**4. Overview of spanish experience dealing with wind power variability: Examples of singular events**

In this section, examples of the Spanish experience of singular events produced by either wind speed variability or operational issues are examined.

#### **4.1. Voltage sags**

**Figure 8.** Comparison between the power produced by two wind power plants with the aggregation of nine wind

Finally, when aggregation includes different types of wind turbines, with different nominal wind speeds and different cut-out speeds, extreme aggregated fluctuations are also reduced significantly.In short, temporal diversities and spatial diversities reduces the peak-to-peak

On the other hand, some regions wind power fluctuations are not related wind speed varia‐ bility. Instead, the power fluctuations are caused by technical and operational challenges

**•** Voltage sags. Voltage sags produce a sudden drop of wind power generation. This drop

**•** Wind power curtailment. This is due to integration issues in the power system, such as limitations on the transmission or distribution networks, inability to ramp up or ramp

Another classification is laid down. Attending to their ramping characteristics, wind power

**•** Wind power die-out. A wind power die-out refers to a persistent drop in wind power.

power plants (including the previous ones) and the whole of Spanish wind power output

rather than by meteorological phenomena.Particularly, these include:

down other generation sources, lack of enough reserves, etc.

magnitudes of power fluctuations.

292 Advances in Wind Power

is usually recovered is quickly.

fluctuations events can be classified in:

Wind turbine manufacturers are required by transmission system operators (TSOs) to equip their turbines with fault ride-through (FRT) capability as the penetration of wind energy in the electrical systems grows [25]. Spain developed a procedure to measure and to evaluate the re‐ sponse of wind turbines and wind power plants subjected to voltage sags [26].The procedure for verification, validation, and certification of the requirements are described in the PO 12.3. This wind power plant commissioning and validation are based on the response of wind pow‐ er plants in the event of voltage sags. The result of wind power plant commissioning leads to the certification of its conformity with the response requirements specified in the Spanish grid code [27]. Some aspects related to that grid code are explained in detail in [28]–[29].

On the other hand, because of the growing impact on power grid operations, the recent rap‐ id expansion of wind generation has given rise to widespread interest in field testing and commissioning of wind power plants and wind turbines. Validation of computer dynamic models of wind turbines is not a trivial issue. Validation must ensure that wind turbine models represent with sufficient accuracy the performance of the real turbine, especially during severe transient disturbances [30]. In [32], different field tests for model validation and standards compliance are categorized according to the main input or stimulus in the test—control stimulus and external physical stimulus. Among these tests, the FRT capability of wind turbines can be performed using factory tests, at the individual wind turbine gener‐ ator terminals, and using short-circuit field measurement data based on operational wind turbines and wind power plants.

Short-circuit field measurement data on operational wind turbines and wind power plants [33]—called opportunistic wind power plant testing in [31]—is performed with measure‐ ment equipment installed at the wind power plant site. The equipment records naturally oc‐ curring power system disturbances that are then used to validate wind turbine models. Power system modeling during the disturbances must be taken into account in the valida‐ tion of wind turbine models. Therefore, monitoring wind power plants and wind turbines can be of interest for turbine manufacturers, wind power plant operators, and TSOs.Both the pre fault and the post fault data and power system network must be represented properly.

An extreme event recorded in Spain related to voltage sags occurred on March 19 and 20, 2007.Within twelve hours, four different disconnections of large amounts of wind power be‐ cause of voltage sags were recorded. Those voltage sags were located in areas with high penetration wind power and during high wind speed periods. Figure 9 shows the recorded Spanish wind power output during these events.

The amount of wind power generation disconnected during these voltage sags were 553 MW, 454 MW, 989 MW, and 966 MW, respectively.

