**1.4. Frequency control**

In offshore and isolated power systems with weak interconnections, the frequency limits tend to be wider to ensure that wind plants (and other forms of generation) can continue to deliver their power and grid support functionalities. This is evident in the blue section of Figure 6, which shows that offshore wind plants are asked to stay connected between 46.5 Hz to 53.5 Hz (± 7%) for up to 10 sec. In Ireland, where the grid is infamous for its wide frequency excursions, wind plants are required to remain connected for frequency devia‐ tions down to 47.0 Hz and during a rate of change of frequency up to 0.5 Hz/sec. These are

Wind power plants are required to have an active power control system capable of receiving set-point commands from the grid operator to limit active power and ramp rate. This is typi‐ cally achieved through pitch angle control and/or by disconnecting some wind turbines.

During periods of transmission congestion or extremely low system loads, constraining con‐ ditions can result in deflated (or even negative) market prices, especially in regions with so‐ phisticated wholesale electricity markets. One technique to address the lack of available transmission, or the excess of wind power at any given time, is to curtail wind plants to low‐ er output levels during periods when it is less economic to keep them producing at full ca‐

To accomplish this, several system operators have integrated wind energy into their securi‐ ty-constrained economic dispatch (SCED). Within the available power from the wind, the output power can be regulated to a specific MW value or a percentage of the available pow‐ er. A fast, robust response of the active power control is important during normal operation to avoid frequency excursions and during transient fault situations to guarantee transient

In one example, AESO specifies that wind plants must be able to limit their active power to real-time MW set-points with an average resolution of 1 MW and accuracy of 2% of rated power on a 1-minute average. It is also specified that wind gusts should not lead to exceed‐ ing the active power limit by more than 5% of rated power [18]. One of the German codes requires wind plants to be capable of operating at a reduced power output without exceed‐ ing 1% change of rated power per minute across the entire range between minimum and rat‐ ed power [15]. The Irish code requires wind plants to commence the implementation of any

Requirements for active power control include the limitation of the ramp rate (rate of change) of active power. Ramp rates are possible for power increase, but operation with a power reserve is necessary in output power decrease, which necessitates sub-optimal eco‐

the most extreme frequency limits specified for 50Hz grids.

**1.3. Active power control**

344 Advances in Wind Power

*1.3.1. Set-point curtailment*

and voltage stability.

*1.3.2. Ramp rate limits*

nomic operation.

set point within 10 sec of receipt of the signal [26].

pability.

System events that include load-generation mismatches often result in transient fluctuations of the system frequency. This can be caused by mechanical failures of generators, sudden load changes, or line losses in the transmission system. The rate and depth of frequency de‐ cline and the time for frequency to return to its target value are all critical bulk power sys‐ tem performance metrics that are affected by the dynamics of the generation mix.
