1. Grid forming
Grid-forming converters can be represented as an ideal ac voltage source with a low-output impedance, setting the voltage amplitude V∗ and frequency ω∗ of the local grid by using a proper control loop.
2. Grid following/feeding
Grid following converters can be represented as an ideal current source connected to the grid in parallel with high impedance. Grid following converters achieves their purposes of power injection by controlling the injected currents.
3. Grid supporting
Grid supporting converters regulate their output current/voltage to keep the value of the grid frequency and voltage amplitude close to their rated values.
Grid supporting grid forming(voltage source grid supporting)
Grid supporting grid following(current source grid supporting)
•If the inverter control structure directly incorporates a voltage tracker/regulator and its frequency is auto-generated (not sensed from the bus), then it's a grid-forming inverter.
The external loop is in charge of regulating the output voltage. In this loop, the error between the reference and the measured voltage is the input to a controller whose output establishes the current reference i∗ to be injected by the converter.
It should be pointed out that the voltage control loop of the grid-forming power converter will be enabled only when the microgrid is disconnected from the main network and works in the island mode. On the other hand, the inner current loop regulates the current supplied by the power converter, tracking the reference current provided by the outer voltage loop.
•If voltage tracker is not employed and frequency reference is obtained from sensed/measured/estimated frequency locked to the corresponding bus by a mechanism like PLL or FLL, then it's grid following.
The inner controller of grid-feeding power converters lays on fast current control loops that regulate the current injected into the grid. The external controller sets the reference current to regulate the power delivered to the grid.
The reference currents [i∗d, i∗q] are usually provided by a power controller that regulates the active and reactive power delivered to the grid. The instantaneous active and reactive power components are calculated by:
•Grid support is an "additional" feature, a grid supporting inverter can be of both types: grid forming or following. If the references/set-points of the inverter (set-points include voltage, frequency, active/reactive power, etc.) are adjusted based on"additional" inputs from the information of other buses in the network, for serving extra ancillary features (apart from local v/f regulation and power injection), then the inverter is said to "support the grid". The additional support includes power quality enhancement, stability support, reserve provision, event ride-through, economic dispatch, etc.
grid-supporting grid-forming converter
This kind of converter can participate in regulating the amplitude and the frequency of the grid voltage, with no need of connecting any grid-forming converter in the grid.
This converter can participate in regulating the voltage amplitude and frequency of both the ac grid and the converter.
J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodriguez, “Control of power converters in AC microgrids,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4734–4749, 2012.
