Protocol_Grid Integration of Wind Energy

8/3/2019 Protocol_Grid Integration of Wind Energy

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TECHNOLOGY AND OPERATION OF WIND TURBINES

Winter Semester

2011

TOPIC: GRID INTEGRATION OF

WIND ENERGYProf: Dr Ing. Shaoqing Ying

Student: Gustavo Cedeo Elizondo. Mat Num: 3042612


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CONTENTS

Introduction .......................................................................................................................... 3

Current state of implementation............................................................................................ 4

Case Study: Denmark .....................................................................................................................5

Case study: Spain ...........................................................................................................................7

Case study: Portugal.......................................................................................................................9

Case study: Ireland.......................................................................................................................10

Case study: Germany....................................................................................................................11

Barriers for continued expansion ......................................................................................... 13

Interaction of wind turbines with the grid.....................................................................................15

Short circuit power level.....................................................................................................................................15

Voltage variations and flicker .............................................................................................................................16Harmonics ...........................................................................................................................................................16

Frequency ...........................................................................................................................................................17

Reactive power ...................................................................................................................................................17

Protection ...........................................................................................................................................................18

Network stability.................................................................................................................................................18

Switching operations and soft starting ...............................................................................................................19

Solutions ............................................................................................................................. 20

Conclusions ......................................................................................................................... 22

References........................................................................................................................... 23


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G R I D I N T E G R A T I O N O F W I N D E N E R G Y

INTRODUCTION

Traditionally the power utilities have focused on providing power to the costumers of high quality

with a high degree of security of supply and tight control of voltage and frequency. Other important

considerations have been redundancy in the network to cope with planned as well as unplanned

outages and long term investment in both transmission and generation capacity.

During the last 30 years more emphasis has been put on the environmental issues, this has

primarily been on emissions and on generating efficiency. This trend has been accelerated with

the process started at the Rio summit and continued at the Kyoto summit that resulted in firm

requirements for emission of green house gases especially CO2.

Even though these goals are ambitious the share of renewable energy have to be much higher in

order to increase the sustainability of the power system.

Another important factor in the utilization of the renewable energy resources is the aspect of

security of supply through diversification of the power production. This will also reduce the

dependency on imported fuels such as coal and oil.

These very significant changes in the power sector of course have a very large impact on most if not

all aspects of the power sector from planning to operation.

One of the keywords in this regard, is integration. Integration takes at least three meanings. The

first one is the integration of the power, heat and transportation sectors, being one of the

prerequisites to obtain high system efficiency as well as sustainability. Second meaning is the

integration of the conventional and new technologies for power generation. Renewable energytechnologies have characteristics that are very different from conventional. They will often have

spatial dependencies and especially wind and solar power have a short term stochastic variation

that is much larger compared to conventional power plants. They also have different characteristics

that can be exploited in order to lower the cost of energy for the complete system (wind energy

production is higher during winter (in Northern Europe), solar energy production is higher during

summer, biomass is a limited resource, however it is dispatchable). Third meaning of the word

integration is the actual integration of a particular renewable energy technology in the power

system and how new strategies can be developed to ease the integration and how the system can be

modified to ease integration.

Wind energy has been very successful during more than 30 years. The initial development of the

technology in the beginning of the 80's soon led to wind turbines that had a high availability at a

relatively low cost. The result was a positive track record that encouraged further investment in

development and implementation of the technology. Currently wind energy is the fastest growing

energy source in the world and in some regions wind energy has a very significant impact on the

power system. This has led to interesting development of the technology in order to integrate it

economically in the power system.


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CURRENT STATE OF IMPLEMENTATION

Wind energy is going through a very rapid phase of implementation. Several countries have very

large wind energy programs and still more countries are initiating such programs. In Europe the

first large market was Denmark. It was succeeded by Germany and later Spain, but also other

European countries are greatly involved in wind energy such as the Netherlands, Greece, Italy andnow France.

The worldwide installed capacity is shown in Figure 1. The rapid increase in installed capacity

can be noticed. In Europe the growth was dominated by Germany and Spain.

Figure 1: World installed wind capacity by 2009. source: World Wind Energy Association. www.wwindea.org

A high rate of increase in the installed capacity is also foreseen in the future. Several references

exist that include predictions of the installed capacity and it is important to notice that the

tendency is an exponential increase in the installed capacity.

The wind turbines are of course concentrated in regions with a good wind resource. This means

that there are areas with very significant amounts of wind turbine capacity installed. This is for

example, the case for Western Denmark, Schleswig-Holstein in Germany and Navarra in Spain. In

these areas the wind power has a significant impact on the power system.

The wind turbine technology is also going through rapid development with continuously increase

in the size of the wind turbines, their blade angle control system and shift from mechanical

gearbox to power electronics drives

The business has also changed. The size of the many of the projects that are being implemented is

much larger now than in previous years. Wind farm sizes larger than 100MW are nowadays

common. Wind farms are also installed offshore in Denmark, Germany, Sweden and also the

UK/Ireland.


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The power systems in Denmark, Portugal, Spain, Ireland and Germany have the highest wind

penetrations in the world. Some important parameters can be seen in table 1.

Denmark (West + East) Portugal Spain Ireland Germany

P eak demand (GW) 3.7 + 2.6 = 6.4 9.4 45.4 4.5 80

Minimum demand (GW) 0.9 + 0.9 = 1.8 3.5 18 1.65 34.6

2010 wind power capacity (GW) 2.7 + 0.97 = 3.7 3.9 20 1.425 26.4

Wind energy produced in 2010 (TWh) 5.9 + 1.9 = 7.8 9.0 42.7 2.9 36.5

Maximum possible instant 204% 111% 110% 86% 76%

penetration (wind/minimumdemand)

(W: 300%,E: 108%)

Capacity penetration (wind/peak 58% 42% 44% 32% 33%

demand) (W: 73%,E: 37%)

Energy penetration (yearly wind 21.9% 17% 16% 10.5% 6.7% (2009)

generation/gross demand) (W: 27.8%,E: 13.4%)

Table 1: Overview of some European wind penetration levels, based on 2010 data. source:IEEE power andenergy society.

