Optimized hierarchical power oscillations control for distributed generation under unbalanced conditions

Dual feedback linearization control based on direct power control (DPC) for three-phase pulse-width-modulation (PWM) voltage-source rectifier (VSR), which connects to grid voltage during network unbalance, is proposed. And there is a non-linear PI controller as the system's outer loop controller, which can accelerate the response of DC voltage in the case of load changing. The positive sequence and negative sequence instantaneous power models of the PWM rectifier are respectively established. The positive sequence active power and reactive power are entirely produced by positive sequence voltages and positive sequence currents. And the negative sequence active power and reactive power are entirely produced by negative sequence voltages and negative sequence currents. The dual power control scheme with feedback linearization control is presented. The positive sequence instantaneous power and the negative sequence instantaneous power achieve decoupling control. By changing reference, the proposed strategy can achieve different control objectives such as: eliminating active and reactive power oscillation without sinusoidal line current, eliminating active power oscillation with sinusoidal line current, eliminating reactive power oscillation with sinusoidal line current or eliminating negative sequence current. Test results show the good performance of the proposed strategy.

In hybrid AC/DC microgrids, parallel interfacing converters (IFCs) with common DC link are commonly used to connect AC and DC subsystems or to interface high-power distributed generations/storage elements to AC subsystem. Under unbalanced condition, the adverse effects of unbalanced voltage on single IFC's operation, such as output power oscillations, DC link voltage oscillations and peak current increase, can be aggregated in parallel IFCs compared to a single IFC. In this paper, a novel control strategy for parallel IFCs is proposed, which is focusing on reducing adverse effects of unbalanced voltage on parallel IFCs' operation based on peak current-based control strategy. In the proposed control strategy, one IFC which has largest power rating among parallel IFCs, named as redundant IFC, cancels out active power oscillations of parallel IFCs. Based on peak current analysis, it is proven in this paper that the collective peak current of parallel IFCs is constant under zero active power oscillation in the fixed average active power output. Therefore, the redundant IFC is controlled based on collective peak current of parallel IFCs to cancel out active power oscillations, which doesn't need communication among parallel IFCs. Moreover, the proposed control ensures the peak currents of IFCs to be approximately in-phase with collective peak current of parallel IFCs. Therefore, it could provide minimum peak current of redundant IFC.

Voltage source converters (VSCs) have been widely used due to their flexibility to control voltages and power independently and bi-directionally. Typically, the control system of VSCs mainly consists of two parts; outer voltage and power controllers and inner current controllers. The vector current control based dq decoupling technique enables to control the active power, reactive power, DC voltage and AC voltage. However, the d- and q-axis of grid voltages and currents comprise AC and DC components under unbalanced grid conditions. The AC components of the d- and q-axis currents make the grid currents unbalanced. In this paper, a novel current control is presented and investigated under unbalanced grid condition for a Back-to-Back Modular Multilevel Converter (B2B-MMC) based HVDC system and validated using the Real Time Digital Simulator (RTDS). Further, the active power oscillation under fault is eliminated by controlling the AC component of grid currents in the dq frame. The RTDS results demonstrate the feasibility of the proposed controllers under unbalanced grid voltage conditions.

This study presents an optimisation control strategy for the power control of voltage source converter‐based high‐voltage direct current transmission under unbalanced grid voltage conditions. To facilitate simple implementation and flexible operation, the current references are derived and synthesised into one generalised equation that introduces two individually adaptable parameters. The two parameters can be adapted to eliminate the oscillations of instantaneous active and reactive power, as well as the currents at the grid side under unbalanced grid voltage conditions. The optimisation method based on particle swarm optimisation algorithm is presented to determine the two parameters. The simulation is conducted with PSCAD/EMTDC. The results show that the proposed strategy is effective and feasible.

Active power oscillation in the hybrid grid-connected system with grid-forming (GFM) and grid-following (GFL) inverters can threaten its safe operation. In this paper, considering the coupling between GFM and GFL inverters, the small signal model of the heterogeneous system is established. Based on the model, the influence mechanism of control parameters on the power oscillation under varying short-circuit ratio (SCR) is qualitatively and quantitatively revealed. In particular, the damping of the GFM inverter has a dominant effect compared with other parameters. Therefore, dual mutual damping control (DMDC), containing angular frequency and active power mutual damping, is integrated into the GFM inverter to enhance system damping. Its stability analysis and key parameters design are also presented. Finally, the experimental results validate the effectiveness of the DMDC, demonstrating a reduction in the interaction power oscillation amplitudes by 38.5% and 29.4% under strong and weak grid conditions, respectively.

