The advent of large scale renewable generation, mainly solar and wind (inverter-based resources), has created a number of discussions both locally and internationally especially with regards to the variability of generation resulting from ambient conditions, as well as the lack of inertia present with inverter-based resources.
With the increased deployment of inverter-based resources, synchronous resources are required to contribute a higher level of ancillary service than originally anticipated. This often comes at a cost to the synchronous resource, either in terms of life of plant or cost of operation.
These costs need to be recovered (e.g. by an ancillary service market) to ensure continued supply of these services. Although certain non-synchronous ancillary services can be provided via flywheels (for frequency control), batteries provide more potential services and show promise for larger deployment in future.
There is no market or provision in South Africa for synchronous generation to recover costs in provision of these services. This leads to the reluctance of utilities to run generators in a mode that provides these services, especially if energy is sourced from renewable resources.
Internationally, market design has moved from energy pricing to capital markets (encouraging the construction of base-load generation) as well as ancillary markets. These designs are continually evolving and a monitoring body is required to adjust and control the use of markets.
The favourable subsidies that enabled the inverter-based resources to develop had the effect of reducing the viability of synchronous resources. This in turn limits the availability of ancillary services necessary for the running of the grid and increases the risk of failure to supply.
For example, it is known that a small percentage of instantaneous penetration of inverter-based resources (under 20%) could cause frequency deviations outside those permitted by the South African grid code.
Another example is the under-frequency load-shedding protection which was designed for synchronous generation sources. Under fault conditions with large-scale instantaneous inverter-based resources providing energy, the rate of change of frequency (ROCOF) may exceed the capability of the protection system to detect and operate. This could lead to the sort of black-outs experienced in South Australia recently.
When introducing inverter-based generation the full grid requirement for energy and ancillary services needs to be taken into account. To force only inverter-based resources without methods to provide and compensate for other necessary services may lead to unacceptable network performance.
In addition, the protection systems currently in use as well as the monitoring of data, analytics and provision of the correct information to operators need to be assessed and decided on. Additional funding is required to provide these systems, tools and skills. Without markets these costs need to be agreed to via the regulatory process.
In costing the resource it is important to include full cost of supply including additional ancillary service support if required and not just the energy cost. This will ensure maximum possible penetration of inverter-based resources with the ability to operate the grid reliably.
A number of organisations, such as the Global Energy Interconnection Development and Cooperation Organisation (GEIDCO) together with Cigré and other institutions, are investigating the possibility of a global grid connecting renewable resources from areas blessed with these resources to countries without them. The reasoning is that with a global grid, wind and solar will always be available at some point on Earth enabling provision of energy to all on a reliable basis. Ancillary services could be provided via large hydro resources such as in the DRC.
Politics may decide the outcome but it is argued that if the countries providing the routes can obtain benefit by local electrification and revenue, it may be possible. Technically it appears such a grid could be a possibility.
The introduction of inverter-based resources needs to be looked at holistically with the full range of services that need to be provided. The mechanisms and costs for provision of these services needs to be decided up front and agreed to. This will ensure a balanced sustainable introduction of these resources to the grid. In addition, we need to be aware of the developments of the global grid which would alter the electricity supply landscape immeasurably.
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