Excitation systems simulated and validated by digital twins can help utilities and power generation companies comply with increasingly strict grid codes
As distributed and intermittent energy sources such as wind and solar continue to grow their share in the energy mix alongside new technologies such as electric cars (EVs), storage systems and zero net energy buildings, transmission system operators (TSOs) are reinforcing their grid codes; the rules to which all players that connect power-generating assets to the grid must comply.
The reasons for this are relatively simple and well-understood: renewable energy sources such as wind and solar PV are gradually replacing the traditional centralised model of power generation. Whereby constant and reliable power from large fossil fuel and nuclear power plants is matched to demand profiles that are well established and understood, thus avoiding frequency fluctuations.
Renewables add to the complexity of grid stability and impact security of supply. Without the back up of grid-level energy storage, they are inherently intermittent because the sun doesn’t always shine, and the wind doesn’t always blow. The need to use the maximum amount of renewable power when available means there are strong fluctuations in the generated power in the grid at any one time.
In addition, electricity demand profiles are changing. As more and more EVs, for example, connect to the grid, the demand for power increases at the same time as becoming more distributed and unstable.
Avoiding RoCoF and FRT events
This changing grid profile increases the likelihood of events such as fast rate of change of frequency (RoCoF) and fault-ride through (FRT). RoCoF refers to a loss of a major part of power generation, due to forced unit outages, for example. Traditionally, the resulting system frequency drop is delayed by inertia from large power stations and industrial facilities, allowing time for additional power reserves to re-establish system equilibrium following RoCoF events, and negating the need for load shedding.
Renewables provide no additional inertia, however. Solar is connected to the grid without rotating mass, while the frequency converter connecting a wind turbine to the grid prevents the kinetic energy of the turbine’s rotating mass from providing inertia during periods of frequency change.
TSOs are also strengthening their requirements related to the ability for power generating units to ride through transmission system faults and other disturbances. Fault-ride through (FRT) is key to system security, and to avoiding voltage and frequency collapses, and potential system shut down.
Digital twins and excitation systems
The reasons for changing grid behaviour may be simple, but the resulting challenges faced by power generators are more complex. Operators must now provide detailed, high-fidelity power system simulation reports based on ‘digital twins’, control systems simulation models that are validated through site measurements and certifications. Tests must also increasingly be conducted during the commissioning and operation of major power plant components and control systems.
To summarise, there are two basic elements to the grid code compliance process. In the first step, the certification body will verify that the digital twin is accurately simulating the actual equipment. Second, the TSO will check if the digital twin is then complying with the grid code requirements.
Digital twins allow power generation companies to accurately replicate and simulate plant processes and network behaviour offline. This means that they can optimise their operations, and predict potential spikes and outages, on a digital model while still ensuring security of supply to the actual power grid.
This modelling also extends to the digital twin itself. By simulating the performance of the solution, power operators can demonstrate to TSOs, using transparent data, that they are able to accurately simulate scenarios and failure modes, and that equipment can effectively counteract events that cause network instability. These simulation reports are then used to validate grid code compliance.
For example, ABB Excitation systems damp voltage and power oscillations and control the grid voltage to ensure the supply of electricity to consumers remains constant. In addition, the simulation of the Excitation system adds an extra level of security by demonstrating it is damping these oscillations effectively.
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