For both grid-following (GFL) and grid-forming (GFM) control, the same hardware structure is used, specifically an IGBT-based two-level voltage-sourced converter (VSC) with a 12 MW rating. This converter is connected to the plant via a 0.69/66 kV transformer, with the grid-side reactance of the LCL filter acting as the step-up transformer.
The converter control is configured to operate in grid-forming mode, using vector control in the dq-reference frame. The control system includes the following components: power calculation, power regulation, voltage regulation, current regulation, and pulse-width modulation (PWM). Additionally, an active damping control utilizing feedback from the series reactor current is considered.
Subsystems of grid-forming converter
Anti-aliasing filtering and sampling: The anti-aliasing filter is a second-order Butterworth filter, with a cut-off frequency set to half the sampling frequency. Sampling delays are approximated using a third-order Padé formula.
Park transformation: Grid-forming control is implemented in a reference frame synchronized to an arbitrary input, provided in per unit (p.u.), where the reference frequency (f*) is set to 1. In practical applications, the phase can be provided by a low-bandwidth phase-locked loop (PLL) that slowly tracks the system frequency.
Power calculation: Instantaneous active and reactive power are calculated based on voltage and current measurements taken at the LCL filter's output.
Power regulation: In GFM mode, power synchronization control generates the reference for the voltage angle, while reactive power control sets the reference for the voltage amplitude. Active and reactive power droop characteristics are combined with a simple low-pass filter.
Voltage regulation: The voltage is controlled through a proportional-integral (PI) regulator, ensuring zero steady-state error. Active damping is also incorporated to enhance noise rejection and system stability.
Current regulation: A proportional (P) controller manages the current, as the integral (I) component is unnecessary; the voltage regulator already handles tracking the voltage reference. Adding complexity to the current regulator may adversely affect converter stability.
PWM: The modulation block calculates the switching functions and sends pulse patterns to the converter's gate drivers. The PWM delay is also accounted for in this block.
Note: The proposed GFM control does not include DC voltage regulation. It is assumed that the GFM converter maintains a stable DC link voltage, such as from a battery energy storage system (BESS), so DC voltage control is unnecessary.
Grid-forming converter parameters
The electrical circuit for GFM unit is assumed to be the same as for GFL converter. The control structure is visualized in figure below.
References
[1] Ł. Kocewiak, Ch. Buchhagen, R. Blasco-Gimenez, J. B. Kwon, M. Larsson, Y. Sun, X. Wang et al., “Multi-frequency stability of converter-based modern power systems,” Technical Brochure 928, Page(s) 1-147, CIGRE, March 2024.
