A PLL-Free Grid-Forming Control With Decoupled Functionalities for High-Power Transmission System Applications

Abstract

This paper proposes a grid-forming control strategy with a generic implementation approach to meet the transmission system requirements asking for the massive integration of power electronic devices into the power systems. In this context, several grid-forming controls have been proposed in the literature either with or without a Phase-Locked Loop (PLL). The PLL-based techniques allow decoupling the different control functionalities (i.e. inertia emulation, frequency support, active power setpoint tracking in steady-state) while the PLL-free schemes, which aim to avoid the PLL drawbacks, create a compulsory coupling between the control functionalities. The proposed grid-forming control in this paper is able to decouple the control functionalities without any dedicated PLL, which makes it more advantageous compared to what have been already proposed in the literature. Since the power converters are exposed to the small and large grid events, the presented control has been tested in both situations. For the small grid events, a simplified small-signal model is developed to assess the active power and frequency dynamics. In case of large grid events, a current limitation algorithm is included to the control in order to protect the power converter. To deal with the transient stability issues linked to the current limitation and enhance the converter performance during the post-fault, a method based on adaptive inertia constant is proposed. To validate the overall approach, time-domain simulations (in Matlab-Simulink) and experimentations are performed. INDEX TERMS Active power regulation, current limitation, fast frequency response, grid-forming control, inertial effect, transient stability. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2020.3034149, IEEE Access T. Qoria et al.: Preparation of Papers for IEEE TRANSACTIONS and JOURNALS Rc, Lc i gabc v gabc Rg, Lg

Grid Forming Inverter Modeling, Control, and Applications

“…Typical GFMIs are capable of providing various control functionalities such as virtual-inertia provision, active power reference tracking, and frequency support in the steady-state. In [48], a PLL-free APC design is proposed to decouple different control functionalities of the GFMIs. Such a decoupling of control functionalities is beneficial for the GFMI owner so that they can opt out from frequency support.…”

New Fault Detection Algorithm for an Improved Dual VSM Control Structure With FRT Capability

“…In this study, the grid is represented by a Thevenin equivalent as standard model used to assess the performance of VSC-FRT [15,16,23,34,42]…”

“…The second approach, which is adopted in this article, is to enhance the control structure, which can enable the FRT capability with lower cost as the same hardware is used. For the FRT capability enhancement of a VSC controlled as a VSM, the researchers tend to build a VSM structure with an inner positive sequence Current Control (CC) loop [13][14][15][16][17][18][19][20][21][22][23], or positive and negative (pn) sequence controllers [18,24]. In addition to the use of the current loop, some ancillary controllers have been used to enhance the FRT performance such as virtual impedance [13,15,17,19,20,[25][26][27], current reference saturation [18,[28][29][30][31], and some structures merging both techniques [14,16].…”

“…For the FRT capability enhancement of a VSC controlled as a VSM, the researchers tend to build a VSM structure with an inner positive sequence Current Control (CC) loop [13][14][15][16][17][18][19][20][21][22][23], or positive and negative (pn) sequence controllers [18,24]. In addition to the use of the current loop, some ancillary controllers have been used to enhance the FRT performance such as virtual impedance [13,15,17,19,20,[25][26][27], current reference saturation [18,[28][29][30][31], and some structures merging both techniques [14,16]. However, some authors reported instabilities regarding the virtual impedance [28,29], and the current reference saturation [20,32].…”

Grid-Forming Control Suitable for Large Power Transmission System Applications

“…Several grid-forming variants have been proposed in the literature. These solutions adopt more or less the idea of mimicking the swing equation of a synchronous machine (SM) [3]- [8] to generate the control angle. Regarding the AC voltage magnitude, it is managed by a lower control level (i.e., inner control).…”