The management of the different power systems to date, with increasing amounts of wind energy,has been successful. There have been no reported incidents in which wind has directly or indirectly

been a major factor causing operational problems on the system. In some areas with high wind

penetration, however, the transmission system operator (TSO) had to increase remedial actions

significantly in order to decrease the loading of system assets during times of high wind power

infeed. In some areas, the risk of faults may have increased. Higher targets for wind power will

mean even higher penetration levels locally and high penetration levels in larger power systems.

In the following sections, the actual situation in the previous mentioned countries is described,

since it constitutes the starting point for the analysis of actual and future challenges for the

increasing integration of wind energy in the power grids.

CASE STUDY: DENMARK

The Danish power system is known for being one of the first systems worldwide to experience a

rapid growth of wind power, beginning in the early 1980s, leading to a significant share of total

electricity demand satisfied by wind power. Wind energy penetration is 28% for western and 13%

for eastern Denmark. The Danish power system is divided into two parts, each belonging to

different synchronous zones, that have been connected by a dc link since September 2010.

Importance of Interconnectors

Both Danish systems have sufficiently strong interconnection capacity to export 40% of the

generated capacity and import 70% of the maximum consumption. The grid will continue to expandover the next four years, in anticipation of increasing wind power: the interconnectors to Norway

and Germany will be expanded, the internal north-south corridor will be reinforced, and another

large offshore wind farm will be connected.

The availability of hydropower in Norway and Sweden via dc interconnectors is ideal and is often

used to balance wind power in Denmark using market mechanisms. The ac connection to the

German thermal system in the south mainly contributes to a stable frequency.


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Wind Power in the Electricity Markets

One of the principal tools for integrating a large amount of wind power into the Danish power

system is a well-functioning common Nordic electricity market among the nations of Norway,

Sweden, Finland, and Denmark. In November 2009, a market coupling between the Nordic

countries and the German power exchange was implemented, resulting in the European Market

Coupling Company (EMCC), which allocated the cross-border transport capacity between Germanyand Denmark. Since November 2010, market coupling has been implemented between the Nordic

countries and the former central West countries, made up of the Netherlands, Belgium,

Luxemburg, France, and Germany, in an interim tight volume coupling (ITVC) arrangement. These

are important steps towards the European goal of integrated electricity markets. Most Danish wind

power is traded on the Nordic power market, which is made up of two main markets: the Nordic

Power Exchange (NPX)itself divided into three marketplacesand the TSOs real-time electricity

markets.

Wind power has contributed to the market design in that a negative price has been allowed since

30 November 2009. Before then, the Nord Pool Spot price was set to zero during hours of excess

generation due to wind. By implementing negative spot prices, suppliers have a stronger incentiveto reduce their supply bids in hours with very strong wind forecasts, and consumers also have a

stronger incentive to use electricity in hours with negative prices. In western Denmark, there were

two incidents of negative prices in December 2009 (a total of nine hours) and five incidents in all of

2010 (a total of 11 hours). In the first half of 2011, western and eastern Denmark have each

experienced ten hours with negative prices.

Forecasting

The probability of an excess or deficit of generation is estimated some days before the day of

operation. With a large share of wind power capacity in the grid, it is important to have good wind

forecasts in order to know whether wind power capacity is available or not.

At present, the overall annual mean absolute error (MAE) on day-ahead forecasts amounts to about5% relative to the installed wind power capacity. The intraday power market, though having lowliquidity, is used from 3 p.m. the day before up until one hour before operation. This market isfollowed by the regulating power market, used several hours before the hour of operation until thehour of operation. Wind power forecasts are updated every 15 minutes, with a five-minuteresolution. By using a scaling-up procedure, the actual wind production is estimated, thusfacilitating optimal trading at the intraday market.

Future Challenges

The Danish government has ambitious plans for transforming Denmark into a country free of fossil

fuels by 2050. Most of the renewable potential will be provided by wind energy, mainly from

offshore, which will be connected to the transmission system. Simultaneously, the transport andheat sectors are planned to become more closely connected to the electricity system, and thus

excess production is expected to be used for transport purposes by electric cars or for heat

production. Neighboring countries will also go through substantial changes of their energy systems,

and therefore it is important to coordinate grid development in an international context.


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CASE STUDY: SPAIN

The interest of Spain in moving to a low-carbon economy has demanded higher levels of renewable

energy penetration. These higher levels of penetration of renewable energy (supplying 35% of

annual consumption in the year 2010) have been achieved primarily by wind generation, making it

one of the main technologies in the Spanish system (it accounts for 21% of total installed electric

capacity). Total installed wind capacity in Spain as of May 2010 was 20,243 MW. This growth is

expected to continue into the future, as more than 40% of the electric energy consumed in Spain

must come from renewable sources by the year 2020 in order to comply with European initiatives.

Operating a system with a large portion of wind generation is complicated due to the inherent

characteristics of both the wind plant and the power system. In the case of Spain, the first challenge

is being weakly interconnected with the rest of Europe and having to provide the required balanc-

ing capacity to compensate for the variability of renewable energy mostly internally. The second

challenge is the startup time and the minimum technical capacity of the thermal units that are the

main source of reserves, along with hydropower generation (which has the drawback of

fluctuations between wet and dry cycles).

In spite of this, the Spanish system has been operated on some days with more than half of its

demand covered by wind generation; a recent example was 9 November 2010, with 54% of

consumption fed by wind. Wind power supplied 20.73% of demand during March 2011, making it

the technology with the highest energy produced during that month.

Experience to Date

The variability of wind and solar generation implies a new uncertainty when the sizing of the

generation reserves is performed, making the wind forecast one of the basic tools for system

operation. Its accuracy affects the required levels of reserve and helps in scheduling manageable

generation to counteract its variability.

Having real-time information about the production of renewable energy is necessary in order to

make reliable production forecasts for this type of installation. Such real-time data let the system

operator distinguish between generation and demand and thus avoid demand forecast errors as

well .

Based on the current legislation, the system operator receives, through the Control Center for

Renewable Energies (CECRE), the telemetry of 98.6% of the wind generation installed, of which

96% is controllable (able to adapt its production to the given set point within 15 minutes). The

telecommunications deployment to almost 800 wind farms spread all around Spain has been

achieved as a result of the aggregation of all the distributed resources of more than 10 MW in

renewable energy sources control centers (RESCCs) and the connection of them with CECRE. This

hierarchical structure, together with the applications developed by Red Electrica de Espaa (REE),is used to analyze the maximum wind generation supported by the system. Supervising and

controlling the wind generation in real time have decreased the number and quantity of

curtailments, maintaining the quality and security of the electricity supply at the same time that

renewable energy integration is maximized.


