Power Supply Challenges

Solutions for Integrating Renewables

POWER SUPPLYCHALLENGES

POWER SUPPLY CHALLENGES

POWER SUPPLYCHALLENGESWind and solar energy are essential to meet energy demand and to cut carbon emissions. But how to merge their variability into decades-old power systems? Searching for answers, this landmark work combines empiric data with theory of power engineering. In a truly cross-disciplinary manner, it reaches from detailed electrotechnical analysis to system-level political and economical issues. Th e award-winning author Jacob Klimstra unfolds

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It is becoming widely understood that future power systems must be FYUSFNFMZFYJCMF8IBUSFNBJOTVOEFSEFCBUFJTXIBUBSFUIFCFTUNFBOTGPSFYJCJMJUZ8JUIDPNQFMMJOHTDJFOUJDFWJEFODFUIJTCPPLQSPQPTFTUIBUGBTUSFBDUJOHNPEVMBSHBTSFEQPXFSQMBOUTBSFPFOUIFNPTUDSFEJCMFanswer.

9 789529 336340www.smartpowergeneration.com

ISBN 978-952-93-3634-0ISBN 978-952-93-3635-7 (pdf)

Th e book addresses both the challenges and possible solutions for future power systems, VTJOHDMFWFSJMMVTUSBUJPOTBOEFYBNQMFTGSPNBSPVOEUIFXPSME JTCPPLJTBHSFBUSFGFSence for anyone involved in the energy industry..JDIBFM8BMTI%JSFDUPS'VUVSF(SJET&JSHSJE*SFMBOE

i SPVHIIJTQSPGPVOEJOTJHIUUPFOFSHZJTTVFTUIFBVUIPSTVDDFFETUPFYQMBJOJOBOFBTZand understandable way the science and facts behind integrating large amounts of variable SFOFXBCMFTJOUPFOFSHZTZTUFNT)FFYQMBJOTJOBOFMFHBOUXBZUIFVOEFSMZJOHQIZTJDTBOEprovides a lot of useful data.1FUFS-VOE1SPGFTTPSJO"EWBODFE&OFSHZ4ZTUFNT"BMUP6OJWFSTJUZ'JOMBOE

"A timely book to demonstrate how to balance the power supply and demand under very complex conditions, such as renewables integration, temperature variation and demand uncertainty."

Chongqing KangProfessorDepartment of Electrical EngineeringTsinghua UniversityChina

"The author does a nice job to describe the changing character of electric power generation while respecting the fundamentals and working principles of an integrated grid."

Thomas KeySenior Technical ExecutiveElectric Power Research Institute EPRIUSA

Solutions for Integrating Renewables

Jacob KlimstraSolutions for Integrating Renewables

Table of Contents:Foreword. ............................................................................................................................................. 8Note to the reader. ............................................................................................................................. 9

1. How to secure the electricity supply in a changing world ....................................... 10 1.1. An affordable, reliable and sustainable supply of electricity ..................................... 12 1.2. Challenges of renewable energy sources ....................................................................... 18 1.3. Faults and failures in electricity supply systems .......................................................... 21 1.4. Balancing supply and demand in energy markets ....................................................... 22 1.5. Conclusions .......................................................................................................................... 26

2. Balancing the electricity supply in case of calamities .....................................................28 2.1. Matching electricity supply and demand ...................................................................... 30 2.2. Primary control reserves compensating for the failure of a power plant ............... 31 2.4. Conclusions .......................................................................................................................... 49

3. Balancing power demand and supply when conditions change ...................................50 3.1. Electricity supply differs depending on local situations ............................................ 52 3.2. Power demand pattern in Finland exemplifying an industrialised nation ............. 52 3.3. Power demand in the Republic of Ireland, exemplifying a system with much wind-based power ................................................................................................... 56 3.4. The 50Hertz transmission system operator region in Germany, a region with much solar-based power .......................................................................... 67 3.5. Effects of photovoltaics on other power plants in Texas and California ............... 72 3.6. Conclusions .......................................................................................................................... 75

4. Active and reactive power ........................................................................................... 76 4.1. Reactive power analogies .................................................................................................. 78 4.2. The three basic load elements in alternating current systems .................................. 78 4.3. The power factor cos ...................................................................................................... 84 4.4. Impedance of electricity transmission systems ............................................................ 86 4.5. Voltage change over a power transmission line ........................................................... 89 4.6. Risks created when insufficient reactive power is supplied by renewable energy sources............................................................................................ 93 4.7. Conclusions .......................................................................................................................... 95

Jacob Klimstra & Wrtsil Finland OyEditorial work: Jussi LaitinenGraphic design: Jiipee MattilaPrinting house: Arkmedia, Vaasa 2014

Publisher: Wrtsil Finland Oy1st editionISBN 978-952-93-3634-0ISBN 978-952-93-3635-7 (pdf)

5. Energy storage .............................................................................................................. 96 5.1. The enormous challenge of energy storage................................................................... 98 5.2. Basic properties of energy storage devices ..................................................................100 5.3. Applications for energy storage devices ......................................................................101 5.4. Methods and costs of energy storage ...........................................................................104 5.7. Discussion on energy storage .........................................................................................117 5.6. Conclusions ........................................................................................................................119

6. Costs of producing electricity ..................................................................................120 6.1. Challenges in determining kWh costs .........................................................................122 6.2. Varying conditions for generating electricity .............................................................122 6.3. Cost analysis for different generating techniques ......................................................124 6.4. The total costs of producing electricity .......................................................................134 6.5. The electricity price for consumers ..............................................................................139 6.6. Discussion regarding electricity production costs ....................................................140 6.7. Conclusions ........................................................................................................................140

7. Future power supply systems ....................................................................................142 7.1. The road towards an optimum power supply system ...............................................144 7.2. An optimised generating portfolio without renewables ..........................................145 7.3. An optimised power plant portfolio design with renewable energy sources ......149 7.4. Discussion regarding the suggested optimum power supply portfolio ................157 7.5. An optimum electricity generation portfolio for emerging economies ...............159 7.6. Conclusions ........................................................................................................................160

8. Power supply challenges A review ........................................................................162 8.1. Realism is needed in the energy debate ......................................................................164 8.2. Low capacity factors escalate balancing issues ...........................................................165 8.3. Flexible local generators of limited size offer excellent backup for renewables .....167 8.4 Natural gas is ideal for backup capacity ........................................................................167 8.5. Integrating power demand and heat demand offers good perspectives ...............168 8.6. Agile, flexible power plants help to ensure a reliable and cost-effective power supply ....................................................................................169 8.7. Outlook for the future .....................................................................................................170

Appendix 1 ......................................................................................................................................172Appendix 2 ......................................................................................................................................176Biograph ...........................................................................................................................................182References .......................................................................................................................................183Glossary ...........................................................................................................................................185

Foreword

Hans ten BergeSecretary General of Eurelectric

Europe's electricity markets are changing. Historically, electricity markets were based on genera-tion capacity with comparatively low fixed costs and high variable (fossil) fuel costs. But the ratio of variable and fixed costs is shifting, as renewable generation based on solar and wind with little to no variable cost increasingly enters the market.

Despite technological advances and efficiency gains, even the most modern, state-of-the-art fossil fuel capacity is finding it increasingly difficult to compete in a market where subsidised capacity is able to generate at zero variable cost. Yet firm capacity will be needed to back up variable generation. In this context, many argue the current market environment is no longer fit for purpose.

The penetration of low-carbon, intermittent, generation capacity is a positive step towards less carbon-intensive electricity systems. However, the change in generation portfolios does pose a number of challenges for the markets. How do we ensure that sufficient capacity is available when the wind doesn't blow and the sun doesn't shine? Currently subsidy schemes remove variable renewable capacity from market price signals. This cannot and should not be a long-term option for any type of electricity generation.

