Transient Stability Analysis of Grid-Tied Converters Considering PLL’s Nonlinearity

Abstract

Transient instability events of grid-tied converters probably occur while riding through grid faults. During lowvoltage ride through (LVRT) period, seeing from the converter terminal towards the grid, the Thévenin equivalent grid impedance becomes pretty significant, accordingly making the converter terminal voltage highly sensitive to the output current. Under such circumstances, it is challenging for the converter to resynchronize with the grid via a phase-locked loop (PLL). This paper develops a reduced-order nonlinear model to elaborate on the dynamic synchronization characteristic of the converters. By considering the impact of grid impedance and analysing spatial vector tracking relation, resynchronization principle of the converters during LVRT is revealed. Besides, the impacts of circuit parameters and controller parameters, including residual grid voltage, grid impedance, current references, and PLL parameters, on the transient stability of the converters are investigated. The results are verified by simulation and experimental results. Index Terms-Grid fault, low voltage ride through, phase-locked loop, power converter, synchronization, transient stability. I. IntroductIonG RID-TIED power converters are playing increasingly important roles in power systems, along with the rapid development of renewable power generation in recent years. Three-phase voltage source converters (VSCs), as one of the most crucial components of renewable energy generation units, often offer quite different performance from the conventional synchronous generator units, such as outstanding rapidity and controllability, but limited fault ride through capability [1], [2]. Grid codes for renewable power generation, in which various facets of the operating characteristics of grid-tied renewable energy generation units are standardized, have been formulated by independent system operators in order to ensure the power system security and stability [2]-[4]. One of the grid codes, namely low-voltage ride through (LVRT) requirement, requires renewable energy generation units to remain connected to the Manuscript received January 25, 2019.

Grid-Synchronization Stability of Converter-Based Resources—An Overview

“…phase of the PLL. Such an approximation is justified by the fact that the timescale of PLL-induced LOS is much slower than that of the inner current control [53], [54].…”

Validation of Novel PLL-driven PI Control Schemes on Supporting VSIs in Weak AC-Connections

“…Then, it is important to quantify the maximum frequency disturbance that the PLL control can withstand without loss of synchronization. According to reference [28], the angle θ must be less than π/2 during a disturbance, otherwise the operating point enters negative damping zones and loss of synchronization is very likely to occur. In order to evaluate an approximate maximum permitted deviation in frequency, we integrate Equation (15) by considering the maximum θ max = π 2 : (ω 0 − ω PLL ) dt < π 2 .…”

“…Obviously, Equations (28) and (29) indicate that for all x ∈ R 9 , conditions of Theorem 1 are satisfied (for w = 2). Thus, according to Theorem 1, the origin of the unforced closed-loop system is proven to be GES.…”

Transient Stability of Vdc – Q Control-Based PV Generator With Voltage Support Connected to Grid Modelled as Synchronous Machine

“…The transient stability of grid connected converter with Power Synchronization Control (PSC) is analyzed through phase portraits in [7]. The risk of transient stability due to Loss Of Synchronization (LOS) of grid connected VSC with PSC is carried out in [8] considering nonlinear model of PLL. Transient stability analysis of grid connected VSCs during Low Voltage Ride Through (LVRT) is presented in [9].…”