**Figure 9.** Spanish wind power during the voltage sags on March 19 and 20, 2007

In addition, nine Spanish wind power plants located in different areas were also analyzed during these events. Nominal power of these wind power plants varied from 6.8 MW at Wind Power plant 9 to 49.5 MW at Wind Power plant 4. In Figure 10, the wind power out‐ put from these nine wind power plants are presented. Highlights include:


can be of interest for turbine manufacturers, wind power plant operators, and TSOs.Both the pre fault and the post fault data and power system network must be represented properly.

An extreme event recorded in Spain related to voltage sags occurred on March 19 and 20, 2007.Within twelve hours, four different disconnections of large amounts of wind power be‐ cause of voltage sags were recorded. Those voltage sags were located in areas with high penetration wind power and during high wind speed periods. Figure 9 shows the recorded

The amount of wind power generation disconnected during these voltage sags were 553

Spanish wind power output during these events.

294 Advances in Wind Power

MW, 454 MW, 989 MW, and 966 MW, respectively.

**Figure 9.** Spanish wind power during the voltage sags on March 19 and 20, 2007

**•** Voltage Sags 1 and 2 affected only Wind Power plant 4.

away from the faults.

put from these nine wind power plants are presented. Highlights include:

In addition, nine Spanish wind power plants located in different areas were also analyzed during these events. Nominal power of these wind power plants varied from 6.8 MW at Wind Power plant 9 to 49.5 MW at Wind Power plant 4. In Figure 10, the wind power out‐

**•** Wind Power plants 1, 2, and 3 are located in the same area. They were at high fluctuating partial load. These power plants were not affected by voltage sags because they were far **•** All voltage sags during this period were located in areas with high wind power penetration.

**Figure 10.** Wind power production of nine wind power plants during voltage sags on March 19 and 20, 2007

The operation of power systems under the effect of voltage sags in wind power has led TSOs to require FRT capability in wind power plants. By the end of 2010, 704 Spanish wind power plants had been certified against FRT capability (19.2 GW and around 95% of the installed ca‐ pacity). A total of 1 GW wind turbines are excluded because of their missed manufacturers, small size, or because they are prototypes turbines. Figure 11 shows the number of power loss‐ es greater than 100 MW from 2005 and the percentage of wind power without FRT. As a result of the FRT implementations, the problem of significant wind generation tripping has been solved; therefore, wind plant curtailments have not been required since 2008.

**Figure 11.** Evolution of wind power with FRT and number of power losses greater or equal to 100 MW by voltage sags in Spain [34]

The implementation of the supervisory control and data acquisition of wind generation in real time have decreased the number and the size of power curtailments, improved the qual‐ ity and the security of the electricity supply, and maximized renewable energy integration. To further enhance wind energy integration, the Spanish TSO (Red Eléctrica de España, or REE) submitted a proposal of a new grid code (P.O 12.2) to the Ministry, with additional technical requirements for FRT, among others. The main purpose this proposal is to antici‐ pate the expected problems in the Spanish power system between 2016 and 2020, by taking into account the incoming plants and new power plants to be deployed during these years to come. It is expected that P.O. 12.2 can be approved and applied in 2013.

#### **4.2. Klaus Storm (January 23, 24, and 25, 2010)**

Meteorological phenomena (e.g., storms or cyclones) are capable of causing large variations in wind power production and very high wind speeds.A storm within this category can affect a large number of wind turbines that have approximately the same cut-out wind speeds. When the cut-out speed is reached, the power generated goes from rated power to zero immediately. If this phenomenon spreads over several wind power plants in a particular area, it can cause a major threat to the power system stability and may lead to a cascading blackout.

The storm Klaus was named after an extra-tropical mid-latitude cyclone that struck between January 23 to January 25, 2009, affecting northern Spain and southern France. Wind speeds of higher than 150 km/h were recorded in the Spanish and French coastlines. The result was the disconnection of many wind power plants in northern areas of Spain, leading to a reduc‐ tion of about 7,000 MW of wind power in a few hours (refer to Figure 12). Figure 13 shows the impact of the storm on a Spanish wind power plant.The wind power plant consists of 30 NEG Micon 82 Wind Turbines with a nominal power of 49.5 MW.

