Synchronous grid forming functionality in modular multi-level converters: design and control requirements

Abstract

This research evaluates the physical limitations on the application of synchronous grid forming control within the present high voltage direct current modular multi-level converters control frameworks. The findings support the viability of implementing synchronous grid forming in modular multi-level converters, but also highlight some limits in the operating points before a frequency event.

The ongoing shift in energy sources has reduced power system stability, necessitating new converter control methods to contribute to system stability as the power system’s inherent inertia decreases. The modular multi-level converter is particularly suitable for this role due to its ability to generate high-quality power and rapidly change its operating mode. In fact, to effectively implement synchronous grid forming, the converter must have the ability to rapidly increase current injection into the AC network. However, power electronic devices are highly sensitive to overcurrents. Additionally, a rapid current increase necessitates an energy source to draw power from. These two limitations question the feasibility of using existing modular multi-level converters, which were not originally designed for synchronous grid forming.

Three investigations have been conducted to assess the synchronous grid forming capabilities of existing modular multi-level converters. A synchronous grid forming controller was modified for application in an modular multi-level converter to collect the results of the initial study, serving as a benchmark. A series of operating points and steps in the grid frequency were simulated to determine the operating area of the converter with respect to (i) arm current magnitude, (ii) the limits in voltage generation from the stacks, and (iii) the energy excursion of the SM capacitors. Of the limits considered, the arm current limit was the most stringent.

One of the core aspects investigated lies in the sourcing of energy required for the synchronous grid forming response to be supplied, whether this energy comes from the DC link or from the charged SM capacitors. The balance of energy draw from either of these two sources was achieved through a controlled delay in the ramping up of the DC current; effectively decoupling the AC and DC power flows with the SM capacitors acting as energy buffers.

The findings support the concept of implementing control synchronous grid forming in modular multi-level converters in some limited circumstances. They indicate that the energy stored in the converter capacitors may be sufficient to support synchronous grid forming for short periods of time, however, it is also shown that this would be significantly more effective in converters operating at reduced power flow before any fault occurs. It was found that in the created test environment, the converter when it already injected 1 pu active power into the AC network could provide a synchronous grid forming response to a frequency step of ¡0.2 Hz. When the converter did not inject any active power prior to fault, it was able to respond to a step in the frequency of the grid of ¡1.5 Hz.