BESS used to support renewables integration through the provision of ancillary services

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POLITECNICO DI MILANO

Scuola di Ingegneria Industriale e dell'Informazione

Corso di Laurea Magistrale in Ingegneria Elettrica

BESS USED TO SUPPORT RENEWABLES

INTEGRATION THROUGH THE PROVISION OF

ANCILLARY SERVICES

Relatore: Prof. Morris BRENNA

Tesi di Laurea di:

Michele AMODIO Matr. 816531

Marco POLENGHI Matr. 816604

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Ci sono sempre due scelte nella vita: accettare le condizioni in cui viviamo o assumersi la responsabilità di cambiarle [Denis Waitley]

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Ringraziamenti

Giunti alla ne di questo percorso universitario, ringraziamo il nostro relatore Prof. Brenna per il suo supporto e tutti i docenti per i preziosi insegnamenti for-niti.

Ci teniamo a esprimere la nostra sincera gratitudine verso i nostri familiari che ci hanno permesso di raggiungere il traguardo tanto atteso.

Un doveroso grazie va anche ai nostri compagni di corso che hanno condiviso con noi momenti di gioia ma anche di dicoltà.

Inne desideriamo ringraziare anche gli amici di sempre per il sostegno che ci hanno dato durante questi cinque anni.

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List of Figures

1.1 Share of generation in Italy . . . 25

1.2 Development of the thermal installed capacity . . . 28

1.3 Development of the hydroelectric installed capacity . . . 30

1.4 Development of the installed capacity of renewable sources during the years . . . 32

1.5 Location of the renewable installed capacity in the Italian regions 33 1.6 Variation of energy produced by renewable sources . . . 34

1.7 Location of the energy produced by renewables in the Italian regions 35 1.8 Development of solar plants . . . 36

1.9 Development of wind plants . . . 37

1.10 Development of plants based on bioenergy . . . 38

1.11 Development of geothermal plants . . . 39

1.12 Share of generation in France . . . 40

1.13 Share of generation in Spain . . . 42

1.14 Share of generation in Germany . . . 43

1.15 Share of generation in Denmark . . . 44

2.1 Example of electrical network for frequency regulation . . . 49

2.2 Hierarchical scheme of the voltage regulation in Italy . . . 55

2.3 Example of transmission system . . . 57

2.4 SVC systems and LC harmonics lter . . . 59

2.5 STATCOM structure based on capacitive storage . . . 60

2.6 Mandatory ancillary services in some European countries . . . 61

2.7 Overview of ancillary services remuneration in some Europian coun-tries . . . 62

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2.9 System electrical frequency in Hz with the increase of the load . . 66

2.10 System electrical frequency in Hz with the decrease of the load . . 66

3.1 PHS system . . . 71

3.2 Flywheel system . . . 72

3.3 CAES system . . . 73

3.4 SMES system . . . 74

3.5 Storage mechanism in EDLC . . . 75

4.1 Electric connections between Europe and Africa . . . 80

4.2 Location of the three pilot projects . . . 82

4.3 Nissan LEAF battery . . . 85

4.4 Tesla Powerwall technical characteristics . . . 86

4.5 Typical connection of a Powerwall . . . 87

5.1 Structure of a module and of a battery . . . 89

5.2 Simplied representation of a battery . . . 90

5.3 Algebraic models . . . 93

5.4 Dynamic model of a battery . . . 94

5.5 Working principle of Li-ion battery . . . 102

5.6 Ragone chart of dierent energy storage technologies including LIC, Li-ion batteries and EDLC . . . 110

5.7 Structure of a LIC cell compared to Li-ion battery and EDLC . . 111

5.8 Parameters of ULTIMO cell models produced by JM Energy com-pany . . . 112

6.1 Illustration of BESS structure composed by many power blocks . 117 6.2 Explanation of the power reversibility in switch-mode DC/AC con-verters . . . 120

6.3 Electrical scheme of a three phase inverter . . . 121

6.4 Illustration of the sinusoidal PWM . . . 122

6.5 Square wave modulation waveforms . . . 124

6.6 Voltage of one phase of a balanced load on the AC side . . . 125

6.7 Electrical connection scheme of an active user with a storage system127 6.8 Capability curve of a BESS in MV network . . . 128

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LIST OF FIGURES 13 6.9 Capability curve of BESS in LV network with inverter power higher

than 6 kW . . . 129

6.10 Regulation of active power for a storage system . . . 131

6.11 Regulation of reactive power in function of the voltage . . . 132

6.12 Behaviour of the inverter in case of a voltage dip in MV network . 133 6.13 Structure of a modular ABB BESS . . . 134

7.1 Single line diagram of the IEEE 30 bus network . . . 141

7.2 Load proles . . . 147

7.3 Percentage of renewables generation prole . . . 148

7.4 Generation proles - Scenario 1 - July 26 . . . 149

7.5 Generation proles - Scenario 1 - December 26 . . . 150

7.6 Generation proles of conventional power plants - Scenario 2 - July 26 . . . 152

7.7 Loading of transmission lines - Scenario 2 - July 26 . . . 153

7.8 BESS characteristic - Scenario 2 - July 26 . . . 154

7.9 Generation proles of conventional power plants Scenario 2 -August 15 . . . 156

7.10 Overproduction - Scenario 2 - August 15 . . . 156

7.11 BESSs characteristics - Scenario 2 - August 15 . . . 157

7.12 Generation proles of conventional power plants with Gen_1 turned o - Scenario 2 - August 15 . . . 158

7.13 Generation proles of conventional power plants Scenario 2 -December 26 . . . 159

7.14 Generation proles of conventional power plants - modied Sce-nario 2 - December 26 . . . 160

7.15 Overproduction - modied Scenario 2 - December 26 . . . 161

7.16 BESSs characteristics - modied Scenario 2 - December 26 . . . . 162

7.17 Generation proles of conventional power plants - Scenario 3 - July 26 . . . 164

7.18 Underproduction - Scenario 3 - July 26 . . . 165

7.19 Generation proles of some photovoltaic power plants - Scenario 3 - July 26 . . . 166

7.20 Generation proles of some photovoltaic power plants with BESSs - Scenario 3 - July 26 . . . 166

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7.21 BESSs characteristics - Scenario 3 - July 26 . . . 168 7.22 Generation proles of conventional power plants Scenario 3

-January 25 . . . 169 7.23 Underproduction - Scenario 3 - January 25 . . . 169 7.24 Generation proles of conventional power plants Scenario 3

-August 15 . . . 171 7.25 Generation proles of some photovoltaic power plants - Scenario 3

- August 15 . . . 171 7.26 Generation proles of some photovoltaic power plants with BESSs

- Scenario 3 - August 15 . . . 172 7.27 BESSs characteristics A - Scenario 3 - August 15 . . . 173 7.28 BESSs characteristics B - Scenario 3 - August 15 . . . 174 7.29 Generation proles of wind and conventional plants Scenario 3

-December 26 . . . 176 7.30 Generation proles of wind and conventional plants with a BESS

- Scenario 3 - December 26 . . . 176 7.31 BESS characteristics - Scenario 3 - December 26 . . . 177

