Medium-Voltage Megawatt Power-Electronic-Based Grid Emulators: Testing Capability Requirements and Dynamics Challenges—A Review

Abstract

Power-electronic-based grid emulators are emerging as a promising way to test the grid-code compliance of renewable energy resources. Yet, the ever-increasing power and voltage levels of device under test pose new requirements and challenges to power-electronic-based grid emulators. This paper first gives an overview of testing capability requirements based on the recent grid codes and standards, then discusses the resulted dynamic interactions and solutions for power-electronic-based grid emulators. Perspectives on the open issues and emerging trends of grid emulators are finally shared.

Adaptive Anti-Saturation Control Design of Transformers in Converter-Based Grid Emulators

“…Taking the wind turbine (WT) test as an example, three types of grid faults are to be reproduced, including the balanced fault without phase jump, the balanced fault with phase jump and the line-to-line fault [1]. Fig.…”

“…10 illustrates the relationship among Rvj, trec and t during these three emulated scenarios. In each scenario, the worst-case point is selected by regulating f(D) and θfj [1]. As a result, three conservative adaptive resistors are identified, i.e., 0.37 p.u., 0.64 p.u.…”

“…Transformer saturation is reported as a common issue in medium-voltage megawatt converter-based grid emulators (GEs) when reproducing grid faults [1]- [5]. The step changes with emulated grid voltage may introduce a significant dc magnetic flux in the transformer of GEs, which leads to a dc magnetizing current surge, reaching up to two or even ten times of the rated current [6], [7].…”

“…To address this issue, the anti-saturation control (ASC) methods are commonly used with converter-based GEs [1]. A typical approach is the magnetizing current control (MCC) [8]- [10], which often utilizes a notch filter [10] or a moving average filter (MAF) [8], [9], along with a proportionalintegral (PI) controller to eliminate the dc component of magnetizing current of transformer.…”

Analysis and Mitigation of Transient Overvoltage and Voltage Imbalance in Submodule Capacitors for MMC-Based Grid Emulators

“…Several research studies have thus focused on identifying the maximum and the minimum SM capacitor voltages to properly design the scaling coefficient, as seen in grid-tied MMC [11], [14], [15] and in MMC-based motor drive systems [12], [16]. Differing from those systems, the modulation reference of the MMC-based GE can be close to zero, yet the DUT still continues to inject fault current for low-voltage ride through (LVRT), which worsens SM capacitor overvoltage or undervoltage [2]. By identifying the worst-case SM capacitor voltage, a scaling coefficient is designed in [6] for MMCbased GEs.…”