IEC/TR 63401-1 Ed. 1.0 en:2022

As the use of inverter-based RE power generation resources increases, the use of low short circuit ratio AC networks is becoming more common. Considering the advantages of short circuit ratio in stability analysis, the low short circuit ratio is an important indication for describing weak AC networks. This document focuses on technologies and standardization aspects of interconnecting inverter-based resources to low short circuit ratio AC networks. A clear definition of low short circuit ratio AC networks with or without a high proportion of inverter-based resources and the calculation method is described. The adaptability of traditional modelling and analytical method for low short circuit ratio AC networks are discussed. Some new characteristics and challenges will be re-examined, and some adapted control strategies will be studied. This document covers the following major aspects.

In terms of defining a weak AC network, for example the (X/R) ratio, voltage sensitivity, system inertia and the short circuit ratio (SCR) are important characteristics. The definition of low short circuit ratio AC networks in IEEE Std 1204TM-1997 [1]1 and in CIGRE B4.62 TB671 [2] is used. Some stability challenges for inverter-based resources in a low short circuit ratio AC network (SCR AC) will be analyzed. There are stability challenges in a low short circuit ratio (SCR) AC network, typically complex static voltage control, risk of failure in fault ride-through situations, strong control interactions and instability.

In terms of identification of low short circuit ratio (SCR) AC networks, some short circuit ratio - like index for various applications is introduced. A wind power plant (WPP) is a power station consisting of a batch of wind turbines or groups of wind turbines, collection lines, main step-up transformers and other equipment. For a single grid-connected WPP system, a fault current based calculation method and an equivalent circuit based calculation method are introduced to make an SCR calculation possible for any given WPP and network topology. For multi grid-connected WPP systems, eigenvalue decomposition based generalized short circuit ratio (gSCR) is then proposed and compared against other approaches referred to as equivalent short circuit ratio (ESCR), composite short circuit ratio (CSCR), and weighted short circuit ratio (WSCR).

In terms of large scale inverter-based resources integration, the steady-state stability analysis methods, including the P-V curve, Q-V curve, and voltage sensitivity analysis, are illustrated. The conventional control strategies of the renewable energy sources are explained. An adaptive controller designed for the photovoltaic (PV) panels, which can maximize the power output capability of PV stations under weak-grid conditions, is presented. Finally, the steady-state voltage stability problem in China that happened recently is illustrated.

In terms of the transient state stability issue for low short circuit ratio AC networks after large
scale inverter-based resources integration, related issues and phenomena that occur need to
be discussed. Undervoltage ride-through (UVRT), overvoltage ride-through (OVRT) and
multiple fault ride-through occur easily in a low SCR AC network, which bring risk of failure to
fault ride-through. Electromagnetic transient simulations to supplement positive sequence
root-mean-square (RMS) simulations are described, and shortfalls of the RMS models and how
to identify them in simulations are considered.
In terms of the oscillatory stability issue for low short circuit ratio AC networks after large scale inverter-based resources integration, the impedance-based method is used to analyze the system stability. For the inverter modelling, three typical inverter models are established, including: a) only considering the current controller (CC); b) considering CC and phase-locked loop (PLL); c) considering CC, PLL and voltage controller (VC). Relying on the impedance analysis method, the effect of PLL, CC, number of inverters, SCR of AC grid is discussed. Finally, the additional active damping control method is proposed for suppressing the oscillation
phenomenon.

This document discusses the challenges of connecting inverter-based resources to low short circuit ratio AC networks, key technical issues and emerging technologies. There are the steady-state stability issue, transient state stability issue, and oscillatory stability issue, which are the most distinct differences compared to inverter-based resources or traditional generators, and accordingly brings new challenges to operation, control, protection, etc. Therefore, technical solutions are needed. The potential solutions will include new technologies, methods and practices, in order to provide more flexibility and improve the efficiency of power systems. It is expected that this document can also provide guidance for further standardization on relevant issues.