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List of Tables

1.1 Energy exchanges with neighbouring countries . . . 24

5.1 Specications of the GS Yuasa Energy Storage Battery Module LIM50E-7G . . . 104

5.2 Specications of the GS Yuasa 1-MW Class Energy Storage System105 5.3 Specications of Saft Energy Storage Li-Ion Cells . . . 106

5.4 Specications of the A123 System Energy Storage Li-Ion Cell AMP 20 . . . 107

5.5 Specications of the Toshiba 20-Ah Class LTO/NCM Cell . . . . 108

5.6 Specications of the Toshiba Battery for Utility Grid Energy Stor-age Systems . . . 109

5.7 Specications of the Altairnano LTO-Type Single Cells for Sta-tionary Use . . . 109

5.8 Specications of the Toshiba Battery for Utility Grid Energy Stor-age Systems . . . 112

6.1 Dependence between the installed power of an active user and the voltage level of the connection point . . . 125

6.2 Specication of BESSs produced by ABB . . . 136

6.3 Specication of BESS produced by Bosch . . . 137

6.4 Specication of BESSs produced by Siemens . . . 137

7.1 Conventional generators data . . . 142

7.2 Loads bus data . . . 143

7.3 Lines data . . . 144

7.4 Transformers bus data . . . 144

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7.6 Lower and upper reserves distribution - Scenario 2 . . . 151 7.7 Data of renewable generators - Scenario 3 . . . 163 7.8 Lower and upper reserves distribution - Scenario 3 . . . 164

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Sommario

Per rispettare i traguardi imposti dai governi riguardo la penetrazione di fonti rinnovabili nei sistemi elettrici, molte nazioni europee hanno programmato la sostituzione di impianti termoelettrici con unità di produzione che utilizzano fonti alternative, come ad esempio generatori eolici e fotovoltaici.

Se da un lato la presenza di questi impianti è positiva per quanto riguarda l'impatto ambientale, dall'altro potrebbe provocare problemi tecnici e logistici alle reti elettriche. Siccome la produzione da fonti rinnovabili potrebbe essere in-controllabile e imprevedibile, si potrebbero vericare squilibri tra la generazione e il carico e alcuni degli impianti termoelettrici tradizionali potrebbero presentare un funzionamento non accettabile. Inoltre la presenza di generazione distributita sui lati di media e bassa tensione potrebbe portare ad un'inversione di potenza dalla rete di distribuzione a quella di trasmissione, causando sovratensioni e in-terventi intempestivi delle protezioni.

I servizi ancillari, come per esempio la regolazione di frequenza e di tensione, sono necessari per supportare l'integrazione delle rinnovabili nei sistemi elettrici in modo da garantirne un funzionamento adabile. Una soluzione per fornire questi servizi è rappresentata dalla combinazione delle rinnovabili con sistemi di accumulo così da poter ridurre l'aleatorietà delle sorgenti e controllare la potenza generata in funzione delle necessità. In particolare i sistemi di accumulo elet-trochimici (BESS), di cui ci si aspetta una futura crescita grazie alla loro ver-satilità e alle buone prestazioni, sono più adatti di altre tecnologie in termini di posizionamento, risposta e essibilità della capacità.

Il ruolo dei BESS è analizzato simulando tre scenari caratterizzati da una penetrazione di fonti rinnovabili molto diversa. I risultati confermano che gli ac-cumuli elettrochimici, a dierenza di una situazione passata contraddistinta da un basso contributo da impianti eolici e fotovoltaici, risultano essere importanti

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quando viene considerata una penetrazione di energie alternative nella stessa percentuale di quella italiana nel 2013. Si sono ottenuti risultati positivi anche simulando un'ipotetica situazione futura in cui la totale potenza installata è cos-tituita principalmente da impianti basati su fonti rinnovabili.

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Abstract

Several European countries have planned to reduce traditional thermal systems by replacing them with production units based on renewable sources, such as solar and wind ones, so that the targets imposed by the governments for the penetration of green energies can be achieved.

On the one hand it is positive from the environmental point of view, on the other hand it could create logistical and technical problems to the electric net-work. As power generated by renewable plants could be not controllable and foreseeable, energy unbalances and dispatching problems for conventional units can occur. Moreover the presence of distributed generation on low and medium voltage sides could lead to a power reversal from the distribution to the trans-mission networks, causing overvoltages and unwanted trips of protections.

Ancillary services, such as frequency and voltage regulation, are necessary to support renewable energy integration in order to make the system reliable. A method to provide these services can be the match between plants based on renewable sources and storage systems, so that the eect of the unpredictable behaviour can be reduced controlling the output power in function of the neces-sities. In particular Battery Energy Storage Systems (BESS), whose installations are expected to increase thanks to their versatility and good performances, oer the exibility in capacity, location and response required to satisfy a wider range of functions than many other types of storage.

The role of BESSs is analysed simulating three scenarios characterized by a dierent penetration of renewable sources. Simulation results show that BESSs, dierently from a past situation characterized by very low installations of wind and solar power plants, turn out to be important when the percentage of re-newables installed power similar to that of Italian network in 2013 is considered. Positive results are also obtained simulating a hypothetical future situation in

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which the total installed power is mainly constituted by plants based on unpre-dictable sources.

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Introduction

The purpose of this thesis is to study the role and the importance of electrochemi-cal storage systems in electric networks characterized by a signicant penetration of unpredictable renewable sources. The work is organized in an initial part that takes into account the topics treated in the literature and in a nal part in which the results of simulations performed with the software Plexosr _{are illustrated.}

In chapter 1 an overview of the electric networks of some European coun-tries in terms of energy production and installed power is given, emphasising the contribution and the problems of power plants based on unpredictable renewable sources.

Chapter 2 deals with the description of the main ancillary services, paying specic attention to how frequency and voltage regulations are performed in the Italian electric network. Moreover a short illustration of the ancillary services market in some European countries is explained, highlighting the compulsory and the optional services. In addition, the role and the problems of unpredictable renewable plants in the provision of these services are studied.

In chapter 3 the role of energy storage systems in electric networks is under-lined, paying attention to the importance that they can have in the provision of ancillary services. Moreover a classication of the most important storage technologies with their positive and negative aspects is shown.

Possible future scenarios in which storage systems are exploited for the im-provement of the network stability are listed in chapter 4. This section initially deals with large-scale applications, such as new intercontinental electrical con-nections and pilot projects in Italy, and then it also shows new small-scale de-velopments, such as domestic electrochemical storages and the use of exhausted batteries of electric vehicles for the network stabilization.

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of the working principle, of the main parameters and of the electric model is per-formed. Dierent types of rechargeable batteries are illustrated, highlighting and comparing drawbacks and advantages of each technology. A particular attention to Li-ion batteries and to Li-ion capacitors is given.

In chapter 6 the structure of a BESS is studied, explaining in particular the operation of electronic converters and of control systems. Furthermore the con-nection and working rules for these systems are analysed, following the Italian standards CEI 0-16 and CEI 0-21. The impositions related to the provision of ancillary services for electrochemical storage systems are also explained. In addi-tion, examples of BESSs produced by some big companies and their applications are described.

