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The power calculation is an indispensable element in droop-controlled inverters because the bandwidth of the measured power has a direct impact on the controller performance. This paper proposes a fast and accurate power calculation algorithm based on the combined Second Order Generalized Integrator (SOGI) filters in stationary coordinates for a three-phase system, which takes into consideration the use of nonlinear loads. The power calculation scheme is formed by the two-stage SOGI filters that are employed for obtaining the active and reactive powers required to perform a droop-based inverter operation, respectively. From the two-stage structure, the first SOGI is used as a band-pass filter (BPF) for filtering harmonics and obtaining the fundamental current of the nonlinear load; The second SOGI is used as a low-pass filter (LPF) for extracting the DC-component, which corresponds with the average power. A small-signal model of a two droop-controlled inverters system is built to obtain the dynamical response and stability margin of the system. And compared it with the dynamical behaviour of a standard droop-control method. Next, the proposed power calculation system is designed in order to achieve the same ripple amplitude voltage as that obtained with the standard droop-control method by adjusting the bandwidth gains. Through simulation and hardware in the loop (HIL) validation, the proposed approach presents a faster and more accurate performance when sharing nonlinear loads, and also drives the inverters’ output voltage with lower distortion.

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current (MTDC) grids, this paper proposes a modular multi-terminal DC power flow controller (MM-DCPFC) with fault current limiting function. The topology structure, operation principle, and equivalent circuit of MM-DCPFC are introduced, and such a structure has the advantages of modularity and scalability. The power balance mechanism is studied and a hierarchical power balance control strategy is proposed. The results show that MM-DCPFC can achieve internal power exchange, which avoids the use of external power supply. The fault characteristics of MM-DCPFC are analyzed, fault current limiting and self-protection methods are proposed, and the factors affecting the current limiting capability are studied. The simulation models are established in PLECS, and the simulation results verify the effectiveness of MM-DCPFC in power flow control, fault current limiting, and scalability. In addition, a prototype is developed to validate the function and control method of MM-DCPFC.

Nonlinear dynamical systems such as weakly coupled oscillators are an interesting approach to be adopted for the regulation of power inverters inside microgrids. Aiming at the synchronization and load sharing in islanded mirogrid, this paper is inspired by oscillator synchronization property to propose a Hopf oscillator controller for the single-phase inverters. The Hopf oscillator dynamic equations are used for providing the inverter's frequency and amplitude voltage references which lead to a robust nonlinear droop behavior for driving the system without using communications. The Hopf oscillator provides better sharing of the load between inverters with higher robustness, less harmonic distortion, and faster time response of the associated limit cycle than the achieved by the other approach made with a Van der Pol oscillator. In addition, global asymptotic synchronization of system is proven by Lyapunov approach. Simulation results of a system composed by paralleled inverters are provided and compared with a Van der Pol oscillator approach reported in literature. Experimental results are also provided to prove the Hopf oscillator based controller under different circumstances.

The dynamic performance of the local control of single-phase voltage source inverters (VSIs) can be degraded when supplying to nonlinear loads (NLLs) in microgrids. When this control is based on the droop principles, a proper calculation of the active and reactive averaged powers (P&ndash, Q) is essential for a proficient dynamic response against abrupt NLL changes. In this work, a VSI supplying to an NLL was studied, focusing the attention on the P&ndash, Q calculation stage. This stage first generated the direct and in-quadrature signals from the measured load current through a second-order generalized integrator (SOGI). Then, the instantaneous power quantities were obtained by multiplying each filtered current by the output voltage, and filtered later by utilizing a SOGI to acquire the averaged P&ndash, Q parameters. The proposed algorithm was compared with previous proposals, while keeping the active power steady-state ripple constant, which resulted in a faster calculation of the averaged active power. In this case, the steady-state averaged reactive power presented less ripple than the best proposal to which it was compared. When reducing the velocity of the proposed algorithm for the active power, it also showed a reduction in its steady-state ripple. Simulations, hardware-in-the-loop, and experimental tests were carried out to verify the effectiveness of the proposal.

The active and reactive powers, P and Q, are crucial variables in the parallel operation of single-phase inverters using the droop method, introducing proportional droops in the inverter output frequency and voltage amplitude references. P and Q, or P-Q, are calculated as the product of the inverter output voltage and its orthogonal version with the output current, respectively. However, when sharing nonlinear loads these powers, Pav and Qav, should be averaged by low-pass filters (LPFs) with a very low cut-off frequency to avoid the high distortion induced by these loads. This forces the droop method to operate at a very low dynamic velocity and degrades the system stability. Then, different solutions have been proposed in literature to increase the system velocity, but only considering linear loads. Therefore, this work presents a method to calculate Pav and Qav using second-order generalized integrators (SOGI) to face this problem with nonlinear loads. A double SOGI (DSOGI) approach is applied to filter the nonlinear load current and provide its fundamental component to the inverter, leading to a faster dynamic velocity of the droop-based load sharing capability and improving the stability. The proposed method is shown to be faster than others in the literature when considering nonlinear loads, while smoothly driving the system with low distortion levels. Simulations, hardware-in-loop (HIL) and experimental results are provided to validate this proposal.