This is no academic debate. Rather, the European electricity industry is already feeling the effects of the recent changes on the ground. Companies have to mothball recently built gas plants, for instance, or put investment projects on hold. The unfavourable market condi-tions are also discouraging external investors from putting their money into the electricity sector. Meanwhile, policy support costs, for instance for renewables or energy efficiency, are increasing the price that end customers pay for their electricity.

In short: the challenges are big and the solutions are as yet unclear. A fresh look at market design and at a 'smarter' energy system in general is needed. This book contributes to that dis-cussion, with a particular focus on the effects for generators. In describing and analysing the current environment and the way that some of the challenges can be addressed, it addresses key concerns of the European and global electricity industry today.

Note to the reader

Jacob Klimstra

Many readers of this book have to make important decisions about electrical energy supply in different regions. Electricity is crucial for creating wealth and comfort in a modern society. With ever-growing demand for electrical energy, its generation has a huge impact on global fuel con-sumption and the related emissions. This demand is expected to double over the next twenty years or so.

During the past hundred years, scientists and engineers have acquired a thorough knowl-edge of the power supply system. Modern power plants achieve high fuel efficiencies, and their emissions have been drastically reduced. Excellent transmission and distribution systems ensure a high degree of reliability in the power supply to consumers. However, measures need to be taken to make the electricity supply more sustainable. Ultimately, the depletion of fossil fuels will occur and the issue of global warming from greenhouse gases cannot be neglected. Therefore, the power sector has to adjust accordingly and find a new path. Decisions made now will have a long-term impact and optimum solutions have to be chosen.

The purpose of this book is to explain the challenges arising from the advent of a large volume of intermittent renewable energy sources. In addition, innovative solutions are offered for keeping the power supply system reliable and affordable. The book also discusses the effects of renewable energy on the cost of electricity per kilowatt-hour. Low costs are very important since energy is so intertwined with the economy any increase in the price of electricity has a significant impact on the cost of products and services.

Power systems are not based on feelings and opinions, but on scientific and technical facts. Therefore, some mathematics and physics are presented in this book. Nevertheless, readers without a technical background should also be able to understand the issues displayed.

Writing this book took much more effort than initially expected. Large levels of intermit-tent power sources in a system have an impact on the requirements for contingency reserves, on balancing electricity production and demand, on supplying reactive power and on costs. This required searching for actual data on the output of solar panels and wind turbines, in combination with actual power demand patterns. Fortunately, most European and USA-based transmission system operators make the necessary information available on the internet, although often heavy number crunching was required to transform this data into a workable format. Also, the International Energy Agency always provides useful information. Yet, this book does not pretend to cover every aspect of the issues at stake. Hopefully it helps the reader to gain more insight into the matter and serve towards achieving the best solutions.

I am indebted to Wrtsil for offering the possibility to write this book. The continuous support from a number of co-workers during its preparation is highly appreciated. In alpha-betical order, I would particularly like to mention Christian Hultholm, Jaime Lopez, Jiipee Mattila, Jussi Laitinen, Krt Aavik, Kenneth Engblom, Kimi Arima, Mats stman, Niklas Wgar and Svante Bethlehem. I am especially grateful to my wife Anna Martha who allowed me to dedicate so much time to writing of this book.

1How to secure the electricity supply in a changing worldThe economy is largely built on a reliable supply of cheap electricity. A challenge is to keep the supply system stable and affordable with the rapid expansion of intermittent renewable energy sources. The new system cannot just be built on top of the old one. To make the integration successful and to ensure prosperity in the future, new technical solutions and market conditions are needed. Business as usual is not an option for the power sector.

12 Power supply challenges

1.1. An affordable, reliable and sustainable supply of electricity

Electricity is all around us. Without electricity, communications, industrial activities and services come to a halt. Households suffer badly when the power supply stops. Thanks to electricity, life in hot regions is bearable. Agricultural products can be treated and stored for extended periods of time with electrical chilling. Soon, electric vehicles will be common on the roads. Before long, electricity will be the major energy carrier for energy consumers.

Electricity production is still mostly based on fossil fuels. Because of the emis-sions that these fuels produce during combustion, legislators and society in general are demanding more renewable energy sources. However, the advent of renewable energy based on, for example, solar radiation and wind is creating challenges in maintaining the delicate balance between electricity production and demand. Power plants charged with the balancing task have to adapt their output faster and more frequently than before. Errors in forecasting electricity production from renewable sources add up to errors in demand prediction. Consequently, more reserve capacity, with a much more responsive character than in the past, is needed. Traditional steam-based power plants lack this flexibility. However, agile generating techniques exist that have the ability to assist in accommodating vast amounts of renewable electricity sources and help, in so doing, reducing use of fossil fuels.

According to the media, energy storage, smart grids, huge transcontinental power transmission lines and demand side management can solve the issues arising from the intermittency of renewable electricity sources. Such news should be judged with great care.

It would be convenient to have affordable storage systems playing a major role in balancing electricity supply with demand. Storing electrical energy directly as electricity is not yet possible in practice. Energy has to be stored chemically as in fuel and in batteries, or mechanically as in flywheels, compressed air and raised water-reservoir levels. Heat from concentrated solar power systems can be tem-porarily stored in molten salts for steam generation at a later point in time. The challenge is to find economic storage systems that have the right properties to serve the balancing of electricity generation and demand. The required storage proper-ties depend on the type of balancing required. Flywheels might help for short-term frequency regulation in time spans of a few seconds, while batteries can help to cover unbalances up to an hour and pumped hydro can take care of smoothing in 24-hour intervals. However, no storage systems exist yet that can substitute the use of fuels such as natural gas and coal in covering, for example, seasonal lacks in power output from renewable sources, such as those occurring with solar PV output during the darker seasons.

High winds can occur in continent-wide areas, so smoothing wind-turbine output with long transmission lines is not an effective option. Using excess electricity during the peak output periods of solar panels and wind turbines for water heating and chilling is a better option. Smart appliances and smart meters in households

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1. How to secure electricity supply in a changing worldy 13

appear to offer only very limited possibilities for balancing the supply of electricity with the demand.

The use of electrical energy is directly linked with economic value, as can be seen in figure 1.1 In contrast to common belief, the domestic use of electricity in households is, on a global average, less than a quarter of the total electricity use. The large remaining portion is consumed by industrial users and by commercial users to create economic value. Electricity is, therefore, primarily a value creator.

A number of conclusions can be drawn from the relationship between gross domestic product and electricity use as shown in figure 1.1 Simply said, if the power supply in Africa would increase by a factor of five, the economy might potentially also grow by a factor of five and much poverty would disappear. In addition, if North America would lower the intensity of electricity used in its economy to the European level, electricity consumption might be lowered by some 20% without losing any wealth. The use of more efficient appliances, better building insulation, and a large-scale introduction of LED lighting are expected to contribute to reduced electricity use in the USA. In Europe, by contrast, the replacing of gas-fuelled heating with electric heat pumps and the advent of electric vehicles might lead to some increase in electricity use. China appears to closely follow the global trend line between elec-tricity use and GDP. The Middle East is clearly an outlier: cheap fuel, hot climate, and relatively low industrial output result in low GDP creation per unit of electric

Figure 1.1. There is a direct relationship between the amount of electrical energy used (kWh) and wealth levels, as expressed in gross domestic product based on purchasing power parity (PPP).

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energy. Nevertheless, although economic boundary condi-tions differ from country to country, electricity use and economic welfare are closely related.

Despite its excellent value to society, electricity has to be affordable. High electricity prices can be a reason for energy-intensive industries to move to a country with lower prices. Distinguishing between the cost, price and value of an economic commodity such as electrical energy, helps in better understanding the mechanisms leading to affordability.