Chapter 7 deals with the simulation of the IEEE-30 bus test system in dierent scenarios. A short description of the used software precedes the modelling of the electric network which is exploited for the implementation of the simulations. For each scenario, that is characterized by a dierent penetration of renewable sources, the obtained results are shown, highlighting the importance that BESSs can have.

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Chapter 1 Overview of European electric

networks

In this chapter the characteristics of the electric networks of some countries in Europe, as the demand and the production of electric energy, and the classica-tion of the power plants installed will be analysed according to the ocial data provided by the national Transmission System Operator (TSO). In this way the positive and negative aspects of each electric system can be underlined, empha-sizing their ability to provide ancillary services and to support the integration of new plants based on renewable sources.

1.1 The Italian electric network

1.1.1 Introduction

Data refer to the year 2013 and they are provided by Terna [1], which is the Italian TSO.

The energy demand, similarly to that registered in 2003, was 318475 GWh with a decrease of 3% compared to the previous year. This trend follows the reduction of 1.9% already registered in 2012.

It is necessary to distinguish:

• gross electricity production. It is the sum of the energy measured at the Italian generators terminals;

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Table 1.1: Energy exchanges with neighbouring countries Imported energy [GWh] Exported energy [GWh]

France 12536.0 857.5 Switzerland 23341.5 1094.7 Austria 1506.2 20.1 Slovenia 5316.5 132.5 Greece 1637.7 95.4 Total 44337.9 2200.2

• net electricity production. It is the dierence between gross electricity pro-duction and the energy absorbed by auxiliary services and power plant transformers;

• nal energy consumption. It is the dierence between net electricity pro-duction and the pumping consumption.

The gross and the net electricity production were respectively 289803 GWh and 278832 GWh, while the nal energy consumption was 276337 GWh (-3.1% com-pared to 2012). It is possible to observe that 86.8% of the total electricity con-sumption has been covered by the national production.

In order to satisfy the demand, 44337 GWh (-2.2% in comparison with 2012) were imported from some European neighbouring countries: France, Switzerland, Austria, Slovenia and Greece. Moreover a small amount of energy (2200 GWh) was also exported from Italy as shown in table 1.1.

In 2013 total losses were 6.7% of total energy demand with a value of 21187 GWh, showing an increasing trend compared to the previous year.

It is useful to give a preview of the national energy production, separately analysing the dierent types of plants and showing the relevant increase of re-newable energy.

A growth of 24.7% led hydroelectric to reach its historic record with 54672 GWh of gross production. In this way it contributed to a development of renew-able sources (for instance hydro, solar, geothermal, bioenergies) with an impact in gross consumption of 33.9%. Driven by incentives established for renewable sources, solar and wind gross energy production reached respectively 21589 GWh (+14.5%) and 14897 GWh (+11.1%). A huge contribution was also given by bioenergies and geothermal with an increased production of 36.9% and 1.2%.

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1.1 The Italian electric network 25

Figure 1.1: Share of generation in Italy

Thermal energy represents at the moment, as in the past, the biggest contri-bution in the Italian electric system, covering 65.8% of the net production. It is in counter-trend compared to the renewable sources analysed here above showing a decrease of 11.5%.

Fig. 1.1 represents the share of gross electricity production in Italy. As already mentioned, it is possible to observe that the most important contribution comes from fossil fuels that are non-renewable sources, while the remaining part is given by renewables. In particular hydroelectric power plants are strongly exploited, combined with wind and solar systems that are more and more increasing in the last few years.

In the past also nuclear plants actively contributed, but since 1987 they were replaced by other types of sources.

The total installed power increased lightly (+0.4%) with a net generating power of 124750 MW. Thermal power plants were partially replaced by renew-able sources: there was a reduction of 2053 MW in thermal and an increase in photovoltaic (+2001 MW), hydroelectric (+129 MW) and wind plants (+440 MW).

The peak of power was recorded at 12 a.m. on July 26, 2013, reaching 53942 MW; lightly lower than the previous year when the maximum was identied on July 10, at 12 a.m.

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1.1.2 Topologies of Italian power plants

Travelling through Italy, it's possible to observe that the landscape and the cli-mate present a lot of variations depending on the position in which one is situated. The country can be simply divided in three big macro areas:

• continental, which is delimited by the Alps and by the imaginary line that connects Rimini to La Spezia. It represents the northern area of the country and it constitutes a connection between Italy and the rest of the continent; • peninsular, which extends in the Mediterranean Sea towards north west

-south east;

• insular, which is characterized by two big islands (Sardinia and Sicily) and some smaller ones.

Italy shows a prevalence of hills (41%), mainly distributed in the central and in the southern part of the country. Also a big part of mountainous areas are present (35.2%) with the Alps in the north and the Apennines along all the peninsular territory. Plain covers the smallest area (23.2%) and it is distributed mainly in the north, where the biggest area is the Po Valley, but also in the south.

The territory is characterized by an area of 324000 km2 _{and it presents a large} vertical extension. For this reason it is clear that climate, solar radiation, wind and water resources are strongly variable in the country. Moreover lakes and rivers are mostly present in the northern region because the rainfall is relatively high and there is a big presence of glaciers in the Alps.

The study of the geographical structure of Italy will be useful to understand the applied choices for the installation of the dierent power plants in the terri-tory.

Thermoelectric power plants

A traditional thermal power unit consists of a steam generator, a thermal prime mover, an electrical generator, thermal-cycle equipment, main and auxiliary ser-vices. The steam generator generally works with fossil fuels as natural gas, coal or oil and derivatives but can also work with radioactive materials. It can be classied [2] in four dierent technologies:

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1.1 The Italian electric network 27 • thermonuclear power plant. It uses a nuclear reactor to generate steam that it is exploited to produce electric energy through a turbine-generator group. This system is characterized by high installation costs but low working costs and the disadvantage is linked to the storage of the radioactive drosses; • traditional thermoelectric power plant. It is also called Rankine cycle and it

is based on the production of steam through the combustion of fossil fuels. It is characterized by lower eciency and higher fuel costs compared to thermonuclear plants;

• combined cycle power plant. It is constituted by a gas turbine and a wa-ter/steam Rankine cycle that uses exhaust gasses of the rst to generate steam with a heat recovery steam generator and to produce electric energy with a steam turbine and a generator unit. This system is characterized by higher eciency compared to the traditional system but also by higher installation costs;

• turbogas power plant. It is constituted by a compressor, a burner and a turbine. Air is previously compressed, then it passes through the burner where it is heated and later expanded in the gas turbine. It is characterized by high working costs and relatively low installation costs.

Thermoelectric power plants were the rst solution applied to produce electricity and nowadays they are the mostly used. Because renewable sources are dicultly foreseeable, the combined cycle and the traditional power plants constitute the basis of the generation of electricity thanks to the relatively high eciency at nominal power and to the high starting times, whereas turbogas is commonly used to cover peaks for a limited number of hours during the year because of the high working costs and of the relatively low starting time.

In Italy the most exploited fuel is the natural gas (37.9%), followed by coal (14.9%) and oil (7.2%). Since 1963 thermonuclear systems gave a little contri-bution to the electric demand and in 1987 they were totally replaced by other sources and technologies.