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Wind Turbine Technology: Voltage Dips

Nowadays, 97.5% of the wind farms installed in Spain have fault ride-through capabilities. As a

result of this technical adaptation, the problem of significant wind generation tripping has been

solvedproduction curtailments for this reason have not been required since 2008.

Voltage Control

Before April 2009, Spanish regulations established that wind farms had to comply with a reactive

power bonus table, receiving a financial bonus or penalty depending on the power factor provided

at each hour of the day. During periods of changing output, simultaneous connections and

disconnections of wind plant capacitors occurred, leading to sudden changes in the network voltage

profile. In order to avoid these situations, and as a short-term measure, in April 2009 it was

established that wind facilities of more than 10 MW must maintain an inductive power factor of

between 0.98 and 0.99, except in certain nodes of the system where particular instructions were

sent due to specific requirements.

Future Challenges

One of REEs goals is to increase the ability of the system to integrate more renewable generation

while maintaining quality of service. This implies:

1. The international exchange capacity among neighboring countries must be increased.This is one of REEs highest priorities, and it is also classified as a top priority by the EU.A reinforcement of the France-Spain interconnection is planned for the year 2014. It willdouble the current exchange capacity between the two countries.

2. Cross-border exchange of balancing energy must be established. Intense work has beencarried out by REE and its neighboring TSOs to develop market-oriented mechanisms forthe exchange of balancing energy as long as there is available international exchange ca-pacity in the required direction.

3. The minimum manageable generation required must be reduced by increasing theflexibility of the manageable generation and reducing its time response.

4. Demand must be turned into a flexible resource. REE set up a demand-side managementdepartment in 2007 with the goal of promoting demand management mechanisms:interruptible service, promotion of efficient integration of electric vehicles, time-of-usetariffs, and smart metering. These technologies and processes must be integrated intothe smart grid of the future.

5. More storage capabilities must be installed, with the objective of maximizing renewableintegration


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CASE STUDY: PORTUGAL

By the end of 2010, the Portuguese power system had a total generation capacity of 18,164 MW,

with 7,407 MW of thermal plants and 4,578 MW of hydropower stations. It had a total of 9,490 MW

of renewable-powered sources (52% of the total installed capacity). It should be noted that, of this

capacity, a large share (3,900 MW of wind capacity and 2,900 MW of run-of-the-river hydropower)

has little or no power regulation capability. During 2010, wind energy contributed 17% of the gross

energy consumed.

Experience to Date

The Portuguese experience in integrating a significant amount of wind generation has been rather

positive, as no major negative system events have occurred. The Portuguese power system is

extremely well prepared for a very high penetration of wind power. The following factors

contribute to this readiness:

1. Wind power plants have been requested to have capabilities for active voltage regulation

from an early phase of the wind deployment. Remote variable reactive power control to

maintain the power factor between 0.98 inductive and 0.98 reactive is also available.

2. Wind production has been aggregated in clusters (called local wind power dispatch

centers) for wind generation monitoring and control.3. The capability to participate in primary frequency control (limited to 5% of the nameplate

power) has been required in contracts signed after 2007.4. Low-voltage ride-through (LVRT) fault capability has been required in contracts signed

after 2007.5. Recently, various solutions for wind and renewable source energy storage, like in pumping

stations, have been introduced when they are available and cost-effective. Electric vehiclesare also being introduced as distributed storage systems.

The power system operators activities regarding grid planning have also helped wind integration.

The TSO promoted the installation of phase shift transformers and uses dynamic line ratings withmonitoring for temperature to manage the main transmission lines. It plans the new lines using a

holistic approach, taking into consideration the spatially distributed generation of renewable

sources and their correlation effects.

Since 2007, new power purchase agreement (PPA) contracts have allowed wind power plants tobe legally curtailed, although only for technical reasons, under severe occurrences, and with the lostenergy not being paid to the producers. To date, curtailment has not been used in Portugal. ThePortuguese power mix shows a very high degree of flexibility, mainly due to the high participationof hydropower generation, which has the capability to balance the wind power.

Future Challenges

Wind targets published in 2010 with the Portuguese National Strategy ENE 2020 document foresaw

the installation of 8.5 GW of wind power by 2020, which would have led to an annual wind energy

penetration above 30%. That ambitious target was slightly reduced in the Portuguese National

Renewable Energy Action Plan (NREAP), which included as a minimum wind objective the

installation of 6.9 GW by 2020. Currently the main concern of the Portuguese power system is the

excess of renewable energy generation during windy, wet winters like the winter of 20092010.


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CASE STUDY: IRELAND

Over the last decade, wind power has become a significant percentage of Irelands overall

generation mix. At the end of 2010, Ireland had installed 1,425 MW of wind power, or enough wind

generation to satisfy 15% of overall system demand. Indeed, at particular time intervals, wind has

produced enough power to meet 50% of system demand and has even reached a high of 37% of

total daily energy production. The current level of installed wind is expected to grow significantly

over the coming years in line with government targets and Irelands obligations under the EU Cli-

mate Change package. By 2020, it is estimated that the synchronous power system of Ireland will

have more than 4,000 MW of wind generation installed, which will meet around 37% of electricity

demand. This anticipated level of wind (in percentage terms) is greater than any other synchronous

region in Europe over this time frame.

The Irish power system has a minimal level of regional interconnection. At present, interconnection

to Northern Ireland is routed through three ac links with effective capacity for 450 MW. Northern

Ireland has a 500-MW HVdc tie to Scotland. There is currently no direct interconnection between

the Republic of Ireland and Great Britain; however, construction work is under way on a 500-MW

HVdc link between the two jurisdictions and is on schedule to be completed by the end of 2012.

Future Challenges and Next Steps

Most of the EU renewable target for Ireland will be met through the electricity sector (the target is

40% renewable electricity). This creates a number of operational, portfolio, and infrastructure

challenges that EirGrid is working to manage.

