**Advanced Wind Generator Controls: Meeting the Evolving Grid Interconnection Requirements**

Samer El Itani and Géza Joós

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51953

**1. Introduction**

#### **1.1. Grid interconnection requirements**

Reliable power system operation requires the continuous, instantaneous balance of supply and demand. Traditionally, power system planners have been familiar with a limited, wellunderstood amount of variability and uncertainty in demand and conventional generation. The large-scale integration of variable generation, such as wind power, gives rise to new challenges, requiring grid planners and operators to modify their traditional activities to maintain a secure, reliable operation of the power system.

#### *1.1.1. Proliferation of wind power*

For more than a decade now, wind power has been driving the change in electric grids worldwide. Currently, wind energy serves 22% of the load energy in Denmark, 17% in Por‐ tugal, 16% in Spain, 10.5% in Ireland and 9% in Germany. Also, with 86 GW of installed wind capacity in Europe, 42 GW in China, and 40 GW in the United States, it is fair to say that wind power has come to stay.

Each year wind power is increasing its share of the global electricity production, Figure 1. As penetration levels increase to the extent that conventional generators are displaced,


**Figure 1.** Global cumulative installed wind capacity, 1996-2011 [1].

In this chapter, we attempt at answering these questions to shed light on the main integra‐ tion challenges that wind power is facing and the opportunities that lie within.

#### *1.1.2. Grid integration challenges & opportunities*

The grid impact of the connection of a wind power plant depends on several factors. These include the technology of the turbines, the plant collector system, the required interconnec‐ tion features/capabilities, and the wind–grid penetration level.

Due to its intermittent nature, wind power blurs the distinction between dispatchable gener‐ ation resources and variable system load. Since the fuel source of wind plants is uncontrolla‐ ble and depends on meteorology, it must be dealt with operationally through mechanisms other than the traditional dispatch or commitment instructions. The challenging characteris‐ tics of wind power itself can be summarized in the following four elements [2]:


The nature of wind power, however, is not the only source of challenge. Some power systems attempting at wind integration are already weak, have limited dispatch flexibility and balanc‐ ing capabilities, or suffer shortage in transmission infrastructure. In some systems, the gap be‐ tween peak and valley loads is already big and the ramping capabilities are already exhausted, leading to a tight load-following capability (e.g. China). The situation is exacerbated by wind integration because the wind power peaks typically occur at load off-peak [3].

In order to tackle these technical challenges while responding to the pressure to accommo‐ date wind power, power system planners and operators have to alter their traditional plan‐ ning methods and operational practices. The dominant philosophy is that wind power plants should have all the technical capabilities needed to contribute to the secure operation of the power system in the same manner as conventional generators do. Thus new grid co‐ des are written, often with supplementary provisions for wind power plants. This is dis‐ cussed in detail in the following sections.

#### *1.1.3. Grid code development*

**Figure 1.** Global cumulative installed wind capacity, 1996-2011 [1].

tion features/capabilities, and the wind–grid penetration level.

*1.1.2. Grid integration challenges & opportunities*

than it is for conventional generation

protection coordination.

onds to hours

338 Advances in Wind Power

In this chapter, we attempt at answering these questions to shed light on the main integra‐

The grid impact of the connection of a wind power plant depends on several factors. These include the technology of the turbines, the plant collector system, the required interconnec‐

Due to its intermittent nature, wind power blurs the distinction between dispatchable gener‐ ation resources and variable system load. Since the fuel source of wind plants is uncontrolla‐ ble and depends on meteorology, it must be dealt with operationally through mechanisms other than the traditional dispatch or commitment instructions. The challenging characteris‐

**• Variability:** The output of wind generation changes in time frames that range from sec‐

**• Uncertainty:** The magnitude and timing of variable generation output is less predictable

**• Location:** Wind farms are often located in relatively unpopulated, remote regions that re‐

**• New technologies:** New technologies are often needed for wind turbines (e.g., doubly fed induction generators), requiring special assessment of their voltage and frequency regula‐ tion capabilities, harmonic emissions, contribution to sub-synchronous resonances, and

The nature of wind power, however, is not the only source of challenge. Some power systems attempting at wind integration are already weak, have limited dispatch flexibility and balanc‐ ing capabilities, or suffer shortage in transmission infrastructure. In some systems, the gap be‐ tween peak and valley loads is already big and the ramping capabilities are already exhausted,

tion challenges that wind power is facing and the opportunities that lie within.