As shown in g. 1.2, power plants based on fuels strongly increased during years reaching an installed gross capacity in 20131 _{of 79274 MW with 4071 units}

1_{The maximum capacity of a power plant or of a group of thermal power plants is the}

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Figure 1.2: Development of the thermal installed capacity

that can be divided in producer (3434 units) and self producer2 _{(637 units).} Between 2012 and 2013 a reduction (-2.6%) of the installed capacity was registered, substituting the polluting and expensive traditional power plants with renewable sources like solar and wind.

In the northern part of Italy, the producer's power plants are 2557 with an installed gross maximum capacity of 34557 MW; whereas in the central and in the southern areas there are respectively 506 power plants with a power of 14138 MW and 371 power plants with a power of 26209 MW. It is clear that the highest production is concentrated in the northern and in the southern part of Italy.

Power plants can also be classied according to the possibility to work with dierent fuels. Most of them work with only one type of combustible that is mainly natural gas (1854 sections with a power of 34237.6 MW).

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1.1 The Italian electric network 29 Hydroelectric power plants

A hydro plant is a complex of hydraulic works, machinery, equipment, buildings and services for the conversion of hydraulic energy into electric energy. It is possible to make a classication [3] depending on the quantity of water available and on the possibility to store it:

• run-o river plants without storage. They use water as it comes when it is available and there is no possibility to store it. For this reason their generation capacity depends on the rate of ow of water and as consequence, it is possible that some water is wasted during rainy seasons with a low electricity demand;

• pondage plants. These plants are similar to the previous ones but it is possible to store water during lean periods and use it during peaks of energy demand. This type of plant is comparatively more useful and its generating capacity is not based on available rate of ow of water. The lling period of these plants, that is the time necessary to provide the reservoir with a volume of water equivalent to its useful capacity, is less than 400 hours and more than 2 hours;

• reservoir plants. They are those with a reservoir classied as seasonal regu-lation, or better with a lling period of 400 hours or more. Generally, water is stored behind a dam and it is available for the plant. This type of plant can be used eciently throughout the year and its capacity can be used ei-ther as a base load plant or as a peak load plant. Most of the hydroelectric plants are of this type. Pumped-storage systems are a particular type of reservoir plants and will be treated later.

Analysing some historical data, it is possible to observe that the production through hydro plants is one of the oldest in Italy, being present since 1887. In that year for a total gross electricity production of 3.5 GWh, 0.2 GWh were provided exploiting water as a source. So unlike other types of renewable sources, which are recording an increase in the last few years, hydroelectric plants have always contributed to the national production. Obviously the quantity of energy produced depends on the availability of water, so it is not possible to give a precise trend during the past few years for the data that refers to the hydroelectric energy production.

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Figure 1.3: Development of the hydroelectric installed capacity

The number of hydro plants installed in Italy on December 31, 2013 was 3258. Only 310 of these units had a power higher than 10 MW with a total installed gross capacity of 19262 MW. All the plants with a smaller capacity can be considered as small hydro-power (SHP) and they are classied by United Nations Industrial Development Organization (UNIDO) in:

• mini hydro, with a power between 100 kW and 1 MW; • micro hydro, with a power between 5 kW and 100 kW; • pico hydro, with a power lower than 5 kW.

SHP represented the majority of installations (2947) with an installed gross ca-pacity of 3121 MW. Moreover mini hydro has doubled between 2008 and 2013 and it is expected to increase further.

As it can be seen in g. 1.3 the maximum gross capacity is increased from 1963 to 2013 from around 13000 MW to 22383 MW. The Northern Italy, with a maximum gross capacity of 16287 MW and an average annual gross energy capability of 44319 GWh, represents the part of the Italian territory in which hydro plants are more productive.

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1.1 The Italian electric network 31 Renewable sources

Renewables play an important role in the national electricity system. There are dierent kinds of renewable sources [4] that can be exploited to produce electric energy:

• photovoltaic. It consists in the direct conversion of light into electricity through the use of a solar panel. Some materials exhibit a property known as photoelectric eect, that causes them to absorb photons of light and release electrons. When this free electrons are captured, a DC electric current is created and it needs to be converted in AC current through an inverter. For this type of plant there is the possibility to be connected to the electric network or to work independently;

• wind. Wind turbine consists of a propeller connected around a rotor that converts the wind force into torque that can be used to produce electric energy with a generator. Since the wind speed is not constant, it is necessary to use adjustable blades and to connect the generator to the grid through an electronic converter, which allows the rated frequency to be maintained; • geothermal. It consists in the exploitation of thermal energy, in the form of hot water and steam from the Earth, that comes mainly from the radioactive decay of some materials (such as uranium, thorium and potassium); • bioenergy. The organic waste of vegetable or animal can be treated to obtain

liquid, solid or gaseous combustible in order to produce heat and electric energy through a process of combustion. It is considered a renewable source because the carbon dioxide formed during the energy conversion is the same one that it is absorbed during the natural growth of the resource. In reality it can't be considered completely renewable because a small amount of pollution is produced with the transportation and the treatment of the materials.

Hydroelectric is also a renewable source but it has been already analysed because in the Italian network it is considered instrumental since the 19th century. It is necessary to underline that the energy generated by water previously pumped to the higher reservoir can not be considered renewable. For this reason on 22383

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Figure 1.4: Development of the installed capacity of renewable sources during the years

MW of gross installed capacity, only 18366 MW were considered as renewables in 2013.

The number of renewable installations throughout the country continues to grow, reaching in 2013 around 600000 installed units, driven especially by the increase of the photovoltaic plants. In 2013 plants powered by renewable sources [5] reached 40.2% of the total power installed in Italy. The gross ecient capacity of renewable plants installed in Italy at the end of 2013 was approximately 50000 MW, increased by almost 2200 MW compared to 2012.

In g 1.4 it's possible to observe the growth of the installed power that reached 49786 MW in 2013. This increasing trend mainly depends on solar and wind energy, even if in the last few years this occurred at a lower rate due to the incentive reduction, while hydroelectric contribution can be considered constant. As shown in g. 1.5 Lombardy, with 16%, was the region with the highest concentration of installed capacity among all Italian regions at the end of 2013. Tuscany, thanks to geothermal, was the region with greater power installed in Central Italy. As the southern Italy is concerned, Puglia was the region which recorded the highest percentage increase.

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Figure 1.6: Variation of energy produced by renewable sources

As depicted in g. 1.6, in 2013 the gross production from renewable registered a record of 112008 GWh, with an increase of 61018 GWh compared to 2000, year in which energy production was 52774 GWh.

This sources contributed with 38.6% to the gross energy production and with 31.3% to the gross domestic electricity consumption3_{. This evolution was mainly} linked to the photovoltaic, wind energy and bioenergy with an increase respec-tively of 35.4% (+21571 GWh), 23.5% (+14334 GWh) and 25.5% (+15586 GWh). The contribution given by the geothermal source remained more or less the same whereas the hydroelectric production showed an increase of 14.1% (+8574 GWh). From the geographical point of view, the regional distribution of renewable energy production is shown in g. 1.7.

