Abstract
With some of the intermittent new energy and large nonlinear loads, grid voltage unbalance, harmonics, and frequency deviation are increasing year by year. The voltage source converter (VSC) is seriously affected by the various unexpected factors, and the presence of grid impedance makes the situation worse. In order to make the VSC track the nonideal grid quickly and accurately, this paper proposes a frequency-adaptive grid-virtual-flux synchronization by multiple second-order generalized integrators (MSOGI-GVFS). Key expressions of the MSOGI-GVFS and its frequency response characteristics are described in this paper. A second-order generalized integrator configured as a quadrature signal generator generates a specific-frequency virtual flux. A harmonics decoupling network achieves fundamental and harmonic components of the virtual flux. The positive- and negative-sequence components are separated by multiple positive- and negative-sequence calculators. A frequency-locked loop is used to track the grid angular frequency. Finally, after compensating the voltage and the flux on the grid impedance and the filtering inductor, it accurately achieves the estimation of the grid virtual flux in the highly polluted grid environment. This method may reduce the voltage sensors, eliminate the influence of grid impedance, and track the grid frequency quickly, which contributes to the stability of the VSC. The good performance of MSOGI-GVFS is verified by simulation and experimental results.
Highlights
The voltage source converter (VSC) is emerging as the main topology of modular and standardized gridconnected converter units in a wide power range
The decoupled double synchronous reference frame phase-locked loop (PLL) (DDSRF-PLL) [10] can detect fundamental negative-sequence components or the specified harmonic components, but when the DDSRF-PLL is extended in multiple reference frames, decoupling cells would be very complicated because feedback decoupling of each component uses a rotation transformation
This approach integrates the estimation of the grid virtual flux with its subcomponent extraction by a harmonics decoupling network (HDN) containing SOGIs [22,23] and the multiple positive- and negative-sequence calculator (MPNSC)
Summary
The voltage source converter (VSC) is emerging as the main topology of modular and standardized gridconnected converter units in a wide power range. Rodriguez et al proposed a second-order generalized integrator-frequency-locked loop (SOGI-FLL) that makes improvements from an enhanced phase-locked loop by using the SOGI as the quadrature signal generator (QSG) This approach detects positive- and negative-sequence fundamental components of the grid, but it takes low-order harmonics expansion into consideration [13]. The literature [18] first proposed a method where the low-pass filter is used to obtain virtual-flux-vector magnitude and a slower PLL is used to extract the positive frequency of the fluxes. This paper proposes frequency-adaptive grid-virtual-flux synchronization by multiple second-order generalized integrators (MSOGI-GVFS) This approach integrates the estimation of the grid virtual flux with its subcomponent extraction by a harmonics decoupling network (HDN) containing SOGIs [22,23] and the multiple positive- and negative-sequence calculator (MPNSC). The simulation and experiment results verify the correctness and feasibility of this proposed approach
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