tics of wind power itself can be summarized in the following four elements [2]:

quire long transmission lines to deliver the power to load centers

In the rush of promoting wind energy, little attention was paid to grid interconnection is‐ sues in many countries. There were no requirements for wind farms to regulate voltage, ride through grid disturbances, or support the system frequency. Even in regions were intercon‐ nection requirements for wind farms where relatively advanced (e.g. Denmark or Germa‐ ny), the provisions were moderate, reflecting the low wind penetration levels and the technology limitations at the time. At several occasions, these wind farms produced unac‐ ceptable voltage fluctuations during normal operation and caused major loss-of-generation events in response to otherwise minor system disturbances [5].

Through extensive experience with interconnection studies, power system operators and planners became increasingly familiar with the concept of a wind power plant; its perform‐ ance characteristics, capabilities, and limitations. Grid codes were updated, requesting that wind plants exhibit similar operational features as conventional synchronous generators and abide by the same minimum performance criteria. The object of these provisions is to maintain the same level of operational security and reliability while minimizing curtailment of wind power. The main requirements relate to fault ride-through, reactive power and volt‐ age control, dynamic behavior, active power and frequency control, and power quality. These requirements are met (either at the level of the wind turbine or the wind plant) through supplementary control loops that are triggered when specific events occur, such as contingencies resulting from grid faults, instabilities or loss of generation. These topics are treated in detail later in this chapter. Other generation controls not yet required from wind power plants include power system stabilizers (PSS), frequency regulation, and automatic generation control. These controls may in the future be incorporated into the core function or provided as ancillary services.

The technical requirements and performance specifications laid out in grid codes relate to the Point of Interconnection (POI), which is the border of responsibility between the net‐ work operator and the wind plant owner. As an example, Figure 2 and Table 1 describe the points of application of the technical rules of the grid operator in the Canadian province of Alberta (AESO). Similarly, Figure 3 shows the points of measurement for voltage ridethrough and reactive power requirements according to the grid operator in the Canadian province of British Columbia.

**Figure 2.** Wind power facility diagram – AESO [7].


**Table 1.** Points of measurement of performance criteria – AESO [7].

Advanced Wind Generator Controls: Meeting the Evolving Grid Interconnection Requirements http://dx.doi.org/10.5772/51953 341

**Figure 3.** Wind power facility diagram – BC Hydro [24]. WGF: Wind generation facility.

For offshore wind power plants, there are two possibilities for the POI depending on how the grid connection is embedded in the regulatory framework. In some countries (e.g. Germany), the local utility is responsible for extending its transmission network offshore to enable the connec‐ tion. In this case, the POI is at the offshore substation of the wind plant so all the offshore trans‐ mission assets are in the scope of responsibility of the network operator. In other countries (e.g. USA), the wind plant developer is responsible for grid connection up to the onshore POI, thus the submarine cables are within the wind power plant in this case [8].

It is challenging to design a wind plant, consisting of many turbines distributed over a large geographical area, so that it behaves like a conventional power plant as seen by the system at the POI. In the following sections, we discuss the different grid code requirements, design considerations, and industry implementations with reference to provisions from several Eu‐ ropean and North American grid codes. Emphasis is placed on the more sophisticated codes that come from countries and regions with high wind penetration levels. For each required control function, solutions are cited from the industry and reserach community.

#### *1.1.4. Power coordination & energy storage*

**Wind Turbine Generators**

340 Advances in Wind Power

WTG's

< 690v

**Wind Turbine Generator Transformer**

25 - 35 kV

External Voltage Regulation / Reactive Power System

Maximum authorized MW X Gross MW X

Over-frequency control X

Reactive power requirements X Voltage regulation X Voltage operating range X

**Table 1.** Points of measurement of performance criteria – AESO [7].

**WIND POWER FACILITY**

MW & ramp rate limiting X

Off-nominal frequency X

Voltage ride-through X Real-time monitoring X X

Meteorological signals X

**Requirement Performance Point** Collector Bus POI WTG

**Transmission System Step-up Transformer**

> **Collector Bus**

**Point of Connection**

69 - 240kV

**Transmission System**

WTG's

WTG's

WTG's

**Figure 2.** Wind power facility diagram – AESO [7].

In addition to requiring a behavior similar to conventional generators from wind power plants, grid operators are looking into energy storage and coordinated generation as intelli‐ gent solutions to facilitate the connection of wind power. The power coming from conven‐ tional generation can be coordinated with the intermittent power from the wind to reduce the minute-to-minute variations. This has been employed in Portugal on multiple occasions where this solution was found technically viable and cost-effective [4]. A recent study per‐ formed in Ireland concluded that pumped hydro storage becomes economically attractive at an average annual wind power penetration of approximately 50% [14].