In 2013 Lombardy was the region with the largest Italian production from renewable sources, representing 15% of the total energy produced. It was followed by Trentino-Alto Adige and Piedmont with a contribution respectively of 10.5% and 9.9%. It means that the Northern Italy was the most productive area of the country, but also The Southern Italy and islands produced an important quantity

3_{Gross domestic energy production represents the sum of gross electricity production and}

net electricity imports. It is dened as net of pumping if the electricity produced by pumping in hydro plants is not included

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Figure 1.8: Development of solar plants of electricity, contributing with 31.2% to the total production.

As the solar power plants are concerned, 591029 PV systems were installed in Italy at the end of 2013 for a total power of 18053 MW. Most of them, approxi-mately 58%, had a power between 3 kW and 20 kW and about 40% of the installed capacity referred to sizes between 20 kW and 200 kW. The power of photovoltaic systems was 36% of the entire renewable generation and contributed with 19% of the total energy produced by these.

In 2013 98% of the total PV plants were installed exploiting the incentives given by the government and a lot of them took advantages from the energy exchanges with the national electric network. Looking at g. 1.8 it is possible to observe that from 2008 to 2011 the number of power plants increased a lot (more than twice every year), whereas in 2013 the growth was reduced due to a cut of the incentives (only +8.2% of power compared to 2012).

The highest concentration of installations was focused in the northern area (54%), whereas in the central and in the southern part there was an installation respectively of 17% and 29% of plants. In the same order, the percentages of installed power was 44%, 18% and 28%. In particular Lombardy was the region with the highest number of PV plants (84338), followed by Veneto (80110). On the contrary, Puglia was the region with the highest installed power with 2555 MW, followed by Lombardy with 1991 MW.

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Figure 1.9: Development of wind plants

In 2013 the production of PV plants in Italy reached a value of 21589 GWh, with an increase of 14.4% compared to the previous year. The region that pro-duced the highest amount of energy was Puglia (17.2%), followed by Emilia Ro-magna (9.2%) and Lombardy (9%).

1386 wind farms were present on the Italian territory at the end of 2013, for a total installed power of 8561 MW. It means that aeolian, with a contribution of 17% on the total installed renewable plants, represented a very important resource for the country. 74% of these plants (1023) were characterized by a power lower than 1 MW, whereas 18.5% (256) had a power higher than 10 MW. All that plants characterized by a power lower than 200 kW that can exploit the national incentives are classied as mini aeolian. They are expected to in-crease in the future due to their low environmental impact and to the low payback time.

As shown in g. 1.9, between 2000 and 2013 a strong development of wind farms was registered in Italy, passing from 55 (363 MW) to 1386 (8561 MW) plants. In the last year there was an increase of 441 MW (+5.4%), that can be attributed fundamentally to plants with a power higher than 10 MW. In spite of this, the growth in number of installations was given by plants with smaller power.

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Figure 1.10: Development of plants based on bioenergy

It is very important to evaluate the environmental and territorial characteris-tics of sites, the presence of wind, the topography and the site accessibility before building wind farms. For these reasons, the Southern regions, with 96.7% of the total national installed power and with 81.2% of installations, turned out to be the most productive areas in 2013. In particular Puglia reached the highest power (2266 MW), followed by Sicily (1750 MW) and Campania (1230 MW). Instead in the northern and in the central area the greater contribution was given by Emilia Romagna, Liguria and Tuscany.

The Wind energy production also increased rapidly in thirteen years, passing from 563 GWh to 14897 GWh and reaching in this way 13% of the entire national generation. Once again, the record was achieved by the three southern regions mentioned before, which alone covered 60.2% of the entire national production.

Bioenergies represented an important resource in 2013, contributing with an installed power of 4033 MW (8% of the entire renewable generation). 2030 plants out of 2409 had a power lower that 1 MW, while only 68 plants were more powerful than 10 MW.

As shown in g. 1.10 starting from 2008 the rate of growth was very high, passing from an installed power of 1555 MW to 4033 MW in 2013. This

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increas-1.1 The Italian electric network 39

Figure 1.11: Development of geothermal plants

ing trend happened thanks to the systems based on biogas, which were realized exploiting the ministerial incentives.

Most of the installations were in the northern part of Italy (74.6%), with the primate of Lombardy, while in the central and in the southern area the highest contribution is respectively given by Lazio and Puglia.

Between 2000 and 2013 the energy production increased rapidly from 1505 to 17090 GWh, with an average rate of growth of 20.6%. Thanks to the increase in production from solid (+1 TWh) and gaseous (+7.45 TWh) biomasses, bionergy reached 15% of the entire renewable production in 2013. In terms of generation, the highest contribution was represented by Lombardy, Emilia Romagna, Veneto, Puglia and Piedmont covering 65% of the total generated energy.

Geothermoelectric gave the lowest contribution to the renewable energy pro-duction. Between 2010 and 2013 the installed power didn't change considerably, since in the last year only one extra plant with a power lower than 20 MW was put on service. Several installations were characterized by a rated power lower than or equal to 20 MW, representing 56.4% of the entire geothermal.

Considering the trend that characterized this source since 2000 in g. 1.11, it's possible to state that the only purposeful changes happened in the rst three years, passing from 627 MW to 707 MW. At the end of 2013, all the 34 plants

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Figure 1.12: Share of generation in France

present in Italy were installed in Tuscany, for a total power of 773 MW.

The energy production followed the trend of the installed power, reaching in the last year 5659 GWh (+1.2% in comparison with 2012) and covering with 5% the total renewable capacity. In spite of the low installed power, geothermal plants are able to produce a signicant quantity of energy, due to the constant availability of the natural source.

1.2 The French electric network

RTE, which is the French transmission system operator, provides the technical data [6] regarding the energy consumption and the installed power in 2014.

National demand fell by 6% due to the extremely mild weather and to the crisis, reaching the lowest value since 2002 corresponding to 465300 GWh. In the same year, energy production was 540600 GWh. The peak in consumption, strongly dependent on the electric heating systems, was 82500 MW on 9 December 2014, as opposed to the record consumption of 102100 MW reached in February 2012.

Nuclear, with an installed power of 63130 MW, held the primate in the na-tional electric eet (48.9%), without any increase compared to the previous year. It was followed by hydroelectric systems that reached 25411 MW (19.7%) and that represented the highest contribution of renewable sources. Thermal plants, due to dominance of nuclear and hydro, were progressively excluded from

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pro-1.3 The Spanish electric network 41 duction (-5% of installed power compared to 2013), reaching at the end of 2014 a value of 24411 MW (18.9% of the entire power). Aeolian and solar represented increasing sources, growing respectively by 11.8% and 21.2% and generating a power of 9120 MW and 5292 MW. A smaller contribution was given by biomasses that contributed with an installed power of 1579 MW (1.2%) with a signicant increase of 6.2%.

In g. 1.12 it can be observed that the order of relevance in energy production was the same of installed power, with a prevalence of nuclear (77%), followed by hydro (12.6%) and by thermal that strongly reduced its production by 39.6% reaching a contribution in the national demand of 5%. This decrease, combined with the boost of renewable energies, that produced 20% of the entire demand, enabled CO2 emissions in the electric energy branch to be further reduced by 40%.

