Inertia and the modern grid: the role of the synchronous condenser in South Africa


  • Traditionally, coal plants in South Africa have provided grid system strength.
  • They rely on the brute force of electromagnetism to couple with the system and produce the necessary inertia to keep the grid stable.
  • Renewable energy technologies such as solar PV, wind, and batteries do not provide grid strength as stand-alone plants (no inertia).
  • The challenge (opportunity) is how to integrate all generation technologies so that reliable power can be delivered cost-effectively.
  • South Africa could deploy synchronous condenser technology to help overcome the problem.

Historically in national power grids, inertia from coal, nuclear, and hydropower generators were abundant as a by-product and thus taken for granted in the planning and operations of the system. But as the grid evolves with increasing penetrations of inverter-based resources—e.g., wind, solar photovoltaics, and battery storage—that do not inherently provide inertia, questions have emerged about the need for inertia and its role in the future grid in South Africa without coal-fired power plants.

A recent survey carried out by the South African Wind Energy Association, the South African Photovoltaic Industry Association SAPVIA and Eskom reveals that 66GW of renewable energy projects are under development which includes 21GW of wind and 7.5GW of wind and battery storage plus 13305MW solar PV and 19572MW of solar PV with battery storage. All projects recorded in the survey are ‘shovel ready’ (construction ready) or close to it. Read more

That is a lot of renewable energy for a transmission infrastructure that has been traditionally designed to manage a steady, continuous stream of reliable power produced from a fleet of coal-fired power stations and one nuclear power plant (all going well).

The challenge is how to integrate intermittent power from renewables into a grid that is designed to manage predominantly coal-fired base load power and, how to manage inertia so that the multiple-generation technologies can work in harmony to deliver reliable power. The problem is not unique to South Africa.

One way is to deploy big batteries. For example, in the Netherlands, Wärtsilä is supplying a 25MW) / 48-megawatt hour (MWh) energy storage system to their client, GIGA Storage BV, to help stabilise the electric grid. The big battery will help to optimise the power system, regulate energy frequency and reliability on the grid, plus improve revenues. Read more

Another option is big, rapid-starting engines that generate power that can regulate grid frequency. Read more

Grid-forming inverters deployed at renewable energy plants can also ensure power is delivered to the grid at the frequency required when it’s needed. Read more

A fourth option is to deploy synchronous condensers which can essentially replicate the physical properties of coal plants without producing any electricity. The system is a DC-excited synchronous motor, whose shaft is not connected to anything but spins freely. Sounds complicated if you are not an electrical engineer like me but it is better explained in what it does. The sole purpose of a synchronous condenser is to adjust conditions (power factor) on the electric power transmission grid. It can store power in the form of kinetic energy and releases it back into electricity within milliseconds. In other words, it can provide the inertia in real time which helps to manage the intermittency of power produced from wind and solar technologies. This ensures that power delivered to the grid is more reliable and more cost-effective.

Unlike big battery and rapid starting engines, synchronous condensers have a long cycle lifetime of up to 20 years or more. They do not degrade and do not require high maintenance costs plus they have a low environmental impact.

A bonus. Retired coal-fired generators at decommissioned plants can be converted to synchronous condensers to support voltage stabilization on the grid for safe, reliable electric power transmission. Read more 

While upfront costs are high because of the expensive metals used in the systems, the value lies in the specific application and the grid stability security provided. Latvia, for example, has ordered three synchronous condensers, with a total cost of 114 million euros, as part of the transmission system infrastructure strengthening projects, which are necessary for the synchronization of the Baltic electricity system with Continental Europe. Read more 

South Africa is facing a massive transmission challenge as its energy transition starts to take shape. Ideally, a mix of technologies can ensure the reliability and security of the grid and an end to blackouts that have plagued the country for over a decade. The grid needs to be modernised to cope with the transition to ensure that when the last coal-fired power station is decommissioned, the national grid will remain resilient. One way of doing this is to include synchronous condensers in the system. They can be deployed in the form of public/private partnerships.

Author: Bryan Groenendaal

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