Stability and Bandwidth Implications of Digitally Controlled Grid-Connected Parallel Inverters

Abstract

Abstract-The increasing use of grid-connected inverter systems is resulting in a desire for parallel-connected inverters that offer greater power capacity while maintaining the high control bandwidth achieved by individual inverters. This paper demonstrates that, in addition to the traditional stability and bandwidth limitations of digitally controlled inverters, further stability and bandwidth limitations occur when LCL inverters with a common set point are connected in parallel to a grid. This paper provides detailed discrete-time derivations for parallel grid-connected inverters and uncovers stability and bandwidth limitations that only occur in grid-connected applications and are not apparent if the system is studied in continuous time. This paper demonstrates that, in a typical application, the voltage bandwidth of an LCL parallel inverter array is 25% lower than a single module or LC parallel configuration. Both simulations and hardware demonstrations on a 105-kVA parallel three-module grid-connected system confirm the findings.

A review on China׳s large-scale PV integration: Progress, challenges and recommendations

“…What is more, harmonic distortion and stability model of parallel inverters were established based on the digital control system. Turner et al [38] analysed the coupling impedance effect in parallel-connected inverters. The existence of coupling impedance reduces inverter control bandwidth and lowers the stability margin, which results in instability problems such as excessive harmonics.…”

Control of Single-Phase Power Converters Connected to Low-Voltage Distorted Power Systems With Variable Compensation Objectives

“…This is due to the physical structure of the PEC is not altered, requiring changes only in the control system and generation of references for compensation. The possibility of the PEI contributing to the improvement of PQ indicators at the connection point of the PEC is particularly important in small scale distributed generation systems, where the AC bus is not strong [33], the loads are predominantly nonlinear, and the intermittency of distributed energy sources is evident. Furthermore, the line impedance is relevant and the grid voltage may be non-sinusoidal voltage.…”

General Unified Integral Controller With Zero Steady-State Error for Single-Phase Grid-Connected Inverters

“…Traditionally, in a PI control the higher is the control bandwidth, the better are the steady-state and dynamic responses. Nevertheless, in practice the high bandwidth usually leads to instability due to the control delay, especially in digital control applications [27]. A practical way to avoid instability is to keep the control bandwidth below one fifth of the sample frequency.…”