| Grid stability
a zero-carbon way of stabilising the grid
Green idling:
The concept of green idling envisages running a green-hydrogen-fuelled gas turbine at full speed, to contribute to grid stability, but without power generation, ie, in ‘idle’ mode. As well as providing grid ancillary services (inertia, frequency stabilisation, voltage control and short circuit capacity) without emitting CO2
,
and at low cost, green idling would also avoid frequent gas turbine starts and stops, with the machine kept hot and spinning, contributing to reduced maintenance costs
Anders Stuxberg Plant Integration Specialist at Siemens Energy
When renewable power meets most of the demand and the residual load (ie, load not met by renewables) is near zero, grid stabilisation services become crucial. Stopping conventional generators typically reduces inertia and increases the need for frequency stabilisation. Traditionally, keeping power plants running at low load addresses this, but that leads to inefficiency and high emissions. An emerging solution that’s gaining attention is the installation of battery systems in conjunction with synchronous condensers. The battery system stabilises frequency, while the synchronous condenser offers inertia, short-circuit capacity, and voltage control. A synchronous condenser operates as a generator synchronised with the grid, but it doesn’t have driving machinery. Instead, it relies on the rotating mass of the generator rotor, which may be combined with a flywheel. A generator connected to a gas turbine (GT) can function as a synchronous condenser when the turbine isn’t operating provided that a clutch is installed between the gas turbine and the generator, with an optional flywheel attached to the generator shaft. This concept requires a battery system for frequency stabilisation.
Another potential method involves operating a gas turbine at full speed and zero power generation (idle), but, instead of using conventional fuel, renewable hydrogen is employed, produced via electrolysis from electricity sourced from the grid at the same time as it’s combusted. As a result, the GT operates on renewable electricity, resulting in zero carbon emissions. The additional equipment required includes an electrolyser system with a capacity sized to meet the GT’s fuel consumption at zero load. When the GT ramps up power for active power dispatch, natural gas, biogas, or other fuels may be used if the hydrogen supply isn’t sufficient for full load operation. In addition to providing essential grid ancillary services at a competitive cost compared to alternatives, this operational mode offers the advantage of avoiding gas turbine maintenance costs
associated with frequent start-ups and shutdowns, as the machine remains hot and spinning. This option, called “green idling”, is explored below.
Renewable power supply: variable and lacking critical grid forming capabilities
To maintain grid stability during periods of high renewable energy supply, inertia, reactive power, short-circuit current capability, and quickly deployable active power supply are required. A balanced use of battery systems and synchronous condensers can manage this. In periods of limited renewable power, residual load must be met, and this can be handled by adding combinations of battery systems (or other storage solutions) and peaking/backup gas turbine plants. Replacing traditional thermal baseload power plants with renewable wind and solar energy requires substantial investments in equipment to provide both intra-day and long duration residual load capacity as well as grid stabilisation services. The effort necessary to produce renewable fuel for the peaking gas turbine plants should also be considered.
The total investment in renewable power generation equipment and all the other necessary assets accumulates, and the levelised cost of renewable electricity must account for these additional grid-stabilising and balancing assets.
Although the anticipated total investment cost is high, the average cost of electricity is expected to be reasonable in the future, because wind, solar, and hydroelectric power sources don’t consume fuel.
Can a gas turbine be used instead of synchronous condensers and batteries?
Yes, definitely. A gas turbine provides grid stabilisation services when operated to meet residual load, and its effectiveness is even enhanced during idle (ie, non power generating) operation. When a GT is connected to the grid and operating at full speed, the combined rotating masses of the turbine rotor, gearbox, and generator contribute significant inertia.
Particularly significant is the inertia that can be provided by single-shaft industrial gas turbines. When the GT is in idle operating
Providing
Frequency stabilisation
Renewable energy from the grid
Voltage control Inertia
Short circuit capacity
Electrolyser Green idling, the basic idea (image: Siemens Energy)
www.modernpowersystems.com | June 2025 | 17 Gas turbine
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