Short-term energy storage facilities (flywheels and batteries) are also gaining momentum in providing ancillary services to assist in power system stabilization and controlled islanding. In one example in the USA [13], a multi-MW battery energy storage system (BESS) was add‐ ed in the grid to allow a large network to operate as a self-powered "island" in the event of transmission feeder loss. The BESS served radially fed distribution feeder loads for several hours during a permanent fault that was experienced on the grid. The BESS was also em‐ ployed for peak shaving, thus helping defer costly transmission and substation transformer upgrades. In another example [14], a 21 MW wind power plant in Hawaii is was designed to utilize a 4 MW BESS to help regulate the variability of the plant's output, thus enhanced the stability of the local grid.

### **1.2. Steady-state tolerance ranges**

## *1.2.1. Frequency & voltage operation ranges*

Wind power plants are required to ride through prolonged frequency excursions without dis‐ connection. This is typically defined through tolerance curves and extended time ranges around the nominal operating point of the power system. When the deviations are large, a reduction of the output power or operation for a limited period may be allowed. For example, Figure 4 shows the frequency tolerance curve of the Northeast Power Coordinating Council (NPCC)1 and that of Hydro-Québec TransÉnergie (HQTE), the system operator in Québec.

**Figure 4.** Required settings of under-frequency protection (log scale) – NPCC [18].

<sup>1</sup> NPCC is responsible for the reliability of the bulk power system in Northeastern North America, governing the grids of several American and Canadian provinces.

The stability of the electric grid can be disturbed if a wind plant is disconnected as a conse‐ quence of a failure due to a voltage perturbation. Thus, a wind plant must be able to run at rated voltage plus an extended voltage range. In Europe, the required voltage and frequency tolerance ranges are often specified simultaneously. For example, the Nordic code2 [19] de‐ mands from wind plants to operate in the voltage-frequency regions described in Figure 5.

Short-term energy storage facilities (flywheels and batteries) are also gaining momentum in providing ancillary services to assist in power system stabilization and controlled islanding. In one example in the USA [13], a multi-MW battery energy storage system (BESS) was add‐ ed in the grid to allow a large network to operate as a self-powered "island" in the event of transmission feeder loss. The BESS served radially fed distribution feeder loads for several hours during a permanent fault that was experienced on the grid. The BESS was also em‐ ployed for peak shaving, thus helping defer costly transmission and substation transformer upgrades. In another example [14], a 21 MW wind power plant in Hawaii is was designed to utilize a 4 MW BESS to help regulate the variability of the plant's output, thus enhanced the

Wind power plants are required to ride through prolonged frequency excursions without dis‐ connection. This is typically defined through tolerance curves and extended time ranges around the nominal operating point of the power system. When the deviations are large, a reduction of the output power or operation for a limited period may be allowed. For example, Figure 4 shows

and that

the frequency tolerance curve of the Northeast Power Coordinating Council (NPCC)1

of Hydro-Québec TransÉnergie (HQTE), the system operator in Québec.

**Figure 4.** Required settings of under-frequency protection (log scale) – NPCC [18].

several American and Canadian provinces.

1 NPCC is responsible for the reliability of the bulk power system in Northeastern North America, governing the grids of

stability of the local grid.

342 Advances in Wind Power

**1.2. Steady-state tolerance ranges**

*1.2.1. Frequency & voltage operation ranges*


**Figure 5.** Voltage / frequency regions for wind power plants – Nordic code [20].

Figure 6 shows the different operation regions as specified by one of the German system op‐ erators.

**Figure 6.** Voltage/frequency tolerance regions of one German system operator. Green: onshore wind plants, Green & blue: offshore wind plants [16], [17].

<sup>2</sup> Until the publication of the ENTSO-E grid code in Fall 2013 [12]-[13], the Nordic code governs the operation of the transmission systems of Denmark, Finland, Iceland, Norway and Sweden .

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 the most extreme frequency limits specified for 50Hz grids.
