Grid Stability

 https://docs.nrel.gov/docs/fy20osti/73856.pdf

The physics that I do not understand is in red. 

During normal grid operation, the supply of power from all the generators equals the demand for electricity, and the frequency remains constant. But just as a vehicle slows when you take your foot off the gas, if there is a loss of a power plant, the supply of power will drop almost instantaneously. However, the demand for electricity has not changed, so the same amount of power will still be extracted from the system. That is where inertia comes in. Stored energy is extracted from the inertia of the spinning generators and can temporarily make up for the lost generator. This action will slow down the generators. Although it cannot be sustained for more than a few seconds, it provides time for the mechanical systems in the grid to detect the imbalance (as reflected in declining frequency) and tell power plants to speed up (or slow down). 

Power system engineers typically describe the inertia of a generator in terms of stored rotational kinetic energy (EPRI 2019), so inertia has the same units of energy (power delivered over a period of time). However, because inertia typically only responds for a short amount of time (seconds), inertia is often measured with units of MWseconds (MW•s) or GWseconds (GW•s). So, a generator with 1 GW•s of inertia can deliver 1 GW of power for 1 second from its stored energy. One GW•s is equal to 0.27 MWh or 278 kWh.

A typical range of response rates is about 0.3% per second for slower-responding units to 2% per second for fast units including certain gas turbines, meaning a 100-MW plant would be able to increase output by between 0.3 MW and 2.0 MW per second (assuming it was not already operating at maximum output).   Natural gas electric plants have a high "response rate" in the sense that they can quickly ramp up or down their output to meet changes in electricity demand. This is particularly true for natural gas combustion turbines (SCGTs), which can quickly respond to peak demand or when output from intermittent renewables is low. Natural gas combined cycle plants (CCGTs) are also relatively responsive, though their ramp-up time is longer than SCGTs

As a result, ERCOT has historically relied on loads as a source of frequency response (ERCOT calls this “load resources”). 19 This consists of equipping certain large industrial loads with sensors that measure frequency and are programmed to disconnect automatically when the frequency drops to a certain level. In contrast with UFLS (which is typically involuntary), using loads for frequency response is voluntary and targeted toward specific non-critical loads, which are compensated for providing this service

https://www.greenlancer.com/post/long-term-energy-storage

Although lithium-ion batteries in utility-scale battery storage systems are great for short-term energy storage, they are not currently cost-effective for long periods of time, and they can experience issues with thermal runaway. Advancing long-duration energy storage (LDES) technologies is critical to the decarbonization of energy by providing system flexibility and managing fluctuations in energy supply and demand

From Grok

What I can not understand is what induces the operating conventional sources to provide more power -- which causes them to slow down.

The phenomenon where conventional generators provide more power, causing their turbines to slow down, is rooted in the physics of how synchronous generators interact with the electrical grid. Let me explain step-by-step why this happens and what induces the generators to deliver more power during a grid disturbance, like the Spanish blackout.

Key Mechanism: Grid Frequency and Turbine Response Synchronous Generators and Grid Frequency: Conventional generators (e.g., in coal, gas, nuclear, or hydro plants) are synchronous machines, meaning their rotors spin at a speed directly tied to the grid’s frequency (e.g., 50 Hz in Europe, corresponding to 3000 RPM for a two-pole generator). The grid’s frequency is a direct reflection of the balance between power supply and demand. When supply equals demand, the frequency stays stable. If a large generator or power source (like a solar plant) suddenly goes offline, the supply drops, and the grid frequency begins to decrease. Why Generators Provide More Power: When a power source trips offline (e.g., during the Spanish blackout on April 28, 2025, when solar plants likely disconnected), the grid’s frequency starts to drop because the remaining generators can’t instantly compensate for the lost power. In a synchronous generator, the rotor is magnetically locked to the grid’s rotating magnetic field. When the grid frequency drops, the rotor experiences a mismatch: the grid’s magnetic field slows down relative to the rotor’s speed. This creates a torque on the rotor, causing it to try to "catch up" with the grid’s demand by delivering more electrical power. To produce this extra power, the generator draws on the kinetic energy stored in the rotor’s inertia. This causes the rotor to slow down slightly, releasing energy into the grid to help stabilize the frequency drop. What Induces the Power Increase?: The key trigger is the drop in grid frequency caused by the sudden loss of power. This frequency drop creates an electrical load on the generators that are still connected. The synchronous generator’s physics automatically responds to this increased load by converting kinetic energy (from the spinning rotor) into electrical energy. This response is instantaneous and automatic—it’s not controlled by human operators or software but by the fundamental electromagnetic interaction between the rotor and the grid. The governor systems on these generators may also kick in within a second or two to adjust fuel or steam input to sustain the power output, but the initial surge comes from inertia. Why the Turbine Slows Down: The extra electrical power comes at the expense of the rotor’s kinetic energy, which is proportional to its mass and the square of its rotational speed (KE = ½ I ω², where I is the moment of inertia and ω is angular velocity). As the rotor releases kinetic energy to produce more electrical power, its rotational speed decreases, slowing the turbine. This slowdown is typically small (a fraction of a percent) but critical in providing a temporary power boost to stabilize the grid. Application to the Spanish Blackout In the context of the April 2025 Spanish blackout, when significant generation (likely solar) was lost in southwestern Spain, the grid frequency began to drop rapidly. The remaining conventional generators (e.g., nuclear, gas, or hydro plants, which made up ~16% of Spain’s power at the time) automatically responded by supplying additional power. Their turbines slowed down slightly as they converted stored kinetic energy into electrical energy to counteract the frequency drop. However, Spain’s grid had low inertia due to its high reliance on renewables (solar and wind, which don’t provide mechanical inertia), so the limited number of spinning turbines couldn’t fully absorb the shock, contributing to the cascading failure.

Why This Matters This inertial response is a natural feature of synchronous generators but is absent in most renewable sources like solar and wind, which use inverters. In a grid with fewer conventional generators, like Spain’s during the blackout, there’s less inertia to buffer frequency drops, making the system more vulnerable. Technologies like grid-forming inverters or synchronous condensers are being developed to mimic this inertial response, but they weren’t sufficiently deployed in Spain at the time.

If you’d like a visual to illustrate how grid frequency drops and turbine speed responds, I can generate a chart—just let me know! Alternatively, I can delve deeper into the math of inertia or the governor response if you’re curious.