Improved system operational tools will need to be developed and deployed as the operation of the

power system becomes more complex with more wind generation. The aim of the tools is to

provide the system operator with more accurate real-time information as well as greater control

and monitoring facilities. These tools include the ability to dispatch wind, to forecast wind output

accurately, and to assess the stability of the power system in real time.

Providing the required transmission and distribution infrastructure forms a major part of the

program of work to deliver on the 2020 renewable policy targets. This grid infrastructure

development includes the Grid25 implementation plan, the East-West interconnector to the United

Kingdom, and the provision of access to the power system for generation using a group processing

approach known as Gate 3 the third renewable group processing directive proposed by the

Commission for Energy Regulation (CER).

Several studies identified deficiencies in system performance capability in terms of frequency and

voltage control out to 2020, as more non synchronous generation becomes embedded on the

system. In terms of frequency control, the analysis has shown that the projected levels of syn-

chronous inertia available in 2020 are less than the amount needed to meet system requirements.At high instantaneous non synchronous generation penetration levels, frequency control becomes

more challenging. This is due in part to the presence of rate of change of frequency (RoCoF)

protection relays that shut down wind turbines under certain scenarios.


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CASE STUDY: GERMANY

The German feed-in tariffs for electricity from renewable sources have enabled a high level of

installed capacity of wind and PV. Currently, more than 27 GW of wind power and almost 18 GW of

photovoltaic (PV) capacity are installed in Germany. The PV units are mainly located in the

southern part of the country. In total, nearly 900,000 individual PV units are connected to the

German grid. Most of them are installed on the rooftops of houses and have a peak capacity of 240

kW. Most of the wind turbines are installed in the northern part of Germany and are connected to

the distribution network. Approximately 80% of the installed wind power is located in the control

areas of the TSOs TenneT and 50Hertz Transmission. Due to this uneven distribution, the four

German TSOs have developed an online sharing of the infeed from wind energy. Each TSO takes a

predefined percentage of the current infeed, derived from the relation between the end consumer

consumption in each control area and total German consumption.

Experience to Date

In Germany, the four TSOs are responsible for marketing and balancing renewable energy. Each

TSO therefore has to have a renewable energy balance group. Since January 2010 the TSOs havebeen obliged by law to sell the day-ahead forecast of renewable energies at the day-ahead spot

market of a power exchange. Deviations between the day-ahead forecast and more accurate

intraday forecasts have to be bought or sold at an intraday spot market of a power exchange. To

maintain system security, it is crucial for the TSOs to have a flexible and liquid intraday spot

market. Remaining differences are balanced through the use of balance energy during real-time

operation.

Due to the large amounts of energy placed on the spot market by the TSOs, there is an

interdependency between the hourly prices and the infeed from renewables within the same

time period. Especially in times with high wind power and low demand, the prices at the power

exchange may turn negative. This phenomenon can be observed for both the day-ahead and

intraday markets.

The root-mean-square error of the day-ahead wind power forecast is currently below 5% of the

installed capacity. Intraday forecast errors are much lower, depending on the forecast horizon.

In some occasions, however, large errors were experienced; probably the most severe of these

occurred during the storm named Kyrill in 2007, when a forecast error of as much as 8 GW (40%

of installed capacity) was experienced when the wind speed exceeded the cutoff wind speeds of

a large number of turbines.

Due to the priority German law assigns to renewable energy, system operators are obliged to

exhaust all conventional measures before reducing the infeed of renewables. If the infeed from

renewables is reduced, the system operator has to pay compensation to the unit operator

amounting to the lost remuneration (from the feed-in tariff). These measures are mainly taken by

DSOs and TSOs in the northern and eastern parts of Germany.


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Future Challenges

The German government has ambitious aims to increase the production of electricity from

renewable energy. By 2020, the share of renewable energy in the electricity production mix should

reach 35%. The amount of installed wind power should reach a level of 37 GW onshore and 14 GW

offshore. For PV, a target installed peak capacity of 51 GW has been announced. This means a

growth of 88% for wind energy and 188% for PV within the next nine years.

Those developments require a massive reinforcement of the transmission system in Germany. The

German dena Grid Study II, initiated by Deutsche Energie-Agentur GmbH (dena) and published in

November 2010, focuses on the requirements for a reliable power supply system in 2020. It

identified the need for about 3,600 km of new transmission lines in Germany in the extra-high-

voltage grid.

Besides the requirements for grid development, there are also important consequences for the

system operation. The TSOs will need more flexible measures to market and balance the

increasingly weather-dependent generation. A monitoring system and control mechanisms for

the units are also required. An increase of the magnitude announced by the German government

also requires an active provision of ancillary services by the units that deliver the variable

infeed.


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BARRIERS FOR CONTINUED EXPANSION

Installation and connection of wind turbines and wind farms meets many barriers from grid

companies and system operators. Many of these barriers stem from the inexperience of handling

wind energy, which has different characteristics than other types of generation, especially the

variations, unpredictability and control properties. Below are listed many, but not all the barriers(with respect to power system integration). Although the list is long, many of the barriers can be

overcome easily and at low costs. However, it is very important that the barriers are handle

seriously if wind energy is to gain confidence at the grid owners and system operators.

The impact of wind power on the power system is at all levels. The main cause for many of the

problems resulting in limitations for the integration of wind power is the wind. The problems have

several origins. One is the concentration of wind energy in some areas (resource availability)

requiring the power to be transmitted to consumers via the transmission grid. The second problem

is the temporal variations of the wind. The problem with the temporal variability is that it imposes

increased regulation capabilities on the rest of the generating system. Finally the wind is very

difficult to predict. This is true in many time scales from seconds to days (weeks or even years).

For smaller wind farms or single wind turbines, the first concern is the grid connection. Usually the

connection of wind turbines at medium voltage level is regulated by the distribution company who

owns the grid.

The thermal capacity of the grid limits the amount of current that can flow in the conductor without

exceeding the thermal limit of the conductors. It means that keeping all other variables equal, there

is a maximum power that can be transmitted through the line.

Another factor that often limits the amount of installed wind power capacity on a particular feeder,

is the voltage level. The voltage level at the consumers site is regulated by international standards

like EN50160. Due to the impedance of the conductors, the voltage level will change along the

feeder depending on the wind power production and the consumer load.

