PSMA Consulting - Power System Studies - Grid Following and Forming Converters

Voltage Source Converter (VSC) Controls

Depending on the operational modes, the voltage source converter can be classified into grid-following converters (also called grid-feeding) and grid-forming converters.

1. Grid Following Converters

Grid-following converters behave like controlled current source and it follows the reference called voltage angle. The grid-following converter can be represented as an ideal current source in parallel with the high impedance (Z) as shown in Fig. 1. Where, P* and Q* are the active power and reactive power reference setpoint.

Fig. 1. Grid Following Converters.

The grid-following converter works as a current source that injects the active and reactive power to the grid according to the reference current or power setpoint. To inject the active and reactive power into the grid, the converter control should be synchronized with the grid voltage through phase-locked loop (PLL). The phase-locked loop can be used to track the angle of the grid voltage (θ) which will be used for the synchronization. The phase-locked loop does not generate its own reference and it always needs a reference (i.e., voltage angle) to follow.

1.2. Limitations of Grid Following Converters

There are some limitations of using the grid-following converter control and that are given below:

The grid-following converter requires stable voltage reference to synchronize with the grid and hence it cannot achieve 100 % penetration on the grid.
Not suitable for weak networks (Low SCR) or networks without machines.
It cannot be operated in the standalone (or islanding) mode.
It cannot contribute to system inertia.
It cannot provide the black-start capability.

2. Grid Forming Converters

Grid-forming converters behave like a controlled voltage source, and it forms the grid like synchronous generators called Virtual synchronous machine. The main difference between grid-forming and grid-following converters is the synchronization method. In grid-forming control, the synchronization is based on the active power transfer and swing equation. The grid-forming converter can be represented as an ideal voltage source in series with a low impedance (Z) which is similar to the synchronous generators. The simplified representation of the grid-forming converter is shown in Fig. 2. Where, ω* and E* are the frequency and voltage amplitude setpoint. The angle of voltage can be calculated using swing equation as given below in (1) - (3).

Fig. 2. Grid Forming Converters.

where, Pm - Pe= ΔP is the generation load balance.

KE is the kinetic energy/rotational inertia in MWs.

Sn is the nominal apparent power of the machine in VA.

J is the moment of inertia in kg.m2.

H is the inertia constant in s.

The Rate of change of frequency (RoCoF) is depending on power balance and acceleration time constant T (2H). Once the magnitude and angle of voltage are determined, the network can be energized. However, the determination of the angle is based on power synchronization which is quite slow due to the inertia time constant.

2.1. Advantages of Grid Forming Converters

The grid-forming converters are essential to provide the robust dynamic support to the grid in the following aspects:

Operation with weak grid networks (Low short circuit ratio).
To stabilize the frequency and voltage of power grid networks.
Fault ride through capability for a large disturbance event in the grid.
To perform the re-synchronization operation the grid.
To achieve black-start capability.

References

Rocabert, Joan, et al. "Control of power converters in AC microgrids." IEEE transactions on power electronics, 2012.
NERC, Grid Forming Technology Bulk Power System Reliability Considerations, 2021.
Pontus Roos, A Comparison of Grid-Forming and Grid-Following Control of VSCs, 2020.
Habibullah, Mohammad, et al. "On Short Circuit of Grid-Forming Converters Controllers: A glance of the Dynamic Behaviour." IEEE PES Innovative Smart Grid Technologies Conference-Latin America, 2021.