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Given that the droop controlled inverters based grid-connected microgrid (DCIGC-MG) presents low equivalent output impedance when connecting to the grid, the grid-connecting current (GCC) is distorted easily by the harmonic voltage components at the point of common coupling (PCC), and results in the increase of THDn of GCC. An active harmonic GCC suppression strategy for DCGC-MG based on hierarchical theory was proposed in this paper. Firstly, the voltage error between bus of DCGC-MG and PCC of grid was transferred to dq frame by the Park transformation. Then, an additional compensator consisting of multiple resonant voltage regulators was added into the secondary control to generate the harmonic voltage reference for inverters in the primary level. PI and multiple resonant controllers were adopted as voltage controller in the original primary level to improve the voltage tracking performance of inverter. As a result, the voltage difference between the PCC and system bus decreased, and the GCC is purified. The proposed control strategy not only reduces the steady-state hormonic current injected into the grid by the microgrid, but also suppressed the harmonic inrush currents when the microgrid connects to the distorted grid for protecting inverter. Finally, the simulation and experimental results from scaled-down laboratory prototype have verified the proposed control strategy.

The principle of injected fundamental and harmonic current of grid-connected inverter with droop controller is analyzed firstly in this paper, and a compound harmonic current suppression strategy of GC-VCI with distorted grid voltage is proposed. A notch filter is firstly designed for the separation of different frequency components of the PCC voltage. Then the fundamental component of the inverter's output is controlled by the droop controller solely. A PIR complex controller is adopted for enhancing the tracking ability for harmonic component. As a result, the difference of harmonic voltage between PCC and the output point of inverter is reduced, and the injected harmonic current is decreased. These theoretical findings are verified through simulations and experiments on the laboratory platform.

This paper addresses the low-frequency relative stability problem of the grid-connected voltage source inverter (VSI) based distributed generation (DG) unit in microgrid, in which the conventional active power-frequency (P-ω) and reactive power-voltage (Q-E) droop controllers are commonly used to share the loads. In the sense of the characteristic equation and eigenvalues, the relative stability and the transient response are remarkably affected by the change of the droop gains and/or stable operating points, resulting in the poor damping and eventually instability. An improved P-ω and Q-E droop scheme based on the static droop characteristics combined with a derivative term is presented to preserve the system stability and obtain an improved damping performance. Subsequently, the dynamic performance of the system can be adjusted without affecting the static droop gains to damp the oscillatory modes of the power sharing controllers. Finally, the desired transient and steady-state responses can be obtained.