[eigenvector]

The primary way that a wind turbine "stops" is by pitching its blades (in some vendors' terminology they call this "pause" because the machine is still connected to the grid just not producing any energy). On modern machines this can reliably be achieved in less than 3 seconds. There is also a mechanical brake that can be applied for emergency stop in 1 second or less. However, mechanical braking is not to be used in normal operations.

However, going to the maximum ramp-down rate (which for a large wind park can approach hundreds of MW/s) is usually not desirable because it will impact grid voltage. At most wind parks voltage control is accomplished using on-load tap changes of the main transformers as well as switched capacitors. Both of these need a time delay of at least 10 seconds to avoid wearing out quickly so if you ramp your whole wind farm from max output to zero in 1 or 2 seconds, you will see a big impact on grid voltage. So if we need to ramp down for a transmission system thermal constraint, we will do it at the slowest possible rate that still meets the transmission utility's needs.

[ghshephard]

Ah, that makes perfect sense. It also explains why we have an expectation of a minimum of 15 seconds before we see any response to a command to stop loading the transmission lines, and why the emergency command to the entire wind-farm doesn't occur until all other options have been exhausted.