With the rising share of renewable energy, grid-forming converters are now key in national grid codes. They emulate voltage source behavior even in weak grids or without traditional synchronous machine support, while also providing inertia and short-circuit current. New grid codes demand that these converters maintain voltage source characteristics, supply instantaneous inertia support, offer robust short-circuit current, and withstand phase jump disturbances.

Technically, the goal is for converters to mimic synchronous generators as closely as possible—a concept originally suggested by “virtual synchronous machine.” However, a major challenge is achieving sufficient overcurrent capability, which requires notable hardware enhancements in two main areas:

1. Power Semiconductor Modules: These must handle large, instantaneous overcurrents and thermal stresses. Solutions include paralleling devices or using higher-specification semiconductors.
2. Filter Inductors and Magnetic Components: These elements need an increased magnetic flux capacity to prevent core saturation and thermal damage during high-current transients.

These upgrades lead to higher costs, larger sizes, and more complex system designs. Insufficient overcurrent capability can jeopardize voltage support during grid disturbances, risk synchronous stability during phase jumps or frequency fluctuations, and limit fault response in short-circuit events—posing challenges for relay protection and fault detection.

Thus, determining the necessary overcurrent capability for grid-forming converters is crucial. This issue affects not only the technical feasibility and safety of converter technology but also equipment investment, operating costs, and the overall reliability and economic efficiency of power systems. Grid operators must ensure stable voltage characteristics, adequate short-circuit current, and synchronism, while converter owners need to balance enhanced overcurrent capability with investment control to benefit from services like inertia support.

To address this issue comprehensively, the paper discusses:

1. Grid Characteristics and Technical Requirements: Analyzing the short-circuit ratio (SCR), the grid impedance X/R ratio, and the diverse overcurrent capability requirements in various national grid codes.
2. Specific Grid Topologies and Fault Scenarios: Examining the challenges posed by instantaneous high currents in typical fault scenarios, such as ground faults.
3. Economic Considerations and Market Returns: Evaluating hardware costs, energy consumption, and maintenance expenses associated with enhancing overcurrent capability, alongside the compensation from providing inertia and rapid frequency response services. This analysis weighs the trade-offs between investment and operating costs.