The basic costs of electricity consist of the cost of the capital investment for the generating unit, the cost of electricity transportation and distribution facilities, the cost of fuel and the cost for operation and maintenance. The price customers pay for electrical energy normally contains at least these basic costs, with profit margins and government-imposed taxes added. The ultimate economic value for the customer per kilowatt-hour delivered should naturally be higher than the price that the customer pays.

A domestic consumer generally pays more per kWh than an industrial user. This is partly because of higher distribution and retail costs, but also because profit mar-gins and levies are generally higher in the case of private customers. For an aluminium smelter, the value, price, and cost of electricity are basically close together since energy costs heavily determine the end-product costs. For a scientist, banker, or family member using a desktop computer, the cost, price and especially the value of electricity can be factors different. Computers raise pro-ductivity so much that the price of the electricity to run them is almost irrelevant

Figure 1.2. An example of where electricity substantially increases productivity.

Figure 1.3. The cost, price and value of electricity compared

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for the user. In households, a 2 kW vacuum cleaner has the same power as 40 people using dustpans and brushes. One hour of vacuum cleaning might cost 0.50 for the electricity, but hiring 40 cleaning people instead might cost at least 500 in wealthy economies.

The social costs of electricity can cause the real costs to be higher than the sum of the costs for capital, fuel and operations plus maintenance. Such social costs include, among other things, the value of the environmental damage caused as a result of pollution from the fuel production and from the emissions. Subsidies for mining jobs also have to be included in the social costs. Politicians might claim the creation of a substantial number of jobs connected with the introduction of renew-able energy, but such jobs can also be seen, at least partly, as social costs as long as subsidies dominate the market for renewables. Ultimately, the integral economic value of a product such as electricity should at least exceed all the costs of making that product. If the cost of electricity exceeds its value, using electricity will be a luxury and a burden on the economy, without creating wealth.

Nevertheless, the acceptability of neglecting social costs depends to a large extent on the actual wealth level of the particular country. When people are starving, items such as food and water are urgently needed, and in such cases some connected

Figure 1.4. The densely populated and polluted environment were created in the new industrial cities during the Industrial Revolution (17601840).

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environmental damage is just taken for granted. The industrial revolution in the 18th and 19th centuries had destructive effects on the environment, but the resulting increase in the level of prosperity ultimately released money for repairs and improve-ments. Enforcing the same environmental standards globally for electricity produc-tion, regardless of whether it is in emerging economies or in the affluent areas of the world is, therefore, not fair if the associated costs are high.

While many people use power as a synonym for electrical energy, this book will distinguish between power and energy. It is scientifically incorrect to state that a machine or a power plant can produce power, since power is the capacity to deliver energy. A car can have an engine with a maximum power capacity of 125 kW (kilowatts), but as long as the engine is not running, no energy is sent to the wheels. Driving the car for one hour at full power means that the engine delivers an amount of energy equalling 125 kW 1 h = 125 kWh (kilowatt-hours). An electric power sta-tion of 360 MW (megawatts) constantly running at full output during 4380 hours, equalling half a year, produces 360 4380 = 1576800 MWh (megawatt-hours), or almost 1.6 TWh (terawatt-hours) of electrical energy.

High reliability in supplying quality electricity is obviously important to energy consumers, who generally require that their need for electric energy is fulfilled at any time. Failure to supply electricity will at least be a nuisance, and generally also results in financial losses. Users in commercial and industrial environments expect a power supply system reliability of at least 99.99%, meaning that the supply fails on average in total only 53 minutes per year. For applications where a constant avail-ability of electricity is crucial, uninterruptable power supply systems and backup generators are common practice. For some applications, such as data centres and hospital operating theatres, a supply reliability of over 99.999% is required.

A reliable supply of electricity also requires that the voltage and frequency are maintained within narrow limits. In addition, the delivered voltage should be clean and not excessively superseded by harmonic or random distortions, i.e. voltage varia-tions with a frequency other than the basic frequency of 50 Hz or 60 Hz. Distor-tions are caused by control electronics and by lighting systems such as LEDs. If the

Figure 1.5. A clean 50 Hz sine wave and a sine wave distorted with harmonics.

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voltage deviates too much in value and shape from the standards, the performance of the users equipment will be detrimentally affected, and the equipment might even be damaged. Quality electricity means high supply reliability of the proper voltage.

The electricity supply should also be sustainable. The burden imposed on the environment should be acceptable, while natural resources have to be used as effi-ciently as possible. Technologies are available nowadays to achieve very low emis-sions of pollutants, including nitrogen oxides (NOX) and sulphur oxides (SOX). Both have negative impact on air quality, and cause acidification of water basins and soil.

As an example of emission reductions, the power sector in the USA was respon-sible for 6.2 Mtonnes of NOX in 1995, but for only 2.2 Mtonnes in 2009, thanks to exhaust gas cleaning and cleaner fuels. Yet, total fossil fuel consumption in the USA, meaning oil, gas and coal together, was roughly the same in 2009 as it was in 1995.

Another issue is global warming. Globally, the power sector is responsible for roughly a quarter of anthropogenic CO2 emissions. The European Union aims to reduce greenhouse gas emissions by some 85% from the 1990 levels by the year 2050. The power sector should be emitting zero greenhouse gases by that time. To achieve this, a reduction in energy consumption, the large-scale introduction of renewable energy sources, and carbon capture and sequestration (CCS) for fossil fuel applications are seen as being the major measures. In this context, it is important to know that power plants are long-term investments with a technical life exceeding 40 years. The EU policy means that newly built power plants that are not prepared for CCS might face early retirement.

However, an excessively abrupt weaning from fossil fuel usage in order to bring down CO2 emissions will disrupt the economy. The reason behind this is that per unit of delivered energy most renewable energy sources are more expensive than

Figure 1.6. The substantial decline in average concentrations of NOX and SO

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fossil fuels. Furthermore, energy sources based on wind, solar radiation, tidal flows, and wave energy are by nature variable in output.

The International Energy Agency (IEA) has estimated that just 3.7 %, or 0.8 PWh, of the total global electrical energy demand was derived from renewable sources in 2010, excluding hydropower. A large part of this is based on biomass, pri-marily wood. Wood is often used in existing coal-fired power plants via co-firing or supplementary firing. Burning wood in power stations is heavily subsidized in some countries, but the positive effect on reducing greenhouse gas emissions is question-able. Estimates are that forestry activities and the transportation costs involved might already result in 200 g/kWh in CO2 emissions, i.e. almost the same amount of CO2 that a natural-gas-fired cogeneration plant emits. If hydropower is included in the renewables, some 19.7 % of electricity is currently derived from renewable sources.

The effort required to increase the amount of renewable energy is huge. Inevi-tably, fossil-based power plants will still be needed for many decades. In any case, fully abstaining from the use of fossil fuels is difficult, since these energy sources can easily be stored in large quantities. In particular, natural gas can serve as a ver-satile, cheap and relatively low-carbon backup battery for balancing the intermittent electricity supply coming from wind, solar radiation and tidal-flow generators. Nev-ertheless, fossil fuel resources are ultimately finite. Expectations are that the global demand for electrical energy will almost double over the coming 20 years. Therefore, maximum fuel efficiency is required and any wasting and flaring of fuels should be avoided. The goal should ultimately be to achieve a gradual shift to affordable renew-able energy sources with mature equipment having sufficient warranties from reliable manufacturers.

1.2. Challenges of renewable energy sources.

Electricity demand has always shown variability. Short-term variations in demand occur because electricity consumers switch their appliances on and off at random. The net effect of this on demand is small and conventional generators can adapt their output accordingly. Moreover, there are daily patterns caused by typical societal behaviour, where people go to work or school in the morning and return home in the evening, and finally go to bed. These daily patterns are affected by the seasons, since in the colder regions more lighting and heat are required in the wintertime. In hot regions, air condi-tioning is needed in the summer.

In many areas, seasonal patterns can clearly be distinguished. Figure 1.7 has 17.520 data points to illustrate how the power demand varies in the north-western part of Germany during a full year. Each weekend, there is a sharp drop in demand. The variability in output of fuel-based power plants is much higher even than the variability in demand shown in figure 1.7 This is because of the intermittent output of a substantial amount of renewable electricity sources in the system. It is notable that at the end of the year, when the labour force stops work for the Christmas

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holidays, the demand is very low. During this holiday period, strong winds were prevalent over Germany, resulting in excess electricity that had to be exported to neighbouring countries for a negative fee of up to 200 /MWh.

Electricity generators based on renewable energy sources such as wind, sunshine and tidal flows are generally granted unrestricted feed-in into the electricity grid. Their output however depends heavily on the weather and the time of day. More-over, their output is never fully predictable and is sometimes even close to zero. Figure 1.8 illustrates the variability in output of wind turbines and solar PV panels in the German 50Hertz TSO (Transmission System Operator) region during week

Figure 1.7. An example of the dynamic pattern in the electricity demand in the Tennet region of Germany (data from Tennet).

Figure 1.8. Wind and solar -based power output, and the remaining supply from other sources in the German 50Hertz TSO region, week 26, 2012 (cumulative curve, the black arrows give the extremes for others).

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26, 2012. Early on Monday, wind turbines generated almost all the power that was needed. On Thursday morning at 8 am, however, the output from wind and solar sources was so low that 10.4 GW had to be derived from other sources.

Since electricity transmission and distribution grids have virtually no energy storage capacity, the production and consumption of electricity have to be precisely matched. If the driving power for the generators exceeds the electricity consumption + system losses, the generators will increase their speed and, simultaneously, the frequency in the system will go up. Alternatively, if demand is higher than supply, the frequency will drop. For the system to operate properly, and for many sensitive applications, the frequency has to remain within narrow limits. Therefore, generators are equipped with controllers that can correct their output depending upon the devi-ation from the desired system frequency. Variation in power plant output is therefore necessary, but it is not economic to run a power plant consisting of a single gener-ating unit in a wide load range. Technical restrictions also limit a single generator from having a wide output range. At low loads, the fuel efficiency is low while the maintenance costs per kWh are high. Therefore, generators are switched off if their load is below a certain threshold. Conversely, if the generators that are online cannot meet an increase in demand, additional generators have to be switched on.

With much intermittent renewable capacity in the system, the balancing task of the fuel-based and hydro-based generators is rapidly increasing. When the sun sets, the output from photo-voltaic cells (PV) drops to zero, while electricity demand gen-erally increases. This results in the dispatchable generating capacity having to ramp up its output significantly. Figure 1.9 gives an example of the rapidly changing output from all the wind turbines in the German Amprion TSO region during the 24 hours of April 28, 2012. This illustrates a typical example of the passage of a depression. Two substantial increases in the wind-power output of up to 1 GW per hour were

Figure 1.9. Large differences in power output from wind turbines in the German Amp-rion TSO region on April 28, 2012.


observed. The large decline in wind-power output, from 1.8 GW to 0.6 GW, in the time span from 11 am to 12 am required a large amount of fast backing-up by power plants. Even worse situations occur when the wind reaches gale force and wind tur-bines have to be stopped in order to avoid physical damage. Because of this, backup power plants have to be increasingly flexible.

Transmission system operators (TSOs) try to introduce Demand Side Manage-ment (DSM) for balancing. Sometimes, it is called Demand System Response. Typical electric appliances, such as refrigerators and air-conditioners, can be switched off for a while. The use of washing machines and laundry dryers can often be postponed to when the general demand for electricity is dropping. This requires smart appliances that respond to a signal from the grid operator. Variable pricing of electricity might also help, and for this smart meters with a momentary tariff indicator are needed.

Nevertheless, such demand management measures can only be part of the solution to keep the system stable. A huge number of appliances would have to be controlled to have any noticeable effect. As an example, using DSM to compensate for the 1.2 GW decrease in wind turbine output, as shown in Figure 1.9, requires switching off the equivalent of 450000 laundry dryers. A laundry dryer runs on average for some 100 hours per year. The probability that a laundry dryer is run-ning at any particular time is, therefore, only 1%. To sum up, it is not expected that domestic electricity demand can be shifted by more than a few percent through the use of smart appliances and smart meters. Better DSM opportunities might be present with industrial users of electricity.

1.3. Faults and failures in electricity supply systems

In addition to the need for normal balancing, faults and failures can and do occur in electricity supply systems. These malfunctions are also called contingencies, and they affect the balance between supply and demand. A failing power plant results in an instantaneous loss of electricity supply. Immediately upon the occurrence of such a loss in generating capacity, the rotating inertia in the system helps to avoid an abrupt change in frequency. A consequent drop in frequency is unavoidable, but system operators allo-cate spare output from the generators that are online to compensate for the lost unit. This spare output is called primary reserve. After application of the primary reserves, additional generating capacity is rapidly activated to restore the frequency to the desired value so that the primary reserves are available again for the next contingency. This addi-tional capacity is called secondary reserve. With a growing fraction of the power capacity derived from non-dispatchable generators in the system, it becomes increasingly diffi-cult to have adequate backup capacity within the system. Having just a few large power plants online is very risky, since then the relative effect of one power plant failing on the dispatchable capacity is large. Modern contingency reserves have to consist of smaller agile power plants that are well distributed across the area to be served.

Cogeneration units are an example of such distributed power plants. Cogenera-tion of heat and power (CHP) is an effective means of improving fuel efficiency and

Figure 1.10. The typical elec-tric power of a laundry dryer is 2.7 kW.

Power plant

Transmission substation

High voltage transmission lines

Transformer

Power substation

Power poles

Transformer drum


reducing greenhouse gas emissions. With an adequate number of such local genera-tors in a system, these units can also be utilised for frequency control and balancing. Again, highly flexible power plants with fast ramping rates and short starting and stopping times will be needed for balancing electricity production and demand.

Faults in the transmission and distribution system cannot be avoided. Trees may fall on high voltage lines, and icy rain in combination with high winds can damage the wires. Excavations frequently cause damage to underground cables. Decentral-ized generation is beneficial in this respect, since it reduces the dependence on a few distant generators and long power lines. Because of their increased contribution to electric generating capacity, decentralized generators should also be able to comply with the grid codes set by transmission and distribution system operators for large power plants. Being able to ride through a short circuit is an example one of the new requirements for local generators (see appendix 2).

1.4. Balancing supply and demand in energy markets

W hen electricity was supplied only by fully integrated utilities, all costs in the system were supposed to be covered by the tariff charged to the customers. In such a system, any profit goes to the system owner, which is often the state, a province or a munici-pality. The system owners are also responsible for any financial losses. In some countries, electricity is even subsidised. Planning expansions to, and renewals of, the generation

Figure 1.11. An integrated power company producing and delivering electricity to end users.

Politicians

Government

Regulator

Transmissionsystem

operator

Distribution systemoperator

Competingelectricity retailer A

Competingelectricity retailer B

Competingelectricity retailer C

Competinggeneratingcompany 1

Competinggeneratingcompany 2

Subsidisedintermittent

renewable generation

Large industrialcustomers

Large industrialcustomers with self

generation

Small industrialcustomers

Commercials

Households

Emission limits,emission tax and

renewable subsidiesEnergy tax and renewable

subsidies

Variable rules Variable electricity owVariable emission limits, tax and subsidies


and distribution system is easy in such circumstances. For this reason some govern-ments and politicians prefer a situation whereby the electricity supply is fully controlled by integrated utilities, with perhaps a few independent electricity producers feeding into the grid.

