time distortion that is caused by the converter switching processes The resultant control system achieves rapid and precise output voltage regulation for both reference voltage and output load changes The theoretical analysis is confirmed by both matching simulation and experimental investigations
174 citations
TL;DR: In this paper, an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter is presented, which takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hystresis bands so as to maintain constant phaseleg switching frequencies.
Abstract: This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal.
87 citations
TL;DR: In this article, an improved current regulation strategy using state feedback concepts to deterministically compensate for pulsewidth modulation (PWM) transport delay so that the controller gains can be significantly increased.
Abstract: AC current regulation for low-pulse-ratio inverters is particularly challenging because the controller bandwidth is often comparable to the target fundamental frequency and this constrains the transient response. This paper presents an improved current regulation strategy to address this issue, using state feedback concepts to deterministically compensate for pulsewidth modulation (PWM) transport delay so that the controller gains can be significantly increased. Controller design principles are presented using discrete linear-quadratic-regulator theory, and an elegant strategy is applied to make the system more robust to PWM saturation. Experimental results for both 50-Hz and 400-Hz systems confirm the theoretical concepts and demonstrate the superior transient performance of the proposed strategy.
50 citations
TL;DR: In this paper, a single-phase flying-capacitor (FC) rectifier is proposed to rectify higher output voltages with lower voltage rated, more efficient, devices.
Abstract: For unity power factor applications such as grid-connected rectifiers, semi-bridge converters offer significant advantages over their full-bridge counterparts because of their reduced active switch count and shoot-through-free phase leg structure. However, semi-bridge rectifiers have intrinsic operating limits that require a tradeoff between current distortion and switching ripple. This paper presents a new single-phase semi-bridge flying-capacitor (FC) rectifier that significantly improves this tradeoff, with effective doubling in switching frequency because of the multilevel topology, and the capability to rectify higher output voltages with lower voltage rated, more efficient, devices. For rectifier applications, the new topology offers a better balance between cost and performance than either a diode rectifier followed by a single-phase-leg three-level boost power factor corrector (PFC) or a full-bridge single-phase five-level FC rectifier. Matching simulation and experimental results are presented to fully validate the new converter structure.
44 citations
28 Oct 2013
TL;DR: In this article, a dual-loop current regulation strategy for coupled-inductor multilevel converters is proposed to enforce continuous conduction operation of the inverter phase legs by dynamically varying the DC bias current to track the AC load current.
Abstract: This paper presents a dual-loop current regulation strategy for coupled-inductor multilevel converters. The basis of the strategy is to recognise that the current in the coupled-inductor can be decoupled into independent DC bias and AC load currents, which can then be regulated using separate feedback processes. This allows the system to enforce continuous conduction operation of the inverter phase legs by dynamically varying the DC bias current to track the AC load current. Design principles are presented to account for the different common- and differential-mode load dynamics, and to maximise the closed-loop bandwidths of both feedback loops. This ensures that the DC bias current can follow rapid load transient events. Matching simulation and experimental results obtained for a five-level flying-capacitor coupled-inductor inverter are presented to validate the proposed control strategy.
15 citations
03 Jun 2013
TL;DR: A fundamental look at the mechanisms underlying the stability of single-edge pulse-width modulated current regulator loops and how computational and other delays in the loop can be compensated for during the design process.
Abstract: This paper takes a fundamental look at the mechanisms underlying the stability of single-edge pulse-width modulated current regulator loops. The analysis applies to continuous-time or highly oversampled digital current regulators. It is shown that pulse-width modulation is essentially a sampling process. Associated with this sampling process is a small-signal z-domain model of the pulse-width modulator which is used to accurately predict the stability of the control loop. A method for designing the current regulator directly in this z-domain is presented. A technique for delay-less ripple feedback compensation is presented. Finally it is shown how computational and other delays in the loop can be compensated for during the design process.
10 citations
01 Sep 2013
TL;DR: In this paper, the effect of differences in droop controller response times on the ability of a microgrid to balance its commanded DG generations with its required load profile is explored, and it is shown that unequal droop response times can substantially degrade the power sharing dynamics and the modularity of the microgrid.