**Figure 12.** Wind power, forecasting, and schedule during the Klaus storm

**Figure 11.** Evolution of wind power with FRT and number of power losses greater or equal to 100 MW by voltage

The implementation of the supervisory control and data acquisition of wind generation in real time have decreased the number and the size of power curtailments, improved the qual‐ ity and the security of the electricity supply, and maximized renewable energy integration. To further enhance wind energy integration, the Spanish TSO (Red Eléctrica de España, or REE) submitted a proposal of a new grid code (P.O 12.2) to the Ministry, with additional technical requirements for FRT, among others. The main purpose this proposal is to antici‐ pate the expected problems in the Spanish power system between 2016 and 2020, by taking into account the incoming plants and new power plants to be deployed during these years

Meteorological phenomena (e.g., storms or cyclones) are capable of causing large variations in wind power production and very high wind speeds.A storm within this category can affect a large number of wind turbines that have approximately the same cut-out wind speeds. When the cut-out speed is reached, the power generated goes from rated power to zero immediately. If this phenomenon spreads over several wind power plants in a particular area, it can cause a

The storm Klaus was named after an extra-tropical mid-latitude cyclone that struck between January 23 to January 25, 2009, affecting northern Spain and southern France. Wind speeds of higher than 150 km/h were recorded in the Spanish and French coastlines. The result was the disconnection of many wind power plants in northern areas of Spain, leading to a reduc‐ tion of about 7,000 MW of wind power in a few hours (refer to Figure 12). Figure 13 shows the impact of the storm on a Spanish wind power plant.The wind power plant consists of 30

to come. It is expected that P.O. 12.2 can be approved and applied in 2013.

major threat to the power system stability and may lead to a cascading blackout.

NEG Micon 82 Wind Turbines with a nominal power of 49.5 MW.

**4.2. Klaus Storm (January 23, 24, and 25, 2010)**

sags in Spain [34]

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**Figure 13.** Wind power in a 49.5-MW wind power plant during the Klaus storm

During this emergency situation, the Spanish TSO REE has deployed several thermal power plants to increase the reserve power generation. Despite of the large difference between the forecast and actual wind power production, the power system continued to operate within the normal operating range.

This example shows the difficulties for forecasting wind power during these types of events.Differences between forecast and real wind power generation reached almost 6,000 MW. Furthermore, wind power decrement during the storm happened in the night, thus, it followed the ramping down of the daily load, so the increased reserves generation (6 GW) were sufficient to maintain the system balance.

## **4.3. Wind power curtailments**

Wind energy curtailments because of integration issues in the power system have appeared in the Spanish power system. Until 2009, major curtailments were due to limitations on dis‐ tribution networks, but since the end of 2009, cuts have been applied in real time to sched‐ uled energy. However, the nature of renewable energy along with the economic and environmental issues, have provoked an interest in adding energy storage (such as pumped hydro storage PHS, a well-known technologies) into the power system mix.Spain accounts for around 5,000 MW (2.75 GW of pure PHS), with 77 GWh capacity. This technology is usu‐ ally deployed because of the limited transmission capacity for exporting or importing power to neighboring countries.

As an example of wind power curtailment, Table 1 indexes orders delivered by the Spanish TSO on February 28, 2010. The initial and end times for every curtailment period are present‐ ed in columns 1 and 2. Column 3 represents Spanish wind power at the beginning of the peri‐ od. In column 4, the Spanish TSO set point for this period is listed. In column 5, the real increase or reduction experimented by Spanish wind power in this period is shown. Finally, the ratio between the real increase/decrease and the increase/decrease obtained if wind power would match the set point is presented in column 6. In decrease periods, this ratio is equal to or high‐ er than 1; in increase periods, it is equal to or less than 1, 1 being the optimum value.