It is to be noticed that there are two possible extreme situations, high load combined with low wind

power production and low load and high wind power production. In this particular case the voltage

at the bus bar of the substation is regulated to the same level at all conditions (automatic tap

changer).

Other power quality measures also have to be taken into account such as flicker and harmonics

when the sizing of the wind farm and the grid connection is done during the design phase. The

flicker is a result of the variations in output power due to variations in the wind or due to

switching operations (cut-in, cut-out and generator switching). In weak grids this can result in

voltage variations at a magnitude and frequency that visibly changes the light emission from

light bulbs. If power electronics are applied in the wind turbines the resulting level of

harmonics have to be below the limits set in the standards. The power quality from wind

turbines and wind farms are calculated on the basis of IEC 61400-21.

Wind turbines are of course mainly installed in places with good wind resources. This can lead to

high concentrations of wind power in some regions as the case with the Western part of

Denmark. This will also in the future be the case in many other places. This means that large

amount of wind power will be generated in regions where the local consumption is low and the


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excess has to be exported or dealt with in another way. In order to do that it is necessary to have

a strong transmission grid designed for the transmission of wind power. This basically means

that it has the right capacity of power transport from the windy regions to the load centers. This

of course is a set of problems that has been known in the past when the transmission grid

capacity was designed to fit the needs of power transmission from large central (coal, natural gas

or nuclear) power plants. The main problem is the rapid implementation of wind power which

means that the reinforcement of the grid has not been planned and that the procedure for

determination of the transmission capacity is different due to the stochastic nature of wind

energy. When wind power is going to be exploited at high levels it means that large amounts of

power has be transmitted over long distances and that in the regions with a high wind power

production there will be very little if any conventional production. This also has an impact on

how inter-area connections are handled with respect to controllability and security of supply.

As the level of penetration increases the wind power will have a significant impact not only on the

local voltage, but it will also have an impact on the system frequency. This is due to the variations

in the wind turbine output, which means that the rest of the system not only has to be able to

regulate the variations in consumption but also the variations in wind power production. At the

same time less conventional capacity will be on-line since the optimal operation (in terms ofeconomy) of the system means that some of the conventional plants have to shut down to save

fuel. The ability of the conventional power plants to control power can therefore be limiting the

amount of wind power that can be integrated in the system. In some areas this can be even worse

since there can be other limitations on the operation of the system. This is the case in the Danish

system. There large amounts of generation is heat bound local CHP (which is participating in the

control of the grid) leaving even less capacity for frequency control. Such areas have to rely on

power exchange with the neighboring areas in order to be able to control the system.

The integration of wind power and energy in the market place is also made difficult by the

unpredictability of the wind. This not so much a technical constraint, but a economic constraint.

In the Nordpool power pool power on the spot market have to be announced at 12.00 for the

next day. This means that a 36 hours prediction horizon is required. In Denmark, the System

Operators have to make the announcement on the spot market. The prediction of the wind

power production therefore covers the System Operators Area. Experience have shown that it is

indeed very difficult to predict the production even considering the spatial distribution of the

wind turbines that will give some averaging effect.

Another barrier in the further implementation of wind power is the planning and operational

procedures of the system/grid operators. Grid codes can be very restrictive for wind power

since many of the grid codes are based on worst case analysis without any reflection on the

probability of occurrence of the worst case. This can lead to very conservative design of the grid

connection, which implies high cost. These costs will usually be borne by the wind turbine

owner resulting in high energy costs. Planning and economic dispatch tools also have to befurther developed to take the characteristics of wind power into consideration in order to

optimize the system operation.


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INTERACTION OF WIND TURBINES WITH THE GRID

In the last 10 years, a steadily increasing number of renewable energy sources such as wind orsolar (photovoltaic) powered generating systems have been added to the systems. A distinctivefeature of electricity is that it cannot be stored as such - therefore must at any instant be balancebetween production and demand. Storage technologies such as batteries, pump storage and fuel

cells all have one common characteristic, the electric energy to be stored is converted to otherforms, such as chemical (batteries), potential energy in form of water in high storage (pumpstorage) and hydrogen (fuel cells). All renewable resources produce when the source is available,for wind power, as the wind blows. This characteristic is of little if any importance when theamount of wind power is modest compared to the total installed (and spinning) capacity ofcontrollable power plants, but it changes into a major technical obstacle as the renewable partgrows to cover a large fraction of the total demand for electric energy in the system.

On the local level, voltage variations are the main problem associated with wind power. Normalstatic tolerances on voltage levels are 10%. However, fast small variations become a nuisance atlevels as low as 0.3% in weak grids, as is often found in remote areas where the wind conditionsare the best. This can be the limiting factor on the amount of wind power which can be installed.In the following, a short introduction is given to each of the electrical parameters which taken

together are used to characterize power quality in a given point in the electricity supply system.

SHORT CIRCUIT POWER LEVEL

The short circuit power level in a given point in the electrical network is a measure of its strengthand while not directly a parameter in the voltage quality, has a heavy influence. The ability of thegrid to absorb disturbances is directly related to the short circuit power level of the point inquestion. Strong and/or weak grids are terms often used in connection with wind powerinstallations. If the line impedance in the equivalent circuit of the grid (zsc) is small in a point ofinterest (p), then the voltage variations in that point will be small (the grid is strong) andconsequently, if ZSC is large, then the voltage variations will be large. Strong or weak are relativeterms. For any given wind power installation of installed capacity P(MW) the ratio RSC= SSC/ P (Short

circuit/active power)is a measure of the strength. The grid is strong with respect to the installationif RSC is above 20 to 25 times and weak for RSC below 8 to 10 times. Depending on the type ofelectrical equipment in the WT they can sometimes be operated successfully under weak conditions.Care should always be taken, for single or few WT in particular, as they tend to be relatively moredisturbing than installations with many units.