Although virtual synchronous generator (VSG) is a promising solution to low inertia and damping of the power electronics-based power system, improper parameter design will lead to instability of such a system. In this paper, a VSG parameter design strategy is proposed by considering both AC and DC restraints for a paralleled inverters system, where the active power output of VSG is restricted by the maximum power limit of the converters in AC side, with the energy supply bounded by the energy margin of power source in DC side. Detailed stable operation boundaries are investigated using small-signal mode by considering the dual physical constraints and the stable constraints. Therefore, the proposed parameter tuning method can not only provide enough virtual inertia for stable operation of the system, but also satisfy the power and energy restraints in practice. Detailed simulation results verified the effectiveness of the proposed strategy.

This paper presents a method which mimics coupled nonlinear Poincaré–Bendixson oscillators synchronization to realize current sharing in islanded three-phase Microgrid. We propose that each inverter be controlled to imitate the dynamics of coupled oscillators. When they are controlled to interact strongly enough, they will all start synchronization. We also introduce the comparison between Van der Pol oscillator corresponding to their physical characteristics. The dual-loop based on the oscillator is designed in αβ frame, and the phase dynamics are derived to observe system initial state performance. The behavior and performance of the proposed method were verified experimentally regarding load connection, inverter removal and current ratios changes.

Conventional autonomous current-sharing controller (ACSC) will induce inevitable frequency and voltage magnitude deviations due to the output currents and virtual impedances. Thereby secondary control has been proposed to restore the frequency and voltage magnitude deviations by using communication links. In this paper, an autonomous deviation eliminative control method is proposed for the parallel-connected inverters in AC islanded microgrids in order to simplify the control scheme and to improve power quality and control reliability. Two high-pass filters are included in synchronous-reference-frame virtual impedance loop and are activated in steady-state operation to attenuate frequency and voltage magnitude deviations, meanwhile maintaining the autonomous current-sharing performance among the parallel inverters. Simulation results based on two parallel-connected voltage controlled inverters from Matlab/Simulink verify the effectiveness of the proposed approach in different scenarios.

In this paper, a voltage modulated direct power control for three-phase pulse-width modulated rectifier is proposed. With the suggested method, the differential equations describing the rectifier dynamics are changing from a linear time-varying system into a linear time-invariant one. In this way, the conventional feedback and feedforward controllers are applicable for the independent control of active and reactive powers. The proposed method is guaranteed that the closed system is globally exponentially stable. A feedback linearization method is also employed for generating the active power reference of inner loops. Finally, some experimental tests are conducted to verify its effectiveness.

A novel coordinated control for energy storage system (ESS) and renewable energy source (RES) units is proposed in islanded AC microgrids without a phase-locked loop system. We use a proportional-resonant control technique for the ESS, which fixes the voltage and frequency of the microgrid. Moreover, we use a voltage-modulated direct power control (VM-DPC) technique, which has not only a good tracking performance but also a good steady-state behavior, for the RESs that generate their maximum power in the normal operation. To ensure the stability of the microgrid, we use the passivity property for RESs when the VM-DPC is sued. Hence, the whole system is still stable and passive after integrating the heterogeneous RESs into the microgrid. To validate the proposed coordinated control architecture, we use MATLAB/Simulink, SimPowerSystems to simulate a microgrid consisting of one ESS, one wind turbine, one photovoltaic and two controllable loads. Simulation results show that the islanded microgrid is operating well and the plug-and-play capability of the RESs is enhanced.

This paper presents an adaptive synchronization for current-controlled grid-connected inverter based on a time-domain virtual oscillator controller (VOC). Inspired by the phenomenon of dynamics of adaptive oscillator under the perturbation effect. Firstly, the fast learning rule of the oscillator frequency is derived in order to make the oscillator evolve to the grid voltage. Thus realizing a phase-locking behavior, but without requiring the use of a dedicated phase-locked loop (PLL). Then, the unit orthogonal components are obtained to generate the current reference, and a current loop is designed by using a PR controller in the αβ frame. Experimental results were obtained with comparison to PI current controller, showing the salient performances of the proposed method in front of grid frequency variations, voltage sags and swells.

In this paper, an improved grid voltage modulated control (GVM) with power compensation is proposed for grid-connected voltage inverters when the grid voltage is unbalanced. The objective of the proposed control is to remove the power ripple and to improve current quality. Three power compensation objectives are selected to eliminate the negative sequence components of currents. The modified GVM method is designed to obtain two separate second-order systems for not only the fast convergence rate of the instantaneous active and reactive powers but also the robust performance. In addition, this method converts the system into a linear invariant system though designing a new voltage input. Simulation results are presented to verify the correctness and validity of the proposed control strategy and power compensation method.