However, the free-market thinking at the end of the twentieth century advocated economic liberalisation, with privatisation and deregulation in all segments of all markets. By having private investors take over the role of the public sector, produc-tivity was supposed to increase and the costs to consumers would then be reduced.

A power supply run by the public sector was, and often is, considered to be bureaucratic, ineffective and less customer friendly. Is this truly the case? That is the big question. Currently, the liberalised and unbundled power sector complains of permanent interference from policy makers with ever-changing rules and high sub-sidies for some types of generation, making it difficult to invest in new power plants. Yet, extensive lobbying continues simultaneously by the different stakeholders for

Figure 1.12. An unbundled power sector affected by many players.


getting preferential rules for their typical facility or technology. Constant changing of the rules creates difficulties for long-term investments.

Power stations have a very long technical life, and transmission and distribution lines last even longer. This is the reason that some countries, especially in Asia, have decided not to adopt the liberalised market model, which is generally dominated by short-term profit making and quarterly results. Moreover, in many countries with a liberalised electricity market, the government still derives much income by taxing energy use and by charging value added tax. The ultimate price of electricity to domestic consumers has, in general, not decreased as a result of the new open markets.

Grid frequency, voltage levels and reliability all have to be guaranteed, even in an open electricity market. Therefore, independent transmission system operators (TSOs) are charged with the control of frequency and voltage, and with setting rules for maintaining grid stability and supply reliability. TSOs estimate the power needs for the near future with elaborate prediction models, and use a market mechanism to ensure that sufficient generating capacity will be available. With the introduction of much intermittent generating capacity from renewables, the uncertainty in predicting the output required from non-renewable power plants is growing. As an example, the large changes in output from wind turbines as shown in Figure 1.9, were not predicted by the forecasting models, as can be seen from figure 1.13.

In the simplest open market approach, a power plant is remunerated only for the energy delivered. The producer that offers the cheapest electricity would be first in the merit order in a national or regional energy market. In this simple market model, the TSO requires electricity producers to include all relevant services, including

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er o

utpu

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d tu

rbin

es(M

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Figure 1.13. An example of a large deviation between the predicted and the actual power output from wind turbines in the German Amprion TSO region, April 28, 2012.

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Figure 1.14. Examples of imbalances caused by electricity trading (data from KEMA report 74100846-ETD/SDA 12-00079, Swissgrid and TENNET.


backup for failing power plants and frequency control in their energy delivery offering.

In a more extended market model, power plants can be remunerated for the avail-ability of reserve power and for their capability to achieve fast ramping up or down of their output. Even factors such as starting up reliability and supply reliability, might be worth rewarding. Manufacturers of energy storage technologies are aiming for financial compensation for the balancing capabilities of their products. Apart from pumped-hydro storage, most technologies for short and medium-term storage are still under development, and researchers are eager to promote their technologies to subsidy providers.

Electricity supply markets generally operate by offering energy in fixed time spans, such as in hourly or even 15 minute intervals. This approach gives rise to periodic deviations in grid frequency. This is illustrated in figure 1. 14. Frequency stability has, therefore, decreased since the introduction of open electricity markets. Each time a trading time span ends, or begins, power plants increase or decrease their power output. This has to be compensated for by the frequency regulation capacity of the power plants, which was originally intended for occasional contingen-cies, such as the loss of a power plant. Sluggishly reacting power plants have diffi-culty in restoring the grid frequency to within its required range.

In chapter 7, this book will show how a properly selected generating portfolio in an electricity supply system can improve system stability with reduced costs and higher reliability. With a proper approach, this stability can even be reached with a high proportion of intermittent renewable generation in the system. Much intermit-tent generation inevitably reduces the utilisation factor of the other power plants. The consequence of a low utilisation factor is higher specific capital costs (/MWh) for fuel-based power plants. Low investment costs will, therefore, be a key element for new power generating capacity.

The shift towards more renewable generation in the system will certainly reduce fossil fuel consumption. However, the consequent decrease in the utilisation factor of the other power plants will inevitably increase the capital costs per kWh produced. Moreover, fuel-based power plants will have to be far more flexible in the future, with frequent starts and stops and high ramping rates in output.

1.5. Conclusions

A steady growth in electricity use, coinciding with concerns for sustainability, creates substantial challenges. Policy makers interfere increasingly with markets and use subsi-dies and levies to achieve their targets. Investors face uncertainty of profitability because of frequently changing boundary conditions. Dispatchers of power supply systems have to live with the challenges of variable outputs of renewable energy sources and the uncertainties from forecasting errors. To compensate for the unpredictability of the markets and to backup the intermittent output of renewables, a new level of flexibility is needed in power systems.

2 Balancing the electricity supply in case of calamitiesStability in electricity supply systems has to be maintained even during disturbances such as a major short circuit, generator failure or losing a large load. In keeping the system stable, the role of rotating inertia is essential. When integrating renewables with no or low inertia to the system, the balancing becomes more difficult. To avoid risks of frequency collapses and blackouts, new solutions are needed for the fuel-based backup generation.

Balance

InertiaGeneration Demand


2.1. Matching electricity supply and demand

The main task of an electric power supply operator is to continuously match electricity generation with electricity demand. Continuous matching is needed since the supply system as such cannot store electrical energy. If electricity demand systematically exceeds the power delivered by the machines that drive the generators, the generating units will respond by decreasing their rotational speed. Consequently, the grid frequency will drop and the system will collapse in a matter of seconds, resulting in a blackout. Fortunately, blackouts wont occur if the unbalance in demand and supply is short-term, since generating units and electric motor drives have energy stored in their rotating mass, the so-called inertia. That buffer limits the rate of change in frequency in the case of an unbalance between generation and demand. The energy stored in this inertia creates time for the engines or turbines that drive the generators to adjust their output in order to restore balance.

The stability of the frequency in alternating current sys-tems is a good measure of balance. Frequency is by definition the number of times that a full sine wave occurs per second in the grid. The international unit denoting frequency is hertz (Hz). The rotational speed of the generators determines this frequency, and the so-called nominal value of the fre-quency depends on the global location. America and Japan operate using 60 Hz, while most other areas of the world have 50 Hz. In reality, the grid frequency varies somewhat around the desired value. Figure 2.2 shows the grid frequency in The Netherlands during a short time span of 5 minutes after 5.00 am on April 12, 2013. Customer demand is never fully con-stant and generators also have some variability in their output. Yet, on average the frequency has to match the desired value.

If the actual frequency deviates for just a small fraction from the desired value, no action is taken to change the output setting of the generators. There are a number of reasons for that. Each measurement system is afflicted with some inaccuracy, while control systems also have some insensitivity. Endeavours to keep the frequency within very narrow limits, meaning real isochronous operation, would result in overactive control of the machines that drive the generators, which in turn would lead to unnecessary wear. Figure 2.2 shows an example of a system with a permitted measurement error range of +/ 10 mHz, with an additional zone of +/ 10 mHz where no action from the generator is required. Consequently, the result is a total dead band of +/ 20 mHz. Nevertheless, the average frequency over a prolonged time span should be exactly 50.000 Hz, and grid operators take action if the cumulative deviation from this desired value becomes excessive.

In Figure 2.2, the grid frequency exceeds the dead band at 20 seconds after 5.00 am on April 10, 2013. At that moment, the so-called primary control reserve power

Figure 2.1. An illustration of the deli-cate balance between electricity demand and production, with rotating inertia as a buffer with some energy stored.

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2. Balancing the electricity supply in case of calamities 31

plants start to automatically slightly reduce their output settings. Some one hundred seconds later, the frequency is back within its allowed limits and the control action of the generators ceases. This frequency regulation action is handled automatically by the so-called primary reserves. Automatic action is the only option because of the fast response required to keep the frequency within its narrow band.

The primary control reserves are also known as frequency containment reserves (FCR). Substantial changes in frequency will occur if a large customer disconnects, or if a large power station suddenly fails. In modern power supply systems, many generators are interconnected via the transmission grid. The number of online gener-ators should be sufficient to ensure that a failure of the largest unit can be absorbed, to a large extent, by the spare capacity of the other generating units.

2.2. Primary control reserves compensating for the failure of a power plant

It is interesting to analyse what happens when a power plant in an electricity supply system fails. To simplify such an analysis, we presume a supply system with ten power plants of the same power capacity. Each of the ten power plants has a nominal power capacity of 500 MW, and they are all running at 90% of their capacity to provide 10% of primary control reserves. That would, at least in theory, be sufficient to compensate for a failure of one of the ten power plants. Nominal power means the nameplate power of the generating unit, while nominal speed means the generating units normal amount of revolutions per minute. In this example, the electricity demand that the ten power plants

Figure 2.2. Example of the rules for frequency control, with an example of actual frequency variations (frequency data source: TENNET)

1 2 3 4 5 6 7 8 9 10

Demand by customers

Transmission linesTransmission lines

Energy in Energy out

Rotational energyEr= l r

2=2 pi f = 2 pi n/60


supply amounts to 90% of 10 500 MW = 4500 MW. It will now be shown what hap-pens if, for example, power plant number 7 suddenly fails and the system immediately lacks 450 MW of the required power supply. This example may appear to be somewhat exaggerated since a sudden loss of 10% of the dispatchable generation is not common. However, with much renewable capacity in a system, such occurrences are becoming increasingly realistic. In addition, the effects of unbalance in a system can be clearly shown with this example.

After the failure of one plant, the nine remaining power plants cannot instanta-neously ramp up the power output of the machines that drive the generators from the initial 450 MW to the newly required 500 MW in order to supply the total system demand of 4500 MW. Power plants need some time to react to a newly desired output value. Therefore, if no energy was available from the rotating mass (the rota-tional inertia) in the system, the unbalance would immediately stop all generators with a resulting blackout. The amount of energy stored in a rotating generating set

Figure 2.3. Ten power plants initially supplying the required electricity demand when one of them, number 7, fails.

Figure 2.4. The energy stored within the power supply system as a result of the fly-wheel effect of the spinning generating units. (f = rotational speed in revolutions per second, Ir = moment of inertia).


is linearly proportional with the moment of inertia I, and the square of the running speed n. Inertia is a property characterising the flywheel effect of the rotating mass. The running speed n gives the number of revolutions per minute of the generator rotor. In a 50 Hz system, a generator with a single pole pair runs at 3000 rpm. In this case, the frequency f equals n/60.

The amount of energy Er stored in the rotating inertia of a generating set is gen-erally expressed as a fraction of the nominal power capacity Pnominal of that generating set. This fraction is called the inertia constant I of a generating set:

=. (2pif)2 .I= =Er

PnominalIr

Pnominal.Ir.2Pnominal

The dimension I of the inertia constant is the same as that of time, and is expressed in joule/watt = J/(J/s) = s (second). The inertia constant of a large gen-erating unit lies in the range between 5 and 10 s. This means that when a 500 MW generating unit is running at its nominal speed, its rotating parts have 2500 MJ of rotational energy in the case of a I of 5 s, and 5000 MJ for a I of 10 s. This is also the amount of energy that has to be transferred to the rotating parts when the generating unit is started up and accelerated to its nominal speed. If the 5000 MJ of energy during this acceleration is supplied to the rotating inertia with a machine that uses natural gas for fuel, we can calculate the amount of gas required. If the fuel effi-ciency of the driving machine is 40% and the natural gas has a lower heating value of 36 MJ/m3, it requires 5000/36 100/40) 350 m3 of gas to bring the rotor up to nominal speed. Such an amount of gas provides enough energy to heat up 35000 litres of water from 20 C to 100 C, for preparing 280000 cups of tea. A cluster of 3000 car batteries can also deliver the required amount of energy. This illustrates that although the energy stored in the rotating inertia of a generating set is not elec-trical, it is nevertheless an impressive amount of energy.

2.2.1. The usefulness of the inertia constant IThe inertia constant I is very helpful in getting a first impression as to how fast a gen-erating unit will change speed in case of an unbalance between the power supply to the generator and power demand. Unbalance occurs when the electrical load changes while the power supplied to the generator shaft from its prime mover, i.e. the driving engine or turbine, remains the same. For an inertia constant of 5 s, the amount of energy in the rotating parts is enough to supply the nominal load of the generator for 5 s without any energy input from its prime mover. After that time span, the generator will come to a standstill. Furthermore, if the prime mover were to supply 90% of the nominal load to the generator, while the load of the generator equals the nominal load, the rotational energy would be enough to cover the unbalance for 50 s. Thus, it takes ten times longer than in the case of no power supply from the prime mover before standstill is reached. However, even in the latter case, the frequency will rapidly reach a value outside the permissible range.

Equation 2.1


Equation 2.1 reveals that the rotational energy of the generating set is propor-tional with the square of the instantaneous running speed. This means that at higher speeds, there is much more energy in the inertia than at lower speeds. Therefore, the frequency will not decrease linearly with time if there is a fixed unbalance between the power supply to the generator and the generator load. Figure 2.5 shows how the frequency of the grid served by the nine remaining generators (Figure 2.3) decreases if each generator receives 450 MW from its prime mover while the combined load remains 4500 MW. Each of the nine running generators should receive 500 MW to avoid an unbalance. Therefore, each generating unit feels a power supply deficit of 50 MW. The frequency curve in Figure 2.5 is a representation of equation 2.2. The derivation of equation 2.2 requires some considerable mathematical manipulation. The interested reader can find the derivation of equation 2.2 in Appendix 1.

. f(t)= ( f 2nominal +f 2nominali

PPnominal . t)

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Figure 2.5. The decline in speed for a generator where the generator load constantly exceeds its driving shaft power by 50 MW (inertia constant

I = 10 s, constant load pre-

sumed).

The drop in frequency during the very first seconds following a major contin-gency event is a perfect indicator of the amount of unbalance. In the beginning, the rotating frequency of the generator set is still very close to the nominal frequency (50 Hz or 60 Hz) and the approximation can, therefore, be made that the initial drop in frequency per unit of time df/dt equals:

f nominalP

2iPnominaldfdt =

Equation 2.2.

Equation 2.3.


Equation 2.3 reveals that in our example of a I of 10 s, a power deficit of 50 MW per generator with a nominal power of 500 MW at a frequency of 50 Hz, results in a change in frequency of exactly 0.25 Hz per second at the start of the occurrence. This is indicated in Figure 2.6 by the brown line. Would the power deficit per gen-erator have been only 25 MW, the decline in frequency would have been halved to 0.125 Hz/s. This simple relationship between unbalance size and the initial frequency change is very convenient, especially in island operation where just a few generators have to maintain grid stability. Based on the value of the inclination in frequency versus time, the fuel supply to the machine driving the generator can be immediately and adequately adapted so that no time is lost in restoring the generator frequency. The horizontal red lines in Figure 2.6 give the maximum allowed dynamic frequency limits in the Continental Europe synchronous area. The green line in Figure 2.6 rep-resents the nominal grid frequency of 50.000 Hz.

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uenc

y (H

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Figure 2.6. A close-up of the first 4 seconds of figure 2.5; the thick brown line gives the decline in grid frequency for a 10% unbalance in the system due to the tripping of a power plant (

I = 10 s).