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VOLTAGE VARIATIONS AND FLICKER

Voltage variations caused by fluctuating loads and/or production is the most common cause ofcomplaints over the voltage quality. Slow voltage variations within the normal -10+6% toleranceband are not disturbing and neither are infrequent (a few times per day) step changes of up to 3%,though visible to the naked eye. Fast and small variations are called flicker. Flicker evaluation is

based on IEC 1000-3-7 which gives guidelines for emission limits for fluctuating loads in mediumvoltage (MV, i.e. voltages between 1 and 36 kV) and high voltage (HV, i.e. voltages between 36 and230 kV) networks. The basis for the evaluation is a measured curve giving the threshold ofvisibility for rectangular voltage changes applied to an incandescent lamp. Disturbances just visibleare said to have a flicker severity factor of Pst= 1 (Pstfor P short term).

Where Pst is measured over 10 minutes and Plt is valid for two hour periods. IEC 1000-3-7 gives both

planning levels, that is total flicker levels which are not supposed to be exceeded and emission

levels, that is the contributions from an individual installation which must not be exceeded.

Determination of flicker emission is always based on measurement. IEC 61000-4-15 specifies a

flickermeter which can be used to measure flicker directly. As flicker in the general situation is the

result of flicker already present on the grid and the emissions to be measured, a directmeasurement requires a undisturbed constant impedance power supply and this is not feasible for

WTGS due to their size. Instead the flicker measurement is based on measurements of three

instantaneous phase voltages and currents followed by an analytical determination of Pst for

different grid impedance angles by means of a flicker algorithm - a program simulating the IEC

flickermeter.

HARMONICS

Harmonics are a phenomenon associated with the distortion of the fundamental sine wave of the

grid voltages, which is purely sinusoidal in the ideal situation.

Harmonic disturbances are produced by many types of electrical equipment. Depending on theirharmonic order they may cause different types of damage to different types of electrical equipment.All harmonics causes increased currents and possible destructive overheating in capacitors as theimpedance of a capacitor goes down in proportion to the increase in frequency. As harmonics withorder 3 and odd higher multiples of 3 are in phase in a three phase balanced network, they cannotcancel out between the phases and cause circulating currents in the delta windings of transformers,again with possible overheating as the result. The higher harmonics may further give rise toincreased noise in analogue telephone circuits.

Highly distorting loads are older unfiltered frequency converters based on thyristor technology andsimilar types of equipment. It is characteristic for this type that it switches one time in each halfperiod and it may generate large amounts of the lower harmonic orders. Newer transistor based

designs are used in most variable speed WT today. The method is referred to as Pulse WidthModulation (PWM). It switches many times in each period and typically starts producing harmonicswhere the older types stop, that is around 2 kHz. Their magnitude is smaller and they are easier toremove by filtering than the harmonics of lower order.

IEC 1000-3-6 put forward guidelines on compatibility and planning levels for MV and HVnetworks and presents methods for assessing the contribution from individual installations to theoverall disturbance level.


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FREQUENCY

The electrical supply and distribution systems used world-wide today are based on alternating

voltages and currents (AC systems). That is, the voltage constantly changes between positive and

negative polarity and the current its direction. The number of changes per second is designated the

frequency of the system with the unit Hz. In Europe the frequency is 50 Hz whereas it is 60 Hz inmany other places in the world. The frequency of the system is proportional to the rotating speed of

the synchronous generators operating in the system and they are - apart from an integer even factor

depending on machine design - essentially running at the same speed: They are synchronized.

Increasing the electrical load in the system tends to brake the generators and the frequency falls. The

frequency control of the system then increases the torque on some of the generators until

equilibrium is restored and the frequency is 50 Hz again.

REACTIVE POWER

Reactive power is a concept associated with oscillating exchange of energy stored in capacitive andinductive components in a power system. Reactive power is produced in capacitive components

(capacitors, cables) and consumed in inductive components (transformers, motors, fluorescent tubes).The synchronous generator is special in this context as it can either produce reactive power (thenormal situation) when over magnetized or consume reactive power when under magnetized. Voltagecontrol is effected by controlling the magnetizing level of the generator, high magnetizing level resultsin high voltage and production of reactive power.

As the current associated with the flow of reactive power is perpendicular (or 90 deg. out ofphase) to the current associated with active power and to the voltage on the terminals of theequipment, the only energy lost in the process is the resistive losses in lines and components. Thelosses are proportional to the squared of total current. Since the active and reactive currents areperpendicular to each other, the total resulting current is the root of the squared sum of the twocurrents and the reactive currents hence contribute as much to the system losses as do the activecurrents. To minimize the losses it is necessary to keep the reactive currents as low as possibleand this is accomplished by compensating reactive consumption by installing capacitors at orclose to the consuming inductive loads. Furthermore, large reactive currents flowing to inductiveloads is one of the major causes of voltage instability in the network due to the associated voltagedrops in the transmission lines. Locally installed capacitor banks mitigates this tendency andincreases the voltage stability in the area.

Many WT are equipped with induction generators. The induction generator is basically aninduction motor, and as such a consumer of reactive power, in contrast to the synchronousgenerator which can produce reactive power. At no load (idling), the consumption of reactivepower is in the order of 35-40% of the rated active power increasing to around 60% at ratedpower. In any given local area with WT, the total reactive power demand will be the sum of thedemand of the loads and the demand of WT. To minimize losses and to increase voltage stability,

the WT are compensated to a level between their idling reactive demand and their full loaddemand, depending on the requirements of the local utility or distribution company. Thus thepower factor of WT, which is the ratio between active power and apparent power, is in general inthe range above 0.96.

For WT with pulse width modulated inverter systems the reactive power can be controlled by theinverter. Thus these WT can have a power factor of 1.00. But these inverter systems also give thepossibility to control voltage by controlling the reactive power (generation or consumption ofreactive power).


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PROTECTION

The extent and type of electrical protective functions in a WT is governed by two lines of

consideration. One is the need to protect the WT, the other to secure safe operation of the network

under all circumstances

The faults associated with first line are short circuits in the WT, overproduction causing thermaloverload and faults resulting in high, possibly dangerous, over voltages, that is earth faults andneutral voltage displacement.

The second line can be described as the utility view, that is the objective is to disconnect the WTwhen there is a risk to other consumers or to operating personnel. The faults associated with thisline are situations with unacceptable deviations in voltage and/or frequency and loss of one ormore phases in the utility supply network.

Depending on the WT design, that is if it can operate as an autonomous unit, a Rate Of Change OfFrequency (ROCOF) relay may be needed to detect a step change in frequency indicating that theWT is operating in an isolated part of the network due for example to tripping of a remote line

supplying the area.

In Germany the grid protection device of WT will be tested accordingly. The test shows thecapability of the WT, to meet grid protection limiting values set by utilities. During this test thereaction of the WT is checked and recorded for voltage and frequency exceeding upper and lowerlimits. Responding levels and response times are recorded and depicted in the final data sheet.The functionality of the complete protection system is also verified and certificated.

NETWORK STABILITY

The problem of network stability has been touched upon briefly above. Three issues are central inthe discussion and all are largely associated with different types of faults in the network such as

tripping of transmission lines (overload), loss of production capacity (any fault in boiler or turbinein a power plant) and short circuits.

Permanent tripping of transmission lines due to overload or component failure disrupts thebalance of power (active and reactive) flow to the adjacent areas. Though the capacity of theoperating generators is adequate large voltage drops may occur suddenly. The reactive powerfollowing new paths in a highly loaded transmission grid may force the voltage operating point ofthe network in the area beyond the border of stability. A period of low voltage (brownout)possibly followed by complete loss of power is often the result.

Loss of production capacity obviously results in a large power unbalance momentarily and unlessthe remaining operating power plants have enough so called spinning reserve, that is generatorsnot loaded to their maximum capacity, to replace the loss within very short time a large frequency

and voltage drop will occur followed by complete loss of power. A way of remedy in this situationis to disconnect the supply to an entire area or some large consumers with the purpose ofrestoring the power balance and limit the number of consumers affected by the fault.

Short circuits take on a variety of forms in a network and are by far the most common. In severitythey range from the one phase earth fault caused by trees growing up into an overhead transmissionline, over a two phase fault to the three phase short circuit with low impedance in the short circuititself. Many of these faults are cleared by the relay protection of the transmission system either bydisconnection and fast re-closure, or by disconnection of the equipment in question after a few


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hundred milliseconds. In all the situations the result is a short period with low or no voltagefollowed by a period where the voltage returns. A large - off shore - wind farm in the vicinity will seethis event and disconnect from the grid immediately if only equipped with the protection describedabove. This is equivalent to the situation loss of production capacity and disconnection of the windfarm will further aggravate the situation. Up to now, no utility has put forward requirement todynamic stability of WT during grid faults. The situation in Denmark today, and the visions for the

future, have changed the situation and for wind farms connected to the transmission grid, that isat voltages above 100 kV, this will be required.

SWITCHING OPERATIONS AND SOFT STARTING

Connection and - to a smaller degree - disconnection of electrical equipment in general and induction

generators/motors especially, gives rise to so called transients, that is short duration very high

inrush currents causing both disturbances to the grid and high torque spikes in the drive train of a

WT with a directly connected induction generator

In this context WT fall into two classes. One featuring power electronics with a rated capacitycorresponding to the generator size in the main circuit and one with zero or low rating power

electronics in a secondary circuit - typically the rotor circuit of an induction generator.

The power electronics in the first class can control the inrush current continuously from zero torated current. Its disturbances to the grid during switching operations are minimal.

Unless special precautions are taken, the other class will allow inrush currents up to 5-7 times therated current of the generator after the first very short period (below 100ms) where the peak areconsiderably higher, up to 18 times the normal rated current. A transient like this disturbs the gridand to limit it to an acceptable value all WT of this class are equipped with a current limiter or softstarter based on thyristor technology which typically limits the highest RMS value of the inrushcurrent to a level below two times the rated current of the generator. The soft starter has a limitedthermal capacity and is short circuited by a contactor able to carry the full load current whenconnection to the grid has been completed. In addition to reducing the impact on the grid, the soft

starter also effectively dampens the torque peaks in the air gap of the generator associated with thepeak currents and hence reduces the loads on the gearbox.


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SOLUTIONS

The key to successfully increase the level of wind energy penetration and move towards an energy

system that is much more sustainable in terms of exploitation of renewable energy sources is to

view the system as a whole and take advantage of the various characteristics that different

technologies provide. The keyword is flexibility.

As presented above the main contributor to the barriers of integration of wind energy is the

variability combined with the unpredictability of the wind. Many solutions to these barriers exist

(available on the market, in development or as changes in the existing practices). In many of these

solutions, power electronics and information technology can be used to mitigate the impact and add

value.

Power electronics installed as part of the wind turbines is increasingly common. This mainly takes

the form of so-called variable speed turbines. The main advantages obtained with this combination

is reduction of mechanical loads on the main wind turbine structure, which leads to reduced

manufacturing cost, and improved grid connection characteristics, which leads to lower the grid

connection costs. The reduced grid connection costs comes from the reduced impact duringconnection of the generator and from reduction of the power fluctuations in the flicker range. The

application of power electronics also adds controllability of the power output both at partial load

and at full load. This can successfully be utilized in wind farm control systems where the total

output from a wind farm is controlled. Power electronics are used to control reactive power.

The concept of wind farm control have been further developed in the grid connection

requirements set by the Danish System Operators for the connection of large wind farms to the

transmission grid. In these requirements it is specified how the wind farm should behave both

during normal operation and during grid faults. In order to increase the ease by which the wind

energy can be integrated it is specified that the output from the wind farm should be controllable

in such a way that it can be reduced in 2s to 20% of rated output, it should also be able to limit

the total output of the wind farm to any value and other similar requirements.

On the Swedish island, Gotland, there has been a extensive implementation of wind turbines,

especially in the Southern part of the island. The load centre is in the Northern part as is the

unidirectional HVDC-connection from the mainland. This has resulted in several problems with

the integration of wind power as well as in some interesting solutions. These include the

application of HVDC transmission of the power and large converter stations based on voltage

source converters using IGBTs. This system makes it possible to transmit power from the

Southern part to the Northern part and due to the power electronics used in the converters it is

also possible to control the reactive power. Also in the Southern part of the island (or rather

offshore) a small wind farm (5*500kW stall controlled wind turbines) has been installed with

wind turbines using power converters. The rating of converters used is one third on the windturbine rating. This makes it possible to run the machines at variable speed at low wind speeds

and produce slightly more power than if the machines were running at fixed speed. When the

wind speed increases and the production from the wind turbine exceeds the rating of the power

electronics, the induction machine is directly connected to the grid and the power electronics is

used for power factor correction. Due to the large penetration of the grid in the Southern part of

the island several other problems with the grid is also handled by the power electronics. The

converters are also used to reduce the impact of the wind turbines on power quality (here taken


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as flicker and voltage level) by controlling the reactive power and if necessary reducing the active

power based on grid parameters.

