Double Active Bridge Operated in Quasi Discontinuous Conduction Mode

The quasi discontinuous conduction mode (QDCM) of the double active bridge (DAB) is addressed in this paper. The DAB converter usually is operated turning on always two semiconductors per bridge, and this leads to a continuous inductor current: However, a similar operation to dc/dc converters can be implemented, for this, less than two switches are turned on, at certain time, in one of the bridges; this leads to the quasi-DCM. With this mode of operation, a natural soft-switching is performed during the whole range of operation, but also low current stress is performed compared to the square modulation, not only at low power range. The modulation is relatively simple compared to other techniques. The proposed technique is described, analyzed, numerically simulated, and experimentally tested

State-feedback-based Low-Frequency Active Damping for VSC Operating in Weak-Grid Conditions

Voltage source converters (VSCs) are nowadays widely integrated in the power grid, nevertheless they can induce low frequency stability problems under weak grid conditions. The interaction of PLL, dc-link voltage control, and ac voltage control generates a positive feedback which threatens the power system stability. The existing researches mainly focus on modeling strategies and stability analyses tools, however still few studies dealt with active damping in the low frequency range. In this paper, a nonlinear state space model of a VSC is presented and linearized around the operating point. From the model, a linear state feedback control law is designed and incorporated in the dc-link and ac voltage control in order to increase the system damping. Eigenvalue analysis is used to investigate the performance of the proposed controller. The simulation results based on a 2 MW grid connected wind generation unit, clearly show the effectiveness of the proposed solution. Experimental results with a 4 kW scaled-down setup validate the analytic and simulation results

Integration and Optimization of Voltage Active Filtering Functionality in a PV Park

The stringent regulations on the power quality declared in the standard IEEE 519-2014 push the companies and the power producers to install active filters to compensate the voltage harmonics distortion in the point of common coupling (PCC). However, in the case of a Photovoltaic (PV) park, the cost for an additional active filter converter can be saved by using the PV converters themselves as active filter. This solution is very attractive, but reserves several challenges. In fact, the harmonic current injection by the PV converters can generate ripples in the DC link which increases the stress on the converter components and affects the MPPT. Moreover, the overcurrent protection of the PV converter must be taken into account. In this paper a centralized optimized strategy to share the harmonic current injection among all the converters in a PV park is investigated. The optimization is formulated as a quadratic programming (QP) problem: the active power consumed by the PV park for the active filtering and the DC link ripple of the PV converters caused by the harmonic currents injection are minimized. The limit on the maximum injectable harmonic current by each PV converter are respected

A Comprehensive Assessment of Multiwinding Transformer-Based DC-DC Converters

Multiwinding-Transfomer-based (MTB) DC-DC converter did emerge in the last 25 years as an interesting possibility to connect several energy systems and/or to offer higher power density because of the reduction of transformer core material and reduction of power converter stages. MTB DC-DC converters can be considered as an interesting compromise between non-modular and a modular DC-DC converter since they are themselves modular in the construction. This eventually leads to some fault-tolerant possibilities since the multiwinding transformer (MWT) connects multiples ports and if one of them is not working anymore and it can be isolated, the others might still continue operating. Unfortunately, it is exactly the MWT that creates most of the technical challenges of this class of DC-DC converters because of the cross-coupling effects among the cells which make especially the resonant-topology very challenging to be designed. This paper reviews the history of the MTB DC-DC and then provides a classification of them, comparing them with Figure of Merits (FOM) and focusing on which is the maximum possible number of windings and which are the most suited magnetic core types. The problems coming from cross-coupling and the possible fault-tolerant operation are analysed with the help of simulation and experimental results

Investigation of load compensation features of smart transformer in medium voltage grid

Harmonics and load unbalance create several adverse issues in power distribution system, which are generally mitigated by bulky shunt active power filter (SAPF). This paper explores load compensation features of smart transformer (ST) in a two-feeder city center, where conventional power transformer (CPT) in one feeder is replaced by the ST whereas other feeder is continued to be supplied through the CPT. The ST is controlled not only to supply its own loads connected at the low voltage side, but also to compensate reactive, unbalance and nonlinear loads of the other feeder. This makes total MV grid currents of the combined city center balanced, sinusoidal, and in unity power factor at the point of common coupling (PCC) voltage. Therefore, in addition to providing continuous and reliable support to the ST based loads, the ST eliminates need of bulky SAPF and isolation transformer from the city center which are used for achieving reactive and harmonic current compensation. A switching model of the system is developed in power system computer aided design (PSCAD) software and implemented on a developed experimental prototype to show the capability of the ST

Scalable State-Space Model of Voltage Source Converter for Low-Frequency Stability Analysis

Low frequency instability phenomena in power electronic based power systems can originate both at converter and at power system level. At the converter level, the interaction between PLL, dc-link and ac voltage control in voltage source converters (VSCs) connected to a weak grid can lead to instability. At the power system level, the interactions among different parallel VSCs can produce oscillatory phenomena, and even result in instability. However, a model to study the instability phenomena at both levels is still under development. In this paper, a scalable VSC state space model, which captures the interactions among PLL, dc-link and ac voltage control is proposed. The proposed model is suitable for interconnection, and an example of power system modeling is shown. The model is then validated through simulations and experimental tests. Eigenvalue analysis is carried out to investigate the influence of the control parameters on the stability

Robustness Analysis of Voltage Control Strategies of Smart Transformer

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs