The grid conditions undergo continuous transformations due to increasing penetration of renewable distributed energy resources (DER). In future grid conditions will be significantly influenced by power generating converters which may potentially compromise the stability of electrical grid. To mitigate that, the power generating converter control strategies must transition from grid following approaches to grid forming control. This shift necessitated the development of new test methods, with emulation of instantaneous changing of grid conditions in real time, in order to validate the correct operation and robustness of grid forming converter (GFC) and their control system.

This research investigates a new operational mode of multi-megawatt power range Power Electronic Grid Simulator (PEGS) based on a modular Medium Voltage converter platform. In addition to the standard functions (high-performance voltage source, controllable grid interface), a current source mode has been enabled. This new operational mode allows the precise controllable injection of current into equipment under-test.

Control platform of PEGS is extended with Auxiliary Application controller based on real-time simulator. This extension leverages a fast real-time interface to an advanced modulator, and realizes application control functions such as:

- synchronization to the external grid;

- closed loop current control;

- reference model.

Consequently, the new operational mode extends the test coverage of grid-forming renewable technologies, particularly by enabling the emulation of active and passive loads, including asymmetrical and unbalanced scenarios.

The primary focus of standard validation tests performed over GFC, is the assessment of operational stability under power flow transients, which are triggered by instantaneous change of grid conditions. These tests are usually simulated with artificial grid, created by grid emulator operating in voltage source mode, allowing controllable variation of voltage parameters.

Another group of tests, focused on assessing GFC operational stability under islanding conditions, requires the disconnection of the GFC from the test grid during the testing procedure. These tests are seen as the potential application for the PEGS current source mode: by integrating PEGS into the test setup it can function as a load emulator, in contrast to its disconnection from the test setup. To facilitate the emulation of various test setup configurations at different stages, it is essential for PEGS to be able to switch instantaneously and seamlessly between voltage and current source modes.

The following new functions of PEGS were investigated through real-time simulation sessions on a controller hardware-in-the-loop stand:

- performance of closed-loop current control in current source mode;

- reference trajectory tracking;

- transients at instantaneous switching between voltage and current source modes.

Real-time simulation results demonstrate a close similarity of the processes observed in the system with the load emulator and those in the idealized system with a variable load. Although the emulator may not fully replicate the realistic load behavior across the entire frequency range, it offers the advantage of replacing a test load array with the single emulating converter. As a result, this solution presents a viable alternative to traditional test setups, allowing a significant extension in the range and diversity of GFC conformity tests.