Subsynchronous Oscillation Analysis of Grid-Connected Converter Based on MIMO Transfer Functions

Xiaofang Wu,Feng Zeng,Zhengchun Du,Guihong Wu,Xiaotian Yuan

doi:10.1109/access.2020.2994219

Xiaofang Wu, Feng Zeng + Show 3 more

Open Access

https://doi.org/10.1109/access.2020.2994219

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Journal: IEEE Access	Publication Date: Jan 1, 2020
Citations: 5	License type: CC BY 4.0

Affiliation: Xi'an Jiaotong University

Abstract

Subsynchronous oscillations (SSOs) induced by the interaction between wind farms based on direct-drive permanent magnet synchronous generators (D-PMSGs) and weak grids have attracted extensive attention. To better understand this SSO phenomenon, this paper derives multi-input multi-output (MIMO) transfer functions between the references of the current controller and the current of a grid-connected converter in the d-q frame, where the dynamic of PLL is considered. The dynamic of PLL lead to positive feedback in the MIMO transfer function, which impairs the stability of the overall system and induces the SSO phenomenon. Using the classical root locus method, the influence of correlated control parameters and different operation conditions on the system stability is substantially analysed. It is found that heavy load, low short circuit ratio (SCR) will intensify the SSO instability of system. Nonlinear electromagnetic transient simulations of a D-PMSG-based weak-grid-connected wind-farm system using PSCAD/EMTDC software effectively verify the correctness of the model proposed in the SSO analysis.

Highlights

The large-scale integration of wind energy into power grids worldwide is highly beneficial for relieving the energy crisis and environmental problems [1]–[3]
This paper studies the subsynchronous oscillations (SSOs) phenomenon by deriving the multi-input multi-output (MIMO) transfer functions shown in Fig. 2, where G(s) is the open-loop transfer function and H(s) is the negative feedback function
The increase in the reference current reduces the stability of the system.The simulation results above agree with the root locus analysis of the MIMO transfer function poles

Summary

INTRODUCTION

The large-scale integration of wind energy into power grids worldwide is highly beneficial for relieving the energy crisis and environmental problems [1]–[3]. Some studies [13], [14] gave the full impedance model of the grid-connected inverter considering the current feedback control and phase lock loop (PLL) dynamic in the frame. Taking into account the control system of the converter and the AC system, the single-input and single-output (SISO) transfer function of the D-PMSG grid-connected system were derived in [18], which provides the reasons for the sub- and super-synchronous frequency components from the stability of the control system. This paper analyses the mechanism of the SSO phenomenon by building a model between the references of the current controller and the current of the grid-connected converter in the d-q frame through multi-inputs multi-outputs (MIMO) transfer functions. It can be seen from (16) and (17) that the MIMO transfer functions are coupled and unsymmetrical when the dynamic of the PLL is considered

MIMO TRANSFER FUNCTIONS WITHOUT CONSIDERING THE PLL

SSOs MECHANISM OF THE D-PMSG GRID-CONNECTED SYSTEM

SIMULATION VERIFICATION

CONCLUSION