Oscillation Mechanism and Mitigation of MMC-HVDC Based on Bifurcation Theory for Wind Farm Integration

Abstract

Modular multilevel converter based highvoltage DC transmission (MMC-HVDC) technology has become a feasible approach from grid integration of large wind farms. As oscillations has been reported on certain number of MMC-HVDC projects with wind power, the mechanism and analytical methods are urgent issues for studying. The current commonly used modeling methods cannot meet the characteristics of MMC topology due to the complex pulse mode shown in a switching period. Therefore, a new method is proposed to effectively solve the stroboscopic mapping modeling problem of multilevel converters. On the basis of traditional proportional resonant (PR) control, a time-delay feedback control (TDFC) loop is added to effectively suppress the current oscillation. Taking the 21 level MMC topology as an example, the discrete model is obtained by the method of switch state traversal, and the correctness of the theoretical analysis and the proposed strategy of oscillation current suppression is verified by simulation. INDEX TERMS Modular multilevel converter; oscillations; time-delay feedback control; stroboscopic map.

Sub-Synchronous Oscillation Characteristics and Analysis of Direct-Drive Wind Farms With VSC-HVDC Systems

“…(18). To analyze the stability of the two feedback systems, the characteristic equations of ( 17) and ( 18) are derived, as shown in (19) and (20), respectively. Applying the Routh's stability criterion, as the coefficients of ( 19) are not all positive, a root or multiple roots exist, which are imaginary or have positive real parts [46].…”

“…As it can be known from ( 19) and ( 20), the coefficients kp4, kp5, ki4, ki5 of the DC voltage controller in the D-PMSG power generation system affect the two feedback systems at the same time. Therefore, it is difficult to analyze the impact of the DC voltage controller parameters on the overall system from (19) and (20). However, the d-axis voltage controller of the VSC-HVDC only affects the negative feedback system.…”

“…This paper focuses on the SSO characteristics and mechanism seen from the small-signal-stability point of view, and thus, the bifurcation and chaos of the DDWFV [20] are not concerned. Two main methods have been widely used to analyze the power system small-signal stability [21], i.e., the impedance analysis [22] and the eigenvalue method [13].…”

A Sub-Synchronous Oscillation Suppression Strategy Based on Active Disturbance Rejection Control for Renewable Energy Integration System via MMC-HVDC

“…In addition, it has been widely utilized in many practical projects to realize the consumption of renewable energy. However, with the large-scale grid connection of renewable energy, sub-synchronous oscillation (SSO) accidents have occurred many times in practical projects such as Nanao MMC-HVDC project [3] and Zhangbei flexible DC power grid [4,5], which leads to wind turbine disconnection, power quality issues, and so on [6].…”

“…Although adding SSO suppression devices has a good suppression impact and a quick response time, it has poor economics [16]. In terms of additional damping control, reference [3] added an SSO suppression controller to the control system of the WFMMC to suppress the SSO of the output current of the wind farm. Although the approach can significantly increase sub-synchronous damping, it pays little attention to the suppression ability of wind turbine converter.…”