Abstract: One increasingly popular approach to integrate distributed generation (DG) energy sources into an electrical grid system, is to assemble them into microgrids which then connect to the main utility grid at a single or multiple points of connection. A microgrid can also operate as a standalone islanded network which is physically disconnected from the grid. During island mode the generated power of each DG unit must be carefully controlled to ensure reliable power distribution and modular operation. Droop control is commonly used for this purpose, commanding individual voltage magnitude and frequency for each DG unit to achieve balanced active and reactive power sharing. A droop controller involves several stages of processing, particularly output power calculation and updating the voltage command setpoint. These stages contribute to a time delay defined in this paper as the droop response time (DR time), which can vary between multiple DG units because of different designs and controller implementations. This paper explores the effect of differences in droop controller response times on the ability of a microgrid to balance its commanded DG generations with its required load profile. It is shown that unequal droop response times can substantially degrade the power sharing dynamics and the modularity of the microgrid.
8 citations
01 Sep 2013
TL;DR: In this article, the authors present a detailed power inverter model developed within the DIgSILENT power network simulation package, which is sufficiently detailed to represent the inverter operation up to the PWM switching frequency, but is still computationally viable for use with larger scale system studies.
Abstract: With increasing levels of small-scale distributed generation (DG) systems connecting into the electrical grid, there is a growing awareness of potentially adverse interactions between these systems and the grid because of their differing responses to steady state and transient network events. To study these issues, it is important to use simulation models of both the grid network and the DG inverter systems that are sufficiently detailed to realistically represent their real world physical system behaviours. However, inverter systems are usually simulated in detail using specialist packages, which are not particularly suited to modelling larger scale power systems. Similarly, power system simulation packages typically represent inverter systems using simpler averaged models, which do not adequately reflect the inverter's real dynamic response to transient events. This paper addresses this issue, by presenting a detailed power inverter model developed within the DIgSILENT power network simulation package, which is sufficiently detailed to represent the inverter operation up to the PWM switching frequency, but is still computationally viable for use with larger scale system studies. The DIgSILENT model has been validated against a detailed PSIM inverter model, which has been verified in previous work against a physical experimental system.
7 citations
03 Jun 2013
TL;DR: In this paper, the authors present an analytical explanation as to why low per-unit filters, specifically LCL filters, generate more baseband harmonic distortion than a conventional L filter.
Abstract: The filter and controller design for a grid connected inverter is a challenging balance between size, cost and complexity with various standard harmonic requirements. Unfortunately, while LCL filters provide exceptional PWM frequency filtering they tend to generate significant baseband harmonics. This paper presents an analytical explanation as to why low per-unit filters, specifically LCL filters, generate more baseband harmonic distortion than a conventional L filter. Firstly it identifies that both grid distortion and baseband distortion created by deadtime and device volt drop contribute similar magnitudes of harmonic voltage, and hence potentially reinforce one another to drive harmonic currents into the grid. Next it is shown how the lower total inductance of an LCL filter significantly reduces its baseband impedance, generating more harmonic currents for a given level of voltage distortion. This outcome is further exacerbated by a reduced allowable controller bandwidth due to the LCL resonance. Consequently the paper proposes that feedback harmonic compensators are the best practical option for mitigation of both grid and converter baseband harmonic current distortion. The analysis is verified by simulation and experimental results.
6 citations
03 Jun 2013
TL;DR: In this article, a sensorless hysteresis current regulator for a single phase grid-connected inverter with a constant switching frequency is presented. But the regulator is not suitable for many applications due to its variable switching frequency and inherent overcurrent protection.
Abstract: While hysteresis current control offers the benefits of a fast dynamic response and inherent overcurrent protection, its variable switching frequency and inherent two-level switching response make it unsuitable for many applications. This paper presents a new approach for three-level hysteresis control of a single phase grid-connected inverter that overcomes these limitations. The strategy adjusts the hysteresis band magnitude in response to variations in the inverter average output voltage, to achieve constant switching frequency using only one hysteresis comparator and without requiring DC offset compensation or current error zero-crossing measurement. The inverter average voltage is also used to estimate the phase of the incoming grid voltage, to create a current reference for the regulator at any required power factor. The result is a robust sensorless hysteresis current regulator suitable for three-level grid connected applications that operates with a constant switching frequency.
4 citations