The combination of wind power and energy storage has also received some attention. The objective

have again been several including elimination of local power quality problems (again flicker and

voltage level) as well as making the wind power schedulable as conventional power plants. The

type of storage depends on the application, but in general the current storage technology is not well

suited for these applications. Some special applications are of interest. One of them is utilization of

pumped storage. In areas where natural two-lake systems exist like in County Donegal, Ireland, this

option is the least cost option when compared to grid reinforcement. For smaller system lead-acid

batteries are an option. In combination with modern power electronics it is possible to provide a

system with significant less power fluctuations.

Other power electronic devices have also been applied in combination with wind farms in order to

improve the grid compatibility. In Denmark to advanced SVC's have been installed in the grid

connection of wind farms. The newest one was installed at the 24MW wind farm Rejsby Hede. The

objective of the project was to reduce grid losses and improve voltage control.

As pointed out in the previous section several other technologies are developing at a fast rate. Many

of these developments have an impact on the challenges and possible solution in the future energy

system.

There has for many decades been worked on storage of electricity for power system

applications. During the last few years new storage technologies have emerged. The most

promising of these are based on so-called redox flow technology. They have three very

important features. The efficiency is high above 80%, the lifetime is long more than 10 years

(and the charge/discharge regime does not have an impact). The third very important feature is

that power rating and energy capacity can be sized independently of each other. The plant

consists of two main parts: A cell stack in which the two electrolytes can interact through a

membrane and storage tanks for the two electrolytes. The power capacity is determined by thearea of the membrane and the energy capacity is determined by the volume of the storage tanks

(or rather by the volume of the electrolyte in the system).

Fuel cells will also play a very important role in the future energy system. To begin with because it

will be possible to have a very high total efficiency of the energy conversion from fuel to electricity

(above 70%) and heat. This will make it attractive to uses in houses as well as in office buildings

and industry. However, from an integration point of view the real perspective is the ability of some

types of fuel cells to work reversibly. In a power system with a high penetration of wind energy

there will be long periods with surplus wind energy. This surplus energy can be fed to the fuel cells

and hydrogen can be produced. This hydrogen can then later be used for power production if there

is a deficit of wind energy, but it can also be used in transportation. In this way renewable energy is

used to produce fuel for cars, trucks and tractors enabling a much higher degree of sustainability.

The event of widespread application of micro CHP and also PV will result in a power system with a

much more distributed nature. This will have to be integrated with wind power that will be

concentrated in regions far from the consumption centers. This will have a huge impact on the

design and operation of the transmission grid and it will also require new technologies to be

developed that can control such a system and further can interact with a energy market.

Information technology will be the key technology in this and its application will not be limited to


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the production side. Many benefits can be gained from including the demand side in the overall

system operation in order to increase the flexibility and there by improve the matching of

production and consumption.

The value of wind energy can be increased significantly by reorganizing the way the power

markets operate. Reducing the lead time will also reduce the error in the prediction of the wind

power produced. Studies have to be made in order to establish a market structure that is fair to

all types of generation. Green certificates have also been investigated as an option, but creating

such a market have shown not to be an easy task. These options have also been investigated in

order to change from a fixed tariff system to a market based system for renewable energy.

It is evident from the above that no single solution or family of solutions exist that can

eliminate or reduce all the barriers. However, it is noticed that integration in the different

meanings previously presented is the key to success. It is also noticed that many of the

solutions require significant research effort if the potential is to be realized.

CONCLUSIONS

There is growing experience with the integration of high amounts of wind generation into power

systems in Europe. Operational challenges are encountered especially in times of high wind and low

load. No incidents in which wind generation has directly or indirectly caused unmanageable

operational problems have been reported to date, however. The system operators face rapidly

growing installed capacity of wind power and must try to maintain the same level of operational

security and reliability while minimizing curtailments from wind power. The key elements for the

future integration of high penetration levels of wind power are:

1. Interconnections and transmission upgrades inside the countries must be enabled. Thedelivery of the required transmission and distribution infrastructure forms a major part of

the work needed to meet the 2020 renewable policy targets in many countries.

2. Enhancing the use of existing grid infrastructure and interconnections to enable operation

at full capacity is also important.3. There must be well-functioning markets offering a range of scheduling periods (i.e., day-

ahead, hour-ahead, and real-time) to accommodate the uncertainty in wind plant forecasts.Establishing cross-border exchange mechanisms for balancing energy is also important.

4. Improved system operational tools will need to be developed and deployed as theoperation of the power system becomes more complex with more wind generation. Thesetools include the ability to forecast wind output accurately, to obtain accurate real-timeinformation on generation levels, and to assess the stability of the power system in real

time in order to control wind plant output when necessary from a system security point ofview.

5. System flexibility must be increased. This means reducing the response time of the

conventional generation plants, turning demand into a flexible resource, and looking for

feasible storage options.6. Policies and capabilities must be put in place to deal with system issues such as transient

stability, voltage collapse, and reactive power support.


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REFERENCES

1. H. Bindner, P. Lundsager. Integration of wind power in the power system. IEEE Xplore. 2002

2. R. Doherty, D. Eleanor, M. OMalley. System operation with a significant wind power

penetration. IEEE, Transactions on Power Systems. 2003

3. H. Siegfried. Grid Integration of Wind Energy Conversion Systems. Wiley & Sons Ltd, SecondEdition, England 2006. ISBN-10: 0-470-86899-6.

4. T. Ackermann. Wind Power in Power Systems. Wiley & Sons Ltd, England 2005. ISBN: 0-

470-85508-8.

5. Y. Zhang, K.W.Chan. The Impact of Wind Forecasting in Power System Reliability. IEEE

plore. 2008

6. The Waves of Wind. An update on Wind Integration. IEEE power & energy magazine.

november / december 2011.

Documents

Protocol_Grid Integration of Wind Energy