Impedance-Based Stability Analysis of Paralleled Grid-Connected Rectifiers: Experimental Case Study in a Data Center

Abstract

Grid-connected systems often consist of several feedback-controlled power-electronics converters that are connected in parallel. Consequently, a number of stability issues arise due to interactions among multiple converter subsystems. Recent studies have presented impedance-based methods to assess the stability of such large systems. However, only few real-life experiences have been previously presented, and practical implementations of impedance-based analysis are rare for large-scale systems that consist of multiple parallel-connected devices. This work presents a case study in which an unstable high-frequency operation, caused by multiple paralleled grid-connected rectifiers, of a 250 kW data center in southern Finland is reported and studied. In addition, the work presents an experimental approach for characterizing and assessing the system stability by using impedance measurements and an aggregated impedance-based analysis. Recently proposed wideband-identification techniques based on binary injection and Fourier methods are applied to obtain the experimental impedance measurements from the input terminals of a single data center rectifier unit. This work provides a practical approach to design and implement the impedance-based stability analysis for a system consisting of multiple paralleled grid-connected converters. It is shown that the applied methods effectively predict the overall system stability and the resonant modes of the system, even with very limited information on the system. The applied methods are versatile, and can be utilized in various grid-connected applications, for example, in adaptive control, system monitoring, and stability analysis.

Identification of Grid Impedance by Broadband Signals in Power Systems with High Harmonics

“…The Maximum-Length Binary Sequence (MLBS) used in this work is a general-purpose signal from the Pseudo Random Binary Sequence group. It has the smallest possible CF making it suitable for sensitive systems [7,24]. It can be generated very easily by feedback shift registers as shown in Figure 3 and offers several degrees of freedom such as selecting the number of bits n of the shift register.…”

Propagation of Supraharmonics in a Data Center With Different Operating Modes of UPS

“…The interferences studied include, audible noise, cable termination failure, RCD tripping and communication failure between devices. Measurements done at the PCC of a new data center installed in Southern Finland showed sustained resonance in the frequency range of 7 to 8 kHz in all the three phase voltages [8]. The supraharmonics can lead to e.g., an increased voltage between PE-N, something that can be an issue in data centers given the type of load that is present there.…”

Hardware-in-the-Loop Methods for Stability Analysis of Multiple Parallel Inverters in Three-Phase AC Systems

“…Usually, the inverters are individually designed based on the stability requirements of the inverter standalone operation. However, the use of multiple inverters in parallel results in dynamic interaction causing system performance degradation or even instability [6], as reported from distribution systems that include photovoltaic plants [7], wind power systems [8], [9], and data centers [10]. Stability issues involved in multi-paralleled grid-connected inverters have been studied extensively in recent years.…”