What’s the problem with an electrical grid with a high share of inverter-based renewable energies?
As the popularity of distributed generation via renewable sources grows, grid operators start to reach the limit at which more variable and inverter-based power sources would jeopardize expected power quality and grid stability. If this technical limit is specific to the energy mix of each electrical grid, it seats usually around a maximum of 30% of the energy produced by renewable sources.
Stabilizing frequency and voltages
Two factors are chiefly responsible for stabilizing the frequency and voltages in traditional electrical grids. The first is the very specific magnetic and mechanical inertial behaviour of a rotating synchronous machine, associated with a prime mover, used to form the grid at a power plant. The second is the control mechanisms that govern the rotational speed and regulate output voltage of those “traditional” power generators. Indeed, synchronous generators can balance their active and reactive energy production automatically, even when there is a sudden change in the demand, using grid voltage and frequency as a sort of “free” communication channel. This aptitude corresponds to the spinning reserve or primary reserve.
On the other hand, today’s PV, wind or storage inverters lack both the physical nature of the synchronous machine and the associated power reserve control. Not only may their power output vary due to the variability of the renewable source, but they simply cannot operate without the support of a strong grid forming unit. Grid instability via the lack of frequency and voltage regulation is the inevitable result if this type of distributed generation starts replacing grid forming capable sources.
The regulatory consequences
Are there regulatory consequences of this problem? Yes.
These stability issues give rise to other barriers that further inhibit the integration of renewable and distributed sources in the grid. In the United States, for example, the California Independent System Operator—responsible for overseeing the operation of California's bulk electric power system, transmission lines, and electricity market, generated and transmitted by its member utilities—handles overgeneration situations by reducing or “curtailing” the generation from renewable energy facilities.
For French island territories, an April 2008 decree allows a maximum penetration rate of 30% instantaneous peak power for intermittent renewable energies, in order to limit the influence of load imbalance on the grid.
Further, some regulatory bodies impose penalties in electro-intensive use cases, such as mines, if the grid does not meet certainly reliability levels, making the integration of renewable energy very risky.
Virtual synchronous generation
How can virtual synchronous generation help? By participating with other generators to form a stable grid.
Virtual synchronous generators (VSGs) can deliver the “virtual” rotational inertia and spinning reserves that are missing from renewable and distributed sources of generation. This can be accomplished by combining the renewable source with short-term energy storage (which stores the reserve), and an appropriate inverter control mechanism that very precisely models a synchronous machine, its associated prime mover and controls. The current flowing from VSGs can then stabilize frequency and voltage as we are used to doing with generators, even during transients phases such as motor starts or short circuits. And this works whether the inverter operates alone or in parallel with other synchronous generators.
That newfound stability means that VSGs allow grids to host a larger share of distributed sources of generation. As a result, more renewable DG penetration becomes possible. Thanks to the reserve and power smoothing capabilities provided by the short term storage, we have decoupled the intermittency of the renewable sources from the grid stability constraint. Thus, it is possible to turn on/off fuel based generation following the average renewable production and load profile, without many concerns about grid stability, control architecture, power quality, or even protections. In fact, if coupled with additional energy storage or intelligent load management, virtual generator technology enable grids to not only surpass the 30% renewable penetration level, but smash that figure completely: Even 100% renewable networks are conceivable.
In the end, VSGs allow utilities to seamlessly and progressively integrate renewable into their perimeter, from PV-diesel hybrid generation offering lower fuel consumption up to zero-emission grids using renewable, storage, and load management.
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