2.2.2. Self-regulating power in an electricity grid

If the grid frequency decreases, electricity demand automatically goes down slightly. This is primarily caused by the synchronous electric motors in the grid demanding less power, since their load declines along with their running speed. This is called the self-regulating power of the supply system. The power requirement of synchronous motor-driven pumps can decrease by 6% per Hz frequency drop. For motor-driven applications with a constant torque, power demand can decrease 2% per Hz. Since synchronous motors form just a fraction of the total load, the self-regulating power of the system in industrialised countries is generally presumed to be 1% per Hz deviation from the

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nominal frequency. In areas with minor industrial activities, the self-regulating power of the grid will be close to zero.

The positive effect of self-regulating power should not be overestimated. If the grid frequency drops from the desired value of 50 Hz to 49.8 Hz, the self-reg-ulating power lowers demand by only 0.2 1% = 0.2%. This 0.2% is only a 9 MW reduction from the 4500 MW total load in our ten generator system example. If, however, the grid frequency would drop from 50 Hz all the way down to 40 Hz, the decrease in electricity demand because of the self-regulating power would be 10 Hz 1% / Hz = 10%, i.e. 450 MW in our example. This renders a 50 MW reduc-tion in demand for each of the nine generators that remained online following the trip of one machine. Consequently, at a frequency of 40 Hz, generation and demand are matched again if the nine power plants keep their power output setting at the initial 450 MW. In reality, it will be difficult for the generators to keep their power output constant when the frequency decreases so much. The power output of the prime mover that drives the generator is proportional with the product of torque M and running speed n. If the grid frequency decreases from 50 Hz to 40 Hz, the driving torque M has to increase by a factor of 50/40 = 1.25 in order to deliver the same power to the generator. This can easily result in mechanical overload. At the same time, turbo machinery in particular is burdened with natural frequencies of the rotor system that limit its range in running speeds. Generators suffer when running at low frequencies because of the so-called magnetic over fluxing, which results in possibly harmful overheating. In practice, the self-regulating power of a grid hardly offers any help in balancing.

Figure 2.7. Mitigation of the decrease in frequency due to self regulation of grid load in the case of a 50 MW initial unbalance between generator output and load.


2.2.3. Explanation of the droop function of generator sets

A decrease in grid frequency from 50 Hz to 40 Hz is excessive and unacceptable for many applications and generators. The maximum deviation from the nominal frequency during dynamic events, such as the loss of a generator or the loss of major load is, therefore, set at +/ 800 mHz in the Continental Europe synchronous control area. If self-regulation of the load alone would be present, this frequency range would be heavily exceeded in the case of calamities. Therefore, each electricity supply system has generators offering primary control reserves that are activated automatically if the grid frequency exceeds its tightly defined limits. The change in output from the primary control reserves depends on the extent of the deviation in grid frequency from the nominal frequency. In other words, the desired output of a generator acting as primary reserve depends on the actual grid frequency; the lower the frequency, the higher the output of the primary control reserves. This dependence of the output set point on the grid frequency is generally called the droop sgenerator (Equation 2.4). The minus sign in equation 2.4 indicates that a decrease f in grid frequency results in an increase P in the power output from the generator providing primary reserves.

Pgenerator = sgenerator1 . f nominal

f.Pgenerator nominal

This is often re-written in terms of the regulating power Pregulating of the system:

Pgenerator = Pregulating . f ,

in which:

Pregulating =Pgenerator nominal

sgenerator . fnominal

The droop value setting of the control system lies generally within a range of 2 to 8%. A droop sgenerator of 4% means, for instance, that the power output of the gener-ator increases from 0% to 100% if the frequency decreases by 4%, say from 50 Hz to 48 Hz. The relationship between output change and frequency deviation is linear. In measurement and control technology language, this is called a proportional action, indicated with the letter P. It means that the extra power from the primary control reserves is only present as long as the deviation from the desired nominal frequency exists.

2.2.4. The function of primary reserves

As explained above, primary reserves are intended to avoid excessive deviations in fre-quency during a major occurrence affecting the balance in the electricity supply system. However, the primary reserves only arrest the frequency temporarily. They cannot return the frequency to its nominal value. Figure 2.8 illustrates how the power output

Equation 2.4.

Equation 2.5.

Equation 2.6.

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4% droop 3% droop 5% droop Frequency limits


from a 500 MW generator running at 250 MW, and acting as a primary reserve, varies with frequency for three different droop settings. For a grid frequency of exactly 50Hz, the output of the generator equals 250 MW. When the grid frequency decreases, the output from the generator will increase linearly along with it. Conversely, if the grid frequency increases, the output from the generator will decrease by following the droop line. Figure 2.8 reveals that the lower the droop percentage is, the heftier the reaction of the generator will be on deviations from the nominal grid frequency.

At first sight, opting for a very low droop value might be the best option to keep the grid frequency as close as possible to the desired 50 Hz. However, if the droop is very small, meaning that the gain factor is very high, the primary reserves may react fiercely to deviations in frequency. This results in extra wear of the machinery, while increasing the risk of oscillations and system instability. Some traditional power plants suffer heavily from rapid changes in output. High-temperature steam boilers can experience cavitation and thermal shock during sudden load changes. Moreover, all generators acting as primary control reserves do not have the same dynamic prop-erties in practice. Each unit has its own delay time when reacting to an increase in the output set point, and each unit will have its own typical ramp up rate. Until now, a close to stepwise increase in output was not possible. Nevertheless, some modern generating techniques can react much faster than traditional units. The typical ramp up rate for primary control reserves in the Continental Europe synchronous area system is 100% within 30 seconds (Figure 2.9). After a short delay of, say 2 seconds, the primary control reserves are supposed to increase their output at a fixed rate.

Figure 2.8. Output from a 500 MW generator acting as a primary control reserve, run-ning at 250 MW at a nominal grid frequency of 50 Hz, with three different droop settings.

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Let us now return to our example of the electricity supply system illustrated in Figure 2.3, whereby ten identical generators were each carrying a load of 450 MW when suddenly one generator tripped. Without any action being taken with respect to the set point of the prime mover that drives each generator, the grid frequency would drop to 40 Hz, as shown in Figure 2.7. However, if each of the nine remaining generators is also used partly for primary frequency control, each with a droop of 4% as depicted in Figure 2.10, the grid frequency will not decrease all the way down to 40 Hz. Due to the droop setting, the set point for full output of each power plant will be reached already at a grid frequency of 49.8Hz, as shown in Figure 2.10. Figure 2.6 reveals that after the contingency of one failing power plant, this 49.8 Hz will already be reached in about 0.5 seconds after the trip of power plant number 7. Therefore, the new set point that asks for full output of the generators can be presumed to be present almost immediately after the loss of one of the ten generators in the system of our example.

However, notwithstanding the quick change in their set points, the nine power plants that use their additional available capacity for primary control reserves will not immediately reach their full output. If we presume that the output of these power plants follow the prescribed ramping up as shown in Figure 2.9, the load from the grid and the power supplied to the generators will equal each other after about 25 seconds (see Figure 2.11). At that point, the grid frequency reaches its minimum.

According to Figure 2.9, the additional 50 MW needed per generator that was lost when one of the original ten generators tripped is fully available from the pri-mary reserves after 30 seconds. This 50 MW per generator is slightly more than the grid load requires for staying balanced at the mentioned minimum in frequency. This is because of the reduction in load created by the self-regulating power. The excess

Figure 2.9. Typical ramp-up rate of primary control reserves.

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500 MW generator running at 450 MW


energy delivered is then used to accelerate again the rotating masses, the flywheels, in the system. Nevertheless, the nominal 50 Hz frequency cannot be reached with primary control reserves following a droop line. In our example, as soon as the grid frequency exceeds 49.8 Hz, the output of the primary reserves once again decreases (see Figure 2.10). That would create another mismatch between power supply and load. Therefore, a deviation between the actual frequency and the nominal grid frequency of slightly less than 0.2 Hz will remain where only primary control reserves are used to restore balance. This deviation between the frequency ultimately reached with the help of primary reserves and the desired frequency of 50.000 Hz is called the quasi steady state deviation. Transmission system operators define the permissible minimum and maximum of this deviation. Extra capacity, the so-called secondary control reserve, is needed for providing the extra power in the system so as to restore the grid frequency to the required narrow band around the nominal frequency.