Electricity trades were especially sustained in 2014 (92.4 TWh exported and 27.3 TWh imported) allowing France to remain the leading electricity exporter in Europe thanks to the exchange with six neighbouring countries among which Switzerland, Italy and Belgium. This is fundamentally supported by the low energy cost, guaranteed by nuclear power plants, and by the central position in the continental area.

1.3 The Spanish electric network

In this section the statistics data [7] regarding the Spanish electricity system dur-ing 2014 are published, considerdur-ing both the peninsular and the non-peninsular part which includes Balearic, Canary and other small islands.

The peninsular demand for electrical energy at the end of the year 2014 was 243486 GWh (-1.2% in comparison with 2013), while the non-peninsular demand was 14581 GWh (-0.9% compared to 2013). In addition exports and imports were respectively of 15772 GWh and 12228 GWh. The maximum instantaneous power recorded in the year occurred at 8:18 p.m. on February 4, when it reached a value of 39948 MW; while the maximum hourly demand was also recorded on February 4, (between 8:00 pm and 9:00 p.m.) with 38666 MWh.

As shown in g. 1.13 in the peninsula, nuclear covered 21.9%, wind 20.4%, coal 16.4%, hydroelectric 15.4% and cogeneration 10.4%. A lower contribution

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Figure 1.13: Share of generation in Spain

was represented by combined cycle with 8.5%, solar thermal and bioenergy which have jointly covered 7% of the demand.

It is possible to observe that renewable energies have continued to maintain a prominent role in the overall production of energy in the electricity system covering 42.8% of the total production. In absolute terms, renewable generation fell by 1% in comparison with the previous year, mainly due to the 6.1% drop in wind production. Despite this decline, it can be conrmed that wind power was the technology which made the largest contribution in the months of January, February and November. As the non-peninsular production is concerned, the total energy required was mostly covered by rankine and combined cycle plants with a low contribution of renewable sources like solar and aeolian.

Hydro and wind, which are the most important renewable sources, stabilised together a production of around 90000 GWh in 2013 and in 2014, after a decrease in the previous year when the climatic conditions were not favourable.

The installed power capacity in Spain remained virtually unchanged com-pared to the previous year and closed 2014 at 102259 MW. The largest variation recorded was that of coal, which reduced its contribution to 11482 MW (-1.4%), as a result of the closure of one generating unit. Combined cycle represented the predominant type of plant, with an installed power of 27206 MW, followed by wind with 23002 MW and by hydro with 17787 MW. The renewable production didn't register a particular growth compared to the previous years.

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1.4 The German electric network 43

Figure 1.14: Share of generation in Germany

1.4 The German electric network

The German electric network is interested by four dierent managing authorities that separate the country in independent areas. Data [8, 9] refers to 2014 and take into account the overall national system without distinguishing the dierent TSOs' networks.

The registered gross electricity consumption was 576300 GWh (-3.8% in com-parison with 2013) with a peak in power of 84000 MW, reached on December 7 at 5:00pm, because of the high heating systems consumption.

As shown in g. 1.14, lignite and hard coal accounted for about 44% of all the electricity produced in Germany in 2014, while nuclear power, which is to be phased out by 2022, produced 16% of the entire energy production. Renew-ables accounted for just over a quarter of all electricity (25.8%), with the highest contribution given by wind and biomasses.

In July, Germany had an installed capacity of 192 GW with a renewable integration of 83 GW and about 44% of this power prevalently given by solar and aeolian. This is the result of Germany's aggressive policies supporting renewable energy, particularly through the feed-in taris. Natural gas, hard coal, brown coal (lignite) also represented a signicant part of the installed power, that is destined to grow in the next few years to compensate the elimination of not insignicant nuclear plants. In order to allow the integration of aeolian and solar, which alone generated in 2014 a peak of power of 34 GW, nuclear power plants had reduced their base load generation by about 10% while lignite plants by about 30%.

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Figure 1.15: Share of generation in Denmark

Germany has signicant interconnection capacity with neighbouring EU mem-ber states such as Austria, Switzerland, Czech Republic, Denmark, France, Lux-embourg, Netherlands, Poland and Sweden. In 2012, Germany had 21.3 GW of available interconnection capacity, that was exploited for less than half. Ger-many was a net energy exporter with 35900 GWh the last year, prevalently with Netherlands and Austria; while it drew net energy imports mainly from Czech Republic.

1.5 The Danish electric network

The statistic data [10, 11] used in this section are provided by Energinet, which is the transmission system operator for electricity and gas in Denmark. In 2014 the energy consumption was 33471 GWh and it has fallen by 1.7% since 2013. In order to supply this demand, the net energy generated was 30615 GWh while the net imported electricity from neighbouring countries was 2855 GWh.

As shown in g. 1.15 the most important contributions came from wind tur-bines and thermal plants based on fossil fuels, producing respectively 13079 GWh and 13054 GWh. Other technologies, such as hydroelectric, photovoltaic and bioenergies provided respectively 16 GWh, 597 GWh and 3871 GWh.

2014 was characterized by a historically low level of thermal electricity gener-ation in Denmark, that followed the previous years production decline, combined with the increase of wind and photovoltaic. In particular, the generation from

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1.6 Importance of ancillary services 45 aeolian increased by 18% from 2013 to 2014, leading to cover 39.1% of Danish electricity consumption. This was a new record and it can be explained through the installation of a new oshore wind farm at Anholt, which was put on service in 2013, but it realized its rst full production in 2014. Moreover, wind power generation is expected to increase up to 23300 GWh in 2024.

As regards installed power in Denmark, in 2014 the capacity of wind tur-bines and photovoltaic cells increased by 76 MW and 49 MW respectively, for a total power of 5500 MW. In comparison, in the same year, the capacity of thermal plants fell by 11 MW, registering a total installed power of 9553 MW. Wind turbines and solar cells capacity is expected to increase up to 6900 MW in 2020, while installed power of thermal plants is expected to decrease up to approximately 5700 MW.

As already mentioned, it's useful to underline the fact that the Danish network is connected to Norway, Sweden and Germany, and there is interchange (import and export) of electricity across the borders every hour of the year.

1.6 Importance of ancillary services

Considering the previous sections, it is possible to state that most of the coun-tries analysed before present an important share of renewables that contribute signicantly to the total energy production. These sources keep on increasing and they are expected to grow more and more in the future, indeed targets such as 20% of renewable energy by year 2020 and 30% by year 2030 are not un-common [12]. This aim is supported by the fact that in Europe, most of the countries currently utilize feed-in taris that either set annually xed prices for generated electricity according to the type of renewable generation technology, or adds technology-specic bonuses.

If on the one hand it is a positive aspect from the environmental point of view, on the other hand it can create logistical and technical problems to the electric network.

First of all power generated by renewable sources could be not controllable. This may cause for example production of energy during low request periods with the consequent necessity to turn o the power plants as combined cycles, that are designated to work continuously.

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Furthermore some renewable sources can be unpredictable because their per-formances strongly depend on weather conditions. Intermittent sources can sud-denly vary their output power or cease to supply the grid causing an energy imbalance between demand and production, with the consequent variation of the network parameters from the rated values. For instance, big power deviations were evidenced in Denmark in January 2005 and in Northern Germany in De-cember 2004 with a decreased capacity respectively of 83% in 6 hours and 58% within 10 hours. These situations were caused by disconnections of large portions of wind farms due to strong wind storms.

