Abstract
Converter–grid interactions tend to bring in frequency-coupled oscillations that deteriorate the grid stability and power quality. The frequency-coupled oscillations are generally characterized by means of multiple-input multiple-output (MIMO) impedance models, which requires using the multivariable control theory to analyze resonances. In this article, instead of the MIMO modeling and analysis, the two-port network theory is employed to integrate the MIMO impedance models into a single-input single-output (SISO) open-loop gain, which is composed by a ratio of two SISO impedances. Thus, the system resonance frequency can be readily identified with Bode plots and the classical Nyquist stability criterion. Case studies in both simulations and experimental tests corroborate the theoretical stability analysis.
Highlights
V OLTAGE source converters (VSCs) have been widely used in the modern power grid for renewable energy generation, flexible power transmission, and energy-efficient power consumption
An important difference between two impedance matrices is that the dq-frame impedance matrices are derived based on the linear time-invariant (LTI) operating points, where the dynamic couplings between different frequencies in the phase domain are hidden in the dq-frame [6]–[8], whereas the αβ-frame impedance matrices are essentially developed based on the linear time-periodic operating trajectories [2], [10], [11], which enables to directly capture the frequency-coupling dynamics
We present first a general two-port network representation of grid-connected VSCs based on the multiple-input multiple-output (MIMO) impedance matrices, and elaborates the principle of deriving the common singleinput single-output (SISO) open-loop gain from the two-port network
Summary
V OLTAGE source converters (VSCs) have been widely used in the modern power grid for renewable energy generation, flexible power transmission, and energy-efficient power consumption. An important difference between two impedance matrices is that the dq-frame impedance matrices are derived based on the linear time-invariant (LTI) operating points, where the dynamic couplings between different frequencies in the phase domain are hidden in the dq-frame [6]–[8], whereas the αβ-frame impedance matrices are essentially developed based on the linear time-periodic operating trajectories [2], [10], [11], which enables to directly capture the frequency-coupling dynamics Both impedance matrices are MIMO systems, which require using the generalized (multivariable) Nyquist stability criterion to predict the system stability, and the Bode plots of the eigenvalues of the MIMO return-ratio matrix were drawn to identify resonance frequencies of the marginally stable system, yet they provide little insight into how the grid impedance affect the system resonance frequencies in [12]. Only the equivalent admittances seen from the terminals is required, and the measurements of the entire MIMO impedance matrices are avoided [16], which significantly facilitates the stability analysis and the resonance frequencies caused by the asymmetric dq-frame control dynamics can be readily identified with the Bode plots of SISO impedance ratios. Simulations and experimental tests validate the effectiveness of the proposed stability analysis method
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