Three-phase Microgrid Research Articles

Phase unbalance mitigation by three-phase damping voltage-based droop controllers in microgrids

Microgrids aggregate grid assets in geographically confined areas, allowing them to locally tackle grid issues and capture the value of their aggregated flexibility by bringing intelligence in the grid in a bottom-up approach. For the control of distributed generation units in single-phase islanded microgrids, the voltage-based droop (VBD) control strategy has already been developed. The VBD strategy can also be used in grid-connected microgrids, which makes a transition between both modes possible. Phase unbalance is a significant issue in three-phase microgrids and will become more pressing with increasing penetration of single-phase renewables. Therefore, the effect of several traditional control strategies for three-phase distributed generation units to phase unbalance is discussed. Subsequently, in this paper, a new control strategy is presented which extends the basic VBD control strategy so it can be used as a three-phase controller which at the same time mitigates unbalance in islanded and grid-connected microgrids, i.e., the three-phase damping VBD control. It is concluded that the presented three-phase damping VBD control method mitigates unbalance, allows the DG units to share the unbalanced currents, and is able to operate both in grid-connected and islanded systems.

An Enhanced Islanding Microgrid Reactive Power, Imbalance Power, and Harmonic Power Sharing Scheme

To address inaccurate power sharing problems in autonomous islanding microgrids, an enhanced droop control method through online virtual impedance adjustment is proposed. First, a term associated with DG reactive power, imbalance power, or harmonic power is added to the conventional real power-frequency droop control. The transient real power variations caused by this term are captured to realize DG series virtual impedance tuning. With the regulation of DG virtual impedance at fundamental positive sequence, fundamental negative sequence, and harmonic frequencies, an accurate power sharing can be realized at the steady state. In order to activate the compensation scheme in multiple DG units in a synchronized manner, a low-bandwidth communication bus is adopted to send the compensation command from a microgrid central controller to DG unit local controllers, without involving any information from DG unit local controllers. The feasibility of the proposed method is verified by simulated and experimental results from a low-power three-phase microgrid prototype.

Virtual Reactance Implementation Method for Droop-Controlled Three-Phase Microgrid Inverters Using SOGI Scheme

Because the line impedance of low-voltage microgrids is mainly resistive and the transfer impedance of microgrid inverters is usually mismatched, a virtual reactance is usually added to the control loop of each microgrid inverter. It is found that the existing two virtual reactance implementation methods introduce harmonic components into the voltage control units of microgrid inverters in the condition of nonlinear loads after the theory and simulation analyses. To solve the problem, a second-order general-integrator (SOGI) virtual impedance implementation method of droop-controlled three-phase microgrid inverters is proposed. Simulation results are provided to show the feasibility of the proposed method.

Oscillator-Based Inverter Control for Islanded Three-Phase Microgrids

A control scheme is proposed for an islanded low-inertia three-phase inverter-based microgrid with a high penetration of photovoltaic (PV) generation resources. The output of each inverter is programmed to emulate the dynamics of a nonlinear oscillator. The virtual oscillators within each controller are implicitly coupled through the physical electrical network. The asymptotic synchronization of the oscillators can be guaranteed by design, and as a result, a stable power system emerges innately with no communication between the inverters. Time-domain switching-level simulation results for a 45-kW microgrid with 33% PV penetration demonstrate the merits of the proposed technique; in particular they show that the load voltage can be maintained between prescribed bounds in spite of variations in incident irradiance and step changes in the load.

Lyapunov Function-Based Current Controller to Control Active and Reactive Power Flow From a Renewable Energy Source to a Generalized Three-Phase Microgrid System

In this paper, a novel current control technique, implemented in the a-b-c frame, for a three-phase inverter is proposed to control the active and reactive power flow from the renewable energy source to a three-phase generalized microgrid system. The proposed control system not only controls the grid power flow but also reduces the grid current total harmonic distortion in the presence of typical nonlinear loads. The control system shapes the grid current taking into account the grid voltage unbalance, harmonics as well as unbalance in line side inductors. The stability of the control system is ensured by the direct method of Lyapunov. A SRC is also proposed to improve the performance of the current controller by estimating the periodic disturbances of the system. The proposed control system (implemented digitally) provides superior performance over the conventional multiple proportional-integral and proportional-resonant control methods due to the absence of the PARK's transformation blocks as well as phase lock loop requirement in the control structure. A new inverter modeling technique is also presented to take care of unbalances both in grid voltages and line side inductors. Experimental results are provided to show the efficacy of the proposed control system.

Multiloop control method for high-performance microgrid inverter through load voltage and current decoupling with only output voltage feedback

This paper proposes a multiloop controller with voltage differential feedback, and with output voltage decoupling and output current decoupling by only using the output voltage feedback. The output voltage differential feedback loop actively damps the output LC filter resonance and thus increases the system stability margin. The decoupling of output voltage and current makes the inner loop equivalent to a first-order system and thus improves the system dynamic response to load disturbance. The pole placement technique has been used here to design the inner loop and outer loop gain, with considering the effect of system control delay. The proposed control scheme possesses very fast dynamic response at load step change and can also achieve good steady-state performance at both linear and nonlinear loads. In addition, it only uses the output voltage as the feedback variable, which reduces the system complexity. The theoretical conclusion has been verified by simulation and experiment results. This method is proved to be an effective solution for voltage control in stand-alone mode of three-phase microgrid inverters.