A versatile method for computation of power pulsations in DFIG under grid imperfections

Power control strategies for grid connected converters applied to full-scale wind energy conversion systems during LVRT operation

This paper presents an analysis and control design of a doubly-fed induction generator (DFIG)-based wind generation system operating under unbalanced network conditions. A DFIG system model in the positive and negative synchronous reference frames is presented. Variations of stator active and reactive powers and generator torque are fully defined in the presence of negative sequence voltage and current. Alternative DFIG control targets during network unbalance, such as reducing stator current unbalance, torque, and power pulsations minimization, are identified. A rotor current control strategy based on positive and negative (dq) reference frames is used to provide precise control of the rotor positive and negative sequence currents. Simulation results using EMTDC/PSCAD are presented for a 2-MW DFIG wind generation system. It shows that conventional vector control of DFIG without considering network unbalance results in excessive oscillations on the stator active/reactive power, electromagnetic torque, and stator/rotor currents even with a small stator voltage unbalance. In contrast, with the proposed control strategy, enhanced system control and operation such as minimizing oscillations in either active power, or electromagnetic torque, or stator or rotor currents can be achieved

—This article presents an enhanced control strategy for a direct-driven permanent synchronous generator based wind-power generation system with a flywheel energy storage unit. The behaviors of the direct-driven permanent magnet synchronous generator system with a flywheel energy storage unit under unbalanced grid fault conditions are investigated. Three control targets for the grid-side converter during network unbalance conditions are identified, including eliminating the oscillations in the total active power or reactive power or no negative-sequence current injecting into the grid. Meanwhile, the DC-link voltage oscillations can be effectively suppressed during the unbalanced grid fault by controlling the flywheel energy storage unit. Furthermore, a proportional–integral-resonant controller is designed for the flywheel motor to eliminate the oscillations in the DC-link voltage. Finally, the proposed coordinated control strategy for the direct-driven permanent magnet synchronous generator system with a flywheel energy storage unit has been validated by the simulation results of a 1-MW direct-driven permanent magnet synchronous generator wind power generation system with a flywheel energy storage unit under unbalanced grid fault.

An enhanced second-order-consensus-based distributed secondary frequency controller of virtual synchronous generators for isolated AC microgrids

This article proposes a novel multiobjective control strategy (MOCS) for improving the performance of grid-tied photovoltaic (GTPV) system without phase-locked loop (PLL) operating under abnormal grid voltage conditions such as harmonically distorted voltages, grid faults, and weak grid. The proposed control strategy effectively mitigates the harmonics in grid injected current, eliminates the oscillations in grid injected power, and enhances the low-voltage ride-through (LVRT) capability simultaneously during abnormal grid conditions. The LVRT capability of the GTPV system is enhanced by incorporating an active power regulator (APR) with dynamic reactive power support (DRPS) controller in the proposed control strategy. The proposed MOCS is designed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> stationary reference frame. Furthermore, the proposed MOCS consists of an advanced phase compensated multiresonant (APCMR) current controller and a phase-shifted reference current generator (PSRCG) integrated with the APR with DRPS controller. The APCMR suppresses the grid current harmonics under weak and distorted grid voltages and the PSRCG eliminates active and reactive power oscillations at the grid side caused by grid faults. The synchronization mechanism of the GTPV system is included in the PSRCG, and therefore, the requirement of PLL is avoided. Consequently, the associated computational burden on the digital controller is reduced. The effectiveness of the proposed control strategy is validated using the OPAL-RT digital simulator and offers superior performance compared to the conventional control strategy (COCS).

Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.

Virtual synchronous generators (VSG) have the potential to solve the stability problems caused by large-scale grid-connected of distributed generation (DG). However, traditional VSG control will lead to current unbalance and power oscillations under unbalanced conditions. Aiming at these problems, an improved comprehensive control strategy of VSG is proposed based on the traditional VSG control method with voltage and current inner loops. By analyzing the relationship among unbalanced current, power oscillation and negative sequence current, the reference currents under different control targets are obtained. On this basis, the comprehensive control of power fluctuation and current quality is realized by the idea of weighted control. Moreover, the proposed control strategy does not change the voltage source properties of VSG, providing voltage and frequency support for the system, and improving the ability of VSG with unbalanced load under islanded conditions. Simulation results verify the effectiveness of the proposed control strategy.

Active power oscillations (APOs) frequently arise in inverter-dominated power systems with multiple converters operating under virtual synchronous generator (VSG) control, posing risks to system stability and protection coordination. While various mitigation strategies have been proposed, many rely on prior knowledge of system parameters, offer limited damping performance, or involve complex models that lack physical interpretability—making them difficult to apply in practice. To address these challenges, this article first introduces a physically intuitive resistor-inductor-capacitor (RLC) equivalent circuit model to explain the root causes of APOs in both stand-alone (SA) and grid-connected (GC) modes. By mapping inertia, damping, and feeder impedance to capacitive, resistive, and inductive elements, respectively, the model reveals how mismatches among converters lead to interunit oscillations characterized by LC resonance. Building on this insight, we propose two mode-specific mitigation strategies: 1) in SA mode, a graph-theory-based impedance control ensures proportional reactive power sharing and effectively suppresses APOs; and 2) in GC mode, adaptive inertia and damping control with feedforward loop is designed to reshape transient power dynamics while preserving frequency stability. The proposed methods are validated through extensive simulations and real-time hardware-in-the-loop experiments, demonstrating their effectiveness in suppressing oscillations and enhancing the robustness of multiconverter power systems.

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.