Online Grid Impedance Measurement Using Discrete-Interval Binary Sequence Injection

Grid impedance affects the stability and control performance of grid-connected power electronic devices, such as inverters used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Such online measurement can be performed by injecting a current perturbation from the inverter into the grid and by reading the grid voltage responses. To minimize the impact on the inverter operation, the injection must be kept as small as possible while producing enough voltage perturbation that can be reliably measured and processed to extract its various frequency components. This paper proposes the use of a discrete-interval binary sequence (DIBS) for this application to minimize the injection. The DIBS is a computer-optimized binary sequence, where the energy is maximized at specified harmonic frequencies based on the expected grid-impedance characteristics. Experimental results based on a three-phase grid-connected inverter are presented to demonstrate the effectiveness of the proposed methods.

I. Introduction

The characteristics of the grid, especially its impedance, have major effects on the operation and control of grid-connected power electronic devices, such as wind and solar inverters [1]. Ideally, the grid should behave like an ideal voltage source and the inverter should be controlled as an ideal current source to avoid any interconnect problems. In practice, however, this ideal condition never exists and stability of the inverter-grid system requires that the ratio of the grid impedance to the inverter output impedance satisfies the Nyquist criterion [2]. One particular problem that has been discussed extensively in [2] and [3] is harmonic resonance between the inverter and the grid. Since the grid impedance varies from location to location as well as over time, one effective solution is to adaptively change the inverter control based on the impedance of the grid it connects to [4]. Such adaptive control requires real-time measurement of the grid impedance, which is the subject of this paper.