The current trend towards inverter-based power supplies, including renewables, batteries and other solutions, is changing the role of power electronics in the grid. As these technologies differ from traditional synchronous generators in that they are not physically synchronized to the grid, new challenges arise. Grid-forming is the answer, increasing grid stability and security of supply by providing flexible and resilient solutions to grid disturbances.
The ever-changing grid is currently shifting towards distributed generation and the implementation of a growing number of inverter-based power plants, including wind turbines, photovoltaic (PV) arrays and batteries. Due to the increasing number of installations, an increase in the occurrence of interaction problems can be expected. It also leads to a reduction in synchronous machines, which weakens the grid and increases the risk of transient voltage instability and converter instability in grid-following systems. Better controls and parameter tuning can reduce these risks, but there is a limit to how far they can go. In addition, the restoration of meshed networks is becoming increasingly important. This leads to the following challenges:
Staying below allowed voltage fluctuations
Power system equipment and loads require regulated voltage for proper operation.
Staying below critical frequency levels
Frequency drift caused by fluctuations in generation or load must be mitigated.
Lack of inertia
All generation resources must synchronize and remain synchronized through network disturbances.
Emergency reaction
System restoration, process of black starting and restoring the power system following a major outage.
Most power electronic systems today use grid-following (GFL) inverter controls. Due to their widespread use and growing installed capacity, it is important to understand the characteristics, dynamic behavior and potential contribution to grid reliability of these inverters. Both grid-forming and grid-following controls aim to supply active and reactive power to the grid. However, their behavior during and after system disturbances differs.
Grid-forming
Grid-following
Grid-forming assets
Portfolio element
TSO description |
HVDC converter stations |
E-STATCOM |
STATCOM |
Topology | Double star / B6 | Double star / B6 + supercapacitors | Delta |
High-level description | Symmetric monopole & bipole solutions for a broad range of power transmission requirements | Superior scalability for energy storage capabilities | Flexible arrangement and scalability of SVC PLUS® branches |
Grid-forming capabilities | Inherent and instantaneous response to voltage disturbances in the AC grid | Inherent and instantaneous response to voltage disturbances in the AC grid | Inherent and instantaneous response to voltage disturbances in the AC grid |
| Stabilizing the AC grid voltage also at very low short-circuit ratio | Stabilizing the AC grid voltage also at very low short-circuit ratio | Stabilizing the AC grid voltage also at very low short-circuit ratio |
| Supporting renewable energy sources and inverter based ressources (IBRs) | Supporting renewable energy sources and inverter based ressources (IBRs) | Supporting renewable energy sources and inverter based ressources (IBRs) |
| Inertial response to frequency disturbances in the AC grid | Instantaneous response to frequency disturbances in the AC grid | - |
| Inherent damping of low frequency oscillations and harmonics, and selective damping of individual harmonic frequency in low frequency range possible | Inherent damping of low frequency oscillations and harmonics | Inherent damping of low frequency oscillations and harmonics |
| Provision of fine-tuned inertial response through power transmission capabilities – response according to system dynamics & selected inertia constant | Provision of powerful and adjustable instantaneous inertial response for frequency support | - |
Inherent active power support | Yes (continuous) | Yes (for several seconds) | No |
Dedicated energy storage | No | Yes | No |
Description | Energy for inherent response & inertia contribution is drawn from the DC cables & DC transmission lines and the connected AC or DC grids – respecting the overall system dynamics and selected inertia constant | Energy for inherent response & inertia contribution is drawn instantaneously from the DC connected supercapacitors, the solution is unrivaled in terms of instantaneous inertia | - |
