I heard once that when a wind turbine power plant doesn't produce enough electricity the power companies are sometimes forced to turn on a couple of jet engines in order to compensate for the loss, is there any truth to that? I imagine stability is a key factor in keeping the production static and efficient, so what would the power company do?
This is correct. When the demand exceeds supply, voltage will sag and frequency will drop (which can risk equipment failure and is certainly an undesirable situation). The operators of power grids will turn on alternative sources of generation in order to correct the imbalance as soon as it is noticed (often under the coordination of a regional transmission organization such as CAISO).
Grid operators are very careful to ensure that the grid frequency is properly maintained (source); even a few seconds of drift (i.e. a few hundred cycles ahead or behind) require RTOs and related agencies to take corrective action where safe. Most of these measures work the same whether demand increases or supply decreases (and thus are relevant whether we are speaking about an increase in consumer load or a decrease in supply from wind or other renewable sources).
In order to understand the mix of energy a bit more thoroughly, it's necessary to take into account the types of generation, which include base-load plants, load-following plants, intermittent sources, and peaker plants:
- Base-load plants are designed to operate at high cost efficiency (not necessarily environmental efficiency or any other measure of efficiency, unless dictated by local laws and priorities), but cannot be adjusted quickly. Examples of these may include large coal and nuclear base load.
- Load-following plants can adjust if they have capacity (e.g. hydroelectric or smaller fuel-burning plants)
- Peaker plants are agile and can be brought online quickly (e.g. gas turbines), but are inefficient. When the base-load plants are insufficient, load-following plants increase their load; if this capacity is exhausted or the grid is experiencing rapid swings in load that the load-following plants cannot keep up with, then peakers will come online and begin burning fuel to achieve enough supply to balance the demand.
Another factor to consider is planning: If an area has consistent winds and enough wind turbines, the wind can be considered part of base-load: It cannot be adjusted, but is relatively predictable and consistent day-to-day. Gaps in the wind are treated the same way as any other shortfall of base-load: first via load-following plants if possible and then with the help of the peakers.
Known gaps and shortfalls can also be handled through trading. For example, Washington State, US has abundant hydroelectric power, and exports energy (as of 2019) to fourteen other states. Its overproduction of energy (which can itself be as harmful as underproduction when it causes overvoltage and overfrequency) is usefully diverted to help make up some of the supply of neighboring states such as California (source). This export includes base-load if the local demand is dropping too quickly for the operating power plants to adjust.
Stored energy also makes a contribution. The sources for such extra energy may be storage sites such as pumped energy storage, batteries (e.g. this), or they may be generation (not necessarily burning fuel).
Lastly, load-shedding is a last-resort. If conditions are adverse (very high demand such as air-conditioning on a hot day, transmission line failures, loss of base-load, etc) then the grid operator may increase the real-time price of industrial energy, or even require that industrial grid users curtail their demand to avoid grid instability. If this is insufficient then blackouts and brownouts will occur, to prevent the total loss of the grid and its most critical users (hospitals, emergency services, communications).
I was going to scold you for not doing a search -- then couldn't find a decent answer! So -- here's a short answer:
First, jet engines -- no. You're thinking of gas turbines, but they are not jet engines (try a web search on "Gas Turbine").
Second, there's not a lot of energy storage on the electrical grid, aside from tanks of gas, piles of coal, uranium rods, and water behind dams. Batteries are starting to look like maybe they'll be practical, eventually. But by and large, when "alternative" energy sources poop out, there needs to be a "traditional" energy source that kicks in. Gas turbines are good for this because they can be brought on line quickly.
This wiki article goes into the grid storage issue.