[Paper Review] Understanding the Nonlinear Behavior and Frequency Stability of a Grid-synchronized VSC Under Grid Voltage Dips

Chen Zhang, Marta Molinas+2 more|arXiv (Cornell University)|Jun 29, 2018

Microgrid Control and Optimization26 references3 citations

TL;DR

This paper investigates the nonlinear dynamics and frequency stability of grid-synchronized voltage source converters (VSCs) during grid voltage dips by decoupling and analyzing the power control loop (PCL) and phase-locked loop (PLL) dynamics separately. It reveals that PLL nonlinearity induces frequency instability under fault conditions, and demonstrates that controller bandwidth and PQ regulation significantly influence transient stability, validated via detailed PSCAD/EMTDC simulations with a switching model.

ABSTRACT

Transients of a grid-synchronized voltage source converter (VSC) are closely related to over- currents and voltages occurred under large disturbances (e.g. a grid fault). Previous analysis in evaluating these transients usually neglect the nonlinear control effects of a VSC (e.g. phase-locked-loop, PLL). Therefore, potential stability issues related with nonlinear dynamics cannot be revealed properly. This work aims to move further in this respect. To better analyze and gain more insights into the nonlinear properties, dynamical analysis of a grid-tied VSC is conducted by parts. Specifically, the nonlinear behaviors of VSC power control loop (PCL) are firstly analyzed, in which the dynamics of PLL are assumed steady. Then, the nonlinear behaviors of PLL-dominant dynamics are further explored in detail, where the PCL is assumed steady. In this case, frequency instability and the mechanisms behind it are revealed. At last, effects of PQ controller regulation as well as controller bandwidth on the frequency stability are discussed. All the analysis and conclusions are verified by time domain simulations in PSCAD/EMTDC, where a switching model of VSC is adopted.

Motivation & Objective

• To address the limitations of linearized models in capturing transient behavior during grid faults.
• To identify and analyze the role of nonlinear control dynamics—particularly PLL—in causing frequency instability under voltage dips.
• To investigate how PQ controller regulation and bandwidth affect the stability of grid-synchronized VSCs during large disturbances.
• To provide a systematic dynamical analysis framework separating PCL and PLL dynamics for deeper insight into instability mechanisms.

Proposed method

• Conducts decoupled dynamical analysis by assuming the PCL is steady while analyzing PLL-dominated dynamics, and vice versa.
• Uses bifurcation analysis and nonlinear system theory to study the emergence of instability in the PLL subsystem.
• Models the VSC using a detailed switching model in PSCAD/EMTDC to simulate real-time fault conditions.
• Analyzes the interaction between the active and reactive power control loops and the PLL under voltage dip scenarios.
• Evaluates the impact of controller bandwidth and PQ regulation on frequency stability through time-domain simulations.
• Validates analytical findings using high-fidelity electromagnetic transients simulation with a full VSC model.

Experimental results

Research questions

• RQ1 What nonlinear behaviors emerge in the VSC's power control loop during grid voltage dips, and how do they affect system stability?
• RQ2 How does the phase-locked loop (PLL) contribute to frequency instability under fault conditions, and what are the underlying nonlinear mechanisms?
• RQ3 What is the influence of PQ controller regulation on the transient frequency response of a grid-synchronized VSC?
• RQ4 How does controller bandwidth affect the stability margins and dynamic performance during voltage dips?

Key findings

• Nonlinear dynamics in the PLL, particularly under voltage dips, can lead to sustained frequency oscillations and instability, even when linear models suggest stability.
• The study reveals that the PLL’s nonlinear behavior is a primary source of frequency instability, with bifurcation points indicating loss of stability under certain fault conditions.
• Controller bandwidth significantly affects the transient response; lower bandwidth increases the risk of instability due to delayed PLL synchronization.
• PQ controller regulation plays a critical role in damping oscillations, and improper tuning can exacerbate frequency instability.
• Time-domain simulations in PSCAD/EMTDC confirm the analytical predictions, showing accurate replication of nonlinear transient behavior including frequency oscillations.
• The decoupled analysis approach successfully isolates and identifies the dominant instability mechanisms, enabling targeted controller design improvements.