Grid-forming inverters (GFI) play an important role as power interfaces for distributed generation units in islanded microgrids, where inductive-capacitive-inductive (LCL) output filters are commonly employed to mitigate the harmonics injected by voltage-source inverters. Due to advantages such as fast dynamic response and straightforward handling of constraints, Finite-control-set model predictive control (FCS-MPC) has become an attractive option for voltage control in GFI systems. However, conventional FCS-MPC algorithms with short prediction horizon have performance limitations in the tracking of ac references in systems with high-order dynamics, such as LCL-filtered GFIs. On the other hand, predictive algorithms with extended prediction horizons suffer from an increased computational burden. This paper proposes a new FCS-MPC algorithm to accurately control the capacitor voltage in an LCL-filtered GFI, using a discrete-time prediction model to dynamically compute the reference for a FCS-MPC inverter-side current controller. The main advantages of the proposed method are its simple implementation without requiring the tuning of weighting factors in its cost function; and its short prediction horizon, which maintains a reduced computational cost. Moreover, active resonance damping elements such as digital filters or ad hoc feedback loops to deal with the LCL filter resonance are not required. Simulation tests and experimental results in a laboratory-scale setup confirm the effectiveness of the proposed control algorithm, yielding lower distortion of output voltage waveforms and increased robustness to modeling errors compared with the conventional FCS-MPC approach.

This paper presents a post-fault current control strategy for dual three-phase Interior Permanent Magnet (IPM) synchronous machines. The current controllers are designed based on the Vector Space Decomposition (VSD) algorithm. This take into account the equivalent harmonic inductance variation, which is then used to introduce the post-fault compensation control. A double three-phase IPM machine is modelled and operated in three different operating conditions: nominal, one three-phase set in short-circuit (SC), and one three-phase set in open-circuit (OC). By keeping the current loop bandwidth constant, the proposed fault-tolerant current controllers guarantee the same dynamic performance of the remaining healthy module after the faults occurred. In OC fault, the proportional and integral (PI) controllers are re-designed while adapting the inductance variation. For the SC case, constant stability margins are provided by cascading a novel lag compensator to the nominal controller. The current dynamics responses are analysed in both healthy and faulty conditions by means of Matlab/Simulink simulations. Experimental results on a 30 $\mathbf {kW}$ six-phase IPM machine prototype with isolated neutral points and supplied by two custom three-phase converters, are demonstrating the effectiveness of the proposed control strategies.

Leakage current measurement and its harmonics have been widely used for the pollution level estimation of HV insulator strings. However, the solutions available to applications into transmission lines and substations are few and it reduces the creepage distance (mm/kV), must be installed by de-energizing the electrical power system or do not allow the characteristic harmonics of the leakage current to be obtained. This paper presents the design, implementation, testing, and experimental validation of a transducer for measuring and online monitoring of leakage currents on high-voltage insulator strings. The transducer is composed of a clamp-type current transformer with a supermalloy material core. The leakage current is reconstructed from the transducer output voltage by applying the Fast Fourier Transform (FFT) and its own inverse transfer function, which allows it to obtain the leakage current with its characteristic harmonics and reduces the electromagnetic noise present in power systems. The output voltage of the transducer was acquired, processed, and transmitted using a dedicated device installed in the HV tower. The proposed transducer was first tested on an insulator inside an artificial fog chamber according to the IEC 60507 standard. The transducer was experimentally validated on an insulator string in a 220 kV substation. The results showed that the designed transducer measured the leakage current and its harmonics with an average error less than 1.76%. With these results the transducer can be used for the on-line monitoring of pollution levels of insulator strings without reduces the creepage distance.

The research on DC-DC power converters has been a matter of interest for years since this type of converter can be used in a wide range of applications. The main research is focused on increasing the converter voltage gain while obtaining a good efficiency and reliability. Among the different DC-DC converters, the flyback topology is well-known and widely used. In this paper, a novel high efficiency modified step-up DC-DC flyback converter is presented. The converter is based on a N-stages flyback converter with parallel connected inputs and series-connected outputs. The use of a single main diode and output capacitor reduces the number of passive elements and allows for a more economical implementation compared with interleaved flyback topologies. High efficiency is obtained by including an active snubber circuit, which returns the energy stored in the leakage inductance of the flyback transformers back to the input power supply. A 4.7 kW laboratory prototype is implemented considering four flyback stages with an input voltage of 96 V and an output voltage of 590 V, obtaining an efficiency of 95%. The converter operates in discontinuous current mode then facilitating the output voltage controller design. Experimental results are presented and discussed.

The development of distributed generation, mainly based on renewable energies, requires the design of control strategies to allow the regulation of electrical variables, such as power, voltage (V), and frequency (f), and the coordination of multiple generation units in microgrids or islanded systems. This paper presents a strategy to control the active and reactive power flow in the Point of Common Connection (PCC) of a renewable generation system operating in islanded mode. Voltage Source Converters (VSCs) are connected between individual generation units and the PCC to control the voltage and frequency. The voltage and frequency reference values are obtained from the P–V and Q–f droop characteristics curves, where P and Q are the active and reactive power supplied to the load, respectively. Proportional–Integral (PI) controllers process the voltage and frequency errors and set the reference currents (in the dq frame) to be imposed by each VSC. Simulation results considering high-power solar and wind generation systems are presented to validate the proposed control strategy.

DC–DC power converters have generated much interest, as they can be used in a wide range of applications. In micro-inverter applications, flyback topologies are a relevant research topic due to their efficiency and simplicity. On the other hand, solar photovoltaic (PV) systems are one of the fastest growing and most promising renewable energy sources in the world. A power electronic converter (either DC/DC or DC/AC) is needed to interface the PV array with the load/grid. In this paper, a modified interleaved-type step-up DC–DC flyback converter is presented for a PV application. The topology is based on a multi-winding flyback converter with N parallel connected inputs and a single output. Each input is supplied by an independent PV module, and a maximum power point tracking algorithm is implemented in each module to maximize solar energy harvesting. A single flyback transformer is used, and it manages only 1/N of the converter rated power, reducing the size of the magnetic core compared to other similar topologies. The design of the magnetic core is also presented in this work. Moreover, the proposed converter includes active snubber networks to increase the efficiency, consisting of a capacitor connected in series with a power switch, to protect the main switches from damaging dv/dt when returning part of the commutation energy back to the source. In this work, the operating principle of the topology is fully described on a mathematical basis, and an efficiency analysis is also included. The converter is simulated and experimentally validated with a 1 kW prototype considering three PV panels. The experimental results are in agreement with the simulations, verifying the feasibility of the proposal.