As a clean energy, wind power has been extensively exploited in the past few years. However, oscillations in wind turbines, particularly those from controllers, could severely affect the stability of power systems. Therefore, oscillation suppression is a recent research focus. Based on the small signal model eigenvalues and participation factors, this paper detects the sub-synchronous interactions (SSI) mainly determined by converters' PI parameters in a grid-connected doubly fed induction generator (DFIG). With the aim of oscillation restraint, a novel optimization model with the reference-point based Non-Dominated Sorting Genetic Algorithm (NSGA-III) and the t-distributed stochastic neighbour embedding (t-SNE) is developed to explore and visualize optimal ranges of PI parameters, facilitating the selection of the appropriate PI parameters to augment the damping. Additionally, to study the adaptability of the optimal PI parameters, interactions performance of the system that uses optimal parameters is studied with different output levels of the wind turbine. Finally, a time domain simulation and a practical experiment are conducted to demonstrate the effectiveness of the proposed approach. Results illustrate that the SSI of a grid-connected DFIG is suppressed by the optimization model. This study is highly beneficial to power system operators in integrating wind power and maintaining system stability.

In an AC/DC hybrid distribution grid (AC/DC HDG), different AC distribution grids are connected through a voltage source converter (VSC) based DC grid to operate the entire system in a secure and economic manner. This paper presents a decentralized optimal operation approach for running AC/DC HDG which can either work individually or collaborate with each other. Using the analytical target cascading (ATC) approach, a three-level hierarchy consisting of DC grids, VSC stations and AC grids, is formulated to decompose the operation problem of AC/DC HDG. The obtained subproblems for these grid levels, as well as the interactions between them, are modeled and solved with a nested solution process. The simulation results show the effectiveness of the proposed decentralized approach.

Power converters in the system-level real-time simulation are usually emulated by L/C-based associated discrete circuit (L/C-ADC). However, the L/C-ADC approaches may suffer from two issues: how to mitigate the unacceptable virtual power loss especially in high-frequency applications, and how to tune LC parameters setting which is affected by the external circuit. This paper proposes a novel generalized associated discrete circuit (G-ADC) model with parameterized history current sources. By utilizing the stability region of the feasible parameter space, the optimized G-ADC models with the best damping characteristic are developed for both two-level and three-level converters. The analytical results also guarantee that the parameters of the optimized G-ADC model are independent of the external circuit for most power grid applications. Furthermore, an FPGA-based real-time simulation platform is built to verify the feasibility of the proposed scheme. Extensive simulation and hardware-in-loop (HIL) experiment results demonstrate the effectiveness and superiority of best-damped models as well as the modeling flexibility corresponding to insensitivity to operating conditions and external system parameters.

The real-time simulation based on the field programmable gate array (FPGA) is receiving more and more attention. However, up to now, the simulation scale for the power electronic system is not so satisfactory due to the real-time requirement and the FPGA resource limitation. This paper proposes a sub-microsecond level real-time simulation method for microgrids. The power converters are modeled with fixed-admittance models and simulated with a compact electromagnetic transients program (EMTP) algorithm. In the meanwhile, the distribution lines/cables are modeled with π-circuit models and simulated with a distributed circuit solution method, called the latency insertion method (LIM). As a result, the distribution generation (DG) systems are decoupled with each other and can be simulated in parallel. The case study shows that the proposed simulation method consumes much fewer FPGA resources compared with the traditional one. Besides, the time step can be much smaller and doesn't need to increase with the simulation scale. With these advantages, the real-time simulation of a microgrid consisting of 3 three-phase converters, 3 boost circuits and 21 three-phase lines can be achieved on the Xilinx Kintex-7 410T FPGA with a minimum time-step of 380ns.

The switching frequency of power electronic devices has become much higher than before, which brings great challenges to real-time simulators. The small time-step in high switching frequency simulations remarkably increases the difficulty to satisfy the real-time requirement. This paper realizes the accurate real-time simulation of a solid-state transformer (SST) with a switching frequency of 50kHz at the time-step of 250ns on an FPGA-based platform. The hybrid companion circuit modeling method and the compact-EMTP (Electro-Magnetic Transient Program) algorithm are proposed in this paper to make sure the simulation loop can complete in less than one microsecond. The hybrid companion circuit modeling method can avoid too many digits used in representing simulation variables. The compact-EMTP algorithm is designed by combining the sequential computation tasks of the traditional EMTP to fully utilize the parallelized hardware structure of the FPGA. Besides, a circuit partition method is adopted to further parallelize the circuit solution of the SST circuit. In these ways, the simulation loop of the SST can be completed in 38 clock cycles (about 237.5ns). The simulation results show that the real-time simulation waveforms are almost consistent with those of the offline simulation software. Besides, the hardware-in-the-loop (HIL) simulation can also be performed on this platform to test the control functions of the SST.

This paper presents the design of a filtered tracking error (FTE)-based robust current controller for three-phase grid- tied inverters interfacing distributed renewable resources (DERs) into the grid. An uncertainty and disturbance modeling based control law is developed for achieving the robustness against non- ideal grid conditions including the grid impedance variations, grid voltage harmonics and fluctuations in grid voltage magnitude (symmetrical/ asymmetrical), frequency and phase. The proposed controller is shown to have superior current tracking performance to directly control the current injected into the grid being pure sinusoidal and three-phase balanced. In addition, high dynamic and tracking performance can be further ensured since all the phase locked loops (PLLs) and multi-loop controllers are eliminated, which also delivers the advantage of a simple implementation. Especially, the system stability is proven by using the Lyapunov function. Both simulation and hardware in the loop (HIL) experimental results of the proposed robust controller, as well as the proportional-integral (PI) controller and the parallel proportional-resonant (PR) controller, are given and compared, which validates the performance and effectiveness of the proposed control strategy.