Conventionally [13], the power ow in electrical grids is unidirectional: current ows from large power generation units over the transmission and distribution grid to the end-consumer causing a voltage drop. With distributed generation, the power ow patterns are altered and they can become bidirectional. If on the one hand it implies a reduction of the net load in lines and of losses, on the other hand it can lead to overvoltages in nodes far from the substation and to undesired trips of protection circuit breakers.

Ancillary services, such as frequency and voltage regulation, are necessary to support renewable energy integration in order to keep the generation and the load demand in balance and to make the system reliable.

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Chapter 2 Ancillary services

Ancillary services are some operational reserved services procured by the TSO [14] needed to support the transmission of power, to maintain reliable operation and to ensure the required level of power quality and safety for the electrical networks.

2.1 Classication of ancillary services

A set of ancillary services that the TSO can provide to the electric network are: • Frequency control (FC). One of the main problems that occur in the power system when there is an imbalance in supply and demand is to maintain the network frequency between imposed limits, by regulating the active power output. Large frequency deviations caused by the excess of generation can lead to disconnections of all generators or to relays trip with the consequent load shedding. Drop in frequency and imbalance conditions can also be caused by the increase in load demand. The control frequency system is divided into primary, secondary and tertiary parts which can be controlled directly through automatic generation controllers or manually by TSO's operators;

• Voltage control (VC). Due to variations of generation and demand, a con-trol service is needed in order to maintain the bus voltage at a certain level, maximizing in this way the active power that can be transferred (increasing or decreasing the reactive power consumption). The industrial tools useful

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to full this need are reactive power compensators based on static con-verters, capacitor banks or synchronous generators with excitation control. Another way to control voltage in the network buses is the use of on-load tap changer transformers;

• Reserve services. They allow to obtain the frequency control through the management of the generator available power. The upper reserve is dened as the dierence between the maximum and the instantaneous power of a production unit, while the lower reserve is the dierence between the instan-taneous and the minimum power of a generator. These services are classied in spinning reserves (SP), non-spinning reserves (NSP) and standing or sup-plementary reserves (ST). Spinning reserves are generally provided by ther-mal power plants that work in part-load operation and they are constituted by generating units on-line and synchronized to the grid. Non-spinning re-serves dier from the previous one only in terms of lack of requirement of being on-line and synchronized to the grid. Standing reserves, which can be considered as backup for the spinning one, are an o-line service performed by gas turbines and pumped hydro storage that in some special cases can be brought on-line to the grid quickly to meet additional contingencies. SP and NSP can start rapidly and be fully available within 10 minutes, while ST can be accessible in 30-60 minutes. The cost of spinning reserves is more than half (almost 52%) of overall operational cost of reserve services, while supplementary reserves are less expensive because they don't require auto-matic generation control and they are not necessarily maintained on-line. These services may also be provided by controllable loads or electrochemical storage systems;

• Black start capability (BS). This service is the capability of a generating unit to start up without external power supply when large blackouts take place in the power system;

• Remote automatic generation control (RG). This is the capability to regu-late the power system frequency through a centralized control far from the network;

• Grid loss compensation (GL). It is a service dedicated to the compensation of transmission losses, that constitute 2% or 3% of the total average losses,

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2.2 Frequency and voltage regulation 49

Figure 2.1: Example of electrical network for frequency regulation

between the sources and the loads. Generators are used to compensate active power losses;

• Emergency control actions (EC). This ancillary service deals with mainte-nance and use of special equipments such as dynamic braking resistors or power system stabilizers to maintain a secure transmission network.

2.2 Frequency and voltage regulation

Among ancillary services, voltage and frequency regulation are the most impor-tant in order to maintain the stability of the grid. As conventional generators are concerned, these two services present dierent characteristic between them and they must be studied separately.

The rst dierence [15] regards the time constants: voltage regulation is char-acterized by a very fast response, in the order of hundreds of milliseconds, while frequency regulation shows a slower behaviour, due to the high thermal and me-chanical inertia of the groups.

Another dierence is linked to the injection or absorption of active and reac-tive power in order to control frequency and voltage of the network. In g. 2.1 a transmission line, in which shunt parameters and longitudinal resistance are neglected, connects together a source and a load. In this condition the active and reactive power which is transferred along the line is:

P = V E x sinδ (2.1) Q = V E x cosδ − V2 x (2.2)

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where x is the longitudinal reactance [Ω] and δ is the load angle [rad] between the voltages V and E [V].

If δ is considered small (so that cosδ ' 1 and sinδ ' δ) and ∆V = E − V , the equations 2.1 and 2.2 become:

P ' V E

x δ (2.3)

Q ' V

x∆V (2.4)

It is possible to observe that:

• P ∝ δ. In order to control the frequency, which is strictly related to the load angle, the active power ows along the line must be changed;

• Q ∝ ∆V. The voltage variation and its regulation are related to the reactive power ows along the electric grid.

In the following, how frequency and voltage regulation are performed will be illustrated, paying special attention to the Italian situation.

2.2.1 Frequency regulation

A generating group [16] is composed by a prime mover driven by various energy sources (including steam, gas or water), which produces a mechanical power (Pm) and creates a mechanical torque (Tm), and by a generator that delivers an electrical power (Pe) and creates an opposite electrical torque (Te).

The dynamic behaviour of the system is described by the swing equation: Jd

2_δ

dt2 = Tm− Te (2.5)

where J is the total moment of inertia of the rotor masses [kg m2_{] and} d2_δ

dt2 repre-sents the rotor acceleration [rad/s2_].

In balance conditions, the electrical frequency f [Hz] of the power system is the same across the entire areas and all the generators rotors rotate at the same angular speed ω [rad/sec]. In this situation, since Tm = Te, the rotor does not change its speed and the frequency remains constant.

Contrary, an unbalance can be caused by a change of the electromagnetic or mechanical torque due to the following reasons:

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2.2 Frequency and voltage regulation 51 • unpredictable variation of the load;

• reconguration of the electric network; • variation of the prime mover power;

• faults and consequent opening of the protections.

This causes a transient in which, in the rst instants, a variation of the kinetic energy stored in the rotating masses occurs. The result is a variation of the frequency from the nominal value. If Tm > Te the rotor accelerates causing an increase of frequency, while if Tm < Te the rotor decelerates causing a decrease of frequency.

As shown in eq. 2.6, the higher is the inertia of the system, the higher is the transient duration:

df dt =

∆P

2Hf0 (2.6)

where df/dt is the rate of change of frequency [Hz/s], ∆P is the power change in per unit, H is the system inertia [s] and f0 is the initial frequency [Hz].

Primary frequency regulation

In the time instants after frequency variation [17], speed regulators of the pro-duction units modify automatically the injected mechanical power, restoring in this way the energy balance. This practice is called primary frequency regulation and it is compulsory for all the production units with an ecient power not lower than 10 MW.

