*1.3.1. Set-point curtailment*

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‐ pability.

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 and voltage stability.

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

#### *1.3.2. Ramp rate limits*

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‐ nomic operation.

For example, wind plants in Québec are required to be able to ramp up rate and down be‐ tween 0 MW and rated power in an adjustable 2 to 60 second interval [23]. In Alberta, AESO specifies that wind plants must be capable of maintaining their ramp up between 5 and 20 %/min of the rated power, taking into account all losses in cables and transformers [19]. The Irish code requirespower curtailment capability with a ramp rate defined project-specifically in the range 1 – 30 MW/min [26]. The Nordic code [20] requires the ability to regulate active power up or down from 100% to 20% of rated power in less than 5 seconds.