**Table 1.** Curtailment schedule on February 28, 2010

#### **4.4. Over-response to wind power curtailments**

During this emergency situation, the Spanish TSO REE has deployed several thermal power plants to increase the reserve power generation. Despite of the large difference between the forecast and actual wind power production, the power system continued to operate within

This example shows the difficulties for forecasting wind power during these types of events.Differences between forecast and real wind power generation reached almost 6,000 MW. Furthermore, wind power decrement during the storm happened in the night, thus, it followed the ramping down of the daily load, so the increased reserves generation (6 GW)

Wind energy curtailments because of integration issues in the power system have appeared in the Spanish power system. Until 2009, major curtailments were due to limitations on dis‐ tribution networks, but since the end of 2009, cuts have been applied in real time to sched‐ uled energy. However, the nature of renewable energy along with the economic and environmental issues, have provoked an interest in adding energy storage (such as pumped hydro storage PHS, a well-known technologies) into the power system mix.Spain accounts for around 5,000 MW (2.75 GW of pure PHS), with 77 GWh capacity. This technology is usu‐ ally deployed because of the limited transmission capacity for exporting or importing power

As an example of wind power curtailment, Table 1 indexes orders delivered by the Spanish TSO on February 28, 2010. The initial and end times for every curtailment period are present‐ ed in columns 1 and 2. Column 3 represents Spanish wind power at the beginning of the peri‐ od. In column 4, the Spanish TSO set point for this period is listed. In column 5, the real increase or reduction experimented by Spanish wind power in this period is shown. Finally, the ratio between the real increase/decrease and the increase/decrease obtained if wind power would match the set point is presented in column 6. In decrease periods, this ratio is equal to or high‐

**Wind Power Set Point**

**Real Increase/Reduction**

**Ratio**

**(MW)**

er than 1; in increase periods, it is equal to or less than 1, 1 being the optimum value.

**(MW)**

1:08 2:07 7796 7331 -796 1.71 2:07 3:48 6470 6099 -718 1.93 3:48 6:08 5175 4904 -720 2.66 6:08 8:45 4036 5904 217 0.11 8:45 9:10 3772 6905 420 0.14 9:10 9:43 3807 7905 276 0.07 9:43 – 4209 Installed capacity – –

the normal operating range.

298 Advances in Wind Power

**4.3. Wind power curtailments**

to neighboring countries.

**Initial Time End Time Wind Power**

**Table 1.** Curtailment schedule on February 28, 2010

**(MW)**

were sufficient to maintain the system balance.

On January 1, 2010, the REE gave instructions for several wind power curtailments consider‐ ing "Non-Integrable Wind Power Excess" as defined in Operational Procedure 3.7 [35]. Dur‐ ing these curtailments, an over-response in the wind power plant power generation was obtained and the reduced power ratio was greater than four times the order required. This kind of event may threaten the power system operation, and from an economical point of view, because reserves generators are used for balancing, increasing costs are produced.

Figure 14 shows the sequence of curtailment instructions provided by the CECRE, the con‐ trol center of renewable energies, together with the wind power generation in the power system. There were four orders with over-response during these hours, with effective wind power reduction from 2.42 to 4.02 times the commanded reduction.

**Figure 14.** Over-response to curtailments in the entire Spanish wind power generation

The main causes of this over-response were:


In Figure 14, an example of over-response to this curtailment is presented for the 49.5-MW wind power plant discussed previously. The TSO set point was ordered during early morn‐ ing (03:00 to 07:00). When wind speed was above the cut-out wind speed (20 m/s), wind power decreased below the set point, reaching half generation and almost no generation. This additional drop must be replaced by the generation reserves.

The sequence of range of production was as follows:


**Figure 15.** Example of an over-response in a 49.5-MW wind power plant on January 1, 2010

Possible solutions to avoid over-response to wind power curtailments, with the actual ca‐ pacity of energy storage and transmission to other countries, are:

