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Power system state-space models are often constructed by interconnection of their subsystems (converters, distribution lines, and grid). The interconnection between L-/LCL-filtered converters with the distribution lines subsystems is often realized through a virtual resistor, because they both have the voltage as input, introducing inaccuracy. Moreover, the parameters variations influence not only the eigenvalues, but also the equilibrium point. In this case, the small-signal model has to be reevaluated around the new equilibrium point. For the computation of the equilibrium point, an additional method, e.g., power flow, is conventionally used. However, the variables computed with power flow (e.g., {rm P},Q,{rm V},text{ and }theta) do not always coincide with the state-space model variables, required for the linearization. Furthermore, the traditional power flow does not consider the influence of the voltage-source-converter control system on the grid equilibrium point. This article proposes a nonlinear grid model that does not need the virtual resistor to be interconnected. The proposed model can be used both for equilibrium point computation through the Newton-Raphson method, and it can be linearized around the computed equilibrium point for small-signal analyses. Simulations and experiments are provided.

To eliminate the weighting factor tuning effort of typical finite-set model predictive control (FS-MPC), this paper proposes a weighting factor-less sequential model predictive control (SMPC) scheme for LC-filtered voltage source inverters. Two independent cost functions for minimizing capacitor-voltage and inductor-current tracking errors are deployed in a cascaded structure, eliminating the weighting factor. First, the optimal cascade order of the cost function is selected by the internal relationship of two control variables. Then, a graphical method is proposed to determine the optimal number of candidate voltage vectors selected from the first cost function. Moreover, to realize the strict current limitation, the current-constraint term is proved to be included in the voltage-related cost function. Another attractive feature of the proposed SMPC is that a smoother inductor-current starting response can be obtained compared to typical FS-MPC. Simulation and experimental results verify the feasibility of the presented approach.

Fault current-limiting dynamic voltage restorers (FCL-DVRs), also named as dual-functional DVRs, involving both the FCL and the voltage compensation (VC) function, have attracted more attention. However, in case of the FCL-DVR operating in VC mode, either the high capacity of storage system in the dc-link or the oversizing of power converter is needed, and in case of the FCL-DVR operating in FCL mode both the high dc-link voltage and the oversizing of power converter are needed. Thus, in order to overcome these drawbacks, an improved FCL-DVR, which consists of an LC filter coupled with series coupling capacitor (LCC) at the ac side, is proposed in this article. The improved FCL-DVR using energy-optimized control strategy can not only perform the dual functions well, but also achieve the capacity optimization of FCL-DVR key components. The operating principle and performance of the proposed FCL-DVR under FCL and VC mode are elaborated, respectively. The optimal design of LCC and selection of FCL branch are studied in detail to ensure high stability and reliability for the proposed FCL-DVR operating in both modes. The simulation and experimental results clearly validate the effectiveness of the proposed topology and method.

Admittance reshaping is a widely used strategy to address the converters low-frequency stability issues in weak grid, caused by the PLL and its interaction with the dc and ac voltage controls. However, the asymmetric control of the d-and q-axis current references and the coupling between the converter ac and dc sides restricts the damping capability of single-input single-output feedbacks. This phenomenon gets even worse in the presence of nearby converters. This article extends the concept of admittance reshaping to multi-input multi-output control. A full state feedback is added to the current reference of the converter to increase the damping of the conventional multiloop control. A systematic offline algorithm is delegated to design the feedback, and a scalar coefficient is employed to activate/deactivate online the reshaping feedback, making the proposed solution user-friendly. The proposed control is analyzed both in time and frequency domains and tested in parallel-operation with other converters, and shows higher damping capability than conventional solutions and good robustness with respect to grid impedance and operating point variations. Experimental tests under ac and dc disturbances are conducted both in lab setup and in hardware-in-the-loop.

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 nonmodular 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 article reviews the history of the MTB dc–dc and then provides a classification of them, comparing them with figure of merits 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 analyzed with the help of simulation and experimental results.

The modular multilevel converter (MMC) has been a highly promising topology in the high-voltage direct-current (HVDC) transmission area, where each arm of the MMC may consist of hundreds of series-connected submodules and an inductor. Due to its parameter inaccuracy, component aging, and so on, the component parameter in different arms of the MMC may be different, which may cause circulating current in the MMC-HVDC transmission system, and result in current deterioration, power losses, and electromagnetic interference, etc. In this paper, the circulating current suppressing (CCS) in the MMC-HVDC system, due to asymmetric arm impedance, is analyzed. Based on the mathematical analysis, a method of using an auxiliary circuit is proposed for the MMC to realize the CCS and improve the performance of the MMC-HVDC system. Simulation studies are conducted with PSCAD/EMTDC in the HVDC system, which confirms the feasibility of the proposed method.

As one of the most representative series active compensators, dynamic voltage restorer (DVR) can solve voltage‐related power quality uses effectively. The in‐phase voltage control and energy‐optimised control are the two typical control strategies of DVR. However, the in‐phase voltage control results in a large amount of active power exchange between the main grid and the dc link of DVR, and hence high capacity of storage system is needed in the dc link while the energy‐optimised strategy requires high injected voltage, resulting in oversizing of the DVR inverter. Thus, in order to avoid the aforementioned drawbacks, this study proposes a modified DVR, which connects an external capacitor in series with the LC filter (LCC) of DVR (LCC‐DVR). Based on the operational principles of the LCC‐DVR, an updated procedure of reference voltage is proposed to avoid overmodulation of DVR caused by load variations, ensuring the continuous operation capability of the LCC‐DVR. The overall control scheme of the LCC‐DVR and the comparative analysis among the proposed strategy and traditional strategies are also presented. The effectiveness of the proposed topology and method is clearly validated by the simulation and experimental results.

The impact of low-carbon technologies, like renewable energies and electric vehicle (EV) charging stations, will be an additional strain in terms of voltage violation and current congestion on the low-voltage network, resulting in a reduced quality of supply and overheating assets. To avoid or defer the replacement of these traditional assets, the network of the future needs an intelligent solution such as smart transformers (STs) [1]. The ST is a solid-state transformer with smart control functionalities as proposed in [2].

To sufficiently utilize a bridge-type fault current limiter (BTFCL) in the distribution grids, this article proposes a dual-functional BTFCL (DF-BTFCL), which can work either at the BTFCL mode to limit faulty currents or at the dynamic voltage restorer (DVR) mode to compensate grid-voltage fluctuations. The proposed DF-BTFCL can flexibly switch between the two modes by either activating a dc capacitive circuit or a dc inductive circuit. The inductive circuit can adjust the equivalent series impedance to limit the faulty current in an acceptable range, and the dc capacitance circuit can provide dc support to enable the DVR operation. The proposed DF-BTFCL can immediately limit the increasing rate of the faulty current without any fault detection delay, and it is similar to the conventional BTFCL. Transient processes between the switch of the two operation modes are analyzed, and the key parameters of the DF-BTFCL system are designed in detail. Simulations and experimental results both clearly prove the feasibility and effectiveness of the proposed DF-BTFCL.

The increasing penetration of distributed generators in low-voltage distribution grids may lead to distribution transformers, also a smart transformer, experiencing the reverse power flow, which is less as compared to the forward power flow in the current electric grid. This article proposes an asymmetrical bidirectional dc–dc (AB-DC/DC) converter, which has two partial-scale rectifiers of an active bridge and a diode bridge connected in parallel at the secondary side, with less cost of power semiconductors and the capability of bidirectional operation to satisfy the realistic condition. The proposed AB-DC/DC converter works as a single active bridge under forward power flow condition, while it works as a dual active bridge under reverse power flow condition. The solutions to share the active power under forward operation and suppress the circulating power under reverse operation are proposed, respectively. The simulation and experimental results both clearly validate the effectiveness and feasibility of the proposed AB-DC/DC converter.