State of the Art of the Techniques for Grid Forming Inverters to Solve the Challenges of Renewable Rich Power Grids

Abstract

To mitigate the fast-growing demand of electrical energy, the use of renewable energy resources, e.g., solar and wind, can offer an environmentally friendly and sustainable solution. Due to their intermittent nature, the grid connected operation of renewable energy resources provides a better performance compared to the standalone operation. However, the massive penetration of power electronic inverter/converter-interfaced renewable resources in power systems introduces new issues, such as voltage and frequency instabilities, because of their inherent low inertia properties. As a consequence, these issues may lead to serious problems, such as system blackouts. Therefore, there is an immediate demand to solve these new issues and ensure the normal performance of the power system with the large penetration of renewable energy resources. To achieve this, grid connected inverters/converters are designed to address these problems and behave as synchronous generators, which is possible with grid forming (GFM) inverters/converters concepts. This paper reviews the recent advancement of GFM converters for solving emerging issues related to the renewable rich power grids. It also provides a comprehensive review on frequency deviations and power system stability issues in low-inertia systems and recent advancements in control methods for harmonic mitigation. It is expected that this paper will help the research community to enhance the technology further to solve the challenges in renewable rich power grids.

Transformer Inrush Current Mitigation Techniques for Grid-Forming Inverters Dominated Grids

“…4a that a common minimum error point exists for this combination around 𝜃 𝐶𝐵 = 210° with (𝜀 𝜙𝑚𝑎𝑥 @ 𝛼 𝐶𝐵 ) ≈ 0.065 𝑝𝑢 in phase A and C. Closing the three-pole CB at this instant should generate a minimum inrush current in all phases. On the other hand, a different residual flux combination (e.g., zeros in all phasesdemagnetized core) is observed to not have a global minimum error points when applying (7) as illustrated in Fig. 4b, and a common angle where the three phases errors 𝜀 𝜙 have near zero values is missing since (𝜀 𝜙𝑚𝑎𝑥 @ 𝛼 𝐶𝐵 ) ≈ 0.86 𝑝𝑢.…”

“…That is to say, the same error in (6) can result into two different per unit inrush current magnitudes for two different transformer types. Equation (7) presents a mathematical condition that is derived to approximate the optimal CB closing instant 𝛼 𝐶𝐵 (defined with respect to phase A voltage waveform), based on minimizing simultaneously the errors in each of the three phases. An additional constraint can be added to limit the application to instances where the maximum flux error is ceilinged to avoid excessive inrush.…”

“…Though, the presented analysis does not take the residual flux (𝜙 𝑟 ) impact into account. 1 Some transformer energization techniques require direct modification to the inverter control to be implemented such as soft energization (SE) through a voltage ramp [7], whereas other techniques require first establishing a 1 pu voltage at the IBR output, followed by inrush mitigation by controlling the circuit breaker (CB) closing instant through controlled switching (CS) [8,9]. Soft energization from IBRs is recently attracting more industrial and research focus since it exploits the inverters voltage control flexibility.…”

Voltage dip propagation in renewable‐rich power systems utilizing grid‐forming converters

“…Consequently, the voltage support task during the faults is going to deteriorate in high penetration scenarios of GFL-based RESs and therefore enhanced technologies or other mitigating solutions might be essentially required to avoid wide scale voltage dip propagation through the networks [9,10]. In this regard, Grid-Forming (GFM) converters have been recently proposed as a promising substitution for the lack of functionalities that SGs have traditionally provided as an alternative for GFL converters that fail to do so [8,11]. Such converters, which act as a controllable voltage source and produce voltage and frequency in the system, are a dependable substitute for grid following converters [11].…”

“…In this regard, Grid-Forming (GFM) converters have been recently proposed as a promising substitution for the lack of functionalities that SGs have traditionally provided as an alternative for GFL converters that fail to do so [8,11]. Such converters, which act as a controllable voltage source and produce voltage and frequency in the system, are a dependable substitute for grid following converters [11]. In the grid-forming mode, the converter is robustly synchronized to the grid, and in the event of any disruption, the issue is immediately resolved without the need for a phase-locked loop (PLL).…”