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One of the primary requirements of dual active bridge (DAB) dc-dc converters is to operate under zero voltage switching (ZVS), to reduce switching losses and increase operating efficiency. However, three-phase DAB converters are known to have limited capability to operate in ZVS, particularly at low power levels, because of their inherent two-level six-step phase shifted square wave modulation strategy...
AC current regulators are always susceptible to controller wind-up, i.e. the condition where the controller dynamic states ramp the controller output up to very large values when the commanded reference cannot be achieved because of inverter saturation. Recovering from saturation can take considerable time once the linear mode has been restored, which degrades the current regulator's transient performance...
Unbalanced transformer impedances for three-phase dual active bridge (DAB) converters cause different zero voltage switching (ZVS) boundaries for each phase leg, which limit the converter soft switching regions. This paper uses harmonic analysis to demonstrate how asymmetrical PSSW modulation can be applied to shape each phase leg's ZVS boundary. The conclusions from the analysis provide guidelines...
Controllers based on resonant Second Order Generalised Integrators (SOGIs) are attractive for AC conversion applications because they provide infinite loop gain at the target resonant frequency, which is required to eliminate AC steady state error in the regulated converter output. Furthermore the simple dual integrator based architecture of a SOGI filter facilitates frequency adaptation, for example...
One of the central requirements of a Dual Active Bridge (DAB) converter is to operate under Zero Voltage Switching (ZVS) conditions, to reduce switching losses and increase operating efficiency. This paper shows how frequency domain analysis can be used to design and operate a DAB to ensure ZVS across the entire operating range by using transformers with a reduced winding coupling factor and adaptive...
Dual Active Bridge (DAB) converters offer an unmatched capability to transfer energy in either direction between two DC sources, especially when they are operated under Zero Voltage Switching (ZVS) conditions. However, the parasitic commutation inductance within the bridge phase leg devices can still lead to significant energy losses at device turn-off despite operating under ZVS conditions. These...
It is well known that level shifted phase disposition pulse width modulation (PD-PWM) achieves the best possible three-phase line-to-line output voltage spectrum for multilevel converters. However the strategy does require post modulation signal decoding to optimally select between redundant switched states and to achieve an even distribution of commutation events across all switching devices. For...
Conventional wind energy conversion systems (WECS) typically employ rotor side synchronous frame current control strategies to regulate the active/reactive power supplied by the WECS generator. Such strategies require accurate information regarding the rotor position and speed in order to set the dq frame reference currents and to perform the necessary frame transformations. This information can be...
While Three-Phase Dual-Active Bridge (DAB) converters are inherently capable of Zero-Voltage-Switching (ZVS), this operating region can be difficult to precisely identify for practical converter systems. This paper presents a new analytical approach based on harmonic analysis that accurately predicts the ZVS boundaries for Three-Phase DAB's with any interconnecting impedance network under all operating...
For energy conversion systems that require bidirectional energy transfer between two DC sources, the generally preferred converter topology is a Dual Active Bridge (DAB), particularly when galvanic source isolation is required. However, for maximum efficiency the two bridges should operate under Zero-Voltage-Switching (ZVS) conditions, and hence it is important to be able to accurately identify this...
This paper presents a theoretical harmonic analysis of phase disposition (PD) and phase shifted carrier (PSC) pulse width modulation (PWM) strategies for MMC converters. It is shown that when these strategies are implemented on a per MMC arm basis, their spectral performances converge because of cancellation of odd carrier sideband groups between each phase leg's arms. An improved PD modulation strategy...
Dual Active Bridge (DAB) converters offer an unmatched capability to transfer energy in either direction between two DC sources, while also providing galvanic isolation and high conversion efficiency. However, to operate at high efficiencies, the bridges must operate with Zero-Voltage-Switching (ZVS) over as wide an operating range as possible. The conventional approach to determine ZVS operation...
This paper presents a novel spectral analysis technique for multilevel modulation. Conventionally, such analyses use a Double Fourier series technique, but this approach can become intractable when complex reference waveforms (e.g. multilevel space vector offsets) and regular sampling processes are considered. In contrast, the strategy proposed in this paper separates the multilevel pulse width modulation...
The usual strategy for current regulation of distributed generation sources in an unbalanced three phase system is to separately control the positive and negative sequence currents using independent positive and negative synchronously rotating frame current regulators. This approach requires the measured phase currents to be separated into positive and negative sequence components before transformation...
Three level Neutral Point Clamped (NPC) inverters exhibit a natural balancing property that can self-regulate the intermediate Neutral Point (NP) voltage to half the overall DC link voltage. However, the underlying mechanisms of this process are not well understood, its performance under various load conditions has not been properly explored, and the requirements and benefits of (RLC) balance booster...
The Distributed Generators (DGs) of an islanded microgrid must be carefully controlled to regulate voltage and frequency, and to share power evenly between the DG sources. This is normally achieved by operating the microgrid Voltage Source Inverters (VSIs) in voltage controlled mode, so that the microgrid voltages and frequency can be explicitly regulated. However, this results in the loss of direct...
Coupled-inductor semi-bridge converters, assembled from magnetically linked pairs of complementary unidirectional phase legs, can achieve a substantially improved harmonic output voltage performance with the same number of silicon devices as traditional converters. However, they must be operated so as to ensure that a continuous common-mode current always flows through each coupled-inductor, to ensure...
Microgrids consisting of distributed generators (DGs) have the ability to operate connected to the utility grid and also in stand-alone (island) mode. In both cases, frequency and voltage regulation across the microgrid must be ensured, and the DGs should share their generation equally. The transition of the microgrid between the two operating modes also needs to be seamless, which is conventionally...
Grid connected voltage source inverters require both current regulation and synchronisation to control the power flow and to maintain stability during grid disturbances. For this type of application, non-linear current control of VSIs using techniques such as hysteresis current regulation offer advantages compared to linear current regulation, such as inherent over-current protection, robustness to...
Despite their increasing levels of penetration into electrical grid distribution systems, PV based distributed generation (DG) systems are still not yet well regarded as a useful contributor to grid voltage regulatory support. However, there is increasing recognition of the potential for these inverter based systems to not only inject real power into the grid, but also to inject/absorb reactive power...
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