All participants must guarantee a reserve of active power higher than or equal to 1.5% of their ecient power. Therefore, in normal operating conditions each generator can work within the following limits:

Pmin = Ptm+ 1.5%Pef f Pmax = Pma− 1.5%Pef f

where Ptm is the technical minimum power, Pef f is the ecient power and Pma is the maximum available power.

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Each production unit must dispense a share ∆Pe of the available primary reserve taking into account the frequency variation ∆f and the degree of statism1 σp imposed in the regulator in function of the following equation:

∆Pe = − ∆f 50 Pef f σp 100 (2.7)

All speed regulators must respect the following characteristics: • measurement precision better than 0.02%;

• insensitivity not higher than ±10 mHz;

• ability to operate with a statism between 2% and 8%, and with a frequency between 47.5 Hz and 51.5 Hz;

• possibility to avoid their operation from 0 mHz to 500 mHz (deadband); • ability to work up to 46 Hz for a limited time.

Moreover the generating units must be able to provide at least half of the required power and the total power respectively within 15 seconds and 30 seconds. They must be also able to provide the regulating contribution for at least 15 consecutive minutes.

Secondary frequency regulation

At the re-establishment of the energy balance, obtained thanks to the primary frequency regulation, the power system operates in a new steady state condition in which frequency diers from the rated value. For this reason a secondary fre-quency regulation, that establishes the nominal value of the frefre-quency, is needed.

The participants must provide a power reserve not lower than:

• the highest between ±10 MW and ±6% of the maximum power for thermal plants;

• ±15% of the maximum power for hydro plants.

1_{Statism: it is the ratio in percentage between the frequency variation, expressed in p.u. of}

the rated frequency, and the variation of the electric power, expressed in p.u. of the ecient power

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2.2 Frequency and voltage regulation 53 Furthermore all units must be able to use the reserve of power for at least two hours.

Speed regulators of the production units that participate to this practice re-ceive a signal from an automatic centralized device (network regulator) and they adjust the generation exploiting their reserve. The regulating signal is used to cancel the error of frequency and of the programmed power exchange between control areas.

If the network operates in island mode and the secondary regulation is not available, the Local Frequency Integral (LFI) function operates in order to re-establish the nominal value of frequency.

Tertiary frequency regulation

The tertiary frequency regulation, dierently from the others, is executed with the imposition of the TSO and it is needed to restore the reserves used for the secondary regulation.

This service is made by generating groups that are already synchronized with the electric grid (with a margin of power available) or by units that are able to be connected in parallel with the network in a short time.

Dierently to primary and secondary regulation, this one is manually con-trolled by TSO, that decides which units participate and how much power they provide.

It is possible to distinguish the reserve of power in:

• ready reserve: it must be available within 15 minutes and it must operate for at least two hours;

• cold reserve: it must be available within 60 minutes from the request and it must operate for at least eight hours.

2.2.2 Voltage regulation

Sudden and slow voltage variations are typical in the nodes of electric systems [18]. The former are generally voltage dips (caused by commutations, switch-ing operations and short circuits) or icker (caused by start of motors and arc furnaces), while the latter are related to the change of load diagrams.

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It is important to maintain voltages around their nominal values in buses, so that the behaviour of electrical devices is ensured and transmission lines can carry their nominal power. This shows the necessity to perform the voltage regulation. Studying the electric grid already used for frequency regulation (g. 2.1), where the load is considered inductive and the line resistance is neglected, it is possible to express the voltage drop as:

∆V =√3(xI sin ϕ) (2.8)

where line to line voltages are considered, I is the current [A] absorbed by the load and ϕ is the power factor angle [rad].

Eq. 2.8 can be written in p.u.: ∆V =. ∆V E = xI sin ϕ E E E = xQ E2 = xQ E2 A A = . xQ. (2.9)

where Q is the reactive power absorbed [Var] and E is the line to line voltage at the load terminals [V].

It is clear that the voltage drop depends on the reactive power ow in the buses. Moreover eq. 2.9 can also be expressed in function of the short circuit power Asc of the bus [VA]:

∆V =. xQ E2 =

Q Asc

(2.10) At constant Q, it is possible to state that voltage drop decreases with the increase of short circuit power of the bus, that is when the network connection is strong. Manual grid voltage control [19], largely used by system operators up to some years ago, typically involves the control of the reactive power produced by each generating unit in function of a forecast. This conventional approach is now considered quite unsatisfactory because:

• the real situation is dierent from the predicted one;

• the dispatching commands are based on written requirements or requested by the system operator. For this reason the control action is not ecient; • there isn't possibility to achieve simultaneous and optimised control actions; • it is not possible to have a direct control of the support given by power

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2.2 Frequency and voltage regulation 55

Figure 2.2: Hierarchical scheme of the voltage regulation in Italy

It is clear that an automatic voltage control system able to monitor in a better way and in real-time the network is needed. In Europe a hierarchical system based on the subdivision of the network in control areas and an automatic coordination of reactive power sources have been introduced. In particular primary (PVR), secondary (SVR) and tertiary (TVR) voltage regulation can be distinguished.

The main reasons supporting coordinated automatic real-time voltage regu-lation can therefore be summarized as follows:

• the quality of power system operation is improved, in terms of reduced variation around the dened voltages prole across the overall transmission network;

• the security of power system operation is enhanced, in terms of reactive power reserves kept available for emergency conditions;

• the transfer capability of power system is improved, with reduced voltage instability and collapse risks;

• the eciency of power system operation is increased, in terms of minimiza-tion of active losses;

• the controllability of the voltage ancillary service is simplied.

In g. 2.2, a schematic diagram of the Italian hierarchical control system is shown in order to explain how the dierent regulators interact between them.

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Primary voltage regulation

This practice consists in a local and independent control of voltage at the genera-tor terminals or at the HV bars and it is mandagenera-tory for all the production units. There is an Automatic Voltage Regulator (AVR) which, together with the gen-erator excitation, controls the reactive power ows and consequently the voltage. Generally the set point of the voltage reference is manually selected, following the imposition of GRTN (Gestore Rete Trasmissione Nazionale). In some cases the reference value takes into account some correction signals as the compound, that increases the voltage reference proportionally to the reactive power generated, and as the one produced by the Power System Stabilizer (PSS), that damp the electromechanical oscillations of the rotor during the transients. Furthermore the voltage reference must be able to vary its value from 80% to 110% in comparison with the nominal one.

Regulators also act as a safety system for the generators, allowing them to work within the thermal and mechanical limits.

Secondary voltage regulation

The design starting point requires a proper subdivision of the overall grid into control areas, in which pilot nodes are dened. These pilot buses have high short circuit power and they are chosen in order to impose voltages on the other electrically close nodes. It is also required to have low electrical coupling between pilot nodes so that dynamic interaction between secondary control loops can be avoided.

Two or more neighbouring areas which are controlled by the same Regional Voltage Regulator (RVR) compose an electrical region. This device allows to keep the desired voltage at the pilot nodes providing each area with a specic reactive power signal, which is in part sent to the electric stations and in part to the power plants, in function of their power capability. When a power plant receives the imposed signal, a voltage and reactive power regulator (REPORT) organises the reactive power produced by each generating units acting on the set-points of their AVR. For instance if the reference is 25% , each unit will provide a reactive power equal to 25% of its maximum.