Deadtime Effect and Impedance Coupling in Dynamic Analysis of Grid-Connected Inverters

Abstract

Grid-connected power-electronic inverters play a crucial role in the transition from the carbon-based energy production to renewable energy production. Because many power generators that utilize renewable power sources cannot be connected directly to the alternating-current distribution grid for power delivery, an inverter is often required as an interface between the renewable energy source and the distribution grid. However, potential dynamic interactions between feedback-controlled power converters and the power grid can lead to stability issues. Detrimental interactions should be prevented in the inverter controller design phase where the interactions can be modeled with equivalent small-signal impedances of the grid and the grid-connected inverters. However, the impedances are usually unknown.

Previous studies have presented methods to measure the terminal impedances in grid-connected-inverter systems for an improved controller design. Recent developments in the impedance measurement have led to broadband measurement methods that can be implemented in a short amount of time using orthogonal binary sequences and with a low computational effort. However, previous non-parametric measurement methods performed with orthogonal sequences have not dealt in depth with a crucial issue of three-phase impedance measurements: distortion between synchronous reference-frame measurement channels.

Conventionally, an impedance that is measured for the control design is assumed to behave linearly in the system operating point where the measurement is performed. However, under low load conditions, a nonlinearity stemming from the deadtime can be significant. The nonlinear deadtime effect adds significant damping, which can lead to a false interpretation of the system stability margins if not modeled correctly.

This thesis presents a novel synchronous-reference-frame impedance measurement method for three-phase grid-connected power-electronic systems. The method makes it possible to measure an equivalent synchronous-reference-frame system impedance within a single measurement cycle, which provides disturbance rejection capability. In addition, a describing function model for the nonlinear deadtime effect is proposed. The model can be used to compute the sinusoidal steady state of an inverter under low load conditions. These methods and models can be applied to the adaptive control, the real-time stability analysis, and the robust control of grid-connected converters.