There has been increasing focus on grid-forming inverter-based resources (IBRs) in the power systems community as we transition to a more sustainable modern grid. New sample simulation cases in the RSCAD FX software make it very straightforward for RTDS Simulator users to perform real-time simulations and hardware-in-the-loop testing involving grid-forming control.
Based on NERC’s definition, grid-forming IBRs act as a controllable voltage source behind an impedance, capable of holding an internal voltage phasor nearly unchanged during the sub-transient to transient time interval. Grid-forming IBRs have many important features including the ability to achieve synchronization without a PLL, black start, and weak system operation. Unlike conventional grid-following converters, the main responsibility of grid-forming (GFM) converters is to maintain the system voltage and frequency.
Many grid-forming control strategies have been proposed in recent years. In general, GFM controls include droop characteristics and are capable of operating autonomously with other active voltage sources, such as synchronous generators or other GFM IBRs. In most cases, they are adapted into one of the two common structures, single-loop or multi-loop, as shown in the figure below.
The outer loop is where grid-forming control is implemented; it produces the phase angle and voltage magnitude. In the single-loop structure, the outputs of the outer loop are directly used to produce PWM switching pulses. In the multi-loop structure, the outputs of the outer loop are fed into a cascaded voltage and current loop.
There are new sample cases available with RSCAD FX Version 2.2 that contain generic GFM models with different control methods:
- Droop control
- Virtual synchronous generator
- Synchronverter
These sample cases can be easily modified to suit different projects, including those with a different control structure. The generic GFM voltage source converters are modelled with the RTDS Simulator’s Universal Converter Model with Improved Firing Pulse input. This model allows for the representation of detailed switching behaviours with a relatively large timestep (and therefore a lighter computational load). Using the UCM in the Mainstep environment with Improved Firing Input would allow users with an entry-level simulation hardware configuration to represent several GFMs in detail simultaneously.
Our generic GFM models include the following features:
- Cascaded control structures as shown in Figure 1
- Per-unitized system and control parameters
- Current limiting strategy
- Convenient modification of ratings and scale / duplication of components
- Suitable for both islanded condition and grid-connected operation
- Black start and resynchronization controls with synchro-check relay
RSCAD FX’s Frequency Scanning Tool was used to conduct a detailed stability analysis on these cases to ensure that the generic GFM models operate under both weak and stiff grid conditions. Download RSCAD FX Version 2.2 to access these exciting new sample cases.
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