As mentioned earlier, this narrow frequency band equals only +/ 20 mHz around 50 Hz in the Continental Europe synchronous area. The application of sec-ondary control reserves increases the frequency further, so that the primary reserves can reduce their output and return to their initial load setting at 50 Hz. In other words, secondary reserves release the primary reserves from their duty. This enables primary reserves to be ready for the next major occurrence, such as the sudden loss of a generator or the losing or receiving of a large load.

The previous example whereby primary control reserves equalled exactly the amount of lost generating capacity, is obviously an exception. In reality, the power

Figure 2.10. The droop function of a generator having a 500 MW nominal power, again with a droop setting of 4%, but now running at 450 MW at 50 Hz.

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Only self regulationTypical point for load sheddingWith primary reserves activated


plants in a system do not all have the same capacity. Primary reserves should be large enough to compensate for at least the loss of the largest power plant in a system. In practice, power plants not dedicated as primary reserves will also provide some compensating power for restoring the grid frequency to its nominal value. Neverthe-less, our example illustrates the way con-tingencies are handled in a grid system.

Transmission system operators specify the maximum allowed dip in frequency, officially called the minimum instantaneous frequency after loss of generation. In the Continental Europe synchronous area, this value equals 49.2 Hz. If the grid frequency drops below this minimum allowed frequency, load shed-ding will be used to avoid too deep devia-tions from the nominal frequency. This means that a group of consumers will have no access to electricity for a while. The example with the 10 power plants of equal size, where one of the units trips, shows that losing 10% of generating capacity gives a frequency dip way below 49.2 Hz. Hence, there should be enough power plants in a system to ensure that the loss of one power plants does not reduce the online gener-ating capacity by more than about 3%. In particular, systems having a large fraction of renewable electricity sources need dispatchable power plants of a limited size, and consequently more of them than in the case of large power plants only.

2.2.5. The consequences of a lower inertia constant

Should the inertia constant of the combined generators in a system decrease, for example, due to the introduction of a large amount of renewable energy sources that are indirectly connected to the grid via frequency converters, the grid frequency can drop to quite low values during a contingency. The red line in Figure 2.12 shows a deep dip in frequency if the inertia constant of the system is 5 s instead of 10 s. The other conditions are the same as in Figure 2.11, with 1%/Hz self regulation, 50 MW unbalance per generator and a nominal generator capacity of 500 MW.

The 46 Hz minimum in the red curve occurs 18 seconds after the calamity that tripped one of the 10 power plants in the example. At that point, the output of the primary reserves has not yet reached its maximum, since that occurs 30 seconds after the trip. After 18 seconds, the output of the primary reserve per generator is therefore only 18/30 50 MW = 30 MW. However, the self-regulating power of the grid equals 4 1/100 500 MW = 20 MW for a frequency drop of 4 Hz and a self-

Figure 2.11. Primary control reserves prevent the system from decreasing too much in frequency after a loss in gener-ating capacity (inertia constant 10 s, 1%/Hz self regulation, 50 MW unbalance for a generator with a nominal capacity of 500 MW).

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Inertia constant 10 sInertia constant 5 s


regulating power sensitivity of the load of 1% per Hz. This means that power demand and power supply are fully matched again at 46 Hz so that the frequency will not decrease further.

The output of the primary reserves continues to increase after the minimum in frequency has been reached. This additional power supply will again accelerate the inertia. This will go faster for an inertia constant of 5 s than for one of 10 s, since less energy is needed to bring the rotors back to nominal speed in case of lower inertia. The deep dip in frequency observed for the lower inertia value is unaccept-

Figure 2.12. The effect of lowering the inertia constant on the frequency dip in case of a calamity (further conditions as in Figure 2.11.).

Figure 2.11. Parts of Manhattan were left in the dark after hurricane Sandy in 2012.


able is most cases. If it is not possible to increase the inertia, the amount of primary reserves has to be increased or the primary reserves have to be made faster with a smaller initial delay.

2.2.6. The effect on frequency deviations of more powerful or faster primary reserves.

Increasing the amount of primary control reserves and having a higher ramp rate in the reserves help mitigate the effects of disturbances. Both measures will reduce the undesired deep dip in frequency, and will shorten the time needed to return to 50 Hz following the loss of a generator.

Doubling the power capacity of the primary control reserves, which results in twice as much capacity as the lost output of a failing generator in our previous example, reduces the dip in frequency by almost a factor of two. This is illus-trated by the dark red line in figure 2.13. The reason that the reduction in dip is not exactly a factor of two is that the self-regulating power decreases the load to a lesser extent when grid frequencies come closer to 50 Hz. Another observation is that with a higher amount of primary reserves, the ultimate quasi steady state frequency will be closer to 50 Hz than in the case where the primary reserves equal just the loss in power from the tripped generator. This is because even above 48.8 Hz, where the output reduction of the primary reserves kicks in because of the chosen droop curve, more power than the power lost from the failing generator is available now.

An additional advantage of higher primary reserves is the shorter time that the grid frequency substantially deviates from the nominal frequency of 50 Hz. Figure 2.13 clearly illustrates that the cumulative, or integral, loss in grid frequency over time is much lower for the red line than for the blue line. This cumulative loss is the area between each curve and the 50 Hz line in figure 2.13. The dimension of cumulative loss is Hz s, because it is the result of a deviation in Hz during a given number of seconds. For the blue line, the cumulative frequency deviation equals 146 Hz s. Doubling the primary reserves brings the cumulative frequency deviation in the example of Figure 2.13 down to only 41 Hz s. Therefore, doubling the primary reserves decreases the cumulative frequency deviation by a factor of 3.5 in this case. A grid operator has to ensure that, at the end of the day, the average frequency is back to 50.000 Hz, to avoid for instance, deviations of synchronous clocks. This means that after a frequency dip, all generators have to run at a higher frequency than 50 Hz for a while to compensate for the dip. With a lower cumulative frequency deviation, less effort is required to compensate for this deviation.

Should more primary reserve capacity be made available than the nominal output of the largest generator in a system in order to avoid excessive frequency dips in case of contingencies, a larger amount of generating capacity must be run below nominal load. This has a negative impact on capital costs, fuel consumption, and operation and maintenance costs.

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Only 15 s response time, minimum primary reserves

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Only 15 s response time, minimum primary reserves

30 s response time, double amount primary reserves


Doubling the power-up ramp rate of the primary control reserves, rendering full output in 15 s instead of 30 s, results in almost the same positive effect on the frequency dip as doubling the capacity of primary control reserves. This is illustrated in figure 2.14. The only slight difference is that the ultimate frequency before the secondary control reserves step in will not be slightly above 48.8 Hz. This is the logical consequence of the 4% droop in the example: if the grid frequency rises above 48.8 Hz, the power output of the primary control reserves automatically decreases again. However, if the droop setting of the more agile primary control reserves is changed to 2%, the exact same positive curve as for doubling the primary control reserves will result.

A major conclusion here is that faster primary control reserves offer an effec-tive option for reducing the frequency deviation caused by a contingency. If the relative amount of the systems rotating inertia decreases, such as when much indirectly coupled renewable electricity sources that do not add to the inertia are introduced, the decline in frequency after a major loss in generation will be faster. In that case, more primary control reserves are required to keep the system stable, or the primary control reserves need to have higher ramping rates and a shorter initial delay.

2.